Biomedical Engineering Reference
In-Depth Information
obstacles is the high voltage losses, also called overpotential [
1
]. Several reasons
contribute to this overpotential. First, the oxidation or reduction reactions at the
electrode incur activation overpotential. Addition of chemicals or employing
biological catalysts can decrease but will never eliminate the activation overpo-
tential. Second, the transfer of electrons through an electrical circuit and ions
through the electrolyte result in ohmic loss. Finally, the supply of substrate or
discharge of protons may become limited at high current density or insufficient
mixing, leading to concentration overpotential. All these add up to voltage losses
and thus decrease the power output [
110
]. Intensive investigations have focused on
these aspects in efforts to reduce the overall voltage losses; for example, searching
for more conductive materials, adopting highly-active catalysts and removing
separation membranes. However, there is a trade-off between the properties and
costs of the materials in most cases. A membrane-less operation can accelerate
proton transfer and reduce anodic inhibition at high pH, but it also incurs the risk
of oxygen intrusion and substrate crossover. One possible solution is to use coarse-
porous separators in place of conventional PEMs, but these materials vary sig-
nificantly in their properties and mostly may not be able to resist hydraulic
pressure. This makes separator design difficult.
Although MFC technologies are also progressing toward low-power utilization,
such as powering implanted medical devices and on-chip instruments, they are still
in the infancy stage. In addition, high current density should be paid more
attention, because it may result in excessive ohmic heating. More detailed study on
optimizing the electrode allocations, power density, and operating stability is
necessary before further applications are possible. Moreover, further studies on
implantation rejection, microbe leakage, and cytotoxicity are needed to investigate
whether mini-MFCs are suitable for practical medical applications.
One common practice to raise overall power output is to connect multiple
MFCs as a stack either in series or in parallel [
20
]. However, voltage reversal may
occur in such stacked systems due to different resistances between stack units or
substrate starvation during operation. The voltage loss is proposed to be a con-
sequence of parasitic current flow due to the substrate cross-conduction effect
when fuel cell arrays share the same electrolyte but have inappropriate connection
with each other. Another crucial parameter to be considered is to reduce the
internal resistance in order to minimize the number of MFCs needed. Thus, key
developments are still required to create highly conductive and scalable scaffolds
with suitable properties for stacking.
4.1.2 Scaling Up
Scale-up of MFCs for practical wastewater treatment and energy production is an
important issue. However, at the present stage, full-scale implementation of this
technology is not straightforward because of several technological and economic
barriers. Scaling up an MFC involves more than just linearly scaling up all reactor
dimensions and increasing the surface area of electrodes. There are many problems
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