Unraveling the Formation Kinetics of the First Intermediate in the Oxygen Evolution Reaction on MnO_x with Different...

In the electrocatalytic oxygen evolution reaction (OER), it has been a long-standing issue to establish the relationship between intermediate kinetics, catalyst structures, and OER activity, which is challenging due to the difficulty in following the intermediate kinetics and charge dynamics simultaneously. Here, using home-built operando electrochemical transient absorption (EC-TA) spectroscopy, we succeeded in resolving the kinetics of the first intermediate (*OH species) in the OER catalytic cycle on MnO_x-based electrocatalysts, which is consistent with microkinetics simulation. In the formation process of *OH species, proton transfer is slower than electron transfer, resulting in the continuous formation of *OH species after the interruption of the potential pulse. Based on a comparison of *OH formation rates in four MnO_x-based catalysts with different electron configurations, the formation rates of *OH species are highly dependent on the coordination symmetry of the corner-shared Mn³⁺O₆ octahedron in the catalyst. The formation rate of *OH species in a catalyst with a corner-shared Mn³⁺O₆ octahedron of D_3d symmetry exhibits a ∼100-fold (2.33 × 10³ s^-1) increase compared with that in a catalyst with a corner-shared Mn³⁺O₆ octahedron of D_4h symmetry (2.45 × 10¹ s^-1). More importantly, the formation rates of *OH species show a positive correlation with the reaction rates of the rate-determining step (RDS) in the OER catalytic cycle. The insight into the formation kinetics of the first OER intermediate in the OER processes indicates the crucial role of the initial rate in the OER catalytic cycle and sheds light on the OER kinetic mechanism on electrocatalysts.