(481f) High-Surface-Area Enzyme Electrode Made of Redox Polymer Grafted Carbon Black for Biofuel Cell Application

Biofuel cells are strong candidates for portable and implantable energy sources. However, power densities of biofuel cells are very low compared to polymer electrolyte fuel cells because of their low current density. The rate-limiting step in mediated enzyme electrodes has been electron conduction via a redox polymer, which shuttles electrons from the enzyme to the electrode, due to the low apparent electron diffusion coefficient in the redox polymer (D_M).[1] We have proposed a high-surface-area three-dimensional enzyme electrode, which uses redox-polymer-grafted carbon black with particle diameters of about 30 nm, to obtain high current density even when the redox polymer used has a low D_M.[2-5] In this electrode, the three-dimensional carbon electrode with high surface area and high electron conductivity plays the primary role in conducting electrons; thus, the electron conduction distance in the redox polymer was reduced, overcoming the rate-limiting step. The electrode was used as a glucose-oxidizing electrode by using glucose oxidase (GOD) as the enzyme. A high current density of the order of 10⁰ mA/cm² was obtained regardless of the redox polymer used.[2, 3] The electrode functioned as a part of an all solid-type membrane electrode assembly (MEA)-style biofuel cell.[2] The effectiveness of the high-surface-area electrode with a thin, grafted redox polymer layer was also verified by modeling the reaction and diffusion processes in the electrode. The model calculation shows that the rate-limiting step of apparent electron diffusion can be overcome by using the electrode with a thin redox polymer, even when the redox polymer used has a low D_M. In addition, an increase in the surface coverage of the enzyme allows an increase in the current density to the order of 10² mA/cm².[4] However, the low enzyme surface coverage limited the current density. Thus, a main factor that caused the low surface coverage was experimentally clarified. The total amount of GOD incorporated in the electrode was shown to be high enough from direct TEM observation and fluorescence spectroscopy. Electrochemical characterization in the presence of free mediator also indicated that poor contact between GOD and mediator in the redox polymer did not explain the low surface coverage. The main reason was, therefore, the deactivation of GOD upon adsorption on the carbon black surface during the GOD incorporation process. Surface treatment of carbon using a surfactant and trehalose reduced the deactivation to some extent.[5] References [1] S. C. Barton, J. Gallaway et al., Chem. Rev., 2004, 104, 4867. [2] T. Tamaki and T. Yamaguchi, Ind. Eng. Chem. Res., 2006, 45, 3050. [3] T. Tamaki, T. Ito, T. Yamaguchi, J. Phys. Chem. B, 2007, 111, 10312. [4] T. Tamaki, T. Ito, T. Yamaguchi, Fuel Cells, 2009, 1, 37., [5] T. Tamaki, A. Hiraide, T. Ito, H. Ohashi, T. Yamaguchi, submitted.