We demonstrated the activation energy approach to evaluate oxygen evolution performance and to probe active site reconstruction at different potentials. Activation energies are acquired following the Arrhenius equation by monitoring the current densities under varied hydroxide concentrations and temperatures. Our complex oxide electrocatalysts (Ag-and Ce-bidoped iron manganese oxyhydroxide) exhibit a much smaller activation energy of 19.12 kJ mol −1 in comparison to FeMnOH (60.01 kJ mol −1 ) at 1.7 V. The higher numbers of doped metal cations show smaller activation energy values corresponding to the higher activities, yet site reconstruction is less likely to occur. Through operando Raman studies, site reconstruction may not be absolutely required to reach a high-performance OER, in contrast to the general recognition. By knowing the potential-dependent activation energy, a quantitative OER activity comparison among reconstructed sites is possible. A substrate with a low background current is useful to experimentally acquire iR-corrected activation energies over a wide potential window.

The microscopic homogeneity of mixed metals in a single-phase oxide is a critical issue in improving material performance. Aqueous alkaline precipitation is the most common approach but it has the limits of microscopic inhomogeneity because of intrinsically different precipitation rates between metal cations. Herein, we demonstrate a new preparation of uniformly structural substituted cobalt iron oxides via acidic redox-assisted precipitation (ARP) upon the interaction of CoII and K2FeO4. This low-pH synthesis features the redox process between Co and Fe, presumably through the formation of inner-sphere complexes such as [(H2O)5CoII–O–FeVIO3]. With the nucleation starting from such complexes, one obtains a product with predominantly mixed-metal Co–O–Fe moieties, which improves the electrical conductivity of the product. This work further analyzes how the properties of the product species evolve during the hydrothermal synthesis step in the ARP process. We see that the Co/Fe ratio slowly increases from about 1:1 to a final value of 2:1, but does not reach the expected redox stoichiometry of 3:1. At the same time, the magnetization also increases, reaching a value of 16.9 emu g–1 for the final superparamagnetic product, which is three times higher than the value of monometallic Co3O4 and Fe2O3. The cobalt iron oxide samples obtained from ARP also possess superior oxygen evolution activity (307 mV overpotential at 10 mA cm–2 μg–1) compared to a mixture of Co3O4 and Fe2O3 (422 mV) or pure cobalt oxide (350 mV), highlighting the structure-induced enhancement of the catalytic activity. The difficult synthesis of evenly blended trinary/quaternary metals in a single-oxide phase may become possible in the future via ARP.