Redox thermochemistry of Ca-Mn-based perovskites for oxygen atmosphere control in solar-thermochemical processes

Sustainable energy supply is a crucial issue in times of climate change and receding fossil energy reserves. The emerging field of solar-driven thermochemical H2O and CO2 splitting cycles is a very promising approach to address this challenge. Providing low oxygen partial pressures is crucial in these processes. This issue is tackled by either high vacuum pumping or inert-gas sweeping. Both techniques come with a rather high energy penalty, leading to lower efficiencies of the whole process. Thermochemical oxygen pumping offers great potential to efficiently reduce oxygen partial pressures in these splitting cycles. In this work a material investigation campaign focusing on earth-abundant, cheap and non-toxic perovskites is presented. The experimental results are complemented with an approach to correlate this performance to inherent material properties, and in particular to the tolerance factor. In this framework, Ca-Mn-based perovskite compositions were demonstrated to function effectively as combined thermochemical oxygen-pumping and energy storage materials. Not only an almost two-fold increase of the reduction extent of ceria as a water splitting material was achieved due to the operation of perovskites as oxygen pumping materials, but this increase was rendered three-fold by applying a suitable temperature swing operation strategy. In parallel, the perovskites' energy storage density can be significantly increased by exploiting specific phase transitions that can be rationally explained via the Goldschmidt tolerance factor. Hence, the work offers a novel approach to reach low oxygen partial pressures with minimal energy penalties and a derived model to evaluate occurring phase transitions and their corresponding heat effects.