Empowering the Future: Cutting-Edge Developments in Supercapacitor Technology for Enhanced Energy Storage

Abstract

The accelerating global demand for sustainable and efficient energy storage has driven substantial interest in supercapacitor technology due to its superior power density, fast charge–discharge capability, and long cycle life. However, the low energy density of supercapacitors remains a key bottleneck, limiting their broader application. This review provides a comprehensive and focused overview of the latest breakthroughs in supercapacitor research, emphasizing strategies to overcome this limitation through advanced material engineering and device design. We explore cutting-edge developments in electrode materials, including carbon-based nanostructures, metal oxides, redox-active polymers, and emerging frameworks such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These materials offer high surface area, tunable porosity, and enhanced conductivity, which collectively improve the electrochemical performance. Additionally, recent advances in electrolyte systems—ranging from aqueous to ionic liquids and organic electrolytes—are critically assessed for their role in expanding the operating voltage window and enhancing device stability. The review also highlights innovations in device architectures, such as hybrid, asymmetric, and flexible supercapacitor configurations, that contribute to the simultaneous improvement of energy and power densities. We identify persistent challenges in scaling up nanomaterial synthesis, maintaining long-term operational stability, and integrating materials into practical energy systems. By synthesizing these state-of-the-art advancements, this review outlines a roadmap for next-generation supercapacitors and presents novel perspectives on the synergistic integration of materials, electrolytes, and device engineering. These insights aim to guide future research toward realizing high-energy, high-efficiency, and scalable supercapacitor systems suitable for applications in electric vehicles, renewable energy storage, and next-generation portable electronics.

1. Introduction

Energy storage systems (ESSs) are critical for addressing efficiency, power quality, and reliability, and they are vital for contemporary power systems, particularly within the context of direct current (DC) and alternating current (AC) systems. Their role in maintaining grid stability and facilitating the integration of renewable energy sources (RESs) is indispensable [1]. Beyond traditional grid applications, ESSs are also critical in specialized domains such as aircraft, shipboard systems, and electric vehicles, where they manage peak load demands and improve overall system reliability and efficiency [1]. In the realm of microgrids, which are gaining prominence as decentralized and resilient energy solutions, ESSs serve to mitigate the intermittency challenges posed by renewables. Their ability to store excess energy during high generation periods and release it during peak demand or low renewable output enhances the reliability and resilience of these systems, thereby ensuring a consistent power supply and optimizing the utilization of locally generated green energy [2,3,4,5]. Microgrids, particularly those utilizing DC power, require careful consideration of the power exchange dynamics influenced by load conditions and power generation characteristics. These dynamics are categorized into low-frequency and high-frequency components, with the former reflecting the gradual variations inherent to renewable sources and regular energy consumption patterns [6]. High-frequency components, on the other hand, are influenced by rapid fluctuations that can impact system stability and necessitate advanced energy storage solutions to maintain equilibrium. Electrochemical capacitors, or supercapacitors, are emerging as a significant energy storage solution due to their high-power density and ultrahigh cyclic stability [7]. These characteristics position them favorably in the quest for high-performance energy devices capable of supporting the growing demands of technology. Despite their lower energy density compared to batteries, supercapacitors are the subject of extensive research aimed at pushing the boundaries of charge storage capabilities. Advancements in the understanding of electrical double-layer formation and pseudocapacitive and intercalation-type (akin to batteries) behaviors have significantly improved the electrochemical performance of supercapacitors. The introduction of innovative nanostructured active materials, such as carbon-, metal-, redox-polymer-, metal–organic framework-, and covalent organic framework-based electrodes, coupled with advanced electrolytes that offer superior stability in conventional aqueous and novel systems, has further bolstered their capabilities [7]. Additionally, the meticulous examination of processes occurring at the electrode–electrolyte interface has led to refined techniques and a roadmap for the next generation of high-performance supercapacitors. The increasing demand for energy and the need for sustainable energy systems have made ESSs an area of renewed interest. The development of sustainable energy systems is critical, due to factors such as declining fuel prices, geopolitical conflicts, and pandemics. Decreasing fuel prices have slowed investment in renewable energy, while geopolitical conflicts and pandemics highlight the need for resilient energy systems that can operate independently. ESSs are essential for providing reliable backup power and enabling microgrids to operate independently of the larger grid. Therefore, developing effective ESS technologies is crucial for creating sustainable energy systems that meet modern society’s demands while mitigating the impact of external factors [8]. A multitude of energy storage solutions are available, categorized into electrochemical, mechanical, electrical, and hybrid systems. Within the electrochemical domain, we find batteries, hydrogen storage fuel cells, and flow batteries as prominent examples. On the mechanical side, pumped hydroelectric energy storage (PHES), gravity energy storage (GES), compressed air energy storage (CAES), and flywheel systems are notable. Additionally, supercapacitor energy storage (SES) and superconducting magnetic energy storage (SMES) represent distinct electrical storage technologies. Hybrid configurations often integrate a combination of these methods, such as merging batteries with PHES or pairing supercapacitors with thermal storage. The selection of the most appropriate technology hinges on various factors unique to each application, such as system requirements, economic viability, and the performance metrics of the chosen solution. These factors are essential to ensure an optimal match between the storage technology and its intended use, which can significantly influence the overall efficiency and effectiveness of the system in question [9]. Batteries are widely used energy storage devices that meet the requirements of different industrial and consumer applications. They offer high energy density, making them suitable for applications requiring long discharge times. However, batteries have limitations such as lower power density, slower charge–discharge speeds, and limited cycle life compared to supercapacitors [10]. Lithium-ion batteries are a common type of battery used in high-power storage applications. Supercapacitors (SCs) are energy storage devices that offer superior power density, faster charge–discharge speeds, and longer cycle life compared to batteries [11]. They store energy through the accumulation of electric charge at the interface between an electrode and an electrolyte [12]. SCs are suitable for applications requiring high-speed energy delivery, such as hybrid vehicles and wearable electronic devices [13]. However, SCs have lower energy storage capability compared to batteries, which has driven research efforts to increase their energy density. Hybrid energy storage systems (HESSs) combine the advantages of batteries and supercapacitors to achieve high energy and power density [14]. A battery–supercapacitor HESS (BS-HESS) is widely used in renewable energy integration, smart grids, and microgrids. HESSs require sophisticated energy management systems (EMS) to coordinate energy flows between the battery and the supercapacitor, ensuring efficient and safe operation [15]. This review is particularly timely in light of several converging developments in the field of energy storage. Recent breakthroughs in hybrid supercapacitor systems, combining battery-like and capacitor-like behaviors, have opened new frontiers for achieving both high energy and power densities. Simultaneously, there is a growing emphasis on sustainability-driven research, including the use of environmentally benign materials and green electrolytes, aligning with global efforts to reduce carbon emissions and enhance energy efficiency. Furthermore, the increasing demand for next-generation applications—such as flexible and wearable electronics, electric vehicles, and grid-scale renewable energy integration—necessitates a comprehensive update that captures the latest innovations in materials, device architectures, and performance optimization strategies. This review aims to bridge these emerging needs by offering a holistic synthesis of current advancements and future directions in supercapacitor technology.

2. Energy Storage Technologies

2.1. Lithium-Ion Batteries

Lithium-ion batteries are vital to a range of contemporary applications, from portable electronic devices to the aerospace sector and electric vehicles, due to their proficient ability to interconvert chemical and electrical energy during discharge and charge cycles [16]. Their design typically features a series or parallel arrangement of electrochemical cells, each encompassing a negative electrode (anode) and a positive electrode (cathode), separated by an electrolyte and a separator within the cell itself [17,18]. The choice of electrolyte and separator has a substantial impact on the battery’s performance and security profile. Solid-state electrolytes represent a burgeoning technology that promises to enhance charge and discharge rates while bolstering safety [19]. However, the challenge with polymer-based electrolytes is their limited electrochemical stability, which restricts the selection of materials that can effectively conduct ions. In contrast, liquid electrolytes present a broader spectrum of options, as solvents boast varying viscosity and dielectric constants that influence performance in distinct ways [20]. In the realm of cathode materials, commonly utilized substances include lithiated metal oxides, with notable examples being LiCoO₂, Li-Mn-O, and LiFePO₄, along with lithium-layered metal oxides [21]. Each of these materials showcases distinct electrochemical properties, which in turn have significant implications for energy density, power density, and safety aspects, as outlined in

Table 1. Comparison between three cathode materials [23].

Parameters	Li-Ion Manganese	Li-Ion Cobalt	Li-Ion Phosphate
Specific energy density (Wh/kg)	100–135	150–190	90–120
Cell voltage (nominal V)	3.8	3.6	3.3
Cycle life (80% discharge)	500–1000	500–1000	1000–2000
Internal resistance (mΩ)	25–75	150–300	25–50
Fast charge time (Hours)	<1	2–4	<1

. For instance, while lithium cobalt oxide boasts high energy density, it is associated with thermal stability and safety concerns. Conversely, lithium iron phosphate offers improved safety and thermal stability, albeit at the cost of a lower energy density. Therefore, the selection of cathode materials requires careful consideration to achieve an optimal balance between various performance metrics and safety requirements. During the charging phase, the cathode material undergoes a transformation into lithium ions, which migrate through the lithium salt electrolyte toward the anode. Upon reaching the anode, these ions combine with incoming electrons from an external circuit. The electrolyte, primarily consisting of organic carbonates of lithium such as LiPF6, plays a critical role in facilitating this reaction [22]. It is essential to note that the properties of the electrolyte, such as the ionic conductivity, viscosity, flammability, and thermal stability, are significantly influenced by the choice of solvent and additives. Consequently, tailoring the electrolyte formulation is crucial for optimizing the battery performance, safety, and longevity. This is achieved by enhancing the efficiency of the ion transfer and thermal stability, which are key factors in prolonging the service life and reducing the risk of failure or degradation.

Lithium-ion batteries are celebrated for their exceptional energy density, swift responsiveness, prolonged cyclic endurance, and remarkable efficiency. Notably, the discharge voltage characteristics of Li-Mn and Li-phosphate variants display a significantly flat profile, with approximately 80% of the total energy capacity being accessible within this range. This feature greatly simplifies the design process for various applications. Their prevalence is particularly evident in the realms of portable electronic devices and as promising components within electric and hybrid vehicles. Despite these benefits, the integration of lithium-ion batteries on a large-scale faces hurdles, primarily due to the substantial costs stemming from the requirement for specialized packaging and sophisticated internal overcharge safeguarding mechanisms. Moreover, the introduction of these batteries into power systems and the intricate management of battery energy storage systems connected to the grid present additional complexities that must be addressed [24,25,26,27].

2.2. Pumped Hydro Energy Storage (PHES)

Pumped hydroelectric storage (PHS) is an established and widely-implemented method in the renewable energy sector for addressing the storage of electrical power. This technology serves a critical function in minimizing the variability inherent in renewable energy sources by accumulating electrical energy during off-peak periods and discharging it during times of heightened demand, as depicted in (Figure 1). The PHS operates through a mechanism that employs two water basins at varying altitudes. The excess electricity generated during low-demand intervals is harnessed to transfer water from the lower basin to the higher one. Conversely, during peak demand, the water is released from the upper reservoir, descending through turbines to produce power as it returns to the lower basin. This process effectively bridges the gap between supply and demand, enhancing the reliability and efficiency of renewable energy systems.

The capacity for energy storage in PHES is contingent upon the water volume retained and the altitudinal disparity between the two interconnected reservoirs. This system plays a critical role in bolstering the stability of electrical grids and enhancing their overall reliability [29,30].

2.3. Compressed Air Energy Storage (CAES)

CAES, or compressed air energy storage, represents a progressive solution for storing energy efficiently. This technology operates by compressing air and storing it in subterranean caverns or designated tanks during off-peak electricity consumption hours. The compressed air is later released during peak demand, undergoing a controlled expansion that propels turbines to produce power. The system is characterized by a two-phase cycle, comprising compression and expansion. During the compression phase, excess electrical energy is astutely utilized to compact and stockpile atmospheric air. In the subsequent expansion phase, the confined high-pressure air is gradually released, facilitating its expansion to drive turbines for electricity generation. It is noteworthy that, despite boasting an energy efficiency spectrum of 40–70%, CAES remains a pivotal component in managing fluctuations in energy demand. Its contributions to the electrical grid’s stability are substantial, illustrating its significance in the broader context of energy management [31,32,33].

2.4. Supercapacitor Energy Storage

Supercapacitors, or ultracapacitors, stand out as a unique category of energy storage devices, bridging the characteristics of typical capacitors and batteries. They leverage electrochemical mechanisms to store energy, which is fundamentally different from the electrostatic approach employed by conventional capacitors. This advanced technology provides supercapacitors with substantially greater energy storage potential. These high-performance devices are known for their exceptional power and specific capacitance properties [34]. The energy storage in supercapacitors is facilitated through a combination of electrical double-layer capacitance (EDLC) and pseudocapacitance. EDLC is characterized by the buildup of ions at the juncture of the electrode and electrolyte, which generates an electrical double layer without involving charge transfer across the boundary [35]. In contrast, pseudocapacitance is a Faradaic process that relies on redox reactions to store charge across the electrode–electrolyte interface. The effectiveness of supercapacitors is contingent upon various factors such as the nature of the electrode material, the composition of the electrolyte, and the intricacies of the device’s architecture [36]. For optimal performance, the electrode material should possess a high surface area to enhance capacitance, excellent electrical conductivity to support efficient charge flow, and robust stability in electrochemical environments [37]. Additionally, the selection of an electrolyte with high ionic conductivity and a broad electrochemical window is crucial for rapid ion movement and elevated operational voltages. In terms of device design, minimizing the internal resistance is vital for reducing energy losses, while optimizing the pathways for ion diffusion is key to achieving superior overall performance.

2.4.1. Types of Supercapacitors

Supercapacitors, also known as ultracapacitors, are distinguished into three primary categories: electrochemical double-layer capacitors (EDLC), hybrid capacitors, and pseudocapacitors, as illustrated in Figure 2. These distinctions arise from variations in their material composition, manufacturing techniques, and specific design features of their electrodes, which in turn influence their suitability for different application domains. Electrochemical double-layer capacitors (EDLCs) are a subset that can be further divided into three types based on the materials utilized for constructing their electrodes. These include capacitors that employ activated carbon, carbon nanotubes, and graphene as the foundation for their energy storage capabilities. Activated carbon is widely recognized for its high surface area and excellent electrical conductivity, making it a popular choice for EDLC electrodes. Carbon nanotubes, on the other hand, offer unique structural properties that enhance the performance of these devices, while graphene-based supercapacitors leverage the exceptional characteristics of graphene, such as high surface area and excellent mechanical and electrical properties, to achieve higher energy densities and power performance [38,39]. Pseudocapacitors, a separate class, are categorized into polymeric and metal oxide/hydroxide types. Polymer pseudocapacitors rely on the redox reactions of conductive polymers, which contribute to their energy storage mechanism. In contrast, metal oxide/hydroxide pseudocapacitors derive their capacitance from the reversible Faradaic interactions between the metal oxide or hydroxide electrodes and the electrolyte, leading to the formation or dissolution of surface species and the movement of electrons or ions. These variations in material selection and the underlying electrochemical processes within pseudocapacitors determine their specific capacities and rate capabilities. Each type of supercapacitor offers distinct advantages and limitations, guiding their application in various fields such as automotive, renewable energy storage, and portable electronics. The choice of supercapacitor type is contingent upon specific performance requirements, such as energy density, power density, cyclic stability, and cost-effectiveness. Understanding these classifications allows for a more informed decision when selecting a suitable technology to meet the demands of a given application [40,41].

Electrical Double-Layer Capacitors (EDLCs)

Electric double-layer capacitors (EDLCs) operate on the principle of charge separation, much like traditional capacitors, but they boast significantly higher capacitance values. This enhanced capacitance is achieved through the utilization of high-surface-area porous materials, such as activated carbon [42], which contrasts with the flat plates found in conventional capacitors. The term “electric double layer” is derived from the mechanism by which EDLCs store energy—electric charge accumulates at the interface between the electrode and the electrolyte, creating a double layer of charge. These capacitors are particularly advantageous in stationary and mobile systems that demand high power. They excel in scenarios requiring rapid energy capture, for instance, during regenerative braking in vehicles, attributed to their low time constant of less than a minute. The storage of energy in EDLCs is an electrostatic process, facilitated by the reversible adsorption of electrolyte ions onto the electrode material. The polarization at the electrolyte/electrode junction results in capacitance that is characteristic of a double layer.

The reasons for EDLCs’ superior energy storage capabilities, when juxtaposed with standard capacitors, are two-fold:

• The extensive surface area of the electrode material, riddled with numerous pores, enables a substantial enhancement in the storage capacity for electric charge. This intricate structure allows for a dramatically increased volume for the formation of electric double layers, which are crucial for energy storage [43].
• The ultrathin electrical double layers that are established at each electrode–electrolyte interface contribute to the elevated capacitance of these devices.

The construction of EDLC supercapacitors is analogous to batteries, comprising two electrodes immersed in an electrolyte and separated by an ion-permeable separator to prevent direct electrical contact (Figure 3a). Upon charging, the anions and cations in the electrolyte migrate towards the respective positive and negative electrodes, forming two distinct double layers. This arrangement can be conceptualized as a series of capacitors within the cell, with each electrode–electrolyte interface acting as an individual capacitor. The separation of ions across the double layers results in a potential difference throughout the cell, which is a fundamental aspect of its energy storage functionality.

Pseudocapacitors

Pseudocapacitors operate on the principle of Faradaic reactions for energy storage, where the electrode–electrolyte interface facilitates the electrostatic storage of charge. The electrochemical process in these devices involves reversible reduction–oxidation (redox) reactions that occur at the electrode surface when a voltage is introduced. These reactions are characterized by a rapid flow of Faradic current, which is the result of charge transfer across the double layer (Figure 3b). In contrast to electrochemical double-layer capacitors (EDLCs), pseudocapacitors exhibit an enhanced rate of electrochemical processes, contributing to higher specific capacitance, and consequently, greater energy densities [45,46].

Hybrid Supercapacitors

Hybrid supercapacitors have gained significant attention in recent times due to their remarkable ability to improve energy density while preserving power density. These advanced devices surpass the capacitance levels of traditional electric double-layer capacitors (EDLCs) and pseudocapacitors (PCs) (Figure 3c), offering a unique solution to the quest for superior energy storage. The asymmetry inherent in their design, which involves the integration of an EDLC and a PC, enhances the overall capacitance values by leveraging the complementary characteristics of each component. This innovative approach opens up a promising avenue for the development of sustainable and high-performance energy storage systems, which are crucial for the growing demand in the field of hybrid electric vehicles (HEVs). The integration of different storage mechanisms in hybrid supercapacitors is a strategic move towards achieving more efficient energy storage. Specifically, the combination of EDLCs, which rely on electrostatic charge storage, and PCs, which utilize fast and reversible Faradaic reactions at the electrode–electrolyte interface, provides a synergistic effect that overcomes the individual limitations of each technology. This synergy is crucial for advancing the performance of these devices and is particularly advantageous for energy-demanding applications such as HEVs. To enhance the performance of hybrid supercapacitors for energy-efficient purposes, the introduction of innovative materials is essential. These materials should facilitate an increase in the surface-to-volume ratio, thereby contributing to enhanced energy storage capabilities. The fundamental concept behind these hybrid storage systems is the combination of the two distinct storage mechanisms: the double-layer capacitance from EDLCs and the pseudocapacitance from PCs. The architecture of hybrid supercapacitors can be tailored to be either symmetric or asymmetric, depending on the desired application and performance criteria. The choice between symmetric and asymmetric designs depends on the specific requirements of the system, such as the voltage window, energy density, and power density [47]. The comparison of various supercapacitor types reveals their relative advantages and disadvantages, as presented in (Figure 4). This comprehensive analysis helps in the selection of the most suitable hybrid configuration for different applications. Understanding these characteristics is pivotal for researchers aiming to design and develop advanced hybrid supercapacitors that cater to the evolving needs of the energy sector, especially in the context of transportation electrification and the push towards a greener future.

2.5. Key Materials in Electrochemical Energy Storage Devices

The performance of supercapacitors is highly dependent on the materials used for their construction, particularly the electrode and electrolyte materials. The electrode material should possess a high surface area, excellent electrical conductivity, good electrochemical stability, and low cost. The electrolyte should have high ionic conductivity, a wide electrochemical window, and good compatibility with the electrode material.

2.5.1. Electrode Materials

Carbon-based materials, transition metal oxides, conducting polymers, metal chalcogenides, and composite materials are some of the electrode materials utilized in supercapacitors [48].

Carbon-Based Materials

Various nanomaterials derived from carbon have been developed and incorporated into the design of energy conversion and storage systems, with a particular emphasis on their role as electrode components in supercapacitors (SCs). Notable examples of such materials include activated carbon (AC) [49], graphene [50], carbon nanotubes (CNT) [51], and carbon nanofibers (CNF) [52]. These materials are favored due to their exceptional properties, which include a substantial specific surface area (SSA), excellent electrical conductivity, tailorable porous architecture, economic viability for large-scale manufacturing, and the ease with which they can be modified through chemical and physical means to suit specific application requirements (Figure 5). Various techniques, such as carbonization, activation processes, functionalization, and doping, are frequently employed to optimize the physical and chemical characteristics of these materials. These methods are crucial in enhancing the electrochemical performance of supercapacitors when utilized as electrodes and are comprehensively discussed in the subsequent sections

Transition Metal Oxides

The utilization of metal oxides in the realm of electrochemical capacitors has gained considerable attention due to their intrinsic merits, such as lower resistivity and improved specific capacitance. A multitude of studies have focused on the potential of various transition metal oxides, which generally outperform conductive polymers in terms of capacitance properties and present higher energy densities than their carbon-based counterparts [53]. Notable entries in this category are nickel oxide (NiO), manganese dioxide (MnO₂), ruthenium dioxide (RuO₂), and iridium oxide (IrO₂). However, while ruthenium dioxide is recognized for its exceptional performance, its high cost and environmental concerns impose significant barriers to widespread application [54]. On the other hand, materials such as MnO₂ and ZnO are naturally abundant and have showcased substantial capacitance values, thereby establishing themselves as appealing alternatives for supercapacitor technologies. These oxides offer the possibility of achieving high energy storage capacity and excellent cyclic stability, which are essential for developing advanced energy storage systems that can meet the growing demands of various industries.

Conducting Polymers

Pseudocapacitors have found notable promise in the realm of energy storage due to the potential of conductive polymers. These polymers stand out from electric double-layer capacitors in terms of their enhanced charge density, simplified manufacturing, and economic viability [55]. Among the key conductive polymers employed as electrodes in such devices are polyaniline (PANi), polypyrrole (Ppy), and polythiophene (PTh). These materials have gained considerable attention as they exhibit a range of desirable characteristics, such as superior conductivity, cost-effectiveness, and adaptability for various applications. Their distinctive properties are succinctly summarized in (Figure 6). Extensive research endeavors have been undertaken to explore the full scope of these polymers, resulting in numerous substantial advancements that have bolstered their performance [56].

Polyaniline (PANi)

Polyaniline (PANi) is a promising conducting polymer that stands out for several favorable attributes, including its flexibility, substantial specific capacitance, cost-effectiveness, ease of synthesis, and adjustable conductivity, with a high doping level of 0.5. Despite these advantages, PANi faces challenges such as its susceptibility to rapid degradation during repeated charge/discharge cycles and the necessity for a protic solvent or ionic liquid to ensure optimal performance [57]. In an effort to mitigate these drawbacks, researchers have integrated PANi with carbon materials and metal oxides, yielding composites that show significant promise. Sivakumar and colleagues employed interfacial polymerization to produce PANi nanofibers, which exhibited a specific capacitance of 554 F/g at a current density of 1.0 A/g. Despite the initial success, it was noted that the specific capacitance value deteriorated rapidly, and the cyclic stability was less than ideal. To address this, they developed a composite of PANi with multi-walled carbon nanotubes (MWCNT), achieving an enhanced specific capacitance of 606 F/g [58]. This demonstrates the potential of combining PANi with other materials to improve its performance characteristics. Li and others conducted both theoretical and experimental studies on the specific capacitance of PANi in sulfuric acid, revealing a discrepancy where the experimental values were significantly lower than the theoretical predictions. The highest theoretical specific capacitance value determined was 2000 F/g, highlighting the substantial potential that remains untapped in PANi [59]. This disparity suggests that further research and optimization are essential to realize the full capacitive capabilities of this material.

Polypyrrole

Polypyrrole (PPy), a notable member of the conductive polymer family, is valued for its flexibility, straightforward fabrication, and enhanced cyclic stability, which contribute to its high volumetric capacitance. Its elevated density is reflected in its substantial capacitance per unit volume. However, PPy also presents certain challenges, namely the complex doping process and a comparatively low specific capacitance on a per gram basis. A notable synthesis approach for PPy-based supercapacitors involves the creation of free-standing films, as suggested by Yang and collaborators. The inclusion of a surfactant in this process has been found to yield more favorable outcomes, as it promotes the formation of smaller pores within the film’s structure. This method showcased a capacitive retention of 75%, along with a peak specific capacitance of 261 F/g [60]. Moreover, the work conducted by Rajesh et al. [61] examined the electropolymerization of polypyrrole doped with phytic acid. This study reported impressive results with a maximum specific capacitance reaching 343 F/g and a capacitance retention rate of 91% at 10 A/g after subjecting the material to 4000 charge–discharge cycles. The use of phytic acid as a dopant in the PPy films not only facilitated the doping process but also significantly improved the film’s electrochemical properties. These findings underscore the potential of PPy in the realm of supercapacitors, despite its limitations, and encourage further exploration into the optimization of its synthesis techniques and doping strategies.

Polythiophene (Pth)

Polythiophenes have been explored for their application in energy storage systems, particularly as electrode materials. These polymers exhibit several favorable properties, such as a high flexibility, ease of synthesis, and environmental compatibility. However, they are also associated with limitations, namely poor electrical conductivity and relatively low specific capacitance [62]. To address these issues, researchers have developed various strategies to enhance their performance. For instance, Laforgue et al. [63] have chemically synthesized poly(thiophene) (Pth) and poly(3,4-ethylenedioxythiophene) (PFPT), achieving specific capacitances of 7 mAh/g and 40 mAh/g, respectively. These values highlight the potential of these materials in the context of energy storage. Further research has focused on improving the capacitance of polythiophenes. Patil’s group employed the successive ionic layer adsorption and reaction (SILAR) method to produce PTh thin films, which demonstrated a notable enhancement in capacitance up to 252 F/g when using FeCl₃ as an oxidizing agent [64]. Another study reported the preparation of PTh films through chemical bath deposition, which yielded an impressive maximum specific capacitance of 300 F/g [64]. These findings underscore the effectiveness of different synthesis techniques in optimizing the properties of polythiophenes for energy storage applications

Metal Chalcogenides

Transition metal sulfides (TMSs) have garnered significant interest as appealing candidates for supercapacitor electrodes, largely attributed to their high theoretical capacitance values, exceptional electrical conductivity, and advantageous redox characteristics. Specific TMSs, such as manganese (Mn), vanadium (V), cobalt (Co), iron (Fe), copper (Cu), nickel (Ni), molybdenum (Mo), zinc (Zn), tungsten (W), and tin (Sn), have exhibited substantial enhancements in their electrochemical performance upon undergoing compositional and structural optimization. Strategic approaches in composite formation and morphological manipulation play a pivotal role in amplifying the efficacy of these TMS electrodes [65].

Binary Composites as Electrode Materials

Composite substances, denoted by their dual- or tri-material structure, are formed by integrating two or three distinct components. Specifically, within this realm, composite materials encompass blends of metal oxides, carbon-based elements, and conductive polymers. These materials can be divided into two primary classifications: (a) binary composites, exemplified by combinations like NiO with Co₃O₄ or MnO₂ with CNT (carbon nanotubes), and (b) ternary composites, which involve more complex arrangements such as GO (graphene oxide) intertwined with MWCNT (multi-walled carbon nanotubes) and PANI (polyaniline) or Co₃O₄ in conjunction with NG (nanographene) and CNTs.

Binary compound electrodes are fabricated by integrating two different substances, which can be any combination of carbon-based materials, metal/metal oxides/metal hydroxides, and conductive polymers. These composite systems typically demonstrate enhanced electrochemical efficacy in comparison to their standalone constituents. This superiority arises from the concurrent engagement of both Faradaic and non-Faradaic charge storage processes within the composite structure. By combining these elements, the resulting material benefits from a synergistic interplay of properties, which can be optimized for various electrochemical applications. Let us delve into some illustrations of binary composites and assess their electrochemical performance characteristics.

Carbon–Carbon Composites

The performance of carbon materials in supercapacitors is largely contingent upon the accessible specific surface area (SSA) for electrolyte ions, which can enhance the energy and power density. To achieve a high SSA, researchers have developed non-covalent functionalized graphene. Despite its remarkable properties, graphene tends to disperse poorly, due to aggregation caused by Van der Waals forces. In a study by Zhang et al. [66], single-walled carbon nanotubes (SWCNTs) were introduced as spacers to prevent restacking and thus maintain the accessible surface area for the electrolyte. This strategy yielded a specific capacitance of 261 F/g and an energy density of 123 Wh/kg in a graphene-based supercapacitor with an ionic liquid electrolyte operating at 3.7 V. Another approach to optimize graphene’s capacitive characteristics involves combining it with activated carbon (AC) to form a composite. Zheng et al. [67] synthesized such a graphene/AC nano-sheet electrode material for supercapacitors. The unique architecture of this composite provided two distinct types of porosity: micropores from AC and mesopores resulting from the interconnection of the graphene sheets. This combination led to specific capacitances of 103 F/g and 210 F/g in organic and aqueous electrolytes, respectively. Moreover, the material demonstrated excellent cyclic stability with a capacitance retention rate of 94.7% after undergoing 5000 charge–discharge cycles.

Carbon–MOs Composites

We are aware of the well-established drawbacks of carbon materials and transition metal oxides (TMOs), including their low specific capacitance, poor electrical conductivity, and restricted electrochemical stability. To overcome these challenges, researchers have turned to the fabrication of composite electrodes for supercapacitors (SCs) that incorporate both TMOs and carbon materials. A notable example is the synthesis of MnO₂/CNT composites through a hydrothermal method, which has been shown to exhibit a specific capacitance of 223 F/g, thereby surpassing the performance of CNT and MnO₂ electrodes used in isolation [68]. The integration of TMOs with carbon materials typically results in composites with higher surface areas compared to their individual components. Several research initiatives have focused on improving the capacitance of these composite materials. For instance, cobalt oxide has been combined with graphene [69] and carbon nanofibers (CNFs) to create Co₃O₄/CNFs [69]. These combinations have displayed enhanced surface areas and correspondingly increased electrical conductivity, which are essential for improved supercapacitor performance. Additionally, the exploration of ternary metal oxides (TMOs) has also shown promise. A research team led by Xiong et al. [70] utilized hydrothermal synthesis to produce nickel–cobalt–manganese triple hydroxide nanoneedles, which exhibited outstanding electrochemical properties. The synthesized ternary metal oxide demonstrated a noteworthy specific capacitance of 1400 F/g, an impressive energy density of 30 Wh/kg at a high-power density of 39 kW/kg, and an exceptional cyclic stability, maintaining 100% of its capacitance over an extensive 3000 cycles. This study highlights the efficacy of such a synthesis approach in creating advanced materials with excellent performance metrics suitable for energy storage applications.

Carbon–CPs Composites

The study conducted by Liangliang et al. [71] has highlighted the potential of nanocomposites comprising activated carbon cathodes and conductive polymer anodes in electric double-layer capacitors (EDLCs) to enhance energy and power density, surpassing alternative materials. Conductive polymers, like polyaniline (PANI), have garnered significant attention in the creation of supercapacitors (SCs) due to their high capacitance and excellent cyclic performance. However, PANI alone confronts issues with rate capability and cycle stability. The integration with carbon-based materials, such as single-walled carbon nanotubes (SWCNT), has been shown to significantly boost these properties. For instance, a PANI–SWCNT blend exhibited a 65% increase in capacitance and a threefold energy density improvement compared to the individual components. An intriguing development in the field is the flexible supercapacitor introduced by Yang et al. [72], which employs a reduced graphene oxide (rGO) aerogel–PANI composite fabricated via electrodeposition. This material boasts a specific capacitance of 432 F/g and an energy density of 25 Wh/kg, while maintaining 85% of its capacitance over 10,000 charge–discharge cycles. The high flexibility stability under various bending conditions underscores its potential for flexible energy storage solutions. Furthermore, researchers have explored ternary composites to optimize SC performance. An example is the work of Yan et al. [73], who synthesized a material through in situ polymerization combining graphene nanosheets (GNS), carbon nanotubes (CNT), and PANI. This GNS/CNT/PANI composite presented a specific capacitance of 1035 F/g, which, although marginally less than the GNS/PANI binary structure (1046 F/g), significantly outperformed the CNT/PANI binary system (780 F/g). Most importantly, the ternary composite displayed remarkable cyclic stability, retaining 94% of its capacitance after 1000 cycles. This suggests that despite the slightly lower specific capacitance compared to the GNS/PANI binary, the GNS/CNT/PANI ternary composition offers superior long-term performance reliability, making it an attractive candidate for supercapacitors that demand robustness and consistent function over numerous charge–discharge cycles.

MOs–CPs

The integration of metal oxides (MOs) with conducting polymers (CPs) is a notable advancement in the realm of electrode materials for supercapacitors (SCs). This synergistic pairing has been observed to offer substantially better electrochemical properties than their individual components. The MOs provide high specific capacitance and stability, while the CPs contribute to improved electrical conductivity and charge storage capabilities [74]. A prime example is the study by Liu and colleagues, who developed a MoO₃ electrode with a polypyrrole (PPy) coating through in situ polymerization [75]. The resultant material demonstrated a remarkable energy density of 20 Wh/kg at a power density of 75 W/kg, along with a specific capacitance of 110 F/g at a current density of 100 mA/g. When configured as an asymmetric supercapacitor employing a 0.5 M K₂SO₄ aqueous electrolyte, the energy density increased to 12 Wh/kg, with power density reaching 3 kW/kg. These findings underscore the potential of such composites for high-performance energy storage solutions. Taking this concept further, the formation of ternary nanocomposites that combine carbon-based substances, MOs, and CPs has yielded electrode materials with remarkable electrochemical performance enhancements. These hybrid materials exhibit a significant improvement in both energy and power density in comparison to their pure counterparts. The careful selection of components and synthesis techniques optimize the synergistic effects between the different materials, leading to enhanced charge storage and transport properties. This approach opens up a new avenue for the design of electrodes with tailored characteristics that can meet the increasing demands of various industrial and electronic applications. The integration of these advanced materials into SCs could pave the way for more efficient and reliable energy storage systems, thereby contributing to the advancement of sustainable energy technologies.

MOs–MOs

Binary metal oxide combinations, particularly the MO–MO configuration, have drawn substantial interest in the realm of supercapacitor research due to their advantageous attributes, including improved electrical conductivity, broad potential ranges, and notable energy and power densities. A prominent illustration is the core-shell structured NiO/Co₃O₄ electrode material introduced by Adhikari et al. [76]. This design, featuring NiO/Co₃O₄@NF as the cathode and activated carbon serving as the anode, showcased outstanding electrochemical properties, achieving a capacitance of 2760 F/g at a 2 A/g current density and an energy density of 81.45 Wh/kg at a power density of 4268 W/kg. The most remarkable aspect of their study was the electrode’s cyclic durability, with the material retaining 95.5% of its initial capacitance throughout an impressive 12,000 charge–discharge cycles. This outcome underscores the promising potential of binary metal composites for the development of supercapacitors with exceptional performance indicators.

Ternary Composite Materials

Recent research has been increasingly focused on ternary composite materials for supercapacitors due to their ability to outperform binary composites. These advanced materials are constructed by combining three individual components to generate synergistic properties that improve the specific capacitance, cyclic stability, rate capability, and energy density in supercapacitor electrodes. Some noteworthy examples of such ternary composites include Mn₃O₄@NiCo₂O₄@NiO, NiO/PANI/CNT, CoO/NiO−Cu@CuO, and MnO₂-AgCNT-CC, each exhibiting distinct advantages that drive the progression of supercapacitor technology. A prime illustration of a ternary composite is the graphene/PANI/Co₃O₄ electrode developed by Lin and colleagues [77]. This material integrates the high stability and capacitance of Co₃O₄ within a 3D porous scaffold formed by graphene and polyaniline. The specific capacitance of this hydrothermally synthesized composite is 1247 F/g at a current density of 1 A/g, with an energy density reaching 190 Wh/kg. Furthermore, it demonstrates excellent cyclic stability, with no loss in capacitance observed after 3500 charge–discharge cycles. These combined features suggest that graphene/PANI/Co₃O₄ has significant potential for the future of supercapacitor electrodes. Another intriguing advancement involves metal–metal–carbon-based composites, such as the one reported by Yang et al. [78]. They developed a ternary system consisting of Co₃O₄, NiO, and graphene oxide (GO), which formed a sea urchin-inspired architecture on a GO substrate that served as the negative electrode in an asymmetric supercapacitor setup with activated carbon as the positive electrode. This design achieved a specific capacitance of 883 F/g at 1 A/g and an energy density of 50.2 Wh/kg. Impressively, it displayed remarkable capacitance retention, with only an 18% decrease after 3000 cycles. The complex microstructures of the sea urchin-like Co₃O₄-NiO and the Co₃O₄-NiO/GO composite can be examined using field-emission scanning electron microscope (FE-SEM) images, revealing their intricate patterns at different scales (Figure 7).

Moreover, the cyclic voltammetry (CV) curves of the Co₃O₄-NiO/GO//AC electrode are presented at varying potentials in (Figure 8). Additionally, this figure showcases the galvanostatic charge–discharge (GCD) profiles of the same electrode material at distinct current densities. A cyclic stability test was also conducted on Co₃O₄-NiO/GO//AC at a rate of 10 A/g to evaluate its performance consistency over multiple charge–discharge cycles.

A study spearheaded by Muhammad Usman and colleagues focused on enhancing the stability of polyaniline (PANI) by creating a ternary material named PANI and Fe–Ni co-doped Co₃O₄ (PANI@FNCO) using an in situ synthesis approach. With its high stability, high processability, and adjustable conducting and optical properties, polyaniline is the most prospective and studied conducting polymer. The concentration of the dopant determines the conductivity of polyaniline, which only exhibits metal-like conductivity at pH values below 3.23. They are classified as leucoemeraldine, emeraldine, and pernigraniline, by their oxidation state, i.e., leucoemeraldine exists in a sufficiently reduced state, and pernigraniline exists in a fully oxidized state. Only when polyaniline is partially oxidized does it become conductive; when it is fully oxidized, it functions as an insulator [79]. The resulting PANI@FNCO nanocomposite presented exceptional electrochemical characteristics, such as a substantial specific capacitance of 1171 F/g at 1 A/g and an energy density of 144 Wh/kg. Furthermore, the cyclic stability was maintained at 84% over an extensive 2000 cycles [80]. Hao and colleagues [81] engineered a ternary carbon-based composite for enhancing supercapacitor performance, which incorporated graphene oxides (GOs), multi-walled carbon nanotubes (MWCNTs), and polyaniline (PANI). This composite material, denoted as GO/MWCNT/PANI, served as the positive electrode in an asymmetrically designed supercapacitor with KOH-activated GO/MWCNT functioning as the negative electrode. The synergistic combination of these components led to an enhanced electrical conductivity and promoted efficient electron transfer. Consequently, the resulting supercapacitor exhibited an impressive specific capacitance of 696 F/g at a scan rate of 20 mV/s, as well as an energy density of 69 Wh/kg and power density of 6.4 kW/kg. Most importantly, the cyclic stability of this material was demonstrated by maintaining 89% of its capacitance over an extensive 3000 charge–discharge cycles, highlighting its robust performance and potential for practical applications. Xu et al. [82] introduced a ternary composite electrode made of graphene (GN), activated carbon (AC), and polypyrrole (PPy). Employing vacuum filtration and anodic constant current deposition techniques, the researchers crafted a GN/AC/PPy material that displayed flexibility and robustness. The unique architecture, with AC intercalated between GN layers to prevent restacking, allowed for a maximum specific capacitance of 178 F/g at 0.5 mA/cm2. Additionally, the electrode material displayed commendable resilience, retaining 83.6% of its capacitance after 500 stretching and bending cycles, indicating its suitability for flexible energy storage devices. In a separate endeavor, a ternary composite of N-doped graphene nanosheets (NG), carbon nanotubes (CNTs), and Co₃O₄, denoted as Co₃O₄/NG/CNTs, was synthesized via hydrothermal methods. The synergy of these components led to a significant enhancement in the electrochemical properties, achieving a specific capacitance of 456 F/g. This metal–carbon–carbon combination showcased remarkable potential in supercapacitor applications.

Another study by Li et al. [83] presented a ternary composite of conductive polymers, namely PANI and PPy, supported by a carbon-based matrix derived from wood. This environmentally friendly and sustainable substrate contributed to the composite’s excellent performance, with a specific capacitance of 360 F/g at 0.2 A/g, highlighting its prospects for eco-friendly supercapacitors.

Some Novel Emerging Materials

The inherent limitation of transition metal oxides (TMOs) and transition metal hydroxides (TMHs) often includes unsatisfactory electrical conductivity levels. MXenes, a novel class of materials, exhibit exceptional mechanical robustness and hydrophilic characteristics. Despite their potential, the production of MXenes faces hurdles primarily due to the insufficient interaction among the sheets and the scarcity of advanced assembly methods [84,85]. The difficulty in achieving proper alignment further complicates their fabrication. To tackle this, researchers have devised several composite approaches that focus on improving MXenes’ electrical properties. Notable among these strategies are the wet-spinning assembly process, the utilization of self-propagating reduction methods, the construction of MXene/CNT sandwich structures, the creation of in situ polymerized CNT-PANI nanocomposites, and the development of self-healing 3D micro-supercapacitors [86]. These innovative techniques strive to overcome the challenges associated with MXene assembly and enhance their overall performance.

Organic Framework (COF, MOF)–Carbon Hybrids

Metal–organic frameworks (MOFs) are created when metal ions bond with organic ligands to produce porous coordination polymers, which can exhibit one-dimensional to three-dimensional structures. The nature of the metal ions and linkers employed significantly affects the pore dimensions, surface area, and electrical properties of these materials, with factors such as the metal ions’ electronic characteristics, ionic radii, and coordination geometries contributing to the determination of MOF morphology and electrochemical behavior [87]. To address the challenge of graphene layers restacking due to van der Waals interactions, researchers have established two main approaches: First, they have developed methods to chemically modify graphene nanosheets (GNSs) to impart repulsive properties that hinder restacking. Second, they have incorporated physical spacers to either preserve or enhance the separation between GNS layers [88]. Among these spacers, covalent organic frameworks (COFs) have emerged as particularly effective, as they are constructed from regularly arranged small organic molecules that form coordination polymers. COFs offer several advantages over MOFs in electrochemical applications, including lower mass, reduced environmental impact, and improved stability. Furthermore, they can be tailored to possess adjustable in-plane porosity and high surface areas. For instance, 2D COFs composed of aminopyridine and substituted-phloroglucinol linkers, synthesized via solvothermal techniques, can achieve substantial surface areas of around 690 m²/g and pore widths exceeding 1 nm [89]. In the realm of supercapacitors, MOFs have been successfully utilized as two-dimensional spacers to prevent graphene sheets from restacking. A notable example is the hybrid material, UPZCNs-K4, synthesized by Li and colleagues, which exhibits a high gravimetric capacitance of 402 F/g at a charge density of 1 A/g. When implemented in supercapacitors, these hybrid carbon sheets demonstrated energy densities of approximately 16 Wh/kg with potassium hydroxide (KOH) and 22 Wh/kg with sodium sulfate (Na₂SO₄) electrolytes [90].

2.5.2. Electrolytes

Electrolytes are vital for the functionality of supercapacitors as they facilitate the movement of ions, thereby influencing the device’s overall performance and operational voltage range. Typically, supercapacitor electrolytes are categorized into three primary classes: aqueous solutions, organic solvent-based systems, and ionic liquids. Each category contributes distinct characteristics to the supercapacitor’s performance profile, such as conductivity and voltage capabilities. These classifications are essential in optimizing the design and selection of electrolytes to meet specific supercapacitor requirements and enhance their efficiency in various applications.

Aqueous Electrolytes

The efficiency of aqueous electrolytes in energy storage surpasses that of solid or gel-like semi-solid alternatives, largely due to their liquid nature and associated low viscosity that facilitates rapid ionic movement. These electrolytes can be further categorized into three distinct groups based on their pH properties: (i) acidic, characterized by a pH value below 7; (ii) alkaline, with a pH above 7; and (iii) neutral, maintaining a pH of 7. This classification is crucial, as it impacts their interaction with electrodes and the overall system performance in various storage applications.

Acidic Electrolyte

H₂SO₄ is a commonly studied aqueous electrolyte, due to its remarkable ionic conductivity and small ionic dimensions. At 25 °C, it showcases the highest ionic conductivity among its counterparts. This high conductivity is advantageous, as it leads to greater specific capacitance and a lower equivalent series resistance (ESR), which are essential for achieving superior energy and power densities. Nevertheless, the elevated conductivity can induce corrosion concerns. To address this, Naik et al. combined 0.2 M H₂SO₄ with 1 M KNO₃ to create a solution that not only mitigates corrosion but also lowers the KNO₃ resistance and enhances the electrolyte’s overall capacitance [91]. Research comparing the electrochemical properties of mesoporous MoS₂ with different electrolytes found that 1 M H₂SO₄ yielded the highest capacitance at 225 F/g, whereas 2 M KOH provided the lowest at 123 F/g, underscoring the substantial influence of the electrolyte on performance, which remained relatively consistent across various electrode studies [92,93]. In a different study, Bakhshandeh and associates used the partial reduction and functionalization of acylated graphene oxide to create a functionalized and partly reduced graphene oxide (FPRGO). The electrochemical analysis of FPRGO in 1 M H₂SO₄, as seen in the charge–discharge curves and cyclic voltammograms (Figure 9 and Figure 10), revealed that the material exhibits a combination of electrochemical double-layer capacitance (EDLC) and pseudocapacitance properties, highlighting its adaptable nature within such an electrolyte environment [94].

The dimension of pores in a material significantly affects its ability to store charge, particularly in supercapacitors. Research has demonstrated that for materials interacting with acidic electrolytes, a pore size of approximately 3 nanometers optimizes the charge-storing capacity. The performance enhancement can be attributed to the superior accommodation of potassium ions in pores of these dimensions compared to hydrogen ions [95]. Phosphate- and nitrogen-doped carbons have showcased higher capacitance in concentrated potassium hydroxide (6 M KOH, reaching 160 F/g) than in 1 M sulfuric acid (138 F/g), which is likely due to their larger pore structure that allows more effective trapping of the larger potassium cations. Investigations into various electrode configurations have been undertaken using a range of electrolytes, with experiments conducted in both two-electrode [95] and three-electrode setups [96]. Despite these variations, it is generally observed that 1 M H₂SO₄ provides the most favorable conditions for achieving high capacitance. Figure 11 illustrates the specific capacitance and internal resistance fluctuations in a symmetric EDLC (electric double-layer capacitor) device utilizing 3 M H₂SO₄ as the electrolyte across different temperatures. A notable increase in equivalent series resistance (ESR) is discernible at −30 °C, which can be ascribed to the electrolyte’s reduced stability in proximity to its freezing point.

The study examining the reactivity of aqueous and organic electrolytes in response to temperature fluctuations determined that aqueous electrolytes demonstrate less reactivity compared to their organic counterparts. Specifically, it was found that while aqueous electrolytes experienced a 15% reduction in capacitance, organic electrolytes such as Et₄NBF₄/PC underwent a more substantial capacitance loss of 32% when exposed to varying thermal conditions [97]. The utilization of H₂SO₄ as an industrial electrolyte is constrained by its narrow potential window, which extends only to 0.6 V. This limitation leads to decreased energy density and an elevated risk of corrosion. On the other hand, sulfuric acid-based electrolytes exhibit higher capacitance in both electric double-layer capacitors (EDLCs) and redox systems, with H⁺ ions significantly participating in the charge storage process and enhancing the overall performance.

Figure 11. (a) Specific capacitance and (b) internal resistance deviation with variation in temperature for 3 M H₂SO₄ solution (Reproduced with permission from [98]).

Figure 11. (a) Specific capacitance and (b) internal resistance deviation with variation in temperature for 3 M H₂SO₄ solution (Reproduced with permission from [98]).

Alkaline Electrolytes

Alkaline electrolyte solutions, such as 6 M potassium hydroxide (KOH), are frequently regarded as suitable candidates for enhancing the performance of supercapacitors, much like their acidic analogs. The appeal of 6 M KOH is attributed to its small ionic radius and exceptionally high ionic conductivity, which is measured at 0.6 S/cm under standard conditions of 25 °C. When the charge storage properties of freeze-dried reduced graphene oxide (rGO) are compared in 6 M KOH, 1 M sodium sulfate (Na₂SO₄), and a protic ionic liquid electrolyte (ILE), it becomes evident that KOH provides the most substantial capacitance, reaching 71.8 F/g at a scanning rate of 50 mV/s. Despite this, the protic ILE exhibits a superior energy density, which is a direct consequence of its broader potential window. This observation underscores the importance of considering both ionic conductivity and potential range when evaluating electrolyte options for supercapacitor applications [99]. Research conducted by Balaji et al. highlighted boron-doped graphene in 20% KOH with an impressive capacitance of 286 F/g and an energy density of 5.3 Wh/kg, whereas in 1-ethyl 3-methylimidazolium tetrafluoroborate (EMIMBF₄), the energy density soared to 43.1 Wh/kg despite a slightly lower capacitance of 138 F/g. This is attributed to the smaller ionic radius of KOH, its reduced viscosity, and enhanced ionic conductivity [100]. Interestingly, while alkaline electrolytes like KOH may offer lower energy density compared to organic variants, they exhibit higher capacitance in acidic environments. This discrepancy is largely due to the more confined potential window of alkaline electrolytes, which in turn limits their charge storage capacity. For instance, the capacitance of a porous carbon electrode in 6 M KOH was measured at 355.6 F/g using a three-electrode configuration at a current density of 1 A/g. When the setup was switched to a two-electrode configuration, the capacitance decreased to 78.15 F/g, underscoring the higher capacitance that KOH can achieve in electrochemical double-layer capacitor (EDLC)-type carbon-based electrodes [101]. Further exploration into the realm of 1-D NiMoO₄/CoMoO₄ nanorod arrays coated with 2-D Ni-Co-S nanosheets revealed that utilizing 1 M KOH as an electrolyte led to a maximum capacitance of 778.1 F/g at a current density of 0.5 A/g. This study also presented an impressive energy density of 33.1 Wh/kg at 0.25 A/g and power density of 3195 W/kg at 4 A/g for flexible asymmetric supercapacitors [102]. These findings suggest that KOH is not only suitable for EDLC electrodes but also for pseudocapacitive ones. Moreover, an alternative to KOH, namely KOH/PVA, has been employed as an electrolyte in Co₃O₄/rGo-based electrodes, resulting in notable capacitance and exceptional energy and power densities of 38.8 Wh/kg and 400 W/kg, respectively [103]. This underscores the potential of alkaline electrolytes in enhancing the performance of supercapacitors. While supercapacitors equipped with alkaline electrolytes like KOH can boast higher power densities, their integration with batteries in hybrid storage systems still requires further investigation. Another alkaline option, LiOH, has been proposed as a viable electrolyte substitute for KOH, warranting additional research in this domain [104].

Neutral Electrolytes

Sulfate-based neutral electrolytes, such as sodium sulfate (Na₂SO₄), are commonly favored in electrochemical systems for their broad operating range and minimal propensity for corrosion. Despite these advantages, they generally exhibit inferior electrochemical performance when compared to acidic and basic electrolytes. Research conducted by Qin et al. focused on the use of nitrogen-doped carbon nanotubes (CNTs) within a neutral electrolyte, achieving a substantial specific surface area of 2608 m²/g and a significant energy density of 27.3 Wh/kg at a power density of 182 W/kg in a 1 M Na₂SO₄ solution. The study contrasted the performance of carbon paper collectors in sulfuric acid (H₂SO₄) with nickel foam collectors in potassium hydroxide (KOH), revealing that the latter’s capacitance was lower. This is because KOH lacks H⁺ ions, which are vital for the occurrence of redox reactions. Consequently, the nickel foam’s charge storage is predominantly influenced by double-layer capacitance (DLC), which is characterized by rapid charge–discharge rates that surpass the redox mechanism. When analyzing the CNT/MnO₂ electrode combination, the researchers found that it demonstrated the highest capacitance in sulfuric acid, reaching 2523 F/g at a 5 mV/s scan rate. This high capacitance can be attributed to the presence of redox peaks, which signify substantial energy storage capabilities. In a neutral Na₂SO₄ electrolyte, however, the capacitance was observed to be 1676 F/g at the same scan rate, with the absence of redox peaks indicating that the charge storage was mainly attributed to electric double-layer capacitance (EDLC). While MnO₂ has higher solubility in H₂SO₄, which could potentially lead to decreased stability, the study showed that Na₂SO₄ offers superior cyclic stability, with the CNT/MnO₂ electrode maintaining 80% capacitance retention following 3000 cycles. This is due to the reduced solubility of MnO₂ in neutral environments, which enhances its stability and longevity in such conditions [105]. Further examination of MnO₂/CNT electrodes in various electrolytes revealed a capacitance of 43.2 F/g in a 0.5 M Na₂SO₄ solution, underscoring the importance of electrolyte selection [106]. An asymmetric device utilizing V₂O₅@3D graphene and Fe₃O₄@3D graphene as positive and negative electrodes, respectively, achieved an energy density of 54.9 Wh/kg and a power density of 898 W/kg in 1 M Na₂SO₄, with minimal ohmic resistance [107]. Different morphologies of MnO₂/carbon electrodes were studied in a 1 M Na₂SO₄ solution, and it was found that the peach-like structure of Mn₂O₃/C offered the highest surface area, which correlated with the highest capacitance [108]. Conversely, the electrode with the lowest surface area displayed the lowest capacitance. KCl is another neutral electrolyte that has been considered for electrochemical applications, with a potential window spanning from −0.2 to 0.3 V (vs. Ag/AgCl) [109]. Although it has a neutral pH, similar to Na₂SO₄, its capacitance is lower due to the less effective conductivity of its anions [110]. However, the cobalt oxide (Co₃O₄) electrode exhibited lower capacitance in KCl than in Na₂SO₄, despite both having a neutral pH [111]. This suggests that the ionic conductivity and size have more influence on the electrochemical capacitance than the electrolyte’s pH. Moreover, while some materials may show high capacitance in acidic conditions, they may not sustain this performance over multiple cycles due to solubility or pH sensitivity issues [112]. Hodaei and colleagues reported a high capacitance of 311 F/g at 1 A/g for nitrogen-doped titanium dioxide (TiO₂) electrodes in 3 M KCl, which also maintained an impressive 98.9% capacitance retention after 4000 cycles (Figure 12). Cyclic voltammetry (CV) studies indicated that the specific capacitance of MnO₂ is dependent on the concentration of cations, with the capacitance increasing with higher cation concentrations across all tested electrolytes. These findings collectively emphasize the significance of ionic conductivity and material stability [113].

Organic Electrolytes

Currently, there is a growing interest in exploring organic electrolytes for various applications, due to their higher energy densities, which are attributed to their extensive potential windows. Organic electrolytes, while beneficial in certain aspects such as high stability potential windows, often suffer from lower ionic conductivity and a higher propensity for ion solvation, which in turn affects their dielectric properties. Furthermore, the larger ionic dimensions of organic electrolytes hinder their diffusion through the narrow pores of typical electrode materials, thereby limiting their overall performance. To address the limitations of organic electrolytes, particularly their lower pseudocapacitive response compared to alternative systems, researchers have focused on optimizing the pore size within the electrode material. This modification is essential to enhance the interaction between the electrolyte and the electrode, thereby improving the electrochemical efficiency. Among the commonly studied organic salts is tetraethylammonium tetrafluoroborate (TeABF₄), which has been examined in combination with solvents such as propylene carbonate (PC) and acetonitrile (ACN). Despite the lower energy density of TeABF₄ in PC due to its higher viscosity and boiling point, it is generally preferred over TeABF₄ in ACN, due to the latter’s elevated toxicity concerns. It has been observed that at a concentration of 1.1 M, TeABF₄ faces solubility issues [114]. To better comprehend the behavior of organic electrolytes, researchers have turned to simulation studies, which provide insights into the underlying mechanisms governing their performance. For instance, Yang and colleagues have investigated the Gogotsi–Simon effect, where they observed an increase in the specific capacitance as the pore width decreased [115]. A study was undertaken to assess the mass ratio and full utilization of the electrolyte potential window. The findings suggest that 1 M TEABF4/ACN can achieve a maximum voltage of 2.9 V, while 1 M TEABF4/PC reaches 2.7 V, indicating a potential enhancement in device operation efficiency [116]. The energy density of the resulting 2D N-doped carbon nanosheets in 1 M TEABF4 was determined to be 34 Wh/kg [117], which is on par with the 29.8 Wh/kg reported by Zou et al. for polymer nanobelt screw electrodes in TEABF4/ACN [118]. Despite the higher capacitance of acidic and alkaline electrolytes, organic electrolytes are preferred for their cost-effectiveness and environmental benefits [119]. To optimize the performance of these electrolytes, researchers have explored various models and experimental methods to determine the suitable pore size for organic electrolytes. By altering the solvents, the interlayer distance of graphene oxide can be adjusted, which in turn affects the capacitance. For instance, when using methanol, the interlayer distance increased to 8.84 Å, whereas in acetone, it expanded to 8.99 Å, and in propanol, it reached 9.5 Å. This tailoring is crucial for ion transport [120]. A notable investigation by Vijaykumar et al. demonstrated a 25% enhancement in gravimetric capacitance when employing the TEABF4/ACN electrolyte [121]. The potential window significantly influences the energy density of supercapacitors, and controlling it through structural manipulation is essential. Researchers examined the areal capacitance of 28 carbon samples with pores sizes ranging from 0.7 to 1.5 nm in 1 M TEABF4/ACN. Interestingly, no substantial variation in capacitance was observed across different pore sizes, possibly due to a diminishing relative permittivity of organic electrolytes, as the pore sizes decreased from mesopores to sub-nanometer dimensions [122]. Furthermore, EMIMBF4 salt has also been employed in organic electrolytes. Through molecular modeling, scientists have compared EMIMBF4 and EMIMBF4/ACN, discovering that the latter exhibits a 55–60% greater capacitance [123]. Another computational study focused on the impact of porosity and decoordination on capacitance and found that monodispersed pores generally yield higher capacitance values compared to polydispersed ones [124]. These findings collectively underscore the importance of understanding and optimizing the relationship between pore structure and electrolyte properties to enhance the performance of supercapacitors. The aforementioned findings cast doubt on prevailing theories concerning the role of pores in contributing to total capacitance, highlighting the need for further exploration of phenomena at the microscale level of the electrode/electrolyte interface [125]. An investigation conducted by Ue and collaborators demonstrated that certain salts, namely 1-ethyl-1-methylpyrrolidinium (MEPYBF₄), triethylmethylammonium (TeMABF₄), and tetramethylene–pyrrolidinium (TMPYBF₄), exhibit superior electrolytic conductivity at practical concentrations around 0.65 M, attributed to their minute ionic dimensions while preserving strong dissociative characteristics. These salts are highly soluble in propylene carbonate (PC), which enables conductivities exceeding 2 M, unlike TEABF₄ that is confined to 1 M solubility [126]. The compact nature of the Li⁺ ionic radius renders organic electrolytes based on lithium salts a prevalent selection for pseudocapacitors and hybrid energy storage systems such as Li-ion capacitors (LICs). This preference is reflected in the exploration of their application within electric double-layer capacitors (EDLCs). The emergence of lithium-ion batteries (LIBs) has significantly shaped the utilization of these salts in various electrochemical storage systems. In this context, Kim and their team engineered a flexible micro-supercapacitor utilizing an organic gel electrolyte constituted of PMMA-PC-LiClO₄-HQ. The incorporation of hydroquinone (HQ) as a redox species within this electrolyte significantly enhanced the capacitance and energy density of the supercapacitor device [127]. The highest specific capacitance for porous activated carbon derived from pea skin was observed in 1 M H₂SO₄, with a notable energy density of 17.86 Wh/kg in 1 M LiClO₄ [128]. Comparative analysis of PANI-NFs electrodes in 1 M H₂SO₄, 1 M Na₂SO₄, and 1 M LiClO₄ in PC showed that LiClO₄ exhibited superior capacitive behavior, potentially due to the smaller ClO₄⁻ anion compared to the SO₄²⁻ anion [129]. Moreover, the thermal properties of LiClO₄ in acetamide displayed remarkable capacitance and a eutectic temperature beneath its melting point (234 °C), which decreased with the increasing acetamide concentration [130]. An electrolyte of 1 M LiClO₄ combined with a vanadium pentoxide (V₂O₅) nanofiber (VNF) electrode achieved an impressive energy density of 78 Wh/kg, and in the context of the Na6V10O28 electrode, 1 M LiClO₄ yielded a commendable energy density of 73 Wh/kg [131,132]. These outcomes suggest that LiClO₄ is a highly suitable electrolyte for vanadium-based electrodes (Figure 13).

LiPF₆ is commonly used in organic electrolytes alongside LiClO₄. When implemented in an electrolyte at a concentration of 1 M for activated carbon/graphene, it yielded a specific capacitance of 19.45 F/g, accompanied by an electrolyte resistance of 4.3 Ω [133]. This was shown in prior research. For vertically aligned carbon nanotube (CNT) electrodes [134]., the capacitance reportedly reached 101 F/g. A hierarchical porous carbon electrode, when paired with a 1 M LiPF₆/EC/DEC electrolyte, demonstrated an energy density of 288 W/kg, with a capacitance of 625 F/g at a current density of 1 A/g [135]. Notably, graphene-deposited mesocarbon nanobead electrodes have shown a high power density of 10,000 W/kg under the influence of LiPF₆ [136]. The conductivity profile of 1 M LiPF6 across diverse solvents at room temperature can be visualized in Figure 14.

Ionic Liquid-Based Electrolytes

Ionic liquids, often known as room-temperature molten salts, are characterized by their ability to dissociate into free ions at temperatures below 100 °C without the formation of discrete molecules. These unique substances have been proposed as viable alternatives to conventional electrolytes in energy storage applications due to their exceptional properties. Typically, an ionic liquid is composed of an asymmetrical organic cation paired with an inorganic or organic anion, which imparts a variety of advantageous features. Some of these include high thermal and chemical stability, significant electrochemical stability, and a broad range of conductivity, generally spanning from 10⁻³ to 10⁻² S/cm. These attributes are contingent upon the specific combination of cations and anions within the liquid. Multiple modeling investigations have been undertaken to elucidate the relationship between the concentrations of anions and cations within ionic liquids and their resulting physical and chemical properties [137,138]. Despite their inherently high viscosity, ionic liquids have found favor in the production of supercapacitors due to their capacity to yield higher energy densities than aqueous counterparts. This is largely attributed to the extended potential window that ionic liquids offer compared to water-based electrolytes. Furthermore, the robust nature of the ions and the strong interactions they form with solvents confer a heightened stability to ionic liquids, effectively resisting volatility and evaporation, which is a prevalent issue with water-based solutions. These factors contribute significantly to the growing interest in ionic liquids as a superior choice for enhancing the performance and durability of energy storage technologies.

Ionic liquids are generally categorized into three major types according to their ionic constitution: aprotic, protic, and zwitterionic. As depicted in (Figure 15), these classifications are based on the nature of their ions. Aprotic ionic liquids are particularly suitable for use in supercapacitors and lithium-ion batteries due to their specific charge-carrier properties. Conversely, protic ionic liquids are typically employed in the realm of fuel cells where their unique ionic interactions play a crucial role. Lastly, zwitterionic ionic liquids find their niche in ion–liquid membranes, which exploit their distinct structural features for optimal performance. The most frequently utilized cations and anions in the formulation of these substances are presented in Figure 16 and Figure 17, highlighting the versatility and tailorability of these materials for various applications [139,140].

When comparing the effects of carbon nanofiber electrodes in aqueous KOH, Na₂SO₄, and (1-ethylimidazolium bis(trifluoromethanesulfonyl)imide) IL-based electrolytes, it was observed that the KOH electrolyte provided a superior specific capacitance of 368.8 F/g at 0.5 A/g, attributed to the elevated K⁺ ion mobility. The performance of these electrolytes in device configurations was then evaluated, with the IL-based device showcasing a noteworthy 24 Wh/kg energy density at a power density of 750.3 W/kg, surpassing the others.

Table 2. The performance of IL electrolyte-based supercapacitors [141].

IL Electrolyte	Electrode	Capacitance	References
[EMIm][SeCN]	Activated carbon	49 F/g for Al 56 F/g for stainless steel	[142]
[EDMF]BF4	GC electrode	126 F/g	[143]
[EMIM]BF4	Nonporous gold	12.1 to 6.6 F/cm3	[144]
EMIM-TFSI	“Peppered”-activated carbon		[145]
[EMIM][TFSI]/FS	MnO2@CF//FeOOH/PPy@CF		[146]
EMIM-TFSI	Mesoporous reduced graphene oxide	104.3 F/g	[147]
[EMIM][TFSI]/LiCl/Al2O3	(MWNT)/V2O5 nanowires (NWs)	10.6 mF/cm2 at 0.5 mA/cm2	[148]
(EMIMTFSI)/ acetonitrile	Manganese oxide (MnOx)-decorated carbonized porous silicon nanowire	635 F/g	[149]
[BMPyr+] [DCA−]	α-Co(OH)2	28.6 F/g @0.15 mA/cm2	[139]
EMIMBF4	Boron-doped graphene	138 F/g	[100]
EMIMBF4	Highly conductive mesoporous activated carbon fiber	204 F/g @0.5 A/g	[150]
(1-Ethylimidazolium bis(trifluoromethane sulfonyl) Imide	Carbon nanofiber	77.1 F/g @0.5 A/g	[99]

compiles a list of prominent ILs that have been explored as electrolytes in contemporary supercapacitor research [141].

The aforementioned progression underscores the vital function of ionic liquids (ILs), a subset of organic electrolytes, in the context of advanced energy storage technologies. Despite certain drawbacks such as limitations associated with large cationic and anionic species, these properties can be cleverly harnessed to augment the energy density of capacitive energy storage solutions, particularly within the realm of supercapacitors that integrate bi-redox ILs [151]. As the demand for high-performance energy storage systems continues to surge, the appeal of supercapacitors utilizing IL-based electrolytes is progressively gaining traction as a competitive alternative to other existing energy storage devices. While ILs have the capacity to act independently as electrolytes, their exclusive use is not a viable approach for various practical considerations. One such hindrance is the inherently high viscosity of ILs, which poses a substantial challenge to their widespread commercialization. Moreover, the prohibitive cost of ultra-pure ILs makes them an economically unfeasible choice as the sole constituent of electrolytes. A more rational strategy involves incorporating ILs as part of a composite electrolyte system to facilitate ionic mobility and enhance overall device efficiency [152].

2.6. Advantages and Disadvantages of Supercapacitors

Supercapacitors offer several advantages over other energy storage technologies, including:

• High Power Density: Supercapacitors have a much higher power density than batteries, allowing for faster charging and discharging rates.
• Fast Charge–Discharge Rates: Supercapacitors can be charged and discharged much faster than batteries, making them suitable for applications requiring rapid energy delivery.
• Long Cycle Life: Supercapacitors have a much longer cycle life than batteries, with some devices capable of withstanding hundreds of thousands or even millions of charge–discharge cycles.
• Wide Operating Temperature Range: Supercapacitors can operate over a wider temperature range than batteries, making them suitable for use in harsh environments.
• High Efficiency: Supercapacitors have high energy efficiency, with minimal energy loss during charging and discharging.

However, supercapacitors also have some disadvantages:

• Low Energy Density: Supercapacitors have a lower energy density than batteries, meaning they cannot store as much energy for a given size and weight.
• Voltage Drop During Discharge: The voltage of a supercapacitor decreases linearly during discharge, which can be a problem for some applications.
• High Cost: Supercapacitors can be more expensive than batteries, although the cost is decreasing as the technology matures.

3. Design and Fabrication Strategies

3.1. The Fundamentals of Supercapacitor Electrode Design

The architecture of supercapacitor electrodes significantly influences their overall effectiveness, energy storage capacity, and operational efficiency [153]. By scrutinizing and refining these fundamental design aspects, scientists are equipped to augment the performance of these electrodes, thereby paving the path for the next generation of high-performance energy storage solutions, as outlined in

Table 3. Comparison of several electrode design principles for supercapacitors. With permission from ref. [153].

Design Principle	Advantages	Applications	Limitations
Surface Area and Porosity	Higher charge storage capacity, faster ion diffusion, enhanced charge–discharge rates, and improved capacitance retention.	Supercapacitor electrodes, energy storage in portable electronics, power buffering in renewable energy systems, hybrid electric vehicles, and grid stabilization and frequency regulation.	Challenging to achieve ultrahigh porosity and limitations in mass loading due to pore structure.
Electrode Material Selection	High specific capacitance, excellent charge–discharge characteristics, wide range of available materials, and tunable properties for various applications.	Energy storage in electric vehicles, portable electronics, renewable energy storage, and aerospace and defense applications.	Limited voltage windows for some materials may exhibit poor cycling stability and high cost for certain advanced materials.
Electrical Conductivity	Low internal resistance, efficient charge transfer, and enhanced power delivery.	High-power applications, rapid charge–discharge cycles, high-frequency applications, and reduced energy losses.	Some materials may suffer from poor conductivity, compatibility with certain electrolytes, and energy harvesting.
Electrolyte Compatibility	Efficient ion transport, improved charge–discharge rates, lower internal resistance, and a wide range of available electrolytes.	Portable electronics, renewable energy systems, electric vehicles, and grid stabilization and frequency regulation.	Limited operating voltage window for certain electrolytes and potential for electrolyte decomposition.
Binder and Additive Selection	Enhanced electrode stability, improved electrode–electrolyte interface, and better mechanical integrity.	Supercapacitor electrodes, portable electronics, renewable energy storage, and hybrid electric vehicles.	Potential for binder and additive decomposition: binder content may reduce the effective surface area.
Electrode Thickness and Mass Loading	Higher energy density, enhanced ion transport, improved power density, and tailored properties for specific applications.	Supercapacitor electrodes, energy storage in portable electronics, renewable energy systems, and electric vehicles.	Limited mass loading may affect overall capacitance, thicker electrodes may lead to slower charge–discharge rates, and mass loading may affect mechanical stability.
Scalability and Cost-Effectiveness	Cost-effective manufacturing, large-scale production feasibility, commercial viability, and potential for integration with existing processes.	Supercapacitor electrodes, energy storage in portable electronics, renewable energy systems, and electric vehicles.	Scalability may affect some material properties, scalability may introduce fabrication challenges, and cost-effectiveness may compromise performance.

.

3.1.1. Surface Area and Porosity

A crucial aspect of supercapacitor electrode design is enhancing the surface area and porosity, which significantly affects the performance. This is because an increased surface area provides a greater number of sites for charge storage, which is essential for improving the device’s energy density. Moreover, an elevated porosity level facilitates better electrolyte infiltration and ion diffusion, leading to enhanced charge transport within the electrode material. To achieve these goals, scientists often employ strategies like constructing hierarchical nanostructures and utilizing various templating approaches. These methods aim to create a porous architecture that allows for optimal interaction between the electrolyte and the electrode material, thereby enhancing the overall efficiency of the supercapacitor [154].

3.1.2. Selection of Electrode Material

Selecting the appropriate electrode material is essential for optimizing the specific capacitance and energy storage capabilities of supercapacitors. Carbon-based substances, including activated carbon, graphene, and carbon nanotubes, are frequently utilized in these devices, as they exhibit exceptional surface areas, electrical conductivity, and longevity. Additionally, researchers have investigated the potential of transition metal oxides, conductive polymers, and composite materials to further improve the efficiency and adapt the characteristics of supercapacitor electrodes to specific requirements. These alternative options contribute to advancing the field by broadening the range of suitable materials for such applications [155].

3.1.3. Electrical Conductivity

The significance of electrical conductivity in the context of supercapacitor electrode design cannot be overstated. It is a fundamental parameter that influences the efficiency of electron transfer across the electrode material, which in turn affects the device’s internal resistance and its capacity to rapidly charge and discharge. In order to optimize this conductivity, electrode architectures often integrate carbon-based substances and conductive additives. These components are meticulously selected and engineered to facilitate superior electron transport, thereby ensuring that the supercapacitor operates with minimal energy loss and achieves high power densities. The strategic incorporation of such materials enhances the electrochemical performance of the device, contributing to the overall effectiveness and longevity of the supercapacitor [156].

3.1.4. Electrolyte Compatibility

Critical to the development of supercapacitor electrodes is the selection of an appropriate electrolyte that is compatible with the chosen material and provides high ionic conductivity. The suitability of the electrolyte is contingent upon the specific requirements of the electrode material and the intended application of the supercapacitor. In this context, a range of aqueous and non-aqueous electrolytes with different salt concentrations are employed to optimize performance. Additionally, the exploration of ionic liquids and gel electrolytes has emerged as a promising avenue to enhance the safety features of these energy storage devices. These alternative electrolyte forms are increasingly being considered for their potential contributions to the overall efficiency and reliability of supercapacitors [157].

3.1.5. Binder and Additive Selection

Essential components known as binders and additives are incorporated within the anode and cathode structures to maintain the integrity of the active material particles and enhance their stability. The careful selection of a suitable binder is crucial as it ensures robust adhesion among particles and significantly reduces the occurrence of detrimental interactions with the electrolyte. Furthermore, the inclusion of conductive additives, such as carbon black or carbon nanotubes, serves to amplify the electrical conductivity of the electrode, thereby contributing to the overall performance of the battery cell. These materials are meticulously chosen to complement the properties of the active electrode substances and foster a cohesive and functional assembly that can efficiently facilitate the flow of electrons and ions, which is pivotal for the battery’s energy storage and release mechanisms [158].

3.1.6. Mass Loading and Electrode Thickness

The energy storage capability of a supercapacitor is significantly impacted by the thickness and mass loading of its electrodes. The optimization process for these factors entails a delicate equilibrium between enhancing the content of the active material to obtain a higher capacitance and preserving efficient ion transport and accessibility. To attain an ideal balance between energy and power density, scientists meticulously calibrate the aforementioned parameters, ensuring that the design enhances both performance metrics without compromising either. This approach is underpinned by a thorough understanding of the underlying physical and chemical principles that govern the operation of supercapacitors [159].

3.1.7. Scalability and Cost-Effectiveness

When engineering supercapacitor electrodes, it is essential to prioritize not only performance enhancement but also scalability and economic viability. The production processes must be adaptable to large-scale manufacturing to maintain performance consistency without incurring significant costs or technical barriers. Furthermore, the selection of materials and implementation of fabrication methods should be based on cost-effective principles, ensuring that supercapacitors remain competitive within the broader energy storage market. This approach is crucial for the commercialization and widespread adoption of supercapacitors as a reliable and efficient energy storage solution. The emphasis on scalability and cost efficiency will facilitate the integration of these devices into various industrial and consumer applications, thereby advancing the overall sustainability of energy storage technologies [160].

4. Electrode Fabrication Techniques and Architecture

The effectiveness of supercapacitor electrodes is significantly shaped by the techniques and designs utilized in their production. This discussion has focused on various manufacturing approaches aimed at creating electrodes with augmented surface area, porosity, and electrical conductivity. These enhancements are crucial for achieving superior performance in energy storage and ensuring long-term reliability. By employing such strategies, researchers are able to optimize the electrochemical properties of these devices, leading to substantial advancements in the field. The emphasis on these fabrication methodologies is essential for the continuous development and refinement of supercapacitor technology [161].

4.1. Chemical Vapor Deposition (CVD)

The chemical vapor deposition (CVD) technique is a widely implemented and highly versatile process that allows for the creation of superior quality thin films, nanomaterials, and nanostructures with meticulous control over their chemical makeup, morphological features, and crystalline organization. In the specific domain of supercapacitor electrode manufacturing, CVD has demonstrated substantial benefits, notably in the synthesis of carbon nanotubes (CNTs) and graphene. These carbon-based materials are celebrated for their remarkable properties, which are highly advantageous in the context of advancing energy storage solutions [162].

4.1.1. Carbon Nanotube (CNT) Growth

The advent of chemical vapor deposition (CVD) has significantly transformed the large-scale synthesis of carbon nanotubes (CNTs), paving the way for their integration into supercapacitors. In this sophisticated technique, a precursor gas rich in carbon is injected into a reaction chamber that maintains an elevated temperature. The precursor gas decomposes under these conditions, resulting in the formation of CNTs on an appropriate substrate. The strategic placement of catalytic nanoparticles on the substrate acts as a nucleation site, facilitating the growth of the CNTs. The CVD process grants researchers precise control over several critical variables such as the temperature, gas flow rates, and growth duration, which in turn influences the CNT properties like the diameter, length, and orientation. Notably, CVD enables the synthesis of vertically aligned CNT arrays. These arrays exhibit remarkable features such as an enhanced surface area for charge storage, rapid ion diffusion for swift charge transfer, and efficient alignment for optimal charge transport, making them highly suitable for use in supercapacitor electrodes [163]. In their 2021 study, Gan et al. [164] developed V₂O₅/VACNT composites employing vanadium(III) acetylacetonate (V(acac)₃) from Adamas as the vanadium source within a supercritical CO₂ (scCO₂) medium. The process involved introducing different precursor quantities (20, 40, and 60 mg) into a 100 mL high-pressure reactor in a N₂-filled glove box, along with a VACNT specimen. A minimal benzene volume was added as a cosolvent, and the system was then sealed and preheated to 50 °C. Following preheating, the CO₂ was pressurized to 12 MPa using a gas injection pump. The reactor was maintained at 100 °C for 12 h, allowing the V(acac)₃ to dissolve and be absorbed by the VACNTs through scCO₂. The adsorbed precursor mass was determined by subtracting the initial mass from the mass of the post-treatment sample. After cooling and depressurization, the V₂O₅/VACNTs composites were formed by annealing in an air atmosphere, converting V(acac)₃ to vanadium oxide. These composites were denoted as VN-T, with T representing the annealing temperature. Yin et al. [165], in 2018, prepared TMOs/VACNTs hybrids utilizing a scCO₂-assisted impregnation technique with tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) [Co(tmhd)₃] from Alfa Aesar as the cobalt precursor. The experimental setup included a 50 mL stainless-steel high-pressure cell in a N₂ environment. Typically, the researchers placed VACNT arrays with 20–40 mg Co(tmhd)₃ inside the cell. The cell was then sealed and connected to a gas supply system, with CO₂ being introduced and pressurized to 11 MPa at 70 °C using a syringe pump. For 6 h, the cell was kept at 200 °C before undergoing a slow depressurization and cooling process to room temperature. To assess the absorbed precursor quantity, the VACNTs’ mass was measured both pre- and post-scCO₂ treatment. The Co(tmhd)₃-impregnated VACNTs were annealed in an air atmosphere within a rapid thermal annealing furnace at temperatures ranging from 200 to 300 °C. The resulting cobalt oxide-containing composites were denoted as CVN−T, with T signifying the annealing temperature.

4.1.2. Synthesis of Graphene

The significant progress in chemical vapor deposition (CVD) has been instrumental in the large-scale manufacture of graphene tailored for various energy storage applications. Graphene’s distinctive architecture, comprised of sp²-hybridized carbon atoms arranged in a two-dimensional honeycomb lattice, exhibits exceptional electrical conductivity, mechanical resilience, and a substantial specific surface area. The CVD process entails subjecting a metal catalyst substrate, such as copper or nickel, to a carbon precursor gas like methane or ethylene. As a result, carbon species adhere to the metal surface, undergoing reactions that culminate in the creation of graphene layers as they build up. Post-synthesis, these layers are meticulously transferred to targeted substrates using suitable techniques. This approach is highly favorable for producing supercapacitor electrodes, as it allows for precise control over the thickness of high-quality large-area graphene films [166]. Graphene, being a single-layered carbon allotrope, showcases an unparalleled combination of high electrical conductivity, carrier mobility, and superior mechanical and morphological traits, due to its sp2 hybridization and two-dimensional hexagonal lattice structure [167]. Its exceptional properties distinguish it from other carbon materials in that it does not rely on pore distribution within the solid state, making it highly suitable for high-performance supercapacitors [168]. This versatile material can be structured into various forms, such as zero-dimensional (free-standing dots), one-dimensional (fiber or tube structures), two-dimensional (graphene nanosheets and nanocomposite films), and three-dimensional (graphene foams) configurations, each offering distinct advantages in different supercapacitor designs [169]. To optimize the performance of supercapacitor electrodes, graphene can be modified through processes like nitrogen doping or graphene oxide reduction or by employing graphene nanosheets. These modifications can enhance the material’s specific surface area and improve its electrochemical properties. For instance, studies have demonstrated that graphene-based electrodes can achieve a substantial theoretical specific surface area of around 2600 m²/g, with a specific capacitance of 205 F/g, resulting in a supercapacitor with an energy density of 28.5 Wh/kg and a power density of 10 W/kg [170]. Furthermore, researchers such as Guevara et al. [171] have successfully fabricated a reduced graphene oxide/polyaniline composite supercapacitor utilizing an infrared laser from a Light Scribe recorder drive for the deposition of PANI nanofibers. This approach exemplifies the innovative ways in which graphene’s properties can be harnessed to create highly efficient energy storage devices (Figure 18).

The reported composite presented a specific capacitance of 442 farads per gram and preserved 84% of this capacitance over an extensive cycling period of 2000 cycles. In a study conducted by Ogata and colleagues [172], an innovative all-solid-state device was engineered, which manifested both supercapacitor (SC) and battery characteristics. This hybrid system comprised reduced graphene oxide (rGO) and graphene oxide (GO) layers. The SC behavior was observed at operational voltages less than 1.2 V, while the device demonstrated battery-like properties above this threshold. The creation of this rGO/GO/rGO sandwich structure involved the photoreduction of a GO film on both sides, resulting in the formation of the rGO electrodes and an intermediate GO layer that functioned as both an electrolyte and a separator (Figure 19).

In a study by S. Zhongting and colleagues from 2022 [173], NiCo₂O₄/rGO composites were synthesized by generating graphene oxide (GO) through a modified Hummers technique, starting with graphite powder. The obtained GO was then diluted in deionized water to form a 0.7 mg mL⁻¹ solution for subsequent use. A combination of 20% ethanol and 80% ethylene glycol was utilized to dissolve Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O in a 1:2 ratio, resulting in a uniform reserve solution. To achieve different concentrations of GO to metal salt, the researchers added varying amounts of the prepared GO dispersion to the solution. The mixture was sonicated for 30 min to ensure homogeneity. For fabrication, an UC320 ultrasonic spraying system from Siansonic, Beijing, China, was employed. The substrate, Ni foam, was cleaned with ethanol and deionized water before being placed on a hot plate. The precursor solution was fed into the system at 0.6 mL min⁻¹ under an ultrasonic power of 1.5 W, and the nozzle was scanned at 2 mm s⁻¹ to produce the composites. The mass loading of the active material on the Ni foam, ranging from 0.5 to 1 mg cm⁻², was regulated by adjusting the spray duration. The synthesized NiCo₂O₄/rGO served as the positive electrode component in the construction of a button-cell asymmetric supercapacitor. The negative electrode, activated carbon (AC), was prepared by combining AC (Super P, YEC-8A), carbon black, and polytetrafluoroethylene (PTFE) in an 8:1:1 weight ratio, followed by grinding with isopropanol and drying at 60 °C for 12 h. The resulting AC sheets were pressed onto the Ni foam to complete the electrode assembly. The electrolyte used for these experiments was 2 M KOH. This approach facilitates the scalable production of such supercapacitors with tailorable properties.

4.1.3. Supercapacitor Electrode Applications

Owing to their distinct characteristics, carbon nanotubes (CNTs) and graphene produced via chemical vapor deposition (CVD) have gained substantial attention in the context of supercapacitor electrode material [174]. The vertical alignment of CNTs offers superior pathways for both charge transfer and ion diffusion, which in turn facilitates high-rate performance and bolsters cycling stability within these devices. Graphene, known for its exceptional specific surface area and electrical conductivity, displays remarkable capacitive properties that significantly enhance the energy storage potential of the electrodes used in supercapacitors. Furthermore, the modulation of CVD growth conditions allows for the optimization of CNT and graphene properties, thereby tailoring their performance to suit a variety of specialized application needs [175].

4.1.4. Future Directions

The continuous development of chemical vapor deposition (CVD) methodologies is expected to lead to significant improvements in the synthesis of carbon nanotubes (CNTs) and graphene, thereby enhancing the performance of supercapacitor electrodes. These advancements will likely result in the fabrication of electrodes with superior characteristics. The exploration of hybrid configurations, like the integration of CNTs and graphene within nanocomposites, presents an attractive opportunity to exploit the synergistic interactions between these materials. This approach is poised to enhance the energy storage capacity of supercapacitors substantially. Additionally, the combination of CVD-produced carbon nanotubes and graphene with diverse nanomaterials, such as metal oxides or conductive polymers, opens up a realm of possibilities for the design of next-generation electrodes. These innovations are expected to introduce tailor-made properties that cater to an extensive array of applications in the energy storage sector [176,177].

4.2. Template-Assisted Synthesis

The utilization of template-guided synthesis stands as a flexible and reliable methodology in the creation of nanostructured materials boasting precise morphologies and architectures [173]. This process involves the application of templates that are predefined in their shape and dimensions to direct the deposition or proliferation of materials intended for use in electrodes. The outcome of such synthesis frequently yields materials that display an enhanced surface area, porosity, and notably superior properties in terms of charge storage. These attributes render these materials highly suitable for integration into the design of supercapacitor electrodes.

4.2.1. Using Alumina Membranes as Models

A prevalent template in the realm of template-aided synthesis is porous alumina membranes, alternatively referred to as anodic aluminum oxide (AAO) templates. These membranes are typically fabricated through the electrochemical anodization of aluminum foils, which yield a structure characterized by nanopores that can be precisely controlled in terms of both the diameter and spatial distribution. The AAO template serves as a scaffold for the deposition or growth of active electrode materials, thereby enabling the production of materials with tailored morphologies and properties. This method involves the infusion of precursor substances into the nanopores, followed by specific processing steps to facilitate the synthesis of a variety of materials such as metal oxides, conductive polymers, and carbon-based compounds [178].

4.2.2. Templates Used as Sacrifices

Aside from alumina membranes, template-assisted synthesis for supercapacitor electrodes often employs sacrificial templates, which are materials that can be simply eliminated post-synthesis to reveal a porous architecture. These templates are constructed from substances that are readily removable, and they include organic polymers, such as polystyrene, as well as inorganic materials like silica particles. The process typically involves incorporating these sacrificial materials within the precursor systems, followed by techniques like thermal annealing or etching to extract them and thus create the desired porous morphology [179].

4.2.3. Applications in Supercapacitor Electrodes

The utilization of template-aided synthesis has demonstrated remarkable efficacy in the production of a multitude of nanomaterials tailored for supercapacitor electrodes. One notable example involves the application of alumina membranes as scaffolding structures, which facilitate the creation of metal oxides exhibiting intricate nanoarchitectures, such as hierarchical nanowires or nanotubes [180]. These metal oxides, with their meticulously controlled morphologies, have been found to possess superior electrochemical characteristics, such as a high specific capacitance and remarkable rate capabilities, rendering them highly suitable for integration into supercapacitor technology. Additionally, the employment of template-assisted synthesis has led to the development of porous conducting polymers with enhanced ion permeability and improved capacitive performance [181]. The high specific surface area and porosity inherent to these polymers significantly bolster their effectiveness in charge storage. This methodology remains a vibrant field of study, with persistent research efforts aimed at discovering innovative template materials and refining synthesis procedures. The flexibility of utilizing sacrificial templates allows for the design and fabrication of electrode architectures with a high degree of customization. Furthermore, the integration of template-assisted synthesis with advanced techniques such as inkjet printing or chemical vapor deposition (CVD) presents a promising avenue for the realization of supercapacitor electrodes boasting hierarchical nanostructures tailored for optimized performance [182]. Moreover, the exploration of hybrid electrode materials, which amalgamate the merits of various nanostructures into a single architecture, opens up exciting prospects for the development of supercapacitors that can achieve enhanced energy storage performance. This approach has the potential to significantly improve the capacitive properties and overall efficiency of these energy storage devices.

4.3. Sol–Gel Process

The sol–gel approach represents a highly adaptable and commonly implemented method for producing metal oxides and various other inorganic substances that exhibit high purity and tailorable structures (Figure 20). Within the realm of supercapacitor electrode manufacturing, this process has seen substantial research interest, particularly for the creation of materials based on transition metal oxides, such as manganese oxide and nickel oxide. Additionally, it has been employed to synthesize mixed metal oxides, encompassing binary and ternary systems, which can significantly enhance the performance characteristics of the electrodes [183].

4.3.1. Procedures for the Sol–Gel Method

The sol–gel methodology is a multi-stage process that transitions from a precursor solution, termed as “sol,” to a solid–gel matrix. This technique is commonly applied in the fabrication of materials and typically involves the following key phases [184]:

• Sol Formation: The initial phase involves dissolving metal alkoxides or salts in an appropriate solvent, which can be alcohols or water, resulting in a uniform mixture known as a sol. To adjust the final material’s characteristics, dopants or additional chemical agents may be introduced into the sol.
• Hydrolysis Reaction: Within the sol, the metal–oxygen bonds of the metal alkoxides or salts are broken down by water molecules in a process called hydrolysis. This generates metal hydroxides, which are essential for initiating the polymerization that follows.
• Polycondensation Process: The metal hydroxides formed during hydrolysis proceed to react with each other, leading to the creation of metal–oxygen–metal (M-O-M) linkages. This step is critical for the formation of a three-dimensional gel framework. The polycondensation reaction can be regulated to achieve the desired porosity and specific surface area in the gel material.
• Solvent Removal and Thermal Treatment: Once the gel has formed, the next step is to eliminate the solvent through a drying process. Following this, the gel undergoes calcination at high temperatures, typically ranging from 200 to 800 degrees Celsius. This thermal process serves to remove any residual organic substances and induces a transformation into a crystalline metal oxide structure.

4.3.2. Applications in Supercapacitor Electrodes

The sol–gel technique holds numerous merits in the context of manufacturing supercapacitor electrodes. This approach provides a high degree of precision in the regulation of the metal oxide compounds’ composition and structure. Through meticulous manipulation of synthesis conditions and the utilization of a range of metal precursors, it is feasible to create metal oxides that exhibit a variety of nanostructures, including nanoparticles, nanowires, and nanosheets. This flexibility in morphology is crucial as it influences the electrical properties of the resulting material. Additionally, the sol–gel method allows for the seamless introduction of dopants or functional species at the gelation stage. This capability is instrumental in modifying the electrochemical characteristics of the material, such as its specific capacitance and the mechanisms responsible for charge storage. These tailored properties are essential in enhancing the performance of supercapacitors, making the sol–gel process an attractive choice for researchers and engineers in the field [185].

4.3.3. Advantages and Limitations

Sol–gel synthesis provides a number of noteworthy benefits in comparison to traditional solid-state procedures for the fabrication of metal oxides, such as the capability to conduct reactions at lower temperatures, the homogeneous distribution of dopants within the material, and the precise manipulation of the nanostructure [186]. These features are particularly beneficial when aiming to optimize the electrochemical properties of the resulting compounds. Nonetheless, this technique can face certain limitations, such as the stringent requirement for the precise regulation of reaction parameters and potential complexities in scaling up to produce substantial amounts of material efficiently.

4.3.4. Future Directions

Continuous investigation in the sol–gel domain centers on refining the synthesis parameters for metal oxide-based supercapacitor electrodes to elevate their functionality. A noteworthy approach involves the study of composite substances, which combine metal oxides with carbon-containing materials to realize synergistic interactions and superior total performance. Moreover, the adaptation of sol–gel procedures to facilitate large-scale manufacturing is pivotal for the widespread application and integration of supercapacitor technology in practical scenarios [187].

4.4. Inkjet Printing

Inkjet printing, a method of depositing materials in a precise and controlled manner, has experienced substantial interest in recent times due to its potential in the realm of additive manufacturing (Figure 20). When applied to the creation of supercapacitor electrodes, this technique presents several key advantages. It enables the production of high-resolution devices, affords great flexibility in design, and boasts scalability, which in turn makes it an attractive choice for the fabrication of customized and high-performance supercapacitor electrodes [188].

4.4.1. Supercapacitor Electrode Advantages

Inkjet printing technology provides significant flexibility in designing electrodes for electrochemical devices. One of its main advantages is the ability to produce customized designs, with precise control over the patterning of these electrodes, including complex geometries and optimized thicknesses. This tailored approach enhances the distribution of current, decreases resistive losses, and ultimately leads to superior electrochemical performance, as noted in past studies [189]. Furthermore, the direct deposition method inherent in inkjet printing eliminates the reliance on intricate lithographic processes and the use of masks. This simplification not only reduces the overall complexity of the manufacturing process but also minimizes the amount of material waste generated during the production of these electrodes. The environmental and economic implications of such a reduction in waste are substantial, contributing to the appeal of inkjet printing in the fabrication of electrochemical devices. Moreover, inkjet printing allows for the sequential deposition of various ink formulations, which is essential for the integration of multilayer structures within the electrode framework. This feature enables the construction of electrodes with hierarchical nanostructures, where different materials can be strategically combined to synergistically improve their electrochemical properties. Such hybrid electrodes can capitalize on the unique advantages of each component material, leading to enhanced overall performance.

4.4.2. Applications in Supercapacitor Electrodes

The application of inkjet printing technology in the creation of supercapacitor electrodes has yielded positive results with a variety of materials, including metal oxides, conductive polymers, and carbon-based nanostructures. This approach has facilitated the production of metal oxide nanoparticle-based electrodes with meticulously regulated porosity and extensive surface areas, which significantly enhances the number of ion-accessible locations and thus the capacitive performance. Moreover, the utilization of conducting polymers in inkjet-printed electrodes has exhibited remarkable charge transfer capabilities and swift ionic mobility, leading to a substantial increase in energy storage capacity. This method’s adaptability allows for tailored designs that cater to the unique properties of each material, thereby optimizing these supercapacitors’ overall functionality [190].

4.4.3. Future Directions

Continuous studies in the domain of inkjet printing for supercapacitor electrode production are concentrated on improving ink compositions, fine-tuning the parameters of the printing process, and examining innovative substances to elevate the efficiency of these electrodes. There is also a significant emphasis on employing eco-friendly solvents and binder components to advance sustainable and environmentally responsible manufacturing methods. Moreover, the integration of inkjet printing with alternative electrode fabrication techniques, such as template-based synthesis or 3D printing, presents a promising approach for the creation of sophisticated supercapacitor devices with customized characteristics suitable for a broad range of uses. This interdisciplinary combination leverages the precision of inkjet technology and the potential of complementary methods to yield supercapacitors with optimized performance and adaptability to various practical scenarios [191].

4.5. Three-Dimensional (3D) Architectures

Emerging as a notable solution to the limitations of conventional planar designs, three-dimensional (3D) architectures have been found to significantly enhance the energy storage capacity and electrochemical performance of supercapacitor electrodes (Figure 20). These innovative structures boast distinct characteristics that effectively tackle critical issues encountered in traditional two-dimensional systems, thereby rendering them highly suitable for next-generation energy storage devices. The intricate 3D configurations provide a substantial surface area for redox reactions, facilitate ion and electron transport, and contribute to an increased volumetric energy density, which are all vital for achieving superior supercapacitor functionality [192].

4.5.1. Types of 3D Architectures

A plethora of 3D architectures have been examined for the purpose of supercapacitor electrodes, each presenting unique advantages in optimizing performance [193].

• Hierarchical Nanostructures: These architectures consist of a combination of diverse nanosized components, such as nanowires, nanotubes, and nanoparticles, which are assembled to create a multifaceted interconnected network. The hierarchical arrangement of pores within these structures enhances ionic access and facilitates superior charge storage due to the synergistic effects of the various pore sizes and high surface-to-volume ratio.
• Mesoporous Materials: Materials characterized by mesoporosity, with pore dimensions ranging typically between 2 and 50 nm, are highly effective in supercapacitor applications. The interconnected nature of these mesopores contributes to an extensive surface area and substantial pore volume, which enable efficient ionic diffusion and bolster the capacitive properties of the electrode.
• Conductive Frameworks: The utilization of 3D conductive frameworks, like graphene foam or carbon nanotube scaffolds, serves as a supportive skeleton for the active electrode material. These frameworks not only ensure rapid electron transfer but also provide mechanical stability to the electrode, as they are designed to accommodate the strain associated with the charge–discharge cycles.
• Additive Manufacturing: Employing 3D printing technology in the fabrication of supercapacitor electrodes allows for the creation of intricate and tailored geometries. This method enables precise control over the deposition of layers of electrode material, thus catering to specific design requirements and potentially leading to superior structural integrity and functional efficiency.

4.5.2. Applications in Supercapacitor Electrodes

Three-dimensional architectures have gained extensive utilization in supercapacitor electrode design, showcasing noteworthy performance enhancements over conventional planar configurations. In particular, metal oxides with hierarchical nanostructures have revealed significant improvements in specific capacitance and rate capability. This is largely attributed to the augmentation of the surface area and the facilitation of the ion diffusion pathways these structures provide. Moreover, mesoporous carbon materials have exhibited remarkable capacitive properties and high energy storage density, establishing them as highly suitable candidates for supercapacitor electrodes. Their distinctive porosity and structural hierarchy contribute to their proficient electrical conductivity and overall capacitive performance [194].

4.5.3. Future Directions

The persistent investigation into state-of-the-art 3D designs for supercapacitor electrodes is anticipated to lead to substantial advancements in energy storage effectiveness. The emphasis of these research endeavors lies in meticulously adjusting the pore dimensions, structural configuration, and material composition of these 3D constructs to optimize their energy storage capacity and power handling characteristics. Furthermore, the integration of various materials within the three-dimensional framework and the utilization of hybrid architectures present promising avenues for realizing enhanced electrochemical performance. This, in turn, opens up new possibilities for their application in a broad spectrum of domains, including cutting-edge energy storage systems, wearable electronic devices, and the burgeoning field of electric vehicles [158].

5. Recent Advancements in Electrode Nanotechnology

Recent years have witnessed significant strides in the customization of nanostructures, the investigation of cutting-edge carbon-based nanomaterials, and the exploitation of the capabilities of two-dimensional (2D) nanomaterials. Furthermore, researchers have focused on creating binder-free and nanocomposite electrodes, which are essential components of supercapacitors.

Table 4. Recent developments in supercapacitor electrode nanotechnology. With permission from ref. [153].

Electrode Material	Specific Capacitance (F/g)	Reference
Graphene oxide	200:300	[160]
Carbon nanotubes	100:300	[160]
Manganese dioxide	350:400	[163]
Nickel hydroxide	2000:3000	[176]

details the latest developments in the realm of electrode technology for supercapacitors [195]. Additionally, the advancement of sophisticated characterization methods and an increased commitment to environmental conservation have paved the way for the innovation and production of high-performance and eco-friendly supercapacitor electrodes, expanding the horizons for the design and fabrication of such devices [196].

5.1. Tailored Nanostructures

The development of customized nanostructures has presented a highly promising path in the realm of electrode nanotechnology for supercapacitors. Through meticulous manipulation of nanomaterials’ dimensions, morphology, and chemical makeup, substantial advancements in both the energy storage capacity and the rapidity of charge–discharge cycles have been realized. This innovative research field, with its pronounced potential to revolutionize supercapacitor electrodes’ efficiency, holds great promise across various energy storage technology applications [197].

5.1.1. Advantages in Supercapacitor Technology

Supercapacitors, also known as ultracapacitors, stand out as an emerging technology with significant potential in the realms of energy and data management. Their versatility and adaptability are continually expanding due to the ongoing discovery and development of innovative materials and techniques. These devices are poised to transform numerous sectors and fields of interest because of their distinctive properties [198].

In comparison to traditional capacitors and batteries, supercapacitors boast a multitude of benefits, including the following:

• Enhanced Energy Density: Through the implementation of tailored nanostructures, the specific surface area is substantially increased, thereby augmenting the number of active sites for ionic adsorption and desorption processes. This leads to a higher specific capacitance, allowing for greater energy storage per unit mass or volume.
• Rapid Charge and Discharge: The meticulously designed nanostructures facilitate efficient ion diffusion through the electrode material. By reducing the diffusion pathways and enhancing the porosity, the ion transport is significantly accelerated. Consequently, supercapacitors are capable of discharging and charging at high rates, rendering them ideal for applications demanding quick energy release.
• Enhanced Cyclability: Supercapacitors equipped with tailored nanostructures often demonstrate improved mechanical robustness and structural stability over numerous charge–discharge cycles. This characteristic contributes to their prolonged lifespan and minimized maintenance needs.
• Customizable Electrode Architecture: The ability to tailor nanostructures permits the fine-tuning of electrode properties to match the particular requirements of diverse applications. Scientists can optimize supercapacitor performance by adjusting the morphology, composition, and porosity of nanomaterials according to specific energy storage demands.
• Multifunctional Electrode Integration: The strategic design of nanostructures allows for the combination of various functionalities within a single electrode. By merging nanomaterials with complementary traits, such as high conductivity and pseudocapacitance, it is feasible to create hybrid systems capable of electric double-layer capacitance as well as Faradaic redox reactions. This synergistic approach enhances the total energy storage capacity of the device.

5.1.2. Applications in Supercapacitor Technology

Nanostructured electrodes have become increasingly prevalent in the realm of portable electronics, including smartphones, tablets, and wearable technology. These meticulously designed nanostructures confer significant advantages such as reduced weight and enhanced energy storage performance, which in turn provide users with extended operational time and the convenience of rapid charging. In the domain of electric vehicles, tailored nanostructured supercapacitors have demonstrated their suitability for applications involving energy recovery and regenerative braking systems. The remarkable feature of these electrodes is their capability to maintain rapid charge–discharge rates while delivering high energy density, thereby contributing to the overall efficiency and power output enhancement of the vehicles’ energy storage systems. Moreover, the integration of renewable energy sources into the electrical grid significantly benefits from the utilization of supercapacitors with tailored nanostructures. By effectively storing surplus energy produced by intermittent sources like solar panels and wind turbines during peak generation periods and releasing it during periods of low production or high consumption, these devices facilitate the stabilization of the grid and enhance the overall functionality of renewable energy systems. At the microscopic level, nanostructured electrodes are revolutionizing the landscape of microelectronics and the Internet of Things (IoT). Their compact nature and high energy density enable the creation of energy storage units that are well-suited for powering small sensors and IoT devices. This advancement allows for the development of self-sustaining and durable power solutions that are essential for the operation of these technologies over extended periods without the need for frequent charging or replacement. The tailored design of these nanostructures is instrumental in achieving the desired performance characteristics, such as power density and energy storage capacity, that are crucial for the successful implementation of IoT networks and various microelectronic applications.

5.2. Advanced Carbon Nanomaterials

The emergence of sophisticated carbon nanomaterials has marked a substantial advancement in the realm of electrode nanotechnology, particularly in the creation of high-performance supercapacitors. These innovative materials exhibit a range of unique structural, electronic, and electrochemical properties, which render them highly desirable for various energy storage purposes. Extensive research has been dedicated to exploring and exploiting these materials, including carbon quantum dots, graphene with strategic dopants, and carbon nitride, among others. The remarkable electrical conductivity, extensive surface area, and superior capacitive characteristics inherent to these substances enable the design of electrodes that boast enhanced energy storage capabilities. These properties contribute significantly to the performance enhancement of supercapacitors, leading to the creation of more efficient and reliable energy storage solutions [199].

5.2.1. Graphene-Based Electrodes

Graphene, a material consisting of a single plane of carbon atoms organized in a two-dimensional honeycomb configuration, is widely recognized as a prominent example of advanced carbon nanomaterials. Its properties include remarkable electrical conductivity and an extensive surface area that facilitate effective charge transfer and offer copious adsorption sites for ions. Electrodes constructed from graphene display high specific capacitance and excellent performance in terms of rate capability, which render them highly suitable for applications requiring rapid storage and discharge of energy [200].

5.2.2. Carbon Quantum Dots (CQDs)

Carbon quantum dots are nanostructures of zero dimensions, which exhibit exceptional properties arising from quantum confinement. Their diminutive size and adjustable energy gap facilitate effective electron entrapment and charge separation, resulting in noteworthy capacitive performance. Given their lightweight nature, eco-compatibility, and robust stability, these materials are highly appealing for utilization in the electrodes of supercapacitors [201].

5.2.3. Doped Graphene and Its Derivatives

The process of incorporating heteroatoms such as nitrogen, boron, or sulfur into graphene’s lattice induces structural imperfections and modifies its electronic characteristics, which can significantly bolster its capacitive performance. Specifically, when graphene is doped with nitrogen, it displays pseudocapacitive behavior as a result of the reversible oxidation–reduction processes that occur at the nitrogen-rich locations. These doped forms of graphene, including reduced graphene oxide (rGO), amalgamate the conductive properties inherent to graphene with an augmented number of redox-active moieties. This synergistic combination contributes to a substantial increase in the energy storage capability of electrodes employed in supercapacitors [202].

5.2.4. Carbon Nitride (g-C₃N₄)

A two-dimensional polymeric material known as carbon nitride possesses a distinctive electronic configuration. Its stratified configuration offers a copious number of active locations that are beneficial for the adsorption of ions. Moreover, the ability to adjust its bandgap enables efficient charge segregation and accumulation. Electrodes constructed from carbon nitride exhibit remarkable specific capacitance and exceptional stability in electrochemical environments. These attributes make carbon nitride a highly promising substance for supercapacitors with prolonged operational lifespans [203].

5.3. 2D Nanomaterials

Electrode nanotechnology has been significantly captivated by the emergence of two-dimensional (2D) nanomaterials, which exhibit remarkable structural and electronic characteristics. Prominent among these materials are graphene, transition metal dichalcogenides (TMDs), and boron nitride, all of which are composed of single- or few-layered atomic arrangements in a 2D lattice. This distinctive configuration confers on them a range of exceptional properties, rendering them highly suitable for utilization in supercapacitors [204].

Benefits

The utilization of two-dimensional (2D) nanomaterials in supercapacitors offers several compelling advantages that enhance their performance and functionality. These materials, due to their high specific surface area, have all their atoms exposed, which provides a multitude of active sites for ion adsorption. This inherent feature is conducive to efficient charge storage and results in a substantially higher specific capacitance, which is a critical factor for energy storage systems. Electrical conductivity is another significant benefit of 2D nanomaterials, particularly in the case of graphene and certain transition metal dichalcogenides (TMDs) such as MoS₂. Their sp2 hybridized carbon atoms and high electron mobility confer remarkable electrical properties, which are essential for rapid electron transfer during charge–discharge cycles. This characteristic minimizes the internal resistance within the electrode material, thereby facilitating swift and efficient energy storage and retrieval processes. Furthermore, the ultrathin nature of these materials leads to quantum confinement effects, which alter the electronic properties of the 2D structures. These effects are manifested as a tunable bandgap and augmented charge separation. As a result, the quantum confined 2D nanomaterials demonstrate superior capacitive behavior, which is crucial for achieving high energy density and power in supercapacitors. This level of electronic control allows for the optimization of the electrochemical performance of the electrode materials. The layered morphology of certain 2D materials, such as TMDs, permits ion intercalation between the layers. This phenomenon not only enhances ionic diffusion dynamics but also expands the availability of active sites for charge storage. Consequently, the layered architecture of these nanomaterials significantly contributes to the enhancement of both capacitance and rate capability. Additionally, some 2D nanomaterials like TMDs and graphene oxide (GO) exhibit redox-active characteristics. This property enables them to support Faradaic charge storage, a mechanism that complements the electric double-layer capacitance. The presence of pseudocapacitance in these materials significantly bolsters the overall energy storage capacity of supercapacitor electrodes, making them increasingly comparable to traditional batteries regarding energy density. Lastly, the mechanical flexibility of 2D nanomaterials is an attribute that makes them particularly suitable for integration into flexible and wearable devices. Their atomically thin nature allows for the preservation of high energy storage efficiency without sacrificing the flexibility and adaptability required for such applications. This flexibility opens new avenues for the development of lightweight and conformable energy storage solutions, which are essential for the burgeoning field of flexible electronics and wearable technology.

5.4. Binder-Free Electrodes

Binder-free electrodes represent a significant advancement in the realm of supercapacitor technology, with the primary objective of overcoming the limitations inherent in their binder-containing counterparts. This innovative design omits the use of binders, which are typically inert substances that serve to cohere the various electrode components. By doing away with these binders, such electrodes present a multitude of benefits, including superior electrochemical properties, elevated energy storage capabilities, and improved mechanical robustness. These enhancements position binder-free electrodes as a highly attractive candidate for the development of next-generation supercapacitor systems [163].

Advantages

The utilization of binder-free electrodes in energy storage systems presents several compelling benefits that distinguish them from conventional binder-containing counterparts. These advantages stem from the maximization of active material content, which enhances specific capacitance due to increased active surface area utilization. The absence of binders in these electrodes also facilitates superior ion diffusion kinetics by eliminating barriers that may impede ion movement, particularly under high-rate charge–discharge conditions. This leads to expedited charge and discharge processes and superior power delivery. Binder-free electrodes typically exhibit reduced internal resistance, which is attributed to the direct contact established between the active materials and the current collectors. This minimizes interfacial resistance, thereby promoting superior charge transport dynamics and overall electrochemical performance. Such electrodes are known for their enhanced electrochemical stability across numerous charge–discharge cycles, as the omission of binders mitigates the likelihood of detrimental side reactions that can degrade the electrode’s structure and impair long-term functionality. Moreover, these electrodes often showcase remarkable mechanical resilience and flexibility, which can be attributed to their open and interconnected structural configuration. This property allows for the efficient distribution of mechanical strain during cyclic operations, thereby lowering the risk of cracks or delamination. Consequently, they are highly suitable for integration into flexible and wearable electronics, as well as other applications requiring adaptable energy storage solutions. The production of binder-free electrodes is generally more straightforward and cost-effective due to the streamlined fabrication process. Techniques such as direct deposition and self-assembly are commonly employed, simplifying the manufacture and enhancing scalability. These electrodes find extensive use in various energy storage devices, including supercapacitors in portable technologies, electric vehicles, and grid-scale storage systems, where their high energy storage density and rapid charge–discharge capabilities are particularly advantageous. Furthermore, the flexibility and mechanical stability of binder-free electrodes make them ideal for incorporation into flexible and wearable electronics, such as smart fabrics and MEMS devices. These qualities also extend to miniature and microelectronic applications, where space constraints necessitate compact yet powerful energy storage solutions. In the realm of electrochemical sensors, the improved charge transport kinetics and stability of binder-free electrodes are highly valuable. They enable rapid response times and efficient ion diffusion, which are essential for the accurate and reliable detection of various analytes. Their robust performance underpins their suitability for use in a broad spectrum of sensor types. When assembled into supercapacitor modules, binder-free electrodes contribute to systems that excel in high-power density and stability, making them highly applicable across diverse sectors such as renewable energy, energy management, and power backup solutions. Their integration into these modules enhances the efficiency and reliability of operations within these domains. In essence, the adoption of binder-free electrodes in energy storage technology represents a significant step forward in terms of performance, longevity, and adaptability, opening up a plethora of potential uses and innovations.

5.5. Electrodes Made of Nanocomposite Materials: Improved Performance via Cooperation

The construction of nanocomposite electrodes presents an intriguing and adaptable strategy within supercapacitor technology, capitalizing on the synergistic outcomes that arise from the amalgamation of various nanomaterials to achieve superior electrochemical efficacy. Through the integration of nanomaterials that exhibit complementary characteristics, these electrodes provide a multitude of merits, such as augmented energy storage density, enhanced electrical conductance, and bolstered mechanical resilience. This segment delves into the conceptual framework, positive impacts, and practical implementations of nanocomposite electrodes in the continuous progression of supercapacitors [205].

5.5.1. Design Principles

Creating effective nanocomposite electrodes involves careful consideration of various fundamental aspects, as detailed in prior research studies [206].

• Material Combination: These electrodes often consist of a blend of two or more nanomaterials with distinct properties. It is essential to select components that complement each other, such as one contributing to high specific capacitance and the other enhancing electrical conductivity. The harmonious interplay of these materials leads to superior composite performance in electrochemical reactions.
• Interface Optimization: The efficiency of charge transfer within nanocomposite electrodes is significantly influenced by interfacial interactions. Therefore, it is critical to engineer interfaces that promote rapid electron movement and ion diffusion. This synergistic behavior ensures that each material’s unique properties are fully utilized to enhance the electrode’s overall electrochemical effectiveness.
• Nanostructure Manipulation: The shape, size, and arrangement of nanomaterials within the electrode are crucial determinants of its electrochemical characteristics. By controlling the nanostructure, researchers can tailor the properties to optimize the electrochemical performance while simultaneously bolstering the mechanical robustness. This precision in design allows for the maximization of the electrode’s potential in various energy storage and conversion applications.

5.5.2. Advantages

Nanocomposite electrodes exhibit several key advantages that significantly impact the performance of supercapacitors. The integration of nanomaterials with distinct capacitive characteristics leads to a synergistic enhancement in specific capacitance, which is greater than what is achievable with single-component systems. This synergistic effect is crucial, as it directly correlates with the energy storage capacity of the device, thereby leading to improved operational efficiency. The electrical conductivity of nanocomposite electrodes is also notably superior due to the incorporation of highly conductive materials such as graphene or carbon nanotubes. This enhancement facilitates efficient electron flow across the electrode’s structure, thereby minimizing the internal resistance and contributing to better power output. The combination of these materials allows for an optimized balance between ionic and electronic conduction, which is essential for high-performance supercapacitors. Moreover, the unique interactions between the various nanomaterials within a nanocomposite can give rise to synergistic effects that amplify the overall energy storage capabilities of the electrode. For instance, a combination of a pseudocapacitive material with a conductive nanostructure can engender a hybrid system that benefits from both electric double-layer capacitance and Faradaic redox processes. This synergy can lead to a significant boost in the supercapacitor’s energy density and rate capability. Mechanical stability is another critical aspect, where nanocomposite electrodes excel. The interplay between different nanomaterials can effectively redistribute mechanical stress during the charge–discharge cycles, thereby diminishing the likelihood of structural damage or deformation. This robustness translates into longer operational lifetimes and improved reliability for supercapacitors. Furthermore, nanocomposite electrodes offer remarkable flexibility and adaptability in their design. Researchers are able to select specific nanomaterial blends and meticulously adjust their proportions to cater to the precise demands of various supercapacitor applications. This tailored approach allows for the optimization of electrochemical properties, enabling the development of supercapacitors that are well-suited to diverse technological contexts, from wearable electronics to electric vehicles [207].

5.6. Advanced Characterization Techniques

The latest developments in the field of electrode nanotechnology involve the utilization of sophisticated analytical methods to probe the intricate electrochemical properties of nanomaterials and the dynamics at electrode–electrolyte interfaces. Techniques such as in situ microscopy, X-ray spectroscopy, and electrochemical impedance spectroscopy are being employed to scrutinize the phenomena of charge accumulation and dispersion within the electrodes throughout charge and discharge cycles. The integration of these approaches facilitates an enhanced comprehension of the underlying mechanisms governing the performance of energy storage devices, thereby allowing for the rational design and optimization of electrode materials and configurations to achieve superior storage capacity and stability [208].

5.7. Sustainability of the Environment: Adopting Green Energy Storage Technologies

The importance of environmental sustainability in the realm of supercapacitor technology is undeniable, as the world faces an urgent need to adopt cleaner and more ecologically responsible energy storage solutions. These devices exhibit characteristics that are in harmony with sustainable objectives, which in turn enhances their allure as alternatives to traditional energy storage systems that prioritize eco-friendliness [209].

5.7.1. Environmental Benefits

Significant environmental benefits are associated with the utilization of supercapacitors. These include the following:

• Minimized Ecological Footprint: The constituent materials of supercapacitors, like activated carbon, carbon nanotubes, and conductive polymers for electrodes, are generally non-toxic and pose a lower environmental risk compared to traditional batteries, which are based on heavy metals and potentially harmful chemicals. This reduces the environmental strain during their entire lifecycle, from manufacturing to disposal.
• Enhanced Energy Efficiency: Supercapacitors boast high efficiency in energy storage and release, which translates to reduced energy waste during charge and discharge cycles. This efficiency contributes to a decrease in the frequency of replacements, thereby lowering the environmental footprint associated with production and disposal.
• Exceptional Endurance: With the capacity to undergo countless charge–discharge cycles without substantial performance loss, supercapacitors demonstrate remarkable durability. This longevity diminishes the necessity for frequent replacements, thus cutting down on the total waste produced and the resources consumed over time.
• Quick Energy Exchange: The rapid charging and discharging rates of supercapacitors facilitate efficient energy capture and release, which is particularly beneficial for stabilizing power grids. Their responsive nature allows for better integration of renewable energy sources and reduces dependency on fossil fuel-based power plants that are typically less flexible and more carbon-intensive.
• Sustainable Disposal: Supercapacitors can be recycled effectively due to the lack of hazardous materials in their construction. This feature enables the recovery of valuable components, such as carbon-based electrodes and current collectors, which can be reused. This recyclability aspect further reduces the environmental impact by curtailing the depletion of resources and the accumulation of electronic waste.

5.7.2. Support for Sustainable Energy Systems

The integration of renewable energy sources, such as solar and wind power, is significantly bolstered by the use of supercapacitors within electrical grids. These devices are instrumental in storing excess energy generated during periods of high production and then discharging it during peak demand, which enhances the stability and efficiency of energy distribution systems [210]. Moreover, supercapacitors are ideal for microgrid applications, offering robust energy management capabilities. Their high-power density and rapid response times facilitate the stabilization and reliability of these localized power networks, thereby reducing dependency on fossil fuels and enhancing energy self-sufficiency within communities or small regions. When it comes to electrified transportation, supercapacitors are a promising component in electric vehicles (EVs) and hybrid electric vehicles (HEVs). Their ability to charge rapidly and provide energy over numerous cycles is particularly advantageous. During regenerative braking, supercapacitors can swiftly store energy and later discharge it for acceleration purposes, which is beneficial in lowering greenhouse gas emissions and enhancing the overall environmental performance of these vehicles. Furthermore, supercapacitors serve as a reliable and efficient energy storage solution for off-grid and remote locations. Their capacity to maintain energy supply effectively and their reduced reliance on fossil fuels makes them a suitable alternative to diesel generators, thus contributing to a decrease in carbon footprint and pollution in such areas. Lastly, within the realm of electronics, supercapacitors are gaining ground in the powering of green and IoT devices. Their eco-conscious design and long operational lifespan, coupled with high-power density, make them an attractive choice for achieving energy-efficient and sustainable performance across various electronic gadgets.

6. Features of Electrode Performance: Assessing the Important Metrics

The evaluation of supercapacitor electrode efficiency is based on several key parameters, which significantly influence their energy storage capacity, power delivery, and cyclic endurance. These performance indicators are essential in guiding the enhancement of electrode architecture and material selection to suit particular use cases, as outlined in

Table 5. Important features used in research applications for supercapacitor electrodes. With permission of ref. [153].

Characteristic	Description	Importance	Measurement Unit	Reference
Specific Capacitance	Energy storage capacity per unit mass or surface area	Fundamental for energy storage efficiency	Farads per gram (F/g) or farads per square centimeter (F/cm2)	[212]
Energy Density	Total energy storage capacity per unit volume or mass	Determines overall device performance	Watt-hours per kilogram (Wh/kg) or watt-hours per liter (Wh/L)	[213]
Power Density	Rate of energy charge and discharge	Crucial for high-power applications	Watts per kilogram (W/kg) or watts per liter (W/L)	[214]
Cycling Stability	Ability to maintain performance over charge–discharge cycles	Ensures long-term device reliability	N/A	[215]
Rate Capability	Ability to handle rapid charge and discharge rates	Important for dynamic energy storage applications	N/A	[216]
Equivalent Series Resistance (ESR)	Internal resistance affecting energy conversion efficiency	Influences power performance	Ohms	[199]
Cycle Life	Number of charge–discharge cycles before performance degradation	Important for device lifespan and cost-effectiveness	N/A	[166]

. The significance of these characteristics lies in their role in the creation of highly effective and dependable supercapacitors. Through the continuous refinement of electrode design and the judicious choice of materials informed by these metrics, researchers aim to promote the widespread integration of supercapacitors across a spectrum of energy storage systems, thereby facilitating their practical application in various industries [211].

7. Advanced Characterization Techniques

7.1. Analytical Characterization Methods

When evaluating the properties of electrode materials, two primary aspects are typically considered: dimensions and morphology. The examination encompasses the size distribution, the tendency towards agglomeration, the presence of surface charges, and the surface chemistry features. These characteristics significantly impact various performance metrics and potential uses. After the synthesis of the electrode materials, their crystalline organization and chemical makeup are meticulously studied, yet a universally accepted protocol remains elusive. Supercapacitors derive their capacitance predominantly from electrode surface interactions. Therefore, understanding the surface properties, including the surface area and pore structures, is critical. Functional groups present on these surfaces are also of great importance. To achieve a comprehensive analysis, researchers often employ techniques such as the Brunauer–Emmett–Teller (BET) method of gas adsorption for examining macropores, mesopores, and micropores within a specified relative pressure range. The density functional theory (DFT)-based approach to interpreting nitrogen adsorption isotherms is particularly useful for elucidating micropore characteristics. Moreover, surface analysis tools like Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are indispensable for the study of pseudocapacitance electrodes, which rely heavily on the nature of their surface functionalities.

7.1.1. X-Ray Techniques

X-Ray Diffraction (XRD)

X-ray diffraction (XRD) is a commonly employed analytical tool for investigating the properties of electrode materials, offering insights into their crystalline structure, phase identification, lattice parameters, and crystallite dimensions. This technique is particularly suitable for powdered samples, providing a volume-averaged representation of the material [217,218,219]. To put it succinctly, the phenomenon of peak broadening in X-ray diffraction patterns can be attributed to various microscopic factors such as the presence of microstrains, nanograins, subgrains, and dislocations, as well as the existence of small crystallites within the material’s structure. This effect is indicative of the material’s microstructure and is influenced by these features that deviate from an ideal perfectly ordered crystalline arrangement [220,221,222,223]. Additionally, shifts in peak positions are often associated with internal stress mechanisms, the occurrence of stacking faults, and the homogeneous distribution of vacancies, which can be a result of chemical variations within the material. The specific location or spacing of these peaks serves as a fingerprint for identifying the material’s crystallographic group, whereas the intensity and morphology of the peaks offer insights into the constituent elements and their spatial distribution within the crystal lattice.

X-Ray Absorption Spectroscopy (XAS)

This methodology presents a straightforward approach to quantifying the chemical states of singular elements within a material matrix and is particularly applicable for observing the oxidation levels of substances participating in supercapacitor electrochemical processes. Since every element exhibits unique absorption edges that are tied to the distinct binding energies of its electrons, XAS is a highly selective and valuable tool for material analysis. EXAFS is recognized for its high sensitivity, allowing it to determine the oxidation state of materials even when present in low concentrations. However, its practical application necessitates the use of synchrotrons for spectrum recording, which results in higher costs and less widespread availability compared to other methods [224,225,226,227,228,229]. In research conducted by Kim and their colleagues, the effect of heat treatment on nickel oxide (NiO) structural defects with respect to their use in supercapacitor electrodes was scrutinized using X-ray absorption spectroscopy (XAS) [230]. The findings from both the EXAFS and XANES analyses indicated that the heat treatment process led to the transformation of non-stoichiometric Ni1-xO into a stoichiometric NiO structure. This is attributed to the inherent defect healing capability of NiO, which suggests that defective NiO structures can potentially enhance the supercapacitors’ electrochemical performance. In a separate investigation, Kim et al. [231] utilized in situ Mn K-edge fluorescence XAS to explore the electronic and structural changes in MnO₂ that were electrodeposited on carbon paper for electrochemical capacitors. Dong and their team [232] also leveraged XAS at the K-edge of C, O, and Mn to evaluate the electronic configuration of MnO₂/functionalized carbon nanotube (f-CNT) nanocomposites. They discovered that the f-CNT functionalization significantly improved the charge storage capacity of the carbonaceous materials, primarily due to the facilitation of electrolyte ion transport by the surface functional groups. The XAS analysis yielded comparable spectral profiles for all the examined samples, signifying a similar local electronic and atomic environment. However, variations in peak intensities were observed, which were attributed to interfacial effects following MnO₂ deposition on f-CNT. The stability of the graphitic framework in f-CNT post-MnO₂ deposition was evidenced by the A5 peak in the in situ XAS spectra, which remained unaffected by any peak shifts. This indicates that the f-CNT’s good conductivity was maintained. Additionally, the A5 peak intensity for MnO₂/f-CNT, which corresponds to the density of unoccupied π* states, was observed to be lower than that of f-CNT alone. This suggests that there was an electron transfer from MnO₂ to the π* state of f-CNT, which is consistent with the metallic nature of MnO₂. The higher intensity of the B5 and C5 peaks in MnO₂/f-CNT, as opposed to f-CNT, pointed towards a robust interaction between MnO₂ and carbon, which caused a substantial electron transfer from C=O π* to C-O σ*, thus creating a highly unoccupied state at the local level. Dong et al. [233] provided an extensive study on the electronic structure of various MnO₂ composites by integrating in situ XAS with electrochemical measurements. They observed that MnO₂ composites with reduced graphene oxide (rGO) experienced the least alteration in peak intensities during the charging/discharging process, compared to those with CNTs. This minimal change can be explained by the homogenous distribution of MnO₂ on the rGO surface, which effectively decreased the ion diffusion pathway at the interface. This structural configuration allows for more efficient charge storage and transport, highlighting the importance of material composition and architecture in optimizing supercapacitor performance.

X-Ray Photoelectron Spectroscopy (XPS)

Pseudocapacitance in supercapacitors is primarily contingent upon the electrochemical reactions occurring at the electrode material’s surface. To thoroughly investigate these processes, X-ray photoelectron spectroscopy (XPS) is frequently utilized due to its proficiency. While standard XPS provides insights into the surface chemistry, angle-resolved XPS allows for the study of even thinner layers. The utility of XPS extends to various aspects of supercapacitor research, such as ligand exchange interactions, surface functional groups, and heteroatom distribution. Researchers like Toupin et al. [234] have employed XPS to examine the chemical state transitions of MnO₂ during charge/discharge cycles, noting shifts from Mn(III) to Mn(IV) due to the redox reactions with electrolyte ions like sodium (Na⁺). Tõnisoo et al. [235] further demonstrated the value of in situ XPS in understanding the charge storage mechanisms in molybdenum carbide-based electrodes for electrochemical double-layer capacitors (EDLCs). They implemented a two-electrode setup within the supercapacitor for real-time monitoring of the electrode/electrolyte interface. This approach underscores the importance of combining XPS with other analytical methods to comprehensively study the complex surface interactions and bulk properties of supercapacitor materials.

7.1.2. Electron Microscopy

Transmission Electron Microscopy (TEM)

The technique known as transmission electron microscopy (TEM) explains how a thin object interacts with an electron beam that has a constant current density. The resulting images are constructed from the information obtained from the transmitted electrons, making TEM a widely applied tool for examining nanomaterial dimensions and morphology, due to its high precision and direct imaging capabilities [236,237]. To mitigate these issues and ensure reliable outcomes, TEM is frequently complemented by other analytical methods like UV–Vis spectroscopy and dynamic light scattering (DLS) that are capable of detecting a higher number of nanoparticles with less intrusive sample preparation. These combined approaches offer a more thorough understanding of the homogeneity and structure of various superlattice composites consisting of isostructural to multi-atomic crystals [238].

High-Resolution TEM (HRTEM)

High-resolution transmission electron microscopy (HR-TEM) is a specialized TEM imaging approach that capitalizes on phase-contrast imaging, an advanced technique that integrates both transmitted and scattered electron waves to construct visual representations. It is particularly indispensable when the goal is to observe and study the intricate crystallographic structure of individual nanoparticles, revealing details beyond the reach of standard TEM resolution. Thus, HR-TEM has become the preeminent tool for elucidating the internal architecture of nanoscale substances [239,240,241].

Electron Diffraction (ED)

Electron diffraction (ED), alternatively referred to as selected area electron diffraction (SAED), is a crucial microscopic tool utilized in the study of crystal structures. This method is typically performed within the confines of a transmission electron microscope (TEM) or scanning electron microscope (SEM), where it leverages the phenomenon of electron backscatter diffraction. However, the technique encounters limitations when dealing with large quantities of particles, as the ensuing diffraction patterns can become complex and challenging to interpret on an individual basis [242,243].

Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is frequently utilized for the detailed examination of nanomaterials, due to its capacity to produce high-resolution images at the nanoscale [244]. The microscopic analysis of electrode materials, as illustrated in (Figure 21, Figure 22 and Figure 23), reveals a significant research focus on supercapacitors utilizing 3D printing technology. Despite the substantial interest in this area, the stringent rheological demands for uniform ink formulation pose challenges to the realization of complex true 3D designs that could potentially enhance energy storage. Xue and colleagues [245] have employed a stereolithography technique to create metal composite lattices with octet-truss configurations through electroless plating, followed by the deposition of 3D hierarchical porous graphene on these structures to form quasi-solid supercapacitors. Examining the morphology of the synthesized reduced graphene oxide (rGO) (Figure 21) through low and high magnification field emission scanning electron microscopy (FESEM), (Figure 21a) clearly demonstrates the establishment of a porous architecture on the metallic frameworks. This unique structure is attributed to the generation of H₂ bubbles during the conversion of H⁺ to rGO in a hydrochloric acid solution, which are subsequently released. This innovative porosity enables the efficient utilization of the active material, thereby leading to an increase in the capacitance. Further insight into the material’s composition and distribution is provided by the scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) images, as presented in Figure 21b,c. These images confirm the presence of the constituent elements and their uniform distribution throughout the lattice. The lattice consists of rGO strands with a diameter of 1 mm, which feature consistent pores of 200 µm, as seen in Figure 21d,e. This structural feature is advantageous for enhancing ion diffusion, a critical aspect for improving the supercapacitor’s performance. The formation of multicavities with diameters of 5 µm on the rGO sidewalls, as depicted in Figure 21f, along with the stacking of nanopores resulting from the folded graphene nanosheets, shown in Figure 21g,h, contribute to the enhancement of the electrochemical properties. This hierarchical porous architecture, coupled with the wrinkled graphene nanosheets, highlighted in Figure 21i, offers multiple pathways for ion diffusion, which is vital for optimizing the material’s performance. These intricate structural characteristics are essential for achieving high capacitance and efficient energy storage in supercapacitors. The combination of lattices and porous rGO facilitates the creation of a supercapacitor with superior electrochemical properties, addressing the limitations imposed by conventional 2D architectures.

Xue and colleagues [246] introduced a streamlined single-step CVD process for producing 3D graphene/CNT hollow fibers with radially oriented CNT arrays encapsulated by graphene cylinders for energy conversion and storage purposes. The synthesis of the catalyst is outlined in a sequence of steps depicted in Figure 22, progressing from (A) to (C). The 3D graphene–RACNT fibers’ structure was examined through various microscopic methods. An SEM micrograph in Figure 22E exhibits a uniform lateral diameter of 100 μm for these fibers. Upon closer inspection at higher magnifications, the open RACNT tip arrays are visible with a pore diameter of about 50 nm, consistent with the AFM measurements displayed in Figure 22F–H,L. Cross-sectional images of the 3D graphene-RACNT fiber, presented in Figure 22I–K, indicate a shell thickness of approximately 6 μm, encircling an unanodized Al core, which is proportional to the RACNT length determined by anodization duration. The homogeneous CNT distribution is evident in the high-magnification views (Figure 22J,K). Moreover, SEM-EDX X-ray mapping, as shown in Figure 22N–P, confirms the central Al composition of the fibers, surrounded by C and O elements, aligning with the theoretical model presented in Figure 22D. This confirms the effective formation of the intended catalyst structure.

The meticulously structured 3D graphene–RACNT fiber, comprising a substantial graphene tube that envelops the RACNT arrays along its inner cavity, is obtained through an aluminum and alumina core removal process in a 1 M KOH solution. The TEM images of these fibers post-extraction are presented in Figure 23A–H. Upon examination of Figure 23A, it becomes evident that the RACNTs are intimately connected to the graphene sheets, which feature a uniform distribution of accessible pores. This observation aligns with the SEM and AFM data. The green indication in the image marks a fragmented section of the graphene–RACNT structure. The formation of CNT bundles and detached pieces is likely a consequence of surface energy-driven aggregation that occurred during the aluminum and alumina dissolution. The stability of the 3D graphene–RACNT arrays may be primarily attributed to the encasing graphene nanotube closely interlinked with the RACNTs. Further TEM imagery, as shown in Figure 23B, provides a closer look at the RACNTs attached to the graphene sheets, with the green marker highlighting a dislodged edge piece. The termination points of the RACNTs arrays are revealed in Figure 23C, illustrating that the nanotubes’ tips situated away from the graphene sheet are all sealed. This configuration corresponds with the 3D architecture depicted in Figure 23D, which serves as the fundamental building element for the creation of these hollow fibers. The open ends of the RACNTs are seen to be flawlessly connected to the graphene nanosheets. The cross-sectional perspective of the synthesized 3D catalyst network, displayed in Figure 23F, indicates an approximate bonding angle of 135° rather than the typical 90° at the graphene–RACNT interface. Lastly, Figure 23G offers a TEM image of an individual hollow 3D catalyst, while Figure 23H presents an elemental map of carbon within the catalyst, showcasing its distribution and structure.

7.1.3. Scanning Probe Microscopy

Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) is an advanced imaging method capable of generating high-resolution three-dimensional surface images. Additionally, it can analyze low-density materials that typically exhibit poor contrast in electron microscopy, thus broadening the scope of its analytical capabilities [246,247,248]. When it comes to supercapacitors, the electrode materials undergo structural and chemical changes during charge/discharge cycles, which can impact their long-term stability. Despite these transformations being crucial to understand in order to improve the device performance, the exact mechanisms underlying these changes remain elusive. AFM provides a valuable tool for examining the intricate details of these electrode materials, potentially shedding light on the phenomena occurring at the nanoscale during the operational lifecycle of supercapacitors. This insight can be instrumental in optimizing their design and enhancing their robustness. Thomas and his colleagues [249] conducted a study focusing on the evolution of two-dimensional (2D) electrode materials during charge/discharge cycles, utilizing a multi-faceted approach. They employed Kelvin probe force microscopy (KPFM) to monitor the surface potential of 2D WS₂ while it underwent periodic charging and discharging, as depicted in Figure 24. The study revealed that a strain developed within the WS₂ layers due to the intercalation and subsequent deintercalation of sodium ions, which in turn caused an increase in the number of accessible electrode active sites and thus an enhancement in the capacity. The AFM topographic images of the synthesized 2D material, presented at various stages of the charge/discharge process in Figure 24a,d,g, showed minor fluctuations in grain size, yet the overall structure remained compact. The KPFM-derived potential difference values, displayed in Figure 24b,e,h, highlight the variations in the estimated surface potential of the electrode, which are attributable to the cyclically induced strains in the 2D material. The intercalation of heteroatoms into the layered structure of the electrode is known to cause strain buildup within the material’s domains. The strain is inversely proportional to the Fermi velocity when considered as a function of wave vector “k”, and it results in an increase in the surface potential. Before the initiation of the charge/discharge process, the mean surface potential of the WS₂ electrode material was approximately 4.82 eV, as seen in Figure 24c. However, as the material experienced the early stages of the charge/discharge process, the potential increased due to the intercalation of sodium ions. This phenomenon is linked to the expansion of the interlayer spacing in WS₂, which provides additional active sites for ion interaction and thus enhances the capacitance. The surface potential reached approximately 4.95 eV after 2500 cycles, as indicated in Figure 24f, demonstrating significant strain in the 2D material. Interestingly, no further noticeable effects of strain on the electrode domains were observed beyond 2500 cycles, suggesting that the electrochemical surface area stabilized, and the material achieved its maximum capacity improvement. After 10,000 cycles, a slight increase in surface potential to approximately 4.97 eV was noted, as seen in Figure 24i. This suggests that the capacitance remained relatively constant once the WS₂ nanosheets had accommodated their maximum strain.

Scanning Electrochemical Microscopy (SECM)

Scanning electrochemical microscopy (SECM) provides high spatial resolution in determining redox properties, making it an instrumental technique in electrocatalytic research, corrosion studies, charge transfer kinetics elucidation, and mechanism dissection. Its utility has extended to energy storage and conversion domains, serving as a sophisticated analytical instrument capable of scrutinizing diverse electrode processes. Unlike cyclic voltammetry, which assesses effective electrode areas rather than geometric, SECM visualizes electrochemical currents directly at the electrode interface, offering insights into the interplay between the in situ structure and surface charge transfer mechanisms [250].

7.1.4. Spectroscopic Techniques

Fourier Transform Infrared (FTIR) Spectroscopy

Fourier transform infrared spectroscopy (FTIR) is an analytical technique that involves examining the absorption of infrared radiation to identify functional groups within materials. FTIR technology boasts several advantages compared to conventional infrared spectroscopy: (i) it enables the acquisition of a complete spectrum in a matter of seconds, as opposed to the minutes required by traditional methods; (ii) it operates at elevated scan rates, which is advantageous when analyzing substances that elute from a column; (iii) enhanced sensitivity is achieved due to the minimal “background noise”; (iv) the use of digital signal processing allows for the subtraction of spectra from solvents or known impurities, thereby improving the data quality; (v) it can produce a spectrum from a minuscule sample by accumulating data from multiple scans to obtain a composite result [251,252,253,254].

Raman Spectroscopy (RS)

Raman spectroscopy (RS) is commonly utilized to explore the correlations between structural and electrochemical properties during electrode cycling processes. In the context of in situ applications, RS is particularly beneficial for analyzing the variation in the distribution of small-diameter nanotubes and the specific capacitance of electric double-layer capacitors (EDLCs) that incorporate chemically functionalized single-walled carbon nanotubes (SWCNT). By monitoring the displacement of the G⁺ line in relation to non-functionalized SWCNT, researchers can gain simultaneous insights into the structural evolution and electrochemical performance of these materials. This approach provides valuable data for enhancing our understanding of the underlying mechanisms governing the behavior of nanotube-based EDLCs during their operational lifecycle [255,256].

Laser Raman Mapping

Laser Raman mapping, also recognized as Raman imaging, is a sophisticated method that offers insights into the intricate properties of two-dimensional materials such as graphene, graphene oxide, and TMDs [257,258]. This enhancement technique allows for the visualization of molecules with lower intrinsic Raman scattering efficiencies, offering a powerful tool in the study of complex systems.

7.1.5. Magnetic Techniques

In Situ Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclear magnetic resonance (NMR) is as a highly effective tool capable of scrutinizing local structures and molecular dynamics. Its elemental sensitivity equips it to independently discern ionic samples, making it a suitable candidate for exploring the charging mechanisms within supercapacitors during their charge and discharge cycles. In the context of ex situ NMR analysis, a supercapacitor is maintained at a specific potential, then dismantled, allowing the subsequent recording of the NMR spectra of the electrolyte species contained within the electrode film. Conversely, in situ NMR offers the advantage of analyzing the variations in the local environment of ions within the electrical double layer (EDL) during the operational phase of the supercapacitor cells [259]. This technique has significantly contributed to the qualitative understanding of charge storage mechanisms across various supercapacitor configurations. Brunner and his colleagues [260] utilized solid-state NMR techniques, specifically H¹, B¹¹, and C¹³, to explore the interaction between electrolyte ions and carbonaceous materials with a well-defined porosity structure at the interface. This method was applied to supercapacitors with AC electrode materials and organic electrolytes to determine the extent of charge storage. Their in situ NMR findings indicated the presence of two distinct charge storage mechanisms: (1) at potentials lower than 0.75 V, the negative electrode’s micropores exhibit a notable desorption of electrolyte ions, while the positive electrode’s ion adsorption remains relatively unchanged with increasing voltage; (2) above 0.75 V, the positive electrode experiences a significant increase in adsorbed electrolyte ions, whereas desorption persists at the negative electrode. Grey and his research team [261] conducted an exhaustive study where they employed in situ NMR to quantify the anionic and cationic species within supercapacitors fabricated from microporous carbon. Their investigation demonstrated that the charge storage processes at the positive and negative electrodes are fundamentally dissimilar. For the positive electrode in a TEABF₄ electrolyte system with an organic solvent, the charging mechanism involves cation exchange with anions, whereas the negative electrode predominantly stores charge through cation adsorption.

7.1.6. Surface Area Measurements

Brunauer–Emmett–Teller (BET) Method

Electrode materials in supercapacitors undergo surface reactions such as the adsorption/desorption of charge carriers and redox processes at the juncture where they meet the electrolyte, exhibiting either electrical double-layer capacitance (EDLC) or pseudocapacitance. The performance of these devices is heavily contingent upon the surface area of the active materials constituting the electrodes. Although electrolytes are crucial for facilitating the storage and transfer of charge, their diffusion is inherently limited within the porous structure of the electrode materials. The extent to which these pores can be effectively utilized is determined by the specific pore dimensions. This technique allows for a precise quantification of the surface properties essential for optimizing the supercapacitor efficiency and performance [262,263].

8. Applications of Supercapacitors

8.1. DC Microgrids

DC microgrids are typically supported by a combination of renewable energy sources and the utility grid to manage their power supply. However, when dealing with fluctuating DC loads, this can lead to variations in voltage or current, which in turn puts transient stress on the DC bus. Some power sources like fuel cells (FCs) and batteries might struggle to accommodate these rapid changes, as they are designed for steady-state operations. The efficiency and longevity of these components can be significantly compromised by such dynamic load conditions. Supercapacitors (SCs), on the other hand, are recognized for their superior power density (PD) in comparison to FCs and batteries. This characteristic allows them to be highly effective in handling transient power needs. In light of this, a transient power supply system based on supercapacitors has been tailored specifically for use in DC microgrids [264]. The multi-bus DC microgrid configuration shown in (Figure 25) incorporates a supercapacitor bank within a dedicated DC bus to effectively address the issue of DC load fluctuations. This setup allows the microgrid to maintain a stable voltage profile and ensures reliable power delivery, even under conditions of varying demand. The integration of SCs enhances the overall performance of the system by providing instantaneous power support, thereby reducing the stress on the FCs and batteries and ultimately extending their operational lifetimes.

The methodology presented involves crafting and implementing a control strategy that employs a rate-limited low pass filter to effectively handle substantial transient high-power loads. The central role of the rate limiter is to regulate the velocity of the current variations, thus preserving the system’s safe operating environment. The architecture of the load converter control system is comprised of an external voltage loop and an internal current loop. The voltage loop provides a voltage or current reference signal to a low pass filter-rate limiter controller (LPFRLC). The LPFRLC’s output is partitioned into a low-frequency segment and a high-frequency segment, which are independently supplied to the corresponding current control loops of the load converter and the solid-state capacitor (SC) converter. Notably, during transient conditions, the high-frequency component acts as the current reference for the SC converter’s regulator. In steady-state operation, the SC’s current contribution, known as the SCESS current, is maintained at zero. The design process for the PI controllers within the voltage and current loops is predicated on the system’s small-signal model. It is essential to ensure that the bandwidth of the inner current loops surpasses that of the outer voltage loop, which contributes to the systemic stability and bolsters the system’s robustness against disturbances emanating from either the power supply or the load. This theoretical framework is corroborated by a synthesis of simulation models and empirical verifications. Additionally, an extensive review of the literature was undertaken by various scholars, scrutinizing the applications of battery-solid-state capacitor hybrid energy storage systems (HESSs). This scrutiny delves into the nuanced workings of their control systems and provides a comprehensive understanding of their overall performance characteristics [265].

8.2. Electric Vehicles

Supercapacitors (SCs) have become an essential component in electric vehicle (EV) systems, primarily due to their ability to recover regenerative energy efficiently during the braking process. In EVs equipped with a combination of fuel cell (FC) stacks and lithium-ion (Li-ion) batteries, the FC stack generally requires a higher power rating than the Li-ion battery pack. This issue is mitigated through the integration of SC modules, which exhibit superior characteristics in managing surge currents in contrast to both batteries and FCs, as indicated by previous studies [266,267]. The various configurations of EVs, with or without the inclusion of SC modules, are illustrated in Figure 26a–c. These configurations demonstrate the flexibility and enhancement that SCs provide to the overall performance of the vehicle. Specifically, SCs are adept at addressing the dynamic peak power demands, offering high charge/discharge rates. They are capable of supplying additional power during acceleration phases and effectively absorbing excess energy during braking. The bidirectional converter plays a crucial role in connecting the SC modules to the direct current (DC) link within the EV’s power system. It operates in two primary modes: buck mode, which allows for voltage step-down, and boost mode, which steps the voltage up. Under steady-state conditions, the Li-ion battery remains responsible for supplying the necessary energy to power the vehicle. However, the integration of SC modules significantly enhances the system’s capability to manage transient power requirements, thereby extending the lifespan of both the battery and the FCs. This is achieved by reducing the load on the battery during peak power situations and preventing the FC from frequently cycling between high and low power outputs. The utilization of SCs in EVs, therefore, not only optimizes energy storage and retrieval but also ensures that the critical components, namely the battery and FCs, are protected from the potential degradation caused by frequent and intense power fluctuations. This synergistic approach leverages the strengths of each component, leading to an overall more robust and efficient propulsion system

A number of research endeavors have underscored the significance of assessing the performance of supercapacitors (SCs) within the framework of regenerative braking (RB) systems, akin to the configuration outlined in [268]. This experimental apparatus encompasses a flywheel, an AC induction motor, an inverter, a bidirectional DC–DC converter, and SC modules. The flywheel is preliminarily accelerated to 3000 rpm with the aid of an external DC supply, which is subsequently disconnected before the RB test begins. The RB process unfolds in five separate sets of commands. It is of considerable note that the state of charge of the SCs has a profound effect on the current and power uptake during RB, largely due to the current constraints imposed by the power electronic converters. The study scrutinizes the system’s overall efficiency under these circumstances. Another investigation, as mentioned in [269], grapples with the power discrepancy between the SC energy storage system (ESS) and the power grid by introducing a multi-parameter collaborative power prediction control system. This cutting-edge approach amalgamates multi-parameter power forecasting with an optimization controller and incorporates under voltage and overvoltage protections. This strategy effectively precludes the grid from receiving energy during RB, thereby slashing unnecessary energy conversion losses. When juxtaposed with conventional double-closed loop control and braking unit energy consumption control methods, this novel control strategy exhibits substantial energy efficiency enhancements of 20% and 45%, respectively. For electric vehicles (EVs), researchers have put forth a modular reconfigurable multi-source inverter (MSI) concept, as presented in [270]. This innovative design allows for the seamless integration of SCs and battery packs without relying on a magnetic structure. The MSI employs space vector modulation (SVM) for precise governance over switching actions, showcasing a lower weight, reduced costs, and streamlined control. Furthermore, a deterministic state of charge (SOC) regulator is developed to ensure the consistent functioning of the SC bank. In the context of EVs, a C-rate control-based battery/SC hybrid energy storage system (HESS) is proposed in [271]. This system is particularly adept at managing the fluctuating power requirements during acceleration and deceleration. Through the utilization of a PWM control technique in tandem with a current-sensing approach, the system reduces the number of load and SC current sensors, thereby simplifying its architecture. The DC–DC converter’s current command is computed using the C-rate control method, which takes into account the SC voltage and the vehicle’s operational states. This ingenious solution not only elongates the battery’s lifespan by 1.5 years but also optimizes the utilization of regenerated energy. Moreover, the effectiveness of an on-board energy storage device (ESD) is scrutinized within the context of electric railway transportation, as detailed in [272]. The researchers constructed an all-encompassing mathematical model of a train equipped with an ESD by employing the Modeltrack simulation tool, integrating factors such as the velocity constraints, track geometry, and train attributes. Additionally, a detailed timetable was factored into the analysis. The crux of the study lay in identifying the optimal ESD size, which in turn engendered cost savings, enhanced the energy utilization, and decreased the CO₂ emissions. This research is pivotal for guiding the design and practical application of optimized ESDs within DC light transportation systems, thereby contributing significantly to the advancement of energy-efficient mobility solutions.

8.3. Smart Phones and Note Computers

The diminished capacity adaptor of portable computing devices offers notable advantages, such as lower cost and compact lightweight designs. Nonetheless, it is associated with significant battery degradation, especially when continuously connected to wall power despite the increased power demands. To address this undesirable issue, the integration of supercapacitors (SCs) with batteries in a hybrid architecture is crucial. The SC–battery hybrid setup, as depicted in the schematic diagram (Figure 27) for smartphones and notebook computers, employs a structured approach to optimize system configuration [273].

This methodology entails the measurement and analysis of the insufficient current profile, followed by the establishment of an upper threshold for compensatory energy. This boundary is set to strike a balance between the hybrid structure’s cost and the battery’s longevity. The design process proceeds with the exploration of the buffer voltage swing and capacitance parameters. Finally, the optimal booster configuration is verified through rigorous analysis. The diagram illustrates J1 and J2 as the charging and discharging DC-DC converters, with J1 charging a supercapacitor array up to a specified voltage limit. The discharging converter, J2, outputs a voltage that matches the fully charged native battery’s nominal voltage. An evaluation of the battery lifespan under idle conditions and with or without the booster reveals that with an ideally designed booster, the battery’s operational life can be enhanced by 49.6%, surpassing the previously reported 46.8% improvement [274].

8.4. Industrial Drives

An open-winding electric vehicle motor with a power rating of 110 kilowatts has been coupled with a supercapacitor–battery hybrid configuration, as presented in the literature [275]. The system’s architecture, shown in (Figure 28), employs a dual inverter drive that interfaces with two distinct energy storage devices. The intricate design allows for a sophisticated power distribution mechanism, which is managed through vector-based algorithms.

Given the substantial disparity in the electrical dynamics of supercapacitors and batteries, the energy management component of this setup assumes precedence to maintain the supercapacitor’s operational voltage within acceptable limits throughout the vehicle’s operational cycle. This approach is essential to optimize the overall performance and lifespan of the hybrid drive system. Supercapacitors (SCs) exhibit remarkable efficiency in both active and reactive power support due to their distinct configuration. This particular setup obviates the requirement for an additional DC/DC converter, which is typically used to either inject or retrieve energy from the SC. This enhancement contributes to a notable increase in light-load efficiency, as the SC inverter is bypassed. Moreover, the elimination of the separate converter reduces the overall switching losses, which further improves the system’s efficiency. This design also enables the utilization of high-voltage electric vehicle (EV) motors, surpassing the capabilities of conventional configurations. To substantiate these claims, comprehensive experimental evaluations were conducted. These tests were primarily focused on validating the rule-based power management system, ensuring its long-term effectiveness in managing SC energy, facilitating stand-still charging capabilities, and safeguarding the SC against potential operational hazards. The integration of advanced intelligent controllers, founded on intricate AI/optimization algorithms, allows for seamless operation and adaptive decision-making, thereby enhancing the overall performance of the power management system. Another significant advantage of this setup is that the power management system can be decoupled from the low-level controls governing the dual inverter drive. This separation streamlines the development process by reducing the dependency on software engineers with specialized expertise in electric drivetrain operations. Consequently, it simplifies the integration and facilitates the adoption of this technology in various electric vehicle applications.

8.5. Wind Power

Supercapacitors (SCs) exhibit remarkable proficiency in countering the volatility inherent in wind power [276]. A typical wind energy conversion system (WECS), as illustrated in (Figure 29), comprises a mimicking converter, supercapacitor modules, a charge regulator, and an energy storage system founded on batteries. The mimicking converter is responsible for transmitting wind energy to the supercapacitors through a DC–DC conversion process. A charge controller facilitates the transfer of stored energy from the supercapacitors to the battery, which can be configured to operate in either a constant voltage or constant current mode. This design ensures that the power electronic converters are safeguarded against current stress during the battery charging phase and significantly enhances the battery’s lifespan by mitigating power fluctuations caused by abrupt load changes and varying wind speeds. Further research indicates that ultracapacitor energy storage systems (ESS) can effectively manage wind speed variations [277]. By employing predictive control, the efficiency of harnessing energy can be notably increased. This control mechanism compensates for the fluctuations in the rotational speed of the induction generator, which is crucial for maintaining a consistent power supply. Comprehensive MATLAB/SIMULINK simulations demonstrate the effectiveness of this approach, revealing an energy increase of 296 J relative to systems lacking predictive control. The extent of this enhancement is contingent upon the turbine’s size, with larger-scale wind farms benefiting from greater energy yields. Consequently, the integration of predictive control within wind energy systems not only amplifies the amount of energy produced but also has the potential to reduce the network costs significantly.

8.6. Multi-Level Inverter with ESSs

The depiction in Figure 30 illustrates a configuration where multilevel inverters (MLIs) are driven by a combination of solar photovoltaic (PV) systems and fuel cell (FC) stacks within hybrid energy storage systems (HESSs) [278]. Each H-bridge module within these MLIs is linked to the direct current (DC) link via a full bridge DC–DC converter, which integrates the distinct energy sources. These power sources are specifically designed to interface with the solar PV and FC stacks. The MLI’s output is then distributed to single-phase microgrid loads. Fluctuations in power delivery from the solar PV systems are a significant concern for grid stability. However, supercapacitors (SCs) play a crucial role in mitigating this issue by either generating or absorbing active power as needed, thereby maintaining a consistent power supply to the MLI during periods of solar PV variation. Furthermore, the SCs have the capability to provide reactive power support, enhancing the overall performance and resilience of the MLI against power fluctuations at the load interface. Under steady and stable solar PV power conditions, the supercapacitors operate in a dormant state, only activating when power fluctuations occur to ensure smooth operation of the microgrid. This interplay between SCs and MLI effectively manages the inherent variability of solar PV power, contributing to the overall stability and efficiency of the HESS-integrated microgrid setup.

Inversely, a supercapacitor (SC) acts as an energy provider for an asymmetrically constructed e-shaped clamped x-type multilevel inverter (AMECXMI)-based dynamic voltage restorer (DVR). The AMECXMI is engineered to produce a voltage waveform that closely resembles a sine wave but with a staircase configuration, requiring a minimal count of switching components. This unique design enables it to generate adjustable staircase sinusoidal voltages, which are vital for addressing a variety of voltage disturbances within the DVR framework. A graphical illustration of the DVR equipped with an SC-integrated AMECXMI is presented in (Figure 31). This setup demonstrates substantial promise for enhancing the fault ride-through (FRT) and power quality (PQ) capabilities of a doubly-fed induction generator (DFIG)-driven wind turbine [279]. The fractional-order super-twisting sliding mode control (FOSTSMC) is employed to regulate the energy supply from the SC, aiming to produce the specified staircase sinusoidal output by the AMECXMI. To evaluate its performance under fault conditions, the DFIG rotor is fitted with a series resistive limiter. Moreover, the DC link is combined with a superconducting DC chopper. This setup addresses various PQ concerns, including voltage fluctuations, voltage flicker, voltage swell, and voltage sag. The effectiveness of the FOSTSMC is then systematically compared with alternative control methods such as fuzzy control, standard sliding mode control (SMC), and proportional–integral–derivative (PID) control to determine its superiority in managing these challenges. The comparison underscores the proficiency of the FOSTSMC in optimizing the FRT and PQ functions within the DVR system.

8.7. Wave Energy Converters

One research endeavor focused on incorporating a supercapacitor/battery hybrid setup within the framework of vented oscillating water column (VOWC) wave energy converters [280]. The study devised a conceptual model of a VOWC array situated in a nearshore detached breakwater environment (VOWCDBW), as illustrated in (Figure 32). The investigation encompassed four primary scenarios: isolated VOWCs, a VOWCDBW configuration with two devices spaced at a threefold separation, a trio of individual VOWCs in an array, and three separate VOWCDBW arrays. The objective was to explore the implications of these different arrangements on the energy storage sizing and power quality. The power management system (PMS) operates by monitoring the state of charge (SoC) of both the supercapacitors (SCs) and batteries. In instances where the SoC of the SCs falls below an upper limit, excessive power is directed to charge them during power excesses. In contrast, during periods of power deficiency, the PMS draws from the SCs until they hit a lower threshold, thus averting detrimental overcharging and overdischarging and thereby enhancing the lifespan of the entire system. The study’s findings suggest that an array configuration of VOWCs is notably efficient in mitigating power fluctuations and diminishing the necessity for energy storage system (ESS) capacity when compared to other arrangements. This leads to an improved power quality delivery to the grid. Another investigation underscored the utilization of supercapacitors as an energy storage solution for wave energy converters (WECs) [281]. The Hamiltonian surface shaping and power flow control (HSSPFC) methodology was implemented to efficiently regulate and optimize the electrical energy transmission from a WEC to the shore-based power grid. This study was predicated upon preliminary wave tank experiments of the mechanical system, which served to establish simulation models to assess the performance and operational conditions. Various SC configurations were examined with respect to the performance metrics, mass, volume, and practical storage capacity. The preliminary analysis favored passive SC energy storage systems due to their inherent simplicity and reduced weight. Through comprehensive simulations, the Skeleton 53 F configuration was identified as the most suitable option, offering superior system efficiency.

8.8. Hand-Held Applications

The supercapacitor (SC) configuration presented is tailored to function as a power supply for portable devices, eliminating the need for conventional external batteries [282]. This design surpasses standard buck converters due to its flexibility in managing diverse input voltages, load currents, operational amplifier supply voltages, and the correlation between the comparator’s supply voltage and the switching pulse amplitude. The system employs a pair of SCs to provide a declining input voltage to the buck converter, which is adept at handling load currents spanning from 0.3 to 1.2 amperes. The output voltage is meticulously maintained at a constant level through an advanced feed-forward closed-loop PWM control system that dynamically adjusts the duty cycle of the switching pulses in accordance with the fluctuating input voltages of the supercapacitors. This unique approach enables the system to deliver a steady voltage for over 4.73 h. Moreover, the operational switching frequency is designed within an eco-friendly and secure range of 1 to 2.5 kilohertz. In the context of similar research, advancements in supercapacitor module technology have been proposed, including a DC–DC converter topology that operates independently of batteries and integrates both feedback and feedforward control systems. These studies underscore the importance of the leakage resistance with respect to the charging current and introduce an innovative charging method based on the gamma function to enhance the efficiency and decrease the charging duration [283].

8.9. Wireless Charging

The SC-based EV wireless charging utilizes resonant inductive coupling, as detailed in a previous study [284]. The research employed MATLAB (MATLAB R2025a) and ANSYS MAXWELL (Maxwell 2025 R1) softwares to perform simulations, with ANSYS MAXWELL focusing on parameter estimation for both the transmitter and receiver coils. MATLAB was then used to calculate the system’s efficiency and output power. This wireless charger boasts a power efficiency exceeding 75% and a high power transfer density and operates with a reduced coil size. It can maintain this performance across a larger distance range of 4–24 cm and has a maximum operating frequency of 100 kHz, delivering an output power of 100 W. In a separate study, an innovative WPT-integrated hybrid ESS (WPT-HESS) was presented [285]. This system combines a battery and SC through a DC–DC converter to interface with the WPT system. The WPT employs an LCC-S compensation network to ensure stable direct current voltage. The power flow between the WPT and HESS is managed by an integral terminal sliding mode controller (ITSMC), which optimizes the system’s efficiency while enabling the SC to charge to full capacity. The system’s stability was scrutinized under the Lyapunov theory framework, and its performance was benchmarked against a standard sliding mode controller (SMC) and proportional–integral–derivative (PID) controller to demonstrate robustness. Furthermore, there was a study on seamlessly integrated wireless charging micro-supercapacitors (IWC-MSCs) that showcased the high performance metrics [286]. These IWC-MSCs are characterized by an impressive energy density (ED) of 463.1 μWhcm⁻², along with an ultrahigh areal capacitance of 454.1 mFcm⁻². To evaluate the practical application, these components were integrated into a toy car model. The wireless charging system exhibited a power transfer efficiency of approximately 52.8%.

8.10. Comparative Study of EES Systems

The comparative study of different types of EES systems are depicted in

Table 6. Comparison of EES systems in terms of efficiency, discharge time, and cost [290].

System	Max. Power Rating (MW)	Efficiency (%)	Discharge Time	Cost/KW (USD)	Cost/KWh (USD)	Energy Density (Wh/L)
PHS	3000	70–85	4 h–16 h	600–2000	5–100	0.2–2
CAES	1000	40–70	2 h–30 h	400–800	2–50	2–6
FES	20	70–95	sec–mins	250–350	1000–5000	20–80
Lead–acid	100	80–90	1 min–8 h	300–600	200–400	50–80
NiCd/NiMH	40		sec–hours	500–1500	800–1500	60–150
Li-ion	100	85–95	1 min–8 h	1200–4000	600–2500	200–400
Metal–air	0.01	50	secs–day	100–250	10–60	500–10,000
Sodium–sulfur	0.05–8	75–90	sec–hours	1000–3000	300–500	150–250
RFB/HFB	100	60–85	hours	700–2500	150–1000	20–70
H2	100	25–45	min–week		10	600
Fuel Cell	50	60–80	secs–day	10,000		500–3000
SMES	10 MW	95	millisec–secs	200–300	1000–10,000	0.2–2.5
Thermal	150	80–90	hours	200–300	30–60	70–210

[287,288,289,290] and

Table 7. Comparison of EES systems in terms of environmental impact [290].

System	Lifetime/Cycles	Environmental Impact	Description of Impact
PHS	30–60 years	-ve	Cutting trees and landscapes for reservoirs
CAES	20–40 years	-ve	Remains from fossil fuel
FES	20,000–100,000	Negligible
Lead–acid	6–40 years	-ve	Toxic residues
NiCd/NiMH	10–20 years	-ve	Toxic residues
Li-ion	1000–10,000	-ve	Toxic residues
Metal–air	100–300	Very small	Slight residues
Sodium–sulfur	10–15 years	-ve	Toxic residues
RFB/HFB	12,000–14,000	-ve	Toxic residues
H2	5–30 years	Yes	Emission of hydrogen in atmosphere can create disturbance in the distribution of methane and ozone, thereby causing imbalance
Fuel Cell	5–15 years	-ve	Remains from fossil fuel
SMES	20 years	-ve	High magnetic field
Thermal	30 years	Small	Releasing charge into atmosphere

[290,291,292,293]. The efficiency, discharge time, cost, and environmental impacts of EES systems are considered for this study.

The Ragone diagram is a graphical representation employed to evaluate the performance of diverse energy storage devices (ESDs) in terms of their power and energy capabilities [294]. It plots energy density on one axis, measured in watt-hours per kilogram (Wh/kg), and power density on the other axis, measured in watts per kilogram (W/kg). This approach, which often utilizes a logarithmic scale for both axes, enables a comprehensive comparison of various ESDs that may exhibit significantly different characteristics. For instance, batteries, capacitors, supercapacitors, and flywheels each occupy distinct regions within the Ragone plot, reflecting their unique energy storage and power delivery properties. The decision on which specific ESD to select is largely influenced by the power density demands of the intended application, as illustrated in the Figure 33, which showcases the relative performances of battery, supercapacitor, and traditional capacitor technologies [295].

The inclined curves (representing characteristic time constants) on a Ragone plot illustrate the differential rates at which devices charge and discharge. On one end of the spectrum, capacitors are capable of transferring power in microseconds, showcasing exceptional dynamic behavior. Conversely, devices exhibiting less impressive performance may need several hours to produce and supply energy. Supercapacitors offer a balanced middle ground between these extremes, providing a practical compromise in terms of power and energy delivery. Portable technology encompasses a diverse set of energy storage requirements. Some devices demand substantial energy reserves to support sustained high-power operations over extended periods, necessitating high-energy storage solutions. In contrast, other applications may only require brief bursts of power before replenishment, focusing on high power density. The Ragone plot effectively visualizes these distinctions, positioning batteries towards the high-energy density and low-power density segment. This makes them ideal for scenarios where prolonged operation without recharge is essential. Batteries, while suitable for long-duration power supply, are not without their limitations. These include a limited lifespan and the need for costly maintenance. Conversely, capacitors boast high power densities and extended lifetimes, with the additional benefit of a substantial number of charge cycles. Despite having lower energy densities, capacitors are advantageous in scenarios requiring quick charge and discharge cycles [297].

9. Future Trends

The illustration labeled as Figure 34 underscores the emerging patterns and hurdles pertinent to solid-state batteries (SSBs). This encompasses the exploration and development of innovative materials for electrodes and electrolytes and efforts to enhance energy density, addressing the issue of cell voltage disparities, the refinement of modeling techniques specific to SSBs, and the establishment of comprehensive industrial standards to govern their production and use. These areas represent the cutting-edge advancements and challenges within the SSB domain.

In recent years, the field of energy storage has undergone rapid evolution, driven by emerging demands in flexible electronics, electric vehicles, and large-scale grid integration. Hybrid supercapacitor systems have attracted growing attention due to their ability to bridge the performance gap between traditional electrochemical capacitors and lithium-ion batteries, offering both high power density and enhanced energy density. At the same time, the global emphasis on sustainability has accelerated research into recyclable electrode materials, green synthesis methods, and environmentally benign electrolytes. These trends underscore the urgent need for a comprehensive and timely review that consolidates recent breakthroughs in material innovation, architectural design, and process scalability. By addressing these developments, this review not only captures the current state of the art but also serves as a strategic guide for future advancements in next-generation energy storage technologies.

9.1. Electrode and Electrolyte Materials

To improve the functionality of supercapacitors (SCs), the advancement of innovative SC materials is crucial. This necessitates an integration of various experimental and computational studies to guide the selection of appropriate electrode and electrolyte components [298,299,300,301,302]. It is imperative to develop fabrication techniques that can produce highly porous and hollow-structured materials, as these architectures enable a greater amount of charge storage and thus enhance energy density (ED). For optimal performance, electrolytes must possess a stable and extensive potential window, as well as excellent ionic conductivity. A smaller equivalent series resistance (ESR) is also essential to achieve both high ED and power density (PD). Additionally, there should be a harmonious relationship between the morphology of the electrode material, the behavior of the electrolyte ions, and the charge storage mechanisms involved. Hybrid supercapacitors (HSCs) have the potential to provide increased PD and ED simultaneously. However, the development of HSCs faces challenges concerning cost-effective and scalable manufacturing processes and the establishment of precise procedures to manage the surface properties of constituent materials. Furthermore, in the context of liquid-based ionic capacitors (LICs), research must concentrate on overcoming the challenges related to novel material innovation, establishing comprehensive characterization benchmarks, and understanding the effects of cycling frequency on longevity. It is also vital to model LICs in the high-frequency domain to assess their suitability for high-frequency applications. Beyond the material aspects, several other factors require investigation to ensure the broader applicability of supercapacitors. These include the lifespan of the devices, the rate of self-discharge, the operational temperature range, and the degradation of separators and current collectors. Moreover, the design and implementation of efficient thermal energy management systems are critical considerations for LICs. Addressing these challenges is essential for advancing the technology and expanding the practical use of supercapacitors. In particular, research efforts should focus on minimizing the self-discharge rate to prolong the operational lifespan of these energy storage systems.

9.2. Energy Density Improvement

Supercapacitors (SCs) are recognized for their remarkable power density (PD) and lower energy density (ED) in comparison to batteries. To overcome the gap between the EDs of SCs and batteries, numerous strategies have been proposed. One promising approach involves the utilization of redox hydrogels constructed on the foundation of sulfur-doped graphene, which has demonstrated a notable enhancement in ED, reaching 21.3 Wh/kg [303]. Additionally, researchers have suggested mass-balancing techniques within graphene composites to further boost the ED [304]. The performance of supercapacitors has also been substantially improved by the integration of thiol-functionalized graphene oxide scrolls, resulting in an impressive ED of 206 Wh/kg and maintaining this performance over an extensive lifespan of 20,000 cycles [305]. Another avenue for advancement lies in the realm of volumetric ED enhancement, which has been achieved through the application of lignin-derived carbon nanofiber in SCs [306]. This material has displayed significant improvements in both volumetric ED and cyclic stability. To gain a deeper understanding of these improvements, a comprehensive review study was conducted, focusing on the enhancement methods applied in hybrid solid-state supercapacitors [307]. The evolution of TMCs/carbon hybrid electrodes is also under scrutiny, with attention being given to structural design strategies such as the electronic structure, interface optimization, and the use of conductive carbon skeletons to enhance the overall performance [308]. These hybrid systems aim to reconcile the high power density and energy density characteristics. However, they face critical challenges concerning the efficient utilization of carbon as a conductive support and the delicate balance required between achieving high mass loadings of nanoscale TMCs and preventing their aggregation. Addressing these issues is essential for the continued development and practical application of these advanced energy storage solutions.

9.3. Cell Voltage Imbalance

The production of a consistent voltage across supercapacitor (SC) cells is essential due to variations inherent in the manufacturing process. Typically, an individual SC cell yields approximately 2.7 volts, necessitating their serial connection to achieve the desired voltage levels for modules. However, this can lead to cell voltage imbalances, which are exacerbated by repeated charging and discharging cycles. To address this issue, advanced voltage balancing systems are essential for reliable and efficient operation. One approach to mitigate this problem involves the implementation of a multiwinding transformer-based equalization circuit for SC modules [309]. This design allows for both inter-cell and inter-module voltage balancing, offering a simple control mechanism, increased speed of equalization, and enhanced reliability. Alternatively, a series LC resonant converter serves as a voltage equalizer to circumvent the intricacies associated with multi-winding transformers [310]. This alternative provides the additional benefit of estimating the time required for voltage equalization. Another notable method involves the use of switched-capacitors as voltage equalizers within the SC framework [311]. This technique leverages the SCs themselves as components of the equalizer system, thereby reducing the size and costs. It also eliminates the need for excessive energy transfer components and offers high efficiency, along with the automatic minimization of the equivalent resistance during the equalization process. Furthermore, a consensus-based voltage equalizer circuit has been proposed for use in reconfigurable supercapacitors [312]. This innovative approach is designed to ensure voltage uniformity across the modules by utilizing a strategy that mimics the decision-making process in a group, thereby providing a more adaptable and robust solution to the voltage imbalance challenge.

9.4. SC Model

Pivotal concerns such as the parameter identification and model selection significantly influence the system design process, as highlighted in the literature [313,314,315,316]. Supercapacitors (SCs) are categorized into various models including thermal, self-discharge, fractional-order, intelligent, equivalent circuit, and electrochemical. These models’ parameters are susceptible to an array of influences stemming from the load characteristics, system stability, environmental conditions, and fluctuating loads. The challenge of accurately estimating the parameters in real time emerges as a crucial aspect. For electric vehicles (EVs), precise model performance is vital for effectively estimating the state of health (SOH) and state of energy (SOE) [317]. In the context of military applications, particularly in satellites and spacecraft power systems, the presence of non-ideal parameters can lead to substantial risks [298]. Ensuring system reliability is paramount when considering both internal and external environmental factors. To address this, researchers have introduced a capacitance prognostic model that depends on power. This approach aims to understand and predict the second-life capacitance dynamics of SC cells once they are no longer used in their primary applications, thereby facilitating their potential reuse [318]. The robustness and sensitivity of the proposed model have been rigorously assessed, employing both quantitative and qualitative evaluation methods to affirm the efficacy.

9.5. Industrial Standard

The performance of SCs can be assessed through several parameters including the cycle life, energy efficiency, power density, energy density, capacitance, and capacity [319]. In contrast, evaluation techniques involve assessments like round-trip efficiency, current interruption, current initiation, power pulse, constant power, current pulse, and constant current. These assessments are influenced by experimental conditions such as the testing temperature, state of charge (SOC), power and voltage operation ranges, hold/rest periods, rebound time, and current density. To ensure uniformity and safety, establishing certain standards is crucial. These encompass a standardized model naming convention, detailed charger specifications, comprehensive safety protocols, and precise methods for gauging the electrical performance. Moreover, production guidelines and material specifications, particularly for electrolytes and electrodes, are essential [320,321,322]. For instance, IEC 62,576 and IEC 62,391-2 serve as benchmarks for SCs used in HEVs and various electronic devices [322]. These standards are complemented by UL 810A, a proprietary standard developed by Underwriters Laboratories for SCs. Additionally, the European Union’s REACH regulation focuses on the production and utilization of chemical substances within the SC industry. There is a significant need for these universal standards to foster the SC industry’s healthy growth and the adherence to safety and quality.

10. Conclusions

Supercapacitors have emerged as indispensable energy storage devices, uniquely bridging the gap between conventional capacitors and batteries. Their hallmark advantages—such as their high-power density, rapid charge–discharge cycles, excellent cycle life, and robust temperature tolerance—have positioned them at the forefront of next-generation energy solutions. This review has highlighted the multifaceted progress in supercapacitor technologies, spanning the development of advanced electrode materials—including carbon-based nanostructures, transition metal oxides, conductive polymers, and composite systems—as well as breakthroughs in electrolyte design, from aqueous and organic systems to ionic liquids. Innovations in device architecture, particularly hybrid configurations and flexible platforms, have further propelled the capabilities of supercapacitors across diverse applications, from wearable electronics to electric vehicles and renewable energy storage. Despite this impressive progress, several critical challenges remain. Chief among them is the need to elevate the energy density without compromising the power performance or longevity. The integration of high-capacitance pseudocapacitive and hybrid materials holds promise, yet issues such as poor long-term stability, complex fabrication methods, and limited scalability persist. Furthermore, ensuring compatibility with sustainable and environmentally benign electrolytes is crucial for real-world adoption. Future research should prioritize the development of low-cost, eco-friendly, and scalable synthesis methods for high-performance materials. A deeper understanding of ion transport mechanisms at the electrode–electrolyte interface, guided by advanced characterization and modeling techniques, will be essential. Additionally, engineering device configurations that support miniaturization, mechanical flexibility, and integration with wearable and IoT technologies will unlock new commercial frontiers.