Host, Suppressor, and Promoter—The Roles of Ni and Fe on Oxygen Evolution Reaction Activity and Stability of NiFe Alloy Thin Films in Alkaline Media

Understanding the oxygen evolution reaction (OER) activity and stability of the NiFe-based materials is important for achieving low-cost and highly efficient electrocatalysts for practical water splitting. Here, we report the roles of Ni and Fe on the OER activity and stability of metallic NiFe and pure Ni thin films in alkaline media. Our results support that Ni(OH)2/NiOOH does not contribute to the OER directly, but it serves as an ideal host for Fe incorporation, which is essential for obtaining high OER activity. Furthermore, the availability of Fe in the electrolyte is found to be important and necessary for both NiFe and pure Ni thin films to maintain an enhanced OER performance, while the presence of Ni is detrimental to the OER kinetics. The impacts of Fe and Ni species present in KOH on the OER activity are consistent with the dissolution/re-deposition mechanism we proposed. Stability studies show that the OER activity will degrade under prolonged continuous operation. Satisfactory stability can, however, be achieved with intermittent OER operation, in which the electrocatalyst is cycled between degraded and recovered states. Accordingly, two important ranges, that is, the recovery range and the degradation range, are proposed. Compared to the intermittent OER operation, prolonged continuous OER operation (i.e., in the degradation range) generates a higher NiOOH content in the electrocatalyst, which is likely related to the OER deactivation. If the electrode works in the recovery range for a certain period, that is, at a sufficiently low reduction potential, where Ni3+ is reduced to Ni2+, the OER activity can be maintained and even improved if Fe is also present in the electrolyte.