Hybrid carbonate and redox system for thermochemical energy storage

Abstract

A hybrid thermochemical energy storage material based on the coupling of carbonate (CaCO₃/CaO) and redox (CuO/Cu₂O) reactions is herein proposed. This integrated CaCO₃/CuO system enables the simultaneous contribution of Calcium Looping and redox processes within a single multifunctional composite. The resulting materials exhibit synergistic enhancements in reactivity, solar absorptance, and thermal conductivity arising from phase interactions between carbonate and oxide domains. Simultaneous operation of both reaction schemes was achieved under atmosphere-switching and fixed-atmosphere conditions, although the extent of conversion varied depending on the reaction conditions. The influence of CuO content and preparation method on multicycle performance was systematically evaluated. A composite containing 50:50 CaCO₃/CuO mass ratio prepared by wet ball milling delivered the most balanced performance over repeated charge/discharge cycles. Conversion and energy storage density analyses demonstrate that both components actively contribute to the total stored energy, with their relative contributions depending on the operating conditions. Beyond providing an additional redox-based energy contribution, CuO mitigates sintering in the CaCO₃/CaO system, thereby enhancing cycling stability. Optical and thermal characterization further revealed a marked improvement in solar absorptance and thermal conductivity compared with CaCO₃ alone. These finding establish CaCO₃/CuO composites as a new materials design platform for integrated thermochemical energy storage.