Bioelectrochemical systems (BESs) are interesting systems that combine electrochemical red-ox reaction with biological activity for generating electricity from organic compounds. In fact, the organic compounds are actually the fuel for the fuel cell in which bacteria on the anode degrade organic molecules and transfer the resulting electrons to the electrode surface. The electrons move through the external circuit generating useful electricity to power devices or sensors. At the cathode, an oxidant is reduced to complete the red-ox reaction. Generally oxygen is used due to its high potential and natural availability.

Interestingly, it has been found that some bacteria, named exoelectrogens, are able to transfer electrons extracellularly to a solid support, generally called the anode electrode, if the substrate oxidation reaction occurs in absence of oxygen. The halfway potential represents the potential at which the electron transfer mechanism of an exoelectrogen is most favorable and thus outputs the most electricity. The more negative the halfway potential, the more energy can be produced. The goal of this project is to maximize microbial energy production considering different anode material-bacteria interactions.

Carbonaceous-based materials are typically used as anode in BESs due to their simplicity, low-cost fabrication, high surface area, high mechanical strength, high chemical resistance to corrosion and biocompatibility. It has been shown previously that both surface chemistry and surface morphology can affect positively or negatively the bacteria attachment on a surface.

Unfortunately, the electrical conductivity of carbonaceous materials is generally low compared to other materials and the durability is often negatively affected in long-term operation mainly due to material deterioration. Materials other than graphite have been proposed as suitable anode materials, but the effect of anode material on the underlying mechanism of extracellular electron transfer (EET) has not been yet addressed. Here, we measure electron transport properties of the model organism, Geobacter sulfurreducens, under turnover (with organic substrate) and nonturnover (without organic substrate) conditions, using an array of materials as working electrodes of an MFC (glassy carbon (GC), graphite (GR), gold (Au), platinum mesh (Pt), nickel (Ni) and indium tin oxide (ITO)). Experimentally, a 1L reactor that accommodates 6 working electrodes was used so all of the working electrodes could be tested under the same conditions with the same reference and counter electrodes. Ag/AgCl (3M KCl) was used as reference electrode while Pt was used as counter. Each material was used as a separate working electrode and connected to a single potentiostat (VMP3, Biologic, Inc., Knoxville, TN) channel.  The reactor was operated using a three-electrode configuration at a set anode potential of +0.3 V (vs. Ag/AgCl) to study each material at stable fixed potential, as opposed to a floating potential observed for MFC anodes.

Preliminary electrochemical tests produced cyclic voltammograms (CV) of all the materials under turnover and nonturnover conditions that displayed differences in slope and in the difference between halfway potential and formal potential, indicating that different materials yielded different electrochemical responses (Figure 1). The observed differences suggest that the bacteria are either using different electrochemical pathways to perform EET or that the material being used as the working electrode is influencing the environment and therefore altering the formal potential. We are currently conducting chemical measurements to characterize the working electrode surfaces along with a detailed study of the Geobacterbiofilm colonization. Finally, we will establish a relationship between the halfway potential and extracellular electron transfer dependence on the surface to which the biofilm is attached. 

Figure 1. Polarization of Geobacter sulfurreducens grown for 14 days on various materials [Preliminary Data]