Title of Invention

"A BIO-FUEL CELL"

Abstract To provide a fuel cell and a method of using the same, which enable such a reaction environment as to exhibit excellent properties as electrode sufficiently, and to provide a cathode for the fuel cell, a device using an electrode reaction, and an electrode for the device using an electrode reaction. A fuel cell 10 includes an electrolyte solution 7 arranged between a cathode 1 and an anode 5. The cathode 1 includes a porous material made typically of carbon and an immobilized thereon. The fuel cell is so configured as to bring at least part of the cathode 1 into contact with a reactant in a gaseous phase. The cathode 1 preferably further includes an immobilized electron-transfer mediator in addition to the enzyme. The reactant in a gaseous phase can be, for example, air or oxygen.
Full Text The present invention relates to a bio-fuel cell.
Technical Field
The present invention relates to a fuel cell, a method of using a fuel cell, a cathode for a fuel cell, an electronic device, a device using an electrode reaction, and an electrode for a device using an electrode reaction. The present invention is advantageously applied to fuel cells for use as power suppliers for electronic devices such as mobile phones. Background Art
Biological metabolism in living beings is a reactionmechanism with high substrate selectivity and remarkably high efficiency. It is characterized by reaction progressing in a relatively moderate atmosphere held at room temperature and neutral. Biological metabolism herein involves aspiration and photosynthesis, among others, which converts oxygen and various nutritional elements such as sugars, fats, oils, and proteins to energy necessary for growth of microorganisms and cells.
Such biological reactions largely rely on biological catalysts made of proteins, namely, enzymes. The idea of using the catalytic reactions of enzymes has been put into practice from early days of the history of humankind.
In particular, ideas and techniques for using immobilized
enzymes have been technologically investigated. Densely
immobilized enzymes exhibit high catalytic performance even in
a small amount, have high specificity, and can be treated as with
solid catalysts generally used in chemical reactions. Such
densely immobilized enzymes are very useful as the usage of enzymes .
Applications of immobilized enzymes range over many
industrial fields including brewing industry, fermentation
industry, fiber industry, leather industry, food industry and
drug industry. Recently, its applications to the field of
electronics suchasbiosensors, bioreactors, bio-fuel cells, which
incorporate the catalytic function into electrode systems, have
come to be examined and brought into practice.
Regarding techniques for utilizing biological metabolism in
fuel cells, there have been reported microbial cells in which
electric energy formed in microorganisms is extracted through
an electron mediator out of the microorganisms, and the resulting
electrons are transferred to an electrode (for example, JP-A No.
2000-133297) . This relates to a technique of using an enzyme for
extracting the energy.
On application of enzymes to electrode systems, high-density
immobilization of an enzyme on an electrode makes it possible
to efficiently catch enzymatic reactions occurring near the
electrode as electric signals.
For researches of electrode systems, in general, it is
necessary to use an electron-accepting compound behaving as an
electron transfer mediator between the protein as the enzyme and
the electrode, where intermediation of electrons is unlikely to
occur. This electron-accepting compound is preferably
immobilized similarly to the enzyme.
To proceed a catalytic action of an enzyme, a reaction is
essentially carried out under such conditions as to enable movement
of electrons and/or protons. The same is also true for the
above-mentioned functional electrodes (enzyme-immobilized
electrodes) each carrying an enzyme and an electron-accepting
compound immobilized thereon.
Evaluations of such enzyme-immobilized electrodes have been
generally carried out in water or a buff er solution, because organic
solvents may adversely affect the activities of enzymes.
In this case, a material serving as a reactant in an
enzyme-catalyzed reaction is preferably dissolved in water or
a buffer solution, because the dissolved reactant may undergo
an enzymatic catalytic act ion more uniformly and more efficiently.
However, when a material serving as a reactant is dissolved
in water or a buffer solution, the solubility of a material is
a parameter specific to the material, thereby the dissolution
of the material is restricted, and this in turn restricts electrode
reactions.
Among re act ants, oxygen has a very low solubility in a solution
(dissolved oxygen concentration) than in air, and this
significantly limits electrode reactions. In addition, oxygen
much more quickly diffuses in a solution than in air.
As is described above, if water or a buffer solution is used
upon use of oxygen as a reactant, electrode reactions have
limitations due to the solubility of oxygen in water or a buffer
solution. It is therefore significantly desirable to solve this
technical problem upon practical use of enzymes in fuel cells.
In addition, the more an enzyme-immobilized electrode
exhibits an excellent catalytic action, the more the supply of
a reactant material to the electrode limits or controls the rate
of an entire reaction. The supply herein is restricted by the
specific solubility of the material in water or a buffer solution,
and excellent properties of the enzyme-immobilized electrode may
not be exhibited sufficiently.
Under these circumstances, a certain technique employs a
system using dissolved oxygen in a solution as a reactant and
is so configured as to improve functions of an enzyme-immobilized
electrode by increasing oxygen partial pressure or stirring a
solution to thereby increase an oxygen supply. This technique
can be found, for example, in N. Mano, H. H Kim, Y. Zhang and
A. Heller, J. Am. Chem. Soc. 124 (2002) 6480.
As is described above, oxygen-reducing electrodes using
enzymes in related art employ a configuration of carrying out
a reaction in a liquid and are so configured as to increase an
oxygen supply to thereby improve the efficiency of electrode
reaction by increasing an oxygen partial pressure or stirring
the solution.
However, the procedure of increasing an oxygen partial
pressure or stirring a solution so as to increase the oxygen supply
is unsuitable in design, from the viewpoint of practical use of
the resulting fuel cell.
Specifically, the oxygen supply is limited, because a reaction
should be carried out in a static state, and the diffusion rate
of dissolved oxygen is limited. It is therefore difficult to yield
a large oxygen-reducing current.
Accordingly, it is desirable herein to provide a fuel cell
that can yield such a reaction environment as to exhibit excellent
properties as electrode sufficiently; a method of using the fuel
cell; a cathode suitable for the fuel cell; and an electronic
device having the fuel cell.
More generally, it is desirable to provide a device using
an electrode reaction, such as a fuel cell, that can yield such
a reaction environment as to exhibit excellent properties as
electrode sufficiently; and an electrode for the device using
an electrode reaction.
Disclosure of the Invention
After intensive investigations to solve the problems in
related art, the present inventors have discovered the direct,
contact supply of a reactant as a gas to an electrode including
an enzyme-immobilized electroconductive and gas-permeable
material, such as an enzyme-immobilized porous material, in an
enzyme-catalyzed reaction. They have experimentally verified
that an enzyme actually undergoes an enzyme-catalyzed reaction
with a reactant in a gaseous phase according to this technique.
Use of a reactant in a gaseous phase in an enzyme-catalyzed reaction
has not yet been reported, and such a reactant is essentially
immersed in water or a buffer solution according to techniques
in related art, as far as the present inventors know. However,
by realizing the use of a reactant in a gaseous phase according
to the technique, the problems can be solved at once.
The present invention has been made based on these findings .
Specifically, according to a first aspect of the present
invention to solve the problems, there is provided:
a fuel cell including a cathode, an anode, and a proton
conductor arranged between the cathode and the anode,
in which the cathode includes an electroconductive and
gas-permeable material and an enzyme immobilized on the material,
and the fuel cell is so configured as to bring at least part of
the cathode into contact with a reactant in a gaseous phase.
According to a second aspect of the present invention, there
is provided:
a cathode for a fuel cell, which includes an electroconductive
and gas-permeable material and an enzyme immobilized on the
material.
According to a third aspect of the present invention, there
is provided:
an electronic device including a fuel cell, the fuel cell
including a cathode, an anode, and a proton conductor arranged
between the cathode and the anode,
in which the cathode of the fuel cell includes an
electroconductive and gas-permeable material and an enzyme
immobilized on the material, and the electronic device is so
configured as to bring at least part of the cathode into contact
with a reactant in a gaseous phase.
Electroconductive and gas-permeable materials for use in the
first, second, and third aspects act as electrode substrates.
The materials can basically be any ones, as long as they have
such a satisfactory electroconductivity as to be used as electrodes
and each have a gas-permeable inner structure. Among them,
preferred are porous materials typically including carbon and
having a high specific surface area, for yielding a satisfactory
reaction area.
The cathode is so configured that at least part thereof is
brought in contact with a reactant in a gaseous phase upon use.
The cathode preferably lies in a wet state upon use, for carrying
out a reductive reaction. More specifically, it is preferred,
for example, to bring into contact and wet the enzyme with a buffer
solution typically by immersing the cathode in the buffer solution
so as to improve the enzymatic activity. Various reactants in
a gaseous phases can be used herein. Oxygen is a representative
reactant which is in a gaseous phase at ordinary temperature and
normal pressure. Oxygen is generally supplied as air or oxygen
gas. Oxygen is very effective for use in a reductive reaction,
because it is inexhaustibly present in air and does not adversely
affect the environment. Instead of or in addition to oxygen, NOX,
for example, can be used as a reactant in a gaseous phase. Any
reactants in a gaseous phase can be fundamentally applied herein,
as long as they are vaporizable substances . It is also acceptable
to supply a reactant in a gaseous phase as bubbles while placing
the cathode in a liquid phase.
A material for a cathode preferably further carries an
electron-transfer mediator immobilized thereon, in addition to
an enzyme. The electron-transfer mediator originally serves to
fetch electrons formed as a result of an enzymatic reaction and
transfer them to the cathode. An electron-transfer mediator, if
immobilized in a sufficiently high concentration on a material
constituting the cathode, can be used as an electron pool for
storing electrons temporarily. Specifically, bio-fuel cells in
related art do not fully utilize the remainder of their power
or capability when they carry out discharge at a power lower than
their critical power or when they are connected to an infinite
resistance. It is difficult for them to generate power equal to
or higher than the critical power. In addition, temporary drop
of oxygen concentration and/or fuel concentration significantly
sensitively affect the output power. These problems can be solved
at once by using an electron-transfer mediator as an electron
pool. In this case, the electron-transfer mediator is preferably
immobilized in a sufficiently high concentration on a surface
of a cathode. More specifically, the electron-transfer mediator
is preferably immobilized in an average amount of, for example,
0.64x 10"6 mol or more per square millimeter (unit area) of the
surface of the cathode. The storage of electrons in the
electron-transfer mediator has following advantages . Electrons
can be temporarily stored by the electron-transfer mediator
spontaneously, in other words, charging can be carried out
spontaneously by maximizing the remainder of catalytic activity
of an enzyme, when an infinite resistance is connected as a load
to an external circuit of a bio-fuel cell or when the fuel cell
supplies a low power. Even if a bio-fuel cell should yield an
output (power) equal to or higher than its critical power, such
power equal to or higher than the critical power can be generated
by using the charged electron-transfer mediator, in addition to
using the catalytic activity of the bio-fuel cell. An
electron-transfer mediator serving as an electron pool may be
immobilized on an anode, in addition to a cathode.
A proton conductor to be arranged between a cathode and an
anode may include, for example, an electrolyte.
Electronic devices for use herein can be basically any ones
and include both portable devices and stationary devices.
Specific examples thereof are mobile phones, portable equipment,
robots, personal computers, vehicle-mounted equipment, and
various household electrical appliances.
According to a fourth aspect of the present invention, there
is provided:
a method of us ing a fuel cell, the fuel cell including a cathode,
an anode, and a proton conductor arranged between the cathode
and the anode, the cathode including an electroconductive and
gas-permeable material, and an enzyme and an electron-transfer
mediator each immobilized on the material, the electron-transfer
mediator constituting an electron pool so configured as to store
electrons, the fuel cell being so configured as to carry out power
generation by bringing at least part of the cathode into contact
with a reactant in a gaseous phase,
the method including the step of feeding electrons from the
electron pool to the cathode when a supply of the reactant to
the cathode stops, when a supply of the reactant to the cathode
decreases, or when an output is to be increased.
Other descriptions relating to the first, second, and third
aspects of the present invention can also be applied to the fourth
aspect, as long as not contrary to their properties.
According to a fifth aspect of the present invention, there
is provided:
a device using an electrode reaction, including a pair of
electrodes,
in which one of the pair of electrodes includes an
electroconductive and gas-permeable material and an enzyme
immobilized on the material, and the device is so configured as
to bring at least part of the one electrode into contact with
a reactant in a gaseous phase.
According to a sixth aspect of the present invention, there
is provided:
an electrode for a device using an electrode reaction, which
includes an electroconductive and gas-permeable material and an
enzyme immobilized on the material.
Examples of a device using an electrode reaction include
bio-fuel cells, biosensors, and bioreactors modeling themselves
on biological metabolism. A pair of electrodes in a device using
an electrode reaction corresponds to, for example, a cathode and
an anode in a bio-fuel cell and to a working electrode and a counter
electrode in a bioreactor, respectively.
Other descriptions relating to the first, second, and third
aspects of the present invention can also be applied to the fifth
and sixth aspects, as long as not contrary to their properties.
According to the present invention configured as above, a
cathode including an electroconduct ive and gas -permeable material
and an enzyme immobilized thereon is so configured as to come
in contact with a reactant in a gaseous phase at least partially.
By satisfying this, the enzyme immobilized on the cathode acts
as a catalyst so as to induce reduction of the reactant in a gaseous
phase, such as oxygen supplied typically as air or oxygen gas.
According to this technique, the amount of a supplied reactant
is not restricted, an enzyme-catalyzed reaction can be carried
out with an excellent efficiency to thereby yield a large reduction
current and to yield a practically excellent efficiency in power
generation. The fuel cell or device using an electrode reaction
has a simple structure.
According to the present invention, there is provided such
a reaction environment as to sufficiently exhibit excellent
properties as electrode of an enzyme-immobilized electrode.
Accordingly, there is provided bio-fuel cells having high
efficiencies, as well as other high-efficiency devices using an
electrode reaction, such as biosensors and bioreactors. Such
devices using an electrode reaction enables to carry out
environmental remediation and decomposition of pollutants in a
gaseous phase. Biosensors using this technique can select
substrates within a broader range and can carry out sensing in
a novel manner.
Brief Description of the Drawings
Fig. 1 is a schematic diagram of a fuel cell according to
an embodiment of the present invention.
Fig. 2 is a schematic diagram showing the configuration of
electrodes in Comparative Example.
Fig. 3 is a schematic diagram showing the configuration of
electrodes in Example.
Fig. 4 is a schematic diagram showing data in current-time
(I-t) measurements of samples according to Comparative Example
and Example, respectively.
Fig. 5 is a schematic diagram of a fuel cell according to
an embodiment of the present invention.
Fig. 6 is a schematic diagram showing data in current-time
(I-t) measurement using the fuel cell.
Fig. 7 is a schematic diagram showing data in current-time
(I-t) measurement at a varying concentration of an
electron-transfer mediator.
Fig. 8 is a schematic diagram showing a more practically
suitable configuration of a fuel cell according to an embodiment
of the present invention.
Best Mode for Carrying Out the Invention
An embodiment of the present invention will be illustrated
with reference to the attached drawings.
Fig. 1 shows a fuel cell according to an embodiment of the
present invention.
With reference to Fig. 1, a fuel cell 10 according to an
embodiment includes a first component 11 and a second component
12. The first component 11 includes a cathode (positive electrode)
1. The second component 12 includes an anode (negative electrode)
5. The fuel cell further includes an electrolyte solution 7 as
a proton conductor arranged between the pair of electrodes (anode
and cathode).
The cathode 1 includes an enzyme-immobilized electrode.
This electrode includes a porous material made typically of carbon,
and an enzyme immobilized on a surface of the porous material
facing the electrolyte solution 7 . The enzyme can be, for example,
an enzyme having an oxidase activity and using oxygen as a reactant,
such as laccase, bilirubin oxidase, or ascorbate oxidase. The
porous material preferably further carries an electron-transfer
mediator immobilized thereon, in addition to an enzyme. An
electron-transfer mediator is more preferably immobilized in a
sufficiently high concentration, for example, in an average amount
of 0 . 64x 10"6 mol/mm2 or more. Any known or conventional procedure
can be used for immobilizing the enzyme and electron-transfer
mediator. In particular, immobilization procedures that are
susceptible to pH or ionic strength of water or a buffer solution
used in related art can be used in the present invention.
In the first component 11, the cathode 1 carrying an
immobilized enzyme is assembled with a current collector 8 made
typically of a titaniummesh. According to this structure, current
collection can be easily carried out.
The anode 5 can be any electrode that is capable of supplying
proton (H+) , such as a hydrogen-platinum electrode or a
methanol-ruthenium-platinum electrode.
The anode 5 is arranged in the electrolyte solution 7 in
the second component 12 . Where necessary, a reference electrode
(not shown) is also arranged in the electrolyte solution 7.
The electrolyte solution 7 may generally be a strongly acidic
solution such as a sulfuric acid solution, or a strongly basic
solution such as a potassium hydroxide solution. This bio-fuel
cell, however, can also work under mild, neutral conditions,
because an enzyme immobilized as a catalyst on the cathode 1 exhibits
a catalytic activity even at a pH of around 7, and the bio-fuel
cell can use a buffer solution or water having a pH of around
7.
The first component 11 and the second component 12 are
separated from each other by the action of a separator 9. The
separator 9 includes an insulative and proton-permeable membrane
made typically of cellophane (methylcellulose) . This prevents
the electrolyte solution 7 from migrating into the cathode 1.
The fuel cell 10 is so configured as to bring the cathode
1 into contact with a reactant in a gaseous phase. Accordingly,
the cathode 1 is brought in contact with a reactant by placing
at least part of the cathode 1 in a gaseous phase. The cathode
1 includes a porous material, and this allows the reactant in
a gaseous phase in contact with the cathode 1 to penetrate the
cathode 1 to thereby react with an enzyme immobilized on the porous
material.
The fuel cell 10 is configured as follows. The cathode 1
is fed with oxygen (O2) from a gaseous phase in contact with the
cathode 1 and is also fed with H+ from the electrolyte solution
7. This induces the following reaction (1) by the catalysis of
the enzyme immobilized on the cathode 1. In the anode 5, there
occurs the following reaction (2) when the anode 5 is a
methanol-ruthenium-platinum electrode, and there occurs the
following reaction (3) when the anode 5 is a hydrogen-platinum
electrode. Thus, electrons are exchanged and thereby a current
passes between the cathode 1 and the anode 5 through an external
circuit.
It has been verified that an oxygen-reducing reaction in the
cathode 1 satisfactorily proceeds in a wet state.
Cathode: O2+4H+ +4e" -> 2H20 (1)
Anode (methanol-ruthenium-platinum electrode): CH3OH+H2O ->
C02+6H++6e" (2)
Anode (hydrogen-platinum electrode): H2 -> 2H++2e" (3)
Specific samples of a fuel cell according to an embodiment
of the present invention, and an enzyme-immobilized electrode
(positive electrode) as a cathode for constituting the fuel cell
were prepared and were evaluated.
The reactivities of following samples were determined upon
use of dissolved oxygen in a buffer solution as a reactant and
upon use of oxygen in a gaseous phase as a reactant, respectively.
To determine whether or not an electrode carrying an
immobilized enzyme for reducing a reactant (oxygen) actually uses
oxygen in a gaseous phase as the reactant, the enzyme-immobilized
electrode should have a catalytic reaction rate sufficiently
higher than the diffusion rate of dissolved oxygen.
In samples shown below, therefore, bilirubin oxidase and
potassium hexacyanoferrate were used as an enzyme and an
electron-transfer mediator, respectively. These were
immobilized on a surface of an electrode by the action of an
electrostatic interaction of poly-L-lysine.
The resulting enzyme-immobilized electrode is known to have
a very high oxygen-reducing capability, in which the diffusion
of dissolved oxygen in a solution acts as a rate-limiting factor
(Nakagawa, T., Tsujimura, S., Kano, K, , andlkeda, T. Chem. Lett.,
32 (1) , 54-55 (2003)) .
Initially, an enzyme-immobilized electrode was prepared in
the following manner.
A commercially available carbon felt (the product of TORAY
Industries, Inc. ; BO030) was used as a porous material, and this
carbon felt was punched into a round piece having a diameter of
6 mm.
Next, the carbon felt was impregnated sequentially with 20
jalof poly-L-lysine (1 percent by weight) , 10 |al of hexacyanoferrate
ion (10 mM) as an electron-transfer mediator, and 10 (J,l (lOOmg/ml)
of a solution of bilirubin oxidase (derived from Myrothecium
verrucaria) , was dried, and thereby yielded an enzyme-immobilized
electrode.
[Comparative Example]
In this comparative example, a system of measurement having
a configuration of electrodes as shown in Fig. 2 was assembled.
An enzyme-immobilized electrode 101 prepared in the above
manner was physically fixed to a commercially available glassy
carbon electrode 102 (the product of Bioanalytical Systems, Inc. ;
No. 002012) using a fixing device 103 made from a nylon net to
thereby constitute a working electrode 100. This has such a
structure as to carry out current collection easily.
Fig. 2 illustrates the enzyme-immobilized electrode 101 as
lying away from the glassy carbon electrode 102, for the sake
of visual observation of the tip of the glassy carbon electrode
102. In an actual configuration, however, the enzyme-immobilized
electrode 101 lies in physical contact with the glassy carbon
electrode 102.
The electrode having the configuration was immersed in a
buffer solution 104 saturated with oxygen. Then a counter
electrode 105 including a platinum wire, and a reference electrode
(Ag/AgCl) 106 were arranged at predetermined positions.
[Example]
In this example, a system of measurement having a
configuration of electrodes as shown in Fig. 3 was assembled.
An enzyme-immobilized electrode 101 prepared in the above
manner was physically fixed to a commercially available glassy
carbon electrode 102 (the product of Bioanalytical Systems, Inc. ;
No. 002012) using a fixing device 103 made from a nylon net to
thereby constitute a working electrode 100. This has such a
structure as to carry out current collection easily.
Fig. 3 illustrates the enzyme-immobilized electrode 101 as
lying away from the glassy carbon electrode 102, for the sake
of visual observation of the tip of the glassy carbon electrode
102. In an actual configuration, however, the enzyme-immobilized
electrode 101 lies in physical contact with the glassy carbon
electrode 102.
The electrode having this configuration was arranged out of
a buffer solution 104 so as to be in contact with air. The
enzyme-immobilized electrode 101 was connected to the buffer
solution 104 through a lead 110 made from a carbon felt. Thus,
a system of electrochemical measurement was configured.
Then a counter electrode 105 including a platinum wire, and
a reference electrode (Ag/AgCl) 106 were arranged at predetermined
positions.
Electrochemical measurements were conducted on the systems
of measurement having configurations shown in Fig. 2 and Fig.
3, respectively.
Specifically, current-time (I-t) measurements were carried
out at a constant voltage of 0.1 V. The results are shown Fig.
4 . In a sample according to Comparative Example having the
configuration shown in Fig. 2, a catalytic current as a result
of oxygen reduction gradually decreased and finally reached
jaA/cm2, as indicated by the broken line Y. This is because the
enzyme-immobilized electrode 101 has such high performance as
to exceed the amount of dissolved oxygen in the buffer solution
104, and oxygen begins being exhausted following the beginning
of measurement.
In contrast, in a sample according to Example, a current as
a result of catalytic action was 3 f^A/cm2 when the lead 110 made
of carbon felt was immersed in the buffer solution 104 (State
A of the solid line X in Fig. 4).
Next, the enzyme-immobilized electrode 101 was wetted with
the buffer solution 104 to thereby activate the enzyme slightly.
As a result, a very high current density of 266 joA/cm2 was obtained
(State B of the solid line X in Fig. 4).
This is because the oxygen concentration in a gaseous phase
is much higher than the concentration of dissolved oxygen in the
buffer solution 104, and the enzyme-immobilized electrode 101
acts to reduce oxygen in a gaseous phase highly efficiently.
These results demonstrate that a reaction can be carried out
with excellent efficiency using an enzyme immobilized on an
enzyme-immobilized electrode 101 and oxygen in a gaseous phase.
[Fuel cell]
Next, the above-prepared enzyme-immobilized electrode 101
was applied as a cathode of a fuel cell by utilizing an
oxygen-reducing reaction thereof, and performance as a cell was
evaluated.
With reference to Fig. 5, a fuel cell 200 structurally includes
an enzyme-immobilized electrode 101 as a cathode (positive
electrode), an anode (negative electrode) 115 containing a
methanol-ruthenium-platinum electrode, and an electrolyte
solution 107 . The enzyme-immobilized electrode 101 and the anode
115 face each other with the interposition of the electrolyte
solution 107.
The enzyme-immobilized elect rode 101 as a cathode was prepared
by cutting a carbon felt into a 10-mm square piece, and immobilizing
an enzyme thereon by the procedure of Example.
With reference to Fig. 5, an enzyme-immobilized electrode
101 was placed over a continuous hole 120 in an electrode contact
region 121 in a cell lower part 201. Next, a titanium mesh as
a current collector 108 was placed on the enzyme-immobilized
electrode 101 and thereby yielded a working electrode having such
a structure as to carry out current collection easily. An
insulative and proton-permeable predetermined film, such as a
cellophane (methylcellulose) film, as a separator 109 was then
arranged on the enzyme-immobilized electrode 101 so as to separate
the same from a cell upper part 202.
In the cell upper part 202, an anode 115 including a
methanol-ruthenium-platinum electrode was arranged in an
electrolyte solution 107, and a reference electrode 106 was
arranged so as to be in contact with the electrolyte solution
107 . These anode 115 and reference electrode 106 have sufficiently
large reaction surface areas . The reference numeral 122 indicates
a lid.
The separator 109 is so configured as to prevent the
electrolyte solution 107 from seeping out of the cell upper part
202. The enzyme-immobilized electrode 101 as a cathode lies in
a gaseous phase.
In the fuel cell 200 having the above configuration, the
enzyme-immobilized electrode 101 as a cathode is fed with oxygen
(02) from a gaseous phase in contact with the enzyme-immobilized
electrode 101 and is also fed with H+ from the electrolyte solution
107. This induces the following reaction (1) by the catalysis
of the enzyme immobilized on the enzyme-immobilized electrode
101. In the anode 115, there occurs the following reaction (2) .
Thus, electrons are exchanged and thereby a current passes between
the enzyme-immobilized electrode 101 as a cathode and the anode
115 through an external circuit.
Cathode: O2+4H++4e" ->• 2H20 (1)
Anode: CH3OH+H20 -> C02+6H"t'+6e~ (2)
Air was fed to the electrode contact region 121 so as to feed
oxygen to the enzyme-immobilized electrode 101 as a cathode through
the continuous hole 120 (Fig. 5). Under this condition, a
current-time (I-t) measurement at a voltage of 0.1 V was carried
out. The result is shown as Curve "a" in Fig. 6.
In this case, oxygen in a gaseous phase acted as a reactant,
and an oxygen-reducing current was observed following the
placement of the electrolyte solution 107 in the cell upper part
202, and a stationary catalytic current of 1. 5 mA/cm2 was obtained
as indicated at the left end of Curve "a" in Fig. 6.
In contrast, another current-time (I-t) measurement at a
voltage of 0.1 V was carried out while an electrolyte solution
was placed in the electrode contact region 121 (Fig. 5) so as
to fully immerse the enzyme-immobilized electrode 101 as a cathode
in the electrolyte solution. The result is shown as Curve "b"
in Fig. 6.
In this case, the catalytic current decreased following the
beginning of the measurement and became constant at 50 lA/cm2 as
indicated at the left end of Curve "b" in Fig. 6. This is because
dissolved oxygen in the electrolyte solution acts as a reactant,
and the diffusion of dissolved oxygen causes the decrease in
catalytic current.
These results verify that a fuel cell using an
enzyme-immobilized electrode 101 as a cathode is so configured
as to efficiently reduce oxygen in a gaseous phase to thereby
yield a current density as high as 30 times that of a fuel cell
in related art using dissolved oxygen in an electrolyte solution
as a reactant.
Next, described is another experimental example. In this
experimental example, the concentration of an electron-transfer
mediator to be immobilized together with an enzyme on a cathode
of a fuel cell was varied, an electron pool constituted by the
electron-transfer mediator was filled with electrons, and a
current was measured continuously with elapse of time from the
stoppage of oxygen supply to the cathode. The result is described
below.
To carry out the experiment, bilirubin oxidase as an enzyme
and hexacyanoferrate ion (Fe (CN) 6
3~/4~) as an electron-transfer
mediator were immobilized on a 5-mm square carbon felt by the
electrostatic interaction of poly-L-lysine as a polycation, to
thereby yield an electrode. The electrode as a single electrode
was evaluated. The results are shown in Fig. 7. In Fig. 7, Curve
(1) shows the result of a comparative example, in which a platinum
catalyst was used in a cathode. Curve (2) shows the result of
a sample in which an electron-transfer mediator was immobilized
to the carbon felt in an amount of 1. 6x 10"6 mol (in terms of an
average concentration of 0.64x 10"7 mol/mm2) , in addition to the
enzyme. Curve (3) shows the result of a sample in which an
electron-transfer mediator was immobilized to the carbon felt
in an amount of 1. 6x 10"5 mol (in terms of an average concentration
of 0 . 64x 10"6 mol/mm2) , in addition to the enzyme. Curve (4) shows
the result of a sample in which an electron-transfer mediator
was immobilized to the carbon felt in an amount of 1. 6x 10~4 mol
(in terms of an average concentration of 0 . 64x 10"5 mol/mm2) , in
addition to the enzyme. Fig. 7 demonstrates that the sample using
a platinum catalyst in the cathode (Curve (1) ) showed a current
which dropped to zero about 20 seconds after the stoppage of oxygen
supply. The sample using the immobilized electron-transfer
mediator in an amount of 1.6x 10"6 mol (in terms of an average
concentration of 0 . 64x 10"7 mol/mm2) (Curve (2) ) showed a current
which dropped about several tens of seconds after the stoppage
of oxygen supply and reached about 5% of the initial current about
100 seconds later. In contrast, the sample using the immobilized
electron-transfer mediator in an amount of 1. 6x 10~5 mol (in terms
of an average concentration of 0.64x 10'6 mol/mm2) (Curve (3))
showed a gradually decreased current after the stoppage of oxygen
supply and maintained a current of about 20% of the initial current
even 600 seconds later. The sample using the immobilized
electron-transfer mediator in an amount of 1.6xlO"4mol (in terms
of an average concentration of 0.64x 10"5 mol/mm2) (Curve (4))
showed a more gradually decreased current after the stoppage of
oxygen supply and maintained a current of about 50% of the initial
current even 600 seconds later. Although only the data up to 600
seconds (10 minutes) are shown in Fig. 7, the sample indicated
by Curve (2) showed currents 2%, 1%, and 1% of the initial current
30 minutes, 1 hour, and 2 hours later, respectively; the sample
indicated by Curve (3) showed currents 6.3%, 5%, and 3% of the
initial current 30minutes, Ihour, and2 hours later, respectively;
and the sample indicated by Curve (4) showed currents 25%, 18.9%,
and 14.1% of the initial current 30 minutes, 1 hour, and 2 hours
later, respectively. These samples can maintain a high current
over a long time. This is because they use a cathode carrying
animmobilizedelectron-transfermediator in a high concentration ;
and the immobilized electron-transfer mediator in a high
concentration acts as an electron pool to store electrons
temporarily; and the electrons temporarily stored in the
electron-pool pass through an external circuit, in addition to
electrons formed as a result of an enzymatic reaction.
These results demonstrate that a current can be maintained
at high level over a long time even after the stoppage of oxygen
supply to a cathode, by immobilizing an electron-transfer mediator
to the cathode in an average amount of 0.64x 10"6 mol/mm2 or more.
Fig. 8 shows a fuel cell 300 having a more practically suitable
configuration. Components in Fig. 8 identical to or corresponding
to those in Fig. 5 have the same reference numerals, and overlapped
description will be omitted as appropriate.
With reference to Fig. 8, the fuel cell 300 structurally
includes an enzyme-immobilized electrode 101 as a cathode
(positive electrode), an anode (negative electrode) 115, and a
separator 109 as a proton conductor. The enzyme-immobilized
electrode 101 and the anode 115 face each other with the
interposition of the separator 109. In this case, the separator
109 includes a proton-conductive film such as a cellophane film.
The anode 115 lies in contact with a fuel 123. The fuel 123 can
be any of various fuels such as glucose. Current collectors 108
are arranged below the enzyme-immobilized electrode 101 and above
the anode 115, so as to carry out current collection easily.
Heretofore, an embodiment and examples have been explained
specifically. However, the invention is not limited to the
embodiment and examples, but contemplates various changes and
modifications based on the technical concept of the invention.

For example, numerical values, structures, configurations,
shapes, materials, and so on are not but mere examples, and any
other appropriate numerical values, structures, configurations,
shapes, materials, and so on may be employed, if necessary.




WE CLAIM:
1. A bio-fuel cell comprising:
a cathode;
a current collector arranged below the cathode;
an anode; and
a proton conductor arranged between the cathode and the anode,
wherein the cathode has an electroconductive and gas-permeable material, an
enzyme immobilized on the material and an electron-transfer mediator immobilized on
the material, and
wherein the fuel cell is so configured as to bring at least part of the cathode into
contact with a reactant in a gaseous phase.
2. The bio-fuel cell as claimed in claim 1, wherein the electroconductive and gaspermeable
material comprises a porous material.
3. The bio-fuel cell as claimed in claim 1, wherein the cathode is in a wet state.
4. The bio-fuel cell as claimed in claim 1, wherein the electron-transfer mediator is
so configured as to store electrons and to constitute an electron pool.
5. The bio-fuel cell as claimed in claim 1, wherein the electron-transfer mediator is
immobilized in an average amount of 0.64 x mollmm2 or more.
6. The bio-fuel cell as claimed in claim 4, wherein the electron pool spontaneously
stores electrons when the fuel cell is connected to an infinite resistance as a load or when
the fuel cell supplies a low electric power.
7. The bio-fuel cell as claimed in claim 1, wherein the anode is in contact with a
biohel.
8. The bio-fuel cell as claimed in claim 7, wherein the biofuel is glucose.
9. The bio-he1 cell as claimed in claim 1, wherein between the catliode and the
anode is a cellophane film which acts as a separator.
10. The bio-fuel cell as claimed in claim 1, wherein the electon-transfer mediator is
hexacy ano ferrate ion.
1 1. The bio-fuel cell as claimed in claim 1, wherein the enzyme is bilirubin oxidase.
12. The bio-fuel cell as claimed in claim 1, wherein the current collector comprises a
titanium mesh.
13. The bio-fuel cell as claimed in claim 1, wherein the cathode is placed over a hole
in an electrode contact region in a part of the fuel cell.

Documents:

1159-delnp-2007-abstract.pdf

1159-DELNP-2007-Claims-(01-01-2013).pdf

1159-DELNP-2007-Claims-(30-04-2012).pdf

1159-delnp-2007-claims.pdf

1159-delnp-2007-Correspondence Others-(01-01-2013).pdf

1159-DELNP-2007-Correspondence Others-(08-11-2011).pdf

1159-delnp-2007-Correspondence Others-(14-05-2012).pdf

1159-delnp-2007-Correspondence Others-(26-12-2012).pdf

1159-DELNP-2007-Correspondence Others-(30-04-2012).pdf

1159-delnp-2007-correspondence-others 1.pdf

1159-DELNP-2007-Correspondence-Others.pdf

1159-DELNP-2007-Description (Complete)-(30-04-2012).pdf

1159-delnp-2007-description (complete).pdf

1159-DELNP-2007-Drawings-(30-04-2012).pdf

1159-delnp-2007-drawings.pdf

1159-DELNP-2007-Form-1-(30-04-2012).pdf

1159-delnp-2007-form-1.pdf

1159-DELNP-2007-Form-13-(30-04-2012).pdf

1159-delnp-2007-form-18.pdf

1159-DELNP-2007-Form-2-(30-04-2012).pdf

1159-delnp-2007-form-2.pdf

1159-delnp-2007-Form-3-(14-05-2012).pdf

1159-delnp-2007-form-3.pdf

1159-delnp-2007-form-5.pdf

1159-delnp-2007-GPA-(26-12-2012).pdf

1159-DELNP-2007-GPA-(30-04-2012).pdf

1159-delnp-2007-gpa.pdf

1159-delnp-2007-pct-210.pdf

1159-delnp-2007-pct-301.pdf

1159-delnp-2007-pct-308.pdf

1159-DELNP-2007-Petition-137-(14-05-2012).pdf


Patent Number 257581
Indian Patent Application Number 1159/DELNP/2007
PG Journal Number 42/2013
Publication Date 18-Oct-2013
Grant Date 17-Oct-2013
Date of Filing 13-Feb-2007
Name of Patentee SONY CORPORATION
Applicant Address 1-7-1 KONAN, MINATO-KU, TOKYO, JAPAN
Inventors:
# Inventor's Name Inventor's Address
1 TAKAAKI NAKAGAWA,ATSUSHI SATO, HIDEKI SAKAI AND MAMORU HATAKEYAMA C/O SONY CORPORATION 7-35, KITASHINAGAWA 6-CHOME, SHINAGAWA-KU, TOKYO,JAPAN
PCT International Classification Number H01M 8/16
PCT International Application Number PCT/JP2005/015207
PCT International Filing date 2005-08-22
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 P2004-242413 2004-08-23 Japan
2 P2005-135726 2005-05-09 Japan