It is particularly challenging to catalyse oxygen evolution under the acidic conditions of polymer electrolyte membrane (PEM) electrolysers. All compounds, apart from IrO_x and RuO_x, are catalytically inactive or unstable. I will present a series of model studies where we aim to elucidate the factors that control oxygen evolution.

Using thin films of RuO₂, we showed that continuous mass losses occur in parallel to oxygen evolution, even when the electrochemical response is stable.¹ This finding demonstrates that short term electrochemical measurements are an insufficient measure of long term stability. Our experiments on mass-selected Ru and RuO_x particles revealed the high sensitivity of the oxygen evolution performance of nanoparticulate catalysts to the exact oxidation treatment.^2,3 X-ray absorption measurements, taken in-operando, provide spectroscopic evidence for the potential dependent coverage of the reaction intermediates. Moreover, tests on oriented thin films indicate that the “coordinatively unsaturated” surface site is responsible for the activity of RuO₂.⁴ Our very recent studies suggest that the dissolution rate of RuO₂ is also highly dependent on the orientation.

The scarcity of RuOx and IrOx could limit the scalability of PEM electrolysers. An alternative strategy could be utilise non-precious metal oxides to catalyse oxygen evolution in acid. However, most non-precious metal oxides exhibit poor stability. Theoretical calculations predicted that MnO_x could be stabilised against corrosion by the presence of TiO_x at its surface, without affecting the catalytic activity. Our experiments on sputter-deposited thin films verify this notion. ⁵

The catalysts under investigation include model-size selected nanoparticles, commercial high surface area catalysts, sputtered films and oriented thin films. We have probed these surfaces with electrochemical measurements, ultra-high vacuum based surface science methods, electron microscopy, synchrotron-based spectroscopy and density functional theory calculations.