Rutile RuO2 has remarkable activity and low overpotential for the oxygen evolution reaction (OER) and is considered a benchmark catalyst for industrial electrolysis and supercapacitors. Understanding the interaction between water and RuO2 surfaces is crucial to control the catalytic activity for OER and realize its large pseudocapacitance in the electrochemically stable potential window of water. However, there is a lack of fundamental understanding of the active sites and atomic scale processes occurring at the oxide-electrolyte interface upon interaction with water since previous work has been confined to the ultra high vacuum environment, and to the thermodynamically stable (110) surface. In this study, we begin by investigating the interaction of the (110) surface with water under applied potential in acidic solution using in situ surface diffraction crystal truncation rod (CTR) analysis and density functional theory (DFT) calculations. Increasing the potential from 0.5 VRHE to 1.0 VRHE and 1.2 VRHE leads to the adsorption and successive deprotonation of water on the coordinatively unsaturated sites (CUS). Finally at OER relevant potentials, an –OO group was detected, which was found to be stabilized by a neighboring –OH group on an adjacent site, suggesting that the final deprotonation step was rate limiting. Having identified the CUS sites as the active sites for OER and establishing that the (110) surface binds oxygen more strongly than optimal, we identify alternative crystallographic facets such as the (100) and (101) with higher CUS site density and weaker oxygen binding to improve OER kinetics. Going beyond tuning the electrode surface, we also consider the role of cations at the interface in stabilizing and/or destabilizing OER relevant intermediates via non-covalent interactions. The degree of order at the interface under applied potential can be measured by normalized surface diffraction spectra and provides the first evidence of how the degree of order at the interface varies with the nature of ions at an electrified oxide interface. The degree of ordering increases with increasing ionic size from Li+ to K+ and then decreases with further increase in size of ions. Thus, by using different experimental and theoretical techniques on model surfaces, this study identifies the active sites for oxygen electrocatalysis and demonstrates how catalyst surface structure and interfacial water structure can be altered to improve OER kinetics.

Towards understanding the electrified RuO2 water interface for the oxygen evolution reaction

2019 Spring National Meeting

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