19-04-2013, 04:55 PM
Bio-Batteries and Bio-Fuel Cells: Leveraging on Electronic Charge Transfer Proteins
[attachment=53210]
Abstract
Bio-fuel cells are alternative energy devises based on bio-electrocatalysis of natural substrates by
enzymes or microorganisms. Here we review bio-fuel cells and bio-batteries based on the recent
literature. In general, the bio-fuel cells are classified based on the type of electron transfer; mediated
electron transfer and direct electron transfer or electronic charge transfer (ECT). The ECT of the
bio-fuel cells is critically reviewed and a variety of possible applications is considered. The technical
challenges of the bio-fuel cells, like bioelectrocatalysis, immobilization of bioelectrocatalysts, protein
denaturation etc. are highlighted and future research directions are discussed leveraging on the use
of electron charge transfer proteins. In addition, the packaging aspects of the bio-fuel cells are also
analyzed and the found that relatively little work has been done in the engineering development of
bio-fuel cells.
INTRODUCTION
Bio-fuel cells are energy-conversion devises based on
bio-electrocatalysis leveraging on enzymes or microorganisms.
1–4 Chemical reactions can proceed by direct
electron transfer (DET), in which case the electron transfer
occurs directly between enzymes and electrodes,5 or
through shuttle mediated electron transfer (MET), in which
electron transfer mediators shuttle the electron between
enzymes and electrodes to reduce the kinetic barrier in the
electron transfer between enzymes and electrodes.Direct
electron transfer (DET) is desirable for efficient communication
between enzymes and electrodes, and eliminating
the need for mediators may simplify the construction of
bio-fuel cells.In terms of applications, of bio-fuel cells
(BFC) will most likely be use in miniature cells to derive
power from biological macromolecules to power small
devices.It may be possible to implant miniature BFCs
within a human patient to power micro sensor/transmitter
devices e.g., glucose sensors for diabetics, to monitor
blood pressure, temperature, metabolite concentrations,
etc.or to power a pacemaker or bladder control valve.It
is also conceivable that these miniature BFCs may have
defense applications.
BIO-BATTERIES AND BIO-FUEL CELLS
Even though fossil fuel (petroleum) meets the majority
of global energy demands, the increasing difficulty of
sustained supply and the associated problems of pollution
and global warming are acting as a major motivation
for research into alternative sustainable energy technologies,
like solar, wind and hydrogen fuel cells.6 Fuel
cells offer a possible (and partial) solution to this problem,
with the fuel needed for conventional cells usually
being either hydrogen or methanol with operating temperatures
<100 C.
In a hydrogen fuel cell, electricity is generated efficiently
from the oxidation of hydrogen, coupled to the
reduction of oxygen, with water as the only by-product
(Fig.1(a)).The most commonly used electrocatalyst in the
fuel cells is platinum.Platinum is very efficient in oxidizing
hydrogen and enabling high currents to be produced
in a fuel cell.The major disadvantage is that platinum
is expensive and its limited availability, making hydrogen
fuel cells an expensive method of energy production.Platinum
is also poisoned by carbon monoxide (CO) impurities
that are often found in industrially produced hydrogen.
Removal of CO adds to the cost of the fuel cell system.
PREVIOUS ATTEMPTS OF
BIO-FUEL CELLS
Several potential applications of BFCs have been reported
or proposed in the literature for implantable devices,
remote sensing and communication devices as a sustainable
and renewable power source.14 However, there are no
BFC design formats or templates that allow for the production
of a working device with a size on the order of
1 cc, which are needed for several “real world” applications.
An enzyme based BFC is very attractive, however it
has been shown that electron flow is too slow to make a
viable fuel cell.This is due to the difficulty for enzymes
to attain direct electrical contact with the electrodes of the
cell and catalyze reactions effectively.
The two largest obstacles with bio-fuel cells which must
be overcome are increasing the power density and increasing
the enzyme stability.In addition, understanding of the
determinants governing the direct electron transfer reaction
and mutation of enzymes to tune the redox potential,
to improve DET kinetics, or to reduce the enzyme size are
also very important challenges facing the commercialization
of bio-fuel cells.15 To address these key issues, various
enzyme immobilization methods have been attempted
for constructing BFCs, such as adsorption, entrapment,
and covalent attachment.
Nanostructured Bioelectrocatalysis
Traditional direct hydrogen fuel cells require noble
metal catalysts both for hydrogen oxidation and oxygen
reduction.17 Similarly, the bio-fuel cells also need catalysts
(bio-catalysts) for the conversion of chemical to electrical
energy.One approach is to use microorganisms and/or
enzymes as biological reactors for the fermentation of raw
materials to fuel products (similar hydrogen fuel reformers);
the second approach is to use the microorganisms
and/or enzymes as catalysts directly in the bio-fuel cells.
The second approach, using purified redox enzymes for the
targeted oxidation and reduction of specific fuel and oxidizer
substrates, is more efficient for bio-fuel cells.Also,
bio-catalysts are an attractive renewable and less expensive
alternative to transition metal catalysts for mediated
electron transfer (MET).
Immobilized Bioelectrocatalysts on CNTs
In order to promote DET of the protein and to improve
the stability, it is necessary that the protein is immobilized
in the form of clusters.4 In this context, glucose oxidase
(GOx) can be attached as crosslinked enzyme clusters
(CECs) onto the surface of carbon nanotubes (CNTs).In
has been demonstrated in a recent study that CEC-GOx did
not manifest any decrease in activity for 250 days.4 The
CEC-GOx based BFC was characterized by potentiostatic
polarization in an unbuffered solution.The open circuit
voltage (OCV) was 0.33 V while a maximum power output
of 120 Wcm−2 occurred at a cell potential of 0.1 V
(Fig.4(a)).The long-term performance of the miniature
BFC with CEC-GOx, constant voltage measurements at
0.1 and 0.25 V (Fig. 4(b)) showed some transient behavior
initially and the cell performance stabilized after about
2 hours.More importantly, at 0.1 V where a heavy load
was applied to the BFC, the performance of the BFC was
very stable without any significant performance decay for
more than 16 hours (Fig.4(b)).
Flavoproteins
Flavoproteins commonly contain one of two prosthetic
groups, FMN (e.g., NADH dehydrogenase, EC 1.6.99.l)
and FAD.The FMN is non-covalently bound in all
known cases. FAD may be non-covalently bound (e.g., in
dihydrolipoamide dehydrogenase (NADH), EC 1.8.1.4) or
covalently bound by a methylene bridge between the benzene
ring of the benzo[g]pteridine-2,4-dione and an amino
acid residue, such as cysteine, histidine or tyrosine, in
the protein (e.g., succinate dehydrogenase, EC 1.3.99.1),
or directly at ring position 6.8-Hydroxy-p yrimidino
[4,5-b]quinoline-2,4-dione functions as prosthetic group
in methanogens and in deoxyribodipyrimidine photolyase
(EC 4.1.99.3). Apart from a few exceptions where the role
of the flavin is not clear, flavoproteins carry out oxidationreduction
reactions, in which one substrate is oxidized
and a second is reduced.F or all these enzymes each catalytic
cycle consists of two distinct processes, the acceptance
of redox equivalents from a reducing substrate and
the transfer of these equivalents to an oxidized acceptor.
Accordingly, the catalyzed reactions consist of two separate
half-reactions: a reductive half-reaction in which the
flavin is reduced and an oxidative half-reaction, in which
the reduced flavin is reoxidized.
Immobilization of Electron Transfer Proteins
We will focus on GOx as a test case for covalent attachment
to substrates in view of its wide spread use in
fuel cells.Co valent attachment of GOx to SWCNT further
enhances efficient transfer of electrons.CNTs have
unique electronic properties, high mechanical strength and
chemical stability, making them attractive for fabricating
of GOx coupled CNT based device elements for use in
bio fuel cells.F or chemically coupling the CNTs with
biomolecules, it is critical to functionalize CNT surfaces,
which are very inert.Se veral strategies have been developed
for functionalizing CNTs, derivatizing them with
biomolecules, and demonstrating the proof of concept
of using bacteriorhodopsin mutants -CNT hybrids24 for
sensing via electrical, electrochemical, and electro-optical
means.In all the cases, however, the CNTs are functionalized
wet-chemically in a spatially random fashion
where molecular bonding is mediated by defect creation,
or hydrophobic adsorption (Fig.11(a)).While such conventional
treatments may be adequate for demonstrating
the proof-of-concept of sensors from CNTs contacted on
substrates, or random dispersions of CNTs in solutions,
they have serious limitations for realizing multifunctional
sensor arrays on a chip that integrates system level logic
operations for data processing.
[attachment=53210]
Abstract
Bio-fuel cells are alternative energy devises based on bio-electrocatalysis of natural substrates by
enzymes or microorganisms. Here we review bio-fuel cells and bio-batteries based on the recent
literature. In general, the bio-fuel cells are classified based on the type of electron transfer; mediated
electron transfer and direct electron transfer or electronic charge transfer (ECT). The ECT of the
bio-fuel cells is critically reviewed and a variety of possible applications is considered. The technical
challenges of the bio-fuel cells, like bioelectrocatalysis, immobilization of bioelectrocatalysts, protein
denaturation etc. are highlighted and future research directions are discussed leveraging on the use
of electron charge transfer proteins. In addition, the packaging aspects of the bio-fuel cells are also
analyzed and the found that relatively little work has been done in the engineering development of
bio-fuel cells.
INTRODUCTION
Bio-fuel cells are energy-conversion devises based on
bio-electrocatalysis leveraging on enzymes or microorganisms.
1–4 Chemical reactions can proceed by direct
electron transfer (DET), in which case the electron transfer
occurs directly between enzymes and electrodes,5 or
through shuttle mediated electron transfer (MET), in which
electron transfer mediators shuttle the electron between
enzymes and electrodes to reduce the kinetic barrier in the
electron transfer between enzymes and electrodes.Direct
electron transfer (DET) is desirable for efficient communication
between enzymes and electrodes, and eliminating
the need for mediators may simplify the construction of
bio-fuel cells.In terms of applications, of bio-fuel cells
(BFC) will most likely be use in miniature cells to derive
power from biological macromolecules to power small
devices.It may be possible to implant miniature BFCs
within a human patient to power micro sensor/transmitter
devices e.g., glucose sensors for diabetics, to monitor
blood pressure, temperature, metabolite concentrations,
etc.or to power a pacemaker or bladder control valve.It
is also conceivable that these miniature BFCs may have
defense applications.
BIO-BATTERIES AND BIO-FUEL CELLS
Even though fossil fuel (petroleum) meets the majority
of global energy demands, the increasing difficulty of
sustained supply and the associated problems of pollution
and global warming are acting as a major motivation
for research into alternative sustainable energy technologies,
like solar, wind and hydrogen fuel cells.6 Fuel
cells offer a possible (and partial) solution to this problem,
with the fuel needed for conventional cells usually
being either hydrogen or methanol with operating temperatures
<100 C.
In a hydrogen fuel cell, electricity is generated efficiently
from the oxidation of hydrogen, coupled to the
reduction of oxygen, with water as the only by-product
(Fig.1(a)).The most commonly used electrocatalyst in the
fuel cells is platinum.Platinum is very efficient in oxidizing
hydrogen and enabling high currents to be produced
in a fuel cell.The major disadvantage is that platinum
is expensive and its limited availability, making hydrogen
fuel cells an expensive method of energy production.Platinum
is also poisoned by carbon monoxide (CO) impurities
that are often found in industrially produced hydrogen.
Removal of CO adds to the cost of the fuel cell system.
PREVIOUS ATTEMPTS OF
BIO-FUEL CELLS
Several potential applications of BFCs have been reported
or proposed in the literature for implantable devices,
remote sensing and communication devices as a sustainable
and renewable power source.14 However, there are no
BFC design formats or templates that allow for the production
of a working device with a size on the order of
1 cc, which are needed for several “real world” applications.
An enzyme based BFC is very attractive, however it
has been shown that electron flow is too slow to make a
viable fuel cell.This is due to the difficulty for enzymes
to attain direct electrical contact with the electrodes of the
cell and catalyze reactions effectively.
The two largest obstacles with bio-fuel cells which must
be overcome are increasing the power density and increasing
the enzyme stability.In addition, understanding of the
determinants governing the direct electron transfer reaction
and mutation of enzymes to tune the redox potential,
to improve DET kinetics, or to reduce the enzyme size are
also very important challenges facing the commercialization
of bio-fuel cells.15 To address these key issues, various
enzyme immobilization methods have been attempted
for constructing BFCs, such as adsorption, entrapment,
and covalent attachment.
Nanostructured Bioelectrocatalysis
Traditional direct hydrogen fuel cells require noble
metal catalysts both for hydrogen oxidation and oxygen
reduction.17 Similarly, the bio-fuel cells also need catalysts
(bio-catalysts) for the conversion of chemical to electrical
energy.One approach is to use microorganisms and/or
enzymes as biological reactors for the fermentation of raw
materials to fuel products (similar hydrogen fuel reformers);
the second approach is to use the microorganisms
and/or enzymes as catalysts directly in the bio-fuel cells.
The second approach, using purified redox enzymes for the
targeted oxidation and reduction of specific fuel and oxidizer
substrates, is more efficient for bio-fuel cells.Also,
bio-catalysts are an attractive renewable and less expensive
alternative to transition metal catalysts for mediated
electron transfer (MET).
Immobilized Bioelectrocatalysts on CNTs
In order to promote DET of the protein and to improve
the stability, it is necessary that the protein is immobilized
in the form of clusters.4 In this context, glucose oxidase
(GOx) can be attached as crosslinked enzyme clusters
(CECs) onto the surface of carbon nanotubes (CNTs).In
has been demonstrated in a recent study that CEC-GOx did
not manifest any decrease in activity for 250 days.4 The
CEC-GOx based BFC was characterized by potentiostatic
polarization in an unbuffered solution.The open circuit
voltage (OCV) was 0.33 V while a maximum power output
of 120 Wcm−2 occurred at a cell potential of 0.1 V
(Fig.4(a)).The long-term performance of the miniature
BFC with CEC-GOx, constant voltage measurements at
0.1 and 0.25 V (Fig. 4(b)) showed some transient behavior
initially and the cell performance stabilized after about
2 hours.More importantly, at 0.1 V where a heavy load
was applied to the BFC, the performance of the BFC was
very stable without any significant performance decay for
more than 16 hours (Fig.4(b)).
Flavoproteins
Flavoproteins commonly contain one of two prosthetic
groups, FMN (e.g., NADH dehydrogenase, EC 1.6.99.l)
and FAD.The FMN is non-covalently bound in all
known cases. FAD may be non-covalently bound (e.g., in
dihydrolipoamide dehydrogenase (NADH), EC 1.8.1.4) or
covalently bound by a methylene bridge between the benzene
ring of the benzo[g]pteridine-2,4-dione and an amino
acid residue, such as cysteine, histidine or tyrosine, in
the protein (e.g., succinate dehydrogenase, EC 1.3.99.1),
or directly at ring position 6.8-Hydroxy-p yrimidino
[4,5-b]quinoline-2,4-dione functions as prosthetic group
in methanogens and in deoxyribodipyrimidine photolyase
(EC 4.1.99.3). Apart from a few exceptions where the role
of the flavin is not clear, flavoproteins carry out oxidationreduction
reactions, in which one substrate is oxidized
and a second is reduced.F or all these enzymes each catalytic
cycle consists of two distinct processes, the acceptance
of redox equivalents from a reducing substrate and
the transfer of these equivalents to an oxidized acceptor.
Accordingly, the catalyzed reactions consist of two separate
half-reactions: a reductive half-reaction in which the
flavin is reduced and an oxidative half-reaction, in which
the reduced flavin is reoxidized.
Immobilization of Electron Transfer Proteins
We will focus on GOx as a test case for covalent attachment
to substrates in view of its wide spread use in
fuel cells.Co valent attachment of GOx to SWCNT further
enhances efficient transfer of electrons.CNTs have
unique electronic properties, high mechanical strength and
chemical stability, making them attractive for fabricating
of GOx coupled CNT based device elements for use in
bio fuel cells.F or chemically coupling the CNTs with
biomolecules, it is critical to functionalize CNT surfaces,
which are very inert.Se veral strategies have been developed
for functionalizing CNTs, derivatizing them with
biomolecules, and demonstrating the proof of concept
of using bacteriorhodopsin mutants -CNT hybrids24 for
sensing via electrical, electrochemical, and electro-optical
means.In all the cases, however, the CNTs are functionalized
wet-chemically in a spatially random fashion
where molecular bonding is mediated by defect creation,
or hydrophobic adsorption (Fig.11(a)).While such conventional
treatments may be adequate for demonstrating
the proof-of-concept of sensors from CNTs contacted on
substrates, or random dispersions of CNTs in solutions,
they have serious limitations for realizing multifunctional
sensor arrays on a chip that integrates system level logic
operations for data processing.