Elucidating the true catalytic nature of first-row transition metal coordination compounds under oxygen-evolution conditions

Abstract

The oxygen-evolution reaction (OER) is the kinetic bottleneck of electrochemical water splitting and, therefore, a central challenge in the development of sustainable hydrogen production technologies. First-row transition metal coordination compounds have attracted considerable attention as potentially low-cost, earth-abundant alternatives to noble-metal-based OER catalysts, owing to their structural tunability and well-defined molecular architectures. However, growing evidence shows that under the highly oxidizing conditions required for OER, many of these coordination compounds do not remain intact. Instead, ligand oxidation, ligand dissociation, hydrolysis, demetalation, and metal-ion redeposition frequently lead to the in situ formation of heterogeneous metal (hydr)oxide or oxyhydroxide phases that serve as candidates for catalytic activity. In this perspective, representative examples of manganese, iron, cobalt, nickel, and copper coordination compounds, as well as metal-organic frameworks and single-atom systems, are presented to clarify the mechanistic pathways that govern catalyst reconstruction under OER conditions. The critical roles of trace metal impurities, electrolyte composition, and transient nanoscale species in shaping the observed electrocatalytic response are discussed. Further, the thermodynamic and kinetic origins of ligand instability are discussed, and strategies that have been explored to enhance catalyst robustness are summarized. Finally, a rigorous analytical framework combining extended electrochemical testing, rinse-and-replace experiments, chelation protocols, microscopic and spectroscopic surface characterization, and operando mass and dissolution tracking was presented to determine the true identity of the catalyst.