Improved Electrocatalytic Water Splitting Reaction on CeO2(111) by Strain Engineering: A DFT+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 H₂ has the highest activation energy along the WSR pathway (E_a) and that the free energy of hydroxyl formation (ΔG_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 (ΔG_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.