Recent Trends in Highly Porous Structured Carbon Electrodes for Supercapacitor Applications: A Review

Abstract

Globally, environmental and energy conservation concerns have sparked a push for more efficient and long-term energy sources. Researchers worldwide have put significant effort into developing supercapacitor-based energy storage devices by fabricating electrode materials from affordable porous carbon. The advantages of porous carbons are low-cost processes, high porosity, high surface area, facilitation of surface modification, high conductivity, high mechanical stability, high chemical stability, facilitation of fast ion transport, high rate capability, and high specific capacitance. Using them as electrodes in supercapacitors (SCs) may lead to better performance in specific capacitance and long-term cyclic stability. This study focuses on the recent development of electrode materials for SCs using porous carbons obtained from several diverse sources, such as biomass, polymers, lignite, metal salts, melamine, etc. Therefore, the topic of this review is the most current development of electrode materials for SCs applications. SCs were subjected to a battery of electrochemical tests, which focused on their performance from a crucial perspective, concentrating on the porous carbon’s surface area and surface functional groups. The report also highlights the supercapacitor’s prospects and challenges.

1. Introduction

The ever-increasing demand for energy on a global scale, the diminishing supply of fossil fuels due to a growing population and modern industries, and the rise in average global temperature resulting from global warming have all contributed to increasing interest in the research and development of renewable and environmentally friendly sources of electricity [1,2]. Therefore, energy storage devices such as batteries and supercapacitors (SCs) can potentially replace fossil fuels as a source of power production and for indoor and outdoor applications. As a result, this change may prevent increased global temperatures brought on by carbon dioxide emissions. Since renewable energy sources possess remarkable electrical properties, it is vital to create low-cost and environmentally acceptable energy storage materials to use these resources [3]. Currently, the most important demand for cutting-edge industrial electrical technologies is using energy storage materials that are environmentally friendly. The stage of present energy-based storage devices is occupied by devices such as batteries and SCs [4,5,6,7,8]. While batteries find widespread use in electric vehicles and other electrically powered devices, the fact that they have short cycle stability and a limited power density (PD) is one of the reasons why more study has to be done on the topic, in addition to the search for other energy sources.

SCs have attracted researchers in recent decades due to their numerous advantages. These include quick charge-discharge (1–10 s), great cycle stability (30,000 h), high power density (10,000 W kg⁻¹), stability at a wide range of temperatures (40–100 °C), and affordability [8]. The components that make up a supercapacitor are two electrodes, an electrolyte, a current collector, and a separator (Figure 1a). SCs are electric devices that store charges at the electrode/electrolyte contact. The ions (anions) collect on the surface of the positive electrode of a supercapacitor device, while the positively charged ions (cations) are attracted to the negative electrode [9]. The high surface area (S_BET) electrodes and active electrolytes are two of the most fundamental parts of a storage device, and their quality greatly affects the functionality of the whole device. Thus, preparing device components, such as a new porous electrode and electrolytes, is crucial for achieving high performance. Each electrical system that relies on power from renewable sources must have access to an efficient energy storage device. In addition, SCs can be subdivided into symmetric and asymmetric SCs according to the structural architecture of the device. Symmetrical devices use the same material for both the positive and negative electrodes.

On the other hand, asymmetric devices have positive and negative electrodes that are made out of completely different materials. It features a high specific PD, enabling it to store or preserve the electrical energy at the electrolyte and electrode interfaces. To meet the rising demand for electric and hybrid electric vehicles worldwide, SCs have emerged as the superior option to batteries. It offers benefits over batteries, including high specific power, quick charging time, high coulombic efficiency, and a long-life cycle [10]. SCs have gained popularity in several fields, including hybrid electric motors, rail, mobile electronics, and shape memory devices [10,11]. Unfortunately, two main problems prevent the practical manufacture of SCs: high cost and extremely low energy density [12]. More research into developing innovative electrode materials and their architecture is necessary to increase the device’s specific energy density without sacrificing its high-power capacitance [13]. SCs need to have a high capacitance, defined by the electrodes’ physiochemical structure, and only then can they perform their functions to the highest possible level [14,15].

Generally, typical electrode materials significantly impact the charge storage and capacitance of SCs [16]. Thus, electrode materials play a vital role in improving the device’s performance, and the developed electrode material should have qualities such as high conductivity, high mechanical strength, high specific S_BET, high thermal stability, and low cost. Some major electrode materials, such as polymers, porous carbon materials, and nanomaterials, were developed for SCs. Among them, highly porous structured carbon materials are recognized as excellent candidates for SCs because of their significant characteristics, such as larger S_BET, hydrophobic and hydrophilic groups on their surface, better conductivity, higher flexibility, low density, higher chemical and thermal capabilities, and high electrochemical characteristics [17,18,19]. Biomass is a high-carbon substance with components such as lignin, cellulose, and hemicellulose that assist in getting carbon through calcination [20]. It is commonly accepted that sustainable and environmentally benign materials, such as shells, wood, bagasse, straw, cotton, and bio-wastes, may be utilized as carbon sources to produce porous carbon for energy storage [21].

An efficient electrode material derived from carbon sources with a larger S_BET allows for high capacitance and charge lodging, while a porous structure allows for quicker ion movement. Nonetheless, despite the vast effort put into building SC-based devices, the cycle and energy density (ED) acquired by the study are still inadequate [22]. By increasing their ED and other electrochemical performance metrics, SCs may benefit from porous carbon’s intrinsic properties. SCs may be classified into electrochemical double-layer capacitors (EDLC), Pseudocapacitors, and hybrid SCs. By bringing electrolytes into contact with the surface electrode, SCs with carbon electrodes might generate the desired capacitance; this phenomenon is known as electrochemical double-layer capacitance (EDLC) [23]. The redox-based techniques known as Pseudocapacitors employ electrode materials, including metal oxide nanoparticles and polymers with conducting characteristics. Pseudocapacitor: a capacitor storing energy by quick redox reactions between the electroactive species and electrolyte on the electrode surface [18]. Because of the weaker Faradaic processes involved, PD is typically poorer than EDLCs. Moreover, Pseudocapacitors are more likely to swell and contract throughout the charging and discharging cycles because of insufficient mechanical steadiness and the relatively limited cycle life of the electrodes; thus, pseudocapacitors may have a shorter cycle life than EDLC. PD and ED performance metrics of SCs are intensely affected by the pore structure of porous carbon since EDLC controls energy storage. Hybrid SCs are devices that combine EDLCs with Pseudocapacitor, and they were developed to increase ED. Manufacturing SCs is costly because of the difficulty of sourcing low-cost, highly efficient electrodes [23]. As a consequence, SCs are more expensive because of the high cost of the materials used in their production. The minimal or no cost, low environmental impact, and promising electrochemical features of this approach have piqued the curiosity of academics and industry.

In real-world SC applications, hierarchically porous carbon has shown considerable advantages over micropore-based carbon materials [24]. Micropores and mesopores, two types of pores, play a crucial role in creating high specific capacitance and performance rates by providing core adsorption active sites and a rapid diffusion path. Carbon-based electrode materials are rigid and have strong conductivity; therefore, carbon-based SCs maintain better cycle stability. Hence, the desired specific capacitance and performance rate of SCs might be obtained by planning the ordered pore structure of carbon materials and increasing the PD of carbon-based SCs has been a key research area in the discipline of energy storage. This information was gathered from Mendeley and demonstrates that the number of research articles on porous carbon-based electrodes for SCs applications has steadily grown over the last several years (Figure 1b). Review reports on using carbon materials in supercapacitor applications, including sulfur-doped carbon, carbon generated from biomass, and graphene, have been published [10,23,25,26,27,28,29]. However, recently, there has been a surge in studies looking into the synthesis and use of porous carbon-based electrode materials to improve SC performance. Therefore, this study has been structured suitably to include the most up-to-date information about advancing porous carbon-based electrode materials for high-function SCs applications. In light of this, we’ve set up the review to concentrate on recent advancements in producing porous carbon from various sources. These porous carbons’ synthesis conditions are covered in detail, their surface areas are identified, and they are discussed. Following that, their specific capacitance for the supercapacitor application was ordered from lowest to highest, and for better understanding, the electrochemical results of the porous carbon were tabulated. Figure 2 shows a schematic representation of porous carbon derived from various sources, including polymers and their composites, biomass, bio-oil residues, metal salt precursors, lignite, and melamine. It also highlights the best findings of each source’s derived porous carbon’s specific capacitance, energy density, and cycle performances.

1.1. Mechanism of Supercapacitor

The charge storage mechanism of supercapacitors is primarily thought to be an electric double-layer capacitor (EDLC), the formation of which is mostly derived from the process of ion adsorption (physical reaction) [30,31]. Another method is a Pseudocapacitor, which stores charges through redox processes and has better specific capacitances than EDLC-based materials [32]. Since EDLCs use meso/micro porous materials for their electrodes, they have very high capacitance. Microporous carbon is often used for making electrodes because of its inexpensive cost. Pseudocapacitors are materials that combine the electrostatic and pseudocapacitive charge storage mechanisms, and examples include functionalized carbon, metal oxide, and conducting polymers [12,33]. Fast redox processes at the electrode surface are responsible for charge storage in this mechanism. Compared to EDLCs, pseudocapacitive materials have poorer performance throughout the cycle life.

1.1.1. Electric Double-Layer Capacitor (EDLC)

The electrochemical process that allows for charge storage in EDLCs depends on the adsorption and desorption of ions at the interface between the electrode and the electrolyte [34]. When producing an EDLC-based device, it is common practice to use electrode materials with a large surface area and high conductivity. A physicist named Helmholtz initially proposed the Helmholtz model in the nineteenth century for EDLC; it implies that the contact between an electrode and an electrolyte is atomically separated into two layers of opposite charge [33]. The Helmholtz model was further developed by two scientists, Gouy and Chapman, and became known as the Gouy-Chapman model [12,33,35]. According to this model, electrolyte ions should be regarded as point charges, and the Boltzmann distribution should be used to determine the electrolyte ions concentration at the electrode’s surface. The Gouy-Chapman model, however, could not characterize the substantial amount of charged double-layer structure. To address the shortcomings of the Gouy-Chapman model, Stern merged both models, Helmholtz and Gouy-Chapman, which separate two regions of ion circulation, namely the Stern and the diffuse layers. In the Stern layer, the electrolyte ions are strongly adsorbed layer by layer on the electrode surface, forming a multilayer structure comprising the internal and outer Helmholtz plane [34,35]. A diffuse layer is located next to the outer Helmholtz plane. The various EDLC mechanism models are shown in Figure 3a. Equation (1) denotes the total DLC (Double-layer capacitance (C_dl)) of the material [35,36]. In this equation, C_dl represents the capacitance of the double layer, C_Stern stands for Stern layer DLC and C_diff for diffuse layer DLC.

$$\frac{1}{C_{dl}}=\frac{1}{C_{Stern}}+\frac{1}{C_{diff}}$$

(1)

A supercapacitor works by an external circuit to carry electrons from the positive to the negative electrode of the charged device [37]. The cations and anions in the negative and positive electrodes produce a double electrode layer, compensating for the external charge imbalance [37,38]. The cation and anion in the surface pores come back to mix during the discharge period because electrons may go to the positive electrode from the negative electrode via the same circuit. This process continues until the device is discharged. Ion movement in the pores of an electrode may differ from that in the bulk electrolyte solution. If the pores are too tiny, ionic transport is hindered and cannot form a capacitively separating double layer [29].

1.1.2. Pseudocapacitance

The basis of pseudocapacitance is a fast and strongly reversible surface or surface redox process. The electrical behavior of a pseudocapacitive-based material is identical to that of an EDLC, i.e., the charge constantly changes with the input of potential and is regarded as capacitance [36]. Materials like functionalized porous carbons, which combine EDLC and pseudocapacitive storage processes, may store a significant amount of charge in a double layer. Various functions may explain pseudocapacitor behavior, such as underpotential deposition, redox reactivity, and intercalation pseudocapacitance [32,39]. Three different mechanisms for charge storage in Pseudocapacitors are shown in Figure 3b. The faradaic adsorption or desorption of electrolyte ions on electrode surfaces to monolayer adsorption across their redox potential is the process for underpotential deposition. Redox reactions develop on the surface of materials in the redox Pseudocapacitor mechanism owing to the accompanying faradaic charge-transfer process. In contrast, an intercalation pseudocapacitance includes the function of the ion intercalation into the layers of a redox-active electrode, which is concurrent with the process of faradaic charge transfer without a change in the crystallographic phase. Equation (2) determines the charge storage process in Pseudocapacitor-based electrodes [35,40,41].

$$E~E^0-\frac{RT}{nF}In\left(\frac{X}{1-X}\right)$$

(2)

Metal oxides, carbons, hydroxides, and high-conducting polymers are often the capacitive materials utilized to fabricate such electrodes [31]. The specific capacitance of the electrode may be raised by faradic processes occurring in tandem with electrochemical double-layer charge storage. As a result, a Pseudocapacitor’s specific capacitance may be larger than an EDLC’s specific capacitance. However, the Pseudocapacitor’s power performance, which involves a weak Faradic response, is often lower than EDLC [42]. Metal chalcogenides, metal chalcogen oxides, metal oxides, and conducting polymers are the electrode components utilized in Pseudocapacitors. As pseudocapacitive electrodes, ruthenium oxide and manganese oxide are often used in supercapacitor applications. Additionally, because of their high conductivity and flexibility, conducting polyaniline, polypyrrole, and polythiophene are used as electrodes in pseudocapacitors [43,44]. Figure 3c illustrates the charge storage materials’ various types of storage methods using cyclic voltammetry (CV) and charge-discharge (C-D) diagrams [45]. During the constant current charging and discharging procedures, the CV and C-D profiles of type-A-based capacitive materials are shown, highlighting rectangular CV curves and linear potential response. The type-B showed faradaic-style pseudocapacitive characteristics. The intermediate behavior between oxidative and reductive action thus indicates the presence of pseudocapacitance in the material. Despite the fact that type-C faradaic does not display capacitive behavior, the large separated peaks caused by concurrent reduction and oxidation of the core metal demonstrated battery performance in the charge storage, hence this material is mostly of the battery type.

Figure 3. (a) Various models of the EDLC, reproduced with permission from [33], Copyright 2016, Elsevier. (b) Several types of charge-storage mechanisms for Pseudocapacitors, reproduced with permission from [41], Copyright 2014, Royal Society of Chemistry. (c) (i,ii,iv,v,vii,viii) Schematic depiction of several types of CV curves and (iii,vi,ix) corresponding galvanostaticdischarge plots for charge storage materials, reproduced with permission from [45], Copyright 2018, American Chemical Society.

Figure 3. (a) Various models of the EDLC, reproduced with permission from [33], Copyright 2016, Elsevier. (b) Several types of charge-storage mechanisms for Pseudocapacitors, reproduced with permission from [41], Copyright 2014, Royal Society of Chemistry. (c) (i,ii,iv,v,vii,viii) Schematic depiction of several types of CV curves and (iii,vi,ix) corresponding galvanostaticdischarge plots for charge storage materials, reproduced with permission from [45], Copyright 2018, American Chemical Society.

2. Synthesis of Porous Carbons

Carbon materials, including porous carbon, nanotubes, activated carbon (AC), carbon aerogels (CAs), graphene (G), etc., are often produced for use in EDLC electrodes [30]. These carbon materials show remarkable conductivity, pore structure, and superior mechanical qualities; as a result, they find widespread use in the production of SCs. The ED of EDLC is also much lower than that of batteries, and the specific capacitance of EDLC is still below the industry standard. To address these challenges, researchers have devised various solutions, including heteroatom doping, composites with other active porous materials, and modifying the pore shape of carbons [46]. Porous carbons generated from biomass that include meso- or micropores and a high-specific S_BET provide excellent materials for SCs applications. Porous carbon has a range of pore diameters; still, micropores are particularly important for electrochemical performance because they may be able to provide ion adsorption sites and encourage the creation of electronic double layers for energy storage applications. Although macropores help with the quick entrance of ions into electrodes, mesopores may function as a diffusion pathway for ion transport and electrolyte transfer. Producing porous carbon with a significant S_BET and high porous structure is thus crucial for applications using SCs. To get a high specific capacitance, a substantial specific S_BET and surface functionalization are required. As a consequence of this, the carbons have to be chemically or physically activated before they can be used in supercapacitor applications. The following section was organized according to the different kinds of materials that were utilized to make porous carbon, and the conditions that were employed in the synthesis were tabulated.

2.1. Porous Carbon-Derived from Polymer-Based Materials

Recently, synergistic effects have been used to improve the electrochemical properties of electrodes made of metal oxide and porous carbon for SCs. Therefore, chitosan-derived carbon (CC) was combined with copper oxide (CuO/Cu) produced at various pyrolysis temperatures (600–900 °C) in an atmosphere (ATM) of N₂ [47]. Initially, the dark blue CuO/Cu-CC gel sample was freeze-dried and then pyrolyzed at various temperatures for 2 h to produce a porous carbon (CuO/Cu-CC). The model was then washed with water many times to purify it. CuO/Cu-CC synthesis is schematically represented graphically in Figure 4a. To prepare different ratios of CuO/Cu-CC for comparison, they adjusted the weight of the CuO precursor to composite with chitosan. At a temperature of 600–700 °C, the S_BET of the CuO/Cu-CC (63–313.2 m²g⁻¹) was increased, and then the S_BET (228.0 m²g⁻¹) was decreased to raise the temperature after 700 °C. Reducing the S_BET may be responsible for the blockage of pores at higher temperatures. It was discovered that the crystalline structure (CuO/Cu-CC) was similar up to 700 °C, and afterward, owing to CuO breakdown, a new peak of Cu₂O creation was discovered. As a result, using a higher temperature (>700 °C) to produce the carbon particles may change their S_BET and crystalline structure.

Recently, coal, biomass, and polymeric resin have been used primarily as precursors in the fabrication of porous carbon [48,49,50,51]. Among them, lignin is a lignocellulosic biomass that may be obtained in significant quantities from the waste products of industries that refine biomass. Lignin is an important and economical precursor for synthesizing porous carbon due to its high carbon content. Anionic functional groups found in lignin may interact with metal ions to form a metal-lignin structure [52,53]. Metal-lignin development permits homogeneous metal ion dispersion and prevents clumping through the carbonization pathway [54]. As a result, Li et al. produced porous carbon with heteroatom doping derived from lignin (e.g., N-LHPC) [55]. Simple steps were used to generate lignin (L)-metal (Mg-Zn)-linked particles, which were then combined with melamine (a nitrogen source) for heteroatom doping (Figure 4b). Moreover, they were carbonized for 2 h at 900 °C with an N₂ ATM to form porous carbon produced from lignin-metal complexes. It was found to be the typical porous carbon with a graphitic structure, indicating that the substance was an amorphous carbon phase. Due to the use of metal salts and the need for a freeze-dryer to dry the materials, this technique might be a rather expensive operation.

To generate N-doped porous carbon (e.g., NPC), KHCO₃-activated agar-l-arginine (Arg) xerogel was carbonized for two hours at 700–900 °C in an N₂ ATM [56]. This investigation employed l-arginine as an N source, while KHCO₃ was used as an activating agent. The synthesis procedure was very simple and uncomplicated, making it possible to synthesize porous carbon using a cost-effective technique. The S_BET (978–3184 m² g⁻¹) increased as the KHCO₃ ratio (0–4.0) increased. Therefore, if a higher proportion of KHCO₃ is activated at a higher temperature of 800 °C, carbon produced from Agar-Arg xerogel may be a good method for increasing S_BET-besides, the structural analyses of this porous carbon show amorphous carbon structure and a low graphitization degree. With further increases in carbonization temperature, the porous carbon’s degree of graphitization constantly rises.

Figure 4. Schematic graphical depiction of the synthesis of porous carbon: (a) CuO/Cu-CC, reproduced with permission from [47], Copyright 2023, Elsevier. (b) N-LHPC, reproduced with permission from [55], Copyright 2023, Elsevier. (c) FeS₂/CoS₂-C, reproduced with permission from [57], Copyright 2023, Elsevier. (d) BBC, reproduced with permission from [58], Copyright 2023, Elsevier. (e) activated carbon (PNOAC), reproduced with permission from [21], Copyright 2023, Elsevier. (f) PC-K, Na, and Ur, reproduced with permission from [59], Copyright 2023, Elsevier.

Figure 4. Schematic graphical depiction of the synthesis of porous carbon: (a) CuO/Cu-CC, reproduced with permission from [47], Copyright 2023, Elsevier. (b) N-LHPC, reproduced with permission from [55], Copyright 2023, Elsevier. (c) FeS₂/CoS₂-C, reproduced with permission from [57], Copyright 2023, Elsevier. (d) BBC, reproduced with permission from [58], Copyright 2023, Elsevier. (e) activated carbon (PNOAC), reproduced with permission from [21], Copyright 2023, Elsevier. (f) PC-K, Na, and Ur, reproduced with permission from [59], Copyright 2023, Elsevier.

2.2. Porous Carbon-Derived from Biomass

The electrical conductivity of metal sulfides may be increased by mixing them with porous carbon made from biomass. As a result, biomass (raw kelp)-derived porous carbon decorated with iron/cobalt disulfides (FeS₂/CoS₂-C) was synthesized for hybrid SCs [57]. The unprocessed kelp used as biomass to make porous carbon (Figure 4c) was made from FeS₂/CoS₂-C, and the anode material was made from N and S-doped carbon particles. Pyrolyzing these materials in an Ar environment at 800 °C produced them. The S_BET of FeS₂/CoS₂-C (468.5 m² g⁻¹) was higher than that of the carbon-doped single forms of FeS₂-C (1968.8 m² g⁻¹) and CoS₂-C (361.5 m² g⁻¹). It could be because the surface area to volume ratio was higher for the composite form and lower for the single states. The crystallinity of FeS₂/CoS₂-C was slightly lower than that of FeS₂-C and CoS₂-C. It could be because the internal structure changed when the two metal sulfides were mixed.

Another study used banana plant waste bract (BB) to generate porous carbon (BBC) by chemical activation and calcination at a range of temperatures between 500 and 800 °C [58]. The ash content and yield of the porous carbon were said to have increased and decreased in response to rising temperatures, respectively. Before activation, a wide diffraction peak with an amorphous carbonaceous structure was seen. In contrast, the graphitic crystalline structure is produced by chemical activation; however, the crystallinity peak decreases with increasing temperature owing to the KOH activation on carbon samples, which results in various defects and destruction of the graphitic structure on the surface of the carbon. The S_BET of activated porous carbon (BBC) grew from 29.5 to 712.1 m² g⁻¹ when the temperature rose from 500 to 700 °C, but further temperature increases had no impact since there was no pore opening. Figure 4d depicts the carbonization procedure used for producing BBC from banana bracts. Evidence shows that N-doped porous carbon may improve wettability, faradaic reactions, and conductivity, enhancing the carbon’s capacitive performance [60]. Accordingly, N-doped porous carbon made from garlic peel and activated with alkali was employed for two hours at a carbonization temperature of 600 °C [61]. This study compared alkali-activated carbon-derived garlic peel treated with hydrodynamic cavitation to N-doped carbon-generated garlic peel. In comparison to other types of low alkali-activated porous carbon with or without N doping and high alkali-activated and hydrodynamically treated porous carbon with N doping, it was discovered that high alkali-activated and hydrodynamically cavitation treated porous carbon (without N doping) showed high S_BET (3272 m² g⁻¹). The appearance of wide diffraction peaks and the peak shifting indicated the amorphous carbon structure and the doping of N into the carbon, respectively.

Recently, activated porous carbons were produced from balsa wood (e.g., PNOAC) by impregnating it with phosphoric acid, an activator, and heating it to 600 °C for one hour [21]. Figure 4e shows a schematic illustration of the carbonization process for making PNOAC. The phosphoric acid mass ratio (0.5–4.0) was adjusted throughout the synthesis without altering the balsa wood powder ratio. Increasing the proportion of phosphoric acid did not change the graphite’s carbon structure, showing that the structure was very stable. However, slight diffraction peak shifts showed that several defects and pore structure deformation could be the reason for a lower degree of graphitization. The impregnation method makes porous activated carbon with a large S_BET (1302 m² g⁻¹) that goes down (from 1302.9 m² g⁻¹ to 1184.8 m² g⁻¹) as the activation agent ratio goes up (from 0.5 to 4.0) because too much active agent blocks the pores on the carbon. The lowest S_BET of this research, however, was shown to be 1184.8 m² g⁻¹ for the used mass ratio (1:4) of balsa wood powder-phosphoric acid-produced carbon. As a result, this research involved synthesizing porous carbon procedures capable of providing a larger S_BET that will benefit effective energy storage applications. In another study, the carbonization temperature (700 °C) was employed for N-doped porous carbon that was made from waste cotton and activated with potassium citrate and urea (PC-K) [59]. Additionally, to create an N-doped carbon, sodium citrate and urea (PC-Na) were combined with heated waste cotton and carbonized at the same temperature (Figure 4f). However, the produced samples PC-K (1727 m² g⁻¹) and PC-Na (1061 m² g⁻¹) in this investigation did not get significant S_BET. In contrast, porous carbon with N doping (PC-Ur) that wasn’t made with citrate salts only had an S_BET of 348.4 m² g⁻¹. Thus, citrate-based salts might increase the S_BET of porous carbon when combined with biomass. When potassium citrate (PC-K) is added, the crystalline graphite is replaced by amorphous carbon, which makes the diffraction peak weaker than for PC-Na and PC-Ur.

An ultra-lightweight carbon aerogel (e.g., CA) was made using naturally occurring Kapok silk at temperatures between 600 and 800 °C in the presence of N₂ [62]. The linked network, high porosity, and rational S_BET of the conductive carbon structure make efficient ion and electron movement possible. S_BET (377–489 m² g⁻¹) was shown to rise as temperatures climbed from 600 to 800 °C. Although the sample’s S_BET was lower than expected, this study’s increased temperature caused the S_BET of the carbon sample to rise. The resultant carbon aerogel sample was subjected to structural analysis at different temperatures, which indicated a reduced GO (rGO) structure. The preparation of typical carbon aerogel from the naturally available Kapok silk could be done reasonably, and the process utilized to manufacture carbon aerogel was a method that did not cost too much.

2.3. Porous Carbon-Derived from Lignite

Carbonizing lignite or coal with KOH at 700 °C recently produced hierarchically organized porous carbon under N₂ conditions [63]. Carbonization is used to make diverse porous carbons from different carbon sources, such as light lignite (LL) or light coal, residue light lignite (LRL) or light residue coal, and soluble portions of light lignite (SP_LRL) or soluble amounts of light coal (Figure 5a). The very high S_BET of 3586 and 2922 m² g⁻¹ were produced from porous carbons derived from the soluble portions of light lignite and coal, respectively, as a result of the formation of micropores and removal of heteroatoms after high-temperature treatment with KOH. In addition, a large diffraction peak and disordered carbon structure were discovered in this investigation. Consequently, lignite or coal-soluble portion-obtained porous carbon with a large S_BET might be a choice for supercapacitor applications. Recent research demonstrates that acid-washed porous carbon was generated from oxygen-doped carbon, hydrothermally treated lignite with phosphoric acid assistance (e.g., POPC), and followed by KOH activation at 700 °C for two hours in an argon environment [64]. A simplified diagram of the carbonization-based synthesis of POPC is shown in Figure 5b. During hydrothermal treatment with temperature increases (160–180 °C), there was a rise in S_BET (2656–2852 m² g⁻¹) for the produced porous carbon; however, after increasing the hydrothermal temperature (200 °C), the S_BET reduced (2742 m² g⁻¹). Without hydrothermal treatment, the porous carbon from lignite had an S_BET of just 2575 m² g⁻¹. So, hydrothermally treating purified lignite with phosphoric acid in a temperature-controlled environment could give better results for increasing S_BET. At the same time, the porous carbon showed extremely amorphous carbon with a high degree of disordered structure, which led to the lack of a diffraction peak and very low crystallinity. While there are several steps in this process, including hydrothermal treatment before activating the porous carbon, it is considered to be a somewhat expensive and time-consuming technique.

Similarly, Liu et al. developed oxygen-enhanced porous carbon by processing lignite (e.g., OHPC) with activation agents such as potassium carbonate (K₂CO₃) and potassium ferrate (K₂FeO₄) [65]. Here, lignite and potassium ferrate were mixed with different amounts of potassium carbonate in a ball mill. The mixture was then carbonized at 900 °C for three hours in an N₂ ATM to make oxygen-rich porous carbon (Figure 5c). Increases in the ratio of potassium carbonate (from 0.25 to 1.0) decreased the S_BET (1638–720 m² g⁻¹), and further increases in the ratio (2.0) increased the S_BET (752 m² g⁻¹), which seems counterintuitive. As a result, increasing the proportion of potassium carbonate in the lignite composition and potassium ferrate may not impact S_BET. Moreover, all porous carbon samples generated showed graphitic carbon structures, confirming that the material was carbonaceous.

2.4. Porous Carbon-Derived from Metal Salts or Precursors

The templating process was used to generate oxygen-doped porous carbon from K/Mg(OH)₂ (e.g., OPCN) [66]. Initially, the carbon sources Mg(OH)₂ were mixed with various KOH ratios (0–30%) to produce a composite K0–30%/Mg(OH)₂. The resulting composite was reacted with catechol and formaldehyde to yield a template polymer composite, which was then carbonized at 700 °C in an argon ATM to obtain oxygen-doped porous carbon (Figure 5d). The S_BET of 20%KOH/Mg(OH)₂ was determined to be 930 m² g⁻¹, while the S_BET of 0%KOH/Mg(OH)₂ was 1386 m² g⁻¹. The rapid polymerization of catechol and formaldehyde might be attributed to a reduction in S_BET for composite materials. Consequently, the produced polymer resin could not fully cover the surface of the template (K/Mg(OH)₂), resulting in a reduction in S_BET. When the KOH ratio grew, the S_BET increased somewhat because it activated the micropores, which increased the S_BET. The prepared composite materials had weak diffraction peaks that matched the graphite structure, confirming that the template used to manufacture oxygen-doped porous carbon was pure amorphous. The chemicals used to synthesize porous carbon in this procedure might be costly, and the templated method used to synthesize carbon material obtained a low S_BET.

Mesoporous carbons, which are made of carbon, have some interesting properties. For example, they have a high S_BET, a high open-pore structure that allows them to adapt to electrolyte ions; they are chemically inert and strong. It has often been said that it is an active material for making electrodes, and this mesoporous-structured electrode can help electrochemical capacitors work better. So, recently, mesoporous carbon (e.g., DMC) was created by pyrolyzing mesoporous silica at 1000 °C in an N₂ environment [67]. The S_BET of this synthetic mesoporous carbon was found to be 1350 m² g⁻¹, and this typical carbonaceous material was found to have a semi-crystalline or imperfect structure. This method was easy, saved time, and did not use toxic materials. An efficient self-template way of producing porous carbon without any physio-chemical activation is to use acceptable carbon sources without adding a template during production. The procedure is straightforward, and the porous structure of the carbon material produced is identical to that of the carbon precursor [68]. As a result, Wang et al. synthesized zinc phytate salt using zinc acetate and phytic acid as starting precursors, which were then employed to generate porous carbon (e.g., PC) at 800 °C in an N₂ ATM [69]. Intriguingly, the porosity of the carbon material could be efficiently regulated by varying the molar ratio (1 and 4 M) of KOH (e.g., PC-K). In this case, a higher KOH molar ratio (4.0 M) showed a very high S_BET (1210 m² g⁻¹) compared to 1.0 M KOH (988 m² g⁻¹) and without KOH activation (1127 m² g⁻¹). The higher KOH molar ratio could have a major impact on the proliferation of micropores, which was attributed to the higher S_BET. A carbon structure was obtained for structural confirmation for porous carbon produced without adding KOH. In contrast, the extended carbon layer spacing causes the diffraction peaks to move to the left after KOH addition. In this research, the low-cost approach and self-templating method utilized to generate porous carbon obtained a lower S_BET when compared to the biomass that was used to produce porous carbon.

Ethylenediaminetetraacetic acid, or EDTA, is often used as a chelating agent. Its high N content can bond with a wide range of metal cations to make metal-EDTA complexes, which are then carbonized to make self-N-doped porous carbon. So, Wei et al. used the reflex method to make the complex coordination structure Zn-EDTA. They then carbonized it at 850 °C for four hours in the presence of N₂ to make N-doped porous carbon, such as NPC-2 [70]. To produce various ratios of N-doped porous carbon, the ratio (1.25–2.0 M) of zinc acetate was changed while EDTA remained constant. Increasing the proportion of zinc acetate in EDTA from 1.25 to 1.59 M increased the S_BET from 756.4 m² g⁻¹ to 1173.8 m² g⁻¹, after which the S_BET decreased dramatically to 563.7 m² g⁻¹ for the molar ratio of 2.0 M. The cause of particle agglomeration at higher ratios may be reduced S_BET. And therefore, a higher molar ratio of the metal compound could impact the S_BET, and it would be preferable to use lower molarity. This study showed that self-N-doped porous carbon had an amorphous carbon (graphite) structure and that acid washing after carbonization got rid of the Zn diffraction peak. It helped make more pores in the carbon materials, which made the sample more porous. This research employed a low-cost method for preparing porous carbon applicable for industrial-scale production.

Another study used the self-template method to create porous carbon sheets (CNS) from commercially available chemicals (e.g., EDTA₄Na) [71]. First, it was carbonized at 700 °C in an argon ATM, and then the resulting carbon was mixed with various KOH concentrations and carbonized at 800 °C in the same argon ATM to produce a porous carbon sheet. Due to an increase in the mass ratio of KOH (4.0–6.0) in the porous carbon, the S_BET of the porous carbon sheet was significantly greater (1828–3800 m² g⁻¹). Later, S_BET was reduced to 3278 m² g⁻¹ for the molar ratio of 7.0 KOH because the pores were clogged at this higher mass ratio. In contrast, the S_BET of porous carbon sheets without KOH was only 468 m² g⁻¹. Therefore, activation of KOH is crucial for carbon materials at elevated temperatures to significantly increase their S_BET. It was observed that the bare Carbon sheet reveals an amorphous carbon structure. After KOH activation, the peaks became attenuated and much broader due to the KOH’s influence on the carbon framework, which lowers the graphitization level. The synthesis method illustrates that a simple and inexpensive carbonization procedure may be used, but this carbonization is necessary to produce porous carbon sheets before and after KOH activation.

2.5. Porous Carbon-Derived from Bio-Oil

Bio-oil distillation residue (BDR) was also used to make N-doped porous carbon by activating it with KOH (e.g., BAC) [72]. In more depth, the BDR was made from walnut shells heated to 500 °C and then burned. Later, it was mixed with KOH and melamine by physical grinding, and the mixture was then pyrolyzed in N₂ to make porous carbon with N added to it. According to this study, S_BET is 1664.1 m² g⁻¹ with a type I isotherm (microporous) and a diffraction pattern with only two peaks that show graphitic planes of an amorphous carbon structure. It was found that this type of process was simple and easy to make porous carbon with a low-cost method, and it may be possible to make it commercially for use. Another work utilized freeze-dried bio-oil dissolved KOH (BK) solution to carbonize porous 3D structured carbon (e.g., HBPC) at 800 °C under Ar flow [73]. The mass ratio of KOH was changed from 1 to 4 g in bio-oil at temperatures ranging from 700 to 900 degrees Celsius so that the influence of preparing porous carbon could be evaluated. The increasing S_BET with increasing temperature (700–900 °C) and increasing KOH dose demonstrated that KOH activation enhances pore formation. The highest S_BET (1758 m² g⁻¹) was discovered for BK carbonized at 900 °C. The 3D structured porous carbon exhibits graphite structure and increases the KOH mass ratio, which decreases the diffraction peak height due to increased carbon density, and the enhancement of graphitization degree in this study was due to carbonization of the samples at higher temperatures (800–900 °C). The cost of the synthesis technique was rather higher owing to the use of a freeze dryer to dry the materials before carbonization.

2.6. Porous Carbon-Derived from Melamine

In recent work, a melamine sponge was utilized to generate N-doped porous carbon (e.g., NCS), which was then activated with KOH at temperatures ranging from 750 to 1000 °C in the presence of N₂ gas [30]. However, the amount of N that was present decreased due to the introduction of KOH. As a result, an effort was made to use further hydrothermal treatment, which consisted of turning urea into a carbon sponge and replenishing the N sponge. It was noted that the produced samples had a low S_BET, such as a carbon sponge (6.8 m² g⁻¹), activated carbon sponge (636.1 m² g⁻¹), and N-doped carbon sponge (158.1 m² g⁻¹). It means that sponge-based carbon compounds may not be able to achieve a greater S_BET. The existence of broader diffraction peaks (activated and N-doped carbon sponge) in the prepared samples was evidence that the material being examined was amorphous carbon in its purest form as compared to carbon sponge, which has a lower carbon concentration and exhibits a less broad peak.

Several studies have attempted to boost the S_BET of porous carbon by widening the pore structure and their size and doping the material with heteroatoms to increase the number of healthy sites for the electrochemical process [74,75]. Leng et al. developed 3-D structured porous carbons that were doped with N and sulfur (S) (e.g., ASNPC) [76]. Sulfonated GO (SGO) was mixed with polyvinyl alcohol and melamine formaldehyde resin to make a 3-D network structure precursor (Figure 6a). This precursor was then carbonized at 800 °C in an N₂ ATM for 1 hour before being activated with KOH. The study’s specific SBET went up (3281 m² g⁻¹), and the carbon’s graphite diffraction pattern showed that an amorphous structure had formed. Nonetheless, the usage of sulfonated GO, the creation of melamine formaldehyde, and the lengthy procedure of heating samples in many steps to make porous carbon may be regarded as costly aspects of this work.

Carbon materials derived from polymers of the C-N group have attracted increasing interest because their S_BET could be high [77]. It is because the N species that make up the C-N groups are easily converted into volatile gasses during the pyrolysis or activation, and their evaporation results in the establishment of the pore structure, which in turn increases the S_BET. Consequently, a typical synthetic polymer (e.g., SP_MA-BQ) was manufactured by Tong et al., and it was carbonized at varying temperatures (between 700 and 900 °C) while being passed through a stream of N₂ gas, after which it was activated with potassium hydroxide [78]. The material was activated and underwent a second carbonization process at the same temperature to produce a porous carbon-derived polymer. It was found that the generated carbon had an S_BET of no more than 2154 m² g⁻¹ and that the S_BET dropped from 2069.6 to 1767.3 m² g⁻¹ when the ratio of KOH used to activate porous carbon went from 3 to 6 using this method. Hence, increasing the proportion of KOH might potentially decrease the S_BET, and it would be preferable to use a low quantity. Before activation, there was only one wide diffraction peak that corresponded to amorphous carbon, but after activation, there is one additional broad diffraction peak that corresponds to graphitic carbon. This procedure can result in a modest cost owing to the synthesis of a polymer and the two carbonization processes that were carried out, which consumed time.

Figure 6. (a) Graphical representation of ASNPC synthesis by carbonization, reproduced with permission from [76], Copyright 2023, American Chemical Society. (b) Various steps of the synthesis of HPC from glucose/acrylamide, reproduced with permission from [79], Copyright 2023, Elsevier.

Figure 6. (a) Graphical representation of ASNPC synthesis by carbonization, reproduced with permission from [76], Copyright 2023, American Chemical Society. (b) Various steps of the synthesis of HPC from glucose/acrylamide, reproduced with permission from [79], Copyright 2023, Elsevier.

2.7. Porous Carbon Derived from Others

The carbon generated by biomasses also has significant drawbacks, such as the fact that they seldom ever have a large S_BET, and after activation of carbon by alkali, the carbon materials formed have a low S_BET and low specific capacitances [80]. Most biomasses, however, include inorganic components that function as self-activating agents and cause extensive porosity during carbonization. Due to this inspiration, glucose was recently mixed with polyacrylamide (pore-forming agent) and activated with potassium hydroxide at a carbonization temperature of 900 °C to produce highly porous structured carbon (e.g., HPC) [79]. This concept comes from the self-activation of natural biomass. Nevertheless, this procedure required many chemicals and two carbonization processes before producing porous carbon. This particular kind of porous carbon was found to have a large S_BET (3381 m² g⁻¹), and the XRD analysis on the carbon sample revealed a disordered carbon structure. In addition, no new impurities were found, and the fact that there was no change in the peaks is evidence that the material’s purity was high and that it had not been subjected to further doping. Figure 6b describes the carbonization process of generating HPC from a combination of glucose and acrylamide.

Semi-coking-based industrial wastewater (SCW) comprises ${\mathrm{S}\mathrm{O}}_4^{2-}$ and ${\mathrm{N}\mathrm{H}}_4^{+}$ ions and was recently utilized to make phenolic resin, revealing that it has self-doping N-O-S characteristics. So, the properties of semi-coking water-based phenolic resin (SWPR) were used to make self-doped heteroatoms on porous carbon for applications in energy storage [81]. Initially, SWPR was generated in this synthesis by reacting SCW with formaldehyde at a temperature of 120 °C. To synthesize heteroatom-doped carbon, a variety of SWPR ratios (two to three) were activated with sodium carbonate (Na₂CO₃) and then calcined at various temperatures (600–800 °C) under N₂ flow. The produced samples had a disordered carbon semi-crystalline structure, and diffraction peak shifts were discovered due to the heteroatoms self-doping of the carbon structure. The S_BET was dependent on the SWPR ratio and temperature, i.e., a rise in the SWPR ratio resulted in a drop in S_BET, and a rise in temperature with a fixed SWPR ratio (2.5) increased S_BET. But this type of research only got 750 m² g⁻¹ of S_BET for a 2:1 ratio of SWPR to sodium carbonate to make porous carbon at 700 °C. It means that the synthesis method used in this study did not affect the production of high S_BET carbon materials.

2.8. Discussion

Table 1. The preparation conditions of porous carbons derived from various sources, their surface area (S_BET) measurement, and cost analyses.

Porous Carbon	Source	Doping/Activation	Pyrolysis Temp. (°C)	ATM & Time	SBET (m2 g−1)	Cost	Ref.
PNOAC	Balsa wood	H3PO4	600	- & 1 h	1302.9	Low	[21]
NCS	Melamine sponge	Urea/KOH	950	N2 & 1 h	158.1	Moderate	[30]
CuO/Cu@C-700	Chitosan & Cu(NO3)2·3H2O	-	700	N2 & 2 h	313.2	High	[47]
N-LHPC	Lignin, MgCl2⋅6H2O & ZnCl2	Melamine	900	N2 & 2 h	-	High	[55]
NPC	Agar-Arg xerogel	L-arginine/KHCO3	800	N2 & 2 h	3184.0	Low	[56]
FeS2/CoS2-C	CoCl2·6H2O, FeCl3·6H2O & Kelp	Thiourea	800	Ar & 2 h	468.5	High	[57]
BBC	Banana bract	KOH	500–800	N2 & 2 h	712.1	Moderate	[58]
PC-K	Waste cotton	Potassium citrate	700	N2 & 2 h	1727.9	Low	[59]
HCGP	Garlic peel	Alkali activation	600	- & 2 h	3272.0	Low	[61]
CA	Kapok silk	-	600–800	N2 & 2 h	489.0	Low	[62]
SPLRL	Runbei lignite	KOH	700	N2 & 1 h	3586.0	Low	[63]
POPC	Lignite	KOH	700	Ar & 2 h	2852.0	Moderate	[64]
OHPC	lignite powder	K2FeO4, & K2CO3	900	N2 & 3 h	1638.0	Moderate	[65]
OPCN	KOH/Mg(OH)2, catechol & formaldehyde	KOH	700	Ar & 2 h	930.0	Moderate	[66]
DMC	Nano silica	KNO3	1000	N2 & 3 h	1350.0	Low	[67]
PC-K	Zinc acetate	KOH	800	N2 & 1 h	1210.0	Low	[69]
NPC-2	EDTA & Zinc acetate	-	850.0	N2 & 4 h	1173.8	Low	[70]
CNS	EDTA4Na	KOH	800	Ar & 1 h	3400.0	Low	[71]
BAC	Bio-oil distillation residue	Melamine/KOH	500–800	N2 & 1.5h	1664.1	Low	[72]
HBPC	Bio-oil	KOH	700–900	Ar & 1 h	1758.0	Moderate	[73]
ASNPC	Melamine & SGO	KOH	800	N2 & 1 h	3281.0	Moderate	[76]
SPMA-BQ	Benzoquinone and melamine	KOH	700–900	N2 & 2 h	2154.1	Moderate	[78]
HPC	Glucose & Polyacrylamide	KOH	900	- & 1 h	3381.0	Moderate	[79]
SWPR	Semi-coking wastewater	Na2CO3	600–800	N2 & 1 h	750.0	Low	[81]

summarizes the synthesis of porous carbon utilizing various sources and their preparation conditions to obtain a significant S_BET, as well as the cost of the synthetic approach. KOH-activated porous carbon may be observed with a large SBET, such as NPC, HCGP, SPLRL, POPC, CNS, HBPC, SPMA-BQ, and HPC. The lignite-based porous carbon (SP_LRL) with the largest S_BET (3586.0 m² g⁻¹) was identified with KOH activation at 700 °C. However, several materials, such as BBC, OPCN, and NCS, did not exhibit a significantly greater S_BET when activated with KOH. It might cause either pore to be closed after activation or the creation of many macropores and a few micropores. It was further found that pyrolysis temperatures ranging from 600 °C to 800 °C may produce a high S_BET. The porous carbon has a large S_BET that may be attributed to active hot spots for ion storage, resulting in superior electrochemical performance. Also, a high number of micropores could lead to an increased adsorption site, which is important for the performance of capacitance, and these pores act as a path for fast ion diffusion and electrolyte movement, which lets the cycle work well. It was found that potassium hydroxide (KOH) was a better activator for making high S_BET porous carbon than potassium bicarbonate (KHCO₃)_, thiourea, phosphoric acid (H₃PO₄), potassium ferrate (K₂FeO₄₎, potassium carbonate (K₂CO₃), potassium nitrate (KNO₃) and sodium carbonate (Na₂CO₃). Also, the cost of synthesis has been summed up in Table 1, which is based on the sources used, the chemicals used, the synthesis process, and the length of the procedure. Most of the techniques have low costs and modest ranges, and it is highly recommended that the production process be easy and make it possible to get effective porous carbon for high-performance supercapacitor applications.

3. Application of Porous Carbon in a Supercapacitor

Fabricating a supercapacitor-based material has been accomplished using either symmetric or asymmetric systems of electrodes. The symmetric system is a device constructed of the same material with similar capacitances at both the positive and negative electrodes. On the other hand, the term “asymmetric” refers to a device with a different construction for the positive and negative electrodes. Asymmetric supercapacitors are a next-generation sophisticated technology due to their high energy density (ED), power density (PD), and amplified cell voltage. Most EDLCs that have been researched have symmetrical configurations, and they were made by fabricating two porous carbon electrodes that are similar to one another. Research into portable electronics and electric vehicles has focused on asymmetric-based SCs because of their fast charging-discharging operations, superior PD, and user-friendliness. To achieve considerable performance, SCs demanded both a high capacitance and extensive voltage, which were determined by the physicochemical characteristics of the electrodes. The following subsection was organized based on the high and low specific capacitances to find the high-performance-based electrodes for the supercapacitor. Accordingly,

Table 2. Electrochemical findings of several porous carbons using 3-electrode and two-electrode systems set up.

Porous Carbon	SC * (F g−1)	CD (A g−1)	EL	ED (Wh kg−1)	PD (W kg−1)	CS and R	System Built	Efficiency	Ref.
PNOAC600-2	263.0	0.5	6 M KOH	9.2	414.0	10,000 & 91.4%	Symmetric	Low	[21]
NCS	253.0	0.5	6 M KOH	4.2	250.0	10,000 & 99%	-	Low	[30]
CuO/Cu@C-700	2479.0	0.5	6 M KOH	76.8	374.5	10,000 & 82.4%	Asymmetric	High	[47]
N-LHPC	235.7	0.5	6 M KOH	5.7	246.6	5000 & 81.3%	Symmetric	Low	[55]
NPC-800-4	443.0	0.5	6 M KOH	35.5	450.0	20,000 & 99.7%	Symmetric	High	[56]
FeS2/CoS2@PC-800	3480.4	0.5	6 M KOH	200.2	463.1	10,000 & 94.7%	Asymmetric	Very High	[57]
BBC-800	472.0	1.0	1 M Na2SO4	86.0	1284.0	5000 & 93.5%	-	High	[58]
PCK	273.0	1.0	6 M KOH	9.9	350.0	5000 & 98%	Symmetric	Moderate	[59]
1:6-HCGP+N	227.0	10.0	6 M KOH	10.1	250.0	5000 & 96%	-	Moderate	[61]
CA8	355.0	1.0	1 M H2SO4 and IL	35.0	65.0	20,000 & 96%	Symmetric	High	[62]
SPLRL	373.0	0.5	6 M KOH	11.5	125.0	10,000 & 97.4%	Symmetric	Moderate	[63]
POPC2	320.0	1.0	6 M KOH	10.7	11.1	25,000 & 88%	Symmetric	High	[64]
OHPC1	283.0	0.5	6 M NaOH	16.5	300.0	10,000 & 64.8%	Symmetric	Moderate	[65]
OPCN-20	375.0	1.0	6 M KOH	25.7	900.0	20,000 & 86.2%	Symmetric	High	[66]
DMC	327.0	0.5	1 M H2SO4 + 0.01 M AO 45	11.4	250.0	2000 & 74%	Symmetric	Moderate	[67]
PC-K4	296.0	0.5	6 M KOH	10.0	230.0	8000 & 96.5%	Symmetric	Moderate	[69]
NPC2	251.9	1.0	6 M KOH	8.7	248.7	10,000 & 100%	-	Moderate	[70]
CNS6-80	290.0	2.0	IL	122.0	1740.0	8000 & 81.1%	Symmetric	Moderate	[71]
BAC-10	442.0	1.0	6 M KOH	14.8	48.5	-	Symmetric	High	[72]
SC@KOH	336.0	0.5	6 M KOH	9.54	130.0	10,000 & 92–98%	Symmetric	Moderate	[73]
ASNPC-1	386.0	0.5	6 M KOH	14.4	700.0	5000 & 84%	Symmetric	Moderate	[76]
SPMA-BQ-5-800	414.6	0.5	6 M KOH	8.8	250.0	7000 & 90.2%	Symmetric	High	[78]
HPC60	419.0	0.5	6 M KOH	10.9	125.0	20,000 & 89.9%	Symmetric	High	[79]
SWPR-700-2.5	129.5	1.0	6 M KOH	5.9	124.9	10,000 & 99.4%	Symmetric	Very low	[81]
So	508.0	1.0	6 M KOH	7.0	586.0	10,000 & 69%	Asymmetric	Moderate	[82]
N/P-HPC-Y:PA(2:1)-800	432.0	1.0	1 M H2SO4	13.6	500.0	10,000 & 93.3%	Symmetric	High	[83]
PC-R6A7	425.0	1.0	6 M KOH	10.5	256.0	10,000 & 92%	Symmetric	High	[84]
TS-HPC	402.0	0.5	6 M KOH	7.0	15,789.5	10,000 & 97.8%	Symmetric	High	[85]
C750	354.0	1.0	6 M KOH	47.2	16,000.0	1000 & 91%	Symmetric	High	[86]
850-24-20-5%	316.0	1.0	6 M KOH	16.3	489.0	5000 & 91.2%	Symmetric	High	[87]
CQD	302.0	0.5	1 M H2SO4	41.9	250.0	5000 & 93.8%	-	Moderate	[88]
TMCK	222.7	1.0	2 M KOH	6.9	600.0	20,000 & 100%	Symmetric	Moderate	[89]
SSL-N/S-K-700	220.0	0.5	1 M KOH	11.2	400.0	10,000 & 99%	Symmetric	Low	[90]
ACTBG	212.0	1.0	1 M KOH	18.8	4900.0	15,000 & -	Symmetric	Low	[91]
ZMB	208.0	1.0	6 M KOH	2.93	61.7	-		Low	[92]
WS-dAC/NiO-0.005	205.0	0.5	6 M KOH	-	-	1000 & 98%	-	Low	[93]
N-HPC	144.9	0.5	6 M KOH	5.0	124.8	15,000 & 96.5%	-	Very low	[94]

SC = Specific capacitance; CD = Current density; EL = Electrolyte; ED = Energy density; PD = Power density; CS and R = Cycle stability and Retention. * Specific capacitance (SC) measurements from a three-electrode system.

was made to show their values. Based on the specific capacitance value of the investigations, the following sections were divided. Table 2 lists the specific capacitance of the survey in descending order to make it easier to discern between higher and lower values. In addition, the device’s overall performance, which includes specific capacitance, energy density, power density, cycle stability, and retention, was utilized for evaluating the device’s efficiency (Table 2).

3.1. Ultra-High Specific Capacitance-Based Porous Carbon

Transition-based metal sulfide cathodes are particularly interesting because of their excellent electrical conductivity and specific capacity. More valence states of metals and sulfur elements are available in this material, which benefits the reversible Faradaic redox operation to increase energy storage. Accordingly, FeS₂/CoS₂ is currently gaining popularity as a cathode material because of its natural availability and eco-friendliness. Therefore, lately, FeS₂/CoS₂ doped biomass (raw kelp) generated porous carbon at 800 °C (FeS₂/CoS₂@PC-800) and further produced N, S doped porous carbon acquired from kelp (NSPC) for the design of the asymmetrical device for practical application [57]. Reversible Faradaic conversion of doped transition metal ions caused two redox peaks in the potential range of 0–0.8 V. Figure 7a shows the FeS₂/CoS₂@PC-800 electrode has a greater CV area and redox current responsiveness than porous carbon with a single metal sulfide (CoS₂@PC-800 and FeS₂@PC-800). Ion transfer and energy storage were also excellent in this investigation. The FeS₂/CoS₂@PC-800 electrode has a high specific capacitance of 3480.4 F g⁻¹ at 0.5 A g⁻¹, compared to 2306.3 and 1666.9 F g⁻¹ for CoS₂ and FeS₂ (Figure 7b). Also, FeS₂/CoS₂@PC-800 displays a greater capacitance retention percentage (60.35%) than CoS₂@PC-800 (44.9%) and FeS₂@PC-800 (52.4%). The asymmetrical device (FeS₂/CoS₂@PC-800//N, S-doped porous carbon-800 (NSPC-800)) was made and tested for its super capacitance at 0.5 A g⁻¹ to be used in real life. It had a specific capacitance value of 685.6 F g⁻¹, compared to 306.7 F g⁻¹ for CoS₂ and 241.6 F g⁻¹ for FeS₂. FeS₂/CoS₂@PC-800//NSPC-800 showed 94.7% cyclic stability over 10,000 cycles, whereas FeS₂@PC-800//NSKC-800 showed 88.74% and 80.05% (Figure 7c). The charge-discharge (GCD) analysis demonstrated high ED (200.2 Wh kg⁻¹) using a 463.1 W kg⁻¹ PD for FeS₂/CoS₂@PC-800/NSPC-800.

Xi et al. used carbonization to generate a CuO/Cu-doped Chitosan-derived carbon (CuO/Cu@C) electrode at varied temperatures (600–900 °C) for electrochemical analyses [47]. All of the prepared materials showed significant pseudocapacitive behavior at 5 mV s⁻¹ in a potential range of 0–0.7 V. CuO/Cu@C prepared at 700 °C (CuO/Cu@C-700) showed a wide cyclic voltammetry (CV) area range, indicating a higher specific capacitance than electrodes prepared at 600, 800, and 900 °C. The galvanostatic charge-discharge (GCD) study revealed that the sample obtained a longer discharge time and a higher charge-discharge platform, representing effective charge storage and ion transport. CuO/Cu@C-700 had the highest capacitance value of 2479 F g⁻¹ at 0.5 A g⁻¹ when compared to CuO/Cu/Cu₂O@C-900 (613 F g⁻¹), CuO/Cu@C-600 (806 F g⁻¹), and CuO/Cu/Cu₂O@C-800 (2122 F g⁻¹), because it has a higher S_BET than others and a distinct flower-like morphology of CuO/Cu on the chitosan-derived carbon. However, CuO/Cu@C-700 had a lower diffusion resistance than others. Furthermore, high scan rates were shown to be unfavorable for capacitance behavior, which reduces the Faraday peak. It was subsequently found in this research that the ions cannot contribute to the redox process, resulting in a drop in specific capacitance (SC) at high current densities (CD) of more than 10 A g⁻¹. Over 10,000 cycles, high cycling stability was observed for asymmetric devices. At 374.5 W kg⁻¹, the device CuO/Cu@C-700 (positive)/AC (negative) demonstrated a high ED of 76.87 W h kg⁻¹.

3.2. High-Specific Capacitance-Based Porous Carbon

In another work, low S_BET (12.09 m² g⁻¹) porous carbon that was generated without pyrolysis from Ni mixed date seed waste (e.g., So) demonstrated higher specific capacitance (508 F g⁻¹) than similar samples that were pyrolyzed at 1400 °C (432 F g⁻¹) when a current density of 1 A g⁻¹ was applied [82]. Even at high CD (5 A g⁻¹), the produced So sample maintained a high specific capacitance, measured at 308 F g⁻¹. Based on the results, an asymmetric device (e.g., So//AC) was manufactured; nevertheless, it exhibited poor capacity retention (69%) over 10,000 cycles at 10 A g⁻¹, while it did not obtain a high ED (7 Wh kg⁻¹) at the PD of 586 W kg⁻¹. According to the research findings, porous carbon with a limited S_BET could acquire a high capacitance despite undergoing pyrolysis.

Porous carbon from banana bracts (BBC) was examined for super capacitance at different temperatures (500–800 °C) with KOH activation [58]. The CV curves of all porous carbons revealed rectangular forms, indicating that they are all EDLCs with high-rate efficacy and swift charge transfer capabilities (Figure 7d). Fast ion movement in the electrolyte damages the diffusion layer of the electrodes, and fast ion transfer to the electrodes may lower specific capacitance (Figure 7e). S_BET was particularly important in this investigation since the BBC-500 and BBC-800 had S_BET of 7.2 m² g⁻¹ and 513.2 m² g⁻¹, respectively, and were assigned specific capacitance values of 15.3 F g⁻¹ and 472.5 F g⁻¹ at a scan rate of 0.001 V s⁻¹. At a faster scan rate (0.5 V s⁻¹), the same materials (BBC-500 and BBC-800) exhibit low specific capacitance (0.9 and 2.13 F g⁻¹). CV at 1 A g⁻¹ revealed a high specific capacitance of 472 F g⁻¹. The charge-discharge curve of the BBC-800 was discovered to have a symmetrical triangular form achieving that typical EDLC distinction in the BBC electrodes. The electrode’s (BBC-800) stability was determined to be almost 93.5% original capacitance after 5000 cycles. As a consequence, the electrode was claimed to have a high potential for maintaining quick charge-discharge with very little deterioration over 5000 cycles (Figure 7f), and it was known that the BBC-800 has a PD of 1280 W kg⁻¹ and a high ED of 86 Wh kg⁻¹.

Recently, agar/L-arginine xerogel carbonization produced N-doped porous carbon, such as NPC800-4, with high specific capacitance [56]. Due to the heteroatom (N and O) presence, the rectangular CV curves obtained in this investigation somewhat deviate (Figure 7g). At a CD of 0.5 A g⁻¹, a high specific capacitance (443 F g⁻¹) was identified (Figure 7h). The high S_BET (3184 m² g⁻¹) and doping of heteroatoms might be the causes of the electrode’s high capacitance. Furthermore, the developed symmetric device based on the NPC-800-4 was shown to have specific capacitance and cycle stability. When the device was tested for charge-discharge strength over 20,000 cycles, the specific capacitance was found to be 78.9 F g⁻¹ at a CD of 0.5 A g⁻¹ and 38.6 F g⁻¹ at a CD of 10 A g⁻¹, and the symmetric system retained 99.7% of its capacitance (Figure 7i). Furthermore, it was discovered that this symmetric device’s calculated ED and PD were also high, at 35.5 Wh kg⁻¹ and 450.0 W kg⁻¹, respectively. Heteroatom-doped porous carbon, like BAC-10, had a bigger CV area curve, perhaps hinting at strong EDLC properties and the capacity for quick charge/discharge [72]. The specific capacitance increased dramatically to 442.0 F g⁻¹ at 1 A g⁻¹ following N₂ doping, compared to 396.0 F g⁻¹ before doping. Well-ordered micropores and the presence of the N group may have contributed to the electrode’s higher capacitance. Testing a BAC-10-based symmetric device that showed quasi-rectangular curves with no deviations at the voltage used of 1.6 V indicated that typical EDLC characteristics apply to the choice of voltage application. Additionally, the BAC-10-based symmetric device obtained a low ED of 14.8 Wh kg⁻¹ at a PD of 48.5 W kg^−1, and in this research, the cycle stability was not evaluated. Self-doping heteroatoms (N and P) using an optimum ratio of yeast and phytic acid to create porous carbon at 800 °C (e.g., N/P-HPC-Y:PA(2:1)-800) were reported to have good electrochemical performance in a 3-electrode system containing 1M H₂SO₄ [83]. As a result, the specific capacitance of the heteroatom doped porous carbon was 432.0 F g⁻¹ at a high CD of 1 A g⁻¹ owing to the high porous structure and N and phosphorous (P) functional groups on the carbon materials that allow redox reactions and high conductivity. However, the device with a symmetric structure with N/P-HPC-Y:PA(2:1)-800 had a low capacitance of 98 and 75 F g⁻¹ at CDs of 1 and 20 A g⁻¹, respectively. Also, the stability test done over 10,000 cycles shows that the device keeps most of its capacity (93.3%), and it can deliver high ED (13.6 Wh kg⁻¹) and PD (500 W kg⁻¹) at the CD of 1 A g⁻¹. As a result, despite their modest S_BET (978 m² g⁻¹), heteroatoms such as N- and P-doped porous carbon produced from yeast and phytic acid might be promising carbon materials for supercapacitor applications.

High specific capacitance (425 F g⁻¹) at a current density (CD) of 1 A g⁻¹ was found in popcorn-derived KOH-activated porous carbon at 700 °C (e.g., PCR6A7), in contrast to porous carbon prepared at carbonization temperatures of 600 °C (320 F g⁻¹) and 800 °C (215.5 F g⁻¹) [84]. Based on the results of this research, a carbonization temperature of 700 °C produces the best specific capacitance on the PCR6A7 due to its higher S_BET (2361.2 m² g⁻¹). Figure 8a shows the PCR6A7 CV curve, which has the biggest rectangular CV area and is a reliable indicator of good capacitance performance at a 6-M KOH 3-electrode system. At a high CD of 20 A g⁻¹, PCR6A7 still maintained a capacitance of 350.2 F g⁻¹ (Figure 8b). The capacitance of the PCR6A7-based symmetric device was found to be high (302 F g⁻¹) at 1 A g⁻¹, but it dropped to 248 F g⁻¹ when the current density was increased to 20 A g⁻¹. The symmetric device (PCR6A7) achieved a high ED of 10.5 Wh kg⁻¹ at a PD of 256 W kg⁻¹ (Figure 8d), and its cyclic stability was so great that 92% of its capacity was still there after 10,000 cycles, which is quite impressive (Figure 8c). Popcorn-derived porous carbon might be cheap with good capacitance and cyclic stability because of its large S_BET. The same specific capacitance value was found when glucose and polyacrylamide made porous carbon [79]. The CV curves of the produced sample (e.g., HPC60) were quasi-rectangular, revealing the pseudocapacitance behavior, and it maintained the quasi-rectangular curve even at high scan rates (100 mV s⁻¹), demonstrating the sample’s superior electrochemical performance. HPC60 had a higher specific capacitance (419 and 441, 331 F g⁻¹ at 0.5, 0.25, and 10 A g⁻¹, respectively) while having a higher current density (CD). According to EIS, the electrode had a low series resistance (0.5 ohms) in the high-frequency area and very low charge-movement resistances. In the low-frequency region, the electrode showed a vertical line, which shows that the sample has EDLC properties. The produced symmetrical device had the same quasi-triangle curves despite not having a very high specific capacitance (78.2 and 46.1 F g⁻¹ at the CD of 0.25 and 5 A g⁻¹, respectively). A symmetric device based on HPC60 achieved a high series resistance (1.8 ohms) and a high charge-movement resistance (1.04 ohm). Over 20,000 cycles, the symmetric device had the highest capacitance retention (89.9%) at 10 A g⁻¹, and the research yielded an ED of 10.9 Wh kg⁻¹ at a PD of 125 W kg⁻¹.

Due to the appearance of quasi-rectangular curves and faradaic reactions of heteroatoms, EDLC and pseudocapacitance behavior have been identified in the investigation of heteroatom (N and O) doped synthetic polymer-derived porous carbon (e.g., SP_MA-BQ-5-800) [78]. Furthermore, the charge-discharge curves exhibit approximately isosceles triangles, showing EDLC features; nonetheless, there was a tiny variation in the curves, demonstrating pseudocapacitance behavior. Similarly, the specific capacitance (414.6–295.0 F g⁻¹) dropped as the CD (0.5-50.0 A g⁻¹) increased. Because a tiny semicircle occurred in the high-frequency findings, the electrode’s resistance was determined to have high charge transfer and electrical conductivity, revealing the electrode has strong electrochemical characteristics. Based on the results, the electrode was made with a symmetric device that shows isosceles triangle curves even at high scan rates (100 mV s⁻¹). It shows that the electrode has the EDLC property. The symmetric device’s specific capacitance was observed to be 254 and 187 F g⁻¹ at CDs of 0.5 and 10 A g⁻¹, respectively, and the stability study revealed significant capacitance retention (90.2%) after 7000 cycles at 1 A g⁻¹. Tobacco-stem-derived chemically activated high S_BET porous carbon, such as TS-HPC, showed high capacitance (402 F g⁻¹) at the CD of 0.5 A g⁻¹ [85]. The EIS study’s finding of very low resistance for the TS-HPC suggests that it has elevated electrical conductivity, and the result of a low charge-transfer resistance also indicates that it has a strong ion-diffusion capability. Also, the symmetric device (TS-HPC) made had a sufficient specific capacitance (204 F g⁻¹) at 0.5 A g⁻¹ and worked better over 10,000 cycles, keeping 97.8% of its capacitance. The symmetric TS-HPC device measured a low CD value of 7.0 Wh kg⁻¹ when operating at a lower PD value of 15,789.5 W kg⁻¹.

Recently, the supercapacitor application of heteroatom (N/S)-doped porous carbon (e.g., ASNPC-1) has been investigated [76]. CV and GCD tests indicated EDLC and pseudocapacitance behavior due to rectangular and triangular curves forming, respectively (Figure 8e). Due to the heteroatom’s functional group enabling the faradaic reaction, the pseudocapacitance is attributable. Even at a scan rate of 200 mV s⁻¹, the rectangular curve remained unchanged, indicating a higher rate capability. The charge-discharge investigation determined the specific capacitance to be 386 F g⁻¹ at 0.5 A g⁻¹ (Figure 8f). Due to the presence of a vertical line in the low-frequency section and the diameter of a semicircle in the high-frequency area, EIS revealed rapid ion diffusion and the least amount of electron transfer resistance in this study. For the symmetric device made with ASNPC-1, the CV showed similar rectangular curves, and even at high scan rates (200 mV s⁻¹), the device didn’t change the shape of the curves. Due to the enrichment of heteroatoms, however, the GCD curve has a slight distortion that reveals pseudocapacitance behavior. At 0.5 and 10 A g⁻¹, the specific capacitance of the device was measured to be 185 and 121.6 F g⁻¹, respectively, indicating exceptional electrochemical capability. When the stability of an ASNPC-1-based symmetric device was tested over 5000 cycles, its capacitance retention stayed at 84%, which shows that the stability is good (Figure 8g). Finally, the better ED and PD of the symmetric device were determined to be 14.4 Wh kg⁻¹ and 700 W kg⁻¹, respectively (Figure 8h). Similar specific capacitance behavior (375 F g⁻¹ at a CD of 1 A g⁻¹) was seen in KOH/Mg(OH)₂/polymer-derived porous carbon (e.g., OPCN-20), and even when the CD was increased to 20 A g⁻¹, the capacitance remained high (304 F g⁻¹) [66]. Through EIS investigation, an electrode (OPCN-20) indicates a significant slope at low-frequency sections that may be caused by the electrolyte’s low diffusion resistance, which encourages high-charge transfer, and low resistance at a high frequency, which the electrode’s high conductivity may cause. When the OPCN-20 was constructed as a symmetrical device, the specific capacitance was determined to be 205.8 and 162 F g⁻¹ at the CD of 1 and 20 A g⁻¹, respectively. The device’s maximum ED of 25.7 Wh kg⁻¹ was discovered at a PD of 900 W kg⁻¹, and its cyclic stability has shown very high performance, maintaining 86.2% of its capacity after 20,000 cycles.

Because of their larger S_BET, porous structure, and self-doping of heteroatoms, hierarchical-based porous carbons are likely to be an excellent choice for SCs. Self-doped heteroatom hierarchical porous structured carbons derived from coal (e.g., SP_LRL) were recently synthesized for supercapacitor applications [63]. In this work that illustrates EDLC behavior, the quasi-rectangular curves were recorded at a scan rate of 10 mV s⁻¹. However, when the scanning rate increased, the quasi-rectangular curves became deformed, suggesting that the electrode had a high electrical conductivity and a good EDLC, as determined by GCD experiments, with specific capacitances of 373 and 300 F g⁻¹ at CDs of 0.5 and 20 A g⁻¹, respectively. SP_LRL demonstrates that the maximum size of the semi-circle appears in the high-frequency area, and the linear curve appears at low frequency, suggesting excellent capacitance behavior. Even at high scan rates (20 mV s⁻¹), the symmetric system with SP_LRL of CV and GCD curves maintains the original quasi-rectangle an isosceles triangle. It has an ED of 11.53 Wh kg⁻¹ and a PD of 125 W kg⁻¹. After 10,000 cycles at a CD of 5 A g⁻¹, the initial capacitance of the SP_LRL-based symmetric device was still 97.4% intact, demonstrating high cycle stability. Another study found that porous carbon aerogel (CA8) made from kapok silk had greater specific capacitance, measuring 355 F g⁻¹ when evaluated at a high CD of 1 A g⁻¹ [62]. According to the EIS study, the redox reaction on the CA8 surface caused high ion diffusion at the electrode and electrolyte interface in the low-frequency section. In the high-frequency region, the stumpy charge-transfer resistance at the interface was due to a good electrical conductivity pathway. The CA8-based symmetric device demonstrated exceptional supercapacitor performance, maintaining an EDLC curve over a high-speed scan (50,000 mV s⁻¹). In this study, a PD of 1500 W kg⁻¹ led to a significant CD of 35 Wh kg⁻¹ and a good cyclic stability of 99% capacitance retention after 20,000 cycles at 5 A g⁻¹. Because of this, electrodes made of porous carbon aerogel could be good candidates for use in SCs because they are very stable over many cycles and have high specific capacitance. Recently, sweet flag (Acorus calamus)-derived activated porous carbon (e.g., C750), was shown to have a high capacitance of 354.4 F g⁻¹ and a high energy density of 47.2 Wh kg⁻¹ after going through the carbonization process [86]. C750 generated a rectangle curve at various scan speeds between 5 and 200 mV s⁻¹, demonstrating the behavior of the EDLC (Figure 8i). It was discovered that the C750 could maintain a capacitance of 245.5 F g⁻¹ even at a high CD of 10 A g⁻¹ (Figure 8j). Despite this, the symmetric device of the C750 produced specific capacitances of 184.8 and 40.0 F g⁻¹ at the CD of 1 and 10 A g⁻¹, respectively. Furthermore, during the stability testing, the device retained 91% of its capacity after being put through up to 1000 cycles of performance testing (Figure 8k). However, N-doped pre-carbonized sweet flag carbon that was hydrothermally produced (such as NDBC) could not reach a high specific capacitance (119 F g⁻¹) at a CD of 1 A g⁻¹. In light of this, a high specific capacitance of the activated carbon might be accomplished via the use of a hypothetical carbonization method carried out at a high temperature (700 °C) in comparison to the preparation of N-doped carbon samples by the hydrothermal process at a low temperature (160 °C). Additionally, it was discovered that C750 has high ED and PD values of 47.2 Wh kg⁻¹ and 16,000 W kg⁻¹, respectively (Figure 8l).

The activated porous carbon from heavy bio-oil (e.g., HBPC4-800) showed significant specific capacitance (336 F g⁻¹ at 0.5 A g⁻¹) under the electrolyte of KOH, and this kind of electrode demonstrated still high capacitance (274 F g⁻¹) at a higher CD of 20 A g⁻¹ [73]. The fabricated symmetrical device with HBPC4-800 showed higher specific capacitance in KOH (70.2 Fg⁻¹) when compared to other electrolytes Na₂SO₄ (46.7 F g⁻¹) and TEABF4/PC (36.7 F g⁻¹) at a CD of 0.5 A g⁻¹ because KOH’s ion sizes were smaller than those of other electrolytes, which are easier and more readily adsorb to the electrode surface. The ED of the device in KOH (9.54 Wh kg⁻¹) was lower than that of the device in other electrolytes, such as Na₂SO₄ (16.1 Wh kg⁻¹) and TEABF4/PC (30.7 Wh kg⁻¹). It is possible to develop high-energy-density devices using these typical electrolytes. The device’s stability was evaluated at a CD of 5 A g⁻¹ for all electrolytes, and after 10,000 cycles, it retained a capacitance of 92–98%.

Recent research has shown that adding acid orange 45 (AO 45) to sulfuric acid (H₂SO₄) as a redox additive can improve the effectiveness of energy storage in SCs made with mesoporous carbon nano-silica (DMC) electrodes [67]. Based on the CV analysis, the electrode displayed a rectangular curve while employing an electrolyte system without AO 45 due to the formation of an electrolyte-electrode interface. Then, utilizing AO 45 in electrolytes, pseudocapacitance behavior was identified by creating a redox peak. However, increasing the concentration of AO 45 in the electrolyte over 0.01 M reduces super-capacitive performance due to ion aggregation. Thus, the optimal redox additive concentration in the electrolyte (H₂SO₄) was 0.01 M. The system’s discharge time was greater when A.O. 45 was combined with H₂SO₄ but less when only H₂SO₄ was used. The specific capacitance value of a supercapacitor system in H₂SO₄ + AO 45 was greater than that of a supercapacitor system in H₂SO₄ at a CD of 0.5 A g⁻¹. As the CD increased, the specific capacitance decreased. The DMC symmetric system was tested for stability over 2000 cycles and showed just 74% initial capacitance. The symmetric system acquired an ED of 11.4 Wh kg⁻¹ in H₂SO₄ + AO 45 but only 4.1 Wh kg⁻¹ in H₂SO₄ alone. The findings revealed that AO 45 significantly impacts the DMC’s supercapacitor performance.

The form of a quasi-triangular curve on oxygen-rich lignite-derived porous carbon (such as POPC2) indicated the behavior of both EDLC and pseudocapacitance [64]. The pseudocapacitance was linked to the redox process since oxygen is present to facilitate it. This investigation determined the specific capacitance to be 320 F g⁻¹ at a CD of 1 A g⁻¹. Since the electrode material was made by heating phosphoric acid, the high specific capacitance of POPC2 could be to blame for the higher amount of S_BET and oxygen in the electrode. Fights for the ions to reach completely generate double-layer capacitance, which causes current density to grow with a drop in specific capacitance. The triangle curves from the charge-discharge investigation for the POPC2 showed excellent electrochemical reversibility even at high CDs. From EIS, a straight line was observed at the low-frequency section for the POPC2 revealing good capacitive performance. For the symmetric POPC2-based device, the electrode’s CV curve remains rectangular even at high scan, indicating that electrochemical reversibility may be possible. At CDs of 0.05 and 20 A g⁻¹, the device’s specific capacitance was discovered to be 390 and 225 F g⁻¹, respectively. As a result, the device performs at a high rate since the average capacitance was kept constant while the CD was raised. At a PD of 11.2 W kg⁻¹, the low ED (10.7 Wh kg⁻¹) was discovered. This investigation showed superior cycle performance throughout 25,000 cycles, showing that 88% of the original capacitance had been maintained. The optimized mass ratio of NaOH (24) and thiourea (20), along with the addition of 5% melamine phosphate, was utilized to make heteroatom (N, P, and S) doped bamboo pulp-derived porous carbon (e.g., 850-24-20-5%), which resulted in a respectable specific capacitance of 316 and 290 F g⁻¹ at current densities of 1 and 10 A g⁻¹, respectively [87]. Doped heteroatoms like N, P, and S are obvious candidates for use as support components in the quest for higher capacitance levels. They improve the active sites and make the porous carbon more conductive. The manufactured symmetric device with 850-24-20-5% porousness produced a high ED and PD of 16.3 Wh kg⁻¹ and 489 W kg⁻¹, respectively, and the stability analysis indicated that cyclic tests done up to 5000 cycles revealed 91.2% capacity retention. In addition, the ED and PD of the symmetric device with 850-24-20-5% porousness were found to be high.

3.3. Medium-Specific Capacitance-Based Porous Carbon

A high specific capacitance may also be seen in carbon quantum dots (e.g., CQD) that were hydrothermally synthesized using biomass (spent tea leaves) [88]. The typical CQD carbon sample could offer a specific capacitance of 302 F g⁻¹ at a low current density (CD) of 0.5 A g⁻¹ under the electrolyte of a 1 M H₂SO₄ 3-electrode system. It can also hold a specific capacitance of 274.1 F g⁻¹ at a high CD of 1 A g⁻¹ (Figure 9a). It was seen to have a high energy density (41.9 Wh kg⁻¹) and power density (250 W kg⁻¹) at a current density of 0.5 A g⁻¹, which indicates that the material is capable of storing a large amount of energy (Figure 9c). The tests were carried out to demonstrate the capabilities of the material. It was observed that the CQDs were stable throughout the cycle test, and after 5000 cycles, they had a high capacitance retention of 93.8% (Figure 9b). CQDs’ increased capacitance and ability to retain that capacitance may be attributed to their superior electronic structure, good electrical conductivity, and high structural strength. Wang et al. investigated the impact of electrolytes on the specific capacitance of porous carbon [69]. As a result, various electrolytes, including 6 M KOH and 1 M H₂SO₄, were evaluated using porous carbon made from zinc acetate/phytic acid and activated with KOH (for example, PC-K4). It was shown that the specific capacitance was higher in 6 M KOH (294 F g⁻¹) than in 1 M H₂SO₄ (247 F g⁻¹). In this study, KOH and H₂SO₄ were found to have EDLC and pseudocapacitance behaviors because their CV curves were square and their redox reaction deviations were small. Because the K⁺ ions (0.42 nm) in KOH are smaller and stick to negatively charged electrode surfaces more easily than the sulfate ions (0.58 nm) in H₂SO₄, KOH was shown to have a higher capacitance. According to EIS, the absence of a semicircle curve for the electrode in both electrolytes confirms that a charge transfer could have occurred. Additionally, the electrode’s inner resistance (PC-K4) is low, suggesting that it may have good conductivity. A good specific capacitance was also detected in the low-frequency region as a result of the electrode’s high slope and vertical observation. The specific capacitance was found to be higher at H₂SO₄ than KOH (47 F g⁻¹) at a CD of 1 A g⁻¹ when it was a symmetrical device (PC-K4). In both KOH and H₂SO₄, the device’s ED was only 10 Wh kg⁻¹ at a PD of 230 W kg⁻¹, and after 8000 cycles, 96.57% capacitance retention was attained.

A symmetrical device was recently developed from porous carbon produced from EDTA4Na and activated with KOH (e.g., CNS6) for studying the use of SCs at room temperature (CNS6-RT) and 80 °C (CNS6-80) [71]. The symmetrical CNS6 device had a larger specific capacitance at 80 °C than it did at room temperature (214 F g⁻¹ at 1 A g⁻¹), likely due to the oxygen groups on the electrode having a greater increase in electroactivity. At a CD of 20 A g⁻¹, CNS6-RT demonstrated greater cyclic stability (84.2% of capacitance kept after 60,000 cycles) than CNS6-80 (81.1% of capacitance retention over 8000 cycles), as seen in Figure 9d. It was also noted that the CNS6-80 (122.2 Wh kg⁻¹) has a greater energy density than the CNS6-RT (90.8 Wh kg⁻¹). As a result, a symmetrical device-based supercapacitor may be active and effective while operating at 80 °C. However, it is generally recommended that devices with substantial capacitance be performed at room temperature for possible real-time application. Using the carbonization process, Liu et al. produced oxygen-doped porous carbon from lignite (such as OHPC1) [65]. In this study, the EDLC and pseudocapacitance were validated based on observing the quasi-triangle curve of CV. Additionally, pseudocapacitance was carried out due to the presence of heteroatom (O). At a current density of 0.5 A g⁻¹, the electrode’s specific capacitance reached a maximum of 283 F g⁻¹ (Figure 9e). The EIS verified the optimum capacitance on the OHPC1 as a steep linear form at low frequencies. Testing was performed on an OHPC1-based symmetric device, and the findings revealed that it produced quasi-triangle curves. These curves were maintained even at high scan rates (500 mV s⁻¹), indicating that the device had excellent EDLC properties. The OHPC-1 symmetric device was able to give an energy density (ED) of 16.5 Wh kg⁻¹ at a power density (PD) of 300 W kg⁻¹. However, the poor cycle stability exhibited on this symmetrical capacitor after 10,000 cycles was maintained at only 64.8% at a CD of 5 A g⁻¹ (Figure 9f).

Recently, the 3-electrode electrochemical performance of potassium-treated waste-cotton-generated porous carbon (e.g., PCK) was explored, and a different symmetric device was constructed for further potential usage in practical applications [59]. The rectangular curve with a redox peak indicated that the electrode exhibited both EDLC and pseudocapacitance behavior. At CD values of 0.5 and 10 A g⁻¹, respectively, the specific capacitance of the study was observed to be 273.7 and 203.8 F g⁻¹, confirming the electrode’s adequate capacity. Due to the electrode’s excellent EDLC characteristic and strong ion-diffusion capacity, a straight line was seen in the low-frequency part. The symmetric PCK-based device could also demonstrate a specific capacitance of 145.86 F g⁻¹ at 20 A g⁻¹, indicating a higher rate capability. With a PD of 350 W kg⁻¹, the symmetric device could produce a maximum CD of 9.9 Wh kg⁻¹. Additionally, at a CD of 5 A g⁻¹, the symmetrical device demonstrated remarkable capacitance retention (98%) after 5000 cycles.

Heteroatoms in activated porous carbon-derived balsa wood (e.g., PNOAC) caused a broad EDLC of rectangular CV curves and a pseudocapacitance effect [21]. Thus, this study indicated EDLC and pseudocapacitance. The activated porous carbon (e.g., PNOAC2-600) had quasi rectangle broader CV curves even at higher CV scans (1–100 mV s⁻¹) owing to its high rate scan capabilities, which allow ions to attach to the electrode surface readily. The GCD discharge curve confirms the active electrode and good conductivity of PNOAC2-600. Electrochemical impedance spectroscopy (EIS) showed that much of the vertical linear portion of the line at low frequencies gives the electrode high capacitance performance. If the activation mass ratio of phosphoric acid exceeded two, the porous carbon’s specific capacitance dropped. Thus, PNOAC2-600’s greatest specific capacitance was 263 F g⁻¹ at 0.5 A g⁻¹. As CD increased, ions did not permeate into all holes and formed EDLC on the electrode, decreasing capacitance. For real-world use, PNOAC600-2 built a symmetrical capacitor that had both EDLC capacitance and pseudocapacitance because there were heteroatoms on the electrode. The symmetrical device demonstrated strong reversibility and high electrochemical capacitance because it kept triangular curves even when CD and quasi-triangles grew with voltage, respectively. Furthermore, the device’s stability was discovered to be that it retains a capacitance of 91.4% upon 10,000 cycles owing to the high activity of the PNOAC2-600 and that doping with heteroatoms assisted in soothing the functional groups of the carbon.

On the other hand, Dong et al. made N-doped carbon sponge-based porous carbon (such as NCS), looked at its electrochemical performance, and then made coin-cell devices to put it to use [30]. Due to the presence of porous carbon with N groups, NCS also showed quasi-triangle curves with a small deviation. This analysis showed that the highest specific capacitance was 253 F g⁻¹ at a CD of 0.5 A g⁻¹. The authors indicated that S_BET and N content impacted the specific capacitance of porous carbon. The greater the diameter of the semicircle, the larger the charge transfer resistance was thought to be. Still, NCS demonstrated a lower semicircle in the high-frequency section, demonstrating a low charge transfer resistance. They also showed a higher slope in the low-frequency section, indicating a high electrical conductivity and a higher charge transfer speed (high capacitance), respectively. Additionally, NCS-based coin cells demonstrated high EDLC behavior since the rectangular curves in the CV remained constant even at high scan rates (100 mV s⁻¹), and the device only produced a small amount of ED (4.20 Wh kg⁻¹) at a PD of 250 W kg⁻¹. However, this coin cell (10,000 cycles, 99% capacity retention at 1 A g⁻¹) may achieve high cycle stability. During testing with a two-electrode system, EDTA/Zinc acetate-derived N-doped porous carbon (e.g., NPC2) exhibited a low specific capacitance of 253 F g⁻¹ at 1 A g⁻¹ [70]. In addition, this study observed both EDLC (generation of a rectangular CV curve) and pseudocapacitance behavior (deviation of the rectangular curve due to redox reaction by heteroatoms on the NPC2). Because NPC2 has a greater S_BET, it can still keep its specific capacitance at 223.6 F g⁻¹ even when the CD is 10 A g⁻¹. However, when the current density (CD) increased, the specific capacitance of the NPC2 steadily decreased. It occurred because there were not enough accessible sites. It was revealed that the cyclic stability was extremely good at a CD of 10 A g⁻¹ with a capacitance retention of 100% over 10,000 cycles. Additionally, a recorded ED value was low at 8.7 Wh kg⁻¹ at a PD of 248.7 W kg⁻¹ when the current density was 1 A g⁻¹. In addition, the NPC2 material was evaluated using a three-electrode setup, which demonstrated a high capacitance of 319.6 F g⁻¹ at a CD of 2 A g⁻¹. As a result, the electrode may operate well when used in supercapacitor applications with three electrodes.

Recently, quasi-rectangular CV curves of heteroatom-doped metal-lignin-driven porous carbon (e.g., N-LHPC) showed both EDLC and pseudocapacitance (oxidation-reduction process of the heteroatom functional group) behavior [55]. N-LHPC has the biggest CV-integrated area, indicating that it is suitable for energy storage. The electrode demonstrated an isosceles triangle with modest fluctuations, suggesting pseudocapacitance and EDLC behavior in the charge-discharge study. The specific capacitance of the electrode rose as the CD increased owing to the doping of heteroatoms (N/O), increasing the hydrophilicity of the electrode surface and encouraging the ions to have greater affinity through a large S_BET, which in effect, supports the capacitance. However, at a CD of 0.5 and 20 A g⁻¹, the specific capacitance of the electrode (N-LHPC) was determined to be only 235.75 and 162.4 F g⁻¹, respectively. The electrode’s ion diffusion and charge-movement speeds were validated using EIS, and it was discovered that vertical alignment of the curve indicated improved electron movement and that vertical lines denote the EDLC characteristic of the electrode in the low-frequency series region. Because of the doping with heteroatoms, the electrode had the lowest electron-transfer resistance, which enabled a good specific capacitance. For the supercapacitor application, an LHPC-based symmetric device was looked at. The EDLC characteristics of the electrode show that it has a nearly rectangular CV curve. However, a charge-discharge analysis confirms that the symmetrical device possesses both ELDC and pseudocapacitance features owing to a slight change in the symmetrical curve. At a current density (CD) of 0.5 A g⁻¹ and 10 A g⁻¹, the specific capacitance of an N-LHPC-based symmetric device was measured to be 166 and 71 F g⁻¹, respectively. It shows that N-LHPC has important electrochemical properties. The stability of the symmetric device was tested over 5000 cycles and found to be 81.36%. The device demonstrated that ED and PD changed depending on the electrolyte employed, with greater values recorded when using 1 M Na₂SO₄ and lower values observed when using 6 M KOH.

For SCs, hydrodynamic cavitation (HC) treatment of N-doped porous carbon-derived garlic peel (1:6 HCGP+N) was explored [61]. Accordingly, the charge-discharge analysis revealed that symmetrical triangular curves indicated strong capacitive performance by the electrode owing to the HC effect and quick ion transference throughout the cycle. Due to changes in the CV’s rectangular curves, N-doped porous carbon samples displayed pseudocapacitance behavior, but undoped carbon did not. At a CD of 10 A g⁻¹, N-doped porous carbon such as 1:6-GP + N (212 F g⁻¹) and 1:6-HCGP + N (227 F g⁻¹) had a higher specific capacitance than porous carbon that didn’t have any N-doped. The low-frequency hydrodynamic cavitation-treated porous carbon curves in the EIS research showed a linear trend, indicating superior EDLC characteristics. HC-treated N-doped porous carbon has an ED of 10.19 Wh kg⁻¹ at a PD of 250 W kg⁻¹. The original specific HC-treated N-doped porous carbon capacitances were maintained at 96% even after 5000 use cycles.

The specific capacitance of sesame shell-derived porous carbon (e.g., TMCK) was measured to be 222.7 F g⁻¹ at a current density of 1 A g⁻¹ [89]. Additionally, it retained a capacitance of 135 F g⁻¹ at a maximum current density of 20 A g⁻¹. It was discovered that the cyclic test performed on the TMCK had a high capacity retention (100%) even after being subjected to a total of 20,000 cycles. The TMCK was built as a symmetrical device after the testing conducted in the initial studies. This device also demonstrated great capacity retention (100%) over 20,000 cycles. The ED (6.9 Wh kg⁻¹) was seen in low at the PD of 600 Wh kg⁻¹, and the specific capacitance (222.7 F g⁻¹) under the electrolyte of 2 M KOH was measured in this investigation. Similar to this, porous carbon made from soybean straw that has been heteroatom-doped (e.g., SSL-N/S-K-700) was observed to have a specific capacitance of 220 F g⁻¹ at the CD of 0.5 A g⁻¹, despite having a high S_BET (1457.6 m² g⁻¹) and heteroatom doping [90]. The sample also had excellent cyclic stability, with 98.5% of the capacitance remaining after 10,000 cycles when used with a three-electrode system at 6 M KOH. The sample was made with a symmetric device that was found to have a low specific capacitance and to be able to keep its capacitance even at higher current densities, which were found to be 125 and 92 F g⁻¹ at the CDs of 0.5 and 5.0 A g⁻¹, respectively. The symmetric device obtained an ED and PD of 11.2 Wh kg⁻¹ and 400.0 W kg⁻¹, respectively, at 0.5 A g⁻¹, and the stability of the device made it possible to demonstrate an exceptional performance, which was 99% capacity retention over 10,000 cycles. It might be attributed to the highly porous structure of the porous carbon that was used in the device.

Recently, Tasmanian blue gumwood (TBG) was used to generate hierarchical porous carbon, which was then activated with KOH (ACTBG) [91]. When compared to Na₂SO₄ (152 F g⁻¹) and H₂SO₄ (110 F g⁻¹), AC-TBG demonstrated a high specific capacitance in KOH (212 F g⁻¹). As a result, an electrolyte solution based on KOH could deliver a high specific capacitance on the electrode for supercapacitor applications. AC-TBG was examined by cycling stability at a current density of 1 A g⁻¹, which exhibited 5000 cycles with an initial capacitance of 89%. Additionally, this research noticed a high energy density (18.8 Wh kg⁻¹) at a power density of 4900 W kg⁻¹. The manufactured symmetric device of AC-TBG was evaluated at 1 M KOH for practical application. The results showed that the device had a specific capacitance of 45 F g⁻¹ at 1 A g⁻¹. Additionally, the device was tested for cyclic stability over 15,000 times and showed good initial capacitance. Low specific capacitance (208 F g⁻¹ at 1 A g⁻¹) could be noticed on the heteroatom (N/O/S) doped biomass-derived porous carbon (e.g., ZMB) under 6 M KOH owing to the ZMB’s low S_BET (508 m² g⁻¹) and the constructed symmetric device exhibiting a low ED (2.9 Wh kg⁻¹) value at the PD of 61.7 Wh kg⁻¹ [92]. This sort of porous carbon demonstrated a lengthy procedure that resulted in significant time consumption and a lack of effectiveness in maintaining high capacitance under 6 M KOH. In contrast, a ZMB-based symmetric device conducted in an electrolyte of 1 M Et₄NBF₄/propylene carbonate showed good capacitance (49.9 F g⁻¹ at 1 A g⁻¹) and a high ED (12.5 Wh kg⁻¹) at a PD of 6.8 W kg⁻¹. Over 20,000 cycles at 5 A g⁻¹, the symmetric supercapacitor preserved 100% initial capacitance. So, the electrolyte system may affect the specific capacitance of the supercapacitor device, and 1 M Et₄NBF₄/propylene carbonate could be a potential electrolyte to improve the electrochemical properties of the electrodes to get a high capacitance and other useful parameters (ED, PD, and cycle stability). Walnut shell-derived porous activated carbon and its modification with 0.005 M NiO (e.g., WS-dAC/NiO-0.005) demonstrated a significant specific capacitance of 205 F g⁻¹ at 0.5 A g⁻¹. Still, the cyclic stability of the prepared sample was tested over a low range of up to 1000 cycles and revealed a 98% capacitance retention [93]. As a result, NiO did not demonstrate effective modification of activated porous carbon to achieve the reasonable specific capacitance of the carbon produced from walnut shells.

3.4. Low-Specific Capacitance-Based Porous Carbon

Chemically activated and N-doped biomass-derived porous carbon (e.g., N-HPC) was recently generated from the carbonization process using KOH and melamine, respectively [94]. The specific capacitance of the N-HPC was investigated in three- and two-electrode systems under 6 M KOH, as well as a two-electrode system under 1 M Na₂SO₄. At a current density of 0.5 A g⁻¹, the 6 M KOH-based three-electrode system had a high specific capacitance (220.5 F g⁻¹) compared to other electrode systems like the 6 M KOH 2-electrode (144.9 F g⁻¹) and the 1 M Na₂SO₄ 2-electrode (103.8 F g⁻¹). The authors did, however, investigate current, power density, and cycle stability for 6 M KOH and 1 M Na₂SO₄ 2-electrode systems. As a result, both 2 electrodes demonstrated strong cyclic stabilities (96% capacitance retention over 15,000 cycles). However, the ED (8.1 Wh kg⁻¹) and PD (187.3 W kg⁻¹) of the 1 M Na₂SO₄ system were greater than those of the 6 M KOH 2-electrode system (5 Wh kg⁻¹ at a PD of 124.8 W kg⁻¹). According to the research findings, a 6 M KOH 3-electrode system could achieve high specific capacitance, and a 1 M Na₂SO₄ 2-electrode system might perform high ED.

EDLC and pseudocapacitance properties were seen in heteroatom-doped porous carbon made from phenolic resin and semi-coke water (SWPR) [81]. In this study, SWPR-700-2.5 had higher CV curves, indicating good capacitance. The charge-discharge investigation showed quasi-triangle curves that illustrate the electrode’s constant reversibility. Still, the specific capacitance (129.5 F g⁻¹) was very low at a current density of 1 A g⁻¹ owing to the electrode’s high porosity and storage regions. According to the investigation, SWPR-700-2.5 electrodes exhibit a high inclination curve at low frequency, indicating good pseudo-capacitance. Quasi-rectangular curves caused EDLC behavior in the symmetric device-based SWPR-700-2.5. From the GCD study, triangular curves maintained with increasing CD showed good charge characteristics and better capacitive performance. EIS was a vertical curve with low frequency, confirming fast ion transport in the electrolyte. The specific capacitance was determined using a symmetric device with values of 170.5 and 117.6 F g⁻¹ at the CD of 0.5 and 20 A g⁻¹, respectively. Hence, increasing CD causes the electrodes’ specific capacitance to decrease. It was discovered that the symmetrical device-based supercapacitor retained 99.4% of its capacity during 10,000 cycles at 20 A g⁻¹. The symmetrical device-based supercapacitor observed an ED of 2 Wh kg⁻¹ at a PD of 124.9 W kg⁻¹. This device’s capacity was determined to have been maintained at 99.4% over 10,000 cycles at 20 A g⁻¹. The SWPR-700-2.5 electrode has heteroatoms and strong electrolyte binding, making strong pseudocapacitance through redox processes.

3.5. Discussion

Table 2 shows the electrochemical findings of several types of porous carbon that have been analyzed using symmetric and asymmetric SCs. When compared to other porous carbons, it is obvious that FeS₂/CoS₂@PC-800 displays greater findings, which achieved an extremely high specific capacitance (3480.4 F g⁻¹) even though it has a low S_BET (468.5 m² g⁻¹). It might be because it contains high functional groups such as N and S. The synthesis of the FeS₂/CoS₂@PC-800 carbon shows a low-cost procedure that may be useful in the commercial application of SCs. So, the raw coal and metal sulfides (FeS₂/CoS₂) used to make porous carbon (FeS₂/CoS₂@PC-800) at 800 °C could be a good electrode for the supercapacitor. However, despite having a relatively low S_BET (228.0 m² g⁻¹), CuO/Cu@C-700 still achieved a very high capacitance (2479 F g⁻¹). The N-group doping was the only kind of doping identified in this combination. As a result, when the porous carbon develops, it may interact with metal-based salts to generate an electrode with a high-performance level. On the other hand, high S_BET materials such as NPC800-4 (3184 m² g⁻¹), 1:6-HCGP+N (3116 m² g⁻¹), SP_LRL (3586 m² g⁻¹), POPC2 (2852 m² g⁻¹), CNS6-80 (3400 m² g⁻¹), ASNPC-1 (3281 m² g⁻¹), and HPC60 (3381 m² g⁻¹) are supposed to exhibit high specific capacitance, but they performed low capacitances such as 443, 227, 373, 320, 290, 386, and 419 F g⁻¹, respectively. Although the S_BET is typically an important factor in supercapacitor performance, there is no clear relationship between it and the specific capacitance of the materials. It could be because not all of the pores in the electrodes allow electrolyte ions to adsorb. Larger S_BETs are more likely to cause electrolyte breakdown during the cycles [90]. As a result, the larger S_BET of porous carbon may not assist in reaching a high specific capacitance. Porous carbons with a high ED, like FeS₂/CoS₂@PC-800, CuO/Cu@C-700, BBC-800, NPC-800-4, OPCN-20, C750, CQD, and CNS6-80, may have a high meso/microporous structure that could help increase energy density and make high energy storage devices. It was found that the 6 M KOH electrolyte employed in experiments had a high capacitance value when compared to Na₂SO₄, H₂SO₄, ionic liquid (IL), and 1 M KOH. As a result, the 6 M KOH electrolyte might be an effective medium for supercapacitor applications. Most of the porous carbons had great cyclic stability, which may have resulted from the porous carbon’s structural stability. As a resul=t, porous carbon has a high mechanical strength, making it potentially beneficial for long-term stability applications and suitable in real time for such applications.

4. Conclusions and Prospects

Producing porous carbon from metal-organic frameworks could be costly and unfriendly. Researchers worldwide are becoming interested in porous carbons since their production is very inexpensive and they have a large S_BET. As a result, the wide availability and cheap cost of porous carbon generated from affordable sources such as biomass, polymers, bio-oil waste, lignite, etc., have increased their popularity. To achieve this, we have researched the literature on the most up-to-date methods for synthesizing porous carbon from the aforementioned inexpensive sources, which will be used in SCs. One could produce heteroatom-doped carbons with the desired property using appropriate precursors, such as thiourea, urea, melamine, or phosphoric acid. Because of their high stability, large S_BET, and layered structure, porous carbons may be an essential electrode substrate in energy production. The customizable surface properties of such carbons might be desirable qualities for an application involving energy storage, and it is also possible that an activation process could form them to build a highly porous structure that could be beneficial for ion transmission in an electrode-electrolyte interface. Porous carbons, which have the potential to be inexpensive, are the most important aspect since they will allow researchers to experiment with developing and testing the efficiency and reliability of the supercapacitor at low risk. Carbon derived from biomass, polymers, and lignite can be produced cheaply, making it a feasible choice for synthesizing porous carbons.

People have become exceedingly intellectual due to the fast rise of industrialization, allowing them to utilize smart technological devices and electric vehicles for prolonged periods. As a result, materials with high energy storage and specific capacitance are in great demand for applications. SCs are preferred over batteries for storing high energies owing to their long-life cycle and high energy density. As a result of that, high-surface-area and porous structured carbon materials with adequate functional groups on their surfaces have substantial capacitance. This review studied porous carbon generated from several low-cost materials as carbon electrodes for supercapacitor applications. It was discovered that an attempt was made to develop an electrode material with a high specific capacitance of 3480 F g⁻¹. Choosing the source material, adjusting the S_BET, selecting a dopant, and selecting an electrolyte might all result in high-performance SCs. The activation process produces a graphitized carbon structure with well-developed porosity, which may be linked to improved charge transfer. A supercapacitor cannot work well without an appropriate design for the electrodes. In addition, porous carbon synthesized via green synthesis should be produced for supercapacitor applications since it is inexpensive and ecologically beneficial.

The high specific capacitance may be seen in materials with a KOH activation temperature doped with heteroatoms. Enhancing the porosity of porous carbons or combining them with other metal salt precursors is possible to make the electrode more effective. Currently, the energy density of SCs is insufficient, and it is extremely rare to find high-energy-density porous carbons in the literature. Doping a material with heteroatoms might potentially boost both the material’s capacitance and energy density. Therefore, surface modifications such as doping or introducing an active functional group may improve the specific capacitance of porous carbon. Although porous carbon materials provide a lot of surface area, their practical uses may be constrained by their low specific capacitance. Therefore, high-surface-area porous materials for pseudocapacitive energy storage need more study. To fill this research gap, scientists are focusing on developing porous carbon to improve the electrochemical characteristics of the electrode by adjusting its surface area, shape, crystallinity, electrolyte affinity, elemental doping, and pore formation. Supercapacitors are characterized by their high energy density, extended lifespan, and quick charging time. However, research shows that porous carbons have a poor energy density, which limits their use in supercapacitors. Therefore, more research is being conducted on hybrid energy storage devices, which combine batteries with supercapacitors (porous carbon electrodes) to address the issues posed by low energy density. It is interesting to note that to achieve rapid electrolyte passage with active sites and fast charges, sources with naturally existing heteroatoms can deliver high performance while also attributing a low-cost procedure, and rapid ion diffusion may be possible from electrolyte to electrode. This results in high energy, power density, and specific capacitance. In addition, the use of a two-electrode system should be developed so that precise outcomes may be obtained. However, there are still a lot of difficulties to overcome when it comes to specific capacitance, long-life stability, and high energy density-based SCs. It is possible to consider the potential outcomes of future studies on SCs, namely those focusing on high energy density and a long-term life cycle. Additionally, the prepared electrode could have high efficiency, stability, environmental friendliness, adaptability, and low cost, and their safety is discarded. Supercapacitors may have vast potential applications; however, process, manufacturing cost, large-scale, economical usage, standards, and other challenges limit their utilization. Large-scale applications in real-world work settings are rare except in certain exceptional cases. Applications of supercapacitors on a large scale need many scientists to work together to address these problems diligently. These are also some important concerns in the production of SCs, and in addition to this, they are also key facts of close research direction. As a result, research into SCs concentrates on low-cost carbon materials, the composition of materials, the scale of synthesis, and the setting up of an approach toward industrial production.