The complex reaction mechanisms and dissolution pathways that drive oxygen evolution reaction on metal and metal oxide surfaces under acidic conditions challenge the development of a highly active, highly stable, and low cost catalyst for proton-exchange membrane (PEM) water electrolyzers. Currently, ruthenium-based materials present lower overpotentials, lower cost and higher global supply compared to iridium-based materials, though they are also considerably less stable. In order to elucidate a way to improve RuO2 stability under the harsh conditions during water electrolysis, we use a density functional theory (DFT) approach to investigate the effects on catalyst structure, OER activity and stability of transition metal substitution within Ru1-xMxO2 (M = Ti, Zr, Nb, Ta, Cr) at different atomic concentrations. Calculations show that M substitution within rutile RuO2 affects the electronic structure resulting in regions of electron accumulation and depletion at the surface and shifts the Ru d-band and O2p band centers, which are highly dependent on dopant characteristics and doped site. Moreover, dissolution calculations on the Ru1-xTixO2 surfaces show that Ti substitution alter the metal dissolution pathway energetics and thermodynamics bringing stability to the catalyst in terms of reducing material loss. Theoretical XRD patterns, Ru d-band and O p-band center calculations, and activation and reaction energy trends are in excellent agreement with experimental results that includes X-ray diffraction analysis, high resolution transmission electron microscopy images, X-ray photoelectron spectroscopy of core and valence bands, and rotating disk electrode measurements. In this presentation, we focus on the theoretical and computational aspects. The evaluation of the thermodynamics and kinetics of the water splitting, and oxygen evolution mechanism is done with spin-polarized plane-wave DFT calculations, while Ab Initio Molecular Dynamics (AIMD) simulations allow following and complementing the understanding of the dynamics of the initial steps of water dissociation on the various M and Ru sites. Electronic structure changes on the surface due to the presence of M before and during the reaction are analyzed based on the density of states and local magnetic moments, and the activation energies are obtained from the climbing Nudge Elastic Band method. Dissolution calculations are carried out with constrained-AIMD simulations using the slow-growth approach within the “Bluemoon ensemble” as implemented in VASP.

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.