CO₂ and H₂O Splitting for Thermochemical Production of Solar Fuels Using Nonstoichiometric Ceria and Ceria/Zirconia Solid Solutions

Abstract

The solar thermochemical splitting of CO2 and H2O with ceria and Zr-doped ceria for CO and H2 production is considered. The two-step process is composed of the thermal reduction of the ceria-based compound followed by the oxidation of the nonstoichiometric ceria with CO2/H2O to generate CO/H2, respectively. As a reference, the reactivity of pure undoped ceria was first characterized during successive thermochemical cycles using a thermobalance. Then, Zr0.25Ce0.75O2 was synthesized using different soft chemical synthesis routes to evaluate the influence of the powder morphology on the reactivity during the reduction and the oxidation steps. The reduction yield of ceria was significantly improved by doping with Zr as well as the CO/H2 production yields, but the kinetic rates of the oxidation step for doped ceria were lower than for pure ceria. CO and H2 production of 241 and 432 μmol/g, respectively, have been measured. A kinetic analysis of the CO2-splitting step allowed one to estimate the activation energy that ranged between 83 and 103 kJ/mol depending on the synthesis route of Zr0.25Ce0.75O2. The powder morphology played an important role on the materials cyclability. In contrast to pure ceria, Zr-doped ceria showed possible deactivation when cycling at 1400 °C, and the influence of the synthesis route on the thermal stability was evidenced. The thermally resistant powders with porous morphology ensured stable reactivity during cycling. The Zr-doped ceria synthesized via pechini process produced the largest amounts of CO/H2 during successive cycles.