High energy density long-term thermochemical energy storage composite based on salt and wood-derived activated biochar

Hygroscopic salts as thermochemical energy storage materials stand out for their substantial energy storage density and long-term storage capability, but challenges like agglomeration, deliquescence, and mass transfer issues during sorption hinder their practical application. To resolve these problems, a nature-inspired, sustainable, and inexpensive composite composed of a wood-derived porous activated biochar matrix and CaCl₂ as active component is developed through an impregnation method. This study investigates how matrix particle size affects the energy storage density and cyclic stability of the composite. In addition, this research analyzes the salt leakage and evaluates the stability of the composite by using an accurate quantitative method. A simultaneous thermal analyzer coupled with a humidifier was used to analyze the energy storage density. Scanning electron microscopy and X-ray micro-computed tomography were used to analyze salt deposition on the outer surface and inner porous structure of the matrices. The surface area and porosity of the matrix and composite samples were characterized by analyzing nitrogen adsorption/desorption isotherms. Derived from cellular and vascular channels of wood, the activated biochar matrix shows a multi-scale porous structure that not only has exceptional surface area but also facilitates mass transfer as well as successful salt impregnation. A particle diameter between 354 to 595 µm leads to synthesis of the composite with optimal performance, exhibiting an initial energy storage density of approximately 2480 J/g. The energy storage density of this composite remains stable at 2310 J/g and 1780 J/g after 10 consecutive hydration/dehydration and 5 leakage test cycles respectively.