(333c) Defect Engineering in Composition and Valence Band Center of Y2(YxRu1-x)2O7-? Pyrochlore Electrocatalysts for Oxygen Evolution Reaction

Unlocking the potential of solid-state structures in oxygen evolution reaction (OER) electrocatalysts is crucial for advancing renewable energy conversion and storage technologies. In this work, we introduce a new design strategy focusing on the controlled cation substitution within the active metal site, specifically targeting the valence band center position of site-mixed Y₂(Y_xRu_1-x)₂O_7-δ pyrochlore, to enhance catalytic activity significantly. Introduction of a lower oxidation state cation (A^(n-1)+) onto a higher oxidation state cation (Bⁿ⁺) site generates acceptors (), which could be charge-compensated through formation of positive defects, such as ionized oxygen vacancies () or trapped holes leading to a higher valence state (e.g., B⁽ⁿ⁺¹⁾⁺). 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 evidenced by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy. Notably, this substitution does not lead to a linear increase in oxygen vacancy concentration (δ), in these oxygen sub-stoichiometric compositions, as quantified by thermogravimetric analysis (TGA) decomposition studies. We found that increased Ru oxidation state leads to a downshift in valence band center. XPS analysis was performed to quantitatively determine the optimal band center which is ~1.27 eV below the Fermi energy level for Y₂(Y_0.2Ru_0.8)₂O_6.6 based on the analysis of valence band edge of these Ru-based Y₂(Y_xRu_1-x)₂O_7-δ OER electrocatalysts. This work indicates that defect engineering is a practical approach to optimize oxidation state and electronic band center for high OER catalytic performance in a quantitative manner.