Electrochemical Impedance Spectroscopy of Geobacter Sulfurreducens Biofilms on Rotating Disk Electrodes

Electrochemical impedance spectroscopy (EIS) has received significant attention recently as a method to measure electrochemical parameters of Geobacter sulfurreducens biofilms. Here, we grew G. sulfurreducens biofilms on rotating disk electrodes to determine the effect of mass transfer processes on the biofilm impedance. We controlled the rotation rate of the electrode up to 530 rpm to control the microscale gradients forming inside G. sulfurreducens biofilms respiring on the electrode. A 24% increase above a baseline of 82 µA was achieved with a rotation rate of 530 rpm. We compared the effect of rotation on the biofilm to a mass transfer-controlled soluble redox mediator, ferrocyanide, to make the distinction between infinite Warburg and pseudocapacitive responses. We observed a 340% increase above baseline using ferrocyanide at 530 rpm, indicating that the primary limitation of electron transfer rates in G. sulfurreducens biofilms was not mass transfer. 

Control of mass transfer processes was also used to quantify the change in biofilm impedance during the transition from turnover to non-turnover conditions. An equivalent electrical circuit model was used to estimate the electrochemical parameters of the biofilm. We found that only one element of the biofilm impedance, the interfacial resistance, changed significantly from 900 to 4,200 Ω under turnover and non-turnover conditions, respectively. We ascribed this change to the electron transfer resistance overcome by the biofilm metabolism and estimate this value as 3,300 Ω. Additionally, under non-turnover, the biofilm impedance developed pseudocapacitive behavior indicative of bound redox mediators. Pseudocapacitance of the biofilm was estimated at 740 µF and was unresponsive to rotation of the electrode. Overall, we tested the hypothesis that the rotating disk electrode can be used as an electrochemical tool that controls mass transfer processes when studying electrochemically active biofilms and facilitates our understanding of EIS in microbially-driven electrochemical systems. The increase in electron transfer resistance and pseudocapacitive behavior under non-turnover could be used as indicators of acetate limitations inside G. sulfurreducens biofilms.