(Invited) Electrochemical Reaction Induced Amorphization to Complex Oxide Surface and Its Impact to Catalyst Activity for Oxygen Evolution Reaction

Water splitting and artificial photosynthesis systems are proposed as promising approaches to the conversion and storage of solar energy into chemical fuels.  However, their efficiency is often limited by the sluggish kinetics of oxygen evolution reaction (OER). Although IrO₂ and RuO₂ can effectively reduce activation barriers for OER, the scarcity of the materials makes it difficult to use them as OER catalysts for large-scale implementation.  Recently there have been a growing interest to investigate complex oxides as alternative OER catalysts and several new catalysts showed activity better or comparable to IrO₂ benchmark.  Along with these efforts, experimental evidences also indicated many of complex oxides were not stable under reaction conditions, of which amorphization was often observed at catalyst/electrolyte interface after cyclic voltammetry (CV) or potentiostatic measurements [1-3].  Our recent results showed that such type of surface amorphization processes seemed to affect catalyst performance in quite different ways dependent on catalyst material, electrolyte pH and compositions.  For instances, in alkaline electrolyte (0.1 M NaOH), the activity of cobalt tungstate reduced after multiple CVs, while copper tungstate, on the other hand, became more active.  Under similar conditions, another lithium-containing OER catalyst, Li₂CoGeO₄, maintained majority of its original activity even after significant loss of lithium and germanium from its surface.  In this study, the surface amorphization process is examined by multiple characterization techniques including HR-TEM, XPS, electrochemical and compositional analyses.  Possible mechanisms leading to these distinctive behaviors will be discussed in detail.

(1)  S. W. Lee, et al., JACS, 2012, 134, 16959-16962

(2)  M. Risch, et al., JPCC, 2013, 117, 8628-8635

(3)  K. J. McDonald, et al., J. Mater. Chem. A, 2014, 2, 18428-18434