Research On Heat Storage Properties Of Nitrogen-Doped Porous Carbon Matrix Composite Phase Change Materials

Nitrogen-doped porous carbon materials possess the performance characteristics of high specific surface area,large total pore volume,high porosity,high thermal conductivity,and low mass density.Meanwhile,doped nitrogen can form hydrogen bonds with organic phase change materials to enhance the adsorption performance of porous materials and improve the stability of composite phase change materials.Therefore,nitrogen-doped porous carbon-based materials is an ideal carrier for the preparation of composite phase change materials with high energy storage density and high stability.In this work,nitrogen-doped porous carbon（NPC）and nitrogen-doped porous carbon modified expanded graphite（NPC@EG）were successfully synthesized by solvothermal and high-temperature carbonization methods.Nitrogen-doped porous carbon-based composite phase change materials with two structures were successfully prepared by adsorbing and encapsulating stearic acid（SA）in porous matrixes.The effects of porous structure and nitrogen doping of nitrogen-doped porous carbon-based materials on the thermophysical properties of composite phase change materials were analyzed,such as heat storage capacity,phase change behavior,supercooling effect,thermal conductivity,and thermal stability.It provides a new idea for the research and application of carbon-based materials in heat storage materials with high heat storage density and high thermal conductivity.The encapsulation capacity of NPC for SA（I）up to 92.7 wt%is higher than that of conventional porous-based shaped composite phase change materials.NPC was obtained from the pyrolysis of amino-functionalized metal-organic frameworks（NH2-MOF-5）.The high proportion of microporous and mesoporous structural features make it have a specific surface area of 1673 m²?g^-1 and a total pore volume of 1.77 cm³?g^-1.The tiny pore size limits the thermal conductivity,resulting in thermal conductivity of0.283 W?m^-1?K^-1 and a certain degree of supercooling for 92.7 wt%SA（I）/NPC.92.7wt%SA（I）/NPC exhibited excellent shape stability without obvious shape-volume change during the phase transition and exhibited good thermal stability due to the decrease of the pyrolysis rate and the increase of the pyrolysis temperature during the pyrolysis process.92.7 wt%SA（I）/NPC has a solidification enthalpy of 181.2 J?g^-1 and a melting enthalpy of 177.3 J?g^-1,and it maintains a high crystallinity of SA.SA（Ⅰ）/NPC has a certain application prospect as a thermal insulation heat storage carrier at a specific temperature.NPC@EG with a 3 D interconnected porous carbon chain network structure has excellent spatial structure stability.NPC@EG was obtained by in situ uniform growth of NH2-MOF-5 nanoparticles on expanded graphite（EG）templates followed by pyrolysis at 1000℃.NPC is constructed with a uniform and sparse nitrogen-doped porous carbon network between the sheets of EG.It modifies the macropore space and adds micropore and mesoporous structure,increasing the specific surface area and total pore volume of the modified expanded graphite to 190 m²?g^-1 and 0.41 cm³?g^-1,respectively.EG with high thermal conductivity acts as the carrier for encapsulating the phase change material.NPC with a porous carbon chain network structure serves as the skeleton for Stabilizing stable phase change material.Therefore,SA（Ⅱ）/NPC@EG shows high stability while maintaining high thermal conductivity.The results showed that 85 wt%SA（Ⅱ）/NPC@EG has high thermal conductivity(2.134 W?m^-1?K^-1)while maintaining considerable melting enthalpy(177.7 J?g^-1)and solidification enthalpy(178.1 J?g^-1).SA（II）/NPC@EG with good thermal stability can be used as an endothermic material to block and absorb heat flow with high energy density to meet the stability and reliability of the working environment.Both nitrogen-doped porous carbon-based composite phase change materials possess excellent stability.SA（I）/NPC exhibits high heat storage capacity according to high specific surface area and total pore volume,while SA（II）/NPC@EG has a single-layer graphite sheet structure to exhibit high thermal conductivity.They can be applied to different heat storage environments according to their performance.