Thermo-kinetic analysis of reductant-driven isothermal solar thermochemical cycles for H₂ production

Producing green hydrogen through solar thermochemical cycles represents a clean and promising avenue for future energy generation. However, several challenges, notably the requirement for elevated reaction temperatures and substantial deoxygenation losses, currently impede the advancement of this technology. Here, we propose a high-efficiency solar thermochemical cycling system assisted by reducing gas for hydrogen production and establish a thermo-kinetic model for isothermal pressure-swing cycles. Carbon monoxide is introduced into the reduction reaction as the reducing gas, chemically facilitating a decrease in Gibbs free energy associated with oxygen vacancies formed by metal oxygen carriers. This process serves to diminish the reduction temperature while concurrently consuming oxygen, thereby establishing an environment characterized by an extremely low oxygen partial pressure. Furthermore, the utilization of industrial waste gas as the source of carbon monoxide input into the cycles is proposed, which represents a potential pathway for the effective utilization of industrial waste gas, concurrently enhancing the efficiency of the thermochemical cycles for hydrogen production. This system mitigates the issue of significant energy expenditures associated with conventional deoxygenation methods, such as the utilization of inert sweeping gases and vacuum pumps, while concurrently achieving a synergistic effect in reducing both the reaction temperature and the oxygen partial pressure. The theoretical solar energy-to-fuel conversion efficiency of this system under isothermal cycles at 1300 ℃ can reach 18.91% and 23.17% with only water heat recovery when CeO_2-δ and Ce_0.80Zr_0.20O_2-δ are used as oxygen carriers, respectively. This work contributes a fresh idea to address the problems of high reaction temperatures and large deoxygenation energy consumption during the solar thermochemical cycles.