Improved Electrocatalytic Water Splitting Reaction on CeO2(111) by Strain Engineering: A DFT plus U Study

Ceria is a promising cathode material in solid oxide electrolysis cells (SOECs) because ceria can become a mixed electronic and ionic conductor through doping, which enables a high surface area for electrocatalysis. Here, we systemically investigate the effect of strain on the electrocatalytic water splitting reaction (WSR) for renewable hydrogen production on CeO2(111) by using density functional theory corrected for on-site Coulomb interactions (DFT+U). We find that tensile strain stabilizes the reduced states of ceria such as oxygen vacancies and surface hydroxyls, while compressive strain destabilizes the reduced states. These trends are explained by a downshift of the Ce 4f orbital energy under tensile strain and agree with the larger size of the Ce3+ ion in comparison to the Ce4+ ion. Our results show that hydroxyl decomposition into H-2 has the highest activation energy along the WSR pathway (E-a) and that the free energy of hydroxyl formation (Delta G(H)) prior to hydroxyl decomposition can act as a thermodynamic barrier to the WSR. Compressive strain (<-3.0%) correlates strongly with increased WSR activity on CeO2(111) because it reduces the total barrier (Delta G(H) + E-a). Strain also effectively engineers the reaction pathway of the WSR at T > 1000 K. By a comparison of the total reaction barrier on different hydroxylated surfaces, the WSR is found to proceed readily via a Ce-H intermediate on excessively hydroxylated CeO2(111) under tensile strain because of the lower barrier, while the WSR proceeds preferentially and readily on the partially or fully hydroxylated CeO2(111) under compressive strain. In addition, a direct mapping between the most efficient WSR pathway and strain at different operating temperatures provides a better understanding of the efficient WSR on the CeO2(111) facet by strain engineering, which sheds light on electrocatalysis on oxide catalysts.