Investigation of Zr, Gd/Zr, and Pr/Zr – doped ceria for the redox splitting of water

There is a renewed interest in CeO₂ for use in solar-driven, two-step thermochemical cycles for water splitting. However, despite fast reduction/oxidation kinetics and high thermal stability of ceria, the cycle capacity of CeO₂ is low due to thermodynamic limitations. In an effort to increase cycle capacity and reduce thermal reduction temperature, we have studied binary zirconium-substituted ceria (Zr_xCe_1-xO₂, x = 0.1, 0.15, 0.25) and ternary praseodymium/gadolinium-doped Zr-ceria (M_0.1Zr_0.25Ce_0.65O₂, M = Pr, Gd). We evaluate the oxygen cycle capacity and water splitting performance of crystallographically and morphologically stable powders that are thermally reduced by laser irradiation in a stagnation flow reactor. The addition of zirconium dopant into the ceria lattice improves O₂ cycle capacity and H₂ production by approximately 30% and 11%, respectively. This improvement is independent of the Zr dopant level, up to 25%, suggesting that above 10% Zr dopant level, Zr might be displaced during the high temperature annealing process. The addition of Pr and Gd to the binary Zr-ceria mixed oxide, on the other hand, is detrimental to H₂ production. A kinetic analysis is performed using a model-based analytical approach to account for effects of mixing and dispersion, and to identify the rate controlling mechanism of the water splitting process. We find that the water splitting reaction at 1000 °C and with 30 vol% H₂O, for all doped ceria samples, is surface limited and best described by a deceleratory power law model (F-model), similar to undoped CeO₂. Additionally, we used density functional theory (DFT) calculations to examine the role of Zr, Pr, and Gd. We find that the addition of Pr and Gd induce non-redox active sites and, therefore, are detrimental to H₂ production, in agreement with experimental work. The calculated surface H₂ formation step was found to be rate limiting, having activation barriers greater than bulk O diffusion, for all materials. This agrees with and further explains experimental findings.