Thermo-kinetic Evaluation of Solar Thermochemical Cycle for Hydrogen Production Driven by Carbonaceous Agents

Solar thermochemical cycling, in which concentrated solar energy is used to split water and produce hydrogen, is considered a promising way to produce green hydrogen in the future. Nonetheless, it encounters issues, including elevated reaction temperatures and significant deoxygenation losses. Here, we propose a high-efficiency solar thermochemical cycling system for hydrogen production driven by carbonaceous agents and establish a thermo-kinetic model for isothermal pressure-swing cycles. Carbon monoxide is introduced as a reducing agent into the reduction reaction, decreasing the Gibbs free energy of oxygen vacancies generated by metal oxygen carriers to lower the reduction temperature while consuming oxygen to create an extremely low oxygen partial pressure environment. This system avoids the problem of large energy losses in traditional deoxygenation methods such as inert sweeping gases and vacuum pumps; moreover, it achieves a synergistic decrease in the reaction temperature and oxygen partial pressure during the thermochemical cycle. The theoretical solar energy to fuel conversion efficiencies of this system under isothermal cycling at 1300 °C can reach 19.89% and 23.77% with only water heat recovery when CeO_2-δ and Ce_0.80Zr_0.20O_2-δ are used as oxygen carriers, respectively. This research contributes a fresh idea to the problem of the high reaction temperatures and large irreversible losses in the solar thermochemical cycle for hydrogen production.