Effective Addition of the Active Sites for Redox Flow Battery By the Electron-Beam Irradiation

Abstract

Introduction In recent years, a mismatch between renewable energy (i.e., solar, wind, etc.) supply and power demand has become a social issue. To overcome this problem, vanadium redox flow batteries (VRFBs) [1] are attracted attention because of their advantages such as flexible design between power density and capacity, accurate measurement of the state of charge, simple process for battery production, etc. One of the issues of VRFB is a low current density due to the high reaction resistance. Thus, the development of an effective oxidation method to add active sites, i.e., surface oxidation groups [2], on the carbon electrode materials is essential. For this purpose, we focused on activation by the radiation chemical reaction induced by the electron-beam. The electron-beam irradiation to the carbon electrode material may cause (i) defects formation by displacement of carbon atoms by the electron beam and (ii) formation of surface oxygen groups on defect sites by reaction with active oxygen species (O3, •OH, etc.) which are synthesized by the irradiation. In this study, we performed the electron-beam irradiation to a carbon cloth in the air. The surface oxygen was analyzed by X-ray photoelectron spectroscopy (XPS). The current-voltage curves of the single cells with the irradiated electrodes were measured. Experimental Electron-beam was irradiated to the electrode materials at Takasaki Advanced Radiation Research Institute (Gunma, Japan), and the acceleration voltage was 2.0 MV. The surface chemical states of C (1s) and O (1s) of the samples were measured by XPS with monochromatic Al Kα radiation. The current-voltage measurement of the single cell was measured. A custom-built single cell with the interdigitated flow field [3] was used. Two-layer of carbon cloth were compressed to ca. 3/4 of their initial thickness and same carbon cloth was used as both the negative and positive electrodes. The Nafion 117 membrane (DuPont) was set as the separator, and 1.0 M vanadium active materials in 3.0 M H2SO4 was used as the electrolyte solution. The current-voltage measurement was carried out at the scan rate of 2 mV s−1 with the electrolyte flow rate per geometric area was 6.2 mL cm−2 min−1. Results and Discussion From the XPS spectra, the surface carbon fraction decreased after the irradiation, in the meanwhile, the surface oxygen fraction increased after the irradiation. Based on the peak separation of XPS C 1s spectra for the irradiated material, C–O (285.9 eV) and COO (288.7 eV) bonds were increased. This result suggests phenol-type hydroxyl group and the carboxyl group, which are considered as active sites for VRFB, were effectively introduced by the radiation chemical reaction. The wettability of the carbon cloth was improved by the irradiation. Figure 1 shows the current-voltage curves of the electrodes (a carbon cloth, Toyobo) with/without electron-beam irradiation. The current density at 1.2 V increased 1.3 times by the irradiation. We confirmed the double layer capacitance of the electrodes were almost the same with/without electron-beam irradiation, thus active surface area did not change by the irradiation. Hence, the increased current is attributed to the addition of surface oxygen groups by the radiation chemical reaction. Acknowledgment This research was supported by JSPS KAKENHI Grant Number JP18K14048 and a research grant from Super-Membrane Project of Gunma University, and a research grant from Element Science Project of Gunma University. We significantly thank this foundation. H. I. appreciate to Toyobo Co., Ltd. for kind supply of the carbon cloth. Figure 1