Transcript Document

Microbial
Fuel Cells
R. Shanthini
26 Feb 2010
Source: http://parts.mit.edu/igem07/images/2/2d/Fuelcell.JPG
Microbial
Fuel Cells
anode
cathode
R. Shanthini
26 Feb 2010
Source: http://parts.mit.edu/igem07/images/2/2d/Fuelcell.JPG
An anode and a cathode are connected by
an external electrical circuit,
and separated
internally by an ion
exchange membrane.
R. Shanthini
26 Feb 2010
Microbes growing in the anodic chamber
metabolize a carbon substrate (glucose in this
case) to produce energy and hydrogen.
R. Shanthini
26 Feb 2010
C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2
or
C6H12O6 → CH3CH2CH2COOH + 2CO2 + 2H2
Hydrogen generated is
reduced into hydrogen
ions (proton) and
electrons.
R. Shanthini
26 Feb 2010
Electrons are transferred to the anodic electrode,
and then to the external electrical circuit.
The protons
move to the
cathodic
compartment
via the ion
exchange
channel and
complete the
circuit.
R. Shanthini
26 Feb 2010
The electrons and protons liberated in the reaction
recombine in the cathode.
If oxygen is to be used
as an oxidizing agent,
water will be formed.
R. Shanthini
26 Feb 2010
An electrical current
is formed from the
potential difference
of the anode and
cathode, and power
is generated.
The anode and cathode electrodes are composed of
graphite, carbon paper or carbon cloth.
The anodic chamber is
filled with the carbon
substrate for the microbes
to metabolize to grow and
produce energy.
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26 Feb 2010
The pH and buffering
properties of the anodic
chamber can be varied to
maximize microbial growth,
energy production, and
electric potential.
The anode and cathode electrodes are composed of
graphite, carbon paper or carbon cloth.
The cathodic chamber
may be filled with air in
which case oxygen is the
oxidant.
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26 Feb 2010
Laboratory substrates are acetate, glucose, or lactate.
Real world substrates include wastewater and landfills.
Substrate concentration,
type, and feed rate can
greatly affect the
efficiency of a cell.
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26 Feb 2010
Microbes should be anaerobic (fermentative type)
because anodic chamber must be free of oxygen.
Microbes tested are:
E. coli
Proteus vulgaris
Streptococcus lactis
Staphylococcus aureus
Psuedomonas methanica
Lactobacillus plantarium
(Many of these species
are known human
pathogens, and pose a
potential safety hazard.)
R. Shanthini
26 Feb 2010
Microbes should be anaerobic (fermentative type)
because anodic chamber must be free of oxygen.
Some bacteria, like
Clostridium cellulolyticum,
are able to use cellulose
as a substrate to produce
an electrical output
between 14.3-59.2
mW/m2, depending on the
type of cellulose.
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26 Feb 2010
Proton Exchange Membrane (PEM)
The PEM acts as the barrier between the anodic and
cathodic chambers.
It is commonly made from polymers like Nafion and Ultrex.
Ideally, no oxygen should be able to circulate between the
oxidizing environment of the cathode and the reducing
environment of the anode.
The detrimental effects of oxygen in the anode can be
lessened by adding oxygen-scavenging species like
cysteine.
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26 Feb 2010
Real-life MFC
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26 Feb 2010
Real-life MFC
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26 Feb 2010
The MFC shown in this tabletop setup can
take common sources of organic waste
such as human sewage, animal waste, or
agricultural runoff and convert them into
electricity (Biodesign Institute).
Real-life MFC
Fuel cells like this are now used by a
leading UK brewery to test the activity of
the yeast used for their ales.
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26 Feb 2010
Real-life MFC
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26 Feb 2010
The black boxes arranged in a ring of the
robot are MFCs, each generating a few
microwatts of power, enough to fuel a
simple brain and light-seeking behaviour in
EcoBot-II.
(http://microbialfuelcell.org).
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26 Feb 2010
Conventional
Fuel Cells
Hydrogen is the fuel for
Proton Exchange
Membrane (PEM) fuel cells.
At the anode,
a platinum catalyst
causes the hydrogen
to split into
positive hydrogen ions
(protons) and
negatively charged
electrons.
R. Shanthini
26 Feb 2010
Conventional
Fuel Cells
The Proton Exchange
Membrane (PEM) allows
only the positively charged
hydrogen ions (protons) to
pass through it to the
cathode.
The negatively charged
electrons must travel along
an external circuit to the
cathode, creating an
electrical current.
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26 Feb 2010
Conventional
Fuel Cells
At the cathode,
the electrons and
positively charged
hydrogen ions
combine with oxygen
to form water,
which flows out of the cell.
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26 Feb 2010
Conventional
Fuel Cells
 Power is produced by an electrochemical process not
by combustion
 Noiseless operation
 50% hydrogen energy content to electrical energy
conversion efficiency
 Multi-fuel (hydrocarbon and alcohols) capability
 Durability, reliability, scalability and ease of
maintenance
 Only water and heat is emitted from a fuel cell (water
is in fact a greenhouse gas)
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26 Feb 2010
Conventional
Fuel Cells
The electrodes are composed of platinum particles
uniformly supported on carbon particles. The platinum
acts as a catalyst.
 Polymer Electrolyte Membrane (Proton Exchange
Membrane) is a thin, solid, organic compound.
 Hydrogen for the fuel cell is produced from fossil fuel
at present (so CO2 emissions are part of hydrogen
energy).
 Power-plant-to-wheel efficiency of 22% if the hydrogen
is stored as high-pressure gas, and 17% if it is stored as
liquid hydrogen
 Hydrogen transportation and refuelling
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26 Feb 2010
Conventional
Fuel Cells
Technological status Proton Exchange Membrane (PEM)
Fuel Cells): commercial in niche
markets
Solid Oxide Fuel Cells (SOFC): market
entering phase in niche markets;
Possible adverse
effects
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26 Feb 2010
disposal of worn-out fuel cells