Increasing energy demand, fossil fuel depletion, and environmental issues require sustainable and efficient energy sources. Hydrogen produced by water splitting is one of the promissing candidates, and electrocatalysts can help to reduce overpotential of the reaction. Electrocatalytic activity can be improved by increasing number of active sites and intrinsic activity. Tuning cation deficiency and strain can enhance intrinsic activity. Here, we explore how cation deficiency and strain can affect electrocatalysis.

Noble metals are representative electrocatalysts, but they are expensive and unstable during oxygen evolution reaction (OER) in alkaline solution. Therefore, new materials for water splitting such as transition metal oxides are needed. Perovskite oxides have been actively studied because of their excellent catalytic activity, attractive physical-chemical properties, low cost, and non-toxicity.

To investigate the effect of La-site deficiency, epitaxial La_1-xNiO_3-δ (x=0, 0.05, 0.1, 0.15) thin films were deposited on NdGaO₃ substrates by pulsed laser deposition (PLD) method. The structural information of each film was verified by X-ray diffraction peaks in θ-2θ scans, Raman spectroscopy and X-ray linear dichroism (XAD) measurements. Each of La-site deficient samples were electrochemically characterized. To compare the effect of La-site deficiency and strain, which can induce e_g orbital polarization, an epitaxial La_0.95NiO_3-δ thin films were grown on a different lattice parameter substrates ranging in lattice parameter from 3.69 to 3.947 Å. They were also characterized physically and electrochemically. Based on experimental results, cation deficiency and strain can cause the e_g orbital polarization. Both can control the metal-oxygen bond strenth and improve catalytic activity.