Introduction new energy that can replace the

                                                                    Introduction                         

Nowadays,
people are too depending on oil,coal and gas as the main sources of power that
are finite and will run out one day. In constrast, the best solution for this
problem is to find a new energy that can replace the current resources that
will never run out and does not bring perile to the environment. There has been
an increase in recent years in the number of reports of microorganism that can
generate electrical current in microbial fuel cells.Since 1970s, the concept of
using microorganisms as a catalyst for fuel cell was explored(2). Human has
discovered the usage of the bacteria to generate electricity for almost a
hundred years ago. In 1911, M.C. Potter, a professor of botany in University of
Durham has discovered that when the bacteria degrading the organic compunds, an
electrical energy is produced. He was able to create an MFCs and then improved
by M. J. Allen and H. Peter Bennetto from Kings College in London in 1980s with
their desired to produce cheaper and reliable power for the third world
countries. Meanwhile, researchers from Korean Institute of Science and
Technology, B-H. Kim in 1990s has discovered there is no need to use mediator
to move the electrons to the electrodes on the MFCs since there are some types
of bacteria are electrochemically active.

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Body Content

Microorganisms
that act as biocatalysts, assist the Mirobial fuell cells (MFCs) to convert
chemical energy to electrical energy. The basic principle is, the organic
substances is oxidized biologically by the microorganisms to transfer electrons
to anode(3). Electrons flow towards the cathode due to the electrochemical
potential between the respiratory enzyme and the electron acceptor at the
cathode. In order to preserved the electroneutrality, electron transfer from
the anode to the cathode and protons moving between the electrodes must be at a
same number(1). The ion exchange membrane as fuel, separate the anode and
cathode.(11) In aerobic conditions, the microorganisms consumed a substance
such as sugar, producing carbon dioxide and water. But, when there is no
oxygen, their product is carbon dioxide, protons and electrons:

C12H22O11 + 13H2O ?
12CO2 + 48H+ + 48e?

One
of the example of microorganisms that can work in the MFCs is Geobacteraceae, that can use electrodes
as electron acceptors for anaerobic respiration(4). There are two types of MFCs
– mediator and mediatorless. The mediator MFCs that aided by a mediator
chemical to transfer electrons from the bacterial cells to the electrode. Some
examples of mediators include natural red, methylene blue, thionine and
resorufin.(5) The problem is, this mediators are too expensive and also toxic
thus make it harder to commercialized.(6) The other type, the mediatorless need
electrochemically bacteria such as the Fe (III) reducer Shewanella putrefaciens that can respire directly into the
electrode. This bacteria is called exoelectrogens as it can transfer electrons
extracellularly.

            Energy is produced by the bacteria
through a process called electron transport chain.( Refer to Figure 1) :

1.      A
biological molecule transport called NADH releases high energy proton and
electron.

2.      The
electron moves in the mitochondrial membrane by following the red path through
the protein(big blob).

3.      The
electron pumps hydrogen ion as it passes through each protein, throughout the
membrane.

4.      In
a bacterial cell, the electron continues it journey along the red path(dotted)
where it mixed with oxygen to produce water.

5.      In
a microbial fuel cell, the electron continues along the red path(solid), where
it is picked up by a molecule of the mediator and brought to the anode.

At
the anode, the bacteria will produce carbon dioxide and water via cellular
respiration by consuming the organic substances such as sugar. However, in the
oxygen void environment, carbon dioxide, protons and electrons instead will be
produced. Thus, we can conclude that an anaerobic condition must be made on the
anode  chamber of the MFCs. Shewanella
oneidensis and Geobacter sulfurreducens, which produce electrically conductive
appendages called bacterial nanowires that assist direct transfer
of electrons to the anode, hence greatly increasing efficiency and reducing
significant costs under anaerobic conditions(7). But, the diversity of bacteria
is much greater than these model iron reducers persisting in the biofilm
community.(8)

 Meanwhile, at the cathode, the positively
charged half of the cell, the cathode chamber consists of an electrode
subjected to a catholyte flow consisting of an oxidizing agent in solution. The
oxidizing agent is reduced as it receives electrons that funnel into the
cathode through a wire originating from the cathode. Protons
migrate across the proton/cation exchange membrane to combine with electrons to
form water if oxygen is provided 13,14 or to form ferrocyanide if
ferricyanide is provided. Therefore, a positive current flow from the positive
terminal to the negative terminal and this direction is opposite to electron
flow. There are several factors that contribute to the performance of the MFCs
including the rates of substrate oxidation, transferred of electron to the
electrode by the microbes, circuit’s resistance, proton transport to the
cathode through the membrane, oxygen supply and reduction in the cathode.(Gil
at.el)

Figure 1 Schematic of MFCs

Using
MFCs to generate power has a lot of benefits compared to the convetional fuel
used nowadays. The organic carbon sources that oxidizes does not contribute net
carbon dioxide to the atmosphere, and there is no need for extensive
pre-processing of the fuel or expensive catalysts.(9)

 

Conclusion

            Beside generating electricity, MFCs
has a lot of usage. It is widely use as a wastewater treatment. It also can
produce hydrogen through a process called electrohydrogenesis.