Synergistic Mo and W Single Atoms Co-Doped Surface Hydroxylated NiFe Oxide as Bifunctional Electrocatalysts for Overall Water Splitting

Two novel defect engineering strategies, surface hydroxylation and single atom (SA) doping, are explored to fabricate a high performance bifunctional water electrolysis catalyst, synergistic Mo and W SAs co-doped surface hydroxylated NiFe oxide (FN-MoWact). The product catalyst achieves ultralow overpotentials of 196 and 246 mV for oxygen evolution reaction (OER) and decent overpotentials of 79 and 246 mV for hydrogen evolution reaction (HER) at current densities of 10 and 500 mA cm-2, respectively. For overall water splitting, the FN-MoWact//FN-MoWact couple requires ultralow cell voltages of 1.504 and 1.729 V to deliver current densities of 10 and 500 mA cm-2, respectively, and remains stable after a 100-hour operation at an initial current density of 534 mA cm-2. The enhanced electrocatalytic activities are attributed to surface hydroxylation and doping of synergistic Mo and W SAs. The two defect engineering modifications, as supported by analyses of density functional theory calculations and in-situ X-ray absorption spectroscopy, lead to a proper reduction in metal-oxygen covalency, thus minimizing the energy difference of the rate-determining step of the OER process, and create surface active sites with a d-band center located closer to the Fermi level, thus properly strengthening proton binding to enhance HER activities.