Hydrophilic modification of 3D honeycomb carbon matrix for solar-driven thermal energy storage based on calcium chloride

“Salt in matrix” composite materials hold considerable promise for utilisation in water-sorption-based energy storage and transformation. The salt-matrix interactions are of pivotal significance in the processes of heat and mass transfer as well as heat storage density. Herein, a type of waste cork-derived macroporous material is reported for loading CaCl₂ via tuning the pore structure and the surface polarity. A carbon matrix with a honeycomb structure and a range of pore sizes was obtained by subjecting the cork to carbonation at temperatures of 700, 800 and 900 °C. Oxidative treatment of the matrix induced a significant increase in oxygen-containing functional groups, resulting in the surface changing from hydrophobic to hydrophilic. A partial degradation of the porous architecture also occurred. The CaCl₂ content within the matrix C800 increased by 68 % after oxidation, under the dual effect of surface properties and pore structure. The highest salt loading (57 wt%) of composite C800-O/Ca was found to exhibit high water uptake of 1.33 g/g and energy storage capacity up to 2427 J/g-dried sorbent under conditions of a solar-driven thermochemical heat storage system (i.e. adsorption at 20 °C, RH 80 %; desorption under 1 kW/m²). The research proposes a valuable approach to synthesising outperforming composite thermal storage materials, whereby waste is transformed into a valuable matrix with controlled properties.