Electricity generation
MFCs are capable of converting the chemical energy
stored in the chemical compounds in a biomass to
electrical energywith the aid of microorganisms. Because
chemical energy from the oxidization of fuel molecules is
converted directly into electricity instead of heat, the
Carnot cycle with a limited thermal efficiency is avoided
and theoretically amuch higher conversion efficiency can
be achieved (>70%) just like conventional chemical fuel
cells. Chaudhury and Lovley (2003) reported that R.
ferrireducens could generate electricity with an electron
yield as high as 80%.
Generation of Electricity
Higher electron recovery as electricity of up to 89% was also reported.
An extremely high Coulombic efficiency of 97% was
reported during the oxidation of formatewith the catalysis
of Pt black. However, MFC
power generation is still very low, that is the rate of electron
abstraction is very low. One feasible way to solve this
problem is to store the electricity in rechargeable devices
and then distribute the electricity to end-users. Capacitors were used in their biologically
inspired robots named EcoBot I to accumulate the energy
generated by the MFCs and worked in a pulsed manner.
MFCs are especially suitable for powering small
telemetry systems and wireless sensors that have only low power requirements to transmit signals such as temperature to receivers in remote locations
MFCs themselves
can serve as distributed power systems for local uses,
especially in underdeveloped regions of the world. MFCs
are viewed by some researchers as a perfect energy supply
candidate for Gastrobots by self-feeding the biomass
collected by themselves.
Researcher Holding a MFC Cell
Realistic
energetically autonomous robots would probably be
equipped with MFCs that utilize different fuels like
sugar, fruit, dead insects, grass and weed. The robot
EcoBot-II solely powers itself by MFCs to perform some
behavior including motion, sensing, computing and
communication. Locally supplied
biomass can be used to provide renewable power for local
consumption. Applications of MFCs in a spaceship are
also possible since they can supply electricity while
degrading wastes generated onboard. Some scientists
envision that in the future a miniature MFC can be im-planted in a human body to power an implantablemedical
device with the nutrients supplied by the human body. The MFC technology is particularly favored
for sustainable long-term power applications. However,
only after potential health and safety issues brought by the
microorganisms in the MFC are thoroughly solved, could
it be applied for this purpose.
Biohydrogen
MFCs can be readily modified to produce hydrogen
instead of electricity. Under normal operating conditions, protons released by the anodic reaction migrate to
the cathode to combine with oxygen to form water.
Hydrogen generation from the protons and the electrons
produced by the metabolism of microbes in an MFC is
thermodynamically unfavorable. Liu et al. (2005c)
applied an external potential to increase the cathode
potential in a MFC circuit and thus overcame the thermodynamic barrier. In this mode, protons and electrons
Schematic of MFC Producing Hydrogen
produced by the anodic reaction are combined at the
cathode to form hydrogen. The required external potential for an MFC is theoretically 110 mV, much lower
than the 1210 mV required for direct electrolysis of
water at neutral pH because some energy comes from
the biomass oxidation process in the anodic chamber.
MFCs can potentially produce about 8-9 mol H2/mol
glucose compared to the typical 4 mol H2/mol glucose
achieved in conventional fermentation. In biohydrogen production using MFCs, oxy-gen is no longer needed in the cathodic chamber. Thus,
MFC efficiencies improve because oxygen leak to the anodic chamber is no longer an issue. Another advan-tage is that hydrogen can be accumulated and stored for
later usage to overcome the inherent low power feature of the MFCs. Therefore, MFCs provide a renewable hydrogen source that can contribute to the overall hy-drogen demand in a hydrogen economy.
Wastewater treatment
The MFCs were considered to be used for treating
waste water early in 1991. Municipal wastewater contains a multitude of
organic compounds that can fuel MFCs. The amount of
power generated by MFCs in the wastewater treatment
process can potentially halve the electricity needed in a
conventional treatment process that consumes a lot of
electric power aerating activated sludges.
Waste Water Plant Using MFC
MFCs yield
50-90% less solids to be disposed of.
Furthermore, organic molecules such as acetate, propi-onate, butyrate can be thoroughly broken down to CO2
and H2O. A hybrid incorporating both electrophiles and
anodophiles are especially suitable for wastewater
treatment because more organics can be biodegraded
by a variety of organics. MFCs using certain microbes
have a special ability to remove sulfides as required in
wastewater treatment. MFCs can
enhance the growth of bioelectrochemically active
microbes during wastewater treatment thus they have
good operational stabilities. Continuous flow and
single-compartment MFCs and membrane-less MFCs
are favored for wastewater treatment due to concerns in
scale-up. Sanitary wastes, food processing wastewater,
swine wastewater and corn stover are all great biomass
sources for MFCs because they are rich in organic
matters. Up to
80%of theCODcan be removed in some cases and a Coulombic efficiency as
high as 80% has been reported.
Biosensor
Apart from the aforementioned applications, another
potential application of the MFC technology is to use it
as a sensor for pollutant analysis andin situ process
monitoring and control. The
proportional correlation between the Coulombic yield of
MFCs and the strength of the wastewater make MFCs
possible biological oxygen demand (BOD) sensors. An accurate method to measure the BOD value of a liquid stream is to calculate its
Coulombic yield.
A Diagram of MFC Sensor
A number of works showed a good linear relation-ship between the Coulombic yield and the strength of
the wastewater in a quite wide BOD concentration
range. However, a high BOD concentration requires a
long response time because the Coulombic yield can be
calculated only after the BOD has been depleted unless a
dilution mechanism is in place. Efforts have been made
to improve the dynamic responses in MFCs used as
sensors. A low BOD sensor can also
show the BOD value based on the maximum current
since the current values increase with the BOD value
linearly in an oligotroph-type MFC. During this stage,
the anodic reaction is limited by substrate concentration.
This monitoring mode can be applied to real-time BOD
determinations for either surface water, secondary
effluents or diluted high BOD wastewater samples. MFC-type of BOD sensors are
advantageous over other types of BOD sensor because
they have excellent operational stability and good repro-ducibility and accuracy. An MFC-type BOD sensor
constructed with the microbes enriched with MFC can
be kept operational for over 5 years without extra
maintenance, far longer in service life
span than other types of BOD sensors reported in the
literature.
