Strategic co-doping of ceria for improved oxidation kinetics in solar thermochemical fuel production

La and Yb were incorporated as co-doping elements into Zr-doped ceria to investigate the influence of doping ions by means of ionic radius and valence on the ceria crystal lattice and potentially on thermodynamic and kinetic properties relevant to solar-driven two-step thermochemical CO2 splitting. Both Zr–La and Zr–Yb co-doped ceria clearly exhibited better reduction capability than ceria and partial molar enthalpy and entropy that were in between those of Zr-doped ceria and undoped ceria. In the case of CO2 splitting kinetics, Zr-doped ceria was the slowest in accordance with relevant literature, but La-doped samples were much faster than Yb-doped counterparts suggesting that extrinsic oxygen vacancy formation, even though important, does not primarily determine the CO2 splitting kinetics unlike the hypothesis postulated by previous works. This work elucidates the role of dopant's ionic radius, lattice constant and valence on thermodynamics and CO2 splitting kinetics via experimental demonstration, and provides a new doping strategy not based on the extrinsic oxygen vacancy formation but based on the dopant ionic radius and valence with an insight for the further doped ceria-based redox material discovery. [Display omitted] •Dopant valence impacts more significantly on the oxygen non-stoichiometry and partial molar enthalpy than the ionic radius.•Extrinsic oxygen vacancy formation does not mainly determine CO2 splitting kinetics as shown via La- and Yb-doping.•Doping larger trivalent cations on the Zr-doped ceria lattice significantly enhances surface exchange during CO2 splitting.•A new doping strategy is suggested based on the dopant ionic radius, valence, and surface oxygen exchange.