(Digital Presentation) Oxygen Evolution Reaction in Ru_1-XM_xO₂ (M = Ti, Zr, Nb, Ta, Cr) Doped Catalysts: Activity and Stability Effects

Abstract

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.