Origin of potential and pH dependence of electrocatalytic rates

Electrochemistry is central to the global transition towards CO2 neutrality, e.g. for the production of green hydrogen. In electrocatalysis, a steep current rise with increasing electrode potential promotes high energy efficiency, but the factors governing the potential dependence of catalytic rates are insufficiently understood. Here, we show how both the electrode potential and pH dependence arise from the formation of interacting oxidized sites within the bulk-active catalyst material and outline a conceptual framework for understanding the steepness of the current-potential relationship that contrasts with classical paradigms. Two prominent electrocatalysts of the oxygen evolution reaction, oxide materials based on cobalt and nickel in synergistic cooperation with varying amounts of iron, were studied by an unconventional ensemble of experiments combining electrochemistry with X-ray absorption spectroscopy during electrocatalytic operation. We find that both electrode potential and pH control the catalytic rate indirectly, first through non-Nernstian oxidation of the catalyst material, and second through the energetic interactions between oxidized sites, resulting in catalytic currents that depend exponentially on the concentration of oxidized sites. Thus, the slope of the Tafel is not determined by the specifics of the reaction mechanisms, as was often assumed in the past, but by the energetic interactions between oxidized sites. The optimization of the interaction energies between oxidized catalyst sites could be targeted in the development of future electrocatalyst materials with high energy efficiency.