(561i) Characterizing Anode Potential Losses in Microbial Fuel Cells Using Electrochemical Techniques

Characterizing Anode Potential
Losses in Microbial Fuel Cells Using Electrochemical Techniques

Sudeep C. Popat and César I.
Torres

Anode-respiring bacteria (ARB) are
unique in their capacity to transfer electrons from organic substrates to a
solid anode.  The result of anode
respiration is an electrical current that can be used for various bioenergy
applications including power generation in microbial fuel cells (MFCs).  An efficient MFC must have minimal potential
losses at the anode, the cathode, or the electrolyte. The anode of an MFC
presents an interesting challenge as it is composed of anode-respiring bacteria
(ARB) catalyzing the oxidation of organic matter into CO₂.  As a living catalyst, ARB form
a porous biofilm that extends tens of micrometers.  Inside the ARB biofilm, concentration and
potential gradients determine the current production by ARB.  Anode resistances can be categorized as those
associated with (1) mass transport, (2) ARB intracellular metabolism, and (3) extracellular
electron transfer (EET).  Although some
efforts have been made to characterize these anodic resistances, their
contributions to MFC potential losses are largely unknown.

Electrochemical
measurements performed recently have begun to provide quantitative measurements
of EET resistances (Torres et al., 2008; Torres et al., 2010; Strycharz et al., 2011; Manklavar
et al., 2011).  Measurements performed
using cyclic voltammetry (CV) in inter-digitated
microelectrode arrays (IDAs) provide biofilm conductivity measurements that
allow us to estimate EET resistances.  In
addition, the studies performed by Manklavar et al.
(2011) complement these measurements and provide specific details on the nature
of the EET conductive matrix.  Our research
group used CVs to interpret anode resistances in MXCs using the Nernst-Monod
equation (Torres et al., 2007; Torres et al., 2008).  All these efforts have one commonality:  the conclusion that EET resistances are low,
indicative of an efficient electron-transport mechanism through the conductive
biofilm.

Although anode
potential losses can be measured with CV, they cannot be characterized
individually.  Theoretical assumptions,
such as those proposed by the Nernst-Monod equation, are the only tools
available to further characterize resistances in CVs.  In order to address this problem of
separating individual polarization losses, fuel-cell researchers rely on Electrochemical
Impedance Spectroscopy (EIS).  EIS has been
used extensively in measuring the polarization losses at the anodes and
cathodes of solid oxide fuel cells.  A
key feature of EIS is that it can help distinguish multiple processes
contributing to polarization loss on a single electrode, if the characteristic
frequency at which each process controls is different.  The use of EIS in MFCs has only begun to be
explored, with the most notable studies being on characterizing total anodic, ohmic, and cathodic polarization losses during long-term
operation (Borole et al., 2010) and in detecting
shuttles produced by ARB (Ramasamy et al., 2009).

 Our preliminary EIS analyses
using G. sulfurreducens as model ARB seem
to indicate that at least two anodic resistances can be resolved using EIS within
typical frequency ranges (Figure 1).  The
resistances we observed reveal very interesting properties of ARB's electron
transfer.  The first resistance
(exhibited at high frequencies) appears to be constant as a function of anode
potential (not shown).  More explicitly,
it acts as a linear resistance. 
Enzymatic processes always have a sigmoidal response to potential;
therefore, this resistance must be attributed to EET (e.g., nanowires acting as
a linear resistor).  The second
resistance is affected by anode potential (not shown) and follows a sigmoidal
response, which is typical of enzymatic processes.  However, the response obtained from this
second resistance is that of a Nernst-Monod sigmoid with n = 2, not n = 1.  Most researchers have assumed an n=1 for the
Nernst-Monod fitting based on the fact that cytochromes carry out a
one-electron transfer and are known to be key proteins in the EET pathway.  These results underscore that we need to revisit
the n = 1 assumption for the Nernst-Monod equation.  If n = 2 for the Nernst-Monod curve, a
two-electron transfer ultimately limits the flow of electrons to the
anode.  This process most likely occurs
within or before the electron-transport chain, in which a few reactions are
known to have n = 2. Thus, we hypothesize that the second resistance observed
in Figure 1 is attributable to an intracellular resistance, basically an energy
loss within the cell.  If this is correct, EIS becomes a method to
differentiate between EET resistances and intracellular resistances. 

Figure 1.  Nyquist plots
example of an ARB biofilm tested at -0.35 vs Ag/AgCl. 

References:

Borole, A. P., D. Aaron,
et al. (2010). Environ Sci Technol 44(7):
2740-2744.

Malvankar, N. S., M. Vargas,
et al. (2011). Nat Nanotechnology 6(9): 573-579.

Ramasamy, R. P., V. Gadhamshetty, et al. (2009). Biotechnol
Bioeng 104(5): 882-891.

Strycharz, S. M., A. P. Malanoski, et al. (2011). Energ Environ Sci 4(3): 896-913.

Torres, C. I., A. Kato Marcus, et al. (2007). Appl Microbiol Biotechnol 77(3): 689-697.

Torres, C. I., A. K. Marcus, et al. (2008). Environ Sci Technol 42(17): 6593-6597.

Torres, C. I., A. K.
Marcus, et al. (2010). FEMS Microbiol Rev 34(1):
3-17.