Understanding the Active Sites and Reaction Mechanism for Water Oxidation on Ruthenium Dioxide

Abstract

Rutile Ruthenium Dioxide (RuO2) is a gold standard catalyst for water splitting in acidic solutions1-3. It can also undergo fast surface redox reactions in the electrochemically stable potential window of water, making it an ideal material for electrochemical capacitors that can charge and discharge in a much shorter time scale than batteries. 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 as previous work has been confined to the ultra high vacuum environment, and to the thermodynamically stable (110) surface4. In this work, we use three different facets of RuO2, (110), (100) and (101) to demonstrate how changing the local environment of surface Ru and O atoms can enable tuning the electronic structure of the active site and alter its intrinsic oxygen evolution reaction (OER) activity. Using synchrotron based ambient pressure X-ray photoelectron spectroscopy and in situ surface diffraction on single crystal surfaces coupled with density functional theory calculations, we show that at 1.0 VRHE, water adsorbs molecularly or dissociatively on the coordinatively unsaturated site (CUS), depending on the surface orientation. This is followed by successive deprotonation occuring at potentials greater than 1.0 VRHE . At oxygen evolution potentials (1.5 VRHE), surface diffraction measurements reveal the presence of an –OO species on the CUS site for the (110) and (100) surface. Computation results show that this –OO group is stabilized by a neighboring –OH group suggesting that the final deprotonation step is rate-limiting. On the other hand, for the (101) surface that binds oxygen more weakly, a completely oxidized surface has been detected, implying a change in the rate-limiting step to the dissociation and deprotonation of the second water molecule on the oxidized surface. The mechanistic insight gained from in situ methods will be used to discuss the difference in OER activity and Tafel slope observed on the different RuO2 facets. Finally, the results obtained using single crystals will be extended to understanding the distinct redox peaks of RuO2 nanoparticles in the electrochemically stable potential window of water as well as the nature of intermediates prior to the evolution of oxygen. 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 can be altered to improve OER kinetics.