Phase change microcapsules with photothermal properties: materials,preparation,applications,and prospects

Abstract

Phase change material microcapsules are converted into phase change materials to provide sealing protection and effectively address the leakage issue during the phase change process. However, phase change microcapsules can only passively absorb heat and store energy through temperature difference, which has a single function and cannot effectively use other energy sources. Phase change microcapsules with photothermal properties, which combine photothermal materials with phase change materials, have gradually entered people’s field of vision. They achieve direct conversion of solar energy into heat energy storage to reduce energy consumption. In this paper, the classification and basic principle of photothermal conversion materials are systematically reviewed, then the preparation methods of photothermal conversion phase change microcapsules (PCPCMs) are summarized and analyzed. Furthermore, the applications in the field of PCPCMs in the textile industry, building energy conservation, thermal management of electronic equipment, and clean energy are introduced in detail. Finally, the future research directions of PCPCMs are proposed.

Keywords

Phase change materials phase change microcapsules photothermal conversion photothermal material thermal management solar-driven interface evaporator

Recently, the rapid development of industry has resulted in the crisis of traditional fossil-fuel energy.¹ Moreover, the massive use of fossil fuels has caused increasing carbon dioxide emissions, which has brought a series of climate change and environmental problems.^2,3 Therefore, the development of a green and low-carbon energy system is one of the important measures to achieve carbon neutrality and alleviate the energy crisis.^4,5

Phase change materials (PCMs), substances with latent heat energy storage, are renewable and reusable resources.⁶ Compared with traditional fossil-fuel energy, PCMs can absorb and release a lot of energy during the phase change process. It also has the advantages of high energy storage density, wide sources, cheap raw materials, and wide phase change temperature, which are widely used in various fields, such as industrial waste heat recovery and utilization,⁷ energy storage,⁸ logistics,⁹ building,¹⁰ food,¹¹ and medicine.¹²

Solid–liquid PCMs have attracted increasing attention, but are limited by defects of easy leakage, large volume change, and unstable shape. Thus, several solutions have been proposed to solve the problem.^13,14 Microencapsulation technology is the use of one or several film-forming materials to cover some liquid, solid, or gas with fluidity, sensitivity, and volatility to form tiny particles, which can realize the permanent solid-state of PCMs.¹⁵ In order to broaden the application field of phase change microcapsules (mPCMs), many researchers have used different methods to prepare functional microcapsules,¹⁶ such as antibacterial mPCMs,¹⁷ magnetic mPCMs,¹⁸ and supercooled protective mPCMs.¹⁹

Solar energy is becoming one of the promising clean energy sources owing to its low cost, nontoxicity, environmental protection, durability, and inexhaustibility.^20,21 We can use solar energy in photoluminescence, photochromism and photothermic. However, the major problem with solar energy is it is not easy to store and is affected by day and night changes, thus the application of solar energy is restricted. Photothermal conversion is one of the most simple and effective solar energy utilization technologies. The Sun radiates energy by emitting ultraviolet, visible, and infrared light, which carries photons with varying vibration frequencies. Upon interaction with a material, a portion of the photons in the light can be absorbed by the material, resulting in an increase in its temperature. In this process, the material can act as a light absorber and effectively transfer light energy into heat, a phenomenon known as photothermal conversion, and the materials are classified as photothermal conversion materials (PHCMs).²² The mechanism of the photothermal conversion process is primarily ascribed to the localized heating of plasma, the nonradiative relaxation of electron–hole pairs, and the thermal vibration of molecules. The main factors affecting the photothermal conversion efficiency include the light absorption coefficient, thermal conductivity, thermal expansion coefficient, light intensity, light duration, ambient temperature, and thermal radiation.^23,24

The introduction of PHCMs into the PCM gives microcapsules with photothermal conversion performance, which can effectively absorb sunlight and transfer to thermal storage, and reduce the loss in the heat transfer process. It is of great significance for the efficient utilization of solar energy.²⁵ At the same time, the PHCM has also enhanced or improved the thermal storage stability of PCM. As can be seen from Figure 1, the combination of PHCM and PCM has also been a research hotspot in recent years.

In this paper, the current research progress and status quo of photothermal conversion phase change microcapsules (PCPCMs) are summarized. First, the PHCMs are introduced, focusing on the preparation method and research progress of PCPCMs. On this basis, application of PCPCMs in the textile industry, building energy conservation, thermal management of electronic equipment, clean energy, and other aspects is described and summarized. Finally, the existing problems and future development direction of PCPCMs are discussed, and it is pointed out that the preparation process should be simplified to further expand the application of PCPCMs in solar, wind, and other clean energy generation.

PHCMs

Localized surface plasmon resonance (LSPR) converts the light energy absorbed by PHCMs into the kinetic energy of electron or hole resonance, or the energy generated by electron transition, and then the vibration energy is scattered through the crystal lattice, which is transmitted to the surrounding environment to increase the ambient temperature.²⁶ PHCMs can be divided into carbon-based materials,²⁷ polymer materials,²⁸ metal-based materials,²⁹ semiconductor materials,³⁰ and other PHCMs³¹ according to their types (Figure 2).

Carbon-based materials have sp² hybrid structure, which has strong photon absorption and conversion ability in the full spectrum of sunlight, and is the focus of current research in the field of PHCMs.³² Polymer PHCMs have high light absorption, light stability, and photothermal conversion efficiency.³³ The surface of metal nanomaterials is filled with a large number of free electrons, forming a free electron gas mass (plasma). When solar irradiation is close to the vibration frequency of the plasma on the surface of metal nanomaterials, local surface plasma resonance effect is generated, which absorbs light energy and releases heat.³⁴ Semiconductor materials possess good photothermal stability, a wide energy gap, and the ability to absorb higher incident light. Due to their variety, high chemical stability, easy of design, low toxicity, and high efficiency, they have been a hotspot in the study of PHCMs.³⁵

Carbon-based materials

Carbon-based material is a kind of nonmetallic material obtained by a series of processes mainly using organic substances such as coal, petroleum, or their processing products as the main raw materials, and its main component is carbon.³⁶ Most carbon-based materials have high electron cloud density of sp² hybrid orbit and π bond, the energy level boundary is not obvious, visible and near-infrared light in solar irradiation has a strong absorption effect, and under light irradiation, sunlight is converted into heat energy by the scattering and reflection of the porous structure of carbon-based materials, thus obtaining excellent solar energy absorption performance. They are the most frequently studied PHCM in recent years.³⁷ According to the source of the material, carbon-based materials can be divided into natural plant-derived carbon-based materials and artificial carbon-based materials two categories, as presented in Table 1.

Table 1.

Classification of carbon-based materials

Material	Features	Principle of light and heat conversion	Common material
Natural plant-derived carbon-based materials	Excellent light absorption ability, easily affected by external environment	The water absorption mechanism and evaporation mechanism of solar energy use the porous channel water transport structure of the fiber and low thermal conductivity for photothermal conversion	Carbonized mushroom,³⁸ carbonized sunflower,^39,40 carbonized wood,⁴¹ etc.
Artificial carbon-based materials	Rich variety, stable physical and chemical properties, light absorption ability after carbonization has been improved, heat insulation ability is relatively common, limiting its application range	The energy level transition and π structure between the band gap are used to make the material absorb sunlight in the full spectrum range	CNTs,⁴² CB,⁴³ graphene,⁴⁴ etc.

Xue et al.⁴⁵ found that the natural wood obtained by simple flame treatment has ultrahigh solar energy absorption rate, and the internal pores transport water to the heating surface, making the wood surface produce high-temperature steam, achieving a photothermal conversion efficiency of 78%. On this basis, Chen et al.⁴⁶ built a double-layer porous structure photothermal conversion system by depositing aluminum phosphate compound on the surface of wood, and it can achieve 90.8% solar energy conversion efficiency under 1 kW/m² light condition. The researchers found that the mushroom umbrella-shaped cap has excellent light absorption capacity, and the light absorption capacity can be further improved after carbonization. Natural and carbonized mushrooms have conversion efficiencies of 62% and 78% per unit of sunlight, respectively. The porous fold not only acts as a bridge to supply water to the cap part, but also provides sufficient steam transmission channels, and reduces the heat loss during the conversion process due to the geometry of the mushroom itself.⁴⁷

The variety of artificial carbon-based materials is rich, the physical and chemical properties are stable, the energy level transition between the band gap and the π structure allow the materials to absorb sunlight in the full spectrum range, showing excellent photothermal conversion ability.⁴⁸ Carbon nanotubes (CNTs), carbon black (CB), and graphene all belong to the artificial carbon-based materials group. Their light–heat conversion principle is shown in Figure 3.

CNTs, a one-dimensional carbon-based material with a special tubular structure, are mainly composed of carbon atoms arranged in a hexagonal shape. They have stable structure, are lightweight, and have good flexibility and mechanical properties.⁴⁹ They have very high absorbance in the whole spectrum range, and can adsorb 99.97% of the incident light in a vertical arrangement, and can efficiently convert sunlight into heat energy. When CNTs are arranged vertically, they can form a very effective light absorber, as light must pass through each CNT and undergo multiple reflections within the tube to be fully utilized and absorbed. When the light enters the blackbody of the CNTs at ambient temperature, it is almost entirely absorbed rather than reflected, being confined to the tube walls where it is continuously deflected until it is ultimately converted into heat (Figure 3(a)).

Researchers have developed CNTs with large specific surface area, low density, and extremely low reflectivity. This material also has a variety of excellent chemical, mechanical, and thermal properties. Its good photothermal conversion ability has resulted in its wide use in the field of photothermal utilization. Yu et al.⁵⁰ prepared the full CNT hybrid film by vacuum filtration method. The prepared heterocyclic films have extremely high solar absorptivity (about 99%), low thermal conductivity and excellent photothermal conversion ability. When the light intensity in the air is 1 kW/m², the temperature of the mixed film increases from 24 to 60°C within 1 s, and the relative photothermal conversion efficiency is 87.4% after 1 h evaporation. When a plurality of small pieces of mixed film self-assembly into a large film with an area of about 42 cm², the photothermal conversion efficiency can be maintained at about 80%. Ghafurian et al.⁵¹ developed this kind of CNT photothermal material and found that the evaporation efficiency of multiwall CNT nanofluid was the highest, reaching 94%.

CB is a kind of amorphous carbon. It is light, loose, and extremely fine black powder, with good electrical conductivity, is the earliest human development, application, and currently the largest output of nanomaterials, has a wide range of uses, has the absorption ability of most wavelengths of sunlight (Figure 3(b)), and, after a series of modifications, little reflection, so is a good light collection material.⁵²

CB powder and mannitol (d-Mannitol) were mixed after 3 h ball milling to prepare a CB/d-Mannitol composite material, and the effects of surface absorption and volumetric absorption on the efficiency of photothermal conversion were explored. The CB/d-Mannitol composite material was irradiated for 400 s under simulated light (915 mW/cm²). The upper surface and bottom temperatures of the CB/mannitol composite material by surface absorption were 195.9 and 92.8°C, whereas the two values of CB/mannitol composite with volume absorption were 173.4 and 110.7°C, respectively.⁵³ Wang et al.⁵⁴ prepared CB bone glue nanocomposites by ball milling using environmentally friendly bone glue and CB. The photothermal conversion efficiency of CB bone glue nanofluid, with a CB mass fraction of 0.006%, can achieve as high as 94%.

Graphene is a two-dimensional honeycomb network single-layer material composed of carbon atoms arranged by sp² hybridization, characterized by high porosity, large surface area, excellent light absorption, and high thermal stability. It is considered to be one of the most promising PHCMs due to its ability to achieve more than 97% light absorption rate in the solar spectral range of 200–2500 nm.⁵⁵ The electron cloud inside graphene can absorb most of the energy photons in visible light to transition into excited electrons under the sunlight. Subsequently, the heat is released through electron–phonon coupling relaxation back to the ground state (Figure 3(c)). Therefore, the absorbed light energy is transferred from the excited electrons to the vibration of the entire lattice, transferring heat to the surrounding space.

Wang et al.⁵⁶ fabricated a graphene aerogel (GA) membrane that exhibits high light absorption rate with less than 4% specular reflection and diffuse reflection in the wavelength range of 250–2500 nm. In addition, the vertical graphene film (VA-GSM) prepared by antifreeze-assisted freezing technology showed excellent light absorption and photothermal conversion ability, which can absorb about 93% of ultraviolet radiation, 98% of visible light, and nearly 100% of near-infrared radiation. Under the irradiation of 1 and 4 kW m⁻² sunlight, the photothermal conversion efficiency is as high as 86.5% and 94.2%, respectively.⁵⁷

It has also been found that graphene oxide (GO) prepared by the modified Hummers method can also be used as a photothermal conversion material.⁵⁸ Polyethylene glycol (PEG) was selected as the matrix and are mixed with GO. The results found that the photothermal conversion efficiency of GO-PEG could reach 75% under the condition of simulating sunlight with a 0.2 W incandescent lamp. Tian⁵⁹ obtained hydrophilic porous graphene composites by a one-step hydrothermal method with graphene/GO composite nanosheets and polyvinyl alcohol as raw materials. In the wavelength range of 250–2200 nm, the solar absorption rate of the composite was 94.5%, and the porosity was as high as 97.2%.

Carbon-based materials have a wide range of sources and possess many excellent characteristics such as broad-spectrum absorption of solar energy, good light absorption stability, high thermal conductivity, biodegradability, and low toxicity. Thus, carbon-based materials are highly attractive for PHCMs. However, the preparation process of nanoscale carbon-based materials is relatively complex, the absorption range is narrow, and the cost is relatively high.

Polymeric materials

Polymer PHCMs possess an electron cloud structure similar to that of carbon-based materials, which can absorb solar irradiation at certain wavelengths. Heat is generated by lattice vibration driven by charge transfer due to electron π–π jump.⁶⁰ Polymer relies on the random motion of its internal molecules for photothermal conversion. Polymer PHCMs possess excellent characteristics, including good flexibility, high light absorption coefficient, low cost, light weight, and ease of chemical processing. Polypyrrole (PPy), polyaniline (PANI), and polydopamine (PDA) are commonly used polymer PHCMs. Table 2 presents a classification table of common polymer materials.

Table 2.

Classification of common polymer materials

Material	Advantage	Photothermal conversion mechanism	Reference
PPy	Excellent photothermal conversion performance, low thermal conductivity, and high optical stability	It has π electron delocalization structure and low optical reflectance in wide wavelength region	Xu⁶¹ used the template method to directly obtain multilayer PPy nanosheets with surface microstructure on a variety of carriers through controlled layer-by-layer interface polymerization. The light absorption rate of 10-layer PPy nanosheets in the solar spectral range exceeded 99%. Under the irradiation of 1 kW/m² sunlight, the photothermal conversion efficiency reaches 95.33%.
PPy			Another study found that a PPy-based solar evaporator has an energy conversion efficiency of 85%.⁶²
PANI	Little loss of light capture energy, mainly converted into heat, high energy efficiency	The main chain of the molecule contains a large number of conjugate P electrons, when exposed to bright light, the electrons in the valence band of PANI will be excited to the conduction band. Additional electron–hole pairs appear, while exciting electrons or holes at impurity levels in the band	PANI nanofiber layered photothermal film is used as an efficient solar membrane for seawater distillation and desalination, which can absorb up to 95% of sunlight.⁶³
PANI			The new low eutectic gallium indium (EGaIn)/PANI composite (EP) and cellulose nanocrystalline (CNCs) are used as absorbents for wide-band light absorption. Under the condition of 1 kW/m² sunlight irradiation for 1 h, the surface temperature of the hydrogel can reach 38.7°C.⁶⁴
PDA	Enables light to penetrate deeper into biological tissues, scattering is relatively low, and effectively absorb near-infrared light energy into heat energy	It has broad-band light absorption properties similar to natural melanin, which is completely consistent with the spectral characteristics of sunlight	The absorption rate of the light absorption layer prepared by Zhao et al.⁶⁵ is about 96% in the wavelength range of 250–1500 nm. When the sunlight intensity is 1 and 4 kW/m², the photothermal conversion efficiency of the interface evaporator using the PDA@ two-dimensional material as the light absorption layer can reach 85.2% and 93.6%, respectively.
PDA			PDA–polyurethane (PDA–WPU) matrix was used as a carrier to adsorb the PCM PEG. PDA has remarkable absorption effect on solar irradiation, and the photothermal conversion efficiency is 72.9%.⁶⁶

Polymer PHCMs are easy to synthesize and process, and the light absorption capacity can be adjusted in a wide range. Most of them are easily biodegradable, making this kind of PHCM more ecofriendly and sustainable.⁶⁷ However, polymer PHCMs are prone to photodegradation after a long time of sunlight irradiation, which limits their application. Recently, various chiral aromatic compounds, including chiral π-conjugated liquid crystals, have been developed for their unique photofunctions. By changing the molecular structure, the optical properties can be adjusted and result in higher stability.^68,69 This provides a new idea for the development of highly stable photothermal polymer materials through the utilization of π-conjugated aromatic compounds.

Metallic nanomaterials

Metallic nanomaterials are a type of metastable intermediate state substance that lies between solids and molecules. Their unique structure gives rise to small size effects, surface effects, quantum effects and interface effects. Metallic nanomaterials exhibit LSPR effect, which absorbs infrared light at a broad wavelength and converts it into heat energy.⁷⁰ At present, metal materials such as gold, silver, aluminum, copper and palladium have been extensively developed, modified and researched. The most commonly used metallic nanomaterials are gold (followed by silver) and they are utilized in photothermal applications in the form of nanoparticles (NPs) or composites.⁷¹

Gold NPs (AuNPs) are an excellent PHCM that can be tuned to absorb light from the visible to the near-infrared. The porous structure of AuNPs obviously enhances near-infrared light absorption performance. In addition, as the diameter of AuNPs in the range of 3–40 nm increases, the photothermal conversion efficiency also increases. For AuNPs dispersed in water under 1 kW/m² simulated light, the photothermal conversion efficiency is only 24%.⁷² In comparison, silver NPs (AgNPs) have more efficient and broad spectrum light absorption capacity based on stronger surface plasmon resonance effect, and can generate 10 times more heat than AuNPs, and the cost is also lower. However, AgNPs are prone to oxidization and have poor chemical stability and tolerance. With the increase of AgNP concentration, the photothermal conversion efficiency increases gradually.

Bae et al.⁷³ modified a gold film using a 40 nm flexible black gold film and 3 µm funnel-shaped metal nanowire to enhance light absorption and reduce reflectivity. The average reflectivity was about 7% of the incident light source, effectively improving the photothermal conversion efficiency. Yu et al.⁷⁴ found that the promotion effect of gold nanochain on light conversion is a factor of 3.5 that of conventional separated AuNPs. In addition, the size or structure could be easily regulated when preparing various composite materials, while also exhibiting excellent biocompatibility. Thus, it has aroused a wide research interest in photothermal applications. However, the preparation process of gold and silver as metal nanomaterials is complicated and the cost is high. Therefore, metal materials such as aluminum, copper, and palladium have been widely developed. Lin et al.⁷⁵ prepared copper NPs (CuNPs) through a simple substitution reaction. The light absorption rate of the CuNPs is about 97.7% in a wide incidence angle and wavelength range (200–1300 nm). The photothermal conversion efficiency of the solar interface evaporator prepared by the CuNPs can reach 93%. Moreover, CuNPs were loaded on the cellulose film, and the photothermal conversion efficiency of the interface evaporation system can reach 73% under the condition of simulated sunlight intensity of 2 kW/m². The CuNPs are recoverable and exhibit high stability of resistance to high temperature, radiation, and salt water corrosion.

Metallic NPs possess excellent light absorption performance, stability, and biocompatability, and are also one of the commonly used PHCMs, especially suitable for photothermal treatment in medicine. However, metallic NPs have high cost, poor stability in complex environments, and limited application scenarios. It is worth considering the synergistic use of these NPs with semiconductor materials to form metal–semiconductor heterojunctions to increase conversion efficiency.

Semiconductor materials

Semiconductor materials refer to materials with conductive properties between insulators and conductors at room temperature, which have a band gap between the valence band and conduction band, rendering them opaque under visible light and transparent under infrared light.⁷⁶ Semiconductor materials achieve photothermal conversion by absorbing incident light with higher energy and stimulating electrons to release heat. A semiconductor with smaller band gap has a wider absorption spectrum and higher photon capture efficiency, resulting in better photothermal conversion ability. Commonly used semiconductor PHCMs include copper sulfide (CuS) and titanium dioxide (TiO₂).

CuS is a type of plasma material as well as a semiconductor material with morphological structure such as microspheres, nanorods, nanochains, nanodendritic crystals, nanowires, and nanodisks,⁷⁷ all of which show a strong full-spectrum absorption capacity, especially in near-infrared light. The increased free carrier interacts with the electromagnetic field, demonstrating a strong near-infrared light absorption capacity.

Wu et al.⁷⁸ used commercial absorbable cotton, CuS nanocages with core–shell structure and agarose to fabricate a stable flexible aerogel. Under the light intensity of 1 kW/m², the photothermal conversion efficiency of the aerogel material as a solar-driven interface evaporator was 94.9%, and this corresponds to a water evaporation rate was 1.63 kg/(m² h). Qiao et al.⁷⁹ developed a drug delivery system (DFO@CuS NP) based on a medium CuS NP loaded with deferoxamine (DFO), aiming to achieve a synergistic treatment for vascular regeneration and photothermal antimicrobial effect. Under the same near-infrared irradiation (980 nm, 2.0 W cm⁻², 5 min), the colonies on the medium gradually decreased with the increasing concentration of DFO@CuS NPs. When the concentration reached 200 µg ml⁻¹, almost no colonies were formed on the plate. The number of colonies in the nonlight group hardly changed, which indicated that the NP itself did not have antibacterial effect, and only had good antibacterial performance under near-infrared light irradiation.

TiO₂, as one of the photoactive metal oxides, has been widely utilized in the fields of environmental pollution treatment and photothermal conversion, which primary mechanism for photothermal conversion through the recombination process of electron–hole pairs excited by sunlight.⁸⁰ However, the light absorption of TiO₂ is limited due to the existence of wide band gap. Therefore, enhancing the light absorption in the visible region by reducing the band gap of TiO₂ is crucial for its application in solar energy conversion.

TiO₂ has strong absorption of ultraviolet radiation at wavelength less than 400 nm. Huang et al.⁸¹ obtained a nontoxic and chemically stable hydrophobic black TiO₂ film, which can absorb part of light at wavelength greater than 400 nm and has increased the absorption of visible and infrared light significantly. Its photothermal conversion efficiency reaches 70.9% under simulated sunlight with 1 kW/m² light intensity. The enhanced visible light absorption of the black TiO₂ film is partly attributed to the reduction of Ti⁴⁺ to Ti³⁺ by the introduction of oxygen vacancy, which results in significantly higher light absorption performance compared with conventional black TiO₂ (H-TiO₂) synthesized by hydrogen reduction. Meanwhile, the light capture effect and nanolattices structure of the black TiO₂ film contribute to improve the photothermal conversion efficiency.

Recently, two or more semiconductor materials can be synthesized into composite materials for photothermal conversion. Studies have shown that a composite material, CuS/TiO₂, has been obtained based on UV-responsive TiO₂ compounds and narrow band gap semiconductor material CuS for CO₂ conversion under full-spectrum irradiation. The results demonstrate that 2%CuS/TiO₂ composite exhibits higher photocatalytic CO₂ reduction efficiency due to solar-induced photothermal synergistic effect.⁸² CuS can absorb infrared light and convert it into heat energy, thus expanding the utilization range of sunlight for CO₂ conversion.

Semiconductor PHCMs are characterized by simple fabrication process, low cost, excellent stability, and adjustable spectral absorption range.⁸³ Furthermore, these materials can incorporate additional functions to achieve multifunctional composite materials, but some of them are difficult to prepare on a large scale.

Other materials

MXene, black phosphorus (BP), and metal–organic skeleton (MOF) are also commonly used PHCMs in recent years.⁸⁴ Figure 4 is a schematic diagram of photothermal conversion of these PHCMs.

MXene materials are a new class of two-dimensional nanomaterials obtained by etching away the A element from layered ceramic materials known as MAX phase. It exhibits excellent photothermal properties in the visible-light and near-infrared region due to the LSPR effect (Figure 4(a)), and has also been proved to have a remarkable photothermal conversion performance.⁸⁵

It has been indicated that a self-floating MXene film was fabricated by a simple vacuum filtration, which produced 84% water–light evaporation efficiency under sunlight with a reasonably selected insulation layer.⁸⁶ Zhang et al.⁸⁷ synthesized a full Ti₃C₂T _x MXene aerogel with a hydrophobic upper layer and a hydrophilic lower layer, achieving a total solar absorption rate of 96%. Zhao et al.⁸⁸ utilized melamine foam as the framework and successively immersed it in PVA solution and MXene colloidal solution to obtain a self- floating interfacial photothermal conversion material with a total solar energy absorption rate of up to 98%. These results all demonstrated that MXene has good light absorption ability.

BP nanosheets are also a novel two-dimensional nanomaterials, characterized by high photothermal conversion efficiency and thermal conductivity (Figure 4(b)), exhibiting strong absorption capacity of sunlight from the ultraviolet to near-infrared band.⁸⁹

BP nanosheets have become a promising nanophotothermal material due to their inherent advantages, including excellent modifiability, miscibility, and reliable biocompatibility, which make them different from other photothermal agents. Some studies have used low-molecular-weight hyaluronic acid (HA) to modify BP NPs to prepare HA-BP NPs with a particle size of about 50 nm, which improves the stability of BP. Compared with BP NPs (29.47%), HA-BP NPs have a higher photothermal conversion efficiency (46.05%), showing excellent photothermal stability.⁹⁰ Yan⁹¹ designed and synthesized two-dimensional selenium-doped BP and formed selenium BP nanosheets (Se@BPNSs) with higher photothermal conversion efficiency by means of liquid phase stripping. A 1.5 W/cm², 808 nm laser was used to irradiate the tumor for 5 min, and the temperature rose rapidly within 5 min, reaching a tumor treatment temperature above 41°C. By injecting a certain dose of Se@BPNSs@RM-1 into tumor under the irradiation of 808 nm laser, Se@BPNSs converted light energy into heat to kill tumor cells at a higher temperature.

MOF is a kind of crystalline material composed of metal nodes and organic ligands through coordination bonds. It present large surface area, high porosity, and excellent chemical stability. MOF could integrate highly photoactive materials into its crystal structure and efficiently dissipates the energy transferred by electron motion and, thus, increase the temperature (Figure 4(c)).^92,93

MOF-derived carbon nanostructured PHCMs have attracted great attention due to their excellent light absorption properties, high chemical stability, and considerable photothermal conversion efficiency, and have shown great potential in the application of solar steam power generation.

Su et al.⁹⁴ synthesized MOF-derived C/TiO₂ composites (carbonized UiO-66-NH₂ (Ti)) with photothermal and photocatalytic functions were designed for the production of fresh water from wastewater. The carbonized UiO-66-NH₂ (Ti) layer has the advantages of large porous structure, excellent sunlight absorption capacity and super hydrophobicity, and the water evaporation efficiency is as high as 94% under 1kW/m2 irradiation

Preparation of PCPCMs

Currently, there are two primary methods for preparing PCPCMs, as illustrated in Figure 5: one is adsorption or modification, and the other is a doping method.⁹⁵ The former mainly forms a photothermal conversion shell through adsorption or deposition of PHCMs, that is, multilayered PCPCMs. The latter method directly incorporates PHCMs in the shell or core material, that is, hybrid shell PCPCMs. Table 3 presents the preparation and classification of PCPCMs.

Table 3.

Preparation and classification of PCPCM

Methods	Characteristics	Materials	Reference
Multilayered PCPCM	Adsorption or modification	PDA	Wang et al.⁹⁸ deposited PDA on the surface of SiO₂-coated n-octadecane mPCM, and the photothermal conversion efficiency reached more than 90%.
			Wu et al.⁹⁹ also found that the photothermal conversion efficiency of n-octadecan@SiO₂ microcapsules deposited with PDA could be increased from 18.6% to 91.8% during the same irradiation time.
			Sun et al.¹⁰⁰ prepared a phase change microcapsule based on n-docosane as the core and CNTs /PDA/SiO₂ as the multilayer shell. The microcapsule achieved the photothermal conversion efficiency of up to 90.1% when the CNTs content was 2 wt%.
		GO	Maithya et al.¹⁰¹ added GO as a photon catcher to polyurea mPCM, and the photothermal conversion efficiency reached 76.03% when the core–shell ratio was 91.
Hybrid shell PCPCM	Doping method	Cu₂O	Zhao et al.¹⁰² prepared stearate-@melamine-formaldehyde/GO mPCM with dual functions of photothermal conversion and superhydrophobic. When the volume of GO water suspension is 5, 10, and 15 ml, the photothermal conversion efficiency of mPCM is increased by 19.85%, 44.01%, and 82.58%, respectively.
		Cu₂O	Xu et al.¹⁰³ used copper formate crystal (Cu(HCOO)₂·4H₂O) as precursor and glucose as reducing agent to prepare photothermal mPCM with a mixture of Cu and Cu₂O as shell and paraffin as core. The coating rate and phase change enthalpy values were 62.97% and 127.55 J/g, respectively.
		SiO₂	Zhang et al.¹⁰⁴ synthesized Ti₄O₇/SiO₂@paraffin mPCM by introducing Ti₄O₇ NPs into SiO₂@paraffin mPCM, and the photothermal conversion efficiency reached 85.36%.
		SiC	Wang et al.¹⁰⁵ used SiC-NP-assisted stabilization emulsion in situ polymerization to prepare SiC modified polymelamine–formaldehyde (PMF) shell mPCM, and the thermal conductivity increased by 60.34%.

PCPCMs use solar energy as the energy source and also possess heat storage and release characters. Compared with composite photothermal PCMs, it has stability advantages. Multishell PCPCMs have high photothermal conversion efficiency and energy storage performance, but there are some problems such as the complexity of modification processes, generally inadequate modification methods, and the contradictory between modification cost and effectiveness.^96,97 Hybrid shell PCPCMs add PHCMs into the shell, and the process is relatively simple. However, the compatibility of photothermal particles in the shell material is poor and the adhesion uniformity is not good. The preparation of PCPCMs by combining solar energy and an energy storage technology can enhance the utilization rate of renewable energy.

Applications

PCPCMs have a wide range of application in the textile industry, building energy conservation, thermal management of electronic equipment, clean energy, and other aspects.¹⁰⁶ The application of PCPCMs is shown in Figure 6.

Textile industry

Thermal management technology enables the human body to maintain a more comfortable temperature range while improving wearing comfort, which is essential for human health and the development of heat storage and thermoregulation textiles. PCPCMs can generate heat through the absorption of sunlight, whereas PCMs can store and release heat. The functional textiles with photothermal conversion and high-efficiency energy storage performance can be manufactured by integrating the PCPCMs with textile materials, which is used for human thermal management and has good application prospects.¹⁰⁷

Zhu and Jiri¹⁰⁸ have incorporated PCPCMs into T-shirts through printing, and the photothermal conversion reached 89.12% under sunlight and low-temperature environments, showing excellent temperature regulation capabilities. Xu¹⁰⁹ synthesized melamine resin/polyphenylene diamine microcapsules and attached these microcapsules onto pure cotton fabric in a printed manner. It was found that the temperature could rise to 38°C under irradiation at an ambient temperature of 18°C and illumination intensity of 7.0 × 10⁴ Lux. Using sodium silicate as the precursor of silicon dioxide, Zhang¹¹⁰ prepared a paraffin @SiO₂(Pn@SiO₂) microencapsulated PCM using a sol-gel method. Using the reducibility of PDA, silver-coated mPCMs were prepared. By wet spinning, silver-coated microcapsules and calcium alginate were combined to prepare a multifunctional photothermal phase change energy storage fabric. When the microcapsule content is 70wt%, its enthalpy ΔH_m is 64.3 J/g, with excellent energy storage performance. Moreover, photothermal conversion test and infrared thermal imaging monitoring results show that the composite fiber present the excellent photothermal conversion and heat transfer ability of and it can be applied in smart textiles.

Architecture

In the field of building energy conservation, PCPCMs can play the role of heat storage and temperature regulation, thereby improving the thermal inertia of buildings and reducing energy consumption.¹¹¹ According to the current research reports, microcapsules can be embedded into various building materials such as concrete mixture, wall panel, cement mortar, gypsum board, sandwich board, and floor slab to meet the energy requirements of buildings for cooling, heating, air conditioning, ventilation, domestic hot water, and lighting systems.

Sun et al.¹¹² used cellulose nanocrystals (CNCs) and GO costabilized Pickering emulsion drops as a template and prepared PCPCMs with melamine formaldehyde resin (MF) as a shell, and coated PDA on the emulsion drops. The obtained PCM@CNC/rGO/PDA/MF microcapsules have uniform size in the micrometer range, excellent leak-proof performance, high phase transformation enthalpy 175.4 J g⁻¹ and encapsulation efficiency of 84.2%. CM@CNC/rGO/PDA/MF microcapsule slurry (15wt%) can reach 73°C under 1 W cm⁻² light irradiation.

Thermal management of electronic equipment

With the rapid development of electronics, communication technology, and the electric vehicle industry, thermal management of electronic equipment has been paid increasing attention by researchers.¹¹³ Fiber supercapacitors attract extensive attention due to their light weight, fast charge–discharge, and excellent safety, which can be considered as potential energy storage devices to power electronic watches and the human motion monitoring system.¹¹⁴ Utilizing high energy storage PCMs in combination with PHCMs can absorb the heat released during the operation of electronic equipment which can prevent thermal runaway and affect the service life.

Xu et al.¹¹⁵ developed an in situ thermal management system for the preparation of flexible solid supercapacitor, and mPCMs were embed into a three-dimensional porous reduced GO/PANI (GP) framework on the surface of CNT films. The addition of mPCMs effectively suppressed the temperature fluctuations of supercapacitors and improved the photothermal conversion performance, so their operating stability at high temperature is improved. Tian¹¹⁶ chose n-docosane as the phase change core material, constructed a multilayer phase change microcapsule with TiO₂ as the inner shell material and electrically active PANI/zinc oxide NPs as the outer composite shell material. Developed PANI multilevel phase change microcapsule modified polyurethane flexible composite film for the field of photothermal conversion and infrared stealth, the results show that the latent heat enthalpy of the multilevel phase change microcapsule reaches 125 J/g, effectively reduce the surface temperature of the target object, delay the thermal response time, and realize infrared stealth and thermal camouflage.

Clean energy

Faced with the problem of energy crisis, PCPCMs store the extra heat generated during the day, and the stored heat is used at night. For the energy storage in high-temperature weather, namely seasonal energy storage, PCPCMs not only solve the problem of continuity of energy supply, but also solve the problem of seasonal price fluctuations of solar energy, reducing the cost of solar energy utilization.^117,118

Tian¹¹⁹ designed an efficient solar-driven interface evaporator, which consists of pearl cotton (EPE), cotton core, and MXene-Me mPCM. Among them, MXene-ME mPCM is composed of n-teixosane and n-eicosane as mPCM core encapsulated in magnetic Fe₃O₄/SiO₂ composite shell, followed by deposited conductive PPy layer and decorated MXene nanosheet. Mxene-Me mPCM can be quickly separated from water through magnetic force to achieve salt-resistant effect, and maintain good evaporation efficiency after washing. It has evaporation rates of 2.04 and 4.11 kg m⁻² h⁻¹ at 1 and 2 kW m⁻² light intensification. Chen et al.¹²⁰ developed a new type of interfacial evaporator, equipped with CB/chitosan (CS) modified phase change microcapsule as a multifunctional photothermal material, the n-docosane PCM core is coated with TiO₂, and then the surface is decorated with a CB/CS nanocomposite layer. Due to the tight sealing of n-docosane as the mPCM core in a perfect core–shell structure microcapsule, which achieves a high thermal energy storage capacity of more than 140 J g⁻¹, the microcapsule synthesized by CS and CB NPs has a solar energy absorption efficiency of 95.04% and good wettability, as well as excellent antibacterial and salt resistance.

Others

The application of PCPCMs in medical treatment is significant, as the combination of chemotherapy and photothermal therapy makes their application as drug carriers in diagnosis and treatment valuable.

Using silica as a template, Li et al.¹²¹ prepared hollow mesoporous PDA (HMPDA) microcapsules modified with AuNPs as NP carriers for drug delivery. HMPDA@AuNPs microcapsule had high photothermal conversion efficiency. The average capacity of the carrier within 24 h was 334.78 mg/g, the release rate was about 44, and the DOX capacity reached 8% within 8 hours under photothermal conditions, demonstrating excellent drug carrying capacity, pH targeted release performance, good photothermal performance and biocompatibility.

Zhao et al.¹²² employed a two-step method to synthesize a novel double-shell phase change microcapsule. The double shell was constructed using polyurethane (PU) and polydopamine/silver (PDA/Ag), while octadecane and paraffin (P/O) served as the core materials. The synthesized microcapsules demonstrated a remarkable latent heat storage capacity surpassing 150 J/g. The coating exhibited outstanding photothermal conversion performance, attributed to the synergistic photothermal conversion effect of PDA and Ag in the first shell. Furthermore, the antimicrobial of the treated textiles against Streptococcus aureus and Escherichia coli were 99.99% and 99.56%, respectively.

Conclusion

MPCMs can only passively absorb and store energy through temperature difference, and their functionality is limited, resulting in ineffective utilization of other energy sources. Recently, the combination of PHCMs and PCMs has gradually gained attention. The introduction of PHCMs can directly and efficiently absorb solar energy into heat energy, which can greatly improve its energy storage efficiency under sunlight irradiation. Based on this, many researchers have developed PCPCMs using PHCMs to enhance their heat storage performance through solar energy. In this paper, the types and mechanisms of the main PHCMs are summarized and compared. At the same time, the preparation method and application of PCPCMs have been introduced, including the application in textile industry, building energy saving, thermal management of electronic equipment, clean energy, and so on. However, PCPCMs still have some problems, such as a complex preparation process, difficult material selection, insufficient stability and durability, poor universality of methods, and contradiction between cost and effect, which restricts the further application and development of PCPCMs.

In the future, the research focus of PCPCMs should be on the following aspects. We should simplify the PCPCM preparation process and techniques to lay the foundation for large-scale industrial applications. Utilizing artificial intelligence methodologies, such as machine learning and numerical simulations, we could forecast the performance of microcapsules and enhance the efficiency of the experimental process.¹²³ We should investigate the potential of using a variety of PHCMs to achieve full spectrum utilization, and explore the feasibility of combining PCPCMs with more functional materials to expand their functional scope. Attention should be paid to improve the efficiency and stability of energy storage and photothermal conversion performance, and broaden its applications. Expanding the application of PCPCM in clean energy such as solar and wind energy, while the biocompatibility, environmental protection, and safety of materials, should be considered to develop low-carbon, green, and energy-saving PCPCMs. In short, PCPCMs have great application prospects, not only can realize the efficient conversion and storage of solar energy, but also expand the application range of mPCMs.

Footnotes

Data availability statement

No new data were created or analyzed in this study.

Declaration of conflicting interests

The author(s) disclosed the conflicts of interest with respect to the research, authorship, and/or publication of this article: There are no conflicts of interest in this study. All authors have read and approved this version of the article, and due care has been taken to ensure the integrity of the work. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financially supported by the Natural Science Basic Research Program of Shaanxi (grant number 2023-JC-QN-0423), Science and Technology Plan Project of Beilin District (grant number GX2303), Opening Project of China National Textile and Apparel Council Key Laboratory for Silk Functional Materials and Technology (grant number SDHY2235), Opening Project of National Engineering Laboratory for Modern Silk (grant number SDGC2243), Opening Project of Hubei Key Laboratory of Biomass Fibers and Eco-Dyeing and Finishing (grant number STRZ202321), Qin Chuangyuan Scientist+Engineer Project in Shaanxi Province (grant number 2023KXJ-005), and Doctoral Scientific Research Starting Foundation of Xi’an Polytechnic University (grant numbers BS201962 and BS201906).

ORCID iDs

Yanli Sun

Hailiang Wu

Tao Liu

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