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Surface interrogation scanning electrochemical microscopy (SI‐SECM) of two electrodeposited manganese‐based electrocatalysts, amorphous MnO_xand perovskite CaMnO₃, was used to investigate the manganese oxidation state relating to the oxygen evolution reaction (OER) under neutral conditions. The results indicate the amounts of Mn^IIIand Mn^IVspecies in MnO_xand CaMnO₃depend on potential. A Mn^Vspecies was identified in both structures during the OER. Time‐delay titration of Mn^Vfurther revealed that MnO_xproduced two types of active sites with different OER reaction rates:k′_fast(MnO_x)=1.21 s⁻¹andk’_slow(MnO_x)=0.24 s⁻¹. In contrast, CaMnO₃perovskites in which the Mn^Vspecies formed at a less positive potential than that in MnO_x, displayed only one kinetic behavior with a faster reaction rate of 1.72 s⁻¹.

We report an electrodeposition protocol for preparing isolated cobalt oxide single molecules (Co₁O_x) and clusters (Co_nO_y) on a carbon fiber nanoelectrode. The as-prepared deposits are able to produce well-defined steady-state voltammograms for the oxygen evolution reaction (OER) in alkaline media, where the equivalent radius (r_d) is estimated by the limiting current of hydroxide oxidation in accordance with the electrocatalytic amplification model. The size of isolated clusters obtained from the femtomolar Co²⁺solution through an atom-by-atom technique can reach as small as 0.21 nm (r_d) which is approximately the length of Co–O bond in cobalt oxide. Therefore, the deposit was close to that of a Co₁O_xsingle molecule with only one cobalt ion, the minimum unit of the cobalt-based oxygen-evolving catalyst. Additionally, the size-dependent catalysis of the OER on Co_nO_ydeposits shows a faster relative rate on the smaller cluster in terms of the potential at a given current density, implying the single molecular catalyst shows a superior OER activity.