Transformation of pseudocapacitance at rate-determining step reveals the onset potential for oxygen evolution reaction at the Pt surface

Onset potential is a measure of electrocatalytic activity and depends on the physicochemical processes occurring at the electrocatalyst/electrolyte interface. Conventionally, techniques like tangent intersection as the current transitions from non-Faradaic to Faradaic region are utilized for establishing onset potential. The electrochemical impedance spectroscopy (EIS) analysis can reveal physicochemical processes at the onset potential with the help of equivalent electrical circuits obtained from reaction mechanism analysis (RMA). The circuits estimated from the RMA are arranged into Ladder, Voigt, and Maxwell models. We demonstrate that in all these model circuits, the capacitance generated by the OH adsorption shows the maxima at the onset potential. The nature of adsorbing species is supported by Tafel slope analysis, wherein the low Tafel region, the Tafel slope of 122.5mV/dec indicates S-OH adsorption step as the rate determining step (RDS).