Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 179

2.5 Process Enhancement Techniques
In biological processes, various physical or chemical enhancement technologies
are usually applied to promote bioactivity and degradation performance.
Such strategies have also been recently investigated in MFC systems to improve
biocatalytic activity and accelerate electrochemical reactions.
Since the anodic potential to some extent determines the degradation rate of the
substrate in the anodic chamber, it is expected that the anodic potential may also
affect the growth and activity of anodic microorganisms. An improved enrichment
of acetate-oxidizing microorganisms on the anode at a positive-poised potential
was first observed by Finkelstein et al. [ 69 ] in a benthic MFC. To obtain a more
comprehensive understanding of this enhancement, the effect of anodic positive
potential on start-up time and performance of the MFC was investigated by
Wang et al. [ 70 ]. The application of a +200 mV (versus Ag/AgCl) poised potential
on the anode was found to shorten the startup time from 59 to 35 days compared to
the control. In this process, a higher voltage output and a lower charge transfer
resistance were obtained, demonstrating that the applied positive potential could
increase the electrochemical activity of anodic microorganisms. This enhancement
was thought to mainly resulted from an increased driving force of substrate
oxidation that favors bacteria growth and bacteria adhesion onto the positively-
charged anode surface. In addition to constant additional voltage application, a
transient external voltage (lasting for only 10 min) was also recently found to
promote the bioanode performance of MFCs [ 71 ]. The enhancement effects were
thought to be mainly attributed to an improved bioanode catalytic activity as well
as an accelerated bacteria adhesion. Notably, employing a transient voltage, which
requires only small amount of energy input, could be economically more attractive
than a permanent voltage application. Such external voltage enhancement tech-
niques offer a possible new avenue for MFC improvement, but the underlying
mechanism is yet to be elucidated and further optimization of the specific
parameters is needed. Apart from external voltage application, an external mag-
netic field was also recently found to improve MFC performance. Li et al. [ 72 ]
applied a 100 mT magnetic field to MFCs, and found it significantly decreased the
internal resistance and enhanced the power generation. The magnetic field is
suspected to have directly accelerated the growth and improved the bioelectro-
chemical activity of a Shewanella strain at the anode through an oxidative stress
mechanism. The exact mechanism likewise needs further in-depth investigation.
In addition, direct chemical dosing may provide another feasible approach to
enhancing MFC performance. Wen et al. [ 73 ] investigated the possibility of
promoting intracellular electron transfer through biosurfactant addition, because
surfactants are known to readily change the cell membrane ultrastructure to
create trans-membrane channels. As expected, the addition of rhamnolipid as a
biosurfactant increased the permeability of bacteria cells, leading to substantially
lowered electron transfer resistance and a 12.5-fold increase in power density
(from 22 to 275 W/m 3 ).