Green Synthesis of Core/Shell Phase Change Materials: Applications in Industry and Energy Sectors

Abstract

Engineered substances that demonstrate superior properties compared with conventional materials are called advanced materials. Thermal energy storage systems based on phase change materials (PCMs) offer an eco-friendly solution to reduce fuel and electricity consumption. PCMs are compounds that can store thermal energy in the form of latent heat during phase transitions. Green synthesis of core/shell composite PCMs is an environmentally friendly method for producing these materials, focusing on reducing energy consumption, minimizing the use of harmful chemicals, and utilizing biodegradable or sustainable materials. Green synthesis methods typically involve natural materials, solvent-free techniques, green solvents, biomimetic approaches, and energy-efficient processes. This review explores green synthesis methods like solvent-free techniques for core/shell PCMs production, highlighting their role in thermal regulation for energy-efficient buildings. Special attention is given to materials derived from biomass that can be used as precursors for PCM synthesis. Moreover, the principles of latent heat thermal energy storage systems with PCMs, in accordance with physical chemistry guidance, are also presented. Furthermore, materials that can be used as PCMs, along with the most effective methods for improving their thermal performance, as well as various passive applications in the building sector, are highlighted. Finally, the focus on the combination of environmentally friendly processes and the performance benefits of composite PCMs that offer a sustainable solution for thermal energy storage and management is also discussed. It was found that PCMs that are synthesized in a green way can reduce emissions and waste during production and disposal. Moreover, waste recycling and its use for another type of synthesis is also a potential green solution.

1. Introduction

The global energy demand of the building sector accounts for 32% of the total. Moreover, 34% of CO₂ emissions that are energy related are attributed to the building sector [1]. Even with a decline in fossil fuel use and an expansion of renewables, electricity consumption, for example for heating coupled with air conditioning, remains high. Additionally, the building floor area continues to grow, contributing to energy consumption, while CO₂ emissions are not being reduced, but rather increased [2]. The cost of energy-efficient technologies remains high and weak regulatory enforcement further slows progress. Therefore, this energy crisis necessitates research on new energy-saving technologies that are cost effective, like phase change materials (PCMs). Green synthesized PCMs can help reduce energy consumption during the heating and cooling of buildings, store thermal energy, reduce peak loads, and support net-zero targets, while not posing a threat to the environment themselves [3].

Core/shell PCMs are contemporary and innovative materials that are fabricated for effective thermal energy storage and release. They consist of a core based on a phase change material, such as fatty acids, paraffin, or inorganic salts, encapsulated into a shell, which offers structural stability and reduces leakage during the phase change process. In general, the synthesis and characterization of core/shell composite PCMs include several considerations and key steps. Some synthesis methods include melt mixing [4], interfacial polymerization [5], and others, such as evaporation of the solvent, coacervation, spray drying, and layer-to-layer assembly, which are microencapsulation techniques.

Many techniques can be used for the characterization of PCMs with respect to their morphology, structure, elemental composition, and thermal behavior. The most widely used physical chemistry methods for thermal analysis are thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), whereas scanning electron microscopy (SEM), transmission electron microscopy (TEM), and optical microscopy are used for structural analysis and morphology characterization [6]. Moreover, Fourier transform infrared (FTIR), ultra–visible (UV), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) spectroscopies are utilized for the analysis of the chemical and structural composition of materials and dynamic mechanical analysis (DMA) is used to test its mechanical properties [7]. The use of DSC aims to evaluate the thermal performance of the synthesized PCMs, assessing the phase change temperature, crystallization and melting temperatures, and latent heat [8]. SEM was employed to visualize the morphology of the core/shell structure and ensure uniform encapsulation. FTIR characterization was performed to confirm the chemical composition of the materials and to verify successful encapsulation.

The green synthesis of core/shell composite PCMs focuses on environmentally friendly methods, wherein energy consumption is reduced, and the use of harmful chemicals is minimized to mitigate environmental damage and consequences. The aim is to develop advanced materials that provide excellent thermal energy storage, durability, and stability, while reducing the environmental impact. It has gained more attention because of its eco-friendly nature and alignment with sustainable development goals, but also for its cost efficiency. By utilizing natural polymers, biobased materials, renewable resources, non-toxic solvents or solvent-free processes, and waste-minimizing techniques, researchers have pioneered more sustainable solutions for PCM production, enhancing their applicability in regard to energy-efficient technologies and building materials.

The green synthesis of core/shell PCMs provides considerable advantages over traditional methods that align with sustainability principles. Several benefits are obtained as a result of green synthesis, such as environmental friendliness owing to biodegradability and a reduction in the use of toxic chemicals, enhanced biocompatibility due to the presence of non-toxic shells, cost-effectiveness via the use of natural precursors and lower synthetic temperatures, and improved material properties that are assigned to the ability to synthesize well-defined core/shell structures with controlled morphology.

Despite their potential in terms of improving energy efficiency, PCMs face several key challenges that may limit their broader industrial application. One of the most pressing issues is the need for reliable thermal performance across repeated heating and cooling cycles. In high-temperature systems like Regenerative Organic Rankine Cycle (RORC) setups, the long-term stability and consistency of the heat transfer characteristics of PCMs become critical. Problems like leakage, material degradation, and changes in thermal properties over time can reduce the reliability of the system. As demonstrated by the thermal energy storage (TES)-RORC pilot plant, PCMs can effectively sustain power generation during thermal input interruptions, but their integration requires careful thermal management and robust containment strategies [9].

This review article highlights the continued research and innovation, particularly in regard to green synthesis methods, that offer promising solutions for enhancing the performance and sustainability of core/shell PCMs (Figure 1). As demand grows for efficient energy systems and greener materials in industrial applications, core/shell PCMs developed through the use of sustainable methods are emerging as a powerful option for thermal management, waste heat recovery, and renewable energy storage. This review paper explores recent advances in the green synthesis of core/shell PCMs and highlights their potential for practical use across various sectors.

2. Synthesis and Characterization of Core/Shell Composite Phase Change Materials

Green synthesis techniques aim to fabricate core/shell composite PCMs with minimal environmental impact. Therefore, green synthesis emphasizes the use of sustainable and energy-efficient methods to design and synthesize these composite materials taking into account principles such as those related to eco-friendly materials, green solvents, biomass-derived materials, emulsion polymerization, solvent-free techniques, and natural/biobased extraction methods (Figure 2).

2.1. Eco-Friendly Materials

In the field of eco-friendly materials, renewable, biodegradable, or non-toxic materials for use as both core and shell components are utilized, such as natural waxes (e.g., carnauba wax, beeswax) and biopolymers (e.g., chitosan, alginate) [10].

Table 1. Tabulated core/shell PCMs with respect to the green synthesis principles.

Core	Shell	Principle of Green Synthesis	Reference
Paraffin	TiO2-chitosan	Eco-friendly materials	[11]
Lauric and stearic acid	PMMA	Eco-friendly materials	[12]
1-dodecanol	Alginate, carboxymethyl cellulose, and chitosan, Al2O3	Eco-friendly materials	[13]
Chia seed oil (fatty acids)	Whey protein, modified tapioca starch	Eco-friendly materials	[14]
Aluminum	Copper slag, bauxite	Eco-friendly materials	[15]
Paraffin	SiO2-PMMA	Green solvents	[16]
Crystal violet lactone, methyl stearate, bisphenol AF	Poly(urethane-urea)	Green solvents	[17]
n-octadecane	SiO2	Green solvents	[18]
Paraffin-Cocos nucifera oil	Industry-generated tea waste	Biomass-derived materials	[19]
n-octadecane	Rice husk silica, rice husk carbon	Biomass-derived materials	[4]
Polyethylene glycol	Carbon-based coconut shell	Biomass-derived materials	[20]
Polyethylene glycol	Baboo flour	Biomass-derived materials	[21]
N-docosane	Biomass waste lotus shells	Biomass-derived materials	[22]
1, 4-butanediol esters	Silica	Emulsion polymerization	[23]
1-dodecanol	Melamine–paraformaldehyde	Emulsion polymerization	[24]
Paraffin wax and n-octadecane	Melamine–formaldehyde	Emulsion polymerization	[25]
Cu–Si–Al	α-Al2O3 and AlOOH	Solvent-free techniques	[26]
Aluminum	Cu(Mn2O4)/Cu(CrO2), FeMn(SiO4)/Fe2(SiO4), and Cr0.75Fe1.25O3	Solvent-free techniques	[27]
Butyl stearate, hexadecane, caprylic acid	Pectin–barium chloride	Natural/biobased extraction methods	[28]
Waste cooking fats	Biosilica, polypropylene	Natural/biobased extraction methods	[29]
Avocado seed oil extraction	Biobased matrix	Natural/biobased extraction methods	[30]

tabulates core/shell PCMs with respect to the green synthesis principles (Figure 3). Taking into account the details in Table 1, it may be remarked that chitosan-based shells were found to consistently improve the thermal stability of PCMs across several studies, as well as that water and ethanol are very common green solvents. Moreover, it can be seen that biomass-derived materials are examined and naturally extracted PCMs are tested. Finally, it is noticed that solvent-free techniques are applied to synthesize Al-based materials.

Chitosan is a natural cationic polysaccharide that is abundant and attractive to researchers due to its ability to meet the requirements of bio-derivation and, thus, was used for the synthesis of micro-PCMs, for a shell made of TiO₂-modified chitosan, by Yu Song and his group. The goal of this study was to compose micro/macro-PCMs that are environmentally friendly, store energy, and are thermally stable. Firstly, the micro-PCMs were synthesized with an oil–water–oil double emulsion and consisted of a paraffin core and the TiO₂–chitosan shell. Another naturally derived material, alginate, was used as a matrix to form macro-PCMs and elevate the thermal stability and compression strength of the already UV- and temperature-resistant micro-PCMs. Alginate was incorporated into the PCMs using the piercing–solidifying method and CaCl₂. These micro/macro-PCMs are difunctional and add an eco-friendly aspect to the core/shell PCM technology [11].

As already mentioned, chitosan, an eco-friendly organic material with a plethora of functional groups, can be found in several studies, used as a shell material. Peng et al. used it to cover a polymethylmethacrylate (PMMA) shell and make bilayer shell core/shell PCMs, where the core consisted of lauric and stearic acid. The capsules were prepared using the microemulsion polymerization method, and a chitosan layer was added through the use of electrostatic attraction. The nanoencapsulated PCMs were then added to the dressings. Through the use of chemical structure and morphological characterization, it was found that they are dense and spherical structures. Additionally, this study shows that they have excellent thermal stability and the chitosan layer adds to their anti-leakage properties, their high-enthalpy preservation (in contrast to other shell materials that reduce the enthalpy of the PCM), and enables their application to more fields [12].

El Bouari et al. studied the encapsulation of PCM in different biopolymer shells for their incorporation into gypsum. The core was made of 1-dodecanol, whereas the biopolymers used were alginate, carboxymethyl cellulose (CMC), and chitosan. Each biopolymer was used to prepare the water phase, and nano-A₂O₃ was added to enhance the thermal conductivity of the materials. After solubilization, 1-dodecanol, in a molten state, was added to form an oil-in-water emulsion. Reticulation solutions were used, which were different for each shell material, and, subsequently, the PCMs were created in a gel form. After characterization, it was proved that the encapsulation of the 1-dodecanol core with biopolymers solved leakage and thermal conductivity problems. Specifically, the materials made with chitosan and Al₂O₃ shells showed a higher compressive strength than those with other types of bio-shells. The phase change temperature was marked as similar to that of non-encapsulated PCMs, with a decrease in latent heat. The CMC and chitosan shells with Al₂O₃ maintained their phase change temperatures, showing long-term stability and a decrease in enthalpies under both melting and freezing conditions. The CMC and Al₂O₃ shell was the one with the best thermal storage and stability properties, with possible uses in building materials [13].

Kaur et al. used chia seed oil as the core and whey protein concentrate with modified tapioca starch as the shell for composite PCMs. Freeze drying was used for synthesis. The correct amount of each ingredient was selected to fabricate the optimum product, with high encapsulation efficiency and high α-linoleic acid content. After the microcapsules were prepared, their fatty acid composition and thermal and oxidative stabilities were characterized. The results were promising, since the materials showed stability in temperatures as high as 700 °C and resistance to oxidation, while a higher content of Ω-3 fatty acids was also observed in contrast to Ω-6 ones. The last characteristic makes them an interesting material for applications in functional foods [14].

Copper slag is a byproduct of pyrometallurgy that poses a threat to the environment and is produced in large amounts. However, Zhang et al., used it to encapsulate commercial solid Al balls, making it eco-friendly. Using a two-step granulation method, a ceramic shell was placed on the core, which had an organic material on it, and sacrificed it to leave a cavity when the metal core melted. Copper slag replaced a part of the bauxite in the ceramic shell. The results showed improved heat transfer, thermal conductivity, and adequate heat storage capacity, which can be attributed not only to the copper slag, but also to the Fe₂O₃ formation on the outer layer of the shell [15].

2.2. Green Solvents

The category of green solvents includes environmentally benign solvents or solvent-free methods. For instance, chemical waste reduction can be accomplished using water as a solvent or performing reactions under solvent-free conditions. Considering that water is a very easily accessible and common solvent, and is renewable and non-toxic, these properties mean that it is eco-friendly. Therefore, it is commonly used as a solvent in research studies. Li et al. explored the synthesis of core/shell phase change materials, wherein the shell was a hybrid of methyl methacrylate (MMA) and SiO₂. The method used for the fabrication was hydrolysis–polycondensation. For the experiment, the aqueous phase contained the surfactant, sodium dodecyl benzene sulfonate, and deionized water. After proper preparation, the oil phase, consisting of the monomer and silicon source, was added to the water phase. The remaining ingredients were added later, and hydrolysis and polymerization occurred. The morphology, chemical composition, crystal structure, surface elements, particle size distribution, crystalline and thermal stability, and leakage were characterized. This green solvent process resulted in hybridized-shell/core microcapsules, with porous shells (structures with many cavities), great durability, energy storage, and thermal conductivity. The products have possible applications as thermoregulation materials and, even more so, because of their porous structure [16].

Water was also used as a solvent in a study by Liu and his group. The purpose of this study was to fabricate microcapsules that are reversible thermochromic mixtures and to apply them in regard to anticounterfeiting. However, the products also had phase change properties and thermal stability, making the study notable. The synthesis of the core/shell phase change materials was performed via interfacial polymerization, avoiding the formaldehyde used in regard to reversible thermochromic materials, owing to its harmful properties. The core contained crystal violet lactone as a color former, methyl stearate as a solvent, and bisphenol AF as a developer, while the shell was composed of poly(urethane-urea) (PUU) (Figure 4) [17].

Ethanol is a widely used green solvent. In a study conducted by Siyi Ju et al. on the fabrication of core/shell PCMs as temperature-controlling materials, it was used as a solvent mixed with deionized water. More specifically, synthesis was performed using the interfacial polycondensation method. The aqueous phase consisted of water, ethanol, and the surfactant, and was poured into the oil phase. After homogenization and sonication, the mixture formed an emulsion. After the hydrolysis and polycondensation reactions, core/shell structures were formed in three different mass ratios. The capsules exhibited excellent thermal properties and strengths. A 2:1 core-to-shell mass ratio was added to the cement to test its compatibility. The results are very promising for the future of temperature cracks, since the addition of these capsules increased the thermal storage capacity and hydration of the cement and can be applied to control the increase in temperature caused by the hydration of cement [18].

2.3. Biomass-Derived Materials

Several studies have been performed using materials derived from biomass, such as cellulose or starch, for the shell, which can provide biodegradability and mechanical strength properties (Figure 5) [31]. The Phukan et al. synthesized a PCM using cocos nucifera oil and then formed stable PCMs using a tea waste matrix. The coconut oil was mixed with paraffin in five different ratios and consisted of a PCM. The mixture was then infused into the tea waste matrix, which was previously subjected to pyrolysis to form biochar via the direct impregnation method. The composite material was characterized and the results showed chemical and thermal stability and energy storage properties. The PCM 7:3 ratio of paraffin and coconut oil was found to be the one with the highest latent heat and impregnation effectiveness. Lastly, the thermal conductivity of the PCM was proved to be better than the biocomposites containing each PCM alone [19].

Research performed by Chaoen Li et al. suggested the use of rice husks in phase change materials. Rice husk is a waste product that is abundant in China, where research has been conducted, and serves as a biomass material for the development of construction core/shell PCMs. This research explores two different applications in regard to the fabrication of porous supporting matrices: rice husk silica and rice husk carbon. The method used to make the silica-based powder involved citrate treatment, while to make the carbon, pyrolysis at a high temperature was applied. Subsequently, melted n-octadecane was added to two vacuumized flasks, each containing one of the supporting matrices. The composite PCMs were ready after air was allowed into the flask and n-octadecane penetrated into the matrix. To prevent leakage, an epoxy resin was added to form a shell around the composite. These composite PCMs were synthesized at low cost and offer excellent thermal conductivity, while being shape stabilized and environmentally friendly [4].

Another approach in regard to phase change materials, supported by Kalidasan et al., used the shell of coconuts to conduct synthesis, involving a green two-step method. Coconut shell is an affordable solid waste agro-based product, which is abundant in many countries like India, where this research was conducted, that when carbonized is converted into biochar. Therefore, in this study, coconut shells were crushed, rinsed in deionized water, dried, and then carbonized at 1000 °C in a tube furnace. Then, the particle size was reduced, and they were added to polyethylene glycol PCM to make the composites. For characterization purposes, coconut shell nanoparticles were added in weight fractions ranging from 0.3% to 1.3%. The composites exhibited excellent thermal features, because the energy storage potential increased during both heating and freezing. The optical absorbance of the composites was also increased and, therefore, the electromagnetic wave transmittance was reduced, with the 1.0% weight fraction enhancing the thermal conductivity a lot. These chemically stable composite PCMs containing coconut shell biochar are a sustainable proposition for future thermoregulation needs [20].

A bio-derived ingredient that has applications in regard to the synthesis of PCMs is bamboo, which is combined with polyethylene glycol (PEG). In their study, Xu et al. formed a phase change material that was shape stable through the use of the vacuum adsorption method. First, bamboo flour was prepared by dry ball milling and then packed with PEG, with adsorption being better at longer milling times. The produced materials showed chemical, thermal, and shape stability, without any leakage, and had a high encapsulation rate and thermal conductivity. The continuous bamboo fiber network structure was what improved these characteristics, compared to pure PEG. This research brings to light the benefits of bamboo utilization in terms of phase change materials, without discarding the parenchyma cells, proposing the application of bamboo flour/PEG products for the composition of new composite materials with potential for use in solar energy storage and incorporation in the building industry [21].

A PCM was produced in the research by He et al., derived from waste lotus shells. Firstly, the lotus was cleaned and then carbonized and activated. N-Docosane was incorporated into different surfaces via a vacuum impregnation method to form shape-stable phase change composites. The materials were characterized and the results showed that the materials with the activated lotus shell had excellent latent heat and thermal stability properties. These phase change materials are promising for utilization in thermal regulation, for example, in heat recovery and solar energy conservation, being sustainable and a recycled waste product [22].

2.4. Emulsion Polymerization

In the literature, natural surfactants have been widely used to stabilize emulsions, allowing for the encapsulation of the core material in a polymer shell. These surfactants are derived from sustainable sources that are eco-friendly, biodegradable, non-toxic, and safe for human use. Several types of natural surfactants exist, such as saponins derived from plants like yucca, soapwort, and quinoa; lecithin found in soybeans and egg yolks; sorbitan esters obtained from sorbitol, a sugar alcohol; rhamnolipids produced by bacteria; proteins and peptides found in milk or silk proteins; fatty acid derivatives derived from oils like palm, coconut, or castor; and microbially produced lipopeptides.

In the work by Zhang Q. et al. on the synthesis of core/shell PCMs, an oil–water emulsion was created using Tween 80, a biobased surfactant. The study followed the esterification of fatty acids with 1,4-butanediol, using an iron chloride catalyst, the product of which was an ester core encapsulated in a silica shell. The encapsulation process was performed using a one-pot method via interfacial polycondensation, wherein an oil–water emulsion was formed. Two different samples were prepared with core/shell ratios of 1:1 and 2:1. The results showed regular spheres with good latent heat, thermal stability, and thermal management and regulation properties, with possible applications for overheating protection [23].

Another green method used to synthesize core/shell PCMs is no-surfactant techniques. Jitendra Singh et al. took this approach in their study to make microcapsules involving a 1-dodecanol core and a melamine–paraformaldehyde (MPF) shell. For the synthesis, the prepolymer was prepared in distilled water, and then PCM was added to the mixture. The prepolymer covered the PCM cores making micelles. The synthesis continued with co-polymerization after the emulsion was formed. The same study included the synthesis of these microcapsules using a Ramsden emulsion method with TiO₂ nanoparticles and compared them to those produced using a surfactant-free method. The results from the characterization of the microcapsules from both methods showed that the second method is not only more environmentally friendly, but is also expected to be less expensive and difficult to prepare [24].

Zhang Zhe et al. used cellulose nanocrystals (CNC), which are natural and sustainable materials, as emulsifiers in regard to the Pickering emulsion polymerization method. Paraffin wax and n-octadecane were used as PCMs. The method followed the addition of drops of a melamine–formaldehyde mixture into diluted CNC–PCM-stabilized emulsions. PCM microcapsules were obtained after in situ polymerization. The CNC not only acted as a surfactant, but also as a reinforcing nanofiller, in the shell. Overall, the PCM microcapsules exhibited excellent thermal properties at temperatures below 200 °C. Furthermore, these materials are flame retardant and, thus, are self-extinguishing and as such are promising materials for many applications [25].

2.5. Solvent-Free Techniques

The production of core/shell PCMs using solvent-free techniques not only reduces environmental hazards, but also simplifies the manufacturing process. Each method has specific operational requirements and material, making the choice dependent on the production scale, desired properties, and target application [32]. Several solvent-free techniques for producing core/shell PCMs can be used, such as spray drying, melt dispersion, co-extrusion, electrostatic coating, fluidized bed coating, solid-state sintering, hot-melt encapsulation, and in situ polymerization [32,33].

The study conducted by Masahiro Aoki et al. explored the synthesis of Cu–Si–Al core/shell phase change materials without solvents. The dry fabrication process is divided into two steps, the first is High-speed Impact Blending (HIB) and the second is heat oxidation treatment. Cu–Si–Al PCM particles were placed into a hybridization system with shell nanoparticles made of α-Al₂O₃ and AlOOH to coat the PCMs. Then, in an oxygen atmosphere, heat oxidation occurred, and the temperature went in 10 min from room temperature to 1000 °C and was maintained for three hours. Thus, the shell was stabilized. Three samples were prepared with different shells: one with alumina, one with AlOOH, and one with both. The characterization results showed optimal thermal stability and cyclic durability of the materials. Alumina and alumina-AlOOH shell PCMs with dual-layered shells exhibited the best thermal stability in high-temperature air. The incorporation of AlOOH enhances the thermal durability of the shell. This solvent-free method is a cost efficient and green method that can be used in the synthesis of core/shell PCMs of different compositions [26].

Another novel form of synthesis of core/shell PCMs was conducted by Du and his co-workers. This study focused on the synthesis of Al-based core/shell composites using a dry-mix extrusion method. In a cylindrical container, a core/shell structure was established between the spherical particles and the metal mixed-phase powder by using the centrifugal force of high-speed rotation. A sieving process was followed to ensure even heating and stable thermal properties, and then a calcination process was applied to reduce the deformation and decomposition risks. The composites were then added to the solar energy receivers, which are analyzed below. The researchers investigated the environmental issues raised from this type of synthesis and noted the recyclability of the particles that is probable and needs further investigation [27].

2.6. Natural/Biobased Extraction Methods

These methods extract phase change materials from natural sources, ensuring that the process does not involve toxic solvents or harsh conditions [34]. The methods involve either biobased PCM extraction, which is used for plant oils and waxes, and fatty acids [35], or bio-derived shell material preparation, which is used for chitosan, sodium alginate, and cellulose [20]. The shell of the PCMs can be synthesized using biobased polymers that are derived from renewable sources, reducing the reliance on fossil fuels. Additional advantages of biobased polymers are their customizability, biodegradability, and the lower greenhouse gas emissions produced during production [30].

Chattopadhyay et al. investigated the use of pectin, a biodegradable, plant-derived biopolymer, to encapsulate organic phase change materials (PCMs) to enhance thermal energy storage. Using an ionic gelation method, pectin was cross-linked with barium chloride to form a shell around three types of PCMs: butyl stearate (an ester), hexadecane (paraffin), and caprylic acid (a fatty acid). The resulting pectin–PCM capsules, with a core content of 83–84 wt%, demonstrated notable thermal stability, sustaining temperatures higher than those of the unencapsulated PCMs. The functionality of each PCM, such as thermal buffering, varied based on its interaction with the pectin shell, with the peak encapsulation efficiency achieved through optimized core-to-shell ratios. The findings emphasize pectin’s versatility and potential in regard to sustainable PCM applications, particularly in temperature-sensitive storage solutions like packaging for perishable goods [28].

The investigation by Bragaglia et al. was based on sustainable PCMs, in which waste fat was extracted after cooking pork sausages. The fat was filtered through a strainer and allowed to cool. The characterization showed that the fat was mostly composed of saturated and unsaturated acids. To test its efficiency as phase change material, they integrated it into two different hosts, with the better one being the filter made of a surgical face mask, and then they layered it onto an exterior building wall. The thermal power that was transmitted with the layered PCM was reduced compared to that of the uncoated wall, proving the thermal energy storage capacity of the waste fat [29].

Avocado seed oil was extracted in a subsequent study to test its application as a PCM for the storage of cold thermal energy. Extraction was performed using steam drag, and the process began with the collected avocado seeds from food establishment waste being cleaned, washed, kept at low temperature, and ground. The product particles were then dried for 3 days, after which they underwent steam distillation. When the oil was extracted during this process, its phase change point was determined (for the solid–liquid transition). The results from tests on thermal boxes showed the oil’s capacity to preserve cold temperatures and, in some quantities, it proved to be better than water [30].

2.7. Comparative Discussion

Green synthesis of core/shell PCMs involves a variety of strategies, each with unique benefits and limitations depending on the goal. Biobased polymers, such as chitosan, starch, or lignin, offer biodegradability and renewable sourcing qualities, making them attractive for environmental applications. However, they often face challenges related to mechanical strength, thermal stability, and uniform encapsulation, especially under repeated thermal cycling. This presents the need for additional reinforcement using other materials. Biomass-derived shells, including those from agricultural residues and waste, can improve thermal conductivity and reduce material costs, but may introduce variability and insecurity in regard to the material’s performance due to compositional inconsistency [36,37].

In comparison, the use of green solvents like water or ethanol reduces toxicity and simplifies post-synthesis processing, while being inexpensive and widely available; however, these systems may limit the variety of compatible materials and require tuning of the reaction conditions [38]. Emulsion polymerization using natural surfactants or cellulose nanocrystals offers sustainability benefits and enables control over the morphology and size of the capsules [39,40]. However, they can be complex to stabilize, and the surfactants might not perform as well as the synthetic ones or in regard to other processing conditions. Solvent-free approaches are very sustainable and useful when metals are involved, but they can be technically demanding, often necessitating precise temperature control and specialized equipment [41]. The optimal synthesis route cannot be based only on the advantages and disadvantages of each method, but also depends on the intended application. The decision requires a balance to be achieved between the environmental impact, performance, scalability, and material reliability.

2.8. Shell Integration

The shell in core/shell PCMs serves as a protective barrier that enhances the durability, stability, and thermal performance of the core material. It, firstly, encapsulates the PCM, preventing leakage during the phase transition, while also improving the thermal and mechanical properties. The advantages of a silica shell were tested in a study by M. Parsamanesh et al., while the best core/shell ratio was also examined. For the experiment, a eutectic core of sodium carbonate decahydrate and disodium hydrogen phosphate dodecahydrate was covered via interfacial polymerization and the sol–gel method, with tetraethoxysilane (TEOS) and trimethoxysilane being used to form a silica shell. The results showed that the optimal core/shell ratio was 4:1. Furthermore, the shell was proven to increase the thermal stability of the PCM, from 35% to 70% at 600 °C [42].

In the next study, the PCMs were encapsulated in hollow steel balls to prevent direct contact with asphalt, thereby inhibiting bitumen property degradation and leakage issues. The steel shell was proven to not only provide a mechanical barrier, but also thermal conduction to facilitate efficient phase change of the PCM. Simulation of the distribution of these spherical shells within the asphalt matrix based on a grid-based pore-scale model was conducted, wherein it provided assurance of uniform distribution and accurate modeling of the thermal response. The encapsulation design considered a finite shell thickness and material properties to resemble field conditions more realistically. While steel is a high thermal conductor, the study revealed that both shell conductivity and thickness changes had minimal impact on the cooling potential, highlighting that the function of the shell is primarily containment and structural integrity and not the enhancement of heat transfer [43].

A proposition to further prevent leakage and heat transfer issues, where a shell might not be enough, was put forward in a study by Aiswarya V and Suden Das. They developed PCMs that featured a magnetized interfaced. The interface, consisting of TiO₂ and MgO modified with magnetized graphene oxide, was integrated between the shell and the core. The goal of the study was to magnetize the thermal conductivity enhancers (TiO₂ and MgO) and, in this way, align the nanoparticles to allow heat conduction along the microcapsules. The experiment process involved the in-situ polymerization technique, while the core consisted of paraffin and the shell of melamine–formaldehyde. Different concentrations of the interface were added and the results of the characterization of the materials showed spherical forms and physical interactions between them. The materials were also thermally stable and leakage was prevented, while the magnetization enabled thermal conductivity and transfer as expected. While improving the shell’s performance using graphite oxide may raise cost concerns, the study suggests that its contribution to thermal stability through the use of small quantities graphite oxide does not impose an economic threat [44].

2.9. Shell Composition

The choice and structure of the shell materials are crucial to ensuring the thermal, mechanical, and chemical stability of core/shell PCMs. The shell composition in this study was carefully designed using a PU/SiO₂ hybrid structure, fabricated via interfacial polymerization, followed by electrostatic self-assembly. Initially, a polyurethane shell was formed from the reaction of isophorone diisocyanate (IPDI) and triethanolamine (TEA), after which hydrolyzed TEOS (Si(OH)₄) was adsorbed and reacted to form a silica layer. This composite approach not only further densified and hardened the shell, but also significantly improved the thermal stability and conductivity of the materials. The SiO₂ incorporation minimized the porosity and suppressed core leakage during phase transitions, making the shell highly suitable for repeated thermal cycling and elevated temperatures [5].

In another study that has already been discussed in regard to the solvent-free synthesis of microencapsulated PCMs (MEPCMs), Cu–Si–Al alloy cores were used, and different alumina-based shell formulations were coated on them, including pure α-Al₂O₃, AlOOH, and their combination. Notably, the mixed α-Al₂O₃–AlOOH system formed a dual-layered shell, wherein AlOOH functioned as a sintering agent during heat oxidation, promoting tighter interparticle bonding and resulting in enhanced cyclic durability. The α-Al₂O₃-only shell formed a robust but moderately porous structure, while the AlOOH-only shell produced a dense but mechanically fragile layer, susceptible to cracking after prolonged cycling. These differences illustrate how strategic shell composition not only affects the encapsulation efficiency and oxidation resistance, but also governs the long-term mechanical durability under thermal stress. Such findings highlight the critical importance of shell design in optimizing MEPCMs for high-performance energy storage applications [26].

In this study, pectin, a biodegradable polysaccharide, was used to encapsulate three organic PCMs with different functional groups: hexadecane, butyl stearate, and caprylic acid. Despite similar core loading capacities, each material showed distinct thermal behavior, due to the molecular interactions between the core and pectin shell. Particularly, caprylic acid capsules exhibited significant supercooling, attributed to the polar interactions between carboxylic acid groups in the core and hydroxyl groups in pectin. Additionally, the use of a barium cross-linked pectin shell delayed the thermal degradation by up to 100°C compared to the PCM without the shell, enhancing the thermal resistance. These findings highlight the importance of selecting shell materials not only for their mechanical protection properties, but also for their chemical compatibility with the PCM core [28].

The use of chitosan as an outer shell in bilayer nanoencapsulated PCMs has demonstrated a significant improvement in both the thermal stability and anti-permeability performance of the materials, as has already been mentioned in Section 2.1. In the referenced study, the PMMA inner shell was reinforced with an electrostatically adsorbed chitosan outer layer, which performed excellently in regard to retaining the core materials, even when subject to extended thermal cycling. Also, a minimal trade-off between the encapsulation strength and energy storage capacity was shown in the results. This highlights the potential of natural biopolymers like chitosan in regard to enhancing capsule integrity without compromising enthalpy, which can be a limitation when it comes to bulkier synthetic shell materials [12].

2.10. Characterization Techniques

Thorough characterization of core–shell PCMs is required to ascertain their thermal behavior and establish their viability for real-world thermal storage systems. In this study, several methods were employed to determine the key thermophysical properties, including differential scanning calorimetry (DSC), densimetry, and rheometry. DSC measurement enabled precise determination of the phase change temperature, latent heat, and specific heat capacity of the encapsulated PCM and the bulk sample. A distinctive exothermic peak upon crystallization of encapsulated octadecane revealed the formation of a metastable rotator phase, nonexistent in the bulk state, demonstrating the effect of shell confinement on phase transition kinetics. Density measurements also corroborated the phase transition ranges via observable breaks in the temperature-dependent density gradients, with linear trends being maintained for single-phase states. Rheological experiments demonstrated the non-Newtonian behavior of the concentrated suspensions, including shear-thinning and shear-thickening phenomena, arising from particle–particle and particle–solvent interactions. These comprehensive measurements not only validate the encapsulation efficiency and thermal responsiveness of the PCMs, but also provide essential input parameters for modeling the heat transfer in engineered systems [45].

In addition to conventional characterization techniques, such as DSC, the study by Du et al. introduced a full set of characterization methods to fully evaluate the structure and performance of core–shell phase change materials (PCMs). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed to confirm the chemical structure and crystallinity of both the synthesized diesters and the MEPCMs. The surface and shell composition were further analyzed using energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS), highlighting the formation of the silica shell. Notably, the core–shell morphology was visualized through the use of scanning electron microscopy (SEM), with the particle size distribution quantified using Nano Measure software. To go even deeper, the authors utilized X-ray microscopy (XRM) for the 3D reconstruction of the microcapsule’s internal architecture. Thermal conductivity was evaluated via the transient planar source method, and thermogravimetric analysis (TGA) was used to assess the materials’ stability, complementing the DSC data. This multifaceted characterization approach focuses on the necessity of combining standard thermal measurements with structural and compositional methods in an effort to generate a comprehensive representation of PCM performance [23].

The microcapsules synthesized in the next study using eutectic lauric–stearic acid cores and melamine–formaldehyde shells were characterized using a wide range of characterization techniques. FTIR spectroscopy proved the absence of chemical interactions between the core and shell, reflecting the chemical stability of the material. DSC provided insights into the latent heat storage and encapsulation efficiency, with the best-performing formulation achieving a 73.26% encapsulation ratio, while TGA was used to assess the thermal degradation behavior. The morphology characterizations of SEM and TEM revealed smooth, crack-free shell surfaces and a well-defined core–shell architecture. The size distribution of the microcapsules was statistically analyzed via the use of image processing software (ImageJ and SPSS), and the average particle diameter was established to be 4.5 µm. Furthermore, thermal cycling and vacuum stability tests demonstrated the capsules’ resilience when subject to repeated thermal stress and low-pressure conditions, underlining their applicability in sensitive environments, such as satellite electronics [46].

Although widely used, a singular characterization method is not sufficient to fully evaluate the material, and each method comes with limitations. In order to ensure that accurate thermal transition data are obtained, DSC is needed, but may not provide an actual picture of the material’s behavior due to the small sample. Thermal stability can effectively be assessed via TGA; however, no spatial details are provided. Both of these methods and thermal cycling tests provide results concerning the thermal properties of materials; however, these properties are not the only properties that affect a PCM’s performance. Both SEM and TEM are important, as they provide structural insights and information on encapsulation efficiency. A disadvantage is that, due to the small sample size, they only provide localized information. The third aspect that is important is the chemical composition and crystallinity of the material, information of which is provided by FTIR and XRD analysis, although interpretation may be difficult in regard to complex composites. Therefore, combining multiple complementary techniques is essential to achieving reliable characterization results on core/shell PCMs’ performance and behavior across practical applications.

3. Application Fields in Industrial and Energy Sectors

Core/shell composite PCMs have a wide range of applications owing to their unique thermal properties and capability to deposit and release thermal energy. The versatility of core/shell composite PCMs means that they are suitable for various applications, being used to significant improve energy efficiency and thermal management across various industries (Figure 6).

3.1. Building and Construction

In the building and construction sector, core/shell PCMs are utilized in building materials to improve energy efficiency through excess heat absorption during the day and its release during the night [43]. In the literature, several studies refer to the incorporation of core/shell PCMs into mortar, cement, and gypsum matrixes (Figure 7).

An investigation related to the use of core/shell PCMs in cement slurries was conducted, wherein natural gas hydrate (NGH) (a methane-containing solid) layers were used as a stabilizing agent during cementing in deep water. The core/shell PCMs contained low-binary PCM cores and BaCO₃ shells. They were synthesized via a self-assembly method and then integrated into cement slurries. The tests revealed phase change temperatures of the microencapsulated PCMs that aligned with those of NGH formations (3.5 °C and 13.9 °C), showing a reduction in the hydration heat generated by cement slurries. More specifically, a reduction of 19% was observed in regard to the heat generated when the added PCM volume was 9%, indicating that the stability of the NGH was sustained. These PCMs proved to be great heat inhibitors, slowing heat release and, at lower concentrations, allowing the recrystallization of NGH back to a stable state [47].

Incorporating nano-enhanced, macroencapsulated PCMs into cementitious composites has demonstrated significant thermal benefits in construction applications, particularly for cement-based mortars. Jong et al. achieved PCM encapsulation using an aluminum box-type macro-capsule, enhancing the strength, durability, and thermal conductivity of the material, while effectively addressing leakage issues. Multi-walled carbon nanotubes were dispersed within the PCM, doubling their thermal conductivity and enabling improved energy storage and release at subzero temperatures. Cementitious mortars incorporating these nano-modified PCMs maintained internal temperatures above 0 °C during thermal cycling, even when the ambient temperatures dropped to 5 °C, highlighting their potential for energy-efficient thermal regulation in extreme climates. By stabilizing temperatures and reducing freeze–thaw damage, this composite approach could reduce heating costs and improve structural resilience in cold environments. Despite the initial cost considerations, the enhanced performance and durability of the aluminum-encapsulated PCM system offer substantial long-term benefits for sustainable, energy-efficient construction in challenging conditions [48].

The work of El Bouari et al., which explored different biopolymers as shells for a 1-dodecanol core, incorporated the materials in gypsum and tested them. Gypsum powder and the core/shell product, with CMC and Al2O₃ shells, were mixed and added to deionized water to form a homogenous composition. Different concentrations of the PCM were added, and the mixtures were left at room temperature to become solid and were later evaluated. The results showed a small decrease in the compressive strength and thermal conductivity of the material, while maintaining the mechanical strength above the minimum requirement. However, they also showed a controlled heat transfer rate during the heating and cooling procedures depending on the PCM amount in the matrix. This proves the usability of gypsum-containing PCMs for the improvement of the management of indoor temperatures [13].

The application of core/shell phase change materials in asphalt binders was tested in the work by Xue and his collaborators. Paraffin core and polymethyl acrylate shell PCMs were fabricated via in situ polymerization and incorporated into asphalt, wherein their effects were characterized. Overall, at high temperatures, the PCMs improved the response of the asphalt to temperature changes, thus preventing damage due to temperature increases. In addition, they improved the viscosity and rutting resistance of the material. However, in low temperatures, they were not effective and reduced the storage stability and ductility of the material; because of the different densities of the materials and the asphalt, these core/shell PCMs have promising applications in regard to road construction for cooling pavements and require further investigation [49].

Ardekane et al. synthesized copolymers containing polyethylene/polymethylmethacrylate (PEG/PMMA) and encapsulated the PCMs. Subsequently, the effect of the produced materials was evaluated in terms of energy conservation when applied to a building’s facade. With the best PEG percentage being 60%, the composites were integrated via a simulation method on the surface of a sample building in regard to six different cases. The annual results revealed that the PCM reduced both the heating and cooling loads, with the first option resulting in the greatest decrease. The number of layers and location of the applied PCM play a significant role in energy preservation, since the incorporation of the PCM in external facades and northern and southern windows showed different results [50].

The work conducted by Yuan Gao and others explores the application of solid–solid phase change materials in windows for the purpose of solar radiation management and thermal energy storage. In this study, an equivalent to the EnergyPlus window was used as a simulator to investigate the energy savings of PCM windows. An s–s PCM used was based on poly(ethylene glycol). The inclusion of this semi-transparent material in translucent windows was tested in three different climates and showed excellent results. Solar absorption was the most efficient energy-saving property in all the climates tested. The thermal response was found to be dependent on many extrinsic factors, one of them being solar radiation heat. Overall, s–s PCMs were proven to be beneficial for use in energy-saving windows, especially for air-conditioning, ventilation, and heating systems, proposing more avenues for research in regard to future building technology [51].

The next study by Kontoleon et al. investigated PCMs’ incorporation in between glass and their effect on heat management and other parameters. Three organic phase change materials were tested in two different climates, facing different directions, and in different states. The calculations were performed using a 3D model that resembled the real structure of a duplex building. Compared to conventional units that are double-glazed, PCMs offer high absorbance, low transmittance, and lower annual heat gain. A cost evaluation was the main objective of this research and the results showed a preference in regard to the organic mixture 30, since overall it minimized the heating and cooling costs in both climates and throughout the year, while it ensured ample access to daylight [52].

Fang et al. impregnated a biobased PCM into an expanded glass aggregate (EGA) that was made of recycled materials, and the produced PCMs were covered with a fly ash shell. The PCMs had good thermal stability and were incorporated into cement mortar and managed to minimize the rate of heat transfer. They have proven to be useful for roofs and walls in buildings, because they reduce the density of cement. The results also showed a lowered and delayed indoor peak temperature and good thermal and storage properties. From an economic perspective, the long-term application of these materials is cost efficient and, from an environmental perspective, the materials are sustainable and have large-scale applications in the circular economy [53].

3.2. Textiles

PCMs can be used to improve the thermoregulating properties of textiles. They can be incorporated into sportswear and apparel to maintain the user’s comfort by adapting the material to variations in body temperature. Moreover, they can be utilized in athletic clothing to manage the user’s temperature and moisture produced during physical activity. Barani et al. examined such an application, while maintaining the integrity of the textiles (Figure 8).

Cotton yarns were doped with core/shell nanocapsules that had phase change properties and then coated with polyacrylonitrile nanofibres. The used PCMs had a polymeric shell and an n-octadecane core. To test the effects of doping, two different compositions were prepared and added to the yarn structures. Polyacrylonitrile was added as a shell to form core/shell nanofibers to test its effect on the yarn. The PCMs were also added to the shell to investigate the effect of doping on the thermoregulation and tenacity of the cotton yarn. As for the results of the core/shell PCM application, they showed lower temperatures on the surfaces of the materials in hot environments, with the yarn that was doped both in regard to the shell and the core being the most cooling. They also had a warming effect at cold temperatures. Overall, the PCM-doped yarn was more temperature regulated during fluctuations in the temperature in the environment. The PCM doping also generally increased the tenacity values and decreased the breaking elongation values of the material [54].

Li et al. investigated the synthesis and application of cross-linked and linear microencapsulated PCMs in textiles. Butyl stearate was used as the core, while the shell monomers were isophorone diisocyanate and triethanolamine for 2,4-toluene diisocyanate and diethylenetriamine, respectively. After fabrication, the materials were characterized, and their effect on the fabrics was examined. The cross-linked PCMs demonstrated better results overall, as their surfaces were dense and smooth, their wrapping effect was good, they did not show leakage, and their thermal stability was significantly better. The fabric that was finished with these PCMs had less yellowing after being subject to high temperatures, highlighting the PCMs’ low yellowing characteristics [55].

In the work by Guo et al., as mentioned above, polyurethane/SiO₂-miniencapsuleted PCMs were prepared via an interfacial polymerization method, using an electrostatic self-assembly technique. The core of the MEPCMs consisted of stearic acid butyl ester, while the shell was composed of polyurethane and SiO₂. The latter was used to increase the thermal and chemical stability, thermal conductivity, and compactness, and reduce the supercooling effect of the core/shell materials. After the cycling tests, the materials exhibited optimal heat storage. These core/shell materials were then used to regulate the temperature of the textiles. The fabric samples were placed in a finishing solution made of PCMs and deionized water, with an adhesive. The results on the effect of the coating on the temperature regulation of the material were very promising [5].

Kumar et al. encapsulated 1-tetradecanol through the use of in situ polymerization and then mixed it with an acrylic binder in three different ratios, using three different techniques. After characterization, the material with the best durability, thermal properties, and add-on level, which was the one with the 75:25 microcapsule/binder ratio, was used to coat the cotton fabric, and the results were examined. The mechanical properties of the fabric were not affected, with the exception of an increase in thickness and a reduction in air permeability. Regarding the thermal properties, the thermal resistance was improved, and the surface temperature was lowered. This research is another example of textile applications of PCMs that paves the way for textiles that are responsive to the climate, which are thermoregulated and sustainable [56].

Wang et al. encapsulated 1-dodecanol cores in a dual shell and incorporated them into a polyacrylate sheath. In this way, multi-core sheaths with a room-temperature phase change point were fabricated. The fabrics containing these structures were tested and characterized. The results showed that the materials had optimal mechanical properties, flexibility, and shape stability, while the cycling tests did not affect their thermal reversibility and regulation. This study offers details on a novel incorporation of core/shell PCMs for the creation of smart textiles [57].

Li et al. designed and fabricated a series of reversible thermochromic micro-PCMs (RT-mPCMs) containing ternary thermochromic mixtures that were designed and fabricated via in situ polymerization. The RT-mPCMs exhibited stable light-to-thermal conversion ability, enhanced thermal storage capability, satisfactory thermal cyclic durability, and thermal reliability. The state of energy storage or release can be monitored through the color change, which is based on the phase transition properties of MeS. The smart adjustment-based garment demonstrated excellent intelligent thermoregulated properties and was used for maintaining a comfortable and constant body surface temperature, due to the fact that it can absorb latent thermal energy from the skin’s surface or environmental heat [58].

Zhang et al. fabricated novel mixed-colorant thermochromic microcapsules (MCTMs), using conventional and thermochromic dyes as cores. The produced mPCMs exhibited enhanced reliability, encapsulation efficiency, and effective overheating protection. The obtained printed cotton fabrics, including MCTMs, can reversibly change color between different tones, with good durability and reliability, as confirmed by repeated cycles of isothermal cooling and heating processes. Moreover, more gorgeous colors were exhibited with respect to common thermochromic materials [59].

3.3. Electronics

Core/shell composite PCMs can be used in power electronic devices for thermal management in order to dissipate heat and maintain optimal operating temperatures, to stabilize temperature fluctuations via the absorption of excess heat and then release it, according to demand when the operating system cools. Moreover, core/shell composite PCMs can be used in batteries and electric vehicles, as well as in consumer electronics, providing uniform and enhanced heat distribution across devices.

The use of microencapsulated PCMs in the management of the temperature of satellite electronic boards was examined in a subsequent study by Mehrali and his collaborators. Lauric and stearic acids were used to form a eutectic core for the mPCMs. At an 82:18 molar ratio, the melting temperature of the mixture was 39 °C and was considered ideal for electronic boards, according to the calculations. The shell was made of a mixture based on melamine and formaldehyde that encapsulated the core in an oil-in-water emulsion via in situ polymerization. Three different mPCMs were fabricated and characterized. The results showed that there was no chemical interaction between the core and the shell and that the encapsulation and latent heat improved as the core/shell mass ratio increased. The shell exhibited great anti-leakage properties, and the mPCMs showed chemical and thermal stability, with a minor decrease in latent heat. The mPCM with the highest encapsulation ratio was used to control the temperature of the electronic board, and the results showed that at different constant electric power inputs, the heat sink in regard to the mPCMs reached the critical temperature, with a significant delay. Furthermore, when subject to pulsed electric power, the maximum temperature of the board decreased by 18 °C. Thus, this study proves the efficacy of mPCMs in controlling the temperature of electronic boards [46].

Core/shell PCMs can also be applied to cold energy storage technology, used in air-conditioning systems. The study by Cui Hongzhi et al. explored the encapsulation of salt hydrate PCM, specifically a type of PCM consisting of NH₄Cl, Na₂SO₄*10H₂O, and nanoclay, in different metals with higher conductivity than that of the PCM, to improve the efficiency of cold storage. After corrosion tests on six metals, stainless steel and aluminum alloys were proven to be the most suitable for encapsulation. After corrosion, the products showed a reduction in the thermophysical properties of the material, while the effect of these properties on heat transfer was minor, with the aluminum alloy providing the best results. This study opens the way for the incorporation of metal-encapsulated salt hydrate PCMs into cold energy storage in air-conditioning systems to improve their durability and efficiency [60].

The work by Xiaoze Du et al. reports on the use of a dry mixing extrusion method to synthesize Al-based core/shell composite particles. Materials with heat transfer/thermal energy storage properties were developed and were used as direct irradiation solid-particle solar receivers. Their efficiency was enhanced, which was confirmed by the improved solar absorption, thermal storage properties, thermal stability, and mechanical strength. The obtained particles remained stable after elevated thermal treatment and exposure to violent collision environments [27].

The study by Xiaoze Du et al. introduced a sustainable, biobased phase change material specifically designed for battery thermal management, addressing both environmental and performance needs. The researchers chose lauric and stearic acids to develop eutectic mixtures with optimal phase transition points, suitable for battery temperatures. Ethylene-vinyl acetate (EVA) acted as a stabilizer, encapsulating the PCM to prevent leakage, while aluminum nitride (AlN) boosted its thermal conductivity. To prepare the composite, fatty acids were blended and heated, with EVA and AlN added incrementally. An appropriate treatment of AlN was conducted to prevent hydrolysis and enhance the thermal stability of the material. The composite showed promising results, with a battery-compatible phase change temperature (37 °C), significant heat capacity (107.94 J/g), and effective cooling, keeping battery temperatures below 45 °C during high-discharge tests. This work offers an innovative solution for prolonging battery life and performance, while minimizing the ecological impact, marking a step forward in the development of sustainable energy storage materials [61].

Other researchers have also addressed the use of microencapsulated PCMs for the thermal management of batteries. The work by Xiufang Ke et al. reports on the use of an organic–inorganic shell made of MQ silicone resin for the encapsulation of a paraffin core, via a precipitation method. At a 2:1 core/shell ratio, the shape stability and thermal properties were the best. The microcapsules were then incorporated into a silicone rubber matrix to form a composite with improved mechanical properties. A 50 wt% composite was tested for its thermal contribution to a prismatic LiFePO₄ battery. The results showed that the microcapsule had efficient thermal storage properties, since the applied composite controlled the temperature within 5 °C, was able to lower the temperature by 10 °C in regard to the maximum temperature, and was able to maintain the difference in temperature between the different parts of the battery [62].

The research by Wenbin Yang et al. introduced microencapsulated PCMs in a silicon rubber matrix to implement the material in regard to lithium-ion batteries. However, the microcapsules were also reversible thermochromic. Microcapsules with a melamine–formaldehyde shell and a thermochromic core were synthesized via in situ polymerization and placed in a silicon rubber matrix. Subsequently, different amounts of Cu₂O were incorporated, and the composites were characterized. The presence of Cu₂O was aided by the possible change in color of the materials during the temperature changes and further improved the photothermal conversion and thermal conductivity of the material. The application of flexible composites in lithium batteries was tested by introducing battery heat. The obtained results showed a reduced surface temperature of the batteries, proving that thermoregulation applications of these phase change materials in batteries lead to flexible, multi-colored, and reversible thermochromic material properties [63].

3.4. Food Packaging

The utilization of core/shell PCMs for food packaging presents several benefits, such as the maintenance of the optimum temperature, the improvement of thermal stability preventing degradation or leakage over time, the reduction of energy consumption because the need for active refrigeration is minimized, an extended shelf life that is attributed to spoilage prevention due to temperature fluctuations, and enhanced sustainability because of the existence of biodegradable or recyclable shells [64].

In the work by Nafiseh Soltanizadeh et al. poly(ethylene glycol) that was used as a PCM was encapsulated into a shell based on alginate and CaCl₂, in three different concentrations. The packages containing the PCM appeared to be better at controlling temperature fluctuations and, furthermore, they had a positive effect on other values, such as maintaining the pH. The results of this research show that the application of encapsulated PCM in fish packaging can be beneficial for short-time transportation without the need for freezers [65].

Another use of PCMs is their incorporation into chocolate packaging. Sujay Chattopadhyay et al. synthesized core/shell PCMs using an ionotropic gelation method and interfacial polymerization. The core consisted of beads that were made of 1-dodecanol-embedded barium alginate, and the shell was made of polyurea, which provided anti-leakage properties and improved the thermal stability and flexibility of the material. The encapsulated PCMs were used in chocolate packaging to test their effects. The PCMs were placed in the inner walls of a box that was temperature controlled, and the goal was to maintain the temperature and the taste of the chocolate for a longer time. The results showed a delay of 86.28 min in regard to the change in steady temperature when the chocolate box was placed in an environment that was approximately 35 °C. Using encapsulated phase change beads can be beneficial for the maintenance of the shelf life of chocolate and for the packaging of other foods during changes in the temperature of the environment [66].

In a study by Du et al., core/shell PCMs with a biodegradable shell were incorporated into meat packaging to control the temperature. Tetradecane was encapsulated in a calcium alginate shell and, after the capsules were characterized, they were integrated into alginate films. The film with the highest concentration was selected for meat packaging. The results showed successful control of temperature increases, physicochemical parameters that did not change as much, and higher chewiness, gumminess, hardness, and lower weight loss compared to samples not containing PCMs [27].

In the research by Saowapa Chaiwong et al. the PCMs’ effect was tested on okra packaging. The study concerned ice water bottles containing commercial gel packs and they were placed in two different thermal boxes and a foam box. After the temperature and quality tests, the foam boxes with or without PCMs experienced CO₂ accumulation. However, the thermal boxes with PCMs, especially those containing ice water bottles, showed reduced temperatures, maintenance of low temperatures, and reduced humidity, while the weight of the okra was almost unaffected. Therefore, PCM application in okra packaging was proven to be beneficial for cold chain management and for the improvement of the food’s shelf life and the maintenance of freshness [67].

In another study, bio-waxes were used as PCMs to make nanofibers to protect butter. Butter being a dairy product has a shelf life that depends on the environment surrounding it; therefore, the insulation provided by its packaging must be effective. For the experiment, soy, carnauba, coconut, spermaceti, and bees’ waxes, in different concentrations, were encapsulated in a matrix, using thermoplastic polyurethane to make the core/shell nanofibers. The most effective material proved to be the nanofibers consisting of carnauba wax at 30% w/v, in the thermoplastic polyurethane shell with a concentration of 15%, which was deduced after the characterization of the materials’ properties. The results showed that the PCMs in the nanofibers helped keep the temperature of the butter stable at 18 °C for 4 h, suggesting potential improvements once again in terms of the storage and transportation of foods that are temperature sensitive [68].

3.5. Automotive

The ability of core/shell PCMs to efficiently manage temperature fluctuations and provide passive heating or cooling in vehicles makes them very useful materials for use in automotive climate control systems. In regard to these systems, comfort improvement can be accomplished through the use of PCMs, together with a reduction in energy consumption. In the research by Srusti and Kumar, considering the provision of the most comfort for the passenger, encapsulated PCMs with thermal melting points within thermal comfort temperatures were incorporated and tested in different positions and orientations in a car cabin. The effects of inorganic and organic PCMs were evaluated in this study. For the simulation, a new method called the “equivalent specific heat method” was developed to reduce the computational time. The solar load distribution and its effect on temperature distribution, as well as the air temperature, phase change temperature, and velocity contours, were examined. The results were generated from seven orientations and sixteen locations in the car cabin and showed that two curvatures of inorganic PCM covering the roof best reduced the car cabin temperature [69].

In a systematic numerical study by Alhamany et al., with respect to battery thermal management, PCMs were incorporated into lithium batteries, involving contemporary shape memory alloys, to improve the performance and safety of electric vehicles. PCMs were added as thermal regulation factors to extend the battery life and prevent excessive overheating. Combined with alloys, they provide thermal and mechanical stability in battery applications, enhance energy storage, the charging speed, and durability, and reduce fatigue. It should be noted that the materials did not add to the weight of the battery and did not affect the design. Overall, this research suggests that the utilization of PCMs in electrical batteries for automotives is valuable, since they showed good results in regard to improving the autonomy, energy storage and, thus, the safety of vehicles, while it mentions the need for the adoption of precautions during their application [70].

The research by Xiaoze Du et al. focused on PCM packages that were put into lithium-ion batteries. The goal of this study was to reduce the cost of battery thermal management systems and CO₂ emissions. The fabricated PCM module was flexible, had a high capacity, and consisted of paraffin and thermally enhanced paraffin, with nano-magnetite, wrapped in a battery block. The testing subject to typical urban battery charge/discharge cycles and at different temperatures, namely 30–40 °C, demonstrated temperature stability at lower temperatures than conventional batteries. In this way, the battery life is increased and leads to a direct annual economic saving for medium-sized electrical and hybrid vehicles and, also, to lower CO₂ emissions, with the exact numbers depending on the type of fuel [27].

3.6. Renewable Energy Systems

Core/shell PCMs have remarkable potential in regard to renewable energy systems, due to the fact that they pinpoint key challenges based on energy storage, energy efficiency, and temperature regulation. Their properties, which includes the provision of thermal energy storage during periods of excess supply and release when there is a shortage, mean that are a significant solution in regard to integrating renewable energy sources, such as wind, solar, hydropower, and tidal (Figure 9).

Metal PCMs are useful in solar energy storage systems because they enable higher temperatures to be achieved by concentrated solar power systems and improve the cycle efficiency of power generation. In a study by Pan et al., aluminum was used as a PCM because it is compatible with concentrated solar power systems that operate above 565 °C, owing to its melting temperature of 660 °C. The matrix used was expanded graphite and an oxidation pre-treatment was used to form a layer of oxide over the Al particles. This layer enhances the encapsulation and thermal properties of PCM composites. At 1100 °C encapsulation, a peak of 80 wt% was reached, while between 600 and 700 °C the energy storage density was 248.5 J/g, making these composite PCMs preferable for use in high-temperature energy storage technology [71].

The study by Hao Bai et al. explored core/shell nanocapsules with a stearic acid (SA) core and a silver (Ag) shell, created to enhance solar water-heating systems. Through the use of a Pickering emulsion and a reduction method, a high-conductivity Ag shell encapsulated the SA and, thus, boosted its thermal conductivity, significantly improving the system’s thermal storage capacity. The characterization results showed effective encapsulation was achieved, producing nanocapsules with diameters of 167–252 nm, which, after the suspensibility tests, were proven to have strong stability and suspensibility in water, which is crucial for consistent energy transfer in suspension systems. This core/shell structure suggests that these SA/Ag nanocapsules hold strong potential for more efficient, eco-friendly applications in solar energy storage [72].

Farzan and others conducted a study to test the exergy and energy performance of a hybrid photovoltaic/solar air heater (PV/SAH) system. Encapsulated PCMs were employed to enhance the electrical efficiency by reducing the PV temperature. PCMs were incorporated as a passive cooling method, absorbing heat and delaying its release, which cooled the PV cells and mitigated efficiency losses due to heat generation. However, the PCMs lowered the outlet temperature of the air heater, slightly reducing its thermal efficiency. In the tests, the PCMs reduced the PV surface temperatures by up to 4 °C, boosting the electrical output by 6–7%, while lowering the daily thermal efficiency by 8–12%. This passive cooling capability boosted the overall exergy efficiency, benefiting the electrical performance of the heater, despite a slight reduction in the thermal output. It was also noted in the study that the size of the PV/SAH was bigger than usual and that this affected the results [73].

In another study by Sui et al., microencapsulated PCMs were incorporated into polyurethane (PU) to make a flexible photothermal film, suitable for solar energy collection. Micro-PCMs were covered with polydopamine (PDA) and then mixed with PU, with different weight ratios, from 30% to 70%. The characterization results showed that the film had high photothermal storage energy because of the PDA. The PDA also improved the elongation of the films, owing to its interaction with PU. The mPCMs-PU films demonstrated desirable results in terms of flexibility, photothermal cyclability, thermal durability, and storage, proving to be ideal for use in thermal energy management and collection systems [74].

In a study by Ahmadi et al., a composite PCM made of paraffin with a mixture of beef tallow and coconut oil was used on the surface of a PV module to increase heat removal. The PCM was a mix of different PCMs to combine their melting points into one material. Thus, they addressed problems such as uneven distribution, hot spots (high melting points), and limited time efficiency (low melting points). The weight effect measurements showed that the optimal ratio was 36.9% beef tallow/coconut oil and 63.1% paraffin [75].

As previously mentioned, in the study by Xiaoze Du et al., Al-based core/shell PCMs were subjected to high temperature tests to evaluate the effect of their integration into solar thermal storage applications. After their dry-mix extrusion synthesis, the tests conducted showed the maintenance of solar absorptivity after several days at high temperatures, reaching 1200 °C, with minimal mass loss and cost. Their compressive strength and energy density reached high values, proving their durability in applications involving circulating flow systems. Their high thermal and solar storage capabilities and thermal conductivity make them ideal for large-scale solar thermal applications [27].

In a study conducted by Z. Al Hajaj and M Ziad Saghir, paraffin wax was used as a PCM that was incorporated into a laboratory simulation of a geothermal energy pile system in order to compare it to a system that did not contain a PCM. The application of the PCMs in the system notably improved its thermal performance; it increased both the stored and extracted energy. Geothermal systems are a very useful source of energy for the heating and cooling of buildings and the application of PCMs to them could further improve the heat exchange and storage capacity of such systems in future large-scale applications [76].

In a study by A. Torbatinejad et al., PCMs were incorporated into a simulation of geothermal energy sources, complementary to the addition of spiral fins to the exterior of a heat exchanger. Microencapsulated PCMs were placed into the backfill material. They were added in four volume fractions, ranging from 0% to 50%. The results of this study showed that the temperature was reduced by 4 K with the use of a PCM. Furthermore, the temperature difference in the backfill material became more moderate as the volume percentage of the PCMs increased. The best results achieved showed that the PCMs increased the efficiency of the heat exchanger and the performance coefficient of the heat pump by 7.5%. In regard to similar large-scale applications, these results show promise in regard to reducing energy consumption, costs, and greenhouse emissions [77].

3.7. Healthcare

In the literature, several studies are based on the utilization of core/shell PCMs for healthcare applications, such as temperature-controlled packaging that is used for transporting sensitive pharmaceuticals and medical supplies that require specific temperature conditions and temperature-sensitive drug release. Mohammed Taghi Zafarani-Moattar et al. focused on using biobased PCMs for the nanoencapsulation of the D3 vitamin, aiming to explore applications in regard to thermosensitive drug delivery. The researchers selected a eutectic mixture of stearic and lauric acids (1:3 mole ratio), with a phase change temperature close to body temperature (35.6 °C), ideal for controlled drug release. Vitamin D3 was encapsulated in this PCM using stabilizers like polyvinyl alcohol and polyvinylpyrrolidone, along with sodium lauryl sulfate as an emulsifier. Characterization showed the attainment of high encapsulation efficiency and the creation of stable core/shell nanocapsules, with smooth morphology. The efficiency of the 35:65 shell-to-core weight ratio was confirmed, while quick, temperature-responsive vitamin D3 release was demonstrated in the release studies at 37 °C, highlighting this PCM’s potential for delivering sensitive drugs within body temperature ranges [78].

Eutectic mixture PCMs were encapsulated together with Rhodamine B, a model drug, in ethyl cellulose nanofibers, via blend electrospinning, in the work by Ping Lu and his collaborators. The goal of this study was to achieve temperature-sensitive drug release in the human body, particularly for antibiotics, anti-inflammatory drugs, and chemotherapy. The PCM loading manipulation led to the achievement of a tunable release rate for the drug, with 5 and 10% showing steady release and 20 and 40% showing burst release at 37 °C. The obtained results are very promising for the future of temperature-responsive drug release [79].

In another study, a PCM was used in a wound-healing drug carrier to enable photothermal-responsive aspirin release, while also reducing the risk of burnt skin. A hydrogel, named Au-Asp@PCM, was designed for non-invasive wound closure in near-infrared conditions. The results of the study were positive, since the hydrogel was effective at wound closure and even enhanced the wound healing, with confirmed wound repair results in the in vivo studies. The PCMs in the drug carrier incorporated into the hydrogel were introduced as thermal regulators. Their application was aimed at accelerating the process of wound healing through the manageable release of antimicrobial agents, showing the potential efficiency of PCMs in regard to further studies on controlled drug release [80].

The work by Shujun Wang et al. reported on the fabrication of natural core–shell sporopollenin microcapsules to control lipid absorption in obesity. Naturally occurring micro/NPs provide an incredible array of potential sources for fabricating pharmaceutical microcapsule carriers. Natural pine pollen was treated with phosphoric acid to yield intact, clean, and monodisperse microcapsules. The intrinsic core–shell structures of natural pine pollen have been used as potential carriers for pharmaceutical microcapsules, owing to their excellent liquid absorption abilities and tunable wetting properties. It was proved that the aforementioned hydrophobic sporopollenin microcapsules are able to selectively adsorb oils in water–oil systems within an organism [81].

3.8. Industrial Processes

Several industries use process heat management as a tool to regulate temperatures to achieve energy efficiency improvements during procedures that generate excess heat (Figure 10).

A study by Karim Emara et al. was conducted to improve thermal systems, using PCMs and water/TiO₂ and AlO₂ nanofluids, and to make them more efficient and save energy. The experiment’s results showed that the PCMs improved the heat transfer of the material and, combined with nanoparticles, they encompass the most effective strategy. Their incorporation into thermal systems suggests potential enhanced thermal performance in future systems, with minimal power use and even lower manufacturing costs and smaller system sizes [82].

The research by Changgui Xie and Xiao Yang explores the effect of nanoencapsulated phase change materials applied to microchannels for bettering the thermal management of fuel cells. More specifically, PCMs consisting of a PMMA shell and an n-octadecane or salt hydrate core were employed in a molten state to test the influence of changes in the Reynolds number, the volume fractions, and the temperature on the bottom plate of the aluminum cast on the absorption of heat. The microchannel used was an aluminum cast structure, with tubular hollows to allow the flow of a water–PCM mixture. The tests as part of the research led to results that included proposed numbers that could be used for improving microchannel heat transfer systems, with numerous potential applications, especially in fuel cells. With the use of PCMs, the improvement of the thermal management maintenance of the cell’s performance, the prevention of thermal degradation, and the uniform distribution of temperature within the cell will also be enhanced [83].

In a study by Peng Zhang et al., PCMs were placed on the shelves of an apple refrigerator warehouse simulator to test their effectiveness in regard to low-temperature storage. This study investigated the performance of PCMs in regard to postponing the alleviation of electricity requirements and its economic, thermal regulation, and preservation properties. The results showed that PCMs can help reduce the cost of operation of the warehouse, reduce the peak air temperature by 1.1 °C, and reduce the maximum temperature increase rate of the apples in comparison to a warehouse refrigerator without on-shelf PCMs. While some defects occur, like a slightly increased temperature in the last 6 h of storage in one day, the on-shelf PCM incorporation in food warehouses shows promise for future energy and food preservation [84].

The work by Shohel Mahmud et al. deals with PCMs that are used in a food storage refrigerator, combined with solar energy and thermoelectric cooling. The PCMs’ goal was to maintain the refrigerator temperature at 5 °C. Fruits and vegetables with moisture levels of 50–99% were selected for testing. The results showed that the average time to solidification of the PCM was approximately 3.5 h and the performance coefficient was 0.69. The water flow through copper pipes for thermoelectric cooling was also proven to be beneficial and even reduced the solidification time of the PCM by one hour. Future research is needed for the optimization of the results and for applications of a large scale [85].

3.9. Smart Materials

Smart buildings demonstrate enhanced energy efficiency due to the fact that they include materials that respond to environmental changes. In a study by Li et al., core/shell thermochromic phase change materials were integrated into wood to induce a phase change in the next study. Microcapsules with a melamine–formaldehyde shell and an ethyl stearate, methyl stearate, and crystal violet lactone core were synthesized and then mixed with polyvinyl alcohol to form a thermochromic solution. The mixture was added to Hinoki wood, forming a coat over it, using the drop-coating method. This new material is now a composite, with thermal energy storage and reversible thermochromic properties. The characterization results showed that the thermochromic phase change wood had thermal stability in the temperature range of 26–270 °C, with a phase change temperature of 34 °C, around human body temperature. This fact makes it applicable to scenarios such as home décor and other locations used by humans because the PCMs make the materials have a low toxicity and environmentally friendly. This composite material also responded to thermal changes by changing color, with an increase over 34 °C it changes from a bluish color to yellow. Its thermal storage and thermochromic properties make it a promising material for smart home systems, the functional material industry, and for use in a sustainable future [86].

Jianwei Zhang et al. investigated the production of core/shell capsules with a core derived from waste cooking oil, wherein they were integrated into asphalt to delay their rate of temperature change and, therefore, prevent damage to asphalt pavements in hot weather. Waste cooking oil was used to extract its waxy components. These components (methyl palmitate) underwent hydrolysis to achieve a phase change temperature and enthalpy closer to that of asphalt (palmitic acid). The phase change materials were made via the orifice coagulation method. The shell consisted of cross-linked calcium alginate. The ratio used was 1:1, making the capsules thermally and mechanically stable, with optimal thermal energy storage [87].

A core derived from cooking oil was also used in the study by Fan Yansong et al. to improve the thermal response of walls in buildings. Microencapsulated PCMs were synthesized this time via the suspension polymerization method, and methyl palmitate itself was incorporated. The shell was made of methyltriethoxysilane to prevent leakage. The microcapsules were put in foamed cement and the characterization results showed improved compressive strength and suitability for the cement’s thermoregulation. While more research is needed, these materials open the way for recycled materials to help reduce energy consumption and make smart building materials [88].

The work by Michele Bottarelli et al. explores the fabrication of a PCM for heating and one for cooling, which were put into a radiant floor as part of a large-scale investigation to test how it stores and releases thermal energy and how it controls heat gains throughout the year. The PCMs were macroencapsulated hydrated salts and they were put in a checkerboard pattern in a snack-bar space, with an air-handling unit that managed the latent loads and ensured certain rates of ventilation. They had different melting temperatures, 27 and 17 °C for summer and winter, respectively. The effective storage of thermal energy was noted, since both during the summer and the winter, the PCM absorbed and released heat, maintaining the temperature and saving energy by limiting the need to use the air-handling unit [89].

4. Challenges

Principally, core/shell composite PCMs provide significant advantages for thermal energy storage and management, but it may also be remarked that several challenges remain [90]. Taking into consideration challenges based on the encapsulation efficiency, thermal stability, the cost of materials, scale-up issues, compatibility, heat transfer efficiency, and environmental concerns, there is the potential for core/shell composite PCMs to become more effective and more widely used in various applications.

Achieving high encapsulation efficiency without compromising the shell’s structural integrity can be difficult. Poor encapsulation may lead to leakage of the core material. To avoid this, a study, firstly, uses an oxide shell as an oxidation pre-treatment. According to this process, the encapsulation ratio was increased, and leakage was prevented [71]. A double shell is another solution to leakage problems. Chitosan is used as the second shell to prevent leakage [12]. Another very crucial factor is the thermal stability of the shell material. Several polymers can be degraded at elevated temperatures, which may limit the operational range of the PCM. To enhance the thermal stability of the biopolymer, for example, in this research [28], it was cross-linked with barium, alumina, and alumina–AlOOH shells.

The shells of biopolymers or advanced materials can be more expensive than conventional materials, which may prevent their large-scale commercial adoption. Balancing affordability with performance is a persistent challenge for manufacturers. To address high costs, waste-derived shells like tea waste [16] could be scaled up, while using materials that are more expensive in small quantities, which will eventually save costs by accelerating the process of saving energy, should be considered [44].

Scaling up the synthesis process from laboratory to industrial levels can be challenging, particularly in regard to maintaining the quality and uniformity of the core/shell structure. Most research has been conducted in laboratories using simulations [84], a refrigerator simulation [52], a 3D model of a building [77], and the simulation of geothermal sources, etc. During large-scale production, issues such as uneven particle size distribution and agglomeration may arise, and different techniques may be needed during the synthesis process. Additionally, reactor designs may require the optimization of emulsification or polymerization processes.

A very important factor is that the shell and core materials have to be chemically compatible in order to avoid reactions that could influence their performance or stability. For instance, hydrophobic and hydrophilic mismatches can lead to phase separation or ineffective encapsulation. Compatibility challenges are also observed when conductive fillers are added, because of their potential to alter the properties of the shell.

Considering their heat transfer efficiency, it may be noted that while PCMs effectively store energy, their heat transfer rates can be slow. Enhancing their thermal conductivity is essential for improving the performance of PCMs. This disadvantage can be overcome by using modified material in the shell or using shell materials that themselves have high thermal conductivity [90]. For example, alumina helped accelerate the heat transfer efficiency of PCMs [26]. Another metal oxide that can be used in the shell to enhance the thermal conductivity of PCMs is SiO₂. Although many PCMs are designed to be eco-friendly, the fabrication procedures and end-of-life disposal of some materials still cause environmental concerns. The biodegradability and recyclability of the PCMs that are produced require further investigation; therefore, after the use and wear of the products, they can be revived, reused, or disposed of in an environmental way.

5. Summary and Outlook

This review reports on the green synthesis of core/shell PCMs, emphasizing environmentally friendly methods of design, production, and application. Traditional synthesis procedures for PCMs usually involve the utilization of toxic chemicals and energy consumption. On the other hand, green synthesis aims to minimize environmental impacts through the use of non-toxic solvents, renewable resources, and low-energy methods. This approach reinforces sustainable development and maintains the main functional performance of the PCMs.

Future directions for core/shell composite PCMs are promising and focus on enhancing their sustainability, performance, and applicability across different sectors. Studies on new, sustainable materials for both the shell and core components, such as biobased polymers or nanomaterials, can improve their performance and minimize the environmental impact. The development of innovative synthesis methods, such as additive manufacturing or self-assembly techniques, can allow for more accurate control of the PCM structure and properties. The incorporation of conductive additives, such as conductive polymers, carbon nanotubes, and graphene, into the PCM matrix can enhance their thermal conductivity without affecting encapsulation. Moreover, investigations based on PCMs that can provide additional functionalities, such as antibacterial properties, fire resistance, and moisture control, could result in a wide range of potential applications. In an already mentioned study, the core/shell materials were not only PCMs, but also thermochromic compounds, providing the produced wood with the same properties [86]. Finally, comprehensive life cycle assessments should be conducted in the direction of better understanding the environmental impacts of PCMs, with the ultimate aim of improving their sustainability.

As for the feasibility of PCM use in industry, it depends not only on the performance metrics, but also on economic and environmental ones. For example, in the construction sector, biobased PCMs can offer a cost-effective and sustainable option, due to their long life cycles and compatibility with building materials; In high-performance applications like electronics or aerospace, more thermally enhanced and complex formulations could be required, which could result in bigger material and processing costs. Even in these sectors where the costs seem to be higher, in the long run, a reduction in energy consumption and improved thermal regulation can offset the initial investments. Taking into account the environmental aspect, PCMs that are synthesized in a green way can reduce emissions and waste during their production and disposal. Recycling the waste and using it for another synthesis is also a potential green solution. Evaluating the feasibility of PCMs must, therefore, consider the balance between the life cycle cost, system efficiency, and the impact on the environment, which can vary from laboratory to industry scenarios and from industry to industry.

The application of core/shell PCMs can be explored further in regard to different and advanced research topics. For example, 3D printing could be implemented for the creation of a precise PCM structure, or the core/shell PCMs could be applied for packaging purposes in the cosmetics sector, for the preservation of agents. Another application in pharmaceuticals and cosmetics sectors involves the release of the active compound based on the temperature. PCMs can also be integrated with other materials, such as the Internet of Things (IoT) for smart thermal management. Sensors (IoT) can be used to detect the temperature, humidity, or energy consumption in real-time, for example, in buildings or batteries, optimizing the system’s response, while PCMs can be used to manage the temperature passively. The integration of PCMs with the IoT can also benefit the actual PCMs, by monitoring their changes through an intelligent system, or may even benefit and prolong the life of IoT systems. Therefore, the use of core/shell PCMs is not limited to the thermal management of buildings, but they can also be explored in regard to other science areas, such as for the benefit of health and well-being or even intelligent systems and technology in order to monitor and improve thermal management. For every application of these PCMs, their compatibility, cost, and environmental impact need further research.