Structure and Activity of Electrolytically Deposited Hybrid Cobalt Hydroxide Nanosheet for Self-Repairing Oxygen Evolution Reaction Catalysts

Introduction

Water electrolysis is a core technology in the conversion of renewable energy to hydrogen. Alkaline water electrolysis (AWE) is one of the most suitable technologies because of its low cost and applicability to large-scale production of hydrogen. However, the AWE system exhibits electrode degradation under fluctuating electricity from renewable energy. Recently, we have demonstrated the use of a hybrid cobalt hydroxide nanosheet (Co-ns) to form a self-repairing catalyst layer on a nickel anode under cycled potential,¹ although formation process of catalyst layer and factors affecting the oxygen evolution reaction (OER) are not sufficiently understood. In this study, we investigated the catalyst formation process and activity via different deposition methods.

Experimental

Co-ns was synthesized according to the literature². Electrochemical tests were performed in a 1.0 M KOH, using a PFA three-electrode cell. A nickel plate, a nickel coil, and a reversible hydrogen electrode were used as the working, counter, and reference electrodes, respectively. The surface Ni oxide/hydroxide was electrochemically removed by at －0.5 V vs. RHE for 3 min. The Co-ns dispersion (final concentration 40 ppm) was then added in the electrolyte. Co-ns was deposited by the two protocols. Cycled method is according to our previous report.¹ The following processes were repeated for 8 or 30 times : i) chronopotentiometry (CP) at 800 mA cm^-2 for 30 min, ii) cyclic voltammetry (CV) between 0.5 and 1.8 V vs. RHE at 5 mV s^-1, iii) CV between 0.5 and 1.6 V vs. RHE at 50 mV s^-1, iv) electrochemical impedance spectroscopy (EIS) at 1.6 V vs. RHE with the frequency range 0.1－10⁵ Hz. The following deposition protocol, called the continuous method, was newly employed, where CP at 800 mA cm^-2 for 4 or 15 hours was performed, followed by the above processes ii－iv once.

Results and discussion

The electrode prepared by the continuous method exhibited higher OER activity than those prepared by cycled method (Figure 1). The Co^2+/3+ redox peaks appeared at different potential, suggesting a change in the composition and/or crystal structure (Figure 2), though little difference was observed in the XAS spectra.

The charge of the anodic peaks due to Co^2+/3+ (Q_a) was used as a measure of the amounts of the deposited Co-ns. The current density at 1.6 V vs. RHE (i_1.6V) exhibited a linear relationship with Q_a by the continuous method (Figure 3). Indicating that the catalytic activity is proportional to the amount of the deposited Co-ns. By the cycled method, the plots appeared on the same line as those obtained by the continuous method. Q_a was monotonically increased up to the 15^th cycle, whereas it turned to decrease after the cycle. Therefore, the OER activity depends hardly on the structure but on the amount of deposited Co-ns.

To investigate the relationship between the morphology of the catalyst film and Q_a, i_1.6V was plotted over the coverage of the electrode surface with Co-ns, estimated by the EDS mapping (Figure 4). The coverage increased after longer electrolysis by the continuous method together with the increase in the OER activity. Contrary, the coverage reached 88% at the 8^th cycle and decreased to 71% at the 30^th cycle by the cycled method, indicating that a part of the catalyst layer was dissolved/delaminated during the process. The surface of the catalyst layer by the continuous method was flat, though that by the cycled method was rough, being consistent with the decrease in Q_a. Thus, potential sweep probably affects the dissolution or delamination of the catalyst layer, causing the decrease in the activity. Interestingly, i_1.6V still increased along with the electrolysis time after the increase in the coverage finished, suggestion that the active sites distribute not only on the surface of the catalyst layer but also inside the layer.

In conclusion, it was found that the OER activity of the Co-ns catalyst layer depends on the amount of deposited Co-ns and potential sweep during the catalyst deposition process causes partial dissolution/detachment and decrease in the activity. These insights will contribute to achieve both higher activity and higher durability for self-repairing catalysts.

Acknowledgements

This work was supported partially by the JSPS KAKENHI from MEXT, Japan.

References

1. Kuroda, T. Nishimoto, S. Mitsushima, Electrochim. Acta, 323, 13481 (2019).

2. Kuroda, T. Koichi, K. Muramatsu, K. Yamaguchi, N. Mizuno, A. Shimojima, H. Wada, K. Kuroda, Chem. Eur. J., 23, 5023 (2017).

Figure 1