Literature the process. Some bacteria have the ability to

Literature review will
provide handy information about MFCs. It will reveal the relevant information
about the technology used in Microbial Fuel Cell. It will let the reader to
understand its design and function.  

MICROBES:

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In order to understand the
fundamental principle of Microbial fuel cell it is important to have
information about the microorganisms. Bacteria are the major microbes which are
involved in this process. Bacteria breakdown organic matter and release energy
in the process. Some bacteria have the ability to generate electricity and to
transfer electrons effectively to anode. The bacteria which have this ability
are known as “Exoelectrogens”. Exoelectrogens have the ability to generate
electricity in microbial fuel cells by extracellular electron transfer to
anode. It directly transfers electrons to a chemical or material that is not
immediate electron acceptor.

These electrogens can be
sourced in sand, water and many other sources. But here we are using soil as a
source.

TYPES OF MFCs:

There are two types of MFCs:

1.      Mediated

2.      Mediator-free

3.      Soil based

Mediated: In this type of microbial
cells a mediator is used o transfer electrons to electrode. Examples of
commonly used mediators are thionine, methyl blue and neutral red etc.

 

 

 

Mediator-free: Mediator free MFCs used electrochemically active
microbes (mostly bacteria) to transfer electrons directly to electrode.
Mediator-free cells can directly obtain energy from certain plants. This is
known as plant microbial fuel cell. Possible plants include sweet grass,
tomatoes and algae etc. It can provide ecological advantages.

Figure 2: A plant microbial fuel cell

 

Soil Based: In soil based MFC, soil
acts as a nutrient rich anodic media, the inoculum and proton exchange
membrane. The anode is placed at the bottom whereas cathode is placed at the
top and is exposed to the air. Soil is filled with diverse microbes, including
electrogenic bacteria, is full of complex sugars and other nutrients obtained from
plants and animal material decay.

Figure 3: Soil based MFC

 

MFC DESIGN:

Double-Chamber
MFC Design:

This type of MFC is used
widely. It contains two chambers. Anode is placed in one chamber whereas
cathode in another and is separated by a proton exchange membrane. The anode
chamber is kept oxygen free for anaerobic breakdown process to occur. Though it
is widely used by it is still challenging because of its impractical
configuration. This
set up can accommodate various electrode shapes, i.e. plane, granular and brush
as it has a dedicated chambers for the anode and cathode. It can also use other
catholyte besides air, which is any source of oxygen. According to a recent
research document, use of algae (seaweed) enhances the oxygen production due to
photosynthetic process in the plant which can be facilitated by this type of
MFC configuration. A. González del Campo, P. Cañizares , M. A. Rodrigo, “Microbial
fuel cell with an algae-assisted cathode: A preliminary assessment,” Nov. 2013

Figure 4: A Double-Chamber MFC

Single-chamber MFC Design:

A one-compartment
MFC eliminates the need for the cathodic chamber by exposing the cathode
directly to the air. We are dealing with sand based MFC which has one Chamber. Anode
is dug inside the soil and cathode is at the top in exposed air as shown in
Figure 3.

 

 

COMPONENTS:

A microbial fuel cell consists of single but effective
components to effectively harness the energy. Components are as follows:

·        
Electrodes: Both in cathode and anode.

·        
Substrate: Any organic matter used as source for
microorganisms such as “Sand”.

·        
Bacteria: Exoelectrogens are most suited for Microbial fuel
cells.

Table 1: Basic components of Microbial Fuel cell

 
Serial no.

 
Item

 
Material

 
1.

 
Anode

Graphite, graphite
felt, carbon paper, carbon-cloth, Pt, Pt black, reticulated vitreous carbon
(RVC).

 
2.

 
Cathode

Graphite, graphite
felt, carbon paper, carbon-cloth, Pt, Pt black, RVC.

 
3.

 
Electrode
Catalyst

Pt, Pt black,
MnO2, Fe3+, polyaniline, electron mediator immobilized on anode.

 

PRINCIPLE OF MICROBIAL
FUEL CELL:

“Microbial fuel
cells (MFCs) are electrochemical devices that use the metabolic activity of microorganisms
to oxidize fuels, generating current by direct or mediated electron transfer to
electrodes.” K. Rabaey and W. Verstraete, “Microbial fuel cells: novel
biotechnology for energy generation” , page 291–298, Jun. 2005. The device
consists of anode, cathode, proton exchange membrane and an external circuit.
The MFC convert biodegradable substrate directly into electricity. Anode holds
the bacteria and the organic matter in an anaerobic environment. Cathode is
exposed to air. Bacteria generate protons and electrons as organic substance
converts to energy. Microbes use this energy for growth. The electrons are
transferred directly to the anode (if mediator-free MFC) and then to copper
electrode via conduction.

Some bacteria
are unable to transfer electrons on their own, so a mediator is used for
electron transfer such as methyl blue, thionine. These are called Redox
mediators. H. J. Mansoorian, A. H. Mahvi, “Bioelectricity generation using
two chamber microbial fuel cell treating wastewater from food processing”, May
2013

Chemical energy
is converted into electricity by microbial activity. Microbes release
electrons. Oxygen is supplied to the cathode by air source. Materials use in
the electrodes influence the energy produced.

Figure 5: Schematic of the basic components of a MFC

 

APPLICATIONS OF MFCs:

The main applications of MFCs are:

Generation of
Bioelectricity:

MFC is most
recent and fantastic technology that uses wide variety of substrates, materials
with bacteria to achieve to produce bio energy despite the fact that power
level in these systems is relatively low. The
main objective of MFCs is to achieve a suitable current and power for the
application in small electrical devices. It is specially used for sustainable long-term power
applications. Rahimnejad and et al. turn on ten LED lamps and one
digital clock with fabricated stacked MFC as power source and both devices were
successfully operated for the duration of 2 days. M. Rahimnejad, A. Ghoreyshi, G. Najafpour
“A novel microbial fuel cell stack for continuous production of clean energy”(Article),
2012

Waste Water Treatment:

Different type
of waste water like sanitary waste, food processing waste water etc. can
contain energy in the form of biodegradable organic matter. MFC can capture
energy as electricity or hydrogen gas. MFCs using specific microbes are
excellent techniques to remove sulfides from wastewater. Up to 90% of the COD
can be removed in some cases.

MICROBIOLOGICAL ASPECT:

As fossil fuels are
depleting soon we are looking for more sustainable methods and one of them is
microbial fuel cell technology for long term energy generation. Microbial fuel
cell concept is possible due to exocellular electron transfer. Microbes are
involved in this activity. Main step i.e. electron transfer, is done by
microbes. All the study of these microbes is done in a microbiology lab.
Without microbes this process is incomplete. It does not produce harmful by
products so it is more sustainable.

 

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