Abstract
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.
Highlights
The generation of hydrogen through water splitting is considered a promising route for storing energy from intermittent renewable sources such as solar, wind, and tidal energy.[1,2] Since one of the main energy losses in water splitting is the kinetic overpotential from the oxygen evolution reaction (OER),[3,4] OER electrocatalysts have been extensively investigated in order to improve the efficiency of water splitting
We have reported the effects of Fe on the surface structure evolution and the OER activity of metallic NiFe films
The analysis of the Fe and Ni species present in KOH shows that the availability of Fe in KOH is necessary for both pure Ni and NiFe films to obtain an enhanced OER activity, while the presence of Ni in the electrolyte is detrimental to the OER
Summary
The generation of hydrogen through water splitting is considered a promising route for storing energy from intermittent renewable sources such as solar, wind, and tidal energy.[1,2] Since one of the main energy losses in water splitting is the kinetic overpotential from the oxygen evolution reaction (OER),[3,4] OER electrocatalysts have been extensively investigated in order to improve the efficiency of water splitting Noble metal oxides, such as RuO2 and IrO2, have been proven to exhibit high catalytic activity toward the OER.[3,5] the scarcity and high cost of these noble metals largely hinder their large-scale application. The alloys have attracted attention because of the simplicity of their preparation by electrodeposition and strong adhesion to the substrate.[13,16] the as-prepared structure of NiFe alloys is not the active phase for OER catalysis, and an electrochemical activation step is usually required to convert the metallic/oxide phase to (oxy)hydroxide in order to achieve better OER activity.[10,16,17] Unlike NiFe (oxy)hydroxides, which have been thoroughly investigated as OER electrocatalysts, few studies have been
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