Photoelectromagnetic multimode triggered phase change materials for thermotherapy

Abstract

Neither pristine phase change materials (PCMs) nor metal‐organic frameworks (MOFs) can be driven by optical/electrical/magnetic triggers for multiple energy conversion and thermal storage, which cannot satisfy the requirements of multi‐scenario applications. Herein, a three‐dimensional interconnected forest‐type array carbon network anchored by Co nanoparticles serving as optical/electrical/magnetic multimode triggers was developed through in situ growth of two‐dimensional MOF nanosheet arrays on pre‐carbonized melamine foam and subsequent high‐temperature carbonization. After the encapsulation of polyethylene glycol, the resulting composite PCMs simultaneously integrate fascinating photothermal, electrothermal, magnetothermal conversion and storage for personal thermotherapy. Benefiting from the synergistic enhancement of forest‐type array carbon heterostructure and Co nanoparticles, composite PCMs exhibit high thermal/electrical conduction and strong full‐spectrum absorption capacities. Resultantly, low‐energy photoelectric triggers are sufficient to drive high‐efficiency photothermal/electrothermal conversion and storage of composite PCMs (93.1%, 100 mW/cm²; 92.9%, 2.5 V). Additionally, composite PCMs also exhibit excellent encapsulation stability without liquid phase leakage, long‐term thermal reliability and multiple energy conversion and storage stability after multiple cycles. The proposed photoelectromagnetic multimode triggers are aimed to inspire innovation and accelerate major breakthroughs in advanced responsive composite PCMs toward multiple energy utilization and personal thermotherapy.

INTRODUCTION

Among various energy storage technologies, heat storage technology has attracted extensive attention, because it cannot only match heat energy supply and demand in time or space, but also be integrated into energy systems including renewable energy sources such as solar, wind, geothermal, and hydropower.^1,2 Due to high density of storage energy per unit mass, small temperature difference between heat storage and heat release, and smaller system size, phase change materials (PCMs) are one of the most suitable materials for the efficient utilization of renewable heat energy.^3,4 Compared with inorganic PCMs, organic PCMs are widely utilized because of their low cost, ecological harmlessness, high heat storage density, good thermal reliability, and easy-to-adjust phase change temperature. However, the disadvantages of liquid PCM fluidity, such as inherently low thermal/electrical conductivity, weak light absorption, and non-magnetism of organic PCMs both weaken their thermal management ability and narrow their multifunctional applications in different scenarios.^4–10

Versatile functional fillers (thermally conductive fillers, electrically conductive fillers, photothermal fillers, magnetic fillers, etc.) have been introduced into organic PCMs to optimize their thermophysical properties.^3,10–19 However, the modification of organic PCMs often raises other problems. Due to the structural instability and poor dispersion of some functional fillers, the permeable conductive network formed inside PCMs will collapse during phase transition, reducing the thermal cycling stability of composite PCMs. Sometimes, a large amount of thermally/electrically conductive fillers are usually added inside PCMs to achieve high thermal/electrical conductivity, which will lead to a reduction in latent heat storage density. Some functional fillers are poorly dispersed and easy to agglomerate, which increase the interfacial transport resistance and reduce the thermophysical properties of composite PCMs. The pore size of some functional fillers is usually too large to provide sufficient surface tension and capillary pore force to effectively prevent the liquid leakage of organic PCMs, resulting in a low loading content of PCMs. In addition, some functional fillers with complex preparation processes and high raw material costs, such as Ag nanoparticles/nanowires,^20–23 MXenes,^24–28 graphene,^29–32 carbon nanotubes (CNTs),^33–36 and so forth, are not conducive to their large-scale applications in thermal energy conversion and storage. Therefore, it is valuable to explore functional fillers with stable structure, good dispersion, abundant pores, and simple and economical preparation process to improve the thermophysical properties of organic PCMs.

Facing these challenges herein, two-dimensional (2D) Co-MOF nanosheet arrays were first grown in situ on the surface of pre-carbonized melamine foam, and then obtained a three-dimensional (3D) interconnected forest-type array carbon network anchored by Co nanoparticles (CF@Co/CNT) through high-temperature carbonization, which serve as optical/electrical/magnetic multimode triggers of PCMs for personal thermotherapy. Commercial personal thermotherapy temperature can be divided into high grade (50°C–55°C), mid grade (40°C–50°C), and low grade (35°C–40°C). Considering that common thermotherapy modes generally do not directly touch the skin, especially solar-driven thermotherapy and magnetically triggered thermotherapy, PCMs with a higher phase change temperature are chosen to ensure that the thermotherapy temperature reaching the human skin is appropriate for mid-grade thermotherapy. The resulting PEG/CF@Co/CNT composite PCMs exhibit high heat storage density, long-term thermal cycling reliability and shape stability without liquid phase leakage. The constructed 3D forest-type heterogeneous carbon conduction network with high graphitization and few defects effectively absorbs the full spectrum, reduces the interface resistance, and accelerates the transport of electrons and phonons. Resultantly, low-energy photoelectric triggers are sufficient to drive high-efficiency photothermal/electrothermal conversion and storage of composite PCMs. Additionally, Co nanoparticles impart PEG/CF@Co/CNT composite PCMs magnetothermal conversion and storage function. The designed photoelectromagnetic multimode triggers are aimed to inspire innovation in advanced responsive composite PCMs toward multiple energy utilization and personal thermotherapy.

RESULTS AND DISCUSSION

Structural and chemical analysis

The synthesis of photoelectromagnetic multimode triggered composite PCMs is schematically shown in Figure 1, including pre-carbonization of melamine foam, in situ self-assembly of Co-MOF, high-temperature carbonation of CF@Co-MOF, and impregnation of PEG. In detail, the carbon foam (CF) derived from melamine foam after high-temperature carbonization is composed of large-scale pores and numerous branches (Figure 2A), which provides numerous binding sites for the growth and anchoring of 2D MOF nanosheets. In this study, 2D Co-MOF nanosheet arrays were uniformly grown and anchored on CF framework using seed growth technology (Figure 2B,C). XRD patterns indicate that the crystal diffraction peaks of CF@Co-MOF are in good agreement with those of Co-MOF (Figure S1a), indicating that 2D Co-MOF nanosheet arrays were successfully anchored to the CF substrate.

[IMAGE OMITTED. SEE PDF]

[IMAGE OMITTED. SEE PDF]

After high-temperature pyrolysis of CF@Co-MOF, 2D Co-MOF nanosheet arrays grown in situ on CF framework have been completely converted into a uniform network structure composed of numerous interlaced carbon nanotube (CNT) forests and Co nanoparticles (Figure 2D–F). This is due to the fact that during the pyrolysis process, Co²⁺ in Co-MOF will be released and reduced to Co nanoparticles. At high temperature, Co nanoparticles act as catalysts to induce the formation of dense CNT forests, while dicyandiamide is decomposed into numerous C- and N-containing gases serving as carbon and nitrogen sources for in situ growth of CNT forests. Compared with CF@Co/CNT-700 and CF@Co/CNT-800, CF@Co/CNT-900 exhibits a longer length and larger diameter CNT structure (Figure 2G–I), because the higher the temperature, the faster the diffusion rate of carbon atoms on the surface of Co nanoparticles. Co nanoparticles with an average diameter of 35 nm are in situ encapsulated in CNTs (Figure 2J–L). The constructed 3D CNT forest cross-linking network is a fast photon/electron/phonon transport channel with low interface resistance. Additionally, the pores constructed in CNT forests are also conducive to the efficient adsorption and stabilization of PCMs, preventing liquid phase leakage during phase change process.

XRD patterns exhibit the phase compositions and crystalline properties of CF@Co/CNT (Figure 3A). The sharp diffraction peaks at 4.3°, 51.5°, and 75.9° can be attributed to the (111), (200), and (220) diffraction surfaces of Co nanoparticles (JCPDS No.15-0806), respectively. The diffraction peak at 26.4° corresponds to the (002) diffraction surface of graphitic carbon, and its diffraction intensity increases gradually with the increase of pyrolysis temperature. In addition, the half-height width of Co nanoparticles in CF@Co/CNT-700 and CF@Co/CNT-800 is smaller than that in CF@Co/CNT-900, indicating that the grain sizes of Co nanoparticles are larger in CF@Co/CNT-900. Moreover, the types of functional groups are similar in different CF@Co/CNT samples (Figure 3B).

[IMAGE OMITTED. SEE PDF]

Raman spectra were detected to explore the carbon structure of CF@Co/CNT (Figure 3C). The peaks at 1356.8 cm⁻¹ and 1592.1 cm⁻¹ can be attributed to the D band and G band, respectively. D-band corresponds to the vibrational mode of carbon atoms in the sp³ hybrid orbital, indicating a disordered and defective structure in carbon materials. G-band corresponds to the vibrational mode of carbon atoms in the sp² hybrid orbital, indicating the degree of order of graphite.³⁷ The strength ratio of D band to G band (I_D/I_G) is usually used to qualitatively assess the graphitization degree of carbon materials. As the pyrolysis temperature increases from 700°C to 900°C, I_D/I_G value decreases from 1.06 to 0.94 (Table S1), indicating that the graphitization degree of CF@Co/CNT is improved. Because Co nanoparticles act as catalysts to promote the formation of highly graphitized CNT forests at high temperature pyrolysis, higher graphitization can accelerate thermal and electrical conduction of carbon materials, thus improving energy conversion and storage capacity of carbon-based composite PCMs.

To further understand the surface compositions and chemical states of CF@Co/CNT, XPS spectra were performed. CF@Co/CNT is mainly composed of C, N, O, and Co elements (Figure S2). C1s XPS spectra can be divided into sp² CC, CN, and CO peaks (Figure 3D). When the pyrolysis temperature increases from 700°C to 900°C, the content of sp² CC in CF@Co/CNT increases gradually, which is consistent with the trend of I_D/I_G values. In general, a high content of sp² CC is beneficial for reducing phonon scattering and prolonging the free path of phonons. N1s XPS spectra can be divided into pyridine-N, pyrrole-N, graphite-N, and oxidation-N (Figure 3E). As the pyrolysis temperature rises from 700°C to 900°C, the content of graphite-N gradually increases, while the content of pyrrole-N decreases (Figure S2). Because unstable pyrrole-N is converted to more stable graphite-N at higher pyrolysis temperature, a high content of graphite-N is also beneficial for improving the thermal conductivity of CF@Co/CNT. In addition, with the increase of pyrolysis temperature, continuous nitrogen cleavage occurs with total N content decreasing from 4.02 at% (700°C) to 3.41 at% (900°C).

Abundant pores are very important for heat storage capacity of PCMs. Nitrogen adsorption–desorption isothermal test was used to explore the pore structure of CF@Co/CNT. The isothermal curves of nitrogen adsorption–desorption of CF@Co/CNT present a typical IV shape with an obvious H3 hysteresis loop (Figure 3F), indicating the existence of mesopores. The nitrogen adsorption capacity of P/P₀ < 0.1 increases rapidly, indicating the existence of micropores. It can also be seen from SEM that CF@Co/CNT is composed of microporous–mesoporous–macroporous hierarchical structure, which can provide strong capillary force to prevent the leakage of liquid PEG and numerous heat transfer channels for triggering the phase transition of PEG molecules.

Thermophysical properties

After the physical infiltration of PEG into CF@Co/CNT, the voids of CF@Co/CNT framework are completely filled with PEG through strong capillary forces and surface tension (Figure S4), forming a special structure similar to reinforced concrete. This special structure effectively resists the internal and external stresses of the system with excellent mechanical properties to improve the stability of composite PCMs. Liquid leakage restricts the practical engineering utilization of PCMs, which will seriously affect the safety of utilization. As shown in Figure S6, cylindrical PEG is completely melted and spread on the filter paper after 20 min of heating, while PEG/CF@Co/CNT still maintains the original cylindrical size and shape without obvious liquid leakage trace on the filter paper, indicating excellent shape stability above the phase change temperature. Furthermore, CF@Co/CNT also constructs a 3D interpenetrated heat transfer network inside PEG to facilitate the storage and release rate of thermal energy.

To explore the interaction between PEG and CF@Co/CNT, the chemical structure was detected using FT-IR (Figure S5a–c). PEG exhibits main absorption peaks of CH₂ at 841 cm⁻¹, CH at 963 and 1466 cm⁻¹, COC at 1112 cm⁻¹, CH₂ at 2891 cm⁻¹, and OH at 3446 cm⁻¹. All the functional groups of PEG and CF@Co/CNT (e.g., COH, CO, and CN) can be observed in PEG/CF@Co/CNT, and no new characteristic peaks appear. XRD patterns were further performed to explore the crystallization behavior of PEG in the hierarchical pores of CF@Co/CNT (Figure S5d–f). PEG exhibits two typical crystalline phases at 19° and 23°, meanwhile all the sharp diffraction peaks of PEG can be distinctly observed in the XRD patterns of PEG/CF@Co/CNT, only accompanied by a decrease in the diffraction intensity. The reduced diffraction intensity is because CF@Co/CNT accounts for a certain proportion, rather than pristine PEG. Interestingly, PEG molecules still retain good crystallinity in the hierarchical pores of CF@Co/CNT without being interfered (Figure 4E). These results indicate that only physical interactions exist between PEG and CF@Co/CNT without chemical reactions, and the two are very compatible.

[IMAGE OMITTED. SEE PDF]

Latent heat and phase change temperature are the most reliable indicators to evaluate the heat storage performance of composite PCMs. Both PEG and PEG/CF@Co/CNT show clear endothermic and exothermic peaks, corresponding to the melting and crystallization of PEG molecular chains. Compared with pristine PEG, the melting enthalpy of PEG/CF@Co/CNT decreases and the melting temperature increases slightly. Because the hierarchical pores of CF@Co/CNT have a certain hindrance effect on the migration of PEG polymer chains, and thus more energy may be required to melt PEG molecules. According to DSC data (Figure 3B–D, Table S2), PEG/CF@Co/CNT composite PCMs have a good heat storage density (120–135 J/g). Thermal stability is also an important criterion to evaluate the reliability of composite PCMs in practical applications. All PEG/CF@Co/CNT samples exhibit two weight losses during heating (Figure 4A). The first stage is a slight weight loss of about 1% at around 100°C, meaning that the absorbed water molecules on the samples are removed. The rapid weight loss in the second stage is attributed to pyrolysis of PEG. PEG/CF@Co/CNT samples do not lose weight below 330°C, indicating that they have good thermal stability below 330°C. In addition, the weight loss of PEG/CF@Co/CNT samples is approximately 80%, corresponding to an 80% loading rate, which is consistent with the theoretical loading rate of PEG.

Excellent thermal conductivity reduces the time required for thermal energy storage and release. Compared with the low thermal conductivity of PEG (0.31 W/m·K), the thermal conductivity of PEG/CF@Co/CNT-700 (0.82 W/m·K), PEG/CF@Co/CNT-800 (0.95 W/m·K), and PEG/CF@Co/CNT-900 (1.02 W/m·K) is increased by 165%, 206%, and 229%, respectively (Figure 4F, Table S3). The results show that the heat transfer capacity of PEG has been greatly improved after the introduction of CF@Co/CNT due to the synergistic effect of CF/CNT heterostructure and Co nanoparticles. On the one hand, CF/CNT constructs a dense 3D interconnected carbon forest-type heat transfer network. On the other hand, Co nanoparticles can form nanoscale hot spots that increase local temperature and reduce interfacial thermal resistance at heterogeneous nodes. Note that the thermal conductivity of PEG/CF@Co/CNT increases sequentially with the increase of pyrolysis temperature. Because forest-type carbon materials with a higher graphitization degree will form more ordered lattice structures, promote phonon–phonon vibration, increase the speed and average free path of phonon transport, thus significantly boosting the thermal conduction (Figure 4H).¹

Photothermal storage and mechanism

The photothermal utilization system mainly includes solar energy absorption, photothermal conversion, and thermal energy storage. Pristine PEG absorbs only a small fraction of the solar spectrum and has little solar absorption capacity in the visible range (380–760 nm). Whereas, PEG/CF@Co/CNT shows enhanced solar absorption over the entire spectral range (200–2500 nm) due to the introduction of CF@Co/CNT with multiple reflection structure (Figure 5C). Therefore, PEG/CF@Co/CNT composite PCMs exhibit great potential for solar-driven energy conversion and storage.

[IMAGE OMITTED. SEE PDF]

Figure 5D shows the temperature–time evolution curves of PEG/CF@Co/CNT at a simulated sunlight intensity of 100 mW/cm². The temperature–time evolution curves have gone through three stages after turning on light switch. In the first stage, the temperature of the samples rises rapidly due to the multicomponent synergistic effect of CF/CNT with conjugation effect and Co nanoparticles with localized surface plasmon effect, which converts light energy into heat energy and store it in sensible heat.³⁸ In the second stage, the heating rate of the samples slows down and the curve becomes flattened, resulting in a distinct temperature plateau. This is because PEG molecules encapsulated in CF@Co/CNT undergo a crystalline–amorphous phase transition, storing the captured light energy in PEG as latent heat. In the third stage, the heating rate of the samples accelerates again, and the light energy is converted into heat energy and stored again as sensible heat. After turning off light switch, the temperature of PEG/CF@Co/CNT drops rapidly, and another temperature plateau is subsequently observed, corresponding to amorphous–crystalline phase transition of PEG molecules.

The calculated photothermal conversion and storage efficiency of PEG/CF@Co/CNT-700, PEG/CF@Co/CNT-800, and PEG/CF@Co/CNT-900 is 84.7%, 89.4%, and 93.1% at 100 mW/cm², respectively (Figure 5E). PEG/CF@Co/CNT-900 has an optimized efficiency thanks to the constructed 3D interconnected accelerated carbon forest-type photon and phonon transfer network with higher graphitization and fewer defects, and converts more light energy into heat energy at the same light radiation. The photothermal conversion and storage efficiency of PEG/CF@Co/CNT is higher than that of most reported solar-driven composite PCMs, due to the strong broadband photon absorption capacity of CF@Co/CNT and the synergistic effect of two fast photon heaters (CF/CNT heterostructure and Co nanoparticles), as well as the local surface plasmon resonance (LSPR) effect of Co nanoparticles (Figure 5G).³⁷ For visualization, PEG/CF@Co/CNT-900 composite PCMs and thermally conductive gel were mixed to prepare phase change film for personal thermotherapy. Distinctly, when the finger wrapping the phase change film is exposed to light irradiation, the temperature can rise to more than 40°C, while the temperature in other locations does not rise significantly, indicating good solar-driven personal thermotherapy (Figure 5H).

Electrothermal storage and mechanism

The inherent low electrical conductivity (10⁻¹³ S/m) of pristine PEG severely limits its application in electrothermal energy conversion and storage. Generally, carbon materials are suitable for the design of electrically driven composite PCMs due to good charge transport property and environmental stability. The electrical conductivity of PEG/CF@Co/CNT-700, PEG/CF@Co/CNT-800, and PEG/CF@Co/CNT-900 is 215, 230, and 265 S/m, respectively, which is 15 orders of magnitude higher than that of pristine PEG (Figure 6C). In addition, the current through PEG/CF@Co/CNT almost does not change with time at a constant voltage of 2.5 V, indicating good current transmission stability with time (Figure S8). Therefore, PEG/CF@Co/CNT composite PCMs exhibit great potential for electrically driven energy conversion and storage.

[IMAGE OMITTED. SEE PDF]

At a constant voltage of 2.5 V, the temperature of PEG/CF@Co/CNT increases rapidly because the current flowing through the conductive composite PCMs generates joule heat and is stored as sensible heat. With the continuous operation of the voltage, the slope of the temperature–time evolution curve gradually decreases until the temperature plateau, during which PEG molecules undergo crystalline–amorphous phase transition, and the joule heat converted into electrical energy is stored as latent heat. After the phase transition is terminated, the temperature of PEG/CF@Co/CNT increases sharply again, and joule heat is stored as sensible heat again. When the voltage is stopped, the temperature of PEG/CF@Co/CNT drops immediately, indicating that sensible heat begins to release. After a period of time, a temperature plateau is observed again, during which PEG molecules undergo amorphous–crystalline phase transition, and the stored joule heat is released as latent heat. After the phase transition is completed, the temperature continues to drop and the final temperature is close to room temperature (Figure 6D).

The calculated electrothermal conversion and storage efficiency of PEG/CF@Co/CNT-700, PEG/CF@Co/CNT-800, and PEG/CF@Co/CNT-900 is 82.2%, 85.8%, and 92.9% at 2.5 V, respectively (Figure 6E). PEG/CF@Co/CNT-900 has an optimized efficiency thanks to the formed 3D interconnected electron and phonon conductive network with higher graphitization and fewer defects that effectively reduces the interface resistance, accelerates the transport of electrons and phonons, and convert more electrical energy into joule heat at the same input voltage (Figure 6G). Compared with the most reported electrically driven composite PCMs, PEG/CF@Co/CNT-900 achieves a high electrothermal conversion and storage efficiency (92.9%) at a relatively low input voltage (2.5 V) due to the constructed 3D interconnected accelerated carbon forest-type electron and phonon transfer network. For visualization of thermotherapy, the prepared phase change film was wrapped around the leg joint. When the electric switch is activated, the temperature at the phase change film quickly rises above 40°C, indicating good electrically driven personal thermotherapy (Figure 6H).

Magnetothermal storage and mechanism

Magnetic materials can capture magnetic energy in the alternating magnetic field and convert it into heat energy. The combination of magnetic materials and PCMs to prepare magnetothermal storage composite PCMs is beneficial to use the magnetocaloric effect to save energy and broaden the application range of PCMs. Pristine PEG cannot be adsorbed by a magnet, while PEG/CF@Co/CNT is easily adsorbed by a magnet due to the introduction of magnetic Co nanoparticles (Figure 7C). The hysteresis loop curve demonstrates the typical ferromagnetism of PEG/CF@Co/CNT (Figure 7C). PEG/CF@Co/CNT exhibits a low magnetic retentivity and coercivity. The saturation magnetization of PEG/CF@Co/CNT is 19.9 emu/g. Therefore, an alternating magnetic field can trigger magnetothermal energy conversion and storage of PEG/CF@Co/CNT. The principle can be attributed to the Néel and Brownian relaxation of Co nanoparticles (Figure 7G).^1,37 These Co nanoparticles are not only a magnetic response unit but a catalyst to induce the formation of dense CNT forest. The Co nanoparticle-catalyzed 3D interconnected carbon forest-type framework can accelerate the transport of phonons. Therefore, Co nanoparticles will convert magnetic energy into heat energy under an alternating magnetic field, and then quickly transfer it to PEG for latent heat storage through 3D interconnected carbon forest-type heat transfer network.

[IMAGE OMITTED. SEE PDF]

Figure 7D shows the temperature–time evolution curves of PEG/CF@Co/CNT composite PCMs under the same alternating magnetic field (AC current of 2.1 A). When an external magnetic field is applied, the temperature of all samples rises rapidly, and the slope of the curves gradually decreases due to the temperature reaching the melting point of PEG, during which time the heat energy converted by the magnetocaloric effect is gradually stored in PEG through solid–liquid phase transition. After the phase transition terminates in the region close to the minimum slope of the curve, the temperature then begins to rise rapidly again. The corresponding magnetothermal conversion rate of PEG/CF@Co/CNT-700, PEG/CF@Co/CNT-800, and PEG/CF@Co/CNT-900 are about 0.54, 0.57, and 0.61°C/s, respectively, because carbon materials with a higher graphitization degree are more sensitive to heat conduction. After the alternating magnetic field is stopped, the temperature of all samples drops. Then, a temperature plateau emerges again due to the liquid–solid phase transition of PEG, accompanied by the release of heat energy stored in melting. For visualization of thermotherapy, the prepared phase change film was wrapped around the neck. When the magnetic switch is activated, the temperature at the phase change film rapidly rises above 40°C, indicating good magnetically triggered personal thermotherapy (Figure 7H).

Long-term durable stability

Long-term durable stability is essential for reliable applications of composite PCMs. In the XRD and FTIR spectra (Figure S7), the shape and position of the characteristic peaks of PEG/CF@Co/CNT-900 are almost coincidental after several heating and cooling cycles, that is, there is no disappearance of old peaks or the generation of new peaks, indicating that the phase structure and chemical structure are stable. The DSC curves of PEG/CF@Co/CNT-900 remain almost unchanged from the first cycle to the last cycle, and the corresponding melting and freezing latent heat is reduced below 2.0%, indicating high thermal energy storage stability (Figure 4F). To further evaluate the stability of multi-energy conversion and storage, PEG/CF@Co/CNT-900 was placed in the light field, electric field, and magnetic field environment for cycle tests. The photothermal, electrothermal, and magnetothermal temperature–time evolution curves of PEG/CF@Co/CNT-900 after several cycles are highly consistent with the initial curves, indicating that the multi-energy conversion and storage performances are highly stable (Figures 5F, 6F, and 7F). These results consistently show that PEG/CF@Co/CNT composite PCMs exhibit excellent thermal reliability and stability, meeting the requirements of multiple reuses in practical applications.

CONCLUSION

This study developed photoelectromagnetic multimode triggered PCMs for personal thermotherapy. 3D interconnected carbon forest-type conduction network anchored by Co nanoparticles was obtained. The corresponding PEG/CF@Co/CNT composite PCMs exhibit a high latent heat storage density (131.87 J/g) and excellent encapsulation stability without liquid leakage. Due to the synergistic enhancement of forest-type CF/CNT carbon heterostructure and Co nanoparticles, PEG/CF@Co/CNT composite PCMs have significantly enhanced thermal conductivity (1.02 W/m·K) and electrical conductivity (230 S/m), as well as strong full-spectrum absorption capacity. Resultantly, the photothermal/electrothermal conversion and storage efficiencies of PEG/CF@Co/CNT composite PCMs are as high as 93.1% (100 mW/cm²) and 92.9% (2.5 V). Furthermore, Co nanoparticles endow PEG/CF@Co/CNT composite PCMs with magnetothermal conversion and storage capability. In addition, PEG/CF@Co/CNT composite PCMs exhibit long-term thermal reliability and stability after multiple cycles. These results consistently show that PEG/CF@Co/CNT composite PCMs have good potential for multiple energy conversion and storage driven by solar/electric/magnetic fields and personal thermotherapy. Our proposed multienergy response design strategy is universal, which can be extended to different kinds of Co-based, Ni-based, and CoNi-based MOFs.

EXPERIMENTAL SECTION

Synthesis of CF@Co-MOF

The synthesis method in this study refers to the literatures with some improvements.^39,40 Specifically, melamine foam (MF) was firstly carbonized at 600°C. Then, the obtained carbon foam (CF) was soaked in cobalt nitrate hexahydrate solution, subsequently 2-methylimidazole solution was added to the above solution, and allowed to stand. Finally, CF was washed three times with deionized water and anhydrous methanol, and dried in a vacuum oven at 60°C to obtain the sample, which was defined as CF@Co-MOF.

Synthesis of CF@Co/CNT

The CF@Co-MOF and dicyandiamide powder were placed in ceramic crucibles. They were pyrolyzed at 700°C, 800°C, and 900°C. Finally, the resulting samples were defined as CF@Co/CNT-X (X is the pyrolysis temperature).

Synthesis of PEG/CF@Co/CNT composite PCMs

The organic phase change material (PEG8000) was first completely dissolved in absolute ethanol at 80°C. Subsequently, CF@Co/CNT-X was immersed in PEG/ethanol solution and stirred thoroughly at 80°C. Finally, the obtained mixture was dried in a vacuum oven at 80°C. The resulting samples were defined as PEG/CF@Co/CNT-X (X is the pyrolysis temperature).

Characterizations

The detailed characterizations are provided in Supporting Information.

ACKNOWLEDGMENTS

This work was financially supported by National Natural Science Foundation of China (No. 51902025). The authors would like to thank Mettler Toledo for the thermal analyzers (DSC3 and TGA/DSC3).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

References

Wang G, Tang Z, Gao Y, et al. Phase change thermal storage materials for interdisciplinary applications. Chem Rev. 2023;123(11):6953‐7024.

Aftab W, Usman A, Shi J, Yuan K, Qin M, Zou R. Phase change material‐integrated latent heat storage systems for sustainable energy solutions. Energy Environ Sci. 2021;14(8):4268‐4291.

Chen X, Gao H, Tang Z, Dong W, Li A, Wang G. Optimization strategies of composite phase change materials for thermal energy storage, transfer, conversion and utilization. Energy Environ Sci. 2020;13(12):4498‐4535.

Aftab W, Huang X, Wu W, Liang Z, Mahmood A, Zou R. Nanoconfined phase change materials for thermal energy applications. Energy Environ Sci. 2018;11(6):1392‐1424.

Xiao C, Piao C, Zhaodi T, Xiaoliang X, Hongyi G, Ge W. Carbon‐based composite phase change materials for thermal energy storage, transfer, and conversion. Adv Sci. 2021;8(9): [eLocator: 2001274].

Wu S, Li T, Zhang Z‐Y, Li T, Wang R. Photoswitchable phase change materials for unconventional thermal energy storage and upgrade. Matter. 2021;4(11):3385‐3399.

Usman A, Xiong F, Aftab W, Qin M, Zou R. Emerging solid‐to‐solid phase‐change materials for thermal‐energy harvesting, storage, and utilization. Adv Mater. 2022;34(41): [eLocator: 2202457].

Xiao C, Han Y, Yan G, Lei W, Ge W. The marriage of two‐dimensional materials and phase change materials for energy storage, conversion and applications. EnergyChem. 2022;4: [eLocator: 100071].

Yuan K, Shi J, Aftab W, et al. Engineering the thermal conductivity of functional phase‐change materials for heat energy conversion, storage, and utilization. Adv Funct Mater. 2020;30(8): [eLocator: 1904228].

Cheng P, Chen X, Gao H, et al. Different dimensional nanoadditives for thermal conductivity enhancement of phase change materials: fundamentals and applications. Nano Energy. 2021;85: [eLocator: 105948].

Shi J, Qin M, Aftab W, Zou R. Flexible phase change materials for thermal energy storage. Energy Storage Mater. 2021;41:321‐342.

Chen X, Tang Z, Liu P, Gao H, Chang Y, Wang G. Smart utilization of multifunctional metal oxides in phase change materials. Matter. 2020;3(3):708‐741.

Chen X, Cheng P, Tang Z, Xu X, Gao H, Wang G. Carbon‐based composite phase change materials for thermal energy storage, transfer, and conversion. Adv Sci. 2021;8(9): [eLocator: 2001274].

Dimberu GA, Beom Yeol Y, Sungwoong Y, Hyeonseong Y, Seunghwan W, Sumin K. Structurally advanced hybrid support composite phase change materials: architectural synergy. Energy Storage Mater. 2021;42:164‐184.

Chen X, Tang Z, Gao H, Chen S, Wang G. Phase change materials for electro‐thermal conversion and storage: from fundamental understanding to engineering design. iScience. 2020;23(6): [eLocator: 101208].

Wang J, Liu X, Xu Q, Luo Q, Xuan Y. MXene reconciles concurrent enhancement of thermal conductivity and mechanical robustness of SiC‐based thermal energy storage composites. DeCarbon. 2023;1: [eLocator: 100005].

Tang Z, Gao H, Chen X, Zhang Y, Li A, Wang G. Advanced multifunctional composite phase change materials based on photo‐responsive materials. Nano Energy. 2021;80: [eLocator: 105454].

Hu Z, Zou Y, Xiang C, et al. Stabilized multifunctional phase change materials based on carbonized Cu‐coated melamine foam/reduced graphene oxide framework for multiple energy conversion and storage. Carbon Energy. 2022;4(6):1214‐1227.

Tang Z, Cheng P, Liu P, Gao Y, Chen X, Wang G. Tightened 1D/3D carbon heterostructure infiltrating phase change materials for solar–thermoelectric energy harvesting: faster and better. Carbon Energy. 2023;5(6): [eLocator: e281].

Liao G, Fang J, Li Q, Li S, Xu Z, Fang B. Ag‐Based nanocomposites: synthesis and applications in catalysis. Nanoscale. 2019;11(15):7062‐7096.

Yang J, Yu F, Chen A, et al. Synthesis and application of silver and copper nanowires in high transparent solar cells. Adv Powder Mater. 2022;1(4): [eLocator: 100045].

Lee S, Sim K, Moon SY, et al. Controlled assembly of plasmonic nanoparticles: from static to dynamic nanostructures. Adv Mater. 2021;33(46): [eLocator: 2007668].

Kalantari K, Mostafavi E, Afifi AM, et al. Wound dressings functionalized with silver nanoparticles: promises and pitfalls. Nanoscale. 2020;12(4):2268‐2291.

Miao X, Li Z, Liu S, Wang J, Yang S. MXenes in tribology: current status and perspectives. Adv Powder Mater. 2023;2(2): [eLocator: 100092].

Wang J, Du C‐F, Xue Y, et al. MXenes as a versatile platform for reactive surface modification and superior sodium‐ion storages. Exploration. 2021;1(2): [eLocator: 20210024].

Chen J, Ding Y, Yan D, Huang J, Peng S. Synthesis of MXene and its application for zinc‐ion storage. SusMat. 2022;2(3):293‐318.

Li L, Cheng Q. MXene based nanocomposite films. Exploration. 2022;2(4): [eLocator: 20220049].

Lu Q, Liu C, Zhao Y, et al. Freestanding MXene‐based macroforms for electrochemical energy storage applications. SusMat. 2023;3(4):471‐497.

Huang P, Li Y, Yang G, et al. Graphene film for thermal management: a review. Nano Mater Sci. 2021;3(1):1‐16.

Li M, Yin B, Gao C, et al. Graphene: preparation, tailoring, and modification. Exploration. 2023;3(1): [eLocator: 20210233].

Zhu Q, Ong PJ, Goh SHA, et al. Recent advances in graphene‐based phase change composites for thermal energy storage and management. Nano Mater Sci. 2023;6(2):115‐138.

Ali A, Liang F, Zhu J, Shen PK. The role of graphene in rechargeable lithium batteries: synthesis, functionalisation, and perspectives. Nano Mater Sci. 2022.

Xie Y, Wang X. Thermal conductivity of carbon‐based nanomaterials: deep understanding of the structural effects. Green Carbon. 2023;1(1):47‐57.

Yu T, Jing L, Jian Y, Lixing K, Qingwen L. Filled carbon‐nanotube heterostructures: from synthesis to application. Microstructures. 2023;3(3): [eLocator: 2023019].

De Volder MF, Tawfick SH, Baughman RH, Hart AJ. Carbon nanotubes: present and future commercial applications. Science. 2013;339(6119):535‐539.

Tasis D, Tagmatarchis N, Bianco A, Prato M. Chemistry of carbon nanotubes. Chem Rev. 2006;106(3):1105‐1136.

Chen X, Xu J, Li Y, Gao Y, Wang G. Integrating multiple energy storage in 1D–2D bridged array carbon‐based phase change materials. SusMat. 2023;3(4):510‐521.

Tang Z, Gao Y, Cheng P, et al. Metal‐organic framework derived magnetic phase change nanocage for fast‐charging solar‐thermal energy conversion. Nano Energy. 2022;99: [eLocator: 107383].

Kim D, Kim DW, Buyukcakir O, Kim M‐K, Polychronopoulou K, Coskun A. Highly hydrophobic ZIF‐8/carbon nitride foam with hierarchical porosity for oil capture and chemical fixation of CO₂. Adv Funct Mater. 2017;27(23): [eLocator: 1700706].

Fang G, Zhou J, Liang C, et al. MOFs nanosheets derived porous metal oxide‐coated three‐dimensional substrates for lithium‐ion battery applications. Nano Energy. 2016;26:57‐65.