CELL-TO-ELECTRODE CONTACT STRUCTURES FOR POWER DENSITY ENHANCEMENTS IN MICROBIAL FUEL CELLS S. Inoue 1, 2 , E.A. Parra 2 , A. Higa 2 , and L. Lin 2 1 TAIYO YUDEN CO., LTD., Akashi, JAPAN 2 University of California, Berkeley, USA ABSTRACT More than 200% power density enhancement in miniaturized microbial fuel cells (MFCs) has been successfully demonstrated by modifying the contact electrode structure using micro and nano processes. Two fundamental issues are addressed in this work: (1) a methodology to enhance power density of MFCs by changing micro and nano structural configurations of contact electrodes, (2) a study on the effectiveness of charge transfer between living cells with organic nanowire-pili and micro/nano interfacial electrodes. As such, this work represents a step forward toward higher energy conversion efficiency as well as practical application of MFCs. INTRODUCTION Compact and highly efficient power sources are integral components for the completion of autonomous sensors and microsystems. A microbial fuel cell (MFC) is one of promising candidates for electrical energy harvesting from ubiquitous organic wastes [1]. The breakdown of organic substances to retrieve energy is a naturally occurring process in nature and the possibility to extract electrical charges in the form of MFCs holds great potential in practical applications such as implantable medical sensors and long-term monitoring systems in remote locations. Previously, miniaturized MFCs have been reported by using baker’s yeasts [2, 3] and Geobacter sulfurreducens [4] as the organic catalysts which broke down hydrocarbon materials during the metabolism process. Electrons were extracted during the process to provide electricity. The efficiencies of these MFCs are very low due to many loss mechanisms, particularly during the electron transfer processes. This work studies and characterizes the influences of cell-to-electrode contact structures in order to provide quantitative guidelines for MFC designs. The goal is to increases living cell-to-electrode contact area and reduces the loss of electron transfer by using micro- and nano-structured electrodes. OPERATION PRINCIPLE Figure 1 shows the schematic diagram of the operation principle of MFC. In the anode compartment, organic fuel such as vinegar is fed into the system and the microorganisms act as bio-catalysts to convert vinegar into glucose. Electrons generated during the metabolism process are transferred to the anode electrode utilizing electron mediator [2, 3]. Figure 2 illustrates the basic operation principle of the MFC by using G. sulfurreducens as the catalyst. G. sulfurreducens are chosen in this work as their own pili can function like “organic nanowires” to directly transfer electrons to the electrodes [5]. Four different kinds of modified cell-to-electrode contact structures have been fabricated as shown in Fig. 2: (A) flat surface as a reference, (B) surface with micro-holes perpendicularly etched into the substrate, (C) surface with tapered micro-channels, and (D) vertically aligned carbon nanotubes (CNTs) array. These contact structures were used as the basic tools to investigate the power density enhancements and effectiveness of charge transfer between living cells with organic nanowire-pili and micro/nano interfacial electrodes. CO 2 Fuel H + e - Bacterium H + O 2 H 2 O e - e - e - Anode Cathode Proton exchange membrane (PEM) H 2 O Air CO 2 Fuel Metabolism Metabolism (Vinegar) (O 2 ) Figure 1: Schematic diagram showing the operation principle of the microbial fuel cell. Vinegar (or other nutrition) is fed into the anode part and bacteria are used to break down foods during the metabolism process to produce electrical charges. A B C D Figure 2: The basic operation principle of the MFC by using Geobacter sulfurreducens as the catalyst. The inset shows four different kinds of modified cell-to-electrode contact structures: (A) flat, (B) hole, (C) channel, and (D) vertically aligned CNTs. Cell-to-electrode contact structures affect the power density of the MFCs.