Ascorbic acid (AA), also known as vitamin C, is an environmentally biologically friendly compound and a stable and non-toxic powder. AA exists in many products such as sports drinks, fruit, and in human blood. Thus, AA is expected to be an alternative fuel for potable power supplies. Fujiwara et al. reported an ascorbic acid fuel cell (AAFC) without catalysts on the anode . However, these AAFCs were not portable. Recently, Hoshi et al. reported a miniaturized AAFC with an area of 0.3 cm2
using a flexible graphene-coated carbon fiber cloth (GCFC) . The AAFC exhibited lower power density of 34.1µW/cm2
in a buffer solution containing 100 mM AA at room temperature and the lifetime is not reported. Further improvement in their power density is needed for their application as portable AAFC and the lifetime should be investigated.
In this study, we optimized a miniaturized and flexible AAFC using a GCFC. The power density of the optimized AAFC in a mcllvaine buffer solution (MBS, pH 5.0) containing 100 mM AA at room temperature was 155.7 µW/cm2 at 0.375 V (Fig. 1a). Although this value is approximately five times greater than that of previous studies Hoshi et al. reported, the lifetime is substantially short. Since AA oxidizes on GCFC electrodes, ion-exchange films is needed for inhibiting crossover. We also fabricated a miniaturized and flexible AAFC incorporating ion-exchange films. The power density of the AAFC incorporating ion-exchange films in a MBS containing 100 mM AA at room temperature was 32.7 µW/cm2 at 0.221 V and the performance lasted about 50 % for 12 hours (Fig. 1b).
 N. Fujiwara, K. Yasuda, T. Ioroi, Z. Siroma, Y. Miyazaki, and T. Kobayashi: Electrochem. Solid-State Lett. 6, A257 (2003).
 K. Hoshi, K. Muramatsu, H. Sumi, and Y. Nishioka: Jpn. J. Appl. Phys. 55(4S), (2016).