Altering Anode Thickness To Improve Power Production in Microbial Fuel Cells with Different Electrode Distances Yongtae Ahn and Bruce E. Logan* Department of Civil and Environmental Engineering, Pennsylvania State University, 212 Sackett Building, University Park, Pennsylvania 16802, United States ABSTRACT: A better understanding of how anode and separator physical properties affect power production is needed to improve energy and power production by microbial fuel cells (MFCs). Oxygen crossover from the cathode can limit power production by bacteria on the anode when using closely spaced electrodes [separator electrode assembly (SEA)]. Thick graphite fiber brush anodes, as opposed to thin carbon cloth, and separators have previously been examined as methods to reduce the impact of oxygen crossover on power generation. We examined here whether the thickness of the anode could be an important factor in reducing the effect of oxygen crossover on power production, because bacteria deep in the electrode could better maintain anaerobic conditions. Carbon felt anodes with three different thicknesses were examined to see the effects of thicker anodes in two configurations: widely spaced electrodes and SEA. Power increased with anode thickness, with maximum power densities (604 mW/m 2 , 0.32 cm; 764 mW/m 2 , 0.64 cm; and 1048 mW/m 2 , 1.27 cm), when widely spaced electrodes (4 cm) were used, where oxygen crossover does not affect power generation. Performance improved slightly using thicker anodes in the SEA configuration, but power was lower (maximum of 689 mW/m 2 ) than with widely spaced electrodes, despite a reduction in ohmic resistance to 10 Ω (SEA) from 51−62 Ω (widely spaced electrodes). These results show that thicker anodes can work better than thinner anodes but only when the anodes are not adversely affected by proximity to the cathode. This suggests that reducing oxygen crossover and improving SEA MFC performance will require better separators. 1. INTRODUCTION Microbial fuel cells (MFCs) are devices that use micro- organisms to covert the energy stored in chemical bonds in biodegradable organic and inorganic compounds to electrical energy. 1 Microbes release electrons to the anodes, and they are transferred through the circuit to the cathode, where they combine with protons and an electron acceptor, such as oxygen, to form water. 1,2 Several types of MFCs with different electrode arrangements have been developed, including two-chamber, single-chamber, flat-plate, and stacked electrode reactors. 3−6 Of these, the single-chamber air cathode MFC is the most commonly used configuration because of its high power output, low internal resistance, and relatively low operational cost as a result of the direct use of oxygen in air. 4,7 Electrode materials play an important role in the perform- ance and cost of a MFC. These materials should have good electrical conductivity, low resistance, chemical stability, corrosion resistance, and high mechanical strength. Various materials have been used, including graphite fiber brushes, graphite rods, carbon paper, carbon mesh, and carbon felt. 8−11 The modification of the surface with chemicals, metals, metal oxide, and non-metals, such as carbon nanotubes (CNTs), supported on different materials (such as textiles and sponges) are effective methods for enhancing power generation by many different types of anode materials by increasing biocompatibility and electron-transfer efficiency. 12−14 For example, the addition of carbon nanotubes to macroporous sponges improved volumetric power production by 12 times (to 182 W/m 3 ) 15 compared to that previously obtained with domestic waste- water. Carbon felt has been used as an electrode material in MFCs 16,17 as well as in other electrolytic cells for ion removal. 18−20 One advantage of the carbon felt anode over other materials is that it has large porosity (∼99%) 21 relative to carbon cloth or paper, allowing more surface area for bacterial growth. In addition, the cost of carbon felt and its performance (maximum power density) are similar to those of other carbon- based materials. 17,22 However, the thickness and placement of these felt materials relative to the cathode have not been well- studied. Reducing the anode−cathode distance can improve the power production by reducing ohmic (solution) resistance, but very close spacing of thin anodes can reduce power. For example, reducing the spacing between a thin carbon cloth anode (0.35 mm thick) and cathode from 3 to 2 cm increased power and decreased internal resistance from 56 to 35 Ω. 23 Although further decreases in electrode spacing reduced the internal resistance to 16 Ω, the power decreased because of oxygen crossover from the cathode to the anode, adversely affecting power generation by bacteria on the anode. One way to reduce oxygen crossover is to place a separator between electrodes, forming a separator electrode assembly (SEA) configuration. Separators are effective at reducing oxygen crossover but not affecting proton transport to the cathode or increasing power densities and Coulombic efficiencies (CEs) compared to systems with larger electrode spacing because of the reduction in ohmic resistance. 24−26 In a SEA MFC, the type of anode used will affect power production and the thickness of the anode size may be a factor in improving MFC performance. A thick (2.5 cm diameter and Received: September 21, 2012 Revised: November 15, 2012 Published: December 12, 2012 Article pubs.acs.org/EF © 2012 American Chemical Society 271 dx.doi.org/10.1021/ef3015553 | Energy Fuels 2013, 27, 271−276