Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO₃ 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 NdNiO₃ 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_{3 z} 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 Ni³⁺ to Ni²⁺ 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.