Electricity Generation in Microbial Fuel Cells Using Neutral Red as an Electronophore

Abstract

Neutral red (NR) was utilized as an electron mediator in microbial fuel cells consuming glucose to study both its efficiency during electricity generation and its role in altering anaerobic growth and metabolism of Escherichia coli and Actinobacillus succinogenes. A study of chemical fuel cells in which NADH, NR, and ferricyanide were the electron donor, the electronophore, and the electron acceptor, respectively, showed that electrical current produced from NADH was proportional to the concentration of NADH. Fourfold more current was produced from NADH in chemical fuel cells when NR was the electron mediator than when thionin was the electron mediator. In microbial fuel cells in which E. coli resting cells were used the amount of current produced from glucose when NR was the electron mediator (3.5 mA) was 10-fold more than the amount produced when thionin was the electron mediator (0.4 mA). The amount of electrical energy generated (expressed in joules per mole of substrate) and the amount of current produced from glucose (expressed in milliamperes) in NR-mediated microbial fuel cells containing either E. coli or A. succinogenes were about 10-and 2-fold greater, respectively, when resting cells were used than when growing cells were used. Cell growth was inhibited substantially when these microbial fuel cells were making current, and more oxidized end products were formed under these conditions. When sewage sludge (i.e., a mixed culture of anaerobic bacteria) was used in the fuel cell, stable (for 120 h) and equivalent levels of current were obtained with glucose, as observed in the pureculture experiments. These results suggest that NR is better than other electron mediators used in microbial fuel cells and that sludge production can be decreased while electricity is produced in fuel cells. Our results are discussed in relation to factors that may improve the relatively low electrical efficiencies (1.2 kJ/mol) obtained with microbial fuel cells.Electricity can be produced in different types of power plant systems, batteries (9, 12), or fuel cells (3). A biofuel cell is a device that directly converts microbial metabolic or enzyme catalytic energy into electricity by using conventional electrochemical technology (2, 16). Chemical energy can be converted to electric energy by coupling the biocatalytic oxidation of organic or inorganic compounds to the chemical reduction of an oxidant at the interface between the anode and cathode (22). It has been shown that direct electron transfer from microbial cells to electrodes occurs only at very low efficiency (1). In microbial fuel cells, two redox couples are required, one for coupling reduction of an electron mediator to bacterial oxidative metabolism and the other for coupling oxidation of the electron mediator to the reduction of the electron acceptor on the cathode surface (where the electron acceptor is regenerated with atmospheric oxygen) (4, 7).The amount of free energy produced either by normal microbial metabolism or by microbial fuel cell systems is determine...

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