Thermodynamic analysis of an isothermal redox cycle for vacuum carbothermal ceria reduction and carbon dioxide splitting for solar fuels production

The transition to sustainable energy systems necessitates efficient solar-driven fuel production technologies. This study presents a detailed thermodynamic evaluation of a novel isothermal ceria (CeO₂) redox cycle, integrating vacuum-assisted carbothermal reduction with carbon dioxide (CO₂) splitting for syngas generation. Unlike conventional nonisothermal cycles, the proposed configuration operates isothermally under subatmospheric pressures (100 mbar) and moderate temperatures (915–965 K), significantly reducing thermal irreversibilities and system complexity. Using equilibrium chemical thermodynamics and a rigorously developed process model, the reduction and oxidation steps were analyzed to quantify the oxygen nonstoichiometry (δ) and determine the equilibrium composition of reactants and products. Vacuum carbothermal reduction enhanced ceria reducibility and syngas selectivity, while CO₂ splitting during oxidation promoted high CO yields, with favorable thermodynamic shifts supported by the Boudouard and reverse water-gas shift reactions. The predicted solar-to-fuel conversion efficiency reached a peak of 48.5% at 965 K, even in the absence of heat recuperation, demonstrating the dominant role of vacuum operation in optimizing cycle performance. Furthermore, the system exhibited minimal efficiency loss under partial methane conversion (25%), underscoring its robustness under practical conditions. This work is the first to thermodynamically assess an isothermal ceria redox cycle for solar fuel production under vacuum conditions. The findings reveal that vacuum-assisted isothermal operation not only simplifies reactor design but also achieves efficiencies exceeding those of previously reported nonisothermal systems. These insights contribute to the development of next-generation solar thermochemical reactors with improved scalability and performance for renewable fuel synthesis.