Proton-exchange membrane fuel cells demand efficient electrode–electrolyte interfaces to catalyse the oxygen reduction reaction (ORR), the kinetics of which depends on the energetics of surface adsorption and on electrolyte environment. Here we show an unanticipated effect of non-specifically adsorbed anions on the ORR kinetics on a Pt(111) electrode; these trends do not follow the usual ORR descriptor, that is *OH binding energy. We propose a voltammetry-accessible descriptor, namely reversibility of the *O ↔ *OH transition. This descriptor tracks the dependence of ORR rates on electrolyte, including the concentration/identity of anions in acidic media, cations in alkaline media and the effect of ionomers. We propose a model that relates the ORR rate on Pt(111) to the rate of the *O to *OH transition, in addition to the thermodynamic *OH binding energy descriptor. Our model also rationalizes different trends for the ORR rate on stepped Pt surfaces in acidic versus alkaline media.

质子交换膜燃料电池需要有效的电极-电解质界面来催化氧还原反应（ORR），其动力学取决于表面吸附的能量和电解质环境。在这里，我们展示了非特异性吸附阴离子对 Pt(111) 电极上 ORR 动力学的意外影响；这些趋势不遵循通常的 ORR 描述符，即 *OH 结合能。我们提出了一个伏安法可访问的描述符，即*O ↔ *OH 跃迁的可逆性。该描述符跟踪 ORR 率对电解质的依赖性，包括酸性介质中阴离子的浓度/特性、碱性介质中的阳离子以及离聚物的影响。我们提出了一个模型，将 Pt(111) 上的 ORR 率与 *O 到 *OH 转换的速率联系起来，除了热力学*OH结合能描述符。我们的模型还合理化了酸性与碱性介质中阶梯 Pt 表面上 ORR 率的不同趋势。