Design of Ni–Fe Hybrid Metal Hydroxide Nanomaterials As Self-Repairing Anode Catalysts For Alkaline Water Electrolysis
Authors: Okada Ryuki , Taniguchi Tatsuya , Sasaki Yuta , Nishiki Yoshinori , Awaludin Zaenal , Nakai Takaaki , Kato Akihiro , Mitsushima Shigenori , Kuroda Yoshiyuki 
Published: February 2024

Abstract

Introduction

To use renewable energy efficiently, the conversion of renewable energy into hydrogen by alkaline water electrolysis has attracted much attention. When an alkaline water electrolyzer is powered by renewable energy, electrodes degrade due to reverse current generated on shutdown. To address this problem, we have reported the usefulness of a hybrid cobalt hydroxide nanosheet (Co-ns) as a self-repairing catalyst that keeps the oxygen evolution reaction (OER) ability under frequent potential fluctuations.[1] However, the OER activity of Co-ns is relatively lower than the state-of-the-art catalysts, such as Ni–Fe layered double hydroxides (LDHs). In this study, we synthesized hybrid Ni–Fe hydroxides with various compositions and structures, and their correlations with the OER activity.

Experimental

Hybrid Ni–Fe hydroxides were synthesized by mixing nickel chloride, iron chloride, and tris(hydroxymethyl)aminomethane (Tris-NH2) similar to the method to synthesize Co-ns.[2]

Electrochemical measurements were performed in 1.0 M KOH, using a three-electrode cell made of PFA. A nickel plate, a nickel coil, and a reversible hydrogen electrode (RHE) were used as the working, counter, and reference electrodes, respectively. The chronoamperometry –0.5 V vs. RHE for 3 min was repeated to remove oxides from the Ni substrate surface. A dispersion of hybrid Ni–Fe hydroxide was added in the electrolyte to adjust the total metal concentration (Ni + Fe) at 40 ppm. Catalysts were deposited by repeating the constant current electrolysis at 800 mA cm-
2 for 30 min, cyclic voltammetries (CVs) and electrochemical impedance spectroscopy for the evaluation of OER activities for 8 times (total electrolysis time is 240 min).

Results and discussion

The synthesis of hybrid Ni–Fe hydroxides were confirmed by XRD, FTIR, elemental analyses, and XAFS. The structure of products were layered, amorphous, and tunnel structures, depending on the metal composition (represented by Ni/(Ni + Fe)) and the degree of modification (represented by Tris-NH2/(Ni + Fe)). Ni/(Ni + Fe) tends to be high when the concentration of metal salts was low and the concentration of Tris-NH2 was high in the preparation solution. The oxidation states of Ni and Fe in all samples were +2.1 and +3.0, respectively.
The amount of catalyst deposited on the electrode by the electrolysis was confirmed by elemental analyses. The amount of deposited catalyst increased along with Ni/(Ni + Fe) (Fig. 1). The lowest deposition amount was 2.74 µg cm-
2 (Ni/(Ni + Fe)=0) and the highest one was 41.5 µg cm–
2 (Ni/(Ni + Fe)=0.82), whereas the values were still lower than that of Co-ns (88.7 µg cm–
2). In our previous study, the oxidation of Co2+ to Co3+ on an electrode surface is crucial for the electrochemical deposition of Co-ns.[3] Thus, it is reasonable that the content of divalent cation (Ni2+) is related with the deposition amount.
From the OER measurements, the mass activity of catalysts was defined as the current at 1.53 V vs. RHE normalized by the amount of deposited catalyst (i
m,1.53). i
m,1.53 decreased along with the increase in Ni content (Fig. 2). The increase in Ni content is beneficial to the electrochemical deposition, though it is negatively correlated with the OER activity. Therefore, there is a trade-off between the amount of deposited catalyst and mass activity. The highest geometrical OER current density was obtained for Ni/(Ni+Fe) = 0.82 and i
1.53 = 296 mA cm–2 (Fig. 3). The deposited amount of catalyst significantly contributed to the highest activity of the electrode.

Conclusion

In this study, we synthesized Ni–Fe hybrid metal hydroxides and demonstrated that the Ni content of the catalysts affect both its deposition behavior and oxygen evolution reaction activity. The highest Ni content catalyst showed the highest activity, indicating a trade-off between the amount of catalyst deposited and mass activity.

Acknowledgement

A part of this study was supported by KAKENHI (Grant-in-Aid for Scientific Research 20H02821) from Japan Society for the Promotion of Science (JSPS)

Reference

[1] Y. Kuroda, T. Nishimoto, S. Mitsushima, Electrochim. Acta, 323, 134812, (2019).
[2] Y. Kuroda, T. Koichi, K. Muramatsu, K. Yamaguchi, N. Mizuno, A. Shimojima, H. Wada, K. Kuroda, Chem. Eur. J., 23, 5023, (2017).
[3] R. Nakajima, T. Taniguchi, Y. Sasaki, Y. Nishiki, Z. Awaludin, T. Nakai, A. Kato, S. Mitsushima, Y. Kuroda, submitted.

Figure 1