Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO 3 Thin Films.

Epitaxial strain can cause both lattice distortion and oxygen non-stoichiometry, effects that are strongly coupled at the heterojunctions of complex nickelate oxides. In this paper, we decouple these structural and chemical effects on the oxygen evolution reaction (OER) by using a set of coherently-strained epitaxial NdNiO3 (NNO) films. We show that within the regime where oxygen vacancies (VOs) are negligible, compressive strain is favorable for the OER while tensile strain is unfavorable, as the former induces orbital splitting, resulting in a higher occupancy in the d3z2-r2 orbital and weaker Ni-O chemisorption. However, when the tensile strain is large enough to promote the formation of VOs, an increase in the OER is also observed. The partial reduction of Ni3+ to Ni2+ due to VOs makes the eg occupancy slightly larger than unit, which is believed to account for this increased OER activity. Our work highlights that epitaxial-strain-induced lattice distortion and VO generation can be individually or collectively exploited to tune OER activities, which is of importance for the predictive synthesis of high-performance electrocatalysts.