In this work, we fabricated flexible electrodes for a miniaturized, simple structured, and flexible glucose biofuel cell (BFC) using a graphene-coated carbon fiber cloth (GCFC). The areas of the anode and cathode electrodes were 3 × 10 mm2. The anode area was coated with the enzyme glucose oxidase, and the cathode area was coated with the enzyme bilirubin oxidase. No ion-exchange film was needed because glucose oxidase selectively oxidizes glucose and bilirubin oxidase selectively reduces oxygen. The power density of the BFC with GCFC electrodes in a phosphate buffer solution of 200 mM glucose solution at room temperature was 34.3 µW/cm2 at 0.43 V. The power density of a BFC using carbon fiber cloth (CFC) without graphene modification was 18.5 µW/cm2 at 0.13 V. The BFC with the GCFC electrode continued to function longer than 24 h with a power density higher than 5 µW/cm2. These effects were attributed to the much larger effective surface areas of the GCFC electrodes that maintain more enzymes than those of the CFC electrodes.

Ascorbic acid (AA) is a biologically friendly compound and exists in many products such as sports drinks, fruit, and even in human blood. Thus, a miniaturized and flexible ascorbic acid fuel cell (AAFC) is expected be a power source for portable or implantable electric devices. In this study, we fabricated an AAFC with anode and cathode dimensions of 3 × 10 mm2 made of a graphene-coated carbon fiber cloth (GCFC) and found that GCFC electrodes significantly improve the power generated by the AAFC. This is because the GCFC has more than two times the effective surface area of a conventional carbon fiber cloth and it can contain more enzymes. The power density of the AAFC in a phosphate buffer solution containing 100 mM AA at room temperature was 34.1 µW/cm2 at 0.46 V. Technical issues in applying the AAFC to portable devices are also discussed.

Enzymatic glucose biofuel cells that generate power in human body fluids have been strongly demanded to supply electric power to microelectric systems such as heart pacemakers. This is because they work under mild conditions of room temperature, neutral pH and atmospheric pressure, which make them amenable to the human body. Porous carbon electrodes have been commonly used in glucose biofuel cells [1]. However, it was not very easy to avoid release of enzymes form these porous carbon electrode during operation of the biofuel cells. In addition, these porous carbon films were not flexible, and they were easily fractured. On the other hand, a carbon fiber woven fabric (CF) has been reported to have high flexibility and conductivity. Moreover, there were some trials to deposit graphene on each carbon fibers in CF, and it is called as graphene-coated carbon fiber cloth(GC) [2]. Thus, it is considered to be possible to increase the effective surface areas to absorb more enzymes on GC surface.      In this study, a glucose fuel cell was fabricated using GC as flexible electrodes in glucose biofuel cells. The SEM image of the GC used in this study is shown in Fig. 1. Many nanostructures were observed on carbon fibers. We used this GC as the anode and cathode of the fabricated micro biofuell cells, and the area of the anode and cathode electrodes were 3 × 10 mm2, respectively. The anode was modified with glucose oxidase (GOD) and ferrocene. The cathode was modified with bilirubin oxidase (BOD). Figure 2 shows comparison of the relationship of the power density and voltage of biofuell cells using GC or CF. The power was measured by immersing the biofuel cells in phosphate buffer solution with 100 mM glucose at room temperature. The maximum power for the CF biofuel cell was 16.3 μW / cm2 at 0.07 V, and the maximum power for GC biofuel cell was 30.7 μW / cm2 at 0.096 V. These results indicated the advantages of using GC on flexible glucose biofuel cells. [1]  M. Togo, A. Takamura, T. Asai, H. Kaji, M. Nishizawa, Electrochim. Acta 52(2007) pp.4669–4674. [2]  http://incu-alliance.co.jp/