The microbial conversion of substrates is a key process to generate electricity in BESs. Despite, the microbial nature of the process, it is affected by electrochemical laws and principles which generally results in a lowering of the attainable voltage. The main electrical principles and the processes governing these losses are briefly described. Subsequently, the various conversions efficiencies are discussed.

Electrical parameters

Due to the positive potential difference (ΔE) between the poles of the MFC, the flow of electrons (I) generates a useful power (P) according to:

P = I x ΔE

The ratio between the voltage and the current is determined by the external resistance (R_ext) according to Ohms law:

ΔE = I x R_ext

When the external resistance is infinite (open circuit conditions) no current flows and the open circuit voltage (OCV) is obtained. Conversely, when the R_ext is zero (short circuit conditions; ΔE = zero) the short circuit current (I_SCC) is generated.

Alternatively, the relation between the cell voltage and the current (density) can be visualized by a polarization curve (Figure I). From the polarization curve, the power performance curve can be calculated (Figure I). The latter presents the relation between the power generation for a given current. The power delivered by a fuel cell is maximized when the external load matches the internal resistance of the fuel cell system (Benziger et al., 2006). Due to losses of any kind (see below), these curves are unfortunately not straight lines but they have a typical pattern (Figure I).

Figure I: The polarization curve (blue), with the respective open circuit voltage (OCV) and i_SCC (short circuit current density) and the power performance curve (red), with the maximum powerdensity (P_max). From the point of maximum power density, the optimal voltage (DE_opt) and optimal current density (i_opt) can be deduced. The striped lines indicate the ideal theoretical polarization curve and power performance curve in case no losses occur (after Clauwaert et al., 2008a).

Mass transfer

The supply of sufficient substrate to the anodic biofilm and electron acceptor to the cathode surface at rates of at least the equivalent of the current generation is crucial to sustain the current generation. In addition, the accumulation of waste products in the biofilm, e.g. oxidized intermediates or protons, needs to be prevented as this might change the redox conditions and hamper the metabolic activity of the biofilm. A limited mass transfer of substrate or electron acceptors towards the anode or cathode respectively, can result in concentration or mass transfer losses (Logan et al., 2006). Generally, mass transfer losses are characterized by a steep decrease of the cell voltage at near maximum current densities during polarization (Region C of Figure II).

In addition, a poor transfer of protons can cause the development of a pH gradient on the electrodes and between the anode and the cathode compartments. Both seriously affect the MFC performance (Freguia et al., 2008, Rozendal et al., 2008a).  

Ohmic losses

The flow of electrons is hampered by the resistance of the electrode material, which introduces an ohmic voltage loss. The higher the conductivity of the electrode material and the lower the contact losses and travel distance of the electrons within the electrode, the more efficient are the electron conduction and the lower the ohmic loss. To convert a substrate loading rate of 10 kg COD m^-3 BES.d^-1 into electrical current (assuming no other losses and a cell voltage of zero volt), the ohmic resistance of a BES should not exceed 0.66 mW.m³ (Clauwaert et al., 2008a) which translates into an ohmic resistance of 3.3 W for a reactor of 200 mL. The ohmic loss of many MFC designs is at least double this value.

In addition, for every negative charge transferred to the anode, an equal amount of positive charges (ideally protons, alternatively cations) needs to flow towards the counter electrode. The resistance which ions experience while flowing through the electrolyte is also part of the ohmic losses of the reactor. Rozendal et al. (2008a) calculated that wastewater with a typical conductivity of 1 mS.cm ¹ and assuming a practical current generation of 10 A.m^-2 results in a voltage loss of ~1V.cm^-1 electrode distance, which is approximately the same as the highest open circuit voltage reported of 0.93 V for MFCs (Clauwaert et al., 2007b). To allow an efficient transport of the ions, the conductivity and the buffer capacity of the electrolyte and a minimal distance between the electrodes are of uttermost importance.

In general the ohmic losses are proportionally most determinative in region B of Figure II. There are several techniques to characterize the ohmic losses. Both a polarization curve, the current interrupt method or electrochemical impedance spectroscopy, allow to determine the ohmic losses in a MFC. However, different techniques can result in different estimations of the ohmic resistance.

Activation losses

In order to start the transfer of electrons from the electrochemical active microorganisms (EAM) towards the electrode or to transfer electrons towards a final electron acceptor, an energy barrier needs to be overcome, which results in a voltage loss or activation overpotential (Logan et al., 2006). Activation losses are characterized by an initial steep decrease of the cell voltage at the onset of the electricity generation (region A of Figure II). As the current steadily increases, the other losses e.g. ohmic and mass transfer losses become proportionally more important. Low activation losses can be achieved by increasing the electrode surface area, improving the electrode catalysis, increasing the operating temperature, and in case of the microbial catalysis, through the establishment of an enriched biofilm on the electrode(s). It is hypothesized that microorganisms can lower the activation overpotential and thus increase their metabolic energy gain by optimizing their electron transferring strategies.

Electron quenching reactions and energy efficiency

Substrate competing processes, such as fermentation or methanogenesis and respiration (if oxygen intrudes), result in a loss of electrons (He et al., 2005, Liu and Logan, 2004). Also, part of the substrate is inherently converted into anodophilic biomass. Moreover, a leakage of substrate towards the cathode results in a potential electron loss. All these processes lower the conversion of substrate into current which is expressed by the coulombic efficiency (CE). The CE is defined as the ratio of the amount of substrate administered and the amount of electrons recovered.

The ratio between the actual cell voltage (DE ) and the OCV is determined as the potential efficiency (PE) (Lee et al., 2008). By multiplying the CE and the PE, the energy conversion efficiency (ECE) is obtained. It is not possible to simultaneously maximize both the power density and the energy conversion efficiency. As a consequence, the energy conversion efficiency of a fuel cell is only 50% when power density is maximized (Benziger et al., 2006). In general, the fuel efficiency increases as the ratio of the external load to the internal resistance increases, however in this case the power output decreases.

Redrafted after:

Aelterman, P. (2009) Microbial fuel cells for the treatment of waste streams with energy recovery. PhD Thesis, Gent University, Belgium  

References cited:

Benziger, J. B., Satterfield, M. B., Hogarth, W. H. J., Nehlsen, J. P. & Kevrekidis, I. G. (2006). The power performance curve for engineering analysis of fuel cells. J Power Sources 155, 272-285.

Clauwaert, P., Van der Ha, D., Boon, N., Verbeken, K., Verhaege, M., Rabaey, K. & Verstraete, W. (2007). Open air biocathode enables effective electricity generation with microbial fuel cells. Environ Sci Technol 41, 7564-7569.

Clauwaert, P., Aelterman, P., Pham, H. T., De Schamphelaire, L., Carballa, M., Rabaey, K. & Verstraete, W. (2008). Minimizing losses in bio-electrochemical systems: the road to applications. Appl Microbiol Biotechnol 79, 901.

Freguia, S., Rabaey, K., Yuan, Z. G. & Keller, J. (2008). Sequential anode-cathode configuration improves cathodic oxygen reduction and effluent quality of microbial fuel cells. Water Res 42, 1387-1396.

He, Z., Minteer, S. D. & Angenent, L. T. (2005). Electricity generation from artificial wastewater using an upflow microbial fuel cell. Environ Sci Technol 39, 5262-5267.

Lee, H. S., Parameswaran, P., Kato-Marcus, A., Torres, C. I. & Rittmann, B. E. (2008). Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs) utilizing fermentable and non-fermentable substrates. Water Res 42, 1501-1510.

Liu, H. & Logan, B. E. (2004). Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ Sci Technol 38, 4040-4046.

Logan, B. E., Hamelers, B., Rozendal, R., Schroder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W. & Rabaey, K. (2006). Microbial fuel cells: Methodology and technology. Environ Sci Technol 40, 5181-5192.

Rozendal, R. A., Hamelers, H. V. M., Rabaey, K., Keller, J. & Buisman, C. J. N. (2008). Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol 26, 450-459.