Electrochemically Engineered Layer of γ–NiOOH with Fe Oxides on Nickel Foam for Durable Oxygen Evolution Reaction Catalyst for Alkaline Water Electrolysis
Authors: Narayanaru Sreekanth , Anilkumar Gopinathan M , Kuroki Hidenori , Tamaki Takanori , Yamaguchi Takeo 
Published: December 2024

Anion Exchange membrane water electrolyzers are reached the stage of commercialization by the advancement of durable anion exchange membranes. Highly active and stable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential to achieve energy efficient and cost-effective hydrogen production. Among the two electrochemical reactions, OER is more sluggish as it involves four electron transfers for the generation of an oxygen molecule.[
1
] Many non-noble metal oxides-based catalysts are displayed OER activity in alkaline medium, but among them Ni, Fe, and Co based mono/bi/tri metallic oxides/hydroxide/oxyhydroxides exhibit high OER activity.[
2–4
] Even though the actual catalytic mechanism is still debated, Ni-Fe oxides have received much attention over all other catalysts because of the synergism between the oxides and oxyhydroxides of Fe and Ni to bring down the OER overpotential.
Several methods were adopted for the synthesis of Ni-Fe oxides. However, most of them lead to the formation of mixed oxides and it affects the overall conductivity of the catalyst. The controlled electrochemical method of preparation possesses, the selective synthesis of conducting Ni-Fe oxides. In this work, we adapted a two-step electrochemical synthesis method to produce free-standing Ni-Fe oxide electrode with high OER activity. At first, a layer of γ–NiOOH was constructed over a cleaned nickel foam by applying a high voltage in 1M KOH. Raman spectra of oxidized Ni foam, represented in Figure 1A show two major peaks at 481 cm–1 and 560 cm–1 corresponding to the bending and stretching vibration of γ–NiOOH. Further, the broad peak band between 850 cm–1 and 1200 cm–1 is ascribed to the active oxygen species (NiOO–) in oxyhydroxide.[5,6] In the second step, a thin Fe layer was electrochemically deposited over the γ–NiOOH. The Raman spectra after Fe deposition also confirmed the existence of γ–NiOOH along with the peak corresponding to FeOOH at around 684 cm–1.[7] Cross-sectional SEM images with EDX of Fe deposited γ–NiOOH (γ–NiOOH-Fe) presented in Figure 1B show an oxide layer of ≈250 nm thickness with a uniform distribution of Fe. For a comparison, Fe deposited directly on a cleaned Ni foam (Ni-Fe) was also prepared thorough electrochemical deposition. The OER activity and stability of both catalysts were tested in 1 M KOH. γ–NiOOH-Fe catalyst attained 10 mA cm–2 at 1.47 V whereas to reach the same current density Ni-Fe took 1.49 V. No degradation in catalytic activity was observed in the case of γ–NiOOH-Fe compared to Ni-Fe catalyst after the accelerated durability test and constant current electrolysis at 10 and 50 mA cm–2 in 1 M KOH (Figure 1C and D). The above results demonstrate that the γ–NiOOH-Fe catalyst exhibits excellent activity and stability as an anode catalyst for anion exchange membrane water electrolyzer.

Acknowledgement

This presentation is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

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Figure 1. (A) The Raman spectra of γ-NiOOH, and γ-NiOOH-Fe (B) Cross sectional SEM- EDX images of γ-NiOOH-Fe, and (C) Linear sweep voltammograms of γ-NiOOH-Fe and Ni-Fe measured before and after the durability test in 1 M KOH at 25 °C.

Figure 1