A Non-Rate-Determining Redox Process Dictates the Oxygen Evolution Tafel Slope of MnO2

Electrocatalytic activity is markedly influenced by the Tafel slope, which dictates the sensitivity of the catalytic current with respect to the potential. Differences in the Tafel slope between electrocatalysts have been rationalized based on redox pre-equilibria preceding the rate-determining step. However, no study has experimentally observed a non-rate-determining redox event that directly correlates with the Tafel slope. Here, we show that the Tafel slope of the oxygen evolution reaction (OER) on alpha-MnO2 is markedly influenced by a Mn(III)/Mn(II) redox process located 500 mV more negative than the OER onset potential. Upon repeated cyclic voltammetry sweeps, a correlation was observed between the Tafel slope and the peak position of a redox event, which was assigned to a Mn(III)/Mn(II) redox couple based on the potential dependence of the UV-vis spectra. Notably, the UV-vis absorption of Mn(III) did not diminish even after the OER was initiated, and another redox process was observed at the OER onset, indicating that Mn(III)/Mn(II) is not the rate-determining step for OER on alpha-MnO2 in alkaline pH. Numerical simulations using physicochemical parameters of Mn(III)/Mn(II) obtained from trumpet plot analyses reproduce the experimental Tafel plots, indicating that even non-rate-determining steps should be considered during catalyst design.