Many fuel cells rely on 4H+/4e− reduction of O2 to H2O, the individual steps of which may involve proton-coupled electron transfer (PCET). For example, O2 can bind a metal catalyst and be converted into a surface-bound *OOH on acceptance of H+ and an e−. When PCET is operative, a plot of catalytic activity versus substrate binding strength will be volcano-shaped and have a peak at the optimal binding strength. If the catalyst binds *OOH or *OH more weakly than the optimum, then these intermediates may form too slowly. Conversely, if binding is too strong then subsequent reactions become too slow. This give-and-take forms the basis of the Sabatier principle, and, with a volcano peak in sight, one can design an active catalyst by adjusting the composition of an alloy to arrive at the sweet spot for binding strength. The volcano is already mapped out for O2 reduction mediated by Pt alloys in acid, but the reactions in alkaline waters are less well chartered. Reporting in Angewandte Chemie International Edition, a team led by Maria Escudero-Escribano and Ifan Stephens found that such a volcano does indeed exist in basic conditions.
With Pt(111)–Cu alloys of varying composition in hand, the team had materials that could bind *OH with a variety of strengths. To identify the optimal strength, the team performed voltammetry on each alloy-covered electrode in the presence of O2, whose transport to the electrode was ensured by rotating the latter at 1600 rpm. Comparing the data for the alloys at 0.9 V versus the reversible hydrogen electrode, the alloy with 0.77 monolayer Cu emerged as the best catalyst, affording a catalytic current density of ∼100 mA cm−2. This catalyst binds *OH (0.1 eV) more weakly than Pt(111) does, a shift identical to that observed at pH 1. “If you assume all the charge transfer steps in the reaction involve PCET, then the thermodynamics of each step should be pH independent,” explain the authors. Thus, PCET is operative not only at low pH but also at high pH, dispelling any thought of a mechanism involving proton-decoupled electron transfer and intermediates such as O2−.
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