Catalytic Properties of FeCoNi Alloy Electrode for Alkaline Water Electrolysis Prepared by Anodizing
Authors: Nagao Tomoya , Kitano Sho , Iwai Mana , Fushimi Koji , Habazaki Hiroki 
Published: December 2024

1.Introduction
Alkaline water splitting, a low-cost and simple hydrogen production method, has been attracting attention to the realization of a sustainable society. However, the overpotential of the oxygen evolution reaction (OER) on the anode is high and the hydrogen production efficiency is low. Catalysts containing Fe, Co, and Ni are expected to be promising candidates for electrodes materials [1]. Recently, we succeeded in fabricating FeNi and FeNiCo alloy electrodes covered with a porous metal fluoride film by anodizing, which exhibited better OER activity and durability than OER catalysts [2].
We reported that the metal fluoride films synthesized by anodizing in fluoride-containing organic electrolytes converted to metal oxyhydroxide during OER in KOH aqueous solution. However, the physical properties of the films and electrode characteristics, which are the factors responsible for the high OER activity of this electrode, remain unclear.
In this study, various commercial FeNiCo and FeNi alloys (Kovar: Fe-17 mass% Co-29 mass% Ni, 42-Invar: Fe-42 mass% Ni, 45-Permalloy: Fe-45 mass% Ni, 78-Permalloy: Fe-78 mass% Ni) were anodized to fabricate highly active OER electrodes and applied to in-situ Raman spectroscopic characterization to understand the activation mechanism of the anodized alloy electrodes.
2.Experimental
Commercial FeCoNi and FeNi alloys were used for electrode preparation. The electrolytes used for anodizing were ethylene glycol containing 0.54 M NH4F and 2.5 M H2O for Kovar, 45-Permalloy, and 42-Invar, and ethylene glycol containing 0.08 M NH4F and 0.02 M H2O for 78-Permalloy. The anodizing process was performed at 10 V for 60 min at 293 K in an ethylene glycol-based electrolyte, and platinum was used as the counter electrode.
The surface morphology and composition of the OER electrodes were measured using field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy.
In-situ Raman measurements were conducted using laboratory-made cells. The measurement was performed using a three-electrode system with a platinum counter electrode and an Hg/HgO electrode as the reference electrode under a flow of 1 M KOH aqueous solution at 24.0 mL min-1. In-situ Raman spectra were measured under potentiostatic conditions every 0.2 V from 1.13 V vs. RHE.
3.Results and discussion
【morphology and compositional characterization】
The surface morphology of the films was highly dependent on the composition of the FeCoNi and FeNi alloys. A uniform and smooth film with an average thickness of 200 nm was observed in the anodized 78-Permalloy electrode, and non-uniform films with an average thickness of 1 µm were observed in the anodized 45-Permalloy and 42-Invar electrodes. The anodized Kovar electrode exhibited a non-uniform and wide crack film with an average thickness of 2.5 µm.
The cationic composition of the films was also dependent on the composition of the commercial FeCoNi and FeNi alloys. The anodized 78-Permalloy electrode showed the lowest Fe content in the film.
【Raman measurement】
The ex-situ Raman measurements revealed the presence of mixed phases of spinel oxide and metal oxyhydroxide for all electrodes after OER, indicating that the metal fluoride films were completely converted to metal oxide and metal oxyhydroxide. Then, in-situ Raman measurements were conducted under the controlled potential in 1 M KOH aqueous solution. The peak intensity of spinel oxides decreased and that of metal oxyhydroxides increased with increasing applied potential. Because the spinel oxide peaks disappeared and those of metal oxyhydroxide were only observed at the OER onset potential for all electrodes, the in-situ measurements revealed that metal oxyhydroxide is an active phase for OER in the electrolysis system. Interestingly, when the electrode potential was returned to negative, the film structure reverted to the mixed phases of spinel oxide and metal oxyhydroxide. Therefore, we confirmed that the anodized electrodes possess structural reversibility. However, since no trend was observed in the correlation between structural change behaviors and the order of OER activities of the electrodes, other factors would also contribute to OER activities in addition to the formation of the metal-oxyhydroxide structure.
References
[1] R. D. L. Smith, M. S. Prévot, R. D. Fagan, S. Trudel, C. P. Berlinguette, Journal of the American Chemical Society, 135, 31 (2013)
[2] M. Nishimoto, Z. Xiong, S. Kitano, Y. Aoki, H. Habazaki, Electrochimica Acta, 427, 20 (2022).