Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 188

associated with internal resistance, substrate diffusion, proton transfer, and
mechanical strength. For example, the overpotentials may increase with size due to
increased electrode spacing and longer distance for proton transfer, the substrate
and proton diffusion would be hindered at a decreased mixing (or it may unfa-
vorably lead to biomass detachment and increased energy consumption), the
separator and air-cathode may be vulnerable and have more leakage under
increased hydrostatic pressure, and there are the economic limitations. For these
reasons, currently the large MFCs generally show poorer performances than
smaller reactors [ 5 , 114 ]. In order to minimize the internal electrical resistances at
the electrodes, it is believed that an arrangement of multiple small reactors in a
stack might ultimately be the most effective design. Indeed, baffled and stacked
types of MFCs may be advantageous because of not only the possible lower
overpotential but also more flexibility for setup and operations. Nevertheless, our
knowledge in this aspect is still rather limited.
While many of the problems encountered can be addressed in laboratory
studies, other issues remain that ultimately require field testing. For example, pilot-
scale studies are needed to address questions like how well materials can perform
at larger scale, how the materials would be influenced by environmental condi-
tions, and how to offer proper maintenance on aeration and to control electrode
fouling. So far, there are already several pilot-scale MFC facilities running [ 29 ],
which are expected to offer valuable information on practical scaling-up of MFCs.
4.1.3 Long-Term Stability
For the MFC to become a commercially-viable technology, sufficient attention
should also be paid to process stability. Currently there are several factors that
unfavorably affect the long-term stability of MFC processes. First, the separator
performance would change over time due to biofilm growth or biofouling [ 22 ].
Second, there is the possibility of cathode fouling due to biofilm development and
electrode material clogging by suspended solids. These two factors would lead to
decreased mass and iron diffusion. Last, catalyst poisoning may occur due to the
complex nature of real wastewater. All these would ultimately impair the system
performance and lead to a decrease in electricity generation or pollutant removal.
Thus, these factors that lead to MFC instability during long-term operation and the
possible countermeasures all warrant further investigation.
4.1.4 Ecological Barriers
Cost is also a critical factor to consider in practical applications of MFCs. Specifically,
the costs of electrode materials, catalysts, separators, buffer agent and the operations all
need to be further reduced to enable commercialization of this technology.
While the properties of the electrode affect the performance of MFCs, it is worth
noting that the economic factor is still a critical criteria for electrode design [ 30 ].