Surface orientation dependent electrochemical stability of RuO2 and IrO2 under acidic oxygen evolution reaction

Ru- and Ir-oxides have considerable attentions as acidic oxygen evolution electrocatalysts for polymer electrolyte membrane water electrolysis. However, there have been no reports of comparative study of electrochemical stability depending on the surface orientations. Here, we prepared the (hkl)-oriented (hkl = 110, 001, 100) RuO₂ and IrO₂ thin films on rutile-TiO₂ single crystal substrates by arc-plasma deposition and investigated the surface orientation dependence of electrochemical stability under oxygen evolution reaction (OER) in 0.1 M HClO₄. At the initial state, RuO₂ showed a lower OER overpotential and smaller Tafel slope compared to IrO₂ for all the surface orientations. However, the overpotential of RuO₂ steeply increased during chronopotentiometry (CP) measurements, while the overpotential increase for IrO₂ was significantly suppressed. After the CP measurement, the overpotential of IrO₂ became smaller than that of RuO₂ for all the surface orientation. RuO₂ shows orientation dependence on the stability number, i.e. ratio of total charges used for CP and dissolution amount of Ru or Ir, while the IrO₂ didn't show a clear trend. The results demonstrated that the surface orientation of both RuO₂ and IrO₂ affects not only the activity, but also the stability, and elemental dissolution and the influence is more significant for RuO₂.