Advancements in form-stabilized phase change materials: stabilization mechanisms, multifunctionalities, and applications - A Comprehensive Review

Phase change materials (PCMs) possess remarkable properties that make them highly attractive for thermal energy storage and regulation purposes. Their ability to store energy in the form of latent heat while maintaining a nearly constant temperature has led to growing interest in their practical applications. However, a signi ﬁ cant challenge in utilizing PCMs lies in their susceptibility to leakage and ﬂ uidity in the melt state. Therefore


Introduction
Phase change materials (PCMs) belong to a class of functional substances that possess a unique capability to absorb and release thermal energy through a phase transition process as their temperature changes.This remarkable attribute renders them highly valuable for applications related to energy conservation and storage [1e3].In recent times, PCMs have attracted significant attention and have been widely utilized across various domains, including the integration and management of renewable energy sources and the advancement of sophisticated energy storage systems [4,5].Particularly promising is their application in the construction sector, where extensive research is underway to explore their potential to reduce the energy demand for air conditioning while ensuring a comfortable indoor environment [6,7].Beyond the construction sector, the versatility of PCMs has made them indispensable in designing efficient thermal management systems for electronic devices [8,9], batteries [10,11], and wearables [12,13].These applications capitalize on the thermoregulatory capabilities of PCMs to maintain optimal operating temperatures and enhance the overall performance and lifespan of these electronic components.Moreover, the integration of PCMs in wearable devices enables improved user comfort and extended battery life.
However, despite their numerous advantages, the widespread use of PCMs faces several significant challenges that hinder their practical implementation.For instance, issues such as leakage and volume changes when PCMs are in a liquid state can lead to operational difficulties and higher costs [14,15].To address these challenges, researchers have focused on developing innovative stabilization techniques for PCMs.One of the primary strategies to prevent or reduce leakage is to encapsulate the PCM within a protective barrier or confine it within porous structures.
Alternatively, blending PCMs with polymers can offer stability and containment benefits [16,17].These techniques involve physically adsorbing, chemically bonding, or ionic crosslinking the PCM to the supporting materials, thus ensuring its stability and contained fluidity.By effectively mitigating leakage, these stabilization methods enable easy handling and broad applicability of PCMs in various practical scenarios.Moreover, the design of form-stabilized PCMs requires a meticulous selection of appropriate supporting materials and shape-stabilizing mechanisms.Embracing these stabilization techniques not only addresses the inherent limitations of PCMs but also paves the way for the development of advanced energy storage materials with multifaceted functionalities, including applications in biomedicine [18], self-healing [19,20], self-cleaning [21e23], electrical conductivity [24], flame retardancy [25], and more.
This comprehensive review aims to explore and examine various PCM stabilization mechanisms, providing detailed insights into different types of support structures, such as microencapsulation materials, porous materials, and polymeric materials, along with the fabrication techniques employed.Furthermore, the review delves into the role of form-stabilized PCMs in diverse applications and addresses key concerns related to stability, leakage prevention, thermal conductivity, and application strategies.The ultimate objective of this review is to offer readers a profound understanding of PCM stabilization for a wide array of practical applications, thereby facilitating the accelerated adoption of these remarkable materials in cutting-edge technologies and energy systems.

Encapsulation of phase change materials
Encapsulation is a highly effective method used to protect and stabilize PCMs through the coating of PCM droplets with shell materials, which can be of an organic or inorganic nature.This process involves employing physical, chemical, and physiochemical techniques to prepare encapsulated PCMs, providing enhanced control over their properties and performance.The encapsulation of PCMs is vital for achieving form-stabilized PCMs, which prevent leakage and enhance their usability.Physical encapsulation methods can be accomplished through various coating and spray-drying techniques.These methods create a protective shell around the PCM, preventing leakage and maintaining the PCM's integrity during phase transitions.Chemical methods involve processes like in situ polymerization and solvent extraction, which result in the formation of a stable shell around the PCM.These chemical techniques enhance the durability and reliability of form-stabilized PCMs.Additionally, physio-chemical methods, such as the sol-gel method [2], have also been employed to encapsulate PCMs.The sol-gel method combines the advantages of both physical and chemical encapsulation, offering a robust means of creating form-stabilized PCMs.
The choice of suitable shell materials for PCM encapsulation is of utmost importance, as specific properties are required to ensure effective encapsulation.The encapsulation material must form a thin, cohesive layer that remains stable and chemically compatible with the core PCM.Moreover, it should exhibit non-reactivity with the PCM and possess solubility in aqueous media or solvents during the preparation process.Furthermore, the shell material should demonstrate desirable coating properties, including strength, flexibility, and stability [26,27].A diverse range of materials, both organic and inorganic, have been utilized for encapsulating PCMs, offering various advantages depending on the specific application.
Examples of inorganic shell materials include metal oxide particles [28e30] and silica [31], which offer excellent thermal stability and durability.These materials are well-suited for hightemperature applications and ensure long-term PCM stability under challenging conditions.On the other hand, organic resins [32,33] and nanocellulose [34,35] have been employed as organic shell materials, providing biocompatibility and versatility for applications where environmental concerns and biodegradability are important factors.To offer a comprehensive overview of the various shell materials commonly used for PCM encapsulation, Table 1 compiles the essential characteristics and advantages of these materials.
Optimizing the energy storage capacity and thermal performance of encapsulated PCMs requires careful consideration of various factors.One crucial aspect is the efficiency of encapsulation and the mechanical strength of the shell material [1].In one of the early research endeavors in this field, the encapsulation process of n-octadecane PCM within an inorganic silica shell was achieved through a sol-gel process.The microcapsules exhibit a spherical morphology with a well-defined core-shell microstructure.The silica-encapsulated PCM exhibited improved thermal conductivity, favorable phase-change performance, and high encapsulation efficiency [36].In another research study [37], the coacervation encapsulation process was utilized to develop microencapsulated-nanoencapsulated PCM by employing chitosan as the shell material and a side-chain crystallizable comb-like polymer as the core material.The process is illustrated in Fig. 1-I.The microcapsules achieved an impressive encapsulation efficiency of 69% and exhibited a melting point of 33 C.

Porous supports for the stabilization of phase change materials
The widespread adoption of porous materials in various applications, particularly in energy conversion and storage, can be attributed to their remarkable attributes, such as a large surface Fig. 1.Synthesis techniques for shape-stable PCMs.I: (a) The encapsulation process of octadecyl acrylate oligomers ( ) with styrene-maleic anhydride copolymer solution ( ) and chitosan ( ); (b) the SEM images depicting the morphology of the resulting capsules [37].With permission from Hua, 2018, Copyright © 2018, Elsevier Ltd.II: Schematic representation of 3D porous carbon composites derived from polyethylene glycol (PEG) and carbon quantum dots (CQDs) [45].With permission from Xia, 2018, Copyright © 2018, Elsevier Ltd.III: The synthesis pathway for hexadecanol/dye-polyurethane (PU) composites achieved by incorporating hexadecanol into a visible-light-driven PCM matrix [46].With permission from Tang, 2016, Copyright © 2016, Elsevier Ltd.IV: Preparation process for the shape-stable blend of paraffin and epoxy [47].With permission from Lian, 2018, Copyright © 2018, American Chemical Society.area and high pore volume.Among the diverse methods employed to prevent leakage of PCMs in their melted state, pore confinement has emerged as an effective approach.This method capitalizes on capillary forces and surface interactions between the porous matrix and PCM molecules, influencing the fluid's physical properties and inhibiting PCM leakage during phase change processes.The synthesis method and characteristics of the pore structure play a critical role in determining PCM loading and form stability.Incorporating PCMs into suitable porous matrices yields significant improvements in various properties, including thermal durability, thermal conductivity, thermal stability, and chemical stability [48,49].Numerous types of porous materials have been extensively investigated for PCM confinement, showcasing their versatility and potential in tailoring PCM-based materials to meet specific requirements in different applications.Examples of such porous materials include metal foams [50e52], expanded graphite (EG) [53,54], graphene aerogels [55,56], carbon nanotubes (CNTs) [57,58], activated carbon [59], porous minerals [60], mesoporous silica [61,62] and polymer porous framework [63].Metal foams offer a unique combination of mechanical strength and high surface area, making them suitable candidates for demanding applications that require both structural integrity and efficient PCM encapsulation.Expanded graphite and graphene aerogels, on the other hand, possess exceptional thermal conductivity and thermal stability, providing ideal platforms for high-performance thermal management systems.Carbon nanotubes exhibit extraordinary mechanical and thermal properties, contributing to enhanced PCM dispersion and stability within the porous matrix.Activated carbon, known for its high surface area and adsorption capabilities, can effectively confine PCM while maintaining desirable thermal characteristics.
Porous minerals, mesoporous silica, and polymer porous frameworks each bring their distinct advantages to the realm of PCM confinement.Porous minerals offer excellent compatibility with a wide range of PCMs, while mesoporous silica provides precise control over pore size and structure, leading to tunable PCM loading and release properties.Polymer porous frameworks, being highly customizable and versatile, enable the development of tailor-made PCM composites for specific applications.For a comprehensive overview of the porous materials used for PCM confinement, Table 2 provides a comprehensive list of porous materials used for PCM confinement.These innovative approaches offer great potential for advancing PCM-based applications in diverse fields.
A highly thermally conductive 3D carbon network with a closely interconnected porous celosia-like structure was developed by Cen et al. [45].As depicted in Fig. 1-II, this remarkable network was formed using CQDs derived from acetone and divinyl benzene.The developed 3D graphitized carbon network exhibited remarkable capabilities in absorbing PEG PCM and enabling controlled crosslinking reactions to achieve complete crystallization.As a result, the method significantly enhanced the thermal conductivity of the PCM by 236%, pushing thermal enthalpy values close to the theoretical limits.The fabrication process of a crosslinked PU-based PCM incorporating in situ reduced GO sheets through a simple solvothermal treatment was reported by Zhou et al. [55].A significant increase in the phase change enthalpy from 86.8 J/g to 127.3 J/ g was achieved by the incorporation of just 3.8 wt% of GO, attributing to the facilitated nucleation and crystallization rate, resulting in a higher energy storage capacity.Moreover, the simultaneous in situ reduction of GO during the solvothermal treatment gives rise to remarkable light-thermal conversion efficiency, reaching up to an impressive 78.7%.

Polymeric hybridization for the stabilization of phase change materials
The hybridization mechanism of PCMs relies on the compatibility and affinity between the PCM and the polymeric matrices, achieved through blending and adsorption processes.By exploiting the miscibility between the PCM and the polymer, it becomes possible to achieve form stabilization through favorable intermolecular interactions.This intermolecular compatibility ensures that the PCM is uniformly dispersed within the polymeric matrix, which is essential for efficient and reliable thermal energy storage.In the context of using polymers for form-stabilizing PCMs, it becomes crucial to ensure that the selected polymer exhibits both thermal stability and chemical compatibility with the PCM [2].The polymer acts as a supporting matrix for the PCM, enhancing its mechanical strength and thermal performance.Consequently, it is imperative to choose a polymer with a suitable glass transition temperature and a decomposition temperature higher than the operational temperature range of the PCM.
This innovative approach offers several advantages over traditional encapsulation methods.By directly blending the PCM with the polymeric matrix, the hybridization technique reduces the need for complex encapsulation processes, making it a more costeffective and simpler strategy for stabilizing PCMs.Furthermore, this approach enhances the thermal conductivity of the PCM/polymer composite, improving its overall thermal energy storage capacity.A wide range of polymers are employed for form-stabilizing PCMs, each offering unique properties and advantages.For instance, polyurethane (PU) [46,76] is commonly used due to its exceptional mechanical strength and flexibility.Polyacrylates [77], on the other hand, provide good chemical resistance and thermal stability, making them suitable for specific applications.Polysaccharides [ 78,79] offer biodegradability and eco-friendliness, which is particularly important for environmentally conscious applications.
Polyvinyl alcohol [80] is chosen for its excellent film-forming properties and good compatibility with various PCMs.Table 3 provides a comprehensive tabulation of various polymer materials commonly employed for the hybridization of PCMs, showcasing their respective characteristics and applications.In a research work by Tang et al. [46] a form-stable composite was created using hexadecanol as the PCM and dye-linked PU as a unique supporting material.The preparation process is schemed in Fig. 1-III.In contrast to previous phase-change composites, where the polymer merely served as a supporting material, it also acted as a functional PCM in this case.The dye-linked PU incorporated in the composite absorbed visible light and converted it into thermal energy, which could then be stored by the PCM.The phase change enthalpy of the composite reached 229.5 J/g.Remarkably, the composite could accommodate a maximum weight percentage of hexadecanol up to 63.8% without any leakage issues.Likewise, paraffin was blended with epoxy resin and poly(propylene oxide) diamine to establish robust intermolecular forces between the PCM and epoxy (Fig. 1-IV), facilitated by their favorable compatibility.The resulting system exhibited a latent heat of 152.6 J/g [47].

Multifunctional applications of phase change materials
PCMs can offer a wide range of multifunctionalities, making them highly versatile for various applications.These multifunctional properties go beyond their primary function of storing and releasing thermal energy during phase transitions.In the subsequent sections, we will explore key functionalities that include selfhealing, self-cleaning, fire-retardancy, and enhanced electrical and thermal conductivity.These functions pave the way for innovative and pioneering applications of PCMs, which we will introduce and discuss in detail.

Self-healing polymers and flexible PCM composites
Mechanical damage and cracking are significant challenges faced by supporting scaffolds subjected to repeated deformation, and these damages often prove irreparable.However, polymers offer a promising solution due to their remarkable self-healing properties [89].The ability to autonomously heal from mechanical damage not only ensures the material's longevity but also enables efficient recycling, making polymers environmentally friendly and sustainable.To tackle the rigidity issue in PCM-based composites used for thermal management in electronic devices, researchers have developed a flexible composite PCM.This novel composite comprises paraffin, olefin, and MXene, a two-dimensional material known for its excellent mechanical strength and electrical conductivity.This composite PCM has demonstrated exceptional thermal management capabilities, ensuring efficient and safe heat dissipation while also exhibiting remarkable self-healing ability (89.6%) and superior flexibility.The self-healing mechanism proposed for this flexible composite PCM involves a sequence of events: Initially, the confined PCM melts under stress, creating a continuous liquid region that facilitates the initial joining of fracture surfaces.As the temperature rises, the increased thermal motion induces the recombination of soft segments within the polymer matrix.Consequently, the hard segment and paraffin domains fuse, forming a co-continuous phase that establishes a dominant connection bridge between fractured regions.This process effectively repairs the mechanical damage and restores the material's integrity [20].In another exciting application, a flexible waterborne polyurethane (WPU)/MXene aerogel was utilized as a supporting scaffold to confine PEG in photo-driven and flexible PCM composites.Some outcomes of this work are illustrated in Fig. 2-I.This flexible composite not only demonstrated efficient and controllable photodriven thermal energy storage capabilities but also showcased an impressive self-healing efficiency of approximately 83% [87].

Self-cleaning superhydrophobic PCM composites
PCMs incorporated with photothermal fillers present a groundbreaking capability to harness sunlight and efficiently store the converted heat.Photothermal fillers, such as MXene, graphene, and CNTs, play a crucial role in enhancing the heat absorption properties of PCM composites while also improving their hydrophobicity and self-cleaning characteristics [21].These enhancements further broaden the potential applications of PCM-based materials and increase their efficiency.One remarkable advantage of incorporating superhydrophobic coatings into the PCM composites is the creation of waterproof materials that preserve the original properties of both the supporting materials and the PCMs.By achieving superhydrophobicity, the composite becomes highly waterrepellent, preventing liquid infiltration and maintaining its stability even in wet or humid conditions.Additionally, the self-cleaning properties associated with superhydrophobic surfaces minimize the accumulation of contaminants, dust, and other particles, ensuring sustained performance and longevity.
An exceptional example of this technology is the development of a superhydrophobic form-stable composite by applying a superhydrophobic coating to its surface.In this case, as schemed in Fig. 2-II, delignified wood was utilized as the supportive material to confine 1-tetradecanol PCM.The resulting composite exhibited an impressively high water contact angle of 155 , indicating excellent water repellency.Notably, the superhydrophobic stability of the composite was maintained within a broad temperature range of 20e100 C and a pH range of 3e12, ensuring robust performance across various environmental conditions.Beyond its hydrophobic properties, this superhydrophobic form-stable composite displayed remarkable thermal energy storage capabilities, with a high energy storage capacity of 125 J/g.Moreover, the incorporation of photothermal fillers enabled efficient conversion of solar energy to thermal energy, making the composite an effective solar heat absorber.The synergistic effects of photothermal fillers and superhydrophobicity contributed to the outstanding overall performance of the material.The exceptional features of this composite make it highly suitable for advanced energy-related devices and systems that require reliable thermal energy storage in wet or humid environments [90].Ongoing research in this field aims further to optimize the composition and design of these composites, exploring new photothermal fillers and superhydrophobic coatings and enhancing their overall performance and stability.With continued advancements, such materials have the potential to revolutionize energy storage technologies, enabling more sustainable and efficient solutions for a wide range of practical applications.

Fire-resistant PCM-polymer composites
Ensuring fire resistance during the shape stabilization of PCMs with polymers is often challenging since both the supporting polymer and enclosed PCMs lack inherent fire resistance [91].One of the most effective and commonly used strategies to enhance the fire resistance of polymers involves incorporating flame-retardant compounds into the polymer matrix.Halogen-based FRs, like brominated aliphatic and aromatic compounds, have demonstrated significant improvements in the flame-retardant properties of polymers.Even at low loadings (e.g., 5 wt%), these additives efficiently minimize their impact on other polymer properties.Despite their widespread use in the plastics industry due to excellent performance and low cost, halogen-based FRs pose serious environmental and health concerns due to the release of toxic materials, leading to efforts to limit or ban their use.
At present, the primary approach is to integrate halogen-free flame retardants, which include phosphorus/nitrogen-based flame materials, nanofillers, and metal hydroxides.Halogen-free flame retardants have gained popularity due to their superior environmental profile and safety features.For instance, traditional and representative flame retardants like red phosphorus (RP) amorphous powders exhibit exceptional characteristics, including non-volatilization, absence of corrosive gas generation, and effective flame retardancy.Another widely employed halogen-free flame retardant is ammonium polyphosphate, which is known for its high phosphorus and nitrogen content, low toxicity, excellent thermal stability, long-lasting flame-retardant performance, and high efficiency in retarding fires.APP serves as an acid source and foaming agent in intumescent flame retardants, creating an intumescent char on the material's surface during combustion.This char hinders the transfer of heat and toxic smoke, thereby enhancing thermal stability and flame retardancy by reducing the concentration of oxygen in the air and blocking its supply [92e96].

Efficient light/electro-thermal energy conversion with advanced PCMs
PCMs exhibit exceptional efficiency in storing thermal energy under isothermal conditions, resulting in minimal temperature differences between the stored and released energy.As a result, they are considered ideal candidates for storing and releasing thermal energy through latent heat during phase changes.Recently, there has been a growing focus on developing composite PCMs with energy conversion properties, such as solar photothermal conversion and electrothermal harvesting techniques.These composite materials are gaining popularity due to their ability to combine a large energy storage capacity with high energy charging efficiency [97e100].
Advanced functional light-/electro-thermal conversion PCMs play a crucial role in sustainable energy utilization by efficiently converting electrical energy to thermal energy [101].However, conventional pristine PCMs, particularly organic ones like paraffin waxes (PWs), non-PWs, fatty acids, alcohols, esters, etc., exhibit inherent limitations such as low thermal conductivity (0.1e0.4 W/ mK) and high electrical resistivity (107e1012 U/m).Consequently, these pristine PCMs have insulating characteristics, making it challenging to initiate the light-/electric-to-thermal energy conversion and storage process directly [102,103].
To address this challenge, researchers have turned their attention to encapsulating PCMs within highly electrically and thermally conductive supporting materials.Various endeavors have been made to enhance the electrical and thermal conductivity of PCM composites using conductive materials, including highly graphitized carbon, graphene, graphite, CNTs, biomass-derived carbon, metal-organic frameworks (MOFs)-derived carbon, and conductive polymers [104e107].For more comprehensive information on the recent advancements in developing conductive PCM composites for electrothermal conversion and storage, please refer to Table 4.

PCMs for enhanced solar energy applications
Renewable energy technology and capacity building are poised to play a pivotal role in mitigating the effects of global warming and climate change.Among the various renewable energy sources like solar energy, wind energy, ocean energy, and geothermal energy, solar energy stands out as the most abundant and readily available source.Its utilization helps meet the escalating energy demand sustainably while reducing emissions linked to the overexploitation of fossil fuel reserves.However, the intermittent nature of solar energy due to the Earth's rotation poses challenges to its continuous utilization.To ensure constant access to solar energy, efficient heat storage solutions are essential for use during nighttime and rainy days, enabling energy peak regulation.TES emerges as a key facilitator in enhancing the performance and feasibility of solar thermal technologies.Utilizing PCMs for latent heat storage proves to be an effective technique for charging, storing, and discharging thermal energy on-demand.PCM-based latent heat storage involves the absorption and release of heat during phase change processes, offering advantages such as high heat storage capacity, minimal temperature fluctuation (similar to isothermal conditions), chemical stability, and safety.PCMs boast excellent heat storage capabilities, with phase transition temperatures remaining relatively constant.As a result, they find wide applications in solar heat collection [122e127].
PCMs offer latent heat by undergoing a phase transition, where they change their state of matter without altering the temperature.This process allows PCMs to absorb or release substantial amounts of latent heat.Depending on their states, PCMs can be categorized into solideliquid, liquidegas, solidesolid, and solidegas PCM.Among these, solideliquid PCMs are the most widely used due to their various phase transition temperatures, significant latent heat of phase transition, and cost-effectiveness [128].Solideliquid PCMs can be further classified into organic, inorganic, and eutectic PCMs [129].They find frequent applications in solar heat collection and storage.However, as mentioned earlier, PCMs have certain drawbacks, such as low thermal conductivity, poor solar-to-thermal conversion efficiency, and the risk of leakage during phase transition.These thermo-physical limitations restrict the potential use of PCMs as effective TES materials.To overcome these challenges, form stabilization using appropriate support materials, along with the development of PCM composites incorporating thermally conductive materials, can effectively address these issues.Consequently, conductive PCM composites hold promise as potential candidates for capturing thermal energy for efficient utilization, converting solar energy to electrical or thermal energy, and storing waste heat for specific applications.
PPy is a conductive polymer known for its unique optical and photothermal properties, making it a captivating material for various applications, particularly in photothermal conversion.Similarly, carbon-based particles such as graphene and CNTs also exhibit remarkable photothermal properties.As a result, extensive research has been conducted to develop composite PCMs containing PPy, carbon-based particles, or a combination of both for solar-/electro-thermal energy conversion and storage.Several examples of such research works can be found in the literature [113,114,118,130,131].For instance, Liu et al. [131] utilized chemical polymerization and physical infiltration methods to create composite PCMs consisting of PPy-coated EG.This combination of PPy and EG led to a significant improvement in both the photothermal conversion and storage capabilities of the PCMs.The composite PCMs exhibited remarkable shape stability and latent heat storage retention, even after undergoing 200 melting-freezing cycles.Moreover, these composites displayed typical photothermal conversion and storage behavior when exposed to light radiation, showcasing the promising potential for solar energy utilization.
In a separate study, Liu et al. [132] developed shape-stabilized PCMs (SSPCMs) using carbonized melamine foam/graphene aerogel as the supporting material and PEG as the PCM.The resulting composite PCM demonstrated exceptional shape stability, high latent thermal storage capacity (181.2J/g), thermal cycling stability, and enhanced thermal conductivity (387% higher than pure PEG).Additionally, the composite PCM exhibited a high solar-to-thermal energy conversion efficiency of 91.7% and a high electric-to-thermal energy conversion efficiency of 87.3%.Furthermore, Shue et al. [133] manufactured highly thermally conductive phase change composites by incorporating vertically aligned graphene/cellulose nanofiber aerogels impregnated with paraffin.This composite PCM demonstrated an exceptional thermal conductivity of 15.9 W/mK at a low graphene loading of 3.35 wt%, which resulted in excellent solar-thermal-electric energy conversion capabilities.Under solar light irradiation of 5 kW/m 2 , the composite PCM achieved a high output voltage of 823.2 mV, further exemplifying its potential for energy-related applications.
In another approach, Tao et al. [117] successfully created composite PCMs with enhanced thermal transfer capabilities by incorporating PPy/Fe 3 O 4 -functionalized hollow KF aerogel supports with PW.The resulting composite PCMs displayed remarkable characteristics, including a high thermal storage density, with a melting enthalpy of 161.4 J/g for PW load mass fractions of 88%.The thermal conductivity of the composite was significantly improved, reaching 1.06 W/mK, which was 307% higher than that of pristine PW.The researchers also assessed the solar-thermal energy conversion performance of the developed PCM and summarized their findings in Fig. 3-I.Under a light intensity equivalent to one sun (~1 kW/m 2 ), the samples showed similar curves, with the surface temperature steadily increasing from 25 C to approximately 50 C within 100 s.After 150 s of irradiation, a temperature plateau formed around 50 C due to the equilibrium between solar thermal conversion and heat dissipation.Subsequently, the surface temperature of the samples continued to rise rapidly, reaching approximately 80 C in 300 s and remaining stable at that level.Notably, the surface temperature of the composite PCMs reached 80 C, whereas it was less than 50 C for the pure PW samples.This significant difference was attributed to the high thermal conductivity and efficient heat transfer of the PCM composites.Upon turning off the light, the surface temperatures of the samples experienced a rapid decrease, except for the pure PW sample.In the case of the composite PCMs, the cooling rate significantly decreased when the surface temperature of the samples dropped to around 52 C, and a cooling plateau appeared.This plateau resulted from the exothermic process of the liquid-solid phase transition, indicating effective temperature control during the phase change period.
MOFs, a novel class of crystalline porous materials composed of central inorganic metal ions and organic ligands, hold tremendous potential in catalysis, adsorption, energy storage, and various other fields.Notably, their high porosity makes them ideal candidates to serve as carriers for PCMs [135,136].As a result, significant research efforts have been dedicated to developing MOFs-based shape-stabilized composite PCMs to enhance solar thermal utilization [134,137e140].For instance, Yan et al. [137] functionally modified MOFs with SA using an in situ hydrothermal method, expanding the scope of photothermal materials for solar energy utilization, particularly in solar water heater systems.The resulting composite PCMs displayed excellent shape stability, high latent heat of 126.4 J/ g, heightened thermal conductivity, outstanding thermal reliability, and remarkable solar-thermal energy conversion capability, reaching an efficiency of 96.6%.
In a separate investigation, a bimetallic organic framework based on nickel and cobalt (NieCo-BTC) was utilized as a supporting material for 1-octadecanol (OD) [141].The NieCo-BTC exhibited a flower-like structure with interconnected networks, providing a large specific surface area, pore volume, and appropriate pore size distribution, which facilitated the encapsulation of OD (up to 70 wt%).To further enhance the photothermal conversion performance of the composite PCM, titanium nitride (TiN) nanoparticles were incorporated.The resulting composite PCM, OD/ NieCo-BTC/TiN, demonstrated a significant TES capacity of 145.8 J/ g and an impressive photothermal energy conversion efficiency of up to 97.75%.Furthermore, Tang et al. [134] developed a remarkable and sustainable solar-driven energy storage system that efficiently supplied both electricity and heat by seamlessly integrating PCMs and magnetic Co-decorated hybrid graphitic carbon and N-doped carbon (Co-GC@NC) nanocages.The magnetic Co nanoparticles and the GC@NC carbon hybrid exhibited a synergistic effect, leading to superior full-spectrum absorption and a high solar-thermal conversion efficiency of 90.7%.Fig. 3-II provides a schematic illustration of the solar-thermal energy conversion mechanism.Remarkably, when a magnetic field was applied, the solar-thermal energy conversion and storage efficiency of the Co-GC@NC-based composite PCMs were significantly enhanced by 115.8% due to the exceptional magnetic manipulation ability of the nanocages.Additionally, the designed solar-thermal energy conversion and storage system achieved outstanding performance with a maximum output voltage of 290 mV and a current of 92.6 mA.
The integration of PCMs, particularly conductive polymer composites, and innovative MOFs-based systems, has showcased promising advancements in overcoming the inherent limitations of PCMs, such as low thermal conductivity and efficiency.These novel materials exhibit notable features, including high thermal storage capacity, shape stability, and efficient thermal conductivity, contributing to their effectiveness in capturing and storing thermal energy for various applications.While these advancements present significant strides in the field, challenges such as addressing the risk of leakage during phase transition and optimizing efficiency persist.Continued research and innovation, particularly in refining the synthesis and application methodologies of these composite PCMs, will likely contribute to further improvements in solar energy utilization and storage.The demonstrated high solar-tothermal energy conversion efficiencies and impressive thermal stability indicate a promising future for these materials in various energy-related applications.

PCMs in building applications for enhancing energy efficiency and thermal comfort
Global warming has triggered a range of adverse environmental consequences, with greenhouse gas emissions, particularly carbon dioxide emissions, being the primary culprit.Regrettably, in the year 2020, the building and construction sector accounted for a substantial 36% of global final energy consumption and 37% of energy-related carbon dioxide emissions.Notably, emissions from building operations contributed approximately 28% of the world's total energy-related carbon dioxide emissions, while HVAC (heating, ventilation, and air conditioning) systems alone were responsible for over 60% of total building energy consumption.Shockingly, the building sector in the United States alone was estimated by the International Energy Agency to produce about 30% of the nation's total CO 2 emissions.This alarming scenario underscores the urgent necessity to enhance the energy efficiency of both commercial and residential buildings on a global scale, aiming to optimize energy usage and drastically reduce CO 2 emissions [142,143].
In pursuit of this imperative goal, TES has emerged as a highly effective pillar of sustainable building practices, capable of significantly curbing greenhouse gas emissions and energy consumption.TES can be categorized into three main types: sensible heat storage, latent heat storage, and thermal chemical heat storage.Among these, PCM energy storage stands out as a passive heat modulation technology that harnesses latent heat storage.Over the past few years, PCMs have gained increasing recognition for their vital role in heat modulation across diverse industries, including aerospace, clothing, military, agriculture, electric power, refrigeration equipment, and electronic devices.Moreover, PCM applications have extended to the construction sector, where they have been instrumental in reducing building energy consumption and promoting a healthier indoor environment.Presently, PCM finds widespread use in the practical realization of net-zero energy consumption buildings, delivering substantial economic benefits and positive social impacts by conserving energy and safeguarding the environment [2,144e146].Solideliquid PCMs are commonly employed in buildings.However, liquid PCMs pose challenges due to potential leakage and heat storage capacity loss during phase change.Therefore, selecting the right PCM incorporation method is crucial.The existing PCM incorporation techniques for buildings include direct methods such as wet mixing and immersion, as well as indirect methods like micro-encapsulation, macro-encapsulation, and form-stable composite PCM [147].
In the context of building applications, there has been significant interest in the utilization of SSPCMs due to their appropriate melting temperature, substantial latent heat capacity, suitable thermal conductivity, and ability to endure multiple thermal cycles without the need for encapsulation.The integration of SSPCMs into building envelopes can significantly enhance overall energy efficiency.However, the widespread adoption of PCMs in building envelopes hinges on the development of cost-effective SSPCMs with anti-leakage properties and excellent compatibility with building materials.SSPCMs consist of both supporting materials and PCMs, with enhancers added to further enhance thermal conductivity.During usage, SSPCMs efficiently absorb and release heat through the working substance, while the supporting matrix prevents leakage and improves the thermal conductivity of the composite PCM.SSPCMs can be classified as organic, inorganic, or eutectic, and various supporting materials can effectively inhibit the flow of liquid PCMs.Examples of such materials include expanded perlite, EG, vermiculite, styrene-butadiene, high-density polyethylene, and bio-based polymers, among others.Incorporating SSPCM elements into construction materials like walls, floors, roofs, and windows is both feasible and advantageous for enhancing building energy efficiency [146,148e150].

PCMs as thermal energy storage systems in building
Hekimo glu et al. [151] effectively integrated carbon-based nanoadditives into fly ash/octadecane shape-stabilized composite PCMs, showcasing the feasibility of employing fly ash, an industrial solid waste, to develop environmentally friendly composite PCMs.Xu et al. [152] developed a composite PCM for floor radiant heating, incorporating diatomite to reduce leakage and aluminum nitride (AlN) to enhance thermal conductivity.The sodium acetate trihydrate-acetamide-AlN/diatomite SSPCM exhibited excellent thermal reliability, with minimal impact on phase change temperature and supercooling degree.Zhang et al. [153] utilized mica as the supporting material to synthesize the KH-550-decorated mica/EG/PEG composite PCM, enhancing thermal conductivity by adding EG.Huang et al. [154] employed artificial culture methods to create a diatom-based biomass/PEG composite PCM, achieving high melting and freezing latent enthalpies of 121.54 J/g and 127.20 J/g, respectively, with an impressive relative enthalpy efficiency of 98.1%.
Wang et al. [155] developed three-dimensional attapulgitebased composite PCMs and investigated their thermophysical properties.They observed that the melting temperature and latent heat of the composite PCMs decreased with reducing carriers' pore size, and the size effect of support matrix was described using the Gibbs-Thomson equation.Zhang et al. [156] proposed a novel SSPCM by incorporating SA in the Kaolinite nanotube, with the SA/ Kaol-nanotube composite PCMs displaying a melting temperature of 52.4 C and a latent heat of 47.5 J/g, indicating the potential of Kaolinite nanotubes as supporting matrices.Shi et al. [157] prepared PEG/modified attapulgite SSPCMs, demonstrating that using modified attapulgite as a supporting material improved the crystallinity of PEG and significantly enhanced the latent heat and thermal reliability of the SSPCM.Luo et al. [158] fabricated a paraffin/sepiolite composite PCM through vacuum impregnation, resulting in a substantial increase in the latent heat of melting and crystallization.The composite exhibited exceptional thermal and chemical stability even after numerous cycle tests.
Gu et al. [159] used fly ash as the supporting material and LA as the working substance to create a composite PCM, incorporating CNTs to enhance thermal conductivity.The prepared composites exhibited high enthalpy values, while the preparation process and raw materials remained cost-effective and environmentally friendly.Zhang et al. [160] utilized raw palygorskite (PAL) as the supporting material to absorb paraffin and form composite PCMs.The resulting paraffin/PAL-3 composite exhibited a melting enthalpy of 89.2 J/g and a temperature of 58.55 C. Remarkably, after 300 testing cycles, the enthalpy of the prepared PCMs still reached 96.2% of the initial value, and the temperature was maintained at 80 C for 12 h.Chen et al. [161] developed a series of formstable PCMs for building applications, using paraffin as the PCM and three types of PE as supporting materials.They extensively studied the compatibility of paraffin with each type of PE and recommended using high-density polyethylene/paraffin form-stable PCMs to minimize interior temperature fluctuations in buildings.Wang et al. [162] utilized PMMA as supporting materials to synthesize form-stable PCMs, and they investigated the relationship between the content of lauric acid-myristic acid (LA-MA) and the properties of the composites, determining that the optimal content of LA-MA was 70%.
Li et al. [163] prepared form-stable PCMs using a paraffin eutectic mixture as PCMs and PP as supporting materials.The resulting paraffin/PP form-stable composite PCMs exhibited a high enthalpy of 126.8 J/g, with a phase change temperature of 24.8 C, making them suitable for building energy conservation in a comfortable indoor environment.Kong et al. [164] conducted a study on PU/LA form-stable PCMs and found that they possessed superior thermal properties, rendering them suitable for use with solar energy in building applications.Tina et al. [165] successfully prepared a form-stable PCM with thermal conductivity by utilizing ethylene-vinyl acetate, paraffin, carbon fiber, and EG.The DSC results showed that the phase-transition temperature of the FSPCM was 45.63 C, with a tested latent heat of 167.4 J/g.

PCMs as thermal insulations in building
Thermal insulation is of utmost importance, especially in hot regions, to ensure optimal thermal comfort within buildings.By effectively insulating buildings, the size of air-conditioning systems can be reduced, leading to substantial annual energy cost savings.Additionally, proper thermal insulation allows for prolonged comfort without overreliance on mechanical air-conditioning systems, particularly during inter-season periods.For buildings located in cold cities, employing optimum insulation thickness and other energy-saving methods can significantly reduce heat losses and result in a remarkable 50% reduction in carbon dioxide emissions.Insulation materials should meet specific criteria, being adaptive, friendly to project site employees, and offering enhanced performance while justifying the invested cost.In passive buildings, extruded polystyrene foams are often preferred for their excellent thermal insulation properties [166].
PCMs play a pivotal role in thermal management applications, particularly in building insulation.These substances can store and release significant amounts of thermal energy during phase transitions (e.g., solid to liquid or vice versa).PCMs absorb excess heat when ambient temperatures are high or when the building's interior is warmer than desired, and during cooler periods, they solidify, releasing the stored heat back into the environment.By absorbing and releasing heat, PCMs efficiently regulate indoor temperature, ensuring a more stable climate and mitigating rapid temperature fluctuations.This leads to reduced energy consumption for heating and cooling systems, making the building's HVAC system operate more efficiently.This, in turn, contributes to a more sustainable and environmentally friendly building design.Moreover, occupants of the building can enjoy increased comfort levels throughout the day and night, regardless of external weather conditions, thanks to the effective temperature regulation provided by PCMs.Notably, PCMs are especially valuable in regions with significant day-to-night temperature variations.They find applications in residential buildings, commercial spaces, and even greenhouses, where maintaining a stable temperature is crucial for optimal performance [167e169].
With the aim of creating an efficient thermoregulating PCM insulation for buildings, Abden et al. [170] assessed the influence of incorporating expanded polystyrene and PCM gypsum board into the building envelope of a typical standalone Australian house.Their evaluation utilized numerical simulations, considering the house's location in three different Australian cities.The results demonstrated considerable cost savings over a 10-year life cycle, with the optimal combination of PCM board and insulation.Similarly, Yasiri et al. [171] investigated the role of traditional expanded polystyrene (EPS) thermal insulation of varying thicknesses in enhancing the thermal performance of building envelopeintegrated PCM during harsh summer months.The PCM-EPS rooms showed enhancements of 143% in maximum indoor temperature reduction, 177.2% in a time lag, 35% in average temperature fluctuation reduction, and 8.5% in average operative temperature reduction.In another study, Ong et al. [172] explored a passive cooling strategy involving the incorporation of microencapsulated PCMs and glass bubbles into paint and coatings on mortar panels in tropical regions.The developed PCM-based mortar panel, comprising 30% PCM and 20% glass bubbles, exhibited significant reductions in both surface temperature (3.2 C) and ambient temperature (7.0 C) in both laboratory-based and outdoor-based parametric studies.Similarly, Wi et al. [173] optimized the external insulation plastering method using PCM to enhance the efficiency of thermal energy use.Meanwhile, Nematchoua et al. [174] conducted a simulation to evaluate, analyze, compare, and discuss the impacts of passive strategies on thermal comfort and energy consumption in coastal tropical climate regions.They concluded that PCMs had a noteworthy effect on thermal comfort and energy consumption in an office under different coastal tropical climates.
The integration of PCMs in building applications for enhancing energy efficiency and thermal comfort represents a crucial step toward addressing the environmental impact of the construction sector.TES technologies, particularly PCM energy storage, have emerged as effective tools for sustainable building practices.The utilization of solideliquid PCMs, especially shape-stabilized PCMs, has garnered attention due to their appropriate melting temperatures, latent heat capacity, and ability to endure multiple thermal cycles.These innovative materials play a pivotal role in developing net-zero energy consumption buildings, thereby contributing to economic benefits and positive environmental impacts.As research to refine PCM formulations, address challenges, and explore novel applications, the potential for widespread adoption of these technologies in building practices remains high, ushering in a more sustainable future for the construction industry.

The power of PCMs in thermoregulating textiles for enhanced comfort and energy efficiency
Clothing, often described as a second skin for humans, is an essential requirement that not only acts as a protective barrier but also provides warmth by insulating the air around the body, reducing heat transfer through convection and radiation from solar energy.In contrast to conventional textiles, smart textiles have the unique ability to respond to various physical stimuli in the surrounding environment, such as thermal, electrical, and mechanical changes.These functional smart textiles, which include thermoregulated, thermo-electric, piezoelectric, and shape memory textiles, have emerged and found diverse applications in energy storage, energy harvesting, personal protection, physiological monitoring, and other areas [175e177].
In hot summer conditions, outdoor workers, like traffic police officers, often require comfortable textiles to cope with high environmental temperatures.While traditional textiles primarily rely on weakening heat conduction and convection to maintain body temperature, they may not suffice in resisting extreme heat.To address this challenge, researchers proposed a viable solution in the form of smart textiles, achieved by integrating functional materials into textiles that can regulate the microenvironment temperature.PCMs have been harnessed in textiles, presenting unique advantages in the realm of clothing and fabric-based applications.Termed as 'smart fabrics' or 'smart textiles,' PCMs offer the ability to respond to temperature changes and deliver enhanced comfort to the wearer.By incorporating PCMs into textiles, the primary objective is to regulate body temperature effectively.When the surrounding environment or body temperature rises, the PCM absorbs excess heat and undergoes a phase change from solid to liquid, storing the heat energy and preventing the wearer from feeling excessively hot.Conversely, when the temperature drops, the PCM solidifies, releasing the stored heat to keep the wearer warm.This temperature-regulating property ensures improved comfort for the wearer, which proves valuable in situations with fluctuating environmental conditions, such as outdoor activities or regions with significant temperature variations.
An essential feature of PCM textiles is their ability to provide a thermal mass effect, effectively buffering temperature changes.This prolonged heat absorption and release over time result in more extended temperature control compared to conventional fabrics.The application of PCM textiles extends across diverse industries, including sportswear, outdoor clothing, bedding, and medical textiles.In sportswear, PCM-enhanced fabrics help athletes stay comfortable during physical activity.In bedding, PCM materials offer more consistent sleeping temperatures.Moreover, medical textiles may utilize PCMs to regulate body temperature for patients in extreme environments.With the integration of PCMs into textiles, the advancement of smart fabrics paves the way for improved comfort, energy efficiency, and enhanced performance in a variety of practical applications, catering to the evolving needs and preferences of consumers across different sectors.
To achieve this objective, Wang et al. [176] developed an innovative smart textile using PEG for UV shielding and temperature regulation.The resulting smart textiles exhibited favorable surface morphology, good compatibility, and excellent thermal stability.Notably, these smart textiles had a latent heat capacity of 51.14 J/g and demonstrated robust thermal reliability, remaining effective even after 500 heatingecooling cycles.Similarly, Lu et al. [175] developed thermo-regulated core-sheath structured smart textiles using PW as the core layer and polyacrylonitrile (PAN) as the sheath layer through a coaxial electrospinning technique.To enhance the utilization efficiency of solar energy, they incorporated hexagonal cesium tungsten bronze with excellent near-infrared (NIR) region absorbing ability into the textiles.The smart textiles exhibited a high encapsulation efficiency of 54.3% and possessed a latent heat capacity of 60.31 J/g.Moreover, they demonstrated excellent stability, retaining their latent heat capacity even after 500 heatingcooling cycles.Additionally, Lu et al. [178] utilized coaxial electrospinning to create luminous thermo-regulated smart textiles comprising a PW core and a PAN/phosphors sheath.These textiles achieved an impressive PW encapsulation efficiency of 52.1%, resulting in an enthalpy of 64.08 J/g.The unique feature of luminosity further enhances the utility of these smart textiles in various applications, ensuring both thermal regulation and visibility.Furthermore, Laza et al. [179] successfully produced thermosensitive and waterproof PU fibers suitable for the textile industry.Fig. 4 illustrates several knitted smart fabrics, with different grammages, obtained from the synthesized PUs loaded with these microencapsulated PCMs.The resulting knitted smart fabrics demonstrated different grammages obtained from the synthesized PUs loaded with these microencapsulated PCMs.The promising results of these thermo-regulating materials open up new possibilities for applications across the textile sector.
In pursuit of creating a smart thermo-regulated textile with specific attributes such as a comfortable phase change temperature (e.g., 25 C), a high latent heat value of up to 80 J/g, excellent shape stability at high temperatures (e.g., 100 C), acceptable waterproof ability, and reliable thermal recycle performance, Zhang et al. [180] achieved a significant breakthrough.In another pioneering study, Baniasadi et al. [18] introduced a smart multifunctional textile comprising a PCM, PEG, with the potential for thermo-regulative biomedical applications.The fabricated mats exhibited a latent heat of 61.7 J/g and demonstrated reliable energy absorptionrelease cyclability over 100 heatingecooling cycles, showcasing their suitability for practical and long-lasting usage.Iqbal et al. [181] reported the development of a smart monofilament fiber by incorporating microencapsulated PCM through the melt-spinning process.By successfully incorporating up to 12% microcapsules into the PP monofilament, they achieved a latent heat of 9.2 J/g.Furthermore, Zhang et al. [182] prepared polyimide/boron nitride composite aerogel fibers with a highly porous structure and excellent mechanical strength using the freeze-spinning technique.This composite textile boasted a high thermal conductivity of 5.34 W/mK, a high enthalpy of 125.2 J/g, and remarkable work stability after 100 heating-cooling cycles, demonstrating the desired thermoregulating performance.
PCM textiles demonstrate significant advancements in creating smart textiles with specific attributes, such as UV shielding, temperature regulation, and energy efficiency.The breakthroughs in developing multicores-sheath nanostructures, form-stabilized nanofiber mats, and monofilament fibers showcase the versatility and potential applications of PCM textiles.They offer a dynamic solution for diverse applications, including sportswear, outdoor clothing, bedding, and medical textiles.Overall, the integration of PCMs in thermoregulating textiles holds promise for widespread adoption, contributing to a future where clothing adapts to environmental conditions, providing unparalleled comfort and functionality.

Application of PCMs in biomedical fields
PCMs have garnered significant attention in biomedical applications due to their unique thermal properties and ability to store and release substantial energy during phase Their thermoregulatory capabilities have proven valuable in improving patient comfort, optimizing drug delivery, and enhancing medical procedures.In the biomedical field, PCMs find utility in various applications, including thermotherapy, cold compress therapy, drug delivery systems, anticancer therapies, wound healing, bone cement, and medical transport.For instance, medical textiles and clothing can incorporate PCMs to regulate body temperature during treatments like chemotherapy, where temperature fluctuations can be common, leading to increased patient comfort.Hyperthermia treatments benefit from PCMs by providing controlled heating to specific body areas, enabling precise targeting and destruction of cancer cells.In drug delivery systems, PCMs control the release of medications at specific temperatures, allowing for more sustained and controlled drug delivery.Additionally, in surgical applications, PCMs can regulate temperatures during procedures, helping maintain a stable and appropriate operating environment through the development of surgical drapes or pads.Furthermore, PCMs are used in wound dressings to provide a cooling effect, reducing inflammation and pain, particularly in burn injuries [183e185].
Apart from direct applications, PCMs play an indirect role in biomedical fields, such as cold storage materials, in maintaining the internal temperature stability of cold storage boxes.Cold storage materials using PCMs with high latent heat ensure the insulation effect and thermal resistance of cold storage boxes, which are crucial for preserving vaccines, medicines, and other temperaturesensitive products during the cold chain process.The development of PCMs with high cold storage capacity, slow cold release, and suitability for different transportation temperatures is a key focus in cold storage material research [186].
While exploring the wide-ranging applications of PCMs in biomedical fields may require a comprehensive review paper, this section provides a glimpse into their use as SSPCMs in biomedical applications.The versatility and potential of PCMs continue to drive research efforts, aiming to improve medical treatments and procedures, enhance patient experiences, and ensure the integrity of temperature-sensitive medical products.
With the goal of achieving controlled drug release, Qiu et al. [187] developed a straightforward method for encapsulating a mixture of two natural fatty acids with a eutectic melting point at 39 C in a biocompatible, silica-based nanocapsule.When subjected to photothermal heating, the fatty acids melted, releasing the payloads.In another study by Zhang et al. [188], they developed fibrous hydrogels, mimicking the extracellular matrix, through electrospinning and UV-crosslinking, followed by the incorporation of fatty acids/aspirin (ASP) encapsulated polydopamine.These hydrogels exhibited a significantly faster release of ASP at 40 C compared to 25 C and 37 C. Baniasadi et al. [18] developed thermoregulating biomedical dressings composed of PEG and polycaprolactone, incorporating gelatin and curcumin to enhance the biomedical performance of the textiles.These curcumin-loaded textiles exhibited a latent heat of 61.7 J/g and demonstrated reliable energy absorption-release cyclability over 100 heating-cooling cycles, with an initial burst release followed by sustained release over one week.
In another study by Lin et al. [189], inspired by the oriented 'brick-and-mortar' structure in bone tissue, they reported a novel approach to developing a form-stable, ultrastrong, and highly energy-stored composite wood-based PCM (PWPCM).The resulting PWPCM showcased a high tensile strength of 81.9 MPa and an elastic modulus of 10.2 GPa along the longitudinal direction, mimicking the structure of bone tissue.Moreover, the PWPCM exhibited a wide phase transition temperature range and high enthalpy of 116.1 J/g, along with exceptional temperature regulation and shape stability properties.In their pursuit of developing flexible and shape-memory PCM with rapid electric heating function for wearable thermotherapy, Lin et al. [190] prepared a novel conductive flexible CPCM.This composite material was formed by absorbing paraffin and silicone oil onto a three-dimensional network of styrene, ethylene butylene styrene, and EG.The CPCM a high energy storage density ranging from 100.8 to 164.5 J/g and sufficient volume conductivity of 383.76 S/m.Testing by volunteers revealed that the material maintained a temperature above 40 C for more than 30 min, making it promising for thermotherapy applications.Similarly, Chen et al. [191] designed advanced CNT bundles assembled with flexible hierarchical framework-based PCM composites for high-performance thermotherapy of allergic rhinitis.The structure was constructed using table salt as a sacrificial template, resulting in a hierarchical CNT sponge that served as an ideal compatible supporting host for PEG.Pristine CNT sponge served as an excellent particulate matter capturer, while PEG-infiltrated CNT sponge acted as a superior thermal regulator.The thermotherapy schematic is shown in Fig. 5.This innovative approach created a multifunctional PCM platform with fascinating properties, offering a new and promising avenue for advanced biomedical applications.
Zhou et al. [192] devised a method for creating stretchable phase change composites with dual-functional layers by combining a photothermal phase change layer with an electrothermal conductive layer.This novel approach resulted in a composite material capable of undergoing a phase change and conducting electricity, making it suitable for applications in flexural heaters, medical electrothermal devices, and defrosting systems.In another study by Yu et al. [193], a textile with phase change and electrothermal functions was fabricated, showing potential for various practical uses.Furthermore, Ong et al. [194] aimed to develop a practical solution for maintaining sub-zero temperatures during the transport of frozen blood samples.They designed a heat-insulated system consisting of 23.3% w/w saltwater as the PCM for encapsulating a 1.5-mL vial of frozen blood samples.Using their model, they predicted that the PCM, pre-cooled in dry ice, and the vial sample kept initially at 20 C before a flight, would maintain a stable transport temperature of À20 C for at least 70 min.
The application of PCMs in biomedical fields has demonstrated remarkable potential, leveraging the unique thermal properties of PCMs to enhance various aspects of medical treatments and procedures.From thermotherapy to drug delivery wound healing, and even the preservation of temperature-sensitive medical products during transportation, PCMs have showcased versatility and efficacy.The integration of PCMs in biomedical research has not only improved patient comfort but has also optimized drug release profiles, enabling more controlled and sustained delivery of medications.Moreover, the use of PCMs in medical textiles, surgical drapes, and wound dressings has demonstrated tangible benefits in maintaining stable operating environments and promoting faster wound healing with reduced inflammation.The continual exploration and refinement of PCM-based technologies in biomedical applications hold promise for advancing medical treatments, improving patient outcomes, and ensuring the integrity of temperature-sensitive medical products in the future.

PCMs in advanced electronics cooling and thermal management
PCMs have been explored and utilized in electronics for various applications due to their unique thermal properties and ability to store and release large amounts of energy during phase transitions.In the context of electronics, PCMs are particularly valuable for their ability to regulate temperatures and manage heat dissipation, which is crucial for the efficient operation and longevity of electronic devices.One common application of PCMs in electronics is in thermal management.Electronic devices generate heat during operation, and if not properly managed, excessive heat can lead to reduced performance, component failure, and even safety hazards.PCMs can be used to absorb and store excess heat from electronic components and release it when the temperature decreases, effectively acting as thermal buffers.This helps maintain a more stable operating temperature and prevents temperature spikes, improving the reliability and efficiency of electronic devices.PCMenhanced heat sinks and thermal interface materials are commonly used in electronic systems to improve heat dissipation.These materials have the ability to absorb and release heat during phase transitions, allowing them to efficiently conduct heat away from hotspots in electronic components and distribute it across a larger area for dissipation.This helps in reducing the risk of thermal throttling and ensuring that the electronic devices can operate at their optimal performance levels.
Additionally, PCMs can be used in electronic devices that undergo frequent heating and cooling cycles.For example, in devices that require repeated rapid charging and discharging, such as lithium-ion batteries in smartphones and laptops, PCM-enhanced battery materials can help manage the heat generated during charging and discharging cycles, improving battery performance and longevity.Furthermore, PCMs have been explored for use in TES in electronics.They can be integrated into electronic devices to store excess heat generated during peak usage periods and release it when the demand for cooling is higher, helping to balance energy consumption and reduce the load on cooling systems [63,195,196].
Despite their significant advantages, the strong rigidity of some phase change composites poses limitations on their widespread application in energy storage and thermal control electronics/systems.To overcome the obstacle of strong rigidity in composite PCMs, researchers have made significant strides in developing flexible and shape-stable PCM solutions.These innovations hold great promise for various thermal management applications in electronics and buildings.For instance, Yang et al. [197] successfully fabricated flexible SSPCMs with excellent thermal management capabilities for electronic devices and buildings.Their composite PCM film effectively reduced the temperature of electronic devices by 5.5 C and buildings by 9.3 C, demonstrating its potential for practical thermal management applications.Similarly, Liao et al. [198] introduced a novel shape-stabilized PCM film with exceptional shape stability and super elasticity, making it ideal for use in new-generation flexible thermal management in electronics and wearable devices.The resulting flexible film demonstrated longterm cycling stability after 700 thermal cycles, along with impressive shape foldability and stretchability, as well as a high enthalpy of 61.48 J/g.In a separate study, Zhang et al. [199] developed flexible phase change films with enhanced thermal and electrical conductivity, specifically designed for efficient thermal management.
In another research endeavor, researchers successfully prepared flexible PCMs with perfect shape stability, high latent heat energy storage density, and excellent 3D printability using a scalable swelling strategy [200].Similarly, Yang et al. [201] developed a novel CPCM with exceptional properties specifically designed for extensive battery thermal management.By incorporating natural rubber as a flexible insulation network between EG and OP44E PCM, they created a CPCM with a high energy storage density of 156.5 J/g, improved resistivity of 2700 U/cm, and excellent thermal conductivity of 3.4 W/mK.Additionally, Deng et al. [104] proposed a multifunctional flexible CPCM with outstanding anti-leakage and thermal conductivity performances.The flexible structure was achieved through a polymerizing and crosslinking reaction involving PEG and hexamethylene diisocyanate, as illustrated in Fig. 6-I.To further enhance thermal conductivity, AlN and CNTs were introduced.The resulting CPCM demonstrated excellent antileakage properties and exhibited remarkable elasticity.Notably, even under a 1.5 C discharge rate, the maximum temperature of the battery module with the multifunctional flexible CPCM was effectively controlled below 45 C, and the corresponding temperature difference was maintained within 4.3 C.
We et al. [203] presented an innovative method to create highly thermally conductive, flexible, and leakage-proof phase change composites suitable for various heat-related applications, including thermal harvesting of renewable energy, building energy management, and thermal management of electronics.The resulting flexible composite film demonstrated efficient and reliable thermal management performance, lowering the working temperature of a commercial lithium-ion battery by over 12 C at high discharge rates.Similarly, Ge et al. [202] proposed flexible PCM composite sheets comprising single-wall carbon nanotubes and the PCM PEG embedded in a polydimethylsiloxane matrix.The PCM composite could easily bend and distort at room temperature while maintaining flexibility due to the large bond angles and bond lengths of SieOeSi backbones (Fig. 6II).Moreover, it exhibited remarkable over À20 dB microwave attenuation with great tunability of working frequency across the entire 2e18 GHz range, achieved at a fixed CNT filling ratio of only 0.2 vol%.Meanwhile, the composite contained 70 vol% of PEG without compromising mechanical robustness and anti-leakage performance, resulting in a latent heat storage density exceeding 110 J/g.
The unique thermal properties of PCMs, such as their ability to store and release large amounts of energy during phase transitions, make them valuable assets in regulating temperatures and managing heat dissipation in electronic devices.The applications range from enhancing the performance and longevity of electronic devices to addressing challenges in energy storage and thermal control.Researchers have successfully addressed challenges related to the strong rigidity of some phase change composites, paving the way for their wider application.Innovations in developing flexible and shape-stable PCM solutions have been noteworthy, demonstrating improved thermal management capabilities for electronic devices.Moreover, the integration of PCMs into electronic devices undergoing frequent heating and cooling cycles, such as lithiumion batteries in smartphones and laptops, has shown positive on managing heat generated during charging and discharging cycles.This not only improves battery performance but also contributes to their longevity.The research efforts underscore the ongoing commitment to overcoming challenges and pushing the boundaries of innovation in PCM technology.As the field continues to evolve, it holds great potential for shaping the future of electronic cooling and thermal management systems.

Other applications
In addition to the aforementioned applications in buildings, electronics, biomedical, textiles, and solar energy, PCM composites exhibit promising potential across a range of diverse fields, providing innovative solutions, notably in electromagnetic shielding (EMI), infrared stealth, desalination, etc. PCM composites have proven effective in electromagnetic shielding applications, offering enhanced capabilities for absorbing or reflecting electromagnetic waves.This property is particularly advantageous in crucial areas such as electronic devices, communication systems, and aerospace applications, where mitigating electromagnetic interference is paramount [204].An illustrative example of this capability is the fabrication of a carbon foam/rGO/PW dual-functional composite phase change material via reductive assembly and vacuum impregnation.The EMI shielding efficiency of this composite, reported to be up to 49 dB at a thickness of 2 mm, significantly surpasses the commercial application standard of 20 dB [205].
Likewise, advancements in infrared detection technology underscore the crucial need for simplified and effective infrared shielding, particularly in military applications.PCM composites play a pivotal role in fulfilling the requirements of achieving infrared stealth, especially in military and defense scenarios.The integration of PCMs with specific transition temperatures into coatings or materials used in military equipment allows for precise management and control of heat release or absorption.This capability becomes instrumental in masking the thermal signatures of objects, rendering them less detectable by infrared sensorsda critical aspect of achieving infrared stealth [206].In a recent breakthrough, a flexible and foldable composite PCM film was innovatively designed, exhibiting substantial improvements in mechanical properties, near-infrared absorption, and infrared photothermal conversion.This development holds significant promise for applications in infrared stealth and thermal camouflage, addressing the evolving needs of military operations [207].
Additionally, PCM composites assume a crucial role in desalination processes, providing a pathway for efficient thermal energy management.Integration into solar desalination systems allows these materials to facilitate the absorption and release of heat during phase transitions.This unique property optimizes evaporation and condensation processes, emphasizing the potential of PCM composites in contributing to sustainable and energy-efficient desalination methods [208,209].
It's worth noting that the applications of PCM composites extend beyond those highlighted in this review.A comprehensive exploration of all applications would necessitate a detailed review article, which diverges from the primary focus of the present review.Within this section, our aim was to study the more prominent applications of PCM composites and provide an overview of recent research in these areas.We trust that this review article will serve as a valuable resource for those interested in delving further into this field.

Future and outlook
Despite significant advancements in the development of formstable PCMs with various functionalities, there are still several challenges that need to be addressed in future research.These challenges include the low thermal conductivity of support materials and PCMs, the supercooling effect of PCMs, the timeconsuming optimization of process parameters, and the limitation of loading capacity.To overcome these challenges, future research efforts should focus on innovative and novel materials.Advanced material engineering techniques, such as the incorporation of high thermal conductivity additives or nanostructured materials, can help improve the thermal conductivity of support materials and PCMs.Additionally, researchers can explore new PCM formulations or nanoencapsulation techniques to mitigate the supercooling effect and enhance the phase change behavior.Furthermore, the development of predictive models and simulations can aid in optimizing process parameters and reducing the trial-and-error approach in form-stable PCM fabrication.This would streamline the design and manufacturing process and expedite the development of high-performance form-stable PCMs.Moreover, researchers can investigate alternative support materials with higher surface areas or tailored porosities to increase the PCM loading capacity while maintaining structural integrity.Additionally, exploring the use of hybrid composites or multilayer structures can offer opportunities to combine the benefits of different materials, overcoming individual limitations and creating multifunctional form-stable PCMs.Addressing these challenges will unlock the full potential of form-stable PCMs and pave the way for their widespread application in various industries, including building and construction, textiles, electronics, and biomedical fields.Ultimately, these advancements will contribute to more efficient and sustainable TES and regulation systems, benefiting both energy conservation and human comfort.

Summary
PCMs offer remarkable properties that make them highly appealing for thermal energy storage and regulation.Their ability to store energy as latent heat while maintaining a nearly constant temperature has generated significant interest in their practical applications.However, the susceptibility to leakage and fluidity in the melt state poses a significant challenge to their widespread utilization.To address this, various effective methods have been explored to create leakage-free form-stabilized PCMs, opening up new possibilities for their use in diverse industries.Encapsulation, by enclosing PCMs within protective shells or microcapsules, emerges as a widely employed technique.This not only prevents leakage but also facilitates controlled energy release when required.Incorporating PCMs into porous matrices offers another promising approach, as the porous structure acts as a confinement mechanism, immobilizing the PCM and ensuring its integrity during phase transitions.This approach also provides advantages like high thermal conductivity and increased surface area for efficient heat transfer.Additionally, hybridization with polymers proves to be an effective means of stabilizing PCMs.Blending PCMs with polymers enhances mechanical strength and structural stability, reducing undesirable leakage and improving thermal performance.
Throughout this review, we have comprehensively evaluated the advantages and disadvantages of these stabilization methods, summarizing key research advancements in the field.Our analysis has highlighted the effectiveness of these techniques in mitigating leakage issues and enhancing the overall performance of formstabilized PCMs.Moreover, we have explored the wide-ranging application areas of form-stabilized PCMs, including solar energy storage, buildings, textiles, biomedical, and electronics.Their efficient energy storage capabilities and ability to regulate temperature render them invaluable assets in these industries, promising advancements in energy efficiency, thermal comfort, and sustainable design.By examining the latest developments and real-world applications, this review sheds light on the immense potential of form-stabilized PCMs in revolutionizing various sectors and contributing to a greener and more energy-conscious future.The ongoing research and advancements in this area pave the way for a more sustainable and efficient energy landscape, where formstabilized PCMs play a crucial role in meeting the demands of modern energy storage and regulation requirements.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 2 .
Fig. 2. Multifunctional applications of PCMs (I) self-healing PCMs and (II) superhydrophobic PCMs.I: (a) Demonstrating the self-healing properties of WPU@MXene3/PEG under near-infrared light irradiation, (b) presenting the tensile stress-strain characteristics of both the original and healed samples, (c) displaying POM images of a cut sample, and (d) revealing POM images of the healed sample.Additionally, (e) showcases images of the healed sample enduring bending deformation, f) illustrates Fourier transform infrared spectra for both the original and healed samples, and (g) provides a mechanism diagram for WPU@MXene/PEG [87].With permission from Hu, 2021, Copyright © 2021, Elsevier Ltd.II: (a) Depicts a schematic representation of the straightforward fabrication process for superhydrophobic PCM-delignified wood composite PCMs, and (b) demonstrates the self-cleaning capabilities through a test of superhydrophobic PCM-delignified wood composites [90].With permission from Yang, 2020, Copyright © 2019, Elsevier Ltd.

Fig. 5 .
Fig. 5. PMCs in biomedical fields.(a) Design depiction of the thermotherapy mask, (b) Schematic representation of the thermotherapy process, (c) Evaluation of thermal release performance comparing the mask with and without PEG encapsulation, (d) Images of the setup for purifying polluted air, (e) Efficiency of air purification using the CNT sponge, (f) Assessment of the actual thermotherapy effect via thermal infrared imaging (the left portion features a thermotherapy mask with PEG encapsulation, while the right portion shows a conventional mask without PEG encapsulation) [191].With permission from Chen, 2020, Copyright © 2020, Elsevier Ltd.

Fig. 6 .
Fig. 6.PCMs in advanced electronics (I) multifunctional flexible composite PCMs and (II) flexible composite PCMs.I: (a) Chemical reaction equation detailing the synthesis of CPCM and (b) and (c) the process involved in preparing CPCM [104].With permission from Deng, 2023, Copyright © 2023, Elsevier Ltd.II: (a) Images capturing the bending and twisting of SPP.(b) Conducting a reversible compression test on SPP and plotting stressestrain curves for (c) strain and (d) compression cycles [202].With permission from Ge, 2023, Copyright © 2023, Elsevier Ltd.
CRediT authorship contribution statement Maryam R. Yazdani McCord: Writing e review and editing, Writing e original draft, Supervision, Resources, Project administration, Investigation, Funding acquisition.Hossein Baniasadi: Writing e review and editing, Writing e original draft, Visualization, Project administration, Investigation, Conceptualization.

Table 1
Encapsulation of PCMs with different shell materials.

Table 2
Confinement of PCMs with different porous materials.

Table 3
Hybridization of PCMs with polymers.

Table 4
Conductive PCM composites for electrothermal conversion and storage.