Enhanced Mass Transportation for the Oxygen Evolution Reaction in Flexible Mesoporous Hydrogel Electrodes

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Water electrolysis is of great importance for the hydrogen production from renewable energy. The efficiency of water electrolysis depends highly on the performance of anodes because of the high overpotential of the oxygen evolution reaction (OER). Mesoporous electrodes with high surface area and relatively small pore size (2-50 nm) have attracted much attention as highly active OER electrodes; however, the mass transportation of generated oxygen molecules and hydroxide ions in mesopores often limit the OER current density.In this study, we propose a novel class of materials, namely flexible mesoporous hydrogel electrodes which consist of assemblies of CoOOH nanosheets. Flexible mesoporous hydrogel electrodes were prepared by electrochemical deposition of cobalt hydroxide nanosheets modified with tris(hydroxymethyl)aminomethane (depicted as Co-ns [1]) on nickel substrates. Co-ns formed a house-of-cards assembly, having characteristics of hydrogels, during the deposition process. The surface modifying group of Co-ns was anodically decomposed and Co-ns was transformed into CoOOH nanosheets. Because of the weak connections among CoOOH nanosheets the resultant assemblies provided flexible mesoporous frameworks.The flexible mesoporous electrodes with different thicknesses (4-37 μm) were prepared, changing the deposition time. The porosity of the flexible mesoporous electrodes were estimated to be approximately 97% because of the very thin thickness of the CoOOH nanosheets. Rigid mesoporous electrodes with Co3O4 frameworks with different thicknesses (11-60 μm) were also prepared by dip-coating method, using Pluronic F127 as a template. The porosity of the rigid mesoporous electrodes was approximately 78%. The electrochemical tests were performed in a three electrode cell at 30 ºC.The OER polarization curves of the flexible mesoporous electrodes and the rigid mesoporous electrodes in 1.0 M KOH electrolyte are shown in the Figure 1. The OER current densities of the rigid mesoporous electrodes depend quantitatively on the thickness of the films at 1.50 V vs. RHE, though the current densities became independent of the thickness of the films at 1.60 V vs. RHE. On the other hand, the OER current densities of the flexible mesoporous electrodes depend quantitatively on the thickness of the films at wide potential region, even at 1.60 V vs. RHE, indicating that the high mass transport ability of generated oxygen molecules and hydroxide ions in the mesopores.In conventional rigid mesoporous electrodes, oxygen bubbles should form within mesopores with defects, i.e. large nanospace and cracks, and the generated bubbles plug the mass transport paths. The flexible mesoporous hydrogel electrodes are expected to have defect-less mesoporous structures because they can change the mesoporous structures flexibly to reduce defects, and defects can be self-healed once they are formed. It is also expected that the flexible frameworks change their mesoporous structure to optimize the mass transportation path during OER.In conclusion, novel mesoporous hydrogel electrodes were prepared for the first time by the electrochemical deposition of Co-ns. The flexible mesoporous hydrogel electrodes exhibited high OER performance, depending on the thickness of the films quantitatively at high current density region. Flexible mesoporous hydrogel electrodes are promising as novel types of electrodes to improve the efficiency of water electrolysis. This work was supported by the JSPS KAKENHI (grant number 24K01580).