Fundamental Studies of Planar Single-Crystalline Oxide Model Electrodes (RuO2, IrO2) for Acidic Water Splitting

Electrocatalytic water splitting allows storing excess energy from renewable energy sources such as wind and solar in chemical bonds of hydrogen molecules. So far, only polymer electrolyte membrane (PEM) water electrolyzers under acidic conditions can cope with the intermittent supply of energy, requiring, however, noble metals at the cathode for the hydrogen (HER) and precious metal oxides at the anode for the oxygen evolution reaction (OER). Prototypical oxides for the OER under acidic conditions are RuO2 and IrO2 that can meet the stringent requirements for long-term stability and high electrocatalytic activity. While RuO2 is substantially more active than IrO2, RuO2 is significantly less stable than IrO2 due to corrosion under OER conditions. Our microscopic understanding of the underlying processes of corrosion is, however, surprisingly poor, while the rate-determining steps in the OER over RuO2 and IrO2 are still under debate. In this perspective, I will be focusing on the discussion of the electrochemical degradation of ultrathin single-crystalline RuO2(110) and IrO2(110) films in the potential regions of both OER and HER. The single crystallinity of the model electrodes allows for identifying structure–property relationships and for a tight connection to theory. Since stability and activity in the OER are reported to be intimately coupled, I will also include a critical review of our present knowledge on the microscopic steps in the OER on RuO2 and IrO2, both from theory and experiments. Ultimately, from these atomic scale insights, the inherent material properties may be uncovered which underlie the observed electrochemical stability and activity of IrO2 and RuO2. It is the hope that these fundamental insights will aid the search for alternative more abundant electrode material systems for acidic water electrolyzers.