Journal Article

Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions

Machala, Michael L., Mueller, David N., Chueh, William C., Bluhm, Hendrik

2015

Overview

Surface redox-active centres in transition-metal oxides play a key role in determining the efficacy of electrocatalysts. The extreme sensitivity of surface redox states to temperatures, to gas pressures and to electrochemical reaction conditions renders them difficult to investigate by conventional surface-science techniques. Here we report the direct observation of surface redox processes by surface-sensitive, operando X-ray absorption spectroscopy using thin-film iron and cobalt perovskite oxides as model electrodes for elevated-temperature oxygen incorporation and evolution reactions. In contrast to the conventional view that the transition metal cations are the dominant redox-active centres, we find that the oxygen anions near the surface are a significant redox partner to molecular oxygen due to the strong hybridization between oxygen 2 p and transition metal 3 d electronic states. We propose that a narrow electronic state of significant oxygen 2 p character near the Fermi level exchanges electrons with the oxygen adsorbates. This result highlights the importance of surface anion-redox chemistry in oxygen-deficient transition-metal oxides. Surface redox centres in metal oxides play a key role in catalytic performance, and the conventional view is that the transition-metal cations dominate this behaviour. Here, the authors perform an in operando spectroscopic study, and find that oxygen anions are a significant redox partner to molecular oxygen.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 140/146 / 639/301/1034/1038 / 639/638/77/886 / Absorption spectroscopy / Anions / Cations / Cobalt / Electrochemistry / Humanities and Social Sciences / Metals / multidisciplinary / Oxides / Oxygen / Science / Science (multidisciplinary) / Thin films