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

Epitaxial strain can cause both lattice distortion and oxygen nonstoichiometry, effects that are strongly coupled at heterojunctions of complex nickelate oxides. Here we decouple these structural and chemical effects on the oxygen evolution reaction (OER) by using a set of coherently strained epitaxial NdNiO ₃ films. We show that within the regime where oxygen vacancies (V _O ) are negligible, compressive strain is favorable for the OER whereas tensile strain is unfavorable; the former induces orbital splitting, resulting in a higher occupancy in the d _{3z ² âr ²} orbital and weaker Ni-O chemisorption. However, when the tensile strain is sufficiently large to promote V _O formation, an increase in the OER is also observed. The partial reduction of Ni ³⁺ to Ni ²⁺ due to V _O makes the e _g occupancy slightly larger than unity, which is thought to account for the increased OER activity. Our work highlights that epitaxial-strain-induced lattice distortion and V _O generation can be individually or collectively exploited to tune OER activity, which is important for the predictive synthesis of high-performance electrocatalysts.