Design of Self-Repairing Anode Catalysts for Alkaline Water Electrolysis with Both OER Activity and Durability Via Colloidal Self-Assembly

Abstract

INTRODUCTION The demand of renewable energy is increasing, yet fluctuating power supply is one of the problems. Therefore, technology for energy storage such as alkaline water electrolysis is required. When an alkaline water electrolyzer is powered by renewable energy, electrodes degrade due to reverse current generated on shutdown.[1] We have reported self-repairing catalysts based on hybrid cobalt hydroxide nanosheets (Co-ns), and β-FeOOH nanorods (FeOOH-nr).[2,3] Our previous studies showed that Co-ns formed thick catalyst layer with high oxygen evolution reaction (OER) activity, whereas FeOOH-nr formed a thin layer with much higher OER activity than that of Co-ns. If the thickness of FeOOH-nr increases, the OER activity and durability are expected to be improved. Here, we demonstrate the design of composite catalysts, consisting of both Co-ns and FeOOH-nr. Co-ns promotes stacking of the catalysts on the electrode, and FeOOH-nr forms active sites. EXPERIMENTAL Co-ns[2] and FeOOH-nr[3] were synthesized according to the literature. 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 Co-ns dispersion, FeOOH-nr dispersion, or their mixture (total concentration of catalyst weights = 80 ppm) was added in the electrolyte. Catalysts were deposited by repeating the following processes 10 times according to our previous report:[2] i) chronopotentiometry 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, and iv) electrochemical impedance spectroscopy at 1.6 V vs. RHE. Furthermore, an accelerated durability test (ADT)[4] was conducted 2000 cycles to examine the degradation. RESULTS AND DISCUSSION The synthesis of Co-ns and FeOOH-nr was confirmed by XRD, FTIR, elemental analyses, and TEM. Co-ns is modified with tripodal ligand, whereas FeOOH-nr is non-modified. The electrophoretic mobilities of Co-ns and FeOOH-nr were 1.98 and –2.80 µm cm V– 1 s– 1 at pH 9.7. Because Co-ns and FeOOH-nr are oppositely charged, they are expected to be self-assembled into a composite in the electrolyte.The OER polarization curves during the repeated electrolysis are shown in the Fig. 1. The OER activity of the electrode prepared from the mixture of Co-ns and FeOOH-nr (Co-ns/FeOOH-nr molar ratio = 10) was higher than those of the electrodes prepared from only one component. The OER overpotential at 100 mA cm–2 (η 100) of the composite catalyst was 291 mV after 300 min of electrolysis. The η 100 of Co-ns and FeOOH-nr were 310 and 353 mV, respectively.The deposited amount of Fe (M Fe) and the η 100 as a function of the deposited amount of Co (M Co) is shown in the Fig. 2. The M Fe using the composite catalysts was much larger than that using FeOOH-nr only. The M Fe was 48.5 µg cm–2 when the Co-ns/FeOOH-nr ratio is 10, although it was 2.00 µg cm–2 when only FeOOH-nr was used. M Fe is correlated with the OER activity. The highest OER activity was obtained at Co-ns/FeOOH-nr = 10.The assembled structure of FeOOH-nr and Co-ns in the deposited catalyst layer was observed by TEM (Fig. 3). A rod-like FeOOH-nr was surrounded by sheet-like Co-ns. Thus, it was suggested that Co-ns was assembled with FeOOH-nr by the electrostatic interactions.The results of ADT with the composite catalyst dispersed in the electrolyte for self-repairing (Fig. 4) shows that there was little degradation after 2000 cycles of start-up and shut-down. Therefore, these composite catalysts have excellent stability. However, when no composite catalyst was dispersed in the electrolyte, meaning that no self-repairing occurred, η 100 gradually increased in the 2000 cycles. The degradation behavior was milder than that of the FeOOH-nr only; thus, the composite catalyst exhibited the improved stability of the FeOOH-based catalyst layer.In conclusion, the deposited amount of FeOOH-nr was successfully increased by the addition of Co-ns in the electrolyte to form composite catalysts. The electrostatic self-assembly is useful to combine the properties of different catalysts for a self-repairing system. 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. Uchino et al., Electrocatalysis 9, 67 (2018).[2] Y. Kuroda et al., Electrochim. Acta. 323, 1348122 (2019).[3] Y. Kuroda et al., J. Sol-Gel Sci. Technol., 104, 647-658 (2022).[4] A. Haleem et al., Electrochemistry. 89(2), 186 (2021). Figure 1