A microbial fuel cell (MFC) or biological
fuel cell is a bio-electrochemical system that drives a current
by using bacteria
and mimicking bacterial interactions found in nature. MFCs can be
grouped into two general categories, those that use a mediator and those that
are mediator-less. The first MFCs, demonstrated in the early 20th century, used
a mediator: a chemical that transfers electrons from the bacteria in the cell
to the anode. Mediator-less MFCs are a more recent development dating to the
1970s; in this type of MFC the bacteria typically have electrochemically active
redox proteins such as cytochromes
on their outer membrane that can transfer electrons directly to the anode.
Since the turn of the 21st century
MFCs have started to find a commercial use in the treatment of wastewater.
History
The idea of using microbial cells in an attempt to produce electricity
was first conceived in the early twentieth century. M. Potter was the first to
perform work on the subject in 1911. A professor of botany at the University of Durham, Potter managed to
generate electricity from E. coli, but the work was not to receive any major
coverage. In 1931, however, Barnet Cohen drew more attention to the area when
he created a number of microbial half fuel cells that, when connected in
series, were capable of producing over 35 volts, though only with a current of
2 milliamps.
More work on the subject came with a study by DelDuca et al.
who used hydrogen produced by the fermentation of glucose by Clostridium butyricum as the reactant at
the anode of a hydrogen and air fuel cell. Though the cell functioned, it was
found to be unreliable owing to the unstable nature of hydrogen production by
the micro-organisms. Although this issue was later resolved in work by Suzuki
et al. in 1976 the current design concept of an MFC came into existence a year
later with work once again by Suzuki.
By the time of Suzuki’s work in the late 1970s, little was
understood about how microbial fuel cells functioned; however, the idea was
picked up and studied later in more detail first by MJ Allen and then later by
H. Peter Bennetto both from King's College London. People saw the fuel
cell as a possible method for the generation of electricity for developing
countries. His work, starting in the early 1980s, helped build an understanding
of how fuel cells operate, and until his retirement, he was seen by many as the
foremost authority on the subject.
It is now known that electricity can be produced directly
from the degradation of organic matter in a microbial fuel cell. Like a normal
fuel cell, an MFC has both an anode and a cathode chamber. The anoxic
anode chamber is connected internally to the cathode chamber via an ion
exchange membrane with the circuit completed by an external wire.
In May 2007, the University of Queensland, Australia
completed its prototype MFC as a cooperative effort with Foster's
Brewing. The prototype, a 10 L design, converts brewery wastewater into carbon dioxide, clean
water, and electricity. With the prototype proven successful, plans are in
effect to produce a 660 gallon version for the brewery, which is estimated to
produce 2 kilowatts of power. While this is a small amount of power, the
production of clean water is of utmost importance to Australia, for which drought is a constant threat.
Types
Definition
A microbial fuel cell is a device that converts chemical energy
to electrical energy by the catalytic reaction of microorganisms.
A typical microbial fuel cell consists of anode and cathode
compartments separated by a cation (positively charged ion) specific membrane. In the anode compartment, fuel is
oxidized by microorganisms, generating CO2, electrons and protons. Electrons
are transferred to the cathode compartment through an external electric
circuit, while protons are transferred to the cathode compartment through the
membrane. Electrons and protons are consumed in the cathode compartment,
combining with oxygen to form water.
More broadly, there are two types of microbial fuel cell:
mediator and mediator-less microbial fuel cells.
Mediator microbial fuel cell
Most of the microbial cells are electrochemically inactive.
The electron transfer from microbial cells to the electrode is
facilitated by mediators such as thionine, methyl
viologen, methyl blue, humic acid,
neutral
red and so on. Most of the mediators available are expensive and toxic.
Mediator-free microbial fuel cell
Mediator-free microbial fuel cells do not require a mediator
but use electrochemically active bacteria to transfer electrons to the
electrode (electrons are carried directly from the bacterial respiratory enzyme
to the electrode). Among the electrochemically active bacteria are, Shewanella putrefaciens, Aeromonas hydrophila, and others. Some
bacteria, which have pili
on their external membrane, are able to transfer their electron production via
these pili. Mediator-less MFCs are a more recent area of research and, due to
this, factors that affect optimum efficiency, such as the strain of bacteria used in the system, type of ion-exchange membrane, and system conditions
(temperature, pH, etc.) are not particularly well understood.
Mediator-less microbial fuel cells can, besides running on
wastewater, also derive energy directly from certain plants. This configuration
is known as a plant microbial fuel
cell. Possible plants include reed
sweetgrass, cordgrass, rice, tomatoes, lupines, and algae. Given that the
power is thus derived from living plants (in situ-energy production), this
variant can provide additional ecological advantages.
Microbial electrolysis cell
A variation of the mediator-less MFC is the microbial
electrolysis cells (MEC). Whilst MFC's produce electric current by the
bacterial decomposition of organic compounds in water, MECs partially reverse
the process to generate hydrogen or methane by applying a voltage to bacteria
to supplement the voltage generated by the microbial decomposition of organics
sufficiently lead to the electrolysis of water or the production of
methane. A complete reversal of the MFC principle is found in microbial electrosynthesis, in which
carbon dioxide is reduced by bacteria using an external electric current to
form multi-carbon organic compounds.
Soil-based microbial fuel cell
A soil-based MFC
Soil-based microbial fuel cells adhere to the same basic MFC
principles as described above, whereby soil acts as the nutrient-rich anodic
media, the inoculum, and the proton-exchange membrane (PEM). The anode is
placed at a certain depth within the soil, while the cathode rests on top the
soil and is exposed to the oxygen in the air above it.
Soils are naturally teeming with a diverse consortium of
microbes, including the electrogenic microbes needed for MFCs, and are full of
complex sugars and other nutrients that have accumulated over millions of years
of plant and animal material decay. Moreover, the aerobic (oxygen consuming)
microbes present in the soil act as an oxygen filter, much like the expensive
PEM materials used in laboratory MFC systems, which cause the redox potential
of the soil to decrease with greater depth. Soil-based MFCs are becoming
popular educational tools for science classrooms.
Phototrophic biofilm microbial fuel cell
Phototrophic biofilm MFCs (PBMFCs) are the ones that make use
of anode with a phototrophic biofilm containing photosynthetic microorganism like
chlorophyta, cyanophyta etc., since they could carry out photosynthesis and
thus they act as both producers of organic metabolites and also as electron
donors.
A study conducted by Strik et al. reveals that PBMFCs yield
one of the highest power densities and, therefore, show promise in
practical applications. Researchers face difficulties in increasing their power
density and long-term performance so as to obtain a cost-effective MFC.
The sub-category of phototrophic microbial fuel cells that
use purely oxygenic photosynthetic material at the anode are sometimes called biological photovoltaic systems.
Electrical generation process
When micro-organisms consume a substance such as sugar in aerobic
conditions, they produce carbon dioxide and water. However, when oxygen is not
present, they produce carbon dioxide, protons, and electrons, as
described below:
C12H22O11 + 13H2O
→ 12CO2 + 48H+ + 48e- (Eqt. 1)
Microbial fuel cells use inorganic
mediators to tap into the electron transport chain of cells and
channel electrons produced. The mediator crosses the outer cell lipid
membranes and bacterial outer membrane; then, it begins
to liberate electrons from the electron transport chain that normally would be
taken up by oxygen or other intermediates.
The now-reduced mediator exits the cell laden with electrons
that it transfers to an electrode where it deposits them; this electrode
becomes the electro-generic anode (negatively charged electrode). The release
of the electrons means that the mediator returns to its original oxidised state
ready to repeat the process. It is important to note that this can happen only
under anaerobic
conditions; if oxygen is present, it will collect all the electrons, as it has
a greater electronegativity than mediators.
In a microbial fuel cell operation, the anode is the
terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore,
the microbial activity is strongly dependent on the redox potential of the
anode. In fact, it was recently published that a Michaelis-Menten
curve was obtained between the anodic potential and the power output of an
acetate driven microbial fuel cell. A critical anodic potential seems to exist
at which a maximum power output of a microbial fuel cell is achieved.
A number of mediators have been suggested for use in
microbial fuel cells. These include natural red, methylene blue, thionine, or
resorufin.
This is the principle behind generating a flow of electrons
from most micro-organisms (the organisms capable of producing an electric
current are termed exoelectrogens). In order to turn this into a usable
supply of electricity, this process has to be accommodated in a fuel cell. In
order to generate a useful current it is necessary to create a complete
circuit, and not just transfer electrons to a single point.
The mediator and micro-organism, in this case yeast, are
mixed together in a solution to which is added a suitable substrate such as
glucose. This mixture is placed in a sealed chamber to stop oxygen entering,
thus forcing the micro-organism to use anaerobic respiration. An electrode is placed
in the solution that will act as the anode as described previously.
In the second chamber of the MFC is another solution and
electrode. This electrode, called the cathode is positively charged and is the
equivalent of the oxygen sink at the end of the electron transport chain, only
now it is external to the biological cell. The solution is an oxidizing
agent that picks up the electrons at the cathode. As with the electron
chain in the yeast cell, this could be a number of molecules such as oxygen.
However, this is not particularly practical as it would require large volumes
of circulating gas. A more convenient option is to use a solution of a solid
oxidizing agent.
Connecting the two electrodes is a wire (or other
electrically conductive path, which may include some electrically powered
device such as a light bulb) and completing the circuit and connecting the two
chambers is a salt bridge or ion-exchange membrane. This last feature allows
the protons produced, as described in Eqt. 1 to pass from the anode
chamber to the cathode chamber.
The reduced mediator carries electrons from the cell to the
electrode. Here the mediator is oxidized as it deposits the electrons. These
then flow across the wire to the second electrode, which acts as an electron sink. From here they
pass to an oxidising material.
Applications
Power generation
Microbial fuel cells have a number of potential uses. The
most readily apparent is harvesting electricity produced for use as a power
source. The use of MFCs is attractive for applications that require only low
power but where replacing batteries may be time-consuming and expensive such as
wireless sensor networks. Virtually any organic material could be used to feed
the fuel cell, including coupling cells to wastewater treatment plants.
Bacteria would consume waste material from the water and
produce supplementary power for the plant. The gains to be made from doing this
are that MFCs are a very clean and efficient method of energy production.
Chemical processing wastewater and designed synthetic wastewater have been used
to produce bioelectricity in dual- and single-chamber mediatorless MFCs
(non-coated graphite electrodes) apart from wastewater treatment.
Higher power production was observed with biofilm covered anode (graphite). A
fuel cell’s emissions are well below regulations. MFCs also use energy much
more efficiently than standard combustion engines, which are limited by the Carnot
Cycle. In theory, an MFC is capable of energy efficiency far beyond 50%
(Yue & Lowther, 1986). According to new research conducted by René
Rozendal, using the new microbial fuel cells, conversion of the energy to
hydrogen is 8 times as high as conventional hydrogen production technologies.
However, MFCs do not have to be used on a large scale, as
the electrodes in some cases need only be 7 μm thick by 2 cm long.The
advantages to using an MFC in this situation as opposed to a normal battery is
that it uses a renewable form of energy and would not need to be recharged like
a standard battery would. In addition to this, they could operate well in mild
conditions, 20 °C to 40 °C and also at pH of around 7. Although
more powerful than metal catalysts, they are currently too unstable for
long-term medical applications such as in pacemakers (Biotech/Life Sciences Portal).
Besides wastewater power plants, as mentioned before, energy
can also be derived directly from crops. This allows the set-up of power
stations based on algae platforms or other plants incorporating a large field
of aquatic plants. According to Bert Hamelers, the fields are best set-up in
synergy with existing renewable plants (e.g., offshore wind turbines). This reduces
costs as the microbial fuel cell plant can then make use of the same
electricity lines as the wind turbines.
Education
Soil-based microbial fuel cells are popular educational
tools, as they employ a range of scientific disciplines (microbiology,
geochemistry, electrical engineering, etc.), and can be made using commonly
available materials, such as soils and items from the refrigerator. There are
also kits available for classrooms and hobbyists, and research-grade kits for
scientific laboratories and corporations.
Biosensor
Since the current generated from a microbial fuel cell is
directly proportional to the energy content of wastewater used as the fuel, an
MFC can be used to measure the solute concentration of wastewater (i.e., as a biosensor
system).
The strength of wastewater is commonly evaluated as biochemical oxygen demand (BOD) values.BOD
values are determined incubating samples for 5 days with proper source of
microbes, usually activate sludge collected from sewage works. When BOD values
are used as a real-time control parameter, 5 days' incubation is too long.
An MFC-type BOD sensor can be used to measure real-time BOD
values. Oxygen and nitrate are preferred electron acceptors over the electrode
reducing current generation from an MFC. MFC-type BOD sensors underestimate BOD
values in the presence of these electron acceptors. This can be avoided by
inhibiting aerobic and nitrate respirations in the MFC using terminal oxidase
inhibitors such as cyanide and azide. This type of BOD sensor is commercially
available.
Biorecovery
In 2010, A. ter Heijne et al. constructed a device capable
of producing electricity and reduce the ion Cu (II) to copper metal.
Microbial electrolysis cells have been demonstrated to
produce hydrogen.
Current research practices
Some researchers point out some undesirable practices, such
as recording the maximum current obtained by the cell when connecting it to a resistance as an indication of its
performance, instead of the steady-state current that is often a degree of
magnitude lower. Often the data about the values of the used resistance is
minimal, or even non-existent, making much of the data non-comparable across all
studies. This makes extrapolation from standardized procedures difficult if not
impossible.
Commercial applications
A number of companies have emerged to commercialize
microbial fuel cells. These companies have attempted to tap into both the
remediation and electricity generating aspects of the technologies. Some of
these are companies are mentioned here.
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