Interfacial and Kinetic Limitations in Alkaline OER: A Case Study of Electronic Modifications Using Fe in CuO Lattice.

Identifying the dominant kinetic bottlenecks in the oxygen evolution reaction (OER) is essential for the rational design of efficient electrocatalysts. Chemical modifications can alter the electronic charge distribution, thereby altering interfacial and surface reaction kinetics. CuO, a semiconducting oxide with localized charge transport and moderate conductivity, serves as an ideal platform for investigating kinetic processes. Fe doping can effectively tune the electronic structure and interfacial charge-transfer kinetics while largely preserving the intrinsic CuO crystal lattice. Density Functional Theory (DFT) calculations reveal that Fe incorporation introduces electronic states near the Fermi level, lowering the work function and facilitating charge transfer, while simultaneously stabilizing oxygenated intermediates at higher Fe concentrations. A promoter-inhibitor crossover, linking electronic structure to kinetic evolution, is analyzed from competing processes identified from a combined investigation using Electrochemical Impedance Spectroscopy (EIS) and Distribution of Relaxation Times (DRT) analysis. Three distinct processes associated with interfacial charge transfer and surface-reaction kinetics are detected. Moderate Fe substitution reduces interfacial resistance and lowers the OER overpotential, whereas higher Fe content increases kinetic resistance, indicating a transition toward surface-reaction-limited behavior. Hence, from DFT, EIS, and DRT, this work provides a mechanistic framework for disentangling interfacial and surface-reaction contributions in semiconducting electrocatalysts.