The document provides an overview of biofuel cells, focusing on their electrochemical mechanisms, including microbial and enzymatic types. It explains how microbial fuel cells harness electricity through bacteria while enzymatic biofuel cells utilize enzymes for energy conversion. Additionally, it highlights applications in wastewater treatment, self-powered biosensors, and potential uses in bioelectronic devices.

1. BIO-FUEL CELLS
Applied Aspects of Biotechnology
SHARAVANAKKUMAR SK
III B.Sc. BIOTECHNOLOGY
PSG COLLEGE OF ARTS AND SCIENCE

2. A Electrochemical cell and its Components
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An electrochemical cell is a device capable of either
generating electrical energy from chemical reactions or using
electrical energy to cause chemical reactions.
The electrochemical cells which generate an electric current
are called voltaic cells or galvanic cells.
The cells that generate chemical reactions, via electrolysis for
example, are called electrolytic cells.
It consists of three components, two electrodes (metal rods)
and a electrolyte (chemical).
The positive electrode is called anode and the negative
electrode is called cathode.

3. Figure: A typical Galvanic Cell

4. Fuel cells
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A fuel cell is an electrochemical cell that converts the chemical
energy of a fuel (often hydrogen) and an oxidizing agent (often
oxygen) into electricity through a pair of redox reaction.
Fuel cells are different from most batteries in requiring a
continuous source of fuel and oxygen (usually from air) to sustain
the chemical reaction.
whereas in a battery the chemical energy usually comes from
metals and their ions or oxides that are commonly already present
in the battery, except in flow batteries.
Fuel cells can produce electricity continuously for as long as fuel
and oxygen are supplied.
In 1932, English engineer Francis Thomas Bacon successfully
developed a 5 kW stationary fuel cell. The alkaline fuel cell (AFC),
also known as the Bacon fuel cell after its inventor, is one of the
most developed fuel cell technologies, which NASA has used since
the mid-1960s

5. Figure: Components and Reactions of fuel cells

6. Bio Fuel cells
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A biofuel cell uses living organisms to produce electricity.
Two types based on their fuel source,
Microbial fuel cell, a bio-electrochemical system that drives a
current by using bacteria and mimicking bacterial interactions
found in nature.
Enzymatic biofuel cell, a type of fuel cell that uses enzymes
rather than precious metals as a catalyst to oxidize its fuel.

7. Microbial Fuel Cells
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In microbial fuel cells, microbes such as bacteria catalyze
electrochemical oxidations or reductions at an anode or
cathode, respectively, to produce an electric current.
Microbial fuel cells can harvest electricity from electrode-
reducing organisms that donate electrons to the anode.
While the microorganism oxidizes organic compounds or
substrates into carbon dioxide, the electrons are transferred to
the anode.
Examples of electrode-reducing microorganisms,
Desulfuromonas acetoxidans
Geobacter sulfurreducens
Geobacter metallireducens
Rhodoferaxf errireducens
Desulfobulbus propionicus

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Enterococcus gallinarum
Shewanella putrefaciens
The iron reducing bacteria such as Shewanella and Geobacter
are mostly used for their high electrochemically active nature.
Microbial fuel-cells reactions:
The mechanism of oxidation and reduction in the MFC is not
clearly understood, and various reactions have been proposed to
explain the process. An example using acetate as the substrate
follows:
Anode: CH3COOH+2H2O →2CO2 +8e
–
+ 8H+
Cathode: 2O2 +8e– + 8H+ → 4H2O
Overall: CH3COOH+2O2 →2CO2 +2H
2
O + Electricity

9. y
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Expression of output:
Power output may be expressed in several ways:
A/m2 of anode electrode surface area.
Area power density W/m2 of anode electrode surface area.
volume power density W/m3 of cell volume.
Three ways of electron transfer in the microbial fuel cells,
1.Electron transfer by Soluble mediator
2.Direct electron transfer
3.Electron transfer by nano-wire or nano-pili

10. 1. Electron transfer by Soluble mediator
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The electrons can be transferred to the anode through a soluble
redox mediator in the solution bathing the electrode.
Soluble Redox Mediators:
Redox mediators are chemicals with electrochemical activity.
In a bioelectrocatalysis process, mediators may exchange
electrons with fuels or oxidants at the reaction sites of
the biocatalysts, and then diffuse to the surface of electrode and
exchange electrons there.
This process is repeated, and the mediator functions as an electron
shuttle between the biocatalyst and electrode.
Mediators should have appropriate redox potentials to coordinate
with the biocatalysts. Their potentials should be within the range
of the thermodynamic potentials of the anode and cathode.
The use of a mediator will enable and accelerate the internal
electron conduction of the biofuel cell.

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Some examples,
Methyl viologen was used to mediate the oxidation of
hydrogen catalyzed by Desulfovibrio vulgaris
2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was
used to mediate the reduction of O2 with bilirubin oxidase.
To achieve highly efficient electron transfer, mediators should
have good activity and diffusivity.
In some cases, a combination of two mediators may provide
better performance. For example, the mediating effects of
mixed Fe(III) EDTA and thionine were studied in a microbial
biofuel cell with E. coli in the anode.
Free cofactors can be viewed as mediators in many ways
despite their catalytic role in the biotransformation reactions.
Many dehydrogenases are NAD(H)-dependent, they can also
used as a redox mediators.

12. 2. Direct electron transfer
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The electrons can be transferred directly to the anode through
proteins found on the outer membrane of the bacteria.
For example,
The bacteria from Geobacteraceae family transfer electrons to
electrodes using cytochromes on the outer membrane.
Shewanella oneidensis also uses cytochrome-c to transfer
electrons but requires an anaerobic environment to convert
lactate to acetate.
The electron transport in bacteria is done by a mechanism called
Periplasmic electron transfer.

13. The Periplasmic electron transport system
in Bacteria
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This a system present in the Periplasmic space of bacterial cell
which helps in electron transport through the electron
transporting proteins e.g. cytochrome-c.
Protons are translocated across the cell membrane, from the
cytoplasm to the Periplasmic space
Electrons are transported along the membrane, through a
series of protein carriers
Oxygen is the terminal electron acceptor, combining with
electrons and H+ ions to produce water
As NADH delivers more H+ and electrons into the ETS,
the proton gradient increases, with H+ building up outside the
cell membrane, and OH- inside the membrane.

14. Figure: Periplasmic electron transport in bacteria

15. 3. Electron transfer by nano-wires or
nano-pili
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In some instances, bacteria form a thick film on the cathode, so it
may be the pili or nanowires that transmit the electrons to the
anode.
Nano-wires and Nano-pili:
Bacterial nanowires or microbial nanowires are
electrically conductive appendages produced by a number
of bacteria most notably from the Geobacter and Shewanella. Other
examples,
Cyanobacterium synechocystis
Pelotomaculum thermopropionicum
Methanothermobacter thermoautotrophicus
Geobacter nanowires are modified pili, which are used to establish
connections to terminal electron acceptors.
Species of the genus Geobacter use nanowires to transfer electrons
to extracellular electron acceptors (such as Fe(III) oxides).

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Figure: The three ways of electron transfer in bacteria

17. Other reactions
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In aerobic chambers, microorganisms can reduce oxygen to
water.
In anaerobic environments, nitrate or sulfate can be reduced
to nitrite, nitrogen, or sulfur ions.
Another potential reduction for these bacteria is the
conversion of carbon dioxide to methane or acetate. The
process uses acetyl-CoA as an intermediate to build even
longer chain fatty acids and alcohols.
Example, G. sulfurreducens reduces fumarate to succinate
with electrons obtained from the cathode.
Interestingly, the substrates that these organisms need for
the redox reactions can be readily obtained from wastewater
or contaminated water, which would both provide energy and
clean up the environment.

18. Figure: The overall reactions of microbial bio-fuel cells

19. Enzymatic bio-fuel cells
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Enzymatic biofuel cells are bioelectronic devices that utilize
oxidoreductase enzymes to catalyze the conversion of
chemical energy into electrical energy.
Enzymatic biofuel cells work on the same general principles as
all fuel cells: use a catalyst to separate electrons from a
parent molecule and force it to go around an electrolyte
barrier through a wire to generate an electric current.
The catalysts and the fuels used by these bio-fuel cells are
enzymes or proteins, whereas most fuel cells use metals such
as platinum and nickel as catalysts, the enzymatic biofuel cell
uses enzymes derived from living cells.
The advantages for enzymatic biofuel cells Enzymes are
relatively easy to mass-produce and so benefit
from economies of scale, whereas precious metals must be
mined and so have an inelastic supply.

20. Working mechanism
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The enzymes that allow the fuel cell to operate must be
immobilized near the anode and cathode in order to work
properly; if not immobilized, the enzymes will diffuse into the
cell's fuel and most of the liberated electrons will not reach
the electrodes, compromising its effectiveness.
Even with immobilization, a means must also be provided for
electrons to be transferred to and from the electrodes.
This can be done either directly from the enzyme to the
electrode by direct electron transfer or with the aid of other
chemicals that transfer electrons from the enzyme to the
electrode by substance mediated electron transfer.
The former technique is possible only with certain types of
enzymes whose activation sites are close to the enzyme's
surface, but doing so presents fewer toxicity risks for fuel
cells intended to be used inside the human body.

21. Figure: Overview of reactions in enzymatic fuel cells

22. Hydrogenase based bio fuel cells
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The enzymes are used as electrocatalysts at both the cathode and
anode. In hydrogenase-based biofuel cells, hydrogenases are
present at the anode for H2 oxidation in which molecular hydrogen
is split into electrons and protons, in H2/O2 biofuel cells,
the cathode is coated with oxidase enzymes which then convert the
protons into water.
The bidirectional or reversible reaction catalyzed by hydrogenase is
a solution to the challenge in the development of technologies for
the capture and storage of renewable energy as fuel with use on
demand
The lack of need for a membrane simplifies the biofuel cell design
to be small and compact, given that hydrogenase does not react
with oxygen ( inhibitor) and the cathode enzymes (laccase) does
not react with the fuel.
The electrodes are preferably made from carbon which is abundant,
renewable and can be modified in many ways or adsorb enzymes
with high affinity.
The hydrogenase is attached to a surface which also extends the

23. Applications
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Self-powered biosensors
The beginning concept applying enzymatic biofuel cells for
self-powered bio-sensing applications was introduced in 2001.
With continued efforts, several types of self-powered enzyme-
based biosensors have been demonstrated. In 2016, the first
example of stretchable textile-based biofuel cells, acting as
wearable self-powered sensors, was described.
The smart textile device utilized a lactate oxidase-based
biofuel cell, allowing real-time monitoring of lactate in sweat
for on-body applications.

24. Figure: Enzymatic biofuel cells

25. Applications of bio-fuel cells in real time
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Implantable Biofuel Cells: Potential Power Sources for Bioelectronic
Devices
There are some methods used to produce electricity from the
human body to provide energy to the bioelectronic devices. Both
physical and chemical methods are used such as converting the
mechanical energy of the body to electrical energy.
Examples, the usage of muscle stretch, blood flow, different
thermoelectric and piezoelectric effect.
These physical systems always rely on the physiological movement
of the body, the most bio-compatible method is using the
biomolecules in the energy production for these bioelectronic
devices.
Natural biological elements such as enzymes are interfaced with
electrodes in implantable bio electrochemical systems, typically
biofuel cells, have illustrated significant importance.

26. Figure: A biofuel cell implanted in a blood vessel for extracting
electrical power by oxidation of glucose

27. Microbial fuel cells in waste water treatment
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All types of waste-water containing organic matter can be
treated by this process, including domestic waste-water,
brewery effluent, and much else.
Use of microbial fuel cells for waste-water requires a design
which allows the waste-water to flow through the cell over
the anode surface.
Various configurations have been adopted for this purpose,
including the tubular microbial fuel cells where the cathode is
placed on the outside of the tube and the anode occupies the
full internal space, waste-water flows through the anode
from one end to the other.
Power output
A disadvantage of the system is relatively low energy
production. However as the purpose of the plant is water
purification, any electricity produced is a bonus.
Developments focused on improving power output are
showing results. Cell voltage and current density vary
depending on cell type, microorganism used, and substrate.

28. Figure: Production of electricity from bio-fuel cells in waster
water treatment plant

29. Electrohydrogenesis
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Producing hydrogen gas is possible at very high yields by
Electrohydrogenesis, in reactors that have various names, usually
referred to as microbial electrolysis cells (MECs).
The MEC is based on modifying a microbial fuel cell (MFC) in two
ways: adding a small voltage (>0,2 V) to that produced by
bacteria at the anode; and by using an oxygen free cathode.
The addition of the voltage makes it possible to produce pure
hydrogen gas at the cathode this MEC system is operated as a
completely anaerobic reactor.
The voltage needed to be added can be produced using power
from an MFC, the protons and electrons produced by the bacteria
are recombined at the cathode as hydrogen gas, a process called
the hydrogen evolution reaction (HER).
Anode: C2H4O2 + 2 H2O → 2 CO2 + 8 e– + 8 H+
Cathode: 8 H+ + 8 e– → 4 H2

30. Electromethanogenesis
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This is a process whereby carbon dioxide is converted to
methane using electric current and a microorganism catalyst.
The process is usually intended for CO2 capture or conversion
and is usually used to convert surplus energy from renewable
sources in to a storable energy carrier.
The process can be combined with the microbial fuel cell to
convert the CO2 generated by the fuel cell to methane.
The simplified reaction is:
CO2 + 8H+ + 8e– →CH
4
+ 2H2O
Figure: The process of Electromethanogenesis
as a bio-fuel cell.
(Electromethanogenesis cell)

31. References
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Microbial fuel cells: A new approach to waste-water treatment. December
4th, 2018, Published in Articles: Energize by Mike Rycroft, EE Publishers.
Environmental Biotechnology, David P. Clark, Nanette J. Pazdernik,
in Biotechnology (Second Edition), 2016.
Katz, E. Implantable Biofuel Cells Operating In Vivo—Potential Power
Sources for Bioelectronic Devices. Bioelectron Med 2, 1–12 (2015).
Enzymatic biofuel cells: 30 years of critical advancements, Author links open
overlay panel Michelle Rasmussen, Sofiene Abdellaoui, Shelley D.Minteer.
Periplasmic Electron Transfer via the c-Type Cytochromes MtrA and FccA
of Shewanella oneidensis MR-1., Bjoern Schuetz, Marcus Schicklberger,
Johannes Kuermann, Alfred M. Spormann, Johannes Gescher.
Engdahl, Anders & Nelander, B. (2002). Electrode-reducing microorganisms
that harvest energy from marine sediments. Science. 295. 482-483.
ELECTROCHEMICAL CELLS, Norio Sato, in Electrochemistry at Metal and
Semiconductor Electrodes, 1998.
Biofuel Cells, Robert F. Service, Science 17 May 2002:
Vol. 296, Issue 5571, pp. 1223.