p-Block Metal Oxide Noninnocence in the Oxygen Evolution Reaction in Acid: The Case of Bismuth Oxide

The Pourbaix diagrams of p-block Pb, Sb, and Bi establish a robust stability for their oxides in acidic solutions. Such oxides have found utility as stable frameworks to support metals that are active for oxygen evolution reaction (OER) catalysis, but they also possess two-electron redox couples, which can potentially engender OER activity. Thus, the use of p-block oxide supports provides an imperative for understanding the OER activity of the unary oxides. Toward this end, we report BiOx films that are able to perform OER catalysis at moderate overpotentials for extended periods of operations (>110 h) in highly acidic solutions (pH 1.0–2.25) with no sign of decreased OER activity during operations at current densities of 1–5 mA cm–2. X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and UV–vis spectroelectrochemistry reveal a change in the Bi oxidation state prior to OER catalysis, from predominantly Bi3+ in the as-deposited films to primarily Bi5+ in the catalytically active films. This transformation is accompanied by a prominent color change from orange to dark brown. Simulations of the cyclic voltammogram catalytic profiles suggest two OER pathways for BiOx involving BiIVOx and BiVOx catalysts, with the BiVOx pathway prevailing after catalyst activation. Together, these results demonstrate the ability of BiOx to facilitate OER in acid with high functional stability and underscore the noninnocent role that p-block metal oxides may play in OER catalysis in acidic media.