Co/N co‐doped flower‐like carbon‐based phase change materials toward solar energy harvesting

The photothermal conversion capacity of pristine organic phase change materials (PCMs) is inherently insufficient in solar energy utilization. To upgrade their photothermal conversion capacity, we developed bimetallic zeolitic imidazolate framework (ZIF) derived Co/N co‐doped flower‐like carbon (Co/N‐FLC)‐based composite PCMs toward solar energy harvesting. 3D interconnected carbon framework with low interfacial thermal resistance, abundant carbon defects and high content of nitrogen doping, excellent localized surface plasmon resonance (LSPR) effect of Co nanoparticles, and light absorber Co3ZnC in Co/N‐FLC synergistically upgrade the photothermal capacity of (polyethylene glycol) PEG@Co/N‐FLC composite PCMs with an ultrahigh photothermal conversion efficiency of 94.8% under 0.16 W/cm2. Uniformly anchored Co and Co3ZnC nanoparticles in carbon framework guarantee excellent photon capture ability. Bridging carbon nanotubes (CNTs) in 2D carbon nanosheets further accelerate the rapid transport of phonons by constructing cross‐connected heat transfer paths. Additionally, PEG@Co/N‐FLC exhibits a thermal energy storage density of 100.69 J/g and excellent thermal stability and durable reliability. Therefore, PEG@Co/N‐FLC composite PCMs are promising candidates to accelerate the efficient utilization of solar energy.


INTRODUCTION
To trade the issue of mismatch between solar energy supply and demand in terms of time and space, photothermal conversion and storage technology is an effective solution.[3][4][5] However, the utilization and conversion efficiency of solar energy in practical applications is still inefficient because of the inherent low solar absorption capacity of PCMs.[14][15][16][17][18] The regular arrangement of metal and organic ligands within the crystal structure of MOFs is one important advantage, which results in a homogeneous distribution of different components (e.g., metal nanoparticles and carbon) in MOF-derived hybrids.21] Therefore, MOFs-derived hybrid materials with various advantages have become a hot topic in the field of photothermal energy conversion.For example, Wang et al. [22] designed MOF-5-derived nanoporous carbon/ZnO nanoparticle hybrid as an efficient photon trap and molecular heater, which synergistically enhances the photothermal conversion and storage capacity of PCMs.Xu et al. [23] prepared Co-MOF-derived magnetic highly graphitized carbon-based composite PCMs with a high photothermal conversion efficiency of 88.1% due to the photothermal synergy between highly graphitized carbon and Co nanoparticles.In addition, MOFs composed of nitrogen-containing organic ligands such as methylimidazole can generate nitrogen-doped carbon nanomaterials after pyrolysis, resulting in higher thermal conductivity. [24,25]Nitrogen doping can effectively induce a large number of defects in the carbon microstructure, which act as composite centers to facilitate the nonradiative relaxation process. [26,27]MOF-derived nitrogen-doped carbon with controllable structure, composition and porosity can be obtained by selecting N-rich organic ligands.
Compared with 3D MOFs, 2D MOFs give PCMs extraordinary physicochemical properties, offering the potential for high performance composite PCMs.Because 2D MOFs have rich functional groups that improve affinity with PCM molecules.Surface functional groups can also be grafted with PCM molecules.What more, 2D MOFs can form a 3D conductive and solar capture network to enhance thermal performance of composite PCMs. [28]Notably, the rough surface/textured sheet and flower-like structures can significantly increase multiple reflections and scattering of light to enhance solar absorption.Furthermore, incorporating thermally conductive fillers (such as CNTs, [29] carbon fibers, [30] and Cu powders, [30] ) along with the unique microstructures (such as honeycomb-like microstructure, [31] well-aligned microstructure, [32,33] ) can further enhance the thermal performances of the composite PCMs.
Based on the above considerations, herein, we prepared bimetallic ZIF-derived cobalt/nitrogen co-doped flower-like carbon-based PCMs (PEG@Co/N-FLC) with excellent light absorption for photothermal conversion and storage.Bimetallic ZIF-derived Co/N-FLC exhibits controllable flower-like morphology through selecting the bimetallic ZnCo-ZIF as the precursor/sacrifice template and controlling pyrolysis temperature.In the PEG@Co/N-FLC composite PCMs, polyethylene glycol (PEG) molecules act as a thermal energy storage unit, and ZIF derived nanoparticle-anchored defective carbon acts as a solar capturer to convert solar energy into thermal energy effectively.Resultantly, PEG@Co/N-FLC composite PCMs exhibit an ultrahigh photothermal conversion efficiency of 94.8% under 0.16 W/cm 2 , showing great potential for solar energy harvesting and conversion application.This design strategy can be extended to other bimetallic ZnCo-MOF-derived carbon-based composite PCMs.

Preparation of PEG@Co/N-FLC composite PCMs
PEG was impregnated into Co/N-FLC using a physical impregnation method.PEG and Co/N-FLC were added into ethanol solution in the calculated ratio, and sonicated for 0.5 h to make Co/N-FLC and PEG completely contact and fully mix.Then, the samples were kept overnight in an oven at 80 • C to yield PEG@Co/N-FLCPEG@Co/N-FLC composite PCMs.Similarly, the corresponding composite PCMs were defined as PEG@Co/N-FLC-600, PEG@Co/N-FLC-700 and PEG@Co/N-FLC-800.

Structural analysis
Bimetallic ZIF-derived cobalt/nitrogen co-doped flower-like carbon (Co/N-FLC) was synthesized by one-step solvother-mal synthesis and subsequent high-temperature pyrolysis (Figure 1A).As seen from SEM images, Co/N-FLC remains the morphological integrity of original ZnCo-ZIF (Figure S2).During the high-temperature pyrolysis, Co ions were converted into carbide (Co 3 ZnC) and metal element while Zn ions were volatilized.With the increase of pyrolysis temperature, Co ions were completely reduced to metal element and then aggregated into nanoparticles (NPs) with the gradual evaporation of Zn species. [34]In the meantime, typical metal Co nanoparticle is one of the most effective catalysts for the growth of CNTs and N-doped CNTs due to the unique electronic structure with few d-vacancies. [35,36]In this study, Co NPs catalyze the reconfiguration of carbon substances to form CNTs on the surface of carbon nanosheets.The embedded Co NPs in the tip of CNTs play a key role as catalysts.Once the NPs are encased in graphite or amorphous carbon, the growth of CNTs is terminated.Therefore, Co NPs as catalysts stimulate the formation of a large number of one-dimensional carbon nanotubes (CNTs) to bridge two-dimensional carbon nanosheets, thereby constructing cross-connected heat transfer paths (Figure 1B-G). [37]In summary, Zn and Co play  1J).In addition, a highly graphitic carbon layer with a thickness of 2-5 nm can be distinctly observed on the outer layer of metal NPs (Figure S3).Since the Co catalyst NPs are embedded on the tips of the CNTs, it can be inferred that the in-situ growth of CNTs follows a tip growth mechanism. [38]In the corresponding EDS mapping images of Co/N-FLC (Figure S4), the elements including C, N, Co and Zn are uniformly displayed in the entire domain, indicating that metal NPs precipitated in situ are uniformly distributed on Co/N-FLC.The N element derived from dimethylimidazole is incorporated into the carbon skeleton.Further, PEG@Co/N-FLC composite PCMs were obtained by impregnating PEG into the interlayered region of Co/N-FLC nanosheets (Figure S5).XRD analysis was conducted to confirm the structural characteristics of ZnCo-ZIF and Co/N-FLC (Figure 2A).The XRD results show that ZnCo-ZIF has a similar crystal structure with ZIF-8 and ZIF-67, without additional phases identified (Figure S6).In addition, both Co 3 ZnC NPs and Co NPs are found in Co/N-FLC-600.Specifically, Co/N-FLC-600 shows obvious diffraction peaks at 41.4, 48.8, and 71.4 • , corresponding to crystal planes of (111), (200), and (220) of Co 3 ZnC NPs (JCPDS No. 29-0524). [39]The diffraction peaks at 44.7, 51.9, and 76.4 • correspond to the crystal planes of ( 111), (200), and (220) of Co NPs (JPCDS: 15-0806). [40,41]It is noteworthy that the characteristic peaks of Co NPs are observed in all Co/N-FLC samples.When the pyrolysis temperature is increased to 700 and 800 • C, only the diffraction peaks of Co NPs emerge in Co/N-FLC-700 and Co/N-FLC-800, which is related to the volatilization of Zn species at higher temperature.The carbon peak at 26.3 • corresponds to the crystal planes of (002).In addition, the characteristic diffraction peak of PEG at 20-30 • is still well retained after PEG is impregnated into the interlayered region of Co/N-FLC nanosheets, indicating that PEG still exhibits excellent crystallinity between FLC layers (Figure S7). [42]t is worth noting all the characteristic diffraction peaks of Co/N-FLC and PEG exist in PEG@Co/N-FLC composite PCMs, indicating that there is only physical interaction between Co/N-FLC and PEG without chemical reaction.This physical interaction successfully prevents the liquid leakage of PEG molecules during solid-liquid phase transition.FT-IR results also indicate that PEG has good chemical compatibility with Co/N-FLC, which is conducive to obtaining high thermal storage density without liquid leakage (Figure S8).
Raman analysis was conducted to evaluate the graphitization degree and structural defects of Co/N-FLC (Figure 2B).Two characteristic bands around 1350 and 1590 cm −1 are assigned to D band and G band, respectively.Raman peak at around 1350 cm −1 reflects the structural defects of sp 3 carbon and 1590 cm −1 corresponds to the in-plane vibration of sp 2 carbon. [43]The I D /I G ratio serves as an index to measure the graphitization degree of Co/N-FLC.The calculated I D /I G ratios of the samples are 2.32 (Co/N-FLC-600), 2.12 (Co/N-FLC-700), and 1.94 (Co/N-FLC-800), respectively.Higher pyrolysis temperature contributes a lower I D /I G ratio of Co/N-FLC, indicating that disordered carbon is gradually transformed into ordered graphitized carbon.An I D /I G ratio higher than 1 in all samples indicates that many defects are generated in Co/N-FLC, which is related to nitrogen doping. [24,44]In general, N content is closely related to the concentration of defects in carbon materials.Co/N-FLC-600 has the highest I D /I G ratio because the lower pyrolysis temperature results in less decomposition of the imidazole ligand and more nitrogen retention, thus producing more structural defects and disordered parts.During the pyrolysis, Co NPs catalyze the formation of abundant interconnected CNTs from organic ligands, promoting the arrangement of carbon atoms from disorder to order state.[47] It can be concluded that pyrolysis temperature has an obvious effect on the spatial arrangement and defect density of carbon atoms in Co/N-FLC.
Suitable pore size and large specific surface area of supporting materials are beneficial to realizing high thermal storage density without PCM leakage.Nitrogen isothermal adsorption-desorption experiments were conducted to evaluate the pore structure of Co/N-FLC.The N 2 adsorptiondesorption isotherms of all samples exhibit a type IV curve with a hysteresis loop, indicating that Co/N-FLC is associated with the mesoporous structure (Figure 2C).The pore size distribution of Co/N-FLC is mainly between 2 and 10 nm (Figure 2D).The specific surface area of Co/N-FLC-600, Co/N-FLC-700 and Co/N-FLC-800 is 179.9, 249.6, and 239.9 m 2 /g, respectively.When the pyrolysis temperature increases from 600 to 700 • C, the specific surface area of Co/N-FLC increases, which is related to the stacking pores formed among Co nanoparticles as well as between Co nanoparticles and carbon.When the pyrolysis temperature is further increased to 800 • C, the agglomeration of partial nanoparticles results in a decrease in specific surface area. [23]inally, Co/N-FLC provides advantageous storage space for PCM to effectively prevent PCM leakage during solid-liquid phase transition.
The chemical state and binding energy of Co/N-FLC were clarified by XPS analysis (Figure 3A).Co/N-FLC is mainly composed of C, N, Co and Zn elements, and the corresponding elemental content is shown in Table S1.Specifically, the C content gradually increases from 77.62% to 86.48% with the increase of pyrolysis temperature from 600 • C to 800 • C, which is consistent with the view that organic matter is carbonized at high temperature.The N content shows a gradual decrease from 9.12% to 5.00% with the increase of pyrolysis temperature from 600 to 800 • C. Nitrogen doping can effectively induce numerous carbon defects in Co/N-FLC.In other words, the number of carbon defects in Co/N-FLC decreases as the pyrolysis temperature increases.In addition, the Co content shows little change, while the Zn content decreases with the increase of pyrolysis temperature, which is attributed to the volatilization of Zn species at high temperature.As shown in Figure S9, C1s peak can be further divided into three different sub-peaks at 284.8, 285.4,and 286.6 eV, corresponding to C-C, C-O, and C-N. [47]The emerging C-N bond indicates the formation of N-doped porous carbon.The N 1s spectrum can be further divided into pyridinic N (398.5 eV), pyrrolic N (399.3eV), and graphitic N (401.3eV) species (Figure 3B). [48]These different types of N species lead to different physical and chemical environments around neighboring carbon atoms. [24]he graphitic N is helpful to accelerate phonon transmission in Co/N-FLC. [49]With the increase of pyrolysis temperature, Co ions located on the metal sites of ZIFs are converted successively into Co 3 ZnC and Co NPs at high temperature, while the content of Zn elements gradually decreases with the increase of pyrolysis temperature due to the volatilization (Figure 3A).The Co 2p spectrum can be deconvolved into two peaks at 778.4 eV and 793.1 eV, corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively, which are attributed to the formation of Co NPs. [48]Two peaks at 779.1 eV and 795.3 eV are attributed to Co 3+/2+ 2p 3/2 and Co 3+/2+ 2p 1/2 , respectively, along with two accessory peaks at 783.1 and 801.1 eV.(Figure 3C). [48]Two characteristic peaks at 1022.2 and 1045.3 eV are attributed to Zn 2p 3/2 and Zn 2p 1/2 , respectively (Figure 3D). [50]

Thermal properties
Thermal stability is an important evaluation index for composite PCMs.As seen from Figure 4A, ZnCo-ZIF precursor shows a weight loss of about 6.1% at the first stage (234−278 • C) due to the removal of physically adsorbed guest molecules. [51]The second stage (448−616 • C) exhibits a dramatic weight loss at around 516 • C due to the decomposition of 2-MI ligand in the skeleton. [48]The weight loss at the third stage (above 616 • C) originates from the evaporation of Zn species and the decomposition of Co 3 ZnC.It is noteworthy that Co 3 ZnC may be decomposed completely around 700 • C. PEG exhibits a dramatical weight loss when the temperature exceeds 385 • C, which is attributed to the thermal degradation of PEG molecules.In addition, PEG@Co/N-FLC composite PCMs exhibit excellent thermal stability below 330 • C due to the structural stability of Co/N-FLC, indicating that PEG@Co/N-FLC composite PCMs are suitable for use at medium and low temperatures (Figure S10).Thermal storage capability is also an important evaluation index for composite PCMs.The actual melting and solidification enthalpies of composite PCMs are 100.69 and 97.61 J/g (PEG@Co/N-FLC-600), 98.44 and 94.47 J/g (PEG@Co/N-FLC-700), 99.09 and 94.67 J/g (PEG@Co/N-FLC-800), respectively (Figure 4B-E).While, the theoretical melting and solidification enthalpies of PEG@Co/N-FLC composite PCMs are 118.87 and 109.69J/g, respectively, as calculated by Equation (1).Compared with the theoretical values, the reduction in melting and solidification enthalpies for all PEG@Co/N-FLC samples is attributed to the influence of nanoconfined interference on the phase change thermal behavior of PEG molecules. [16,49]The corresponding crystallinity of PEG molecules in Co/N-FLC is calculated by Equation ( 2).The calculated crystallinity of PEG@Co/N-FLC composite PCMs with different temperatures is located in the range of 82.81%−84.71%(Figure 4F).The similar crystallinity is associated with the similar pore size distribution in Co/N-FLC.The equations for calculating  2), respectively.In addition, PEG@Co/N-FLC composite PCMs are highly consistent in the phase change enthalpies and phase change temperatures before and after undergoing multiple heating-cooling cycles, indicating excellent thermal storage stability (Figure 4G-I).Both FT-IR and XRD results were also analyzed after 50 cycles and exhibited excellent thermochemical and crystal structure stability of the composite PCMs (Figure S11).
ΔH theory is the theoretical phase change enthalpy of composite PCMs, ΔH PEG is the phase change enthalpy of pristine PEG, and R is the encapsulation rate of PEG in Co/N-FLC.F C is the crystallinity of PEG in Co/N-FLC, and ΔH PEG@Co/N-FLC is the measured phase change enthalpy of composite PCMs.
The shape stability needs to be considered in evaluating the practical application of composite PCMs. Figure S12 shows the shape stability test of PCMs, where all samples were pressed into a cylinder and placed in an oven at 75 • C. The shape changes were recorded every 10 min using a digital camera.As shown in Figure S12, PEG completely melted after 50 min, while Co/N-FLC-600, Co/N-FLC-700, and Co/N-FLC-800 maintained their original shapes at 75 • C for 50 min, indicating that the Co/N-FLC composite PCMs have excellent shape stability without liquid phase leakage above the melting point.

Photothermal conversion and storage
The UV-Vis-NIR absorption spectrum was conducted to verify the optical absorption capacity of Co/N-FLC composite PCMs.Compared with weak optical absorption of pristine PEG, PEG@Co/N-FLC composite PCMs display remarkably strong optical absorption in the full spectrum range, and PEG@Co/N-FLC-600 is optimal (Figure 5A).The hybrid structure consisting of metal nanoparticles and high graphitized defective carbon synergistically promotes the photothermal conversion.Therefore, the addition of Co/N-FLC in PCMs greatly enhances the optical absorption performance of PEG@Co/N-FLC composite PCMs, thus obtaining high-efficiency photothermal energy conversion and storage.PEG@Co/N-FLC composite PCMs, respectively, and P is the power of the simulated solar irradiation.The starting and terminating phase change points (t 2 and t 1 ) are obtained by a tangential method.The calculated photothermal conversion efficiency of PEG@Co/N-FLC-600, PEG@Co/N-FLC-700 and PEG@Co/N-FLC-800 composite PCMs are 70.8%,67.1%, 63.5% under the simulated sunlight of 0.10 W/cm 2 , respectively (Figure S13).Additionally, we also measured the photothermal conversion curves of PEG@Co/N-FLC-600 under different simulated solar intensities (Figure 5C,D).The photothermal conversion efficiency of PEG@Co/N-FLC-600 is correspondingly improved from 70.8% to 94.8% with the increase of simulated sunlight from 0.10 W/cm 2 to 0.16 W/cm 2 .This is because higher light intensity can excite more photothermal electrons and PEG@Co/N-FLC-600 can absorb more light waves per unit time. (3)

Proposed mechanism
The photothermal conversion mechanism of PEG@Co/N-FLC composite PCMs is shown in Figure 5E.Firstly, the narrow 3D multistage penetration structure formed by the ultra-thin nanosheets of Co/N-FLC and the tangled CNTs with strong full spectrum absorption greatly enhances the multiple reflections of incident sunlight, thus effectively absorbing light energy.][54] As seen from XPS results, the nitrogen element content in Co/N-FLC decreases with the increase of pyrolysis temperature due to the decomposition of 2-MI ligand, so the nitrogen doping content is the highest in Co/N-FLC-600.When PEG@Co/N-FLC is exposed to light radiation, Co/N-FLC rapidly absorbs incident photons through π-π* transition in the graphite region, and the excited electrons relax mainly through nonradiative transition, thus releasing energy in the form of heat.In this regard, the structural defects in Co/N-FLC promote the nonradiative relaxation process.When the energy is released through phonons, it causes local heating of the lattice, thereby enhancing the photothermal conversion properties of PEG@Co/N-FLC.Raman results also prove that Co/N-FLC-600 has the most abundant carbon defects.In addition, the uniformly anchored Co NPs in in Co/N-FLC can further enhance the photothermal conversion property through their excellent LSPR effect.The uniformly anchored transition metal carbide Co 3 ZnC NPs in Co/N-FLC also have excellent photothermal conversion performance due to the metallic properties and strong light absorption. [55,56]NTs bridge in 2D carbon nanosheets further accelerates the rapid transport of phonons by constructing cross-connected heat transfer paths with low interfacial thermal resistance.Therefore, benefiting from the synergistic effect of 3D interconnected carbon framework with low interfacial thermal resistance, abundant carbon defects and high content of nitrogen doping, and uniformly anchored light absorber Co 3 ZnC and Co nanoparticles with excellent LSPR effect, Co/N-FLC-600 as efficient photon absorber and molecular heater assigns the optimal photothermal conversion and storage capacity to PEG@Co/N-FLC-600 composite PCMs.

Photothermal conversion and storage application
Given the efficient photothermal conversion ability of PEG@Co/N-FLC-600, we performed a practical test using simulated sunlight to heat water.It was mixed and stirred with a water-based varnish in a ratio of 1:10.The resulting mixture was then applied to the surface of a 5 mL clear glass tube and allowed to sit undisturbed for 48 h (Figure 6A).The prepared PEG@Co/N-FLC coated glass tube containing 3 mL water was irradiated under simulated sunlight (0.16 W/cm 2 ) for 20 min.The temperature of the water in the tube was recorded using a thermocouple and the surface temperature evolution of the coating was recorded using IR camera.Resultantly, the temperature of the water inside the tube rose from 22.1 to 46.5 • C after 20 min of light radiation (Figure 6B).The temperature of outer PEG@Co/N-FLC coating rose rapidly to 45.2 • C within 5 min, and finally reached 54.2 • C within 20 min (Figure 6C).However, the uncoated glass tube only reached 30.7 • C within 20 min.The study shows that PEG@Co/N-FLC coating has good photothermal conversion capability and provides a useful reference in expanding the application of bimetallic MOF-derived carbon-based composite PCM for solar water heating.

CONCLUSION
In summary, we designed bimetallic ZIF derived Co/N co-doped flower-like carbon-based composite PCMs (PEG@Co/N-FLC) for photothermal conversion and storage by simple high-temperature pyrolysis and physical impregnation methods.A large amount of defective carbon, in situ anchored Co nanoparticles with LSPR effect and photosensitive Co 3 ZnC nanoparticles were synchronously obtained in Co/N-FLC.Their synergistic effect enables PEG@Co/N-FLC-600 composite PCMs to reach a high photothermal efficiency of 94.8% under the simulated solar illumination of

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no competing financial interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

F I G U R E 1
(A) The preparation schematic diagram of ZnCo-ZIF and Co/N-FLC, (B and E) scanning electron microscope (SEM) images of Co/N-FLC-600, (C and F) SEM images of Co/N-FLC-700, (D and G) SEM images of Co/N-FLC-800, (H and I) transmission electron microscopy (TEM) images of Co/N-FLC-600, (J) HRTEM image of Co/N-FLC-600.10 • C/min in N 2 atmosphere.The pore structure of Co/N-FLC was analyzed using an N 2 adsorption and desorption analyzer (Autosorb IQ).The graphitization degree of Co/N-FLC was studied by Raman spectra at an excitation wavelength of 532 nm.The chemical compositions and valence states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, ThermoKalpha, USA).The light absorption capacity was assessed by a Ultraviolet-Visible-Near-infrared (UV-Vis-NIR) spectrometer (Agilent, Cary 5000).The photothermal energy conversion and storage capacity was tested under analogue solar (CEAULIGHT, CEL-S500) and the temperature-time curves were measured with a digital data collector (R2100).

F I G U R E 2
Structure characterization of Co/N-FLC.(A) X-ray diffraction (XRD) patterns, (B) Raman spectra, (C) N 2 adsorption-desorption isotherms, (D) Pore size distribution.crucial roles in the high-temperature pyrolysis of ZnCo-ZIF, with Zn contributing to the formation of Co 3 ZnC and some nanopores, and Co acting as a catalyst for the formation of CNTs.TEM images show that a large number of metal NPs are anchored on the carbon nanosheets or partly encapsulated in the tips of CNTs (Figure 1H,I).HRTEM image shows that the lattice spacing of 0.20 nm corresponds to the (111) crystal plane of Co NPs and 0.22 nm corresponds to the (111) crystal plane of Co 3 ZnC NPs (Figure

F
I G U R E 3 X-ray photoelectron spectroscopy (XPS) spectra of Co/N-FLC.(A) Total spectrum, (B) N 1s, (C) Co 2p, (D) Zn 2p.theoretical phase change enthalpy and crystallinity are shown in (1) and ( Figure5Bpresents the photothermal energy conversion and storage curves of PEG@Co/N-FLC composite PCMs under the simulated sunlight of 0.10 W/cm 2 .It can be observed that the temperature of PEG@Co/N-FLC composite PCMs rises rapidly and reaches a temperature plateau once the light switch is turned on.This temperature plateau at about 55-60 • C corresponds to the phase change temperature of PEG@Co/N-FLC composite PCMs.At this stage, Co/N-FLC converts the captured solar energy into heat energy and is then stored as latent heat by PEG.Compared with PEG@Co/N-FLC-700 and PEG@Co/N-FLC-800, PEG@Co/N-FLC-600 takes a minimum of time when reaching the same temperature of 65 • C, indicating its faster heat transfer rate.The photothermal conversion efficiency (η) of composite PCMs is also an important evaluation parameter towards solar energy utilization.Photothermal conversion efficiency is calculated according to the Equation (3), in which m and ΔH are the weight and melting enthalpy of

F I G U R E 4
Thermal properties of polyethylene glycol (PEG)@Co/N-FLC composite phase change materials (PCMs).(A) TGA and differential thermogravimetric curves, (B and C) differential scanning calorimetry (DSC) curves, (D and E) phase change enthalpy and phase change temperature, (F) crystallinity.(G) Cycling DSC curves of PEG@Co/N-FLC-600, (H and I) phase change enthalpy and phase change temperature of PEG@Co/N-FLC-600 after heating-cooling cycles.

F I G U R E 5
(A) UV-Vis-NIR absorption spectra, (B) photothermal temperature-time curves under 0.10 W/cm 2 , (C) photothermal temperature-time curves of PEG@Co/N-FLC-600 composite phase change materials (PCMs) under different illumination intensities, (D) photothermal conversion efficiency of PEG@Co/N-FLC-600 composite PCMs under different illumination intensities, (E) mechanism illustration of photothermal conversion and storage.

F
I G U R E 6 (A) Experimental schematic device of PEG@Co/N-FLC coating for simulating sunlight heating water, (B) temperature-time curves of the water inside the glass tube, (C) infrared thermal images of PEG@Co/N-FLC coating on the glass tube.0.16 W/cm 2 .Additionally, PEG@Co/N-FLC-600 composite PCMs exhibit a thermal energy storage density of 100.69 J/g and excellent thermal stability and durable reliability.This design strategy can be universally extended to other bimetallic organic framework systems.Therefore, our designed ZIF-derived Co/N co-doped flower-like carbon-based composite PCMs will open up a promising possibility for the efficient utilization of solar energy.A C K N O W L E D G M E N T SThis work was financially supported by Beijing Natural Science Foundation (grant number: 2232053) and National Natural Science Foundation of China (grant number: 51902025).

2.2 Preparation of Co/N-FLC
Methanol and ethanol were supplied by Beijing Tongguang Fine Chemical Co., Ltd.Water-based varnishes were supplied by Hebei Yuling Waterproof Material Co., Ltd.All reagents were used without further purification in this study.