Defect Engineering in Composition and Valence Band Center of Y2(YxRu1–x)2O7−δ Pyrochlore Electrocatalysts for Oxygen Evolution Reaction

Oxygen evolution reaction (OER) takes place in various types of electrochemical devices that are pivotal for the conversion and storage of renewable energy. This paper describes a strategy in the design of solid-state structures of OER electrocatalysts through controlling the cation substitution on the active metal site and consequently valence band center position of site-mixed Y₂(Y_xRu_1-x)₂O_7-δ pyrochlore to achieve high catalytic activity. We found that partially replacing the B-site Ru⁴⁺ cation with A-site Y³⁺ in pyrochlore-structured Y₂Ru₂O_7-δ modifies the oxidation state of B-site Ru from 4⁺ to 5⁺, as observed by electron paramagnetic resonance (EPR) spectroscopy but does not continuously increase the oxygen vacancy concentration in these oxygen substoichiometric compositions, as quantified by thermogravimetric analysis (TGA) decomposition studies. We found the increased Ru oxidation state leads to a downshift in valence band center. X-ray photoelectron spectroscopy (XPS) analysis was performed to quantitatively determine the optimal band center to be ∼1.27 eV below the Fermi energy level based on the analysis of the valence band edge of these Ru-based Y₂(Y_xRu_1-x)₂O_7-δ OER electrocatalysts. This work highlights that defect engineering can be a practical, effective approach to the optimization of oxidation state and electronic band center for high OER catalytic performance in a quantitative manner.