A microbial fuel cell 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 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, and neutral red. 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.
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 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, this variant can provide additional ecological
advantages. = Microbial electrolysis cell =
A variation of the mediator-less MFC is the microbial electrolysis cells. 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 =
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. 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 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. Sediment microbial fuel cells
application for wastewater treatment is a relatively new field. SMFCs with
simple structures can generate electrical energy while decontaminating
wastewater. Most SMFCs used for wastewater treatment contain plants to
mimic constructed wetlands. Both synthetic and real wastewaters have been
used as substrates in SMFCs that achieved satisfactory performance in
organic removal. SMFC tests have reached more than 150 L.
In 2015 researchers announced an SMFC application that extracts energy and
charges a battery. Salts found in the waste dissociates into positively and
negatively charged ions in water, and move and adhere to the respective
negative and positive electrodes, charging the battery and making it
possible to remove the salt effecting microbial capacitive desalination. The
microbes produce more energy than is required for the desalination process.
= Phototrophic biofilm microbial fuel cell =
Phototrophic biofilm MFCs 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. = Nanoporous membrane microbial fuel
cells = The United States Naval Research
Laboratory developed the nanoporous membrane microbial fuel cells which
operate the same as most MFCs, but use a non-PEM to generate passive diffusion
within the cell. The membrane used instead is a nonporous polymer filter
which generates comparable power densities as Nafion while remaining more
durable than Nafion. Porous membranes allow passive diffusion thereby reducing
the necessary power supplied to the MFC in order to keep the PEM active and
increasing the total output of energy from the cell.
MFCs that do not use a membrane can deploy anaerobic bacteria in aerobic
environments however, membrane-less MFCs will experience cathode contamination by
the indigenous bacteria and the power-supplying microbe. The novel
passive diffusion of nanoporous membranes can achieve the benefits of a
membrane-less MFC without worry of cathode contamination.
Nanoporous membranes are also eleven times cheaper than Nafion (Nafion-117,
$0.22/cm2 vs. polycarbonate,