A Quantitative Study of Extracellular Electron Transfer Kinetics in Electroactive Biofilm-Electrode Interface by Nonlinearly Fitting Discharging Current of Microbial Fuel Cells

In this abstract, we present a quantitative kinetic modeling of the extracellular electron transfer (EET) in electroactive Geobacter sulfurreducens enriched biofilm-electrode interface for a series of biofilm growth stages and utilizing the kinetic equations to study kinetics parameters of EET through nonlinear fitting of discharging current profiles of microscale microbial fuel cells (MFC). The quantitative EET rate constants and the amounts of redox cofactors associated with individual EET steps are obtained for initial to fully-grown biofilms. The quantitative study reports the rate-limiting step in EET transitions during the biofilm growth. In early to mid-stage biofilms, having current densities of less than 2.2 Am^-2 and between 2.2 and 3.1 Am^-2, respectively, the rate-limiting step transitions from irreversible acetate turnover to the electron transfer from inside exoelectrogen to extracellular redox cofactors (ERCs) within the biofilms. For fully-grown biofilms, having current density of more than 3.1 Am^-2, the rate-limiting step is the electron transfer from ERCs in biofilm to ERCs at the anode.

Microbial fuel cells (MFC) have been extensively studied in the past two decades to address the global warming and energy crisis. Although significant performance improvement has been achieved, the current and power densities are still lower than those of other power sources, such as lithium ion batteries. Further improving the performance of microbial fuel cells requires studying the bottleneck of current and power densities of MFCs. However, although many studies have studied the bottlenecks of microbial fuel cells by electrochemical or optical approaches, the bottlenecks are still not clear, which limits the further improvement of MFC performance.

The microscale MFC utilized to obtain discharging current has a chamber volume of 50 μL. We model the extracellular electron transfer (EET) on electroactive Geobacter sulfurreducens enriched biofilm-electrode interface for a series of biofilm growth stages by mathematical kinetic equations, and obtain the discharging current profiles by discharging the MFCs at different biofilm growth stages. Afterwards, nonlinear fitting is performed on the experimental discharging current profiles to obtain the quantitative EET rate constants and the amounts of redox cofactors associated with individual EET steps during different growth stages of biofilms. The results show that the rate-limiting bottleneck for EET changes as biofilm grows thicker. In early-stage biofilms, having current densities of less than 2.2 Am^-2, the rate-limiting bottleneck is the irreversible acetate turnover. In mid-stage biofilms, having current densities between 2.2 and 3.1 Am^-2, the rate-limiting bottleneck transitions from irreversible acetate turnover to the electron transfer from inside exoelectrogen to extracellular redox cofactors (ERCs) within the biofilms. For fully-grown biofilms, having current density of more than 3.1 Am^-2, and the rate-limiting bottleneck is the electron transfer from ERCs in biofilm to ERCs at the anode. The nonlinear fitting results provide important guidance on future work of improving the current generation capability of Geobacter sulfurreducens enriched biofilms: alleviating the bottleneck of electron transfer from ERCs in biofilm to ERCs at the anode will improve the current generation capability of Geobacter sulfurreducens enriched biofilms.