The Role of Ru Redox in pH-Dependent Oxygen Evolution on Rutile Ruthenium Dioxide Surfaces

Rutile RuO2 is known to exhibit high catalytic activity for the oxygen evolution reaction (OER) and large pseudocapacitance associated with redox of surface Ru; however, the mechanistic link between these properties and the role of pH is yet to be understood. Here, we report that the OER activities of the (101), (001), and (111) RuO2 surfaces increased, whereas the potential of a pseudocapacitive feature just before OER shifted to lower potentials ("super-Nernstian'' shift) as pH increased on the reversible hydrogen electrode scale. This behavior contrasts with that of the (100) and (110) surfaces, which showed pH-independent Ru redox and OER activity. The link between catalytic and pseudocapacitive behavior illustrates the importance of this redox feature in generating active sites, thus building new mechanistic understanding of the OER.