Towards Identifying the Active Sites on Oriented Ruthenium Dioxide Surfaces in Catalyzing Oxygen Evolution
Authors: Rao Reshma R , Kolb Manuel J , Halck Niels , Hwang Jonathan , Pedersen Anders Filsøe , Mehta Apurva , You Hoydoo , Corchado-Garcia Juan , Hansen Heine A. , Feng Zhenxing , Zhou Hua , Rossmeisl Jan , Vegge Tejs , Chorkendorff Ib , Stephens Ifan , Shao-Horn Yang 
Published: February 2020

The unique interaction between water and rutile Ruthenium Dioxide (RuO2) affords high pseudocapacitance and catalytic activities for a number of reactions such as the oxygen evolution reaction (OER)1,2,3. While the low energy, RuO2 (110) and (100) surfaces have been studied as model systems for gas phase catalysis and ultra high vacuum surface science studies4,5, the nature of adsorbed species in aqueous solutions remains to be understood.
In this work, we examine the structural and chemical charge transfer processes occurring on oriented RuO2 single crystal surfaces as a function of potential, in acidic electrolyte, using in situ surface X-ray diffraction measurements. In order to validate experimentally observed changes in the nature of adsorbed oxygen, we use density functional theory to compute surface Pourbaix diagrams that show the most stable surface termination at any given potential. We start with the thermodynamically stable (110) surface, where we show that the coordinatively unsaturated sites (CUS) are the active sites for oxygen evolution. We also detect the formation of an –OO like group on the CUS site, which is the probable precursor of the evolved oxygen. We thus propose a four proton-electron mechanism for OER, where the final proton-electron removal is found to be rate limiting6. Using this evidence, we can rationally tune the oxygen evolution reaction (OER) activity of RuO2 surfaces by modifying the surface structure to increase the density of coordinatively unsaturated sites and optimize the binding strength of oxygenated species on the surface, in order to decrease the energetic barrier of the proposed rate-limiting step. We thus explore different orientations of RuO2, namely the (100), (101) and (001) terminations that have a higher CUS site density and lower oxygen binding energy compared to the (110) surface. Using in situ surface X-ray diffraction measurements we can detect the change in the nature of surface adsorbates just prior to OER for the different orientations. Probing the OER active state, supplemented by density functional theory calculations provides insight into the change of the rate limiting step and reaction mechanism for different orientations. Our work shows that modifying the surface orientation is an effective way to tune the active site density and energetics of key OER intermediates to obtain higher activities. Through this synergistic experimental and computational study, we provide an atomistic understanding of charge transfer processes prior to oxygen evolution and its implications on the oxygen evolution pathway on RuO2 for different surface orientations.

References

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[6] Rao RR, et al., Energy & Environmental Science, 2017, DOI: 10.1039/C7EE02307C (accepted)