Polypyrrole‐boosted photothermal energy storage in MOF‐based phase change materials

Infiltrating phase change materials (PCMs) into nanoporous metal–organic frameworks (MOFs) is accepted as a cutting‐edge thermal energy storage concept. However, weak photon capture capability of pristine MOF‐based composite PCMs is a stumbling block in solar energy utilization. Towards this goal, we prepared advanced high‐performance pristine MOF‐based photothermal composite PCMs by simultaneously integrating photon absorber guest (polypyrrole [PPy]) and thermal storage guest (1‐octadecanol [ODA]) into an MOF host (Cr‐MIL‐101‐NH2). The coated PPy layer on the surface of ODA@MOF not only serves as a photon harvester, but also serves as a phonon enhancer. Resultantly, ODA@MOF/PPy composite PCMs exhibit intense and broadband light absorption characteristic in the ultraviolet–visible–near‐infrared region, and higher heat transfer ability than ODA@MOF. Importantly, the photothermal conversion and storage efficiency of ODA@MOF/PPy‐6% is up to 88.3%. Additionally, our developed MOF‐based photothermal composite PCMs also exhibit long‐standing antileakage stability, energy storage stability, and photothermal conversion stability. The proposed coating strategy and in‐depth understanding mechanism are expected to facilitate the development of high‐efficiency MOF‐based photothermal composite PCMs in solar energy utilization.

organic PCMs (such as paraffins, polyethylene glycol [PEG], fatty acids, fatty alcohols, and esters), inorganic PCMs (such as salt hydrates, salts, and metallics), and eutectic PCMs. Inorganic PCMs often encounter phase separation, supercooling, and corrosion problems. [5] Comparatively, organic PCMs are currently the most widely utilized. However, organic PCMs often encounter liquid leakage, low thermal conductivity, and weak solar absorption capacity in solar energy utilization. [6][7][8] To address these shortcomings, numerous researchers have been devoted to developing porous supporting materials with high thermal conductivity and strong solar absorption for the encapsulation of PCMs to prepare shape-stabilized composite PCMs, especially graphite, graphene, graphene derivatives, and carbon nanotubes (CNTs). [9][10][11][12][13][14] Recently, graphite, graphene, graphene derivatives, and CNTs-based photothermal PCMs have taken a big step forward as the most competitive candidates. [15] For instance, Li and Wang [16] prepared graphene oxide (GO) @PEG composite PCM for photothermal conversion. Zhang and Liu [17] used CNT sponge to prepare sebacic acid/CNT sponge composite PCM for photothermal conversion. Expect single photoresponsive materials, photoresponsive hybrid materials for PCMs are expected to harvest more excellent photothermal conversion capabilities. In this regard, Zhou et al. [18] simultaneously encapsulated PEG and zinc oxide (ZnO) into graphene aerogel to prepare composite PCM with an excellent photothermal conversion efficiency of 80.1%, due to the whole band visible (Vis) light absorption of graphene aerogel and ultraviolet (UV) light absorption ability of ZnO. Similarly, Yang et al. [19] designed arrayed GO/ boron nitride (BN) hybrid scaffold for the encapsulation of PEG. Resultantly, arrayed GO/BN hybrid scaffold endowed composite PCM with an excellent photothermal conversion efficiency of 83.8%. Qian et al. [20] used graphene and CNT hybrid to encapsulate PEG, and the obtained composite PCM exhibited a high photothermal conversion efficiency of 86.0%. Bao et al. [21] fabricated three-dimensional (3D) interpenetrating networked scaffolds by integrating GO aerogels, CNTs, and carbon spheres. After encapsulating PEG, the obtained composite PCM exhibited a high photothermal conversion efficiency of 89.3%, 36.10% higher than that of GO aerogel@PEG. However, complex preparation process, easy agglomeration, and difficult pore regulation of graphene, graphene derivatives, and CNTs severely hinder their large-scale applications in photothermal conversion and storage field. Therefore, more promising nanoporous supporting materials with easy dispersibility and precise pore regulation are urgently sought for harvesting excellent thermophysical properties.
Three-dimensional metal-organic frameworks (MOFs) that are self-assembled through the coordination of organic linkers and metal-containing nodes can be rationally designed by screening the appropriate organic linkers and metal centers. [22][23][24][25] Advanced cluster chemistry has facilitated the booming development of MOFneighboring interdisciplinary with expanding potential applications. [26][27][28][29][30][31][32] Benefiting from the attractive merits of adjustable pore size, ultrahigh surface area, large pore volume, and customizable chemical features, MOFs have been recently applied to the field of phase change thermal energy storage. [33]

| Preparation of MOF
Cr-MIL-101-NH 2 was synthesized using a one-step hydrothermal method. [34] First, 1.44 g (8.0 mmol) H 2 BDC-NH 2 , 3.20 g (8.0 mmol) Cr(NO 3 ) 3 ·9H 2 O, and 0.80 g (20.0 mmol) NaOH were dissolved into 60 mL deionized water, followed by stirring at room temperature for 30 min. Then, the mixture solution was transferred to a polytetrafluoroethylene-lined autoclave and reacted at 180°C for 12 h. Finally, the obtained solid product was purified several times using N,N-dimethylformamide, methanol, and deionized water, respectively. The centrifuged solid product was dried to obtain MOF powder in a vacuum oven at 100°C for 24 h.

| Characterizations
The structural morphologies were observed by the scanning electron microscope (Regulus 8100, HITACHI) with an accelerating voltage of 5 kV. The chemical functional groups and phase compositions were analyzed by Fourier transform infrared spectrometer (FTIR, Nicolet 6700) and X-ray diffractometer (XRD, Bruker D8 ADVANCE), respectively. The light absorption capacity was evaluated by a UV-Vis-NIR spectrometer (Agilent, Cary 5000). The pore structure was measured through nitrogen adsorption/desorption instrument (BET, AUTOSORB-1C). Thermal storage properties were measured by differential scanning calorimetry (DSC3, METTLER TOLEDO) with a heating/cooling rate of 10°C/min from 20°C to 100°C. The thermogravimetric analysis (TGA, METTLER TOLEDO) was used to evaluate the thermal stability at a heating rate of 10°C/min from 25°C to 1000°C under N 2 atmosphere. An infrared thermal imaging camera (FLUKE Ti480 PRO) was used to record thermal response rate and temperature distribution. A solar simulator (CHF-XM500, Beijing Changtuo) was used to evaluate solarthermal conversion capability of composite PCMs, and a digital thermometer was used to record temperature curve with time.

| Morphological and structural analysis
Pristine MOF-based photothermal composite PCMs were prepared by integrating photosensitizer PPy and thermal storage agent ODA in nanoporous Cr-MIL-101-NH 2 ( Figure 1A). First, ODA@MOF composite PCM was prepared by hydrothermal reaction and physical infiltration strategies. Subsequently, PPy was uniformly coated on the surface of ODA@MOF composite PCM through the chemical polymerization of pyrrole under the oxidation of ferric chloride. After coating PPy, ODA@-MOF/PPy composite PCMs are capable of realizing efficient photothermal conversion and storage compared to ODA@MOF composite PCM with almost no photothermal conversion capability due to the highly sensitive photon capture capability of PPy. Figure 1B-E depicts the morphological evolution of ODA@MOF/PPy with different contents of PPy under the same magnification. Compared with ODA@MOF, it can be observed that the morphological size of ODA@MOF/PPy with different contents of PPy is nearly analogous due to the small amount of coated PPy on the surface of ODA@MOF. However, ODA@MOF/PPy exhibits more and more black with the content increase of PPy in the macroscopic morphology, indicating the valid coating effect of PPy.
Considering that the pore structure of MOF plays an important role in the thermal storage capacity of the composite PCM system. The pore characteristics of Cr-MIL-101-NH 2 were evaluated by measuring nitrogen adsorption-desorption isotherm. Figure S2 exhibits a type-I nitrogen adsorption-desorption isotherm with a pore size distribution ranging from 0.6 to 3.0 nm (the molecular size of ODA is about 2.5 nm) and a specific surface area of 823.19 m 2 /g, indicating that the nanoporous space of Cr-MIL-101-NH 2 is sufficient to allow the encapsulated ODA molecules to rotate freely without significantly destroying the crystallization behavior. Meanwhile, microporous and mesoporous MOF-induced strong capillary force can guarantee high PCM-loading capacity and thermal energy storage density. It is worth mentioning that ODA molecules are mainly infiltrated into the nanopores of MOF. While PPy is mainly polymerized on the surface of ODA@MOF rather than inside the nanopores of MOF. Because the specific surface area and pore size distribution of ODA@MOF is close to zero compared with pristine MOF, indicating that the pore of MOF is fully filled by ODA.
FTIR spectra were conducted to define the types of functional groups of ODA, MOF, PPy, ODA@MOF, and ODA@MOF/PPy ( Figure 1F). ODA exhibits the symmetrical stretching vibration of C─H at 2919 cm −1 and the antisymmetric stretching vibration of C─H at 2849 cm −1 . [36] Cr-MIL-101-NH 2 exhibits the stretching vibrations of N─H at 665 cm −1 , C─N at 1580 cm −1 , C═O at 1649 cm −1 , and C─OH at 3460 cm −1 . [34,37] PPy exhibits the stretching vibrations of C─C at 1543 cm −1 and C─N at 1458 cm −1 of pyrrole ring. [38] XRD patterns were conducted to define the phase compositions of ODA, MOF, PPy, ODA@MOF, and ODA@MOF/PPy ( Figures 1G and S3-S5). The two strong and sharp diffraction peaks of ODA are located at 21.8°a nd 24.7°( Figure S3). The main diffraction peaks of Cr-MIL-101-NH 2 are located at 9.1°and 16.6°( Figure S4). An obvious characteristic peak of PPy at 25°( Figure S5) is attributed to the scattering of bare polymer chains on the plane spacing. [39] On the whole, the main characteristic peaks of ODA are obviously observed in ODA@-MOF composite PCMs without significant new peaks, indicating that ODA@MOF is just the physical integration of ODA with MOF, and no chemical reaction occurs. The lack and location shift of some PPy peaks further indicates the successful chemical polymerization fabrication of PPy on the surface of ODA@MOF. More importantly, after coating PPy, the main characteristic peaks of ODA are still obviously observed in ODA@-MOF/PPy composite PCMs, indicating that both PPy and MOF have no influence on the crystalline structure of ODA. It is worth noting that the characteristic peak of PPy in ODA@MOF/PPy is almost unobservable because the excessively strong diffraction peak intensity of ODA covers PPy ( Figure 1G).

| Thermal storage properties
Phase change enthalpy is the most reliable indicator to evaluate the thermal storage capacity of composite PCMs. The detailed latent heat values and phase change temperatures obtained from DSC curves are shown in Figure 2 and Table S1. It can be observed that there is almost no significant difference in the latent heat values and phase change temperatures among ODA@MOF/PPy composite PCMs with different contents of PPy, indicating that the introduction of PPy does not interfere with the thermophysical properties of ODA@MOF. Specifically, the melting enthalpies and freezing enthalpies of ODA@MOF/PPy composite PCMs with different contents of PPy are both ranging from 127 to 131 J/g. The melting temperatures and freezing temperatures of ODA@MOF/PPy composite PCMs with different contents of PPy are ranging from 62°C to 64°C and from 50°C to 53°C, respectively. These thermophysical results are reasonable because PPy is mainly polymerized on the surface of ODA@MOF in the form of coating, and the coated amount is very small (4%-6%). Resultantly, the coated PPy is difficult to interfere with the thermophysical behaviors of ODA molecules due to their contactless space configurations.
Amino-functionalized Cr-MIL-101-NH 2 can provide sufficient hydrogen bond interaction sites compared with amino-free functionalized Cr-MIL-101, [37] thus guaranteeing high thermal storage density (about 130 J/g) of ODA@MOF composite PCM, showing outstanding application prospect in the thermal storage field. During the preparation process of ODA@MOF composite PCM, the hydrogen bond interaction will effectively adsorb and stabilize ODA molecules in the nanopores of Cr-MIL-101-NH 2 to prevent leakage. The leakage-proof evaluation experiment can confirm this statement ( Figure 3C). Additionally, compared with ODA@MOF composite PCM, ODA@MOF/PPy composite PCMs exhibit a slight reduction in the melting enthalpy and freezing enthalpy because a small amount of PPy has no latent heat storage capacity. Moreover, compared with pure ODA, the supercooling degree of ODA@MOF/PPy composite PCMs is reduced by about 5°C due to the evolution of nucleation modes from homogeneous nucleation of ODA to heterogeneous nucleation of ODA@MOF/PPy.

| Photothermal conversion and storage
Considering the weak photon capture capability of pristine PCMs and MOFs, we coat excellent photosensitizer PPy on the surface of ODA@MOF composite PCM for photothermal conversion and storage. Nanoporous Cr-MIL-101-NH 2 serves as a supporting material for leakage-proof of ODA, and PPy serves as a photon trapping agent to trigger photothermal conversion and storage of ODA. As seen from UV-Vis-NIR absorption spectra ( Figure 4A), ODA@MOF/PPy composite PCMs exhibit intense and broadband light absorption characteristic. While ODA@MOF composite PCM exhibits very weak and narrowband light absorption characteristic, meaning that solar radiation is difficult to trigger the photothermal conversion and storage of ODA@MOF composite PCM. The intense and broadband light absorption characteristic of ODA@MOF/PPy composite PCMs is attributed to PPy with conjugated structure. [40][41][42] It is worth noting that the light absorption capacity of ODA@MOF/PPy composite PCMs is gradually enhanced with the increase of PPy content. The photon capture properties are closely related to photothermal conversion performance of composite PCMs.
The fundamental photothermal conversion and storage mechanism of ODA@MOF/PPy composite PCMs is shown in Figure 4B. As an excellent photon absorber, organic PPy has a conjugated structure with alternating C─C and C═C bonds. [42][43][44] The π electrons in the conjugated double bonds are not fixed to a certain carbon atom, and they can be transferred from a carbon atom to another carbon atom. That is, π electrons tend to extend across the entire molecular chain. When exposed to light radiation, PPy molecules can utilize the transition of electrons in molecular orbitals to absorb light energy, and then convert the absorbed light energy into thermal energy, exhibiting a photothermal conversion effect. In other words, under the light radiation, π electrons on the π-bonding molecular orbitals of PPy absorb the light energy, and then transition to π* antibonding molecular orbitals. [45] During the process of the excited electrons F I G U R E 3 (A) TGA curves, (B) DTG curves, (C) shape stability evaluation, and (D) heat transfer capacity evaluation. DSC, differential scanning calorimetry; DTG, derivative thermogravimetric; MOF, metal-organic framework; ODA, 1-octadecanol; PPy, polypyrrole; TGA, thermogravimetric analysis. falling back to the ground state, part of the energy is released in the form of heat, resulting in a photothermal conversion effect. The generated thermal energy is then transferred to MOF and ODA crystals through heat conduction. When the converted thermal energy heat composite PCMs to a temperature higher than the melting point of ODA@MOF/PPy, the transferred thermal energy is stored by ODA in the form of latent heat through the phase change of ODA, exhibiting a good photothermal storage effect.
The photothermal conversion and storage tests were performed under a solar simulator (Xenon lamp) radiation at a constant intensity of 150 mW/cm 2 . The temperature evolution over time inside the samples was recorded by a digital thermometer. As seen from photothermal conversion and storage curves ( Figure 4C), ODA@MOF/PPy exhibits a faster heating rate compared with ODA@MOF, and the corresponding temperature is higher at the same light irradiation time. This phenomenon indicates that PPy is an effective photon trap that can efficiently absorb and convert light energy into heat energy, which is consistent with UV-Vis-NIR absorption spectra ( Figure 4A). For ODA@MOF/PPy, once the light irradiation switch is turned on, the radiated light energy is absorbed by PPy and then converted into thermal energy. The generated thermal energy is then transferred to MOF and ODA crystals through heat conduction. When the temperature reaches the melting temperature of ODA@-MOF/PPy, the transferred thermal energy begins to be absorbed in the form of latent heat through the phase change of ODA, corresponding to a melting phase transition plateau. Once the light irradiation switch is turned off, the temperature of ODA@MOF/PPy descends and then reaches another plateau corresponding to a solidification phase transition plateau. It is worth noting that the photothermal conversion and storage capacity of | 429 ODA@MOF/PPy composite PCMs is gradually enhanced with the increase of PPy content. Obviously, a higher content of PPy can absorb more light energy and convert more heat energy per unit time. Unlike ODA@MOF/PPy composite PCMs, the thermal energy converted by light irradiation cannot reach the melting temperature of ODA@MOF composite PCM, indicating that light radiation fails to trigger the photothermal conversion and storage of ODA@MOF composite PCM due to the weak photon capture capability of ODA and MOF.
The photothermal energy conversion and storage efficiency (η) of ODA@MOF/PPy can be calculated by the following equation. m is the quality of ODA@MOF/ PPy, ΔH represents the latent heat of ODA@MOF/PPy, P is the intensity of simulated light, S is the area of ODA@MOF/PPy exposed to light, and t is the phase transition duration. After calculation, the photothermal conversion and storage efficiencies of ODA@MOF/PPy-4%, ODA@MOF/PPy-5%, and ODA@MOF/PPy-6% are 79.8%, 85.6%, and 88.3%, respectively, while the photothermal conversion and storage efficiency of ODA@MOF is zero ( Figure 4D). It can be concluded that the high content of photosensitizer PPy enables more efficient photothermal conversion and storage due to its intense and broadband light absorption characteristic, which is consistent with UV-Vis-NIR absorption spectra ( Figure 4A). Subsequently, we mixed ODA@MOF/PPy with waterborne elastic polyurethane (PU) to prepare ODA@MOF/PPy-PU composite film for wearable thermal management. The prepared ODA@MOF/PPy-PU composite film was attached on the finger. As seen from Figure 4E, after being exposed to the sun for about 120 s, the surface temperature of the composite film increased from 32°C to 43°C, while the temperature of other parts of the finger exhibited very little change, indicating excellent photothermal conversion capability of ODA@MOF/PPy-PU composite film in practical application.

| Thermal stability and shape stability
Thermal stability is a crucial factor to consider in the practical applications of PCMs. As seen from the TGA and DTG curves ( Figure 3A,B), the mass loss of the original MOF before 100°C is attributed to the evaporation of water molecules adsorbed by the MOF framework. As the temperature gradually increases, the MOF framework begins to decompose until pyrolysis is complete. The initial pyrolysis temperature and termination pyrolysis temperature of pristine ODA are about 150°C and 350°C, respectively, and there is no residue at the end. The two obvious thermal decomposition steps (150-250°C and 300-500°C) of ODA@MOF correspond to ODA pyrolysis and MOF pyrolysis, respectively. The two obvious thermal decomposition steps (200-300°C and 300-500°C) of ODA@MOF/PPy correspond to ODA pyrolysis and MOF/PPy pyrolysis, respectively. It is worth noting that the thermal stability of ODA@MOF/PPy drops after coating PPy on the surface of ODA@MOF due to the poor thermal stability of PPy. Importantly, all the MOF-based composite PCMs show no thermal decomposition phenomenon below 150°C, indicating that our developed MOF-based composite PCMs are suitable for applications in medium and low-temperature thermal storage scenarios. In addition, shape stability is another key factor to consider in the practical applications of PCMs. As a typical solid-liquid PCM, ODA is prone to leakage during phase transition, which is one of the most difficult problems in practical applications. Herein, nanoporous MOF with high specific surface area and pore porosity is used to encapsulate ODA to prepare shape-stabilized composite PCMs for leakage-proof. To visually demonstrate the excellent shape stability of MOF-based composite PCMs, ODA, ODA@MOF, and ODA@MOF/ PPy with the same size were placed in an oven at 80°C, and the process was recorded with a digital camera. As seen from Figure 3C, pristine ODA is white, ODA@MOF is dark green, and ODA@MOF/PPYi is black due to the introduction of PPy at 25°C. The color evolution from dark green to black also demonstrates the successful coating of PPy on the surface of ODA@MOF. After heating for 10 min, pristine ODA starts to melt, and it is difficult to maintain a solid state. In contrast, ODA@-MOF and ODA@MOF/PPy can remain a solid state well without any liquid leakage. When heating for 30 min, pristine ODA almost completely melts, indicating its extremely poor shape stability. While ODA@MOF and ODA@MOF/PPy still maintain their original shape well at different times even though the ambient temperature is higher than the melting point of ODA, indicating their excellent shape stability of MOF-based composite PCMs due to the strong encapsulation capability.
To evaluate the heat transfer capacity, ODA@MOF and ODA@MOF/PPy with different PPy contents were placed on a heating stage at 80°C. The thermal imaging cameras were used to record the temperature changes of samples at different times. As displayed in Figure 3D, ODA@MOF/PPy exhibits higher heat transfer ability than ODA@MOF. As PPy contains a large π-conjugated structure that can contribute to fast heat transfer through electronic heat conduction mechanism, thus enhancing the thermal conductivity of ODA@MOF composite PCM. [42,46] It can be concluded that the coated PPy not only serves as a photon harvester, but also as a phonon enhancer.

| Thermal cycle stability
Thermal cycle stability of composite PCMs is also very important for their practical application. To evaluate the thermal cycle stability of ODA@MOF/PPy composite PCMs, we recharacterized photothermal conversion curves, DSC curves, FTIR spectra, and XRD patterns after ODA@MOF/ PPy composite PCMs underwent 50 melting/freezing or photothermal conversion cycles, as shown in Figure 5. After comparison, the phase change enthalpies and phase change temperatures of ODA@MOF/PPy composite PCMs are highly consistent before and after 50 melting/freezing cycles, indicating the excellent energy storage stability. The photothermal conversion curves of ODA@MOF/PPy composite PCMs almost overlap before and after 50 photothermal conversion cycles, indicating the excellent photothermal conversion stability. Similarly, FTIR spectra and XRD patterns of ODA@MOF/PPy composite PCMs are highly consistent before and after 50 melting/freezing cycles, indicating the excellent chemical structure stability. On the basis of the above experimental cycle data, it can be F I G U R E 5 (A) Temperature-time curves of ODA@MOF/PPy-6% after 50 photothermal conversion cycles. (B) DSC curves of ODA@ MOF/PPy-6% after 50 melting/freezing cycles. (C, D) Phase change enthalpies and phase change temperatures of ODA@MOF/PPy-6% after 50 melting/freezing cycles. (E) FTIR spectra of ODA@MOF/PPy-6% after 50 melting/freezing cycles. (F) XRD patterns of ODA@MOF/PPy-6% after 50 melting/freezing cycles. DSC, differential scanning calorimetry; FTIR, Fourier transform infrared spectrometer; MOF, metal-organic framework; ODA, 1-octadecanol; PPy, polypyrrole. concluded that our prepared ODA@MOF/PPy composite PCMs exhibit excellent thermal cycle reversibility.

| CONCLUSION
In this work, we prepared advanced pristine MOF-based photothermal composite PCMs by simultaneously integrating photosensitizer guest and PCM guest into MOF host. The PCM-based photothermal conversion and storage system is composed of photothermal conversion unit (PPy), latent heat storage unit (ODA), and supporting framework (MOF). High content (6%) of PPy is more conducive to the improvement of these thermophysical properties of ODA@MOF/PPy composite PCMs. Resultantly, ODA@MOF/PPy composite PCMs exhibit intense and broadband light absorption characteristics in the UV-Vis-NIR region, high thermal storage density of 127-131 J/g, and high photothermal conversion efficiency (88.3%). Additionally, MOF-based photothermal composite PCMs also exhibit excellent thermal energy storage stability and photothermal conversion stability, showing great application prospects in solar energy utilization.