In living organisms, transport of electrons from oxidoreductases creates a proton gradient across the mitochondrial inner membrane. This proton gradient drives adenosine triphosphate (ATP) synthesis from adenosine diphosphate (ADP). Inspired by these energy conversion systems in living organisms, electrons are transferred from oxidoreductase to the electrode in biofuel cells. In other respects, biofuel cells are fuel cells that use enzymes as catalyst. Since enzymes are able to oxidize a variety of fuels, such as glucose or ethanol, and operate under moderate conditions, biofuel cells are strong candidate for portable and implantable energy sources. Until now, one of the important issues to be addressed is their low power density, which is due to their low current density. This is because the amount of enzyme effectively used in electrode reactions was restricted by electron conduction via a redox polymer, which shuttles electrons from the enzyme to the electrode.

In this work, we have developed a novel biofuel cell electrode system to overcome the problem described above. This electrode consists of three elements: a three-dimensional carbon electrode made of a small particle-size carbon black, short-chain redox polymer grafted onto the carbon black surfaces, and enzymes. In this electrode, conduction of electrons is divided between a short-chain redox polymer and carbon, and thus the electron conduction distance in the redox polymer is decreased. The three-dimensional carbon electrode, having a high electron conductivity, plays the major role in conducting electrons, while the short-chain redox polymer only transports electrons between the enzyme and the carbon electrode.

The electrode was used as a glucose-oxidizing anode, using glucose oxidase (GOD) as the enzyme and a poly(vinylferrocene-co-acrylamide) as the redox polymer. The electrode was electrochemically characterized using cyclic voltammetry. To determine whether this method increased the amount of effective enzyme, we estimated the electrochemically active GOD surface coverage per unit geometric surface area from the current density obtained in cyclic voltammetry. The surface coverage was estimated to be about 10⁴ times higher than that of a densely packed GOD monolayer. Using this novel electrode, we successfully obtained a high glucose electro-oxidation current density. Furthermore, a membrane-electrode assembly-style biofuel cell using this novel electrode was fabricated, and its performance was tested. We succeeded in achieving power generation. This shows that our novel GOD-incorporated electrode can function as a part of biofuel cell.