Charge-Transfer Effects in Ni–Fe and Ni–Fe–Co Mixed-Metal Oxides for the Alkaline Oxygen Evolution Reaction

Abstract

Ni–Fe and Ni–Fe–Co mixed-metal oxide (MMO) films were investigated as electrocatalysts for the oxygen evolution reaction (OER) in 0.1 M KOH. In an effort to optimize MMO morphology, aniline was used as a capping agent to produce high-surface-area Ni–Fe–Co films on Raney nickel supports. This catalyst exhibits enhanced mass activity in comparison to the Ni–Fe OER electrocatalysts reported to date. Cyclic voltammetry shows changes in the potential of the Ni2+/3+ transitions in Fe- or Co-containing MMO films. In situ X-ray absorption spectroscopy (XAS) analysis confirms that Fe acts to stabilize Ni in the 2+ oxidation state, while Co facilitates oxidation to the 3+ state. The results of this study support the recent claims that Fe (not Ni) is the OER active site. The OER enhancement of the ternary Ni–Fe–Co catalyst results from two effects: (1) the charge-transfer effects of Co result in the formation of the conductive NiIIIOOH phase at lower overpotential, thus activating the Fe sites which are otherwise inaccessible to electron transfer in the nonconductive NiII(OH)2 host lattice, and (2) XAS analysis shows that the presence of Co effectively “shrinks” the Ni and Fe local geometry, likely resulting in an optimized Fe–OH/OOH bond strength. In addition, analysis of heat-treatment effects indicates that calcination at 400 °C improves the OER activity of Ni–Fe–Co but deactivates Ni–Fe. Annealing studies under argon show that MMO surfaces with a hydrated Ni­(OH)2 phase and a crystalline NiO phase exhibit nearly identical OER activities. Finally, the morphology of the MMO catalyst film on Raney Ni support provides excellent catalyst dispersion and should result in high active-site utilization for use in technologically relevant gas-diffusion electrodes.

Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films

“…S5). Additionally, the edge shift upon Fe incorporation is less than that previously observed (40,49) with the caveats that the results of Fig. 4 have been obtained under OER conditions with less Fe incorporation.…”

NiFe‐Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non‐Acidic Electrolytes

“…[77] Consistently, it has been shown that Fe ions in solution easily incorporate in Ni hydroxides compounds, and therefore NiFe-based oxyhydroxide can be prepared by electrochemical cycling or aging of Ni hydroxide electrode in iron containing electrolyte. [80] The supported catalyst was prepared by impregnation method involving chemical reduction to a mixed metal alloy obtained by NaBH 4 addition and in presence of aniline as capping agent to control the morphology. [78] Another recently reported method consists in pulsed laser ablation in liquids.…”

“…[25,31,45] Completely different is the route proposed in a previous study by Demourgues-Guerlou et al who reported a chimie douche method to obtain NiFe LDH by first preparation of iron substituted sodium nickelate, followed by oxidizing hydrolysis and a final chemical reduction. [80] Similarly, metallic film obtained by sputter deposition can also be transformed in NiFe hydroxides by electrochemical conditioning in 0.1 m KOH. [79] In this method only one of the metal ion precursors is dispersed in the liquid while the second metal is introduced in suspensions as metal powder.…”

Nanostructured catalysts for electrochemical water splitting: current state and prospects

“…As demonstrated by Lu et al 213 , amorphous Ni-Fe hydroxide nanosheets prepared by potentiostatic method had high surface area and fast electron transport. Mukerjee et al 216 considered that the charge transfer of Co cations could influence the formation of conductive NiOOH phase, and NiOOH phase was prerequisite for activating Fe sites; on the other hand, the existence of Co may adjust the Ni and Fe local geometry, thus it optimized the bond strength of Fe−OH/OOH. Nickel oxides and oxyhydroxide as other popular hosts of foreign iron dopants had attracted considerable attention.…”