(547a) Comprehensive Modeling of a Microbial Fuel Cell

Bacteria consume electron donors (fuel) which are oxidized to gain energy. This electrochemical reaction and the reduction of terminal electron acceptors typically occur inside the bacterial cells. Recent discoveries, however, revealed that some bacteria such as Geobacter can directly transfer electrons out of their cell onto solid electrode surfaces [1]. This extra-cellular electron transfer facilitates the use of these bacteria as catalysts in electrochemical cells which oxidize a variety of electron donors and to capture the electrical energy produced. Such “microbial fuel cells” (MFCs) can be used not only for electrical power production but also for biosensors [2] and even waste treatment [3].

Preliminary experiments in our laboratory using Geobacter metallireducens as a biocatalyst showed that stable operation of a semi-batch MFC was possible when the electron donor and the medium were regularly replenished. Electricity production, however, was approximately two orders of magnitude smaller than the estimated theoretical limit despite an excess of electron donor. This suggests that a process other than the microbial respiration rate is rate-limiting. Unfortunately, little is known about the processes involved in MFCs, especially electron transfer. Zhang and Halme modeled an MFC system by considering only reaction kinetics under the complete mixing condition [4]. This model did not consider mass transfer through a biofilm or a diffusion boundary layer, which could be rate-limiting in some systems. The researchers assumed that electron transfer to the electrode surface was carried out solely by an electron-transfer mediator. Recent research, however, suggests that electron transfer may be facilitated by nanowires that provide electrical connections between bacteria and the electrode surface through the biofilm [5]. Therefore, more work is needed to develop a comprehensive model of MFCs in order to improve our understanding of MFCs and to optimize their design and operation.

In this research, an MFC with a membrane-electrode assembly was modeled under quasi-steady state and non-steady state conditions. The model assumed the presence of a thin biofilm on the anode surface, a diffusion boundary layer, diffusion and consumption of the electron donor in the biofilm, and cross-over of the electron donor through the membrane. For electron transfer, direct electron transfer via the moderately conductive biofilm and indirect transfer via a hypothetical mediator were considered. The model is based on fundamental theories of mass transfer, electron transfer, and microbial respiration and incorporates experimentally determined parameters as well as estimated parameters. The results of this modeling explain trends in the experimental data, provide direction for future experiments, and give insights into more efficient MFC designs and operational strategies.

[1] Bond, D. R.; Lovely, D. R. (2003) "Electricity Production by Geobacter sulfurreducens Attached to Electrodes," Appl. Environ. Microbiol. 69, 1548-1555. [2] Gil, G.; Chang, I.; Kim, B.; Kim, M.; Jang, J.; Park, H.; Kim, H. (2003) "Operating Parameters Affecting the Performance of a Mediator-Less Microbial Fuel Cell," Biosens. Bioelectron. 18, 327-334. [3] Rabaey, K.; Boon, N.; Siciliano, S. D.; Verhaege, M.; Verstraete, W. (2004) "Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer," Appl. Environ. Microbiol. 70, 5373-5382. [4] Zhang, X.; Halme, A. (1995) "Modeling of a Microbial Fuel Cell Process," Biotechnol. Let. 17, 809-814. [5] Reguera, G.; McCarthy, K. D.; Mehta, T.; Nicoll, J. S.; Tuominen, M. T.; Lovley, D. R. (2005) "Extracellular Electron Transfer via Microbial Nanowires," Nature. 435, 1098-1101.