Electrochemically active biofilms: facts and fiction. A review

Abstract

This review examines the electrochemical techniques used to study extracellular electron transfer in the electrochemically active biofilms that are used in microbial fuel cells and other bioelectrochemical systems. Electrochemically active biofilms are defined as biofilms that exchange electrons with conductive surfaces: electrodes. Following the electrochemical conventions, and recognizing that electrodes can be considered reactants in these bioelectrochemical processes, biofilms that deliver electrons to the biofilm electrode are called anodic, ie electrode-reducing, biofilms, while biofilms that accept electrons from the biofilm electrode are called cathodic, ie electrode-oxidizing, biofilms. How to grow these electrochemically active biofilms in bioelectrochemical systems is discussed and also the critical choices made in the experimental setup that affect the experimental results. The reactor configurations used in bioelectrochemical systems research are also described and the authors demonstrate how to use selected voltammetric techniques to study extracellular electron transfer in bioelectrochemical systems. Finally, some critical concerns with the proposed electron transfer mechanisms in bioelectrochemical systems are addressed together with the prospects of bioelectrochemical systems as energy-converting and energy-harvesting devices.

Figure 1

EABs can be studied using four different configurations: (A) an MFC with an anode and a cathode; (B) an MFC with an anode, a cathode and a reference electrode (RE) used to monitor individual electrode potentials (against the RE); (C) an MFC with an anode and a cathode and an RE connected to a potentiostat; and (D) an electrochemical cell with a working electrode (WE) covered by biofilm and a counter electrode (CE) and RE immersed in the same solution. This is called a three-electrode bioreactor.

Figure 2

Current generation by S. oneidensis MR-1 biofilm on a graphite electrode under anaerobic conditions in the reactor configuration shown in Figure 1C. The current increased steadily over a period of 9 days. The polarization potential was 0 mV_Ag/AgCl.

Figure 3

(A) In situ CV of S. oneidensis MR-1 (black trace). The biofilm was physically removed from the biofilm electrode and a second CV was performed (red dashed trace). A glassy carbon electrode (10 mm × 10 mm) was used. The scan rate was 10 mV s⁷¹. (B) In situ CV of G. sulfurreducens (black trace). The biofilm was physically removed from the biofilm electrode and a second CV was performed (red dashed trace). A glassy carbon electrode (25 mm × 25 mm) was used. The scan rate was 10 mV s⁷¹ for the first CV and 1 mV s⁷¹ for the second CV.

Figure 4

Example scan rate dependence of the peak potentials of the CV in Figure 3A. Red arrows show the peak potentials used for scan rate analysis. The inset shows that the peak potential is a linear function of scan rate.

Figure 5

(A) Choosing the initial potential of the CV can yield information on anodic and cathodic peak coupling. (B) Choosing different specific potential windows can show different CV behaviors that may be interpreted in different ways.

Figure 6

SWV of riboflavin in pH 4.5, 50 mM citric buffer at a glassy carbon electrode. The inset focuses on the electrochemical response. The midpoint potential was −277.7 mV_Ag/AgCl, very close to the theoretical value of −267.5 mV_Ag/AgCl at pH 4.5.

Figure 7

SWV of the S. oneidensis MR-1 biofilm described in Figure 4. The CV is the same as that in Figure 4 and is shown for comparison.

Figure 8

(A) Forward and reverse currents for S. oneidensis MR-1 in situ SWV. The difference current is the same as that in Figure 7. (B) Forward and reverse currents for G. sulfurreducens in situ SWV. Note the difference in the y-axis scales.

Figure 9

Bode (● and ○) and phase angle (■ and □) plots from EIS showing the complexity of electron transfer in G. sulfurreducens grown on glassy carbon electrodes (Figure 7 in reference). From Marsili et al. 2008b. Reproduced with permission from the American Society for Microbiology.

Figure 10

The redox potential inside a S. oneidensis MR-1 biofilm grown on a graphite electrode. Reprinted (adapted) with permission from Babauta et al. (2011). Copyright (2011) American Chemical Society.

Figure 11

(A) Diagram of a H₂O₂ microelectrode. (B) H₂O₂ concentration measured ~100 μm above a glassy carbon electrode during a CV scan. The inset shows current vs H₂O₂ concentration.

Figure 12

Absorbance spectra of G. sulfurreducens biofilms exposed to different electrode potentials under turnover (a, b) and non-turnover (c,d) conditions. Reprinted from Jain et al. (2011), with permission from Elsevier.

Figure 13

Potential losses at both the anode and the cathode restrict the amount of power that remains for the MFC when a resistor is connected. Activation, ohmic, and concentration losses reduce the anode and cathode potentials, lowering the cell potential from the maximum at OCP. The distances on the line are not drawn to scale.