Recent progress on ceria doping and shaping strategies for solar thermochemical water and CO2 splitting cycles

Thermochemical redox cycling for either water or CO2 splitting is a promising strategy to convert solar energy into clean fuels. Such splitting reaction can convert water and recycled CO2 into H2 and CO respectively, the building blocks for the preparation of various synthetic liquid fuels. Attractively, CO2 is valorized in this way and can be used as a carbon-neutral fuel. However, the efficiency of the solar thermochemical process has to be improved to achieve an economically viable fuel production. For this purpose, an optimization of the reactive materials regarding both their chemical activity and long-term stability is a key requirement. To date, ceria is considered as the benchmark material for thermochemical redox cycles. Indeed, it is able to maintain a single cubic fluorite phase during thermal cycling over a large range of oxygen non-stoichiometry and also provides thermodynamically favorable oxidation. However, it suffers from a high reduction temperature and a low reduction extent. Several doping strategies of ceria have been developed to increase its redox activity and long-term performance stability. This paper provides an overview of the efforts made to enhance the thermochemical performance of ceria by investigation of dopant incorporation and material shaping for designed morphologies and microstructures.