Solar fuels production: Two-step thermochemical cycles with cerium-based oxides

Solar CO₂/H₂O splitting via two-step thermochemical cycles of metal oxides is a promising path for solar energy conversion to carbon-neutral, liquid hydrocarbons from virtually inexhaustible resources, water and (waste) carbon dioxide, with high theoretical efficiency potential. Cerium-based oxides have seen enormous interest and research efforts since they were proposed for this application, mainly due to their good stability at high temperatures and fast kinetics in redox reactions. The current state-of the-art review on the advancements of thermochemical cycles performed with the aid of cerium-based oxides is presented in this work, with emphasis on the latest developments during the last decade. Reaction principles, material modifications, reaction kinetics and finally solar reactors developed and operated are discussed in detail to provide a comprehensive understanding of the nature of the specific material and the factors impacting on the system efficiency. This efficiency depends on a combination of redox material/solar reactor/operation mode. With respect to the material issue, even though most studies have been targeted on improving the reduction conditions by suitable doping (e.g. Zr and Hf), the experience accumulated so far points to the direction of improving the oxidation step, provided the reduction step is performed below a critical, operationally feasible temperature. Thus the efficiency-optimal solar operation mode should be based on a trade-off between material reduction and oxidation performance and on another trade-off between solid and gas heat requirements and suitable recuperation strategies. The latter are highly dependent on the concept of solar reactor chosen and have to be demonstrated efficiently in real cyclic, field-test operation. The development of more effective oxygen removal strategies to lower the oxygen partial pressure during reduction may bring great improvement to efficiency.