Development and Evaluation of Gel Fuel and Liquid Fuel Hybrid Structure of Fructose Fuel Cell

Abstract

An enzyme-type biofuel cell (EBFC) is a device that converts chemical energy into electrical energy by a redox reaction, using an enzyme as a catalyst to generate electricity. EBFCs have the features that it can operate at normal temperature, normal pressure, and neutrality. In addition, fuel can be selected depending on the type of enzyme, and the structure is simple and the easy to miniaturize. Therefore, EBFCs are expected to be a power source for medical and mobile devices. EBFCs have problems of low output and low lifetime. Hoshi et al. proposed using GCFC (large specific surface area) for the electrode, to improve the output [1]. EBFCs generally use oxygen in the cathode reaction. Therefore, oxygen supply is important in EBFCs. Kuroishi et al. reported on the improvement of output in air exposure type EBFCs using gel fuel [2]. However, the gel did not function well after a few minutes owing to drying i.e., they have a short lifetime. In this study, a new structure to solve these problems was proposed.In this study, fructose was used as the fuel. Figure 1 shows the operating principle of EBFCs using enzymes of fructose dehydrogenase (FDH) and bilirubin oxidase (BOD). Figure 2 shows the hybrid structure of liquid fuel and gel fuel, which was proposed in this work. The electrode uses GCFC. The distance between the two electrodes and the thickness of the gel fuel is 3 mm. The specifications of the used enzyme, liquid fuel, and gel fuel are shown in Table 1. Figure 3 shows the cyclic voltammetry (CV) of the anode to determine the optimum concentration of the fuel (200 mM). Next, we compared the power density of a conventional gel fuel cell (original gel fuel cell) and a fuel cell with liquid fuel dropped on the cathode side of gel fuel cell (cathode wetting gel fuel cell). Figure 4 shows the power density of the original gel fuel cell and the cathode wetting gel fuel cell. The power density of the cathode wetting gel fuel cell is extremely reduced. This indicates that the gel fuel cell requires the cathode to be dry to provide sufficient oxygen. Therefore, the hybrid structure was adopted to make the gel dry on the side of cathode and to prevent the drying of the gel by contacting with the liquid fuel on the side of the anode. Figure 5 shows the power density of the three types of EBFC. The power density of the hybrid was 74.0 μW / cm2, which is almost equivalent to that of just using gel. Figure 6 shows the time history of the maximum power density. The gel fuel type halved power density after 45 minutes. However, the hybrid type achieved a power density of 61 μW/cm2, even after 120 minutes. This was the most important feature of this study. Figure 7 confirms that the high level of the maximum power density, even after 24 hours, was maintained to at least 62.3% (46.1 μW/cm2) of the initial value.[1] K. Hoshi et al. Jpn. J. Appl. Phys, 55 02BE05,2016[2] K. Kuroishi et al. IEICE Transactions on Electronics, pp151-154,2019 Figure 1