Oxygen evolution on Fe-doped NiO electrocatalysts deposited via microplasma
The oxygen evolution reaction (OER) in alkaline media was investigated on nanostructured Fe2O3, NiO, and Ni1−xFexO (Fe-doped, rocksalt NiO, x = 0.05–0.19) electrocatalysts deposited via microplasma on indium tin oxide. A detailed investigation of film morphology, structure, and chemical surface state using SEM, XRD, and XPS, respectively, was carried out to understand catalytic activity, which was assessed using cyclic voltammetry and chronopotentiometry. Iron was seen to be fully incorporated into the parent rocksalt NiO lattice during microplasma deposition, and overpotentials (η) decreased from 360 mV for NiO to 310 mV for Ni1−xFexO at 10 mA cm−2. Interestingly, overpotential did not change significantly for Fe compositions from 5–19%. The Ni1−xFexO films displayed relatively low Tafel slopes of 20–30 mV dec−1 at 0.01–1 mA cm−2, demonstrating their high activity for (OER). Turn-over-frequency (TOF, i.e., O2 molecules per Ni atom per s) at η = 350 mV revealed a continuous improvement in activity of the NiO surface with increasing Fe content, where values of 0.07 and 0.48 s−1 were measured for undoped NiO and Ni0.81Fe0.19O films, respectively. Chronopotentiometry measurements followed by SEM and XPS verified that the as-deposited Ni1−xFexO catalysts were mechanically and chemically stable for OER under alkaline conditions. This work highlights that microplasma-based deposition is a general approach to realize conformal coatings of nanostructured, doped oxides with high activity for OER.