Effects of Titanium Substitution within Ruthenium Oxide on Structure, Oxygen Evolution Activity and Stability

Abstract

The development of acidic oxygen evolution reaction (OER) electrocatalysts with high activity, extended durability, and lower costs furthers the development and utilization of proton-exchange membrane (PEM) water electrolyzers. Although iridium-based catalysts are currently the primary OER catalysts used within PEM electrolyzers, ruthenium has a higher activity, lower cost, and higher global supply compared to iridium; however, ruthenium-based catalysts generally have lower stability compared to iridium-based catalysts. Within acidic OER catalysts, stability remains a significant challenge for non-iridium materials, particularly under the highly corrosive environment of highly acidic conditions and high anodic potentials encountered during PEM water electrolysis. As an approach to obtain high OER activity and improved stability, we investigated the effect of titanium substitution within rutile ruthenium oxide as a model system of oxide structure that combines a highly OER active, unstable element (Ru) with a highly stable but OER-inactive element (Ti). Titanium-substituted ruthenium oxides, Ru1-xTixO2, at different Ti substitution ratios were synthesized via wet-chemistry and subsequent thermal treatments. X-ray diffraction analysis and high resolution transmission electron microscopy images support Ti is substituted within the RuO2 structure. X-ray photoelectron spectroscopy of core and valence bands shows Ti substitution alters the surface electronic structure. From rotating disk electrode measurements, Ti substitution lowers the OER mass activity, OER specific activity, and electrochemical surface area. In addition to affecting OER activity, Ti substitution increases the OER stability and lowers Ru dissolution. Density functional theory (DFT) calculations of the titanium-substituted ruthenium oxides show that effects of Ti substitution on the reaction energies and activation energies are highly dependent on the site. Theoretical analysis supports that specific sites may predominately act as catalytic sites for the OER, while other sites influence metal dissolution. Further understanding how the structure of bimetallic oxides influences OER activity and stability provides a pathway to electrocatalysts with higher activity, improved stability, and lower cost.