Biofuel cells transforming biological fuels such as ethanol or sugar into electricity are safe and ecofriendly source of power. However, biofuel cells are often limited to low voltage and are insufficient to provide necessary power for daily use. Like other traditional electrolyte batteries, stacking up the biofuel cells may boost their single cell voltage to an applicable level. Miyake et al. (2013) performed experiments on multiple ways to improve the voltage of biofuel cells utilizing fructose as the energy source. When stacking cells, each cell has to be isolated with proper wrapping to prevent short-circuits. The authors layered the biofuel cells with enzyme-modified carbon fabric strips and hydrogel sheets to ensure the ion-conduction between the anode and cathode fabric layers; the hydrogel sheet also served as a fuel tank. The modification effectively improved the performance of both bioanodes and biocathodes and maximizes cell power to 0.64 mW at 1.21V. —Chieh-Hsin Chen
Miyake, T., Haneda, K., Yoshino, S., Nishizawa, M., 2013. Flexible, layered biofuel cells. Biosensors and Bioelectronics 40, 45-49.
To increase the efficiency and the power of biofuel cells, the authors made three modifications to the cells; to prevent short-circuits due to ion-conduction. The first modification was the preparation carbon fabric anodes, where are multi-walled with carbon nanotubes to increase the reaction surface area; they are heated in 400 °C and immersed in multiple solutions such as D-fructose dehydrogenase to increase the efficiencies. The result of this modification is significant: it almost doubles the current density. In the first modification, the authors also found that the performance of cells is significantly affected by buffer concentration; buffer is added to stabilize the local pH level change caused by the oxidation process. With a stabilized pH level, enzymes in the carbon fabrics perform with the highest efficiency with a 0.5M buffer with maximum current produced at 0.6V of 15.8 mA.
The second modification is the gas-diffusion of carbon fabric cathodes. This process followed the process used for Biliruben oxidase (BOD) cathodes. BOD can catalyze the reaction of O2 to H2O without election transfer mediators. The cathode is also treated with heat and multiple solutions including BOD and a surface coat of carbon nanotube solution to make it hydrophobic. To test the effect, the electrode strip was put in an oxygenic pH 5 buffer testing the electric potential versus the current capacity. The performance of a BOD-modified strip reaches to about 1.9 mA cm-2; the additional carbon nanotube coating onto the BOD-modified cathode strip was enhanced to 2.9mA cm-2. The hydrophobic carbon nanotube coating controls the penetration of excess solution into the carbon fabric electrodes allowing the conduction to optimize. The authors also conclude that control of the buffer concentration may optimize the performance with maximum current of 4.6 mA cm-2 at 0V using a .25M buffer solution utilizing an oxygen supply from the ambient air through the carbon fiber.
The third modification is through double-network hydrogel films that contain fructose. This modification is prepared through a three-step process: first, the formation of one layer hydrogel film, then another layer of film, lastly with loading of 500 mM fructose. The hydrogel film is later treated with three stock solutions to secure the fructose solution in the film. Both cathodes and anodes went through the process of lamination with double-network hydrogel sheet; the lamination provides the cells with moisture, fuel sources, and buffering for the reaction. The cells are tested with 0.74V, which is about the electric potential difference between fructose oxidation and oxygen reduction. The performance of the biofuel cell is fairly good; it reached a maximum power density of 0.95 mW cm-2 at 0.36 V. However, the stability of the cell decreases drastically after a few hours due to drying of the hydrogel. More importantly, the authors found that bending the cell sheet into a cylinder effectively increases the performance of the cell. The laminated bent cell produced a maximum power of 0.64 mW at 1.21V, which is sufficient to light an LED unit. With these types of modification, we may expect a more powerful biofuel cell in the future.