Phase Change Materials in Electrothermal Conversion Systems: A Review

Abstract

Green energy harvesting is one of the most important and evolving research areas. Solar energy is an inexhaustible and environmentally friendly energy source, and phase change materials (PCMs) are capable of improving photovoltaic devices by heat storage and could have a positive impact on sustainable energy utilization. This review presents the current state of the art on PCMs and their modifications for electrothermal energy conversion applications. The paper focuses on PCMs characteristics and their properties required for electrothermal energy conversion systems, and it presents various methods of PCMs modification intended to obtain multifunctional systems based on these materials as well as electrothermal conversion and energy storage mechanisms and selected applications. The goal of this review is to present different types of PCM modifications to obtain multifunctional PCM-based systems for electrothermal energy conversion.

1. Introduction

Global energy consumption is continuously increasing, driven by population growth, industrialization, and economic development. This rising demand, coupled with a significant dependence on fossil fuels, has led to serious environmental concerns, including greenhouse gas emissions and climate change. The projections indicate that global energy consumption will continue to increase in the coming years, further exacerbating the challenges [1,2,3]. According to the International Energy Agency (IEA), global energy demand is projected to increase by approximately 50% between 2020 and 2050 [4]. The building sector plays an essential role, accounting for 40% of global total energy consumption. Consequently, the decarbonization of the building sector is of paramount importance in mitigating the adverse effects of climate change. To address this challenge, it is imperative to reduce energy consumption from conventional sources, necessitating development based on sustainable technologies, passive systems, or net-zero energy solutions [2,5,6,7]. To mitigate environmental impacts and ensure a sustainable energy future, it is an urgent need to investigate and adopt alternative energy sources, particularly renewable options, with solar energy currently playing a predominant role. Thermal energy storage (TES) systems, which store excess heat for later use, represent a promising solution for more efficient utilization of solar energy. By integrating TES with solar and photovoltaic (PV) systems, intermittency issues can be addressed, enabling a more reliable and consistent energy supply. Additionally, TES can play a crucial role in the decarbonization of various sectors, including industrial processes, residential heating and cooling, and electricity generation. This integration of TES with renewable energy systems has the potential to significantly improve energy efficiency and reduce dependence on fossil fuels in multiple applications [8,9,10,11,12,13,14,15].

The issue of energy conversion and the possibility of storing heat is becoming of increasing interest these days. Implementing these technologies requires a multifaceted approach, including advanced materials, innovative system designs, and optimized control strategies. Research efforts should focus on improving the efficiency, cost effectiveness, and scalability of TES systems to facilitate their widespread adoption in all industries. Phase change materials (PCMs) represent a promising group capable of reversibly storing and releasing substantial thermal energy by utilizing phase transitions, particularly solid–liquid and solid–solid phase transitions. The use of latent heat in phase transition for thermal energy storage allows for the storage of large amounts of heat in a small mass and volume of material [15,16,17]. The heat storage capacity of PCMs can be 5–14 times greater per unit volume compared to sensible heat storage materials, and one of their main advantages is the ability to absorb and release heat at a nearly constant temperature [18,19,20]. These materials have been known for over 50 years. Significant breakthroughs have since been made in PCM materials engineering, including the use of organic materials [16,21,22,23]. The most important characteristic for practical PCM application is its phase change energy and the temperature range in which it occurs. When selecting a PCM for a specific application, the operating temperature of the material must be considered. One of the most important aspects but one that is often overlooked by researchers is resistance to thermal degradation, which translates into cyclic stability and the ability to convert energy in countless cycles and phase transitions. Undesirable changes also include PCM composition instability, phase separation, and evaporation of PCM components.

This publication aims to focus on the characteristics of PCMs and their properties. The most important issues related to application of PCMs in electrothermal conversion systems, such as mechanism of energy conversion and storage in PCMs, PCMs modification with carbon nanotubes, graphene, graphite, carbon aerogels and MXenes, multifunctional PCMs capable of electrothermal conversion and storage, as well as applications and future trends, are herein presented.

2. Energy Conversion and Storage Mechanism in PCM-Based Materials

The electrothermal conversion mechanism in PCMs involves the efficient conversion of electrical energy into thermal energy, which is subsequently stored as latent heat during the phase transition of the PCM. In the initial phase, upon application of voltage to the PCM, the temperature rapidly increases, and the converted heat from electrical energy is stored as sensible heat, while the PCM itself remains in the solid state. The electrothermal conversion in PCMs is primarily driven by the Joule heating effect, which is the process of heat generation due to the flow of electric current through a resistive material. When an electric current flows through a conductive PCM, the moving electrons collide with the atoms/molecules of the material, transferring kinetic energy to them. This kinetic energy manifests itself as vibrations of the atoms/molecules, generating heat within the PCM [24,25,26,27].

The amount of heat generated by the Joule heating effect is proportional to the square of the current and the electrical resistance of the material, as described by the following equation:

Q = I²Rt

(1)

where Q is the heat generated, I is the electric current, R is the electrical resistance, and t is the time duration.

As the temperature continues to rise, the PCM reaches its melting point, and a tilted temperature plateau is observed. During this stage, the PCM undergoes a solid–liquid phase change, and the converted heat from electrical energy is stored as latent heat. Once the phase transformation process is complete, the temperature rises rapidly again until equilibrium is reached and the transformed heat is stored as sensible heat. During the exothermic (heat releasing) process, as soon as the applied voltage is removed, the temperature rapidly decreases until it reaches the freezing point of the phase change materials (PCMs). At this point, another temperature plateau is observed, which corresponds to the crystallization process occurring within the PCMs. There are the number of factors affecting electrothermal conversion. A critical voltage is required to initiate the electrothermal conversion process. A higher applied voltage results in higher conversion efficiency due to a shorter phase-transition period and lower heat loss. The electrical conductivity itself is important. The inherent low electrical conductivity of PCMs (10⁻⁵ to 10⁻¹⁰ S/cm) limits their electrothermal conversion capability. Therefore, it is crucial to increase the electrical conductivity by adding conductive fillers or creating composite structures. Materials with greater electrical conductivity allow the electric current to flow more easily. When subjected to the same voltage, these highly conductive materials generate more heat through the Joule heating effect compared to less conductive materials. In turn, the thermal conductivity of PCM materials affects the rate of thermal charging and discharging during the phase transition. Materials with superior heat-conducting properties facilitate faster energy absorption and dissipation during thermal storage processes. This enhanced heat transfer capability naturally contributes to improved efficiency in energy conversion systems. Strategies to increase thermal conductivity, such as the use of high thermal conductivity fillers or the creation of porous structures, can improve overall performance [27,28,29,30,31,32].

Generally, the electrothermal conversion and storage efficiency (η) can be calculated through following equation [27]:

η = mΔH/UIt

(2)

where m is the total mass, ΔH is the phase change enthalpy, U the applied voltage, I the applied current, and t is the complete phase-transition time.

PCMs store thermal energy through the latent heat of phase transition. When heat is supplied to a solid PCM, its temperature rises until it reaches the phase-transition point. At this stage, the PCM starts to absorb a large amount of heat without any significant increase in temperature. This heat is absorbed as latent heat of phase transition, causing the phase change. The amount of heat absorbed during this phase transition is directly proportional to the latent heat of the phase transition of the PCM. The mechanism of latent heat storage involves a transformation in the state of the storage medium due to the absorption or release of heat from the system. This transformation occurs at a nearly constant temperature, allowing for the preservation of an adequate heat transfer temperature over a specified duration. The total heat accumulated, denoted as Q_PCM throughout this process, can be expressed as follows:

$$Q_{PCM}={\int }_{T_i}^{T_m}{m}_{PCM}·{Cp}_{sPCM}·dT+m_{PCM}·L+{\int }_{T_m}^{T_{fin}}{m}_{PCM}·{Cp}_{IPCM}·dT$$

(3)

$$Q_{PCM}=m_{PCM}·{Cp}_{sPCM}·\left(T_m-T_i\right)+m_{PCM}·L+m_{PCM}{·Cp}_{IPCM}·\left(T_{fin}-T_m\right)$$

where Q_PCM is the amount of heat stored; m_PCM is the mass of the phase change material; Cp_sPCM and Cp_lPCM are the specific heat capacity of the accumulation material in the solid and liquid phases at the average temperature, respectively; T_m is the melting point of a phase change material; and L is the heat of fusion [33].

The modeling of thermal performance in phase change materials is the focus of considerable discourse and scientific investigation. Numerical methods such as finite-difference or finite-element methods are commonly employed to solve these equations under various boundary conditions. The numerical approach typically involves discretizing time and space to track the changes in temperature and phase fractions over time. The predominant approaches to modeling the latent heat of PCMs involve fixed grid techniques, which encompass the heat capacity method and the enthalpy method. The heat capacity method can be further classified into the square heat capacity method, the triangular heat capacity method, and the differential scanning calorimetry (DSC) heat capacity method [15,34,35,36,37,38]. Hu et al. [39] in their research emphasized the utilization of the quadratic method to model the heat capacity curve in their research. They asserted that this method is both rapid and straightforward to implement numerically, making it the most frequently employed technique in numerical simulation tools and scientific investigations. Furthermore, many manufacturers of materials such as construction products provide only a limited set of key parameters regarding the properties of PCMs due to a lack of standardized metrics. From the essential parameters, including the melting/freezing temperature range, the maximum melting/freezing temperature, the specific heat capacity without phase change, and the total latent heat, a simplified heat capacity curve can be easily deduced. The fundamental equations applied to PCMs exhibiting stable shapes in simulations are the Navier–Stokes momentum equation, the continuity equation, and the energy equation. The calculation of heat capacity is achievable through a quadratic function, which is articulated in the subsequent equation:

$$Cp\left(T\right)=\left\{\begin{array}{cc}{C}_s& T<T_s\\ {\displaystyle \frac{C_s+C_l}{2}}+{\displaystyle \frac{L}{T_l-T_s}}& T_s<T<T_l\\ C_l& T>T_l\end{array}\right\}$$

(4)

where C_l is the specific heat capacity of PCM in liquid phase; C_s is the specific heat capacity of PCM in solid phase; L is the latent heat; T_l is the PCM melting temperature; T_s is the PCM clotting temperature.

The method described for solving Equation (4) has notable limitations, particularly regarding convergence. If the time step is excessively large or if the phase change range is minimal, the solver may fail to converge. This can lead to a phenomenon known as a “step-jump”, where critical phase transitions are overlooked due to skipping temperature ranges during calculations. To mitigate this problem, an alternative approach involves redefining heat capacity using a triangular function that incorporates hysteresis effects [39,40,41,42]. The equations governing this triangular heat capacity curve are presented in the following equations:

$$Cp\left(T\right)=\left\{\begin{array}{cc}{C}_s& T<T_s and T>T_l\\ C_s+M+{\displaystyle \frac{2M}{T_l-T_s}}(T-{\displaystyle \frac{T_s-T_l}{2}})& T_s\le T\le {\displaystyle \frac{T_s+T_l}{2}}\\ C_s+M-{\displaystyle \frac{2M}{T_l-T_s}}(T-{\displaystyle \frac{T_s-T_l}{2}})& {\displaystyle \frac{T_s+T_l}{2}}\le T\le T_l\end{array}\right\}$$

(5)

where M is the melting peak factor.

The works of Musial et al. [14,43] also show an interesting approach to the calculation of modeling the functioning of heat accumulators. The following equation presented by the authors allows the determination of the heat flux flowing through an anisotropic composite:

$$Q_{PCM}=\left\{\begin{array}{cc}{A}_O{\int }_{t=1}^{t=O}{q}_{r}dt& T_p<T_l\\ m_L{\int }_{t=O}^{t=n}\Delta H_{E.r}dt& T_p=T_l\\ A_O{\int }_{t=1}^{t=O}{q}_{r}dt& T_p>T_l\end{array}\right\}$$

(6)

where T_l is the phase change temperature; T_p is the sample temperature; A_O is the external surface of the PCM sample; q_r is the heat flux density flowing through the sample from the PCM; m_L is the mass of the PCM being melted; ΔH_E.r is the PCM melting/solidification heat.

Organic PCMs, such as paraffins and fatty acids, have relatively high latent heat, allowing them to store a significant amount of thermal energy in a small volume. Once the PCM has completely transformed to another phase, any additional heat supplied will raise the temperature of the PCM, storing heat through sensible heating. However, the heat storage capacity in this sensible heating mode is much lower than during the phase change process [44,45,46,47,48]. When the PCM starts losing heat to the surroundings, its temperature drops until it reaches the phase-transition point, which is slightly lower than the phase-transition point connected to heating due to supercooling effects. At this stage, the PCM releases the stored latent heat and undergoes a phase change without a significant temperature drop. The cyclic phase-transition process allows organic PCMs to efficiently store and release thermal energy through their latent heat, making them attractive for various thermal energy storage applications. It is important to note that the performance and efficiency of PCMs can be influenced by factors such as thermal conductivity, phase separation, and supercooling effects, which are active areas of research to enhance their practical applications [44,49,50,51,52,53]. Summary of transformations regarding electrothermal conversion process and storage process is shown in Figure 1.

3. Overview of PCMs Capable of Electrothermal Conversion and Storage

The need to apply technological change in the context of the rise of renewable energy over conventional energy brings with it inevitable research developments. PCMs are substances that can store and release large amounts of thermal energy during their phase transitions, typically between the solid and liquid states. They are most often used for thermal energy storage rather than for direct conversion of thermal energy to electrical energy or vice versa. However, PCMs can be integrated into thermal energy storage systems that work in conjunction with other components to enable conversion between thermal and electrical energy [27,54,55,56]. These materials, due to their unusual properties and enormous potential, have become the topic of many scientific works. The following paragraphs outline the achievements in the development of PCMs, taking into account the potential for complex modification to enhance the performance of the described applications containing PCMs.

The chemical structure plays a crucial role in determining their thermophysical properties. Properties such as melting point, latent heat, thermal conductivity, and chemical stability govern the suitability of PCM for specific applications, including thermoelectric conversion and thermal storage. Based on the general chemical structure, PCMs can be divided into three main categories: organic, inorganic, and eutectic [57,58,59,60,61,62].

Inorganic PCMs include salt hydrates, metals, and metal alloys. Salt hydrates are the most common type of inorganic PCMs, consisting of metal salts such as nitrites, sulphates, phosphates, carbonates, chlorides, and acetates bonded with water molecules (e.g., CaCl₂·6H₂O and Na₂HPO₄·12H₂O). During melting, salt hydrates partially or completely lose their water molecules by absorbing latent heat. During solidification, the water molecules reattach, releasing latent heat [63,64,65,66,67]. Metals such as aluminum and copper and metal alloys can also be used as inorganic PCMs, particularly for high-temperature applications. These materials have high thermal conductivity and high latent heat of fusion per unit volume, which is beneficial for efficient heat transfer during the phase change process [68,69,70,71]. Generally, inorganic PCMs have a higher heat storage capacity per unit volume compared to organic PCMs as well as a higher thermal conductivity, and they are often nonflammable. On the other hand, the use of inorganic PCMs is associated with certain drawbacks, such as corrosiveness, phase segregation, and supercooling issues, affecting heat storage capacity, the phenomenon of the higher vapor pressure and volume change during the phase transition, or variable chemical stability. The characteristics and costs often limit the use of inorganic PCMs for industrial processes or specialized thermal management systems [72,73,74,75].

Organic PCMs are typically derived from paraffin waxes, fatty acids, esters, alcohols, and their eutectic mixtures. These materials have moderate to high latent heat of fusion, ranging from around 100 to 250 J/g, which allows them to store significant amounts of thermal energy during phase change. Their phase change temperatures can range from around −20 °C to 150 °C, covering a wide range of applications. Generally, organic PCMs are noncorrosive, chemically stable, and have good thermal and chemical stability over repeated phase change cycles [59,76,77,78,79]. Especially promising in recent years have become electrothermal conversion PCMs (EPCMs). These are a class of advanced functional PCMs that can directly convert electrical energy into thermal energy through Joule heating or other mechanisms. They typically consist of composite materials where the PCM is combined with electrically conductive additives. The performance of PCMs for electrothermal conversion is influenced by several factors, including the type and concentration of conductive additives, the dispersion and interfacial interactions between the additives and the PCM, and the overall structure and morphology of the composite. Optimization of these factors is essential to achieve high energy conversion efficiency, thermal conductivity, and shape stability [80]. The current limitations of PCM materials are the efficiency of such PCM systems and their lifespan. Some PCM batteries that are enclosed in special containers cannot degrade due to the exclusion of oxygen from the system. The density of the systems is also important. Each system of scaffolding, stabilizers, conductive modifiers, or the housing itself systematically reduces the efficiency of the entire system because these additives themselves do not participate in energy storage and transformation but increase the volume and mass of the system.

In general, the most promising PCM modifications for electrothermal conversion include the following:

• Nano-enhanced PCMs: Adding nanoparticles (e.g., carbon nanotubes or metal oxides) increases the thermal conductivity, improving heat transfer rates. Studies show significant increases in conductivity (up to several hundred percent) with certain nanoparticle loadings;
• Composite PCMs: Combining PCMs with supporting matrices (e.g., metal foams or porous materials) improves structural stability, prevents leakage, and can improve thermal conductivity. Evidence includes improved shape stability and reduced supercooling in composite PCMs.

These modifications address key PCM limitations like low thermal conductivity and leakage, leading to more efficient and reliable electrothermal systems. The modification of composite PCMs results in the formation of an internal conductive network, which facilitates the movement of electrons. As electrical current travels through these conductive pathways, Joule heating occurs due to collisions between electrons and other particles or molecular groups. During this process, the PCM absorbs and stores the generated heat as latent heat. This mechanism enables the composite materials to perform two functions: converting electrical energy into thermal energy and storing that energy for later use [27,80,81,82]. Based on their ability to undergo electrothermal conversion and chemical structure, this publication will focus on describing the two main, most promising groups: PCMs modified with MXenes and carbon-based materials (Figure 2). There are other techniques that can also be used in combination, such as metal foams and expanded surfaces [83,84].

3.1. PCMs Modified with Carbon-Based Materials

PCMs modified with carbon-based materials have emerged as promising candidates for electrothermal conversion and heat storage applications because of their unique properties and versatile structures. These materials typically consist of a carbon scaffold or matrix that encapsulates or supports PCM, enabling efficient conversion of electrical energy to thermal energy and vice versa. The carbon component can take various forms, including carbon nanotubes (CNTs), graphene, biomass-derived carbon, graphite, highly graphitized carbon, and metal–organic framework (MOF)-derived carbon structures. The efficiency of these carbon-based PCM systems depends on factors such as the type and orientation of carbon fillers, their distribution within the PCM matrix, and the overall composite structure. These carbon-based scaffolds address several key challenges in PCM technology, such as improving thermal conductivity, providing electrical conductivity for direct Joule heating, and improving shape stability. The electrothermal conversion process in carbon-based PCMs involves the application of electrical current to the conductive carbon network, which generates heat through Joule heating. This heat is then efficiently transferred to the encapsulated PCM, where it is stored as latent heat during phase transition. The high thermal conductivity of the carbon scaffold significantly improves heat transfer within the PCM, addressing the inherent low thermal conductivity of many organic PCMs. Furthermore, the electrical conductivity of the carbon network enables direct electrothermal conversion, allowing for efficient energy input and potential thermoelectric applications. PCMs modified with carbon-based materials also offer advantages such as increased surface area for heat transfer, mechanical support to prevent PCM leakage, and, in some cases, additional functionalities like electromagnetic shielding. However, challenges remain to optimize the carbon–PCM interfaces, enhance the energy density, scale production, and ensure long-term stability over multiple thermal cycles. As research in this field continues to advance, PCMs modified with carbon-based materials are poised to play a crucial role in addressing the challenges of efficient energy conversion and storage in various technological domains, from energy management to electronic thermal regulation [27,80,85,86].

3.1.1. PCMs Modified with CNTs

Carbon nanotubes (CNTs) possess exceptional mechanical strength, electrical conductivity, and thermal properties, making them highly attractive for improving the performance of PCMs [31,85,87]. CNT/paraffin composites have emerged as promising materials for electrothermal conversion and energy storage applications. These composites leverage the excellent thermal and electrical conductivity of CNTs along with the high latent heat storage capacity of paraffin wax. The degree of enhancement depends on various factors, including CNT concentration, type, and dispersion method. For pure paraffin, the thermal conductivity is typically around 0.2–0.25 W/mK. With the addition of CNTs, this value can be significantly increased. The addition of 2.0 wt.% multi-walled carbon nanotubes (MWCNTs) to paraffin increased the thermal conductivity by 35.0% in the solid state and 40.0% in the liquid state [88]. Another research reported thermal conductivity enhancements of up to 76.5% for a composite with 0.3 wt.% surface-modified MWCNTs compared to pure paraffin [89]. In contrast, the use of ultra-long CNTs by Kuziel et al. [90] resulted in increased thermal conductivity of up to 161% compared to pure paraffin. The CNTs form conductive pathways within the paraffin matrix, enabling efficient Joule heating when an electric current is applied. This mechanism allows for effective conversion of electrical energy into thermal energy, which is then stored as latent heat in the paraffin-based PCM. Additionally, CNT/paraffin composites can be designed as form-stable materials, maintaining their shape even when the paraffin melts, which is crucial for preventing leakage and ensuring long-term stability. The preparation of these composites involves mixing CNTs with paraffin at temperatures around 60–65 °C, above the melting point of paraffin. Although CNTs improve thermal conductivity, they can slightly reduce the latent heat capacity of the composite. However, the composites still retain a high latent heat storage capacity. Unmodified paraffins typically have a latent heat capacity of around 200 J/g, while CNT/paraffin composites maintain latent heat capacities in the range of 88.5 to 160 J/g, depending on the paraffin grade and CNTs concentration. The phase change temperature of CNT/paraffin composites remains similar to those of pure paraffin, typically in the range of 45–65 °C, depending on the specific grade of paraffin used. In addition, the incorporation of CNTs improves the thermal stability of the composites, as evidenced by the higher decomposition temperatures observed in thermogravimetric analysis (TGA). These improvements in thermal conductivity and stability position CNT/paraffin composites as effective materials for thermal management and energy storage applications [81,87,88,90,91,92,93].

Also, the significant potential of CNTs and poly(ethylene glycol) (PEG) composites for electrothermal conversion and energy storage applications was demonstrated. These materials exhibit high latent heat capacity (up to 158.3 J/g), efficient electrothermal conversion (with energy storage efficiencies reaching 92.3%), and improved thermal conductivity (up to 252% increase). CNT/PEG composites can operate at low voltages (1.5–2.0 V) with conversion efficiencies of 58.3–70.2%, and demonstrate good thermal stability with minimal changes in phase change enthalpy and electrothermal conversion efficiency after multiple thermal cycles. Some composites also show potential for photothermal conversion, with energy storage efficiencies of up to 85.6%. The incorporation of cellulose nanofibers modified with chitosan has been reported to create a three-dimensional network structure enhancing the composite’s performance, as presented in work by Kiu et al. [94] (Figure 3).

The CNFs underwent modification and cross-linking with chitosan, resulting in the formation of a three-dimensional network structure that significantly enhances the support for the CNF/CNT)/PEG composites. Importantly, an increase in voltage correlates with a rise in the electrothermal energy conversion efficiency of these composites, as illustrated in Figure 3c,d. Specifically, when the voltage was elevated to 10 V, the electrothermal energy conversion efficiency reached 92.3%. Furthermore, Figure 3e demonstrates that the heating rate of the CNF/CNT/PEG composites at 10 V (26.8 °C/min) was substantially greater than that observed at 6 V (9.93 °C/min) over a duration of 3 min. This enhancement is attributed to the higher voltage, which shortens the phase change duration and reduces convective heat loss from the composite to its surroundings, thereby facilitating an increased heating rate. These properties, combined with their multifunctional capabilities, make CNT/PEG composites promising candidates for various thermal management and energy storage applications [27,94,95]. The brittleness of composites dedicated to electrothermal conversion is often a characteristic that limits their application despite obtaining good values of key physicochemical properties. Excessive rigidity of the material can, among other things, cause problems in creating good contact with the surface during installation at the site of intended use. Therefore, the development of materials with sufficient flexibility has also become crucial. Polyurethane-based flexible and conductive phase change composites (PCCs) have emerged as promising materials for energy conversion and storage applications, combining the advantages of polyurethane’s flexibility and solid–solid phase transitions with enhanced thermal conductivity and energy storage capabilities. These composites incorporate functionalized CNTs into the polyurethane matrix, resulting in significant improvements in thermal conductivity and solar-driven thermal conversion efficiency. The incorporation of CNTs can increase thermal conductivity by up to 2.3 times with only 5% weight content, while the efficiency of photothermal energy storage can reach as high as 85.89% under specific irradiation conditions. Furthermore, the integration of graphene oxide (GO) into polyurethane-based solid–solid PCMs has been shown to further enhance energy storage capacity and photothermal performance. The combination of GO and polyurethane creates a composite material with improved thermal conductivity, latent heat storage, and solar energy absorption capabilities [87,96,97,98].

3.1.2. PCMs Modified with Graphene

Recent advances in graphene-based PCCs have shown promising developments for thermal energy storage and management applications. These composites combine the exceptional thermal properties of graphene with the energy storage capabilities of PCMs. Graphene, known for its high thermal conductivity, enhances heat transfer within the composite, addressing one of the primary limitations of organic PCMs, their poor thermal conductivity. The incorporation of graphene into PCMs such as PEG and paraffins has been shown to significantly improve thermal conductivity and overall heat management efficiency [99,100,101,102,103]. For instance, graphene oxide (GO) has been used to encapsulate PCMs, preventing leakage during the solid–liquid phase transition, another common issue with organic PCMs [104]. The latent heat capacity of these composites can reach 142 J/g, which is approximately 98% of pure PCM, indicating minimal compromise in energy storage capacity despite the addition of graphene. Notably, after undergoing 200 cycles of heating and cooling, both the latent heat values exhibited no significant changes, indicating that the prepared PCM composites maintained excellent stability. Furthermore, these composites exhibit excellent thermal stability, maintaining their performance over numerous heating and cooling cycles. The integration of CNTs alongside GO in PCCs has been explored to create electrothermal conversion materials, combining the benefits of electrical conductivity with phase change properties in a study by Guo et al. [100]. The measured thermal conductivities of the obtained GO/CNTs/PEG composites were as high as 0.46 and 0.45 W m⁻¹ K⁻¹ at PEG contents of 70 and 78 wt.%, respectively, which is a 2-fold increase with respect to the pure PCM organic material (Figure 4). Moreover, the composites enable effective self-heating through Joule heating, with temperature increases correlating to applied voltage. For instance, at an applied voltage of 5.4 V, the temperature reaches 32 °C, while at 7 V, it rises to 56 °C. The Joule heat storage efficiency is about 70% at 6.6 V, decreasing to 63% at higher voltages due to thermal losses. In another study, Yang et al. [105] developed scaffolds composed of boron nitride (BN) micro-sheets interconnected with reduced graphene oxide (rGO) to create a thermally conductive framework for encapsulating PEG-based PCMs. This rGO/BN scaffold, referred to as rGBP, not only demonstrated significant thermal conductivity and structural stability but also exhibited effective electrothermal conversion capabilities. The synthesis of rGO/BN involved initially bridging BN with GO sheets through freeze drying to produce a GO/BN composite designated as GBP. Following this, microwave irradiation was applied to facilitate the in situ reduction of GO within GBP to yield rGBP. Comparative analysis revealed that rGBP outperformed both GBP and pre-treated GBP (pGBP) in terms of heating and cooling rates, achieving an enhanced thermal conductivity of 1.06 W/(m·K) at a BN loading of 14.4 wt.%. Furthermore, the electrothermal conversion efficiencies of rGBP reached the impressive value of 87.9%. The electrothermal assessment indicated a rapid temperature increase upon applying a constant voltage of 7 V to rGBP, highlighting its swift response time in electrothermal applications.

3.1.3. PCMs Modified with Graphite

Graphite/PCM composites have demonstrated significant enhancements in thermal conductivity, electrical conductivity, and overall stability compared to conventional PCMs. These improvements address the limitations of traditional PCMs, making them more suitable for thermal energy storage and electrothermal conversion applications. The study by Nishad et al. [106] reported that paraffin wax/expanded graphite (PW/EG) composites achieved thermal conductivity enhancements up to 677 times in the axial direction and 22 times in the radial direction compared to pure paraffin wax. This anisotropic thermal conductivity is a characteristic feature of graphite-based composites because of the orientated structure of graphite sheets. It is worth noting, however, that a shorter melting time means less PCM and composite and therefore less ability to store heat during phase change. In another publication, You and Tao [107] reported that the thermal conductivity of paraffin/expanded graphite composites can be significantly increased, reaching up to 7.82 W/m·K when the EG content is 20%, which is 47.8% higher than the conductivity perpendicular to the EG lamellar direction. This enhanced conductivity is crucial for efficient heat transfer and temperature control, as evidenced by the average surface temperature during the melting process being 44.8 °C, which is lower compared to other configurations. The latent heat capacity, another important parameter for thermal energy storage, varies depending on the type of paraffin wax used. Nishad et al. [106] observed latent heat capacities ranging from 88.5 to 102.7 J/g for their composites. This range ensures effective thermal energy storage while maintaining good thermal conductivity, and the graphite-based skeleton effectively prevented liquid paraffin wax leakage, ensuring form stability during phase transitions. Wu et al. [108] developed a form-stable PCC that achieved an electro-to-heat conversion efficiency of up to 85% at lower voltages (1.5–2.0 V). This high efficiency demonstrates the potential of these materials for energy harvesting applications. Moreover, the obtained PCC reached stable thermophysical properties after 200 frequent melting/solidification cycles. Maleki et al. [109] further improved the electrothermal conversion efficiency by developing graphitic domain-rich carbon foams. Their composites exhibited superior electro/photothermal energy conversion capabilities, reaching an electrothermal efficiency up to 85% at low voltage (3–3.6 V). The stability of these composites is also noteworthy. Nishad et al. [106] reported that their graphite-based skeleton effectively prevented liquid paraffin wax leakage, ensuring form stability during phase transitions. In comparison, the composites of Wu et al. [109] reached stable thermophysical properties after 200 frequent melting/solidification cycles. This stability is crucial for long-term performance and reliability in practical applications. Tabassum et al. [110] introduced a multifunctional PCC based on methyl stearate, which demonstrated a synergistic effect in the tailoring of thermal properties. Although specific values were not provided, this study highlights the potential for fine-tuning thermal properties through careful material selection and composition. In terms of photothermal conversion, some composites have shown remarkable efficiency. For example, certain graphene-based composite PCMs have achieved photothermal conversion efficiencies as high as 86.9% [27,109]. Another study conducted by Li et al. [111] presented a synergistic approach for the synthesis of scalable, highly conductive PCCs and the optimization of thermal transport properties through the alignment of self-assembled, large, reticulated graphite nanoplatelets (RGNPs) within the PCCs. The vertically aligned and layered RGNPs facilitate directional thermal and electrical conductivities in the PCCs, achieving values of 33.5 W/mK for thermal conductivity and 323 S/cm for electrical conductivity at RGNP loadings below 25 wt.%, surpassing current leading PCC technologies. These PCC-based energy devices are capable of harnessing sunlight for direct photo-thermal energy harvesting and storage at elevated temperatures exceeding 186 °C without the need for optical concentration. Additionally, they enable rapid electrothermal energy conversion and storage at ultra-low voltages (less than 0.34 V) with high efficiency, reaching approximately 92.7%. These advanced graphite-based composite PCMs exhibit a combination of high thermal conductivity, substantial latent heat capacity, high electrothermal conversion efficiency, and improved structural stability. The ability to tailor these properties by adjusting composition and structure offers flexibility in the design of materials for specific thermal energy storage and conversion needs [85,112].

3.1.4. PCMs Modified with Carbon Aerogel

PCMs modified with graphene aerogels (GA) have recently become popular and very promising materials, which have demonstrated superior electrothermal conversion efficiency compared to other materials because of their unique structural and conductive properties. The electrothermal conversion efficiency of GA-based PCMs can reach up to 85.4% under a voltage of 3.0 V, which is significantly higher than that of traditional carbon-based materials. For instance, carbon aerogels typically require much higher voltages, around 15 V, to achieve similar phase change effects, and carbon nanotube sponges have a critical voltage of approximately 1.5 V for complete phase change. The high efficiency of PCMs/GA systems is attributed to the continuous conductive network formed by GA, which facilitates efficient electrical conductivity and rapid heat transfer. Furthermore, these composites exhibit a low voltage threshold of about 1.0 V to initiate phase change, further highlighting their efficiency. In comparison, other materials such as hybrid aerogels composed of melamine foam and graphene oxide/graphene nanoplatelets (GO/GNPs) show lower electrothermal conversion efficiencies, around 62.5%, even though they significantly enhance thermal conductivity and electrical conductivity. Moreover, GA-based PCMs can achieve a photothermal conversion efficiency of up to 91.8%, indicating their versatility in the conversion of various forms of energy into thermal energy for storage. These properties make PCMs/GA systems particularly advantageous for applications requiring efficient energy conversion and storage [113,114,115,116,117]. Schematic of compositon PW/SEBS/LGA with diagram of LED brightness, in LGA circuit and in PW/SEBS/LGA circuit was presented on Figure 5.

The PCM/graphene aerogel systems described in these studies exhibit remarkable improvements in thermal conductivity, energy conversion efficiency, and shape stability compared to traditional PCMs. Li et al. [118] developed graphene-aerogel composite fibers with tunable thermal conversion and storage properties. These fibers exhibited superhydrophobicity and responsiveness to multiple stimuli, including heat, light, and electricity. Li et al. [119] reported anisotropic graphene aerogels with a thermal conductivity of 2.94 W m⁻¹ K⁻¹ and a thermal conversion efficiency of 80.4%. Min et al. [120] achieved even higher thermal conductivity (4.92 W m⁻¹ K⁻¹) and photothermal conversion efficiency (93.4%) using anisotropic graphene aerogels. Wang et al. [121] synthesized “graphene-like” mesoporous carbons with exceptionally high paraffin loading capacities of up to 91.7 wt.%. Furthermore, they reported an improved latent heat of 104.5% compared to pure paraffin, indicating an improved energy storage capacity. Yang et al. [122] reported hybrid graphene aerogel/PCM composites, resulting in a composite with a thermal conductivity of 1.43 W m⁻¹ K⁻¹, a 361% improvement over pure PEG. In another paper, Zhong et al. [123] developed an innovative PCC by combining three-dimensional GA with octadecanoic acid (OA) for thermal energy storage applications. This novel material exhibited remarkable thermal properties, including a significantly enhanced thermal conductivity of approximately 2.635 W/m·K, which was 14 times higher than that of pure octadecanoic acid (0.184 W/m·K). Despite the addition of GA, the composite maintained an impressive heat storage capacity of 181.8 J/g, nearly matching the capacity of pure octadecanoic acid (186.1 J/g). This combination of improved thermal conductivity and preserved heat storage capacity makes the GA-OA composite a promising material for efficient thermal energy storage systems. Zhou et al. [124] developed polyurethane-based PCMs with halloysite nanotubes hybrid GAs, achieving efficient light and electrothermal conversion and storage. The electrothermal conversion efficiency of material reached 67.2%, with a latent heat parameter of 92.1 J/g. When comparing the above-mentioned materials, it is evident that anisotropic GA structures generally outperform isotropic ones in terms of thermal conductivity and energy conversion efficiency. All these studies collectively demonstrate that GA-based materials can significantly enhance the performance of PCMs across various parameters, including thermal conductivity, energy conversion efficiency, loading capacity, and multifunctionality. The choice of optimal material would depend on the specific requirements of the intended application, which balance factors such as thermal performance, energy storage capacity, and additional functionalities.

Reducing the carbon footprint and recycling have become extremely important issues these days, so the creation of modern materials should also be accompanied by the search for sustainable sources of raw materials. Biomass-derived carbon/PCM composite have shown promising potential for electrothermal conversion and storage applications. This group of materials is sourced from wastes, making them a renewable and cost-effective option. This not only reduces the environmental impact but also provides a sustainable alternative to synthetic materials, reducing reliance on fossil fuel-based carbon materials. The production of biomass-derived carbons typically requires less energy compared to the synthesis of CNTs or graphene. For example, biochar can be produced at low pyrolysis temperatures, resulting in lower cumulative energy demand (100–1000 MJ/kg), and it often involves fewer harsh chemicals compared to the synthesis of other carbon materials, potentially reducing environmental pollution and health risks associated with production. Furthermore, biomass-derived carbon can be a cheaper counterpart to other raw materials used in the creation of phase change materials, making them more accessible for large-scale applications [20,125,126,127].

3.1.5. PCMs Modified with Biomass-Derived Carbon

In many applications, biomass-derived carbon/PCMs show comparable or even superior performance to those using other forms of carbon, such as CNTs or graphene. It is important to note that their overall impact depends on factors such as the specific biomass source, production methods, and end-of-life management. A comparative analysis of these materials reveals significant variations in their physicochemical parameters and properties. Biomass carbon-based PCM are often prepared using PEG, paraffins, and fatty acids, which offer excellent thermal properties and leakage resistance [128,129]. For example, PEG-based PCMs have demonstrated a phase change temperature of 58.5 °C and a latent heat of 51.5 J/g, showcasing their high thermal cycle stability and minimal latent heat loss after multiple cycles. Garlic peel-derived carbon/PCM system exhibited phase change enthalpies of 52.5 J/g during melting and 51.9 J/g during freezing, with a low latent heat loss rate of about 1.5% after 200 thermal cycles, indicating excellent thermal cycle stability [130]. Moreover, Fabiani et al. [131] reported biochar and lignin-based PCCs impregnated with paraffin that displayed stable behavior up to 100 °C, with significant thermal buffer capabilities, particularly for paraffin, with a melting temperature of 21 °C. Additionally, the incorporation of nanoparticles such as Fe₃O₄ not only improves magnetothermal and photothermal conversion abilities but also improves thermal conductivity, demonstrating multifunctional capabilities. For instance, a PCC based on lauric acid (LA) and Fe₃O₄ nanoparticles demonstrated a phase change temperature of 44.6 °C and a latent heat of 167.1 J/g, retaining over 95% of pure LA’s heat storage capacity [132]. The study by Wei et al. [133] explored the development of leakage-proof PCC supported by biomass carbon aerogels derived from succulents. The microstructure of the succulent-based carbon aerogel (SCA), which includes epidermis, palisade tissue, and spongy tissue, provides a high loading efficiency of up to 95 wt.% for organic PCM. This structure effectively prevents leakage, resulting in a mass loss of as low as 1.3 wt.% during phase change. The PCM composites exhibit a high latent heat close to that of pure paraffin and maintain 100% thermal cycling performance after 20 cycles. Furthermore, SCA enhances the thermal conductivity of PCM composites and significantly increases their light-to-thermal energy conversion efficiency, making it a multifunctional scaffold promising for practical applications of PCM composites. Li et al. [134] presented a novel approach to synthesizing high-performance materials for energy conversion and storage using biomass. The authors developed lightweight, three-dimensional (3D) carbon aerogels (CAs) through a green, two-step process involving the hydrothermal carbonization and postpyrolysis of melons. These CAs exhibited high electrical conductivity (3.4 S m⁻¹) and were subsequently infused with paraffin wax to create form-stable PCC with a high melting enthalpy of 115.2 J/g. The resulting CA–wax composites demonstrated exceptional performance in both photothermal and electrothermal energy conversion and storage applications (Figure 6). They absorbed 96% of incident solar radiation across the UV–vis–NIR spectrum, storing it as thermal energy, and achieved an electric–heat conversion efficiency of 71.4%. Furthermore, these composites exhibited good thermal stability and cycling performance. The authors posited that the low cost, green synthesis, low density, and excellent electrical conductivity of these 3D CAs make them promising candidates for various energy-related devices. Another interesting application was presented by Umair et al. [135], who developed advanced 3D hollow carbon fiber (HCF) network scaffolds through an alkaline treatment followed by the pyrolysis of raw cotton at 900 °C to enhance the electrothermal conversion efficiency of biomass-derived carbon/PCM composites. Alkaline treatment significantly improved the specific surface area, pore volume, and pore size distribution of the HCF. In the HCF/paraffin composite PCMs, the 3D HCF network provided highly interconnected thermally and electrically conductive paths. These composites exhibited a phase change enthalpy of 182.22 J/g with an 85 wt.% paraffin load. The electrical conductivity of the composite PCMs reached 19.6 S/m, a substantial increase from the approximately 10¹⁴ S/m of pristine paraffin, indicating that the insulating nature of paraffin was effectively overcome by the conductive HCF network. Consequently, the electrothermal conversion efficiency of the HCF-based composite PCMs reached 81.1% at 3 V, attributed to the efficient Joule heating effect facilitated by the long conductive paths within the HCF. This study highlights the potential of HCF-based composites for high-performance thermal management applications. One interesting application showed the use of wood and was presented by Yang et al. [136]. They investigated properties of a carbonized wood-based PCC designed for thermal energy storage applications. This novel material, known as TDCW, exhibits a significantly improved thermal conductivity of 0.669 W/m·K at 50 °C, which represents an 114% improvement over pure TD. The material also shows a high latent heat of 165.8 J/g, indicating its capacity to store and release substantial amounts of thermal energy during phase transitions. TDCW maintains good thermal reliability after 200 thermal cycles, which indicates its durability and efficiency in repeated use. Furthermore, the composite shows favorable thermal stability below 120 °C, making it suitable for various thermal management applications. These properties highlight the potential of TDCW as a cost-effective and efficient solution for thermal energy storage, utilizing the inherent advantages of carbonized wood structures to enhance thermal performance.

3.2. PCMs Modified with MXene

A promising area for the development of PCMs in the context of increasing energy conversion efficiency and energy storage capabilities in the context of electrothermal applications has been the use of MXenes in the creation of composite structures. MXenes are a class of two-dimensional inorganic compounds consisting of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. They have the general formula M_n+1X_nT_x, where M is a transition metal, X is carbon and/or nitrogen, and T represents surface terminations such as O, F, OH, or C [137,138]. The overall synthesis process involves the use of MAX phases, which have the general formula M_n+1AX_n, where M is an early transition metal, A is an element from group 13 or 14, and X is carbon and/or nitrogen). The A layers are selectively etched from the MAX phase using an etchant solution such as hydrofluoric acid (HF) or a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl). This etching process results in the formation of multi-layer (ML) or few-layer (FL) MXene structures with an accordion-like morphology. In the following step, the etched MXene layers can be further delaminated into single- or few-layer sheets by sonication or mechanical agitation in water or organic solvents. The surface of MXenes is typically terminated by functional groups such as O, F, OH, or Cl. Alternative synthesis methods, such as bottom-up approaches using chemical vapor deposition (CVD), template methods, or plasma-enhanced pulsed laser deposition, have also been reported. These methods enable the synthesis of high-quality thin 2D transition metal carbide and nitride films, including compositions that cannot be obtained by selective etching [139,140,141,142].

MXenes have quickly become attractive for a variety of electrothermal effect research fields because of their exceptional properties. High thermal conductivity, large surface area, tunable surface chemistry, thermal stability, and compatibility with different PCMs make these materials an extremely promising material for enhancing the energy conversion and storage capabilities of PCMs. MXenes can be incorporated into various types of PCMs, including organic paraffins and inorganic salts, expanding their applicability. Because of the large specific surface area, greater interaction with the PCM matrix is possible, increasing heat transfer. Moreover, the surface chemistry of MXenes can be tuned to optimize their interaction with the PCM, further improving thermal properties [143,144,145]. MXenes are excellent conductors of both electricity and heat, which is crucial for efficient heat transfer and storage in PCMs. Aslfattahi et al. [146] in their work modified the organic PCM (PLUSICE A70) with MXene Ti₃C₂ to improve its thermophysical properties. Their composite material achieved an increase in thermal conductivity by 16% with a mass fraction of 0.3 wt.%. In addition, they revealed that the specific heat capacity of the nanocomposite was increased by 43% compared to pure organic PCM. Interesting solutions for the use of Ti₃C₂ material were presented by Krishna et al. [147]. A two-step process of preparation was used to combine the prepared MXene nanoflakes with palmitic acid to form the nanocomposite. The results demonstrated significant improvements in the thermal properties of the nanocomposite. The addition of MXene nanoflakes enhanced enthalpy by 4.34% and thermal conductivity by 14.45%, indicating the suitability for TES applications. FTIR analysis revealed no chemical reaction between palmitic acid and MXene, suggesting an improved stability of the composite. MXene nanoflakes create a network within the palmitic acid matrix, providing additional paths for heat conduction and helping to reduce the thermal resistance within the PCM, allowing for more efficient heat transfer. These enhancements in thermal properties make the palmitic acid/MXene nanocomposite a promising candidate for solar thermal and solar photovoltaic thermal applications, addressing the challenge of low thermal conductivity in conventional PCMs. Other work describes the development of novel PCC composed of dodecylamine, Ti₃C₂, and pectin. The research demonstrates that the incorporation of 3 wt.% Ti₃C₂ into the composite results in a phase change temperature of 25.47 °C, which is considered comfortable for the human body. This characteristic makes the material potentially suitable for thermal management applications in buildings or wearable devices. The study also highlighted the enhanced light-to-thermal conversion properties of the composites, suggesting improved energy efficiency. Furthermore, research indicates that the inclusion of Ti₃C₂ and pectin in the composite structure leads to improved mechanical properties. These advances in material properties could potentially expand the range of applications for PCMs in various fields, including renewable energy and thermal management systems [148].

The difference is notable between the MXene Ti₃C₂ and Ti₃C₂T_x compounds, which in both forms are encountered in the applications described in scientific publications. Ti₃C₂ refers to the bare MXene structure, while Ti₃C₂T_x includes surface termination groups (T) such as -O, -OH, and -F. The “x” in T_x denotes that the exact composition and stoichiometry of these termination groups are often undetermined. The presence of surface terminations in Ti₃C₂T_x leads to an increased interlayer spacing compared to the theoretical Ti₃C₂ structure. This can be observed as a downshift of the (002) reflection peak in X-ray diffraction (XRD) patterns. It is important to note that the bare Ti₃C₂ structure without any surface terminations is largely theoretical, as most synthesized MXenes will have some form of surface functionalization, resulting in Ti₃C₂T_x [149,150,151]. In another publication, the researchers created PEG2000-based PCMs composites supported by Ti₃C₂T_x/poly(vinyl alcohol) (PVA) foam support by freeze drying after PEG’s introduction into the skeleton via vacuum impregnation. The obtained PCM demonstrates good performance characteristics compared to pure PEG. Notably, it maintains a high phase change enthalpy, addressing a common limitation observed in many composite PCMs. The composite exhibits an exceptional photothermal conversion efficiency of 96.5%. Upon incorporation of 7.68% (w/w) of the filler material, the thermal conductivity of the composite PCM increases by 423.8% while simultaneously achieving a phase change enthalpy of 131.1 J/g. Moreover, the Ti₃C₂T_x/PEG composite displays structural stability, effectively mitigating the risk of PCM leakage during the melting process. These properties collectively contribute to the enhanced functionality and applicability of the developed composite PCM in thermal management systems [152]. The use of the MXene Ti₃C₂T_x compound can also be seen in other works written by Wu et al. [153] and Lu et al. [154]. The first study focused on the large-scale fabrication of flexible EPDM/MXene/PW (ethylene-propylene-diene terpolymer/MXene/paraffin wax) PCC using a water-assisted melt-blending technique. The melting, latent heats, and freezing latent heats of EMP-0.8 are 120 J/g and 115 J/g, respectively. The second study introduced a novel composite material combining PEG with two-dimensional MXene nanosheets (PEG/MXene) (Figure 7). This composite exhibited enhanced thermal and electrical conductivity compared to pure PEG, with an electrical conductivity of 10.41 S/m and a thermal conductivity reaching 2.052 W/mK, which is 7.3 times more than the conductivity of pure PEG. The incorporation of MXene nanosheets acted as heterogeneous crystal nuclei, promoting crystallization of PEG without chemical reactions between the components. The composite demonstrated high photothermal and electrothermal conversion capabilities as well as latent heat storage capacity, with melting and freezing latent heats of 131.2 and 129.5 J/g, respectively.

Two other interesting studies have explored the use of MXene-coated melamine foam (MF) as a support for PCMs to create functional flexible composite structures [155,156]. Shao et al. [155] created a PCM composite by introducing MXene-coated melamine foam into PEG. This composite demonstrated excellent encapsulation properties, with a dimension retention ratio of 90% and a phase change enthalpy of 194.1 J/g. The three-dimensional MXene network along the MF skeleton significantly improved the composite’s solar absorption capabilities, achieving a solar–thermal conversion efficiency of 92.7%. Additionally, the composite exhibited remarkable shape fixation and recovery effects because of the combination of MF/MXene sponge resilience and PEG phase transition. The researchers further developed a flexible heat eye-patch application, showcasing the potential for thermal therapy. In a related study, Du et al. [156] explored a similar concept by synergistically modifying melamine foam/PEG composites with polydopamine (PDA)/MXene, further enhancing the photothermal conversion efficiency and thermal energy storage capabilities of the material. The researchers developed a new supporting material by integrating MF and MXene using PDA’s adhesive properties, which was then used to encapsulate PEG. The resulting CPCM, designated as PEG/MPMF, demonstrated good heat storage capabilities with a melting enthalpy of 186.2 J/g, reaching 99.5% of the pure PEG’s enthalpy. Furthermore, the material exhibited remarkable shape stability and reusability, maintaining its performance after 100 thermal cycles. Gong et al. [157] showed an interesting approach to creating a robust and flexible structure. The study introduced novel flexible polyurethane/MXene solid–solid PCMs (SSPCMs) with enhanced photothermal conversion efficiency and mechanical strength. These SSPCMs were successfully prepared using a solvent-free method and exhibit leakage-proof properties because of the formation of a three-dimensional crosslinking network by HDIT and PEG. Research demonstrates that SSPCMs containing 2.0 wt.% MXene (2.0-MPH) achieve a melting phase-transition enthalpy of 127.97 J/g and a photothermal conversion efficiency (θ) of 90.45%. In particular, the materials maintain their enthalpy values and θ value after 200 thermal cycles and 50 light cycles, indicating good photothermal reliability and cycle reusability. The improved mechanical properties of these composites further extend their potential use in electrothermal applications. All characteristics of the materials obtained, combined with good photothermal and electrothermal conversion capabilities, make the Ti₃C₂T_x MXene-based composite a promising candidate for advanced thermal energy management and energy storage solutions.

Undoubtedly, modified aerogels have now become interesting materials because of their unusual properties and low density. PCM/MXene aerogel systems have emerged as a promising technology for solar energy conversion and thermal energy storage. Lin et al. [158] developed a PCM/MXene aerogel systems using MXene nanosheets and PEG. The MXene nanosheets demonstrated excellent light absorption properties with an extinction coefficient of 25.67 L/(g·cm) at 808 nm, enabling efficient conversion of solar energy into thermal energy. The PEG component acted as a medium for thermal energy storage and release during phase transitions. The resulting MXene/PEG aerogels were lightweight, with a density of approximately 30 mg/cm³, and they exhibited improved thermal stability compared to pure PEG, with thermal decomposition temperatures increased by 40 °C. The aerogels achieved high fusion and solidification enthalpies of 167.72 J/g and 141.51 J/g, respectively, with a remarkable photothermal storage efficiency of 92.5%. This innovative approach combines the excellent light absorption properties of MXene nanosheets with the thermal storage capabilities of PEG, offering a new direction for developing efficient solar energy conversion and storage materials. The study by Luo et al. [159] presents the development of flame-retardant and form-stable PCCs using MXene aerogel as a supporting skeleton and chemically modified stearyl alcohol (SAL) with a phosphorus-containing molecule. These composites were manufactured using a vacuum impregnation method, resulting in materials with enhanced thermal conductivity (0.486 Wm⁻¹K⁻¹) and form stability up to 90 °C. The incorporation of phosphorus and MXene significantly improved the flame retardancy of the PCMs, reducing the maximum heat release rate and total heat release by 42.8% and 32.1%, respectively. This improvement is attributed to catalytic charring, barrier effects in the condensed phase, and free radical quenching in the gas phase. The developed MXene-based flame-retardant PCMs demonstrate potential for safe and efficient solar energy storage applications, addressing the critical issues of leakage, poor thermal conductivity, and high flammability that have previously hindered the widespread use of PCMs. Another interesting approach was the development of hybrid PCMs with anisotropic properties [160]. The researchers synthesized vertically oriented network composite PCMs using rGO/MXene hybrid aerogels as carrier materials and encapsulating stearic acid (SA) as the PCM (Figure 8).

The vertical alignment of MXene and rGO hybridized backbones significantly improved the thermal conductivity of the composite, reaching 1.21 W/(mK), which is a 317.24% increase compared to pure SA. In particular, the composite maintained a high melting enthalpy of 168.25 J/g, nearly equivalent to that of pure SA. The addition of rGO and MXene substantially improved the photothermal conversion capability of composite PCMs, achieving up to 90.19% efficiency. The study also revealed anisotropic thermal behavior, with a faster temperature rise and a higher equilibrium temperature (4.8 °C difference) observed in the orientation direction compared to the non-orientated direction under irradiation. These properties make this new material highly promising for energy storage. An overview of PCMs capable of electrothermal conversion is summarized in

Table 1. Overview of PCMs capable of electrothermal conversion.

Type of PCM	Modification Materials	Melting Enthalpy (J/g)	Thermal Conductivity (W/mK)	Input Voltage (V)	Conversion Efficiency (%)	Reference
Paraffin	CNT	138.2	1.2	1.50	40.6	[93]
Paraffin	CNT	138.2	1.2	1.75	52.5	[93]
N-eicosane	CNT	217.3	1.89	1.3	49.0	[98]
N-eicosane	CNT	217.3	1.89	1.7	74.7	[98]
PEG2000	CNT	89.8	0.91	1.5	58.3	[95]
PEG2000	CNT	89.8	0.91	2.0	94.0	[95]
Polyurethane	CNT	132	2.40	1.5	49.0	[96]
PEG4000	CNT/CNF aerogel	158.3	-	6	75.2	[94]
PEG1000	GO/CNT	120.7	0.37	6.6	70.0	[100]
PEG10000	GO/BN	164.1	1.06	7	87.9	[105]
Paraffin	GO/GNPs	161.7	1.46	2.9	62.5	[113]
Pentaerythritol	GNPs	225.3	26.63	0.22	11.33	[111]
Pentaerythritol	GNPs	222.8	26.63	0.34	92.73	[111]
Methyl Stearate	EG	147	3.6	1.7	72	[110]
Paraffin	EG	145.7	0.75	4.8	61.89	[161]
N-eicosane	EG	199.2	3.56	2.1	65.7	[162]
PEG6000	Graphene/cellulose	178.9	0.26	20	66.1	[163]
Paraffin	Graphene aerogel	193.7	2.99	1.5	50.5	[119]
Paraffin	Graphene aerogel	193.7	2.99	3.0	85.4	[119]
Paraffin/SEBS	Graphene aerogel	212.4	0.41	8	73.5	[114]
PEG4000	Graphene aerogel	92.1	-	10	67.2	[124]
Polyurethane	Graphite foam	60.3	10.86	1.8	85.0	[108]
PEG8000/polyurethane	Graphite foam	80.3	3.40	1.4	48.0	[164]
PEG6000/polyurethane	Graphite foam	76.1	3.40	1.4	88.0	[164]
PEG4000/polyurethane	Graphite foam	64.5	3.50	1.4	69.0	[164]
PEG6000	Graphite foam	163.9	-	3.0	52.0	[164]
Paraffin	Graphitic carbon foam	120.2	-	3.0	56.0	[109]
Paraffin	Winter melon-based carbon aerogel	115.2	-	15	71.4	[134]
Paraffin	Cotton-derived carbon scaffold	182.22	0.42	3	81.1	[135]
PEG 4000	MXene	131.2	2.05	7.2	-	[154]
Stearic acid	MXene/GO	168.25	1.21	-	-	[160]
n-octadecane	MXene/Ag nanowire	165.7	0.75	1.5	-	[165]

.

4. Applications and Future Trends

The rapid development of electronics and the continual expansion of the applicability of sustainable energy sources are driving the search for new and effective technological solutions. PCM-enhanced systems generally exhibit improved performance metrics compared to conventional systems. Energy conversion efficiency can increase as a result of reduced thermal losses and better temperature control. Thermal regulation is significantly enhanced, with PCMs providing more stable temperatures and reduced temperature fluctuations. This leads to improved performance and lifespan of temperature-sensitive components. However, the degree of improvement depends on the specific design of the system and PCM properties. PCMs capable of electrothermal conversion and energy storage fit perfectly into this model, capable of providing solutions to many existing problems [81,166,167,168].

4.1. Solar Energy

One of the most interesting development paths is the use of PCM in heating systems that convert surplus electrical energy from solar panels into thermal energy, which is then stored for later use. The integration of PCM-based thermal storage systems with solar energy systems allows efficient utilization of surplus energy. For example, during peak solar production hours, excess energy can be directed to heat the PCM. This stored thermal energy can then be used during periods of low solar availability, effectively bridging the gap between energy supply and demand. Cutting-edge electrothermal conversion PCMs can effectively manage these issues by converting surplus electrical energy into thermal energy for later storage and then reverting this stored thermal energy back into electrical energy when it is necessary. Research indicates that such systems can significantly enhance the efficiency of heating systems, especially in buildings where energy consumption is high [81,85,111,169]. The study reported by Wang et al. [170] investigated the integration of PCMs for distributed electric energy storage in building heating systems. The research highlighted several key physicochemical parameters, including the thermal conductivity, specific heat capacity, and conversion efficiency of the materials utilized. For example, the thermal conductivity of the PCM-based composite used in the study was observed to be approximately 0.92 W/m·K, which is essential for effective heat transfer. The total heat capacity within 50–150 °C was measured at around 445.3 kJ/kg, indicating a significant ability to store thermal energy. Furthermore, the prepared material after experiencing 500 cycles had no significant differences in both heat storage capacity and heat transfer capability compared to the freshly prepared one. These values demonstrate the potential of PCMs in improving energy efficiency and sustainability in building heating systems, particularly when combined with solar energy technologies, thus contributing to reduced carbon emissions and improved energy management in urban environments. One of the primary applications of PCMs in solar panel installations is their ability to regulate the temperature of photovoltaic (PV) panels. Solar panels often experience efficiency losses as a result of overheating. PCMs can absorb excess heat during peak sunlight hours and release it during cooler periods, maintaining the panels at an optimal operating temperature. This thermal regulation can lead to significant improvements in electrical performance. For example, studies have shown that PCM-based cooling approaches can reduce the temperature of photovoltaic panels, thus improving their efficiency [171,172,173,174]. Solar energy conversion involves heat accumulation, which heats the entire installation and integrated circuits, reducing system efficiency and shortening the lifespan of the entire system. This heat can be managed by PCMs and converted into electrical energy. Additionally, the PCM system protects the solar system from overheating and also utilizes this heat, which increases the efficiency of the entire solar system. The development of PCM materials capable of electrothermal conversion can also effectively contribute to the protection of photovoltaic installations against overvoltage and the build-up of excessive electrical charge [81,175,176]. PCMs are also used in concentrated solar power (CSP) plants to store thermal energy at high temperatures. In these systems, PCMs can absorb and store the intense heat generated by solar concentrators, which can then be used to produce steam and generate electricity during periods of low solar intensity. The use of PCMs in CSP plants can lead to improved efficiency and more stable power output. The key element to employ latent heat storage in concentrated solar thermal plants lies in the interaction between PCMs and heat transfer fluids during the charging and discharging phases [7,35,177,178,179]. A schematic of a CSP plant that utilizes latent heat storage alongside a Rankine cycle is depicted in the article written by Mofijur et al. [171]. Additionally, as illustrated in Figure 9, this system can be integrated with a supercritical carbon dioxide cycle (s-CO₂), which may also connect to a Brayton cycle for electricity generation. The process encompasses a heliostat field, a central tower (solar receiver), a thermal energy storage system, and a power block. Solar energy is focused onto the solar receiver by heliostats, with the heat transfer fluid (e.g., molten salt) circulating the absorbed heat throughout the system to supply either a steam Rankine cycle or an s-CO₂ Brayton cycle, facilitating the conversion of thermal energy into electrical energy.

4.2. Automotive and Electoronic

An interesting and future-oriented area for the use of PCMs is automotive and electronic. The automotive industry uses PCMs primarily for thermal management in electric vehicles (EVs) and internal combustion engine (ICE) vehicles. One of the primary applications of PCMs in this area is in air conditioning systems. Modern vehicles often have automatic start–stop systems to reduce fuel consumption and CO₂ emissions. However, when the engine stops, the air conditioning compressor also stops, leading to a rise in the cabin temperature. PCMs can mitigate this issue by absorbing excess heat when the engine is off, thus maintaining a comfortable cabin temperature. The PCM remains solid during normal operation and melts when the engine stops, absorbing heat and preventing hot air from entering the passenger cabin [180,181,182,183]. The thermal management of lithium-ion batteries in electric vehicles is another critical application of PCMs. Batteries generate significant heat and overvoltage during charging and discharging cycles, which can lead to thermal runaway and reduced battery life. PCMs can absorb and dissipate this heat, keeping the battery within an optimal temperature range and enhancing its performance and longevity. This action directly prevents temperature spikes that can damage battery cells. Moreover, by evenly distributing heat and excess electrical charge, PCMs help maintain a uniform temperature across all battery cells, improving overall efficiency and safety. Unlike active cooling systems that require additional power, PCMs provide passive thermal management, which is more energy-efficient and reliable [184,185,186,187,188]. Research conducted by Schweitzer and Al Hallaj et al. [189,190] investigated the thermal performance of various PCCs in a small battery system using commercial cylindrical 18,650 lithium-ion cells. The study examined PCCs with different melting points (37 °C, 48 °C, and 55 °C) and thermal conductivities to assess their heat rejection capabilities. During a 2 C (2A) constant discharge test, PCCs with melting points at 37 °C and 55 °C yielded similar results, maintaining a system temperature of 67 °C due to their comparable latent heat values (156 and 153 kJ/kg, respectively). The PCC48 material, with a higher latent heat of 183 kJ/kg, demonstrated superior performance by absorbing more heat during the exothermic phases. On the contrary, a system without thermal management reached 115 °C, highlighting the thermal advantages of using PCMs in battery applications. Lin et al. [191] further explored a passive thermal management system (TMS) for LiFePO₄ battery modules using PCMs. To address the low thermal conductivity of PCMs, they incorporated an expanded graphite matrix and graphite sheets, which increased thermal conductivity by a factor of 24 compared to paraffin. Their results showed temperature reductions of 32% and 37% at discharge currents of 40 A and 80 A, respectively. Additionally, idle tests demonstrated that the PCM-equipped battery module cooled more slowly than the PCM-equipped module, suggesting potential benefits for electric and hybrid vehicles during cold weather conditions. Another study written by Luo et al. [192] reported an electric-conductive paraffin/EG PCC for thermal management of the lithium-ion battery in a harsh thermal environment ranging from the ultralow −40 °C to ultrahigh +50 °C. The incorporation of 20 wt.% EG significantly reduces the resistance of non-conductive paraffin to 0.1–0.28 Ω mm and boosts its thermal conductivity by as much as 960%. This PCC functions by integrating both heating and cooling mechanisms, using Joule heating for preheating the batteries while simultaneously employing thermal storage to preheating the batteries and simultaneously employing thermal storage to prevent overheating. When operating at a voltage of 3.4 V, the PCC can raise the temperature of an eight-cell module at a rate of 13.4 °C/min, achieving a maximum temperature variation of 3.3 °C between cells. Additionally, during high-rate discharge (3 C), it effectively lowers the battery temperature from 77 °C to 43 °C, maintaining it within a range of 20 °C to 55 °C throughout the discharge cycle (Figure 10). This research expands the role of PCMs from merely cooling applications to comprehensive thermal management solutions.

4.3. Textiles

Recent studies indicate that the integration of PCM with textiles represents a significant advancement in fabric technology, providing innovative solutions for thermal management in various applications. As research continues to evolve, the potential for more sophisticated electrothermal conversion textiles will likely expand, paving the way for smarter and more sustainable textile products [193,194,195]. MXene materials have exhibited significant potential as Joule heating agents in various formats, including textiles, fibers, and films, owing to their metallic-like electrical conductivity. To fabricate flexible and sewable MXene-coated polyethylene terephthalate (PET) threads, Park et al. [196] employed an electrostatic assembly technique to deposit negatively charged MXene flakes onto the positively treated surface of PET fibers. These MXene/PET threads can be sewn and woven into various configurations to meet a range of wearable heating applications. Notably, the superior photothermal conversion efficiency of MXene materials can synergistically enhance their Joule heating capabilities, enabling all-weather heating solutions. For example, the application of 3D MXene aerogel facilitates simultaneous Joule and photothermal heating, allowing for continuous steam generation under varying environmental conditions. By modulating the applied voltage and solar irradiation levels, the temperature of the MXene aerogel can be precisely controlled, enabling steam production even in low-light scenarios (Figure 11). Integration of solar energy with Joule heating significantly enhances both the evaporation rate and the energy conversion efficiency. This synergistic effect is mainly attributed to the elevated surface temperature of the sample and the reduced relative humidity in the vicinity of the system [197]. Another study by Wang et al. [198] presented a novel approach to enhance the functionality of textile materials through the integration of MXenes. This study demonstrated that the in situ polymerization of polypyrrole (PPy) on MXene sheets significantly improves the electrical conductivity of the textiles, achieving approximately 1000 S m⁻¹. The resultant textiles exhibit remarkable electromagnetic interference (EMI) shielding efficiency, reaching up to 90 dB at a thickness of 1.3 mm, which is attributed to the unique layered structure and high conductivity of MXene. Furthermore, a silicone coating enhances the hydrophobic properties of textiles while maintaining adequate air permeability. The materials also show effective Joule heating performance under moderate voltage conditions (the applied voltage can be adjusted to range from 40 °C under 2 V to 79 °C under 4 V), indicating their potential for applications in wearable electronics, personal heating solutions, and clothing that is protective against electromagnetic radiation.

4.4. Building Industry

One of the most promising areas for the future has become the building industry. The incorporation of PCMs into the building envelope, such as walls, roofs, and floors, aims to increase thermal performance and energy efficiency. Publications show that this measure allows effective energy savings and a reduced carbon footprint [199,200,201,202,203,204,205,206]. It is suspected that the development of PCMs capable of electrothermal conversion will not only improve the properties of the current application of these materials in the building industry but also enable additional benefits such as the protection of electrical equipment and installations from surges or being able to effectively dissipate surface energy by providing grounding. In addition, integration of electrothermal conversion PCMs into smart building frameworks enables dynamic temperature regulation based on real-time data from environmental sensors. This capability allows adaptive responses to changing conditions, optimizing both comfort and energy use [27,85,113,162,201].

Considering the critical importance of thermal energy in total energy consumption, it is urgently necessary to identify and implement thermodynamically efficient pathways to convert electrical energy into thermal energy. The complex nature of energy conversion processes involving thermal energy presents significant challenges in the assessment, analysis, and optimization of such systems. In light of the aforementioned context, the development of scientific research concerning electric heating integrated with thermal energy storage has become a compelling topic. Within this domain, PCMs represent one of the intriguing pathways for improving energy efficiency and thermal management. An objective evaluation of various energy conversion methods requires the analysis of energy, exergy, and entransy, which measure loss and irreversibility in different ways, each with unique strengths and weaknesses. Given the importance of converting electrical energy to thermal energy, applying the entransy theory alongside energy and exergy comparisons can yield valuable insights [207,208,209,210,211,212]. Kou et al. [212] in their publication conducted a comprehensive comparison of three different paths of electric heating in combination with thermal energy storage (Figure 12).

To ensure a fair comparison of input–output results across these pathways, identical boundary and output conditions were applied. The analysis assumed that all thermal storage systems have similar charge/discharge efficiencies and temperature change ratios, as shown in Equations (7) and (8) below. Any reduction in thermal energy quantity or temperature is attributed to heat loss to the environment.

$${\left({\displaystyle \frac{Q_4}{Q_3}}\right)}_{Path 3}={\left({\displaystyle \frac{Q_4}{Q_3}}\right)}_{Path \mathrm{1,2}}={\eta }_Q\le 1$$

(7)

$${\left({\displaystyle \frac{T_4}{T_3}}\right)}_{Path 3}={\left({\displaystyle \frac{T_4}{T_3}}\right)}_{Path \mathrm{1,2}}={\eta }_T\le 1$$

(8)

where η_Q is the charge and discharge efficiency of the thermal energy storage device, and η_T is the ratio of the output temperature to the input temperature. η_Q∙η_T represents the entransy efficiency attributed to the thermal energy storage device.

The findings of the researchers’ investigation indicate that both theoretical and empirical analyses consistently demonstrate the enhanced efficacy of systems that integrate heat pumps with intermediate thermal energy storage. This methodology capitalizes on ambient thermal energy while effectively minimizing both the quantitative and qualitative losses of energy. Significantly, this configuration has the potential to improve energy conversion efficiency and exergy efficiency by as much as 70% while simultaneously decreasing the entransy dissipation associated with thermal energy storage by 30% within specific heat supply scenarios in Shanghai. It is worth noting that there is also an interesting possibility of using other heating sources, such as induction heating or magnetic heating of metal nanoparticles.

The applications of PCMs are vast, but there are ongoing challenges and areas for improvement. Enhancing the thermal conductivity of PCMs, ensuring long-term stability, and reducing costs are critical areas of research. The development of composite PCMs with nanoparticles and other advanced materials holds promise for overcoming these challenges and expanding the use of PCMs in new and existing applications. As research continues to advance, the potential applications of PCMs in construction are expected to expand, paving the way for smarter and more energy efficient buildings in the future.

5. Conclusions

As demonstrated in this review, PCMs hold significant promise for electrothermal conversion and storage, offering innovative solutions for sustainable energy management. Recent advances have focused primarily on improving the efficiency and functionality of PCMs through the integration of carbon-based materials and other composites. These materials, including carbon nanotubes, graphene, and MXene integrated with PCMs, have demonstrated improved electrical conductivity and thermal management capabilities, making them suitable for a wide range of applications. The incorporation of electrically conductive materials into PCMs significantly boosts their electrothermal conversion efficiency. This enhancement is crucial for applications requiring rapid and efficient energy storage and release, such as microelectronic devices and thermal energy storage systems. The development of composite PCMs, particularly those using carbon-based supports, has led to substantial improvements in the thermal and electrical properties of these materials. Innovations such as the use of graphene aerogels, biomass-derived carbon, and MXene have been pivotal in achieving these advancements. The key to further development is a deeper understanding of the electrothermal conversion mechanisms, linking the structural design of PCMs to their performance. This understanding is essential for the rational design of high-performance materials tailored to specific applications. Despite the progress, several challenges remain. These include the need for scalable production methods, long-term stability, and the integration of PCMs into existing systems. Future research is expected to address these challenges by exploring new materials, optimizing composite structures, and developing novel fabrication techniques.

In conclusion, the potential of PCMs for electrothermal conversion and storage is vast, with ongoing research continually pushing the boundaries of what is possible. Advances in material science and engineering are paving the way for more efficient, reliable, and versatile energy storage solutions that contribute significantly to the field of sustainable energy. In the literature, various researchers have attempted to categorize PCMs based on their characteristics, leading to the creation of insightful review articles that systematically enrich the knowledge regarding contemporary research on PCM materials [6,29,47,75,80,81,85,143,187]. A significant number of intriguing studies are emerging on PCM topics, with each review facilitating comparisons of different approaches to PCM development and their respective investigation methodologies. In conclusion, the potential of PCMs for electrothermal conversion and storage is vast, with ongoing research continually pushing the boundaries of what is possible. Advancements in materials science and engineering are paving the way for more efficient, reliable, and versatile energy storage solutions, contributing significantly to the field of sustainable energy. Their applications in building construction, renewable energy, electronics, automotive, food, infrastructure, and electrothermal conversion highlight their versatility and importance in the advancement of sustainable and efficient energy solutions.