Effect of Pore Size on the Oxygen Evolution Reaction Activity of Hydrogel Electrodes Composed of Hybrid Cobalt Hydroxide Nanosheets

Abstract

Introduction Alkaline water electrolysis (AWE) is a promising technology for large-scale production of hydrogen from renewable energy; however, the sluggish oxygen evolution reaction leads to a large overvoltage. Porous electrodes with a large specific surface area are useful to achieve high current densities in AWE, whereas mass transport in small pores often reduces the overall current density. We have reported that hydrogel electrodes, possessing a flexible framework composed of interconnected hybrid cobalt hydroxide nanosheets (Co-ns), exhibit enhanced mass transport at high current density.1 Hydrogel electrodes can be regarded as porous electrodes swollen by electrolytes. They are expected to repair cracked surfaces by recombination of microparticles and to optimize the framework structure suitable for mass transport. In this study, we demonstrate the relationship between the pore size of the hydrogel electrodes and the OER activity at high current density. Experiment Co-ns was synthesized by mixing aqueous solutions of 1.0 M tris(hydroxymethyl)aminomethane and 0.1 M CoCl2·6H2O, followed by heating at 90 °C.2 The heating time was varied as 1, 5 and 10 days in order to control the particle size.Electrochemical measurements were performed in a 1.0 M KOH, using a PFA three-electrode cell. Co-ns was dispersed in the electrolyte (200 ppm). Nickel plate, nickel coil, and reversible hydrogen electrode (RHE) were used as working, counter, and reference electrodes, respectively. Co-ns was deposited on the electrode by constant current electrolysis at 800 mA cm–2 for varied time (10 min–100 h). The OER activity was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy for iR correction. Result and Discussion TEM images of the synthesized Co-ns exhibited that the diameter (long axis) of Co-ns increased along with the heating time. Therefore, the samples were named as 1d_21, 5d_34 and 10d_42, based on the heating time (days) and average diameter (nm). The FESEM images of the hydrogels after drying showed that nanosheets were assembled into house-of-cards structures (Figure 1). The assembled structures of Co-ns with different diameters were analogous to each other.The charges of the cathodic peaks (Q c) at 0.8–1.5 V vs. RHE due to Co2+/3+ and Co3+/4+ in cyclic voltammograms were used as a measure of the amount of deposited Co-ns. The linear relationship between Q c and the thickness of catalyst layers exhibited that deposited Co-ns presented uniformly along the thickness of catalyst layers (Figure 2). The slope of this line corresponds to the porosity of the hydrogels. The porosity increased along with the diameter of Co-ns; therefore, the pore size is expected to depend on the diameter of Co-ns because of the analogy of the assembled structure.Polarization curves of representative hydrogels are shifted from the Tafel equation at high current densities, indicating the influence of mass transport (Figure 3). The OER current densities at 1.6 V vs. RHE (i geo-1.6 V) of relatively thin hydrogels were linearly correlated with their thicknesses (Figure 4), though they were saturated for hydrogels with larger thicknesses. In the thicker hydrogels, OER is limited by mass transport of generated O2 and OH– in pores. The order of the limiting current densities were 1d_21 < 5d_34 < 10d_42; thus, the influence of mass transport is lowered for hydrogels with larger pores.In conclusion, the size of Co-ns was used to control the porosity and pore size of hydrogel electrodes. Hydrogel electrodes with larger pore size exhibited better mass transport to achieve higher OER activity at high current density. These insights will contribute to designing an optimal hydrogel as an oxygen evolving electrode. Acknowledgement This work was supported partially by the JSPS KAKENHI from MEXT, Japan. References R. Nakajima, T. Taniguchi, Y. Sasaki, Y. Nishiki, A. Zaenal, T. Nakai, A. Kato, S. Mitsushima, and Y. Kuroda, Autumn Meeting of the Electrochemical Society of Japan, 1P15 (2022).Y. Kuroda, T. Koichi, K. Muramatsu, K. Yamaguchi, N. Mizuno, A. Shimojima, H. Wada, and K. Kuroda, Chem. Eur. J., 23 (2017) 5023. Figure 1