WO2011141818A1

WO2011141818A1 - Fuel cell and method of producing fuel cell

Info

Publication number: WO2011141818A1
Application number: PCT/IB2011/001241
Authority: WO
Inventors: Masaaki Mori
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2010-05-14
Filing date: 2011-05-05
Publication date: 2011-11-17
Also published as: JP2011243289A

Abstract

A fuel cell includes an anode (3) and a cathode (5) that is provided on the opposite side of an ion-conductive membrane (4) from the anode, and a hydrophilic gel membrane (7) that is provided between at least one of the anode and the ion-conductive membrane and between the cathode and the ion-conductive membrane.

Description

FUEL CELL AND METHOD OF PRODUCING FUEL CELL

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a fuel cell and a method of producing the fuel cell.

2. Description of the Related Art

[0002] A polymer electrolyte fuel cell is an example of a fuel cell in which an anode and a cathode are provided on opposite sides of an ion-conductive membrane. In general, the anode and the cathode are laminated on opposite surfaces of the ion-co ductive membrane (for example, an electrolyte membrane made of an ion-exchange resin).

[0003] A fuel (hydrogen gas) is supplied to the anode, where the fuel is formed into protons (H⁺) through the action of a catalyst, and two electrons (e^") are discharged toward the cathode. The protons pass through the ion-conductive membrane to reach the cathode. The protons receive the two electrons (e^") from the anode through the action of a catalyst at the cathode, and forms water with oxygen ions that are generated from oxygen that is supplied to the cathode. The movement of the electrons through an external circuit as a result of the reaction is taken out as electric current.

[0004] That is, the reaction H₂ -^» 2H⁺ + 2e^" occurs at the anode, and the reaction 2H⁺ + l/20₂ + 2e^" -» H₂0 occurs at the cathode. Overall, the reaction H₂ + l/20₂ → ¾0 generates electricity. In order to efficiently promote the chemical reactions, a catalyst is used at the electrodes as described above. For example, platinum is often used as a catalyst in the polymer electrolyte fuel cell.

[0005] In recent years, attention has been drawn to biological metabolism as an efficient energy conversion mechanism. It is proposed to apply the biological metabolism to the fuel cells. The biological metabolism provides high energy-utilization efficiency, and metabolic reactions are promoted under moderate conditions'/ such as room temperature. However, because microorganisms and cells involve a large number of unnecessary reactions in addition the desired reactions for converting chemical energy into electrical energy, it is difficult to achieve high energy-conversion efficiency. Thus, there is proposed a fuel cell (bio fuel cell) in which only the desired reactions are performed using an enzyme as a catalyst. In the bio fuel cell, a fuel is dissolved into protons and electrons by the enzyme. Various types of fuel that include alcohols such as methanol and ethanol, monosaccharides such as glucose, and polysaccharides such as starch are developed.

[0006] In the bio fuel cell, immobilization of the enzyme on the electrode is very important, and affects the output power characteristics, the life, the efficiency, and so forth of the bio fuel cell very significantly. Thus, it is very important to immobilize the enzyme while minimizing damage to the enzyme during the production process for the immobilized-enzyme electrode. Japanese Patent Application Publication No. 2009-48833 (JP-A-2009-48833) describes a fuel cell in which an enzyme is immobilized on a positive electrode and a negative electrode by a photo-curable resin and a thermosetting resin. In addition, Japanese Patent Application Publication No. 2007-225444 (JP-A-2007-225444) describes a fuel cell in which an enzyme is immobilized on an electrode surface using an ion-conductive gel. This enables both a reaction for transferring electrons from the enzyme to the electrode and a reaction that allows the electrode to discharge protons.

[0007] In fuel cells that use hydrogen as fuel and a precious metal such as platinum as the catalyst, as well as fuel cells that use an alcohol, a monosaccharide such as glucose, or a polysaccharide such as starch as fuel and an enzyme as the catalyst, improving the efficiency of transport of the fuel and the protons between the ion-conductive membrane and the anode and the cathode leads to the improvement of the cell output. In studies of fuel cells that have been conducted so far, however, it has not been fully studied how the ion-conductive membrane and the anode and the cathode should contact each other.

[0008] In particular, because of the dimensional instability of the ion-conductive membrane which may occur when the^" membrane is wet, air bubbles that may be trapped between the membrane and the electrode, or the like, there may be insufficient contact between the electrode and the membrane. Thus, ions that are produced in the reaction at the anode may not be efficiently transported to the cathode, which reduces the power output of the cell. However, no adequate measures have been taken to ensure contact between the electrode and the membrane.

SUMMARY OF THE INVENTION

[0009] The present invention provides an improved fuel cell in which an ion-conductive membrane and an anode and a cathode are contacted in a novel manner to improve the power generation efficiency in comparison to conventional fuel cells.

[0010] The invention is directed to a fuel cell in which an enzyme catalyst is immobilized on an electrode using a gel material to significantly improve the power generation efficiency by interposing a hydrophilic gel membrane between an ion-conductive membrane and at least one of the anode or the cathode.

[0011] A first aspect of the present invention relates to a fuel cell that includes: an ion-conductive membrane; an anode that is provided on one side of the ion-conductive membrane; a cathode that is provided on the opposite side of the ion-conductive membrane from the anode; and a hydrophilic gel membrane that is interposed between the membrane and at least one of the anode and the cathode.

[0012] A second aspect of the invention relates to a method of producing a fuel cell that includes an anode and a cathode that are provided on opposite sides of an ion-conductive membrane, the method including: forming a hydrophilic gel into a thin membrane; disposing the hydrophilic gel membrane between the ion-conductive membrane and at least one of the anode and the cathode; and bringing the elements into pressure contact with each other in normal temperature.

[0013] The fuel cell according to the aspect of the present exhibit improved cell power compared to that of the fuel cell according to the related art. The fuel cell according to the related art includes a fuel cell that does not include a hydrophilic gel membrane that is interposed between an electrode and an ion-conductive membrane, and a fuel cell that has a structure in which a gel material is provided between an electrode and an ion-conductive membrane but in which an enzyme is directly immobilized on an electrode surface using the gel material such as the fuel cell according to JP-A-2007-225444 mentioned above

[0014] The reason that the cell power of the fuel cell according to the above aspects is improved compared to that of a fuel cell that has a structure in which a catalyst that contains a precious metal or an enzyme is directly immobilized on an electrode surface using a gel material has not been fully clarified yet, but may be considered as follows. In general, an ion-conductive membrane and a gel material have a complex mesh structure. In order that a catalyst that contains a precious metal or an enzyme is to be directly immobilized using a gel material, it is necessary that the gel material should have a finer mesh structure so that the catalyst does not fall off. As a result, there is resistance to the transport of a fuel to the reaction field, which is believed to cause a decrease in output power. According to the above aspects, the hydrophilic gel membrane is interposed between the electrode that contains the catalyst and the ion-conductive membrane. This allows control of the mesh structure, which is considered to make it easy for both the fuel and the protons to move.

[0015] That is, according to the above aspects, the contact area between each electrode and the ion-conductive membrane may be increased without increasing the fuel transport resistance. Thus, ions produced by the reaction at the anode may be efficiently transported to the cathode, which improves the cell output power.

[0016] In the above aspects, the gel material is formed between the catalyst-containing electrode and the ion-conductive membrane not by applying a gel material which has been melted by heating onto an electrode but by using a hydrophilic gel membrane that is formed in advance in a separate process. Therefore, the work of interposing the hydrophilic gel membrane between the electrode and the ion-conductive membrane is significantly facilitated.

[0017] In the above aspects, the hydrophilic gel membrane may be formed using an gel material that conducts ions. Examples of suitable gel materials include, for example, Nafion resin (trade name; manufactured by E. I. duPont de Nemours and Company), which contains positive ions, photo-crosslinked poly (vinyl alcohol) with pendent styrylpyridinium, polyethylene oxide, polyvinyl alcohol, agarose gel, alginate gel, and acrylamide gel.

[0018] In the above aspects, the hydrophilic gel membrane may be formed by impregnating a carrier membrane with a hydrophilic gel. The carrier membrane provides high shape retention properties to the hydrophilic gel membrane in order to facilitate the handling of the hydrophilic gel membrane. Suitable carrier membranes include materials such as filter paper and a porous membrane filter. The carrier membrane also facilitates the adjustment of the thickness of the hydrophilic gel membrane. The thickness of the carrier membrane is not specifically limited, but may be about 1 μπι to 1 mm, and may be about 1 μπι to 10 μηι. By immersing a carrier membrane that has a thickness in the above range in a melted hydrophilic gel to impregnate the carrier membrane with the hydrophilic gel and then drawing up the carrier membrane, it is possible to form a hydrophilic gel membrane that is substantially as thick as the carrier membrane.

[0019] If no carrier membrane is used, a hydrophilic gel membrane of the required thickness may be prepared by pouring a gel into a mold with the required thickness and solidifying the gel.

[0020] In the above aspects, as discussed above, an enzyme may be used as the catalyst. In this case, a so-called bio fuel cell such as disclosed in JP-A-2009-48833 or JP-A-2007-225444 is provided. Many naturally-occurring enzymes are very susceptible to heat, and may be deactivated if subjected to high temperatures. In the above aspects, a hydrophilic gel membrane that is formed in advance is used, rather than a gel material that is melted at a high temperature is applied onto an electrode. Further, the hydrophilic gel membrane is disposed between the electrode, which contains the enzyme catalyst, and the ion-conductive membrane, and then bringing the elements into pressure contact with each other in normal temperature environment. Thus, deactivation of the enzyme may be avoided.

[0021] Suitable enzymes include, for example, oxidation-reduction enzymes such as glucose dehydrogenase, alcohol dehydrogenase, formic dehydrogenase, laccase, and bilirubin oxidase.

[0022] According to the above aspects, the ion-conductive membrane and the anode and the cathode are contacted with the hydrophilic gel membrane interposed between the electrode and the membrane to improve the power generation efficiency of the fuel cell in comparison to that of the fuel cell according to the related art. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 illustrates a fuel cell for evaluation used in Examples and Comparative

Examples;

FIG. 2 shows the current density vs. power density characteristics of Examples 1, 2, and 3 and Comparative Examples 1 and 2, in the case where a 2M aqueous sodium ascorbate solution (manufactured by Wako Pure Chemical Industries, Ltd.) was used as an anode reactant solution;

FIG. 3 shows the current density vs. voltage characteristics of Examples 1, 2, and 3 and Comparative Examples 1 and 2, in the case where a 2M aqueous sodium ascorbate solution (manufactured by Wako Pure Chemical Industries, Ltd.) was used as an anode reactant solution;

FIG. 4 shows the current density vs. power density characteristics of Example 4 and Comparative Example 3, in the case where 1M NADH (manufactured by Nacalai Tesque, Inc.) and 100-mM mPMS (manufactured by Dojindo Laboratories) were used as an anode reactant solution; and

FIG. 5 shows the current density vs. voltage characteristics of Example 4 and Comparative Example 3, in the case where 1M NADH (manufactured by Nacalai Tesque, Inc.) and lOOmM mPMS (manufactured by Dojindo Laboratories) were used as an anode reactant solution . DETAILED DESCRIPTION OF EMBODIMENTS

[0024] Examples 1 to 4 of the present invention will be described below in conjunction with Comparative Examples. However, the present invention is not restricted to the described Examples.

[0025] First, the fabrication of a separation membrane-hydrophilic gel-electrode assembly will be described. Hereinafter, a "separation membrane" functions as the "ion-conductive membrane" according to the present invention, and an "electrode" functions as the "anode" or the "cathode" according to the present invention.

[0026] To fabricate the electrode, an appropriate amount of a carbon slurry (e.g., ketjen black (manufactured by Lion Corporation) slurry) composed of 50 mg of ketjen black, 222 ul of a 10% (w/v) PVP solution, and 3 ml of N-methyl-pyrrolidone was applied to a Trayca Mat 50 (manufactured by Toray Industries, Inc.) that was cut into a circular shape with an area of 1.0 cm², and dried in a drying machine at a temperature of 60 °C to remove the solvent.

[0027] The ketjen black was used after being grounded in an agate mortar to an appropriate particle diameter. The slurry was obtained by processing a mixture of the above components in an ultrasonic disruptor so that the components were sufficiently dispersed. The 10% (w/v) PVP solution was obtained by dissolving poly(4-vinyl pyridine) with a molecular weight of 160000 (manufactured by Sigma- ALDRICH Corporation) in N-methyl-pyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.).

[0028] A hydrophilic gel membrane was prepared by impregnating the following three types of filters (that function as a carrier membrane) that were cut into a circular shape with an area of 1.0 cm² with a hydrophilic gel composed of 0.5% (w/v) Ca-Alginate, an aqueous Na-Alginate solution, and a lOOmM aqueous CaCl₂ solution. Examples 1-3 of the invention were prepared in this manner using GLASS

MICROFIBRE FILTERS GF/C (manufactured by Whatman, Inc.) in Example 1,

Nitrocellulose Filter 45 μΜΗΑ (manufactured by Millipore Corporation) in Example 2, and FILTER (manufactured by Advantec Toyo Kaisha, Ltd.) in Example 3 as the carrier membrane. The hydrophilic gel membrane is then brought into pressure contact between

Nafion 115 (manufactured by Sigma-ALDRICH Corporation) (that functions as an ion-conductive membrane) that serves as a separation membrane and the electrode fabricated in process 1-1 at normal temperature to form a separating membrane-hydrophilic gel (filter impregnation type)-electrode assembly.

[0029] The electrode-gel (filter impregnation type)-membrane was formed only on the anode, and the cathode was directly brought into pressure contact with the separating membrane at normal temperature.

[0030] The hydrophilic gel membrane is prepared by immersing the filter in the aqueous Na-Alginate solution, and then in the aqueous CaCl₂ solution. Consequently, a hydrophilic gel membrane that is attached to the filter which is a semi-permeable membrane that is not easily soluble in water was formed from alginate ions and calcium ions.

[0031] A test fuel cell having the configuration shown in FIG. 1 was fabricated using the electrode-hydrophilic gel (filter impregnation type)-separating membrane assembly as described above. The compositions of reactants that were used for the anode and the cathode are as described below. As described below, two types of anode reactant solutions were used.

[0032] In FIG. 1, reference numeral 1 denotes a fluorine resin plate with an anti-static function, 2 denotes a current collector (titanium mesh), 3 denotes the anode, 4 denotes an ion-conductive membrane, 5 denotes the cathode, 6 denotes a silicone plate, 7 denotes the hydrophilic gel membrane (filter impregnation type), and 8 denotes a fuel tank.

[0033] The anode reactant solution used in Examples 1-3 is a 2M aqueous sodium ascorbate solution that is prepared by dissolving sodium ascorbate (manufactured by Wako Pure Chemical Industries, Ltd.) in water The 2M aqueous sodium ascorbate solution refers to a solution obtained dissolving by 2 moles of sodium ascorbate per 1 liter of a solution, and the same applies hereinafter The anode reactant solution used in Example 4 is prepared by dissolving NADH (reduced nicotinamide adenine dinucleotide; manufactured by Nacalai Tesque, Inc in the 1M sodium phosphate buffer solution. The mPMS solution was obtained by dissolving l-methoxy-5-methyl phenazinium methyl sulfate (manufactured by Dojindo Laboratories) in the 1M sodium phosphate buffer solution (pH: 7.0)

[0034] The cathode reactant solution used in all of Examples 1 to 4 is a 1M aqueous potassium hexacyanoferrate (III) solution that is prepared by dissolving potassium hexacyanoferrate (III) (manufactured by Wako Pure Chemical Industries, Ltd.) in water.

[0035] To evaluate the test cell, an external load device such as an ELECTRONIC LOAD PLZ164WA (manufactured by ikusui Electronics Corporation) is connected in series to the test cell fabricated as described above, and software such as Wavy for PLZ-4W (produced by Kikusui Electronics Corporation) was used to vary the external resistance applied to the test cell from 4 kQ to 1 Ω at appropriate intervals. The current and voltage at each time point were measured using a 34970A Data Acquisition/Switch Unit (manufactured by Agilent Technologies, Inc.). The current and voltage were measured under room temperature conditions (about 25°C).

[0036] The electrode for Comparative Examiner 1 was prepared in the same way as the electrodes for Examples 1 to 4.

[0037] In contrast, the test fuel cell does not include the filter impregnation type hydrophilic gel membrane 7 that is included in the test fuel cell shown in FIG. 1 was fabricated using the electrode which was fabricated in the above process The anode and cathode reactant solutions used are described below.

[0038] As the anode reactant solution, a 2M aqueous sodium ascorbate solution (that is obtained by dissolving sodium ascorbate (manufactured by Wako Pure Chemical Industries, Ltd.) in water) was used. [0039] The cathode reactant solution used in Comparative Example 1 is a 1M aqueous potassium hexacyanoferrate (III) solution prepared by dissolving potassium hexacyanoferrate (III) (manufactured by Wako Pure Chemical Industries, Ltd.) in water.

[0040] The test cell as fabricated above was evaluated in the same manner as in Examples 1 to 4.

[0041] The electrode used in Comparative Examples 2 and 3 were prepared in the same way as the electrodes for Examples 1 to 4.

[0042] An appropriate amount of a hydrophilic gel having the same composition as that of the hydrophilic gel in Examples 1 to 4 which has been melted by heating is applied directly onto the electrode, then left to stand until normal temperature was reached, and brought into pressure contact with Nafion 115 (manufactured by Sigma-ALDRICH Corporation) that serves as a separating membrane at normal temperature to form a separating membrane-hydrophilic gel-electrode assembly. The electrode-hydrophilic gel-membrane assembly may be formed only on the anode, with the cathode directly brought into pressure contact with the separating membrane at normal temperature.

[0043] A test fuel cell having the same configuration as the test fuel cell shown in FIG. 1 was fabricated using the electrode-hydrophilic gel-separating membrane assembly prepared as described above. The compositions of the reactant solutions used for the anode and the cathode are as described below. In addition, it should be noted that two types of anode reactant solutions were used.

[0044] (Comparative Example 2) As anode reactant solutions, a 2M aqueous sodium ascorbate solution (that is obtained by dissolving sodium ascorbate (manufactured by Wako Pure Chemical Industries, Ltd.) in water) was used.

(Comparative Example 3) As anode reactant solutions, A 1M NADH solution, a lOOmM mPMS solution, and a 1M sodium phosphate buffer solution (pH: 7.0) was used. The NADH solution was obtained by dissolving NADH (manufactured by Nacalai Tesque, Inc.) in the 1M sodium phosphate buffer solution (pH: 7.0). The mPMS solution was obtained by dissolving l-methoxy-5-methyl phenazinium methyl sulfate (manufactured by Dojindo Laboratories) in the 1M sodium phosphate buffer solution (pH: 7.0).

[0045] A cathode reactant solution of 1M aqueous potassium hexacyanoferrate (III) solution is prepared by dissolving potassium hexacyanoferrate (III) (manufactured by Wako Pure Chemical Industries, Ltd.) in water.

[0046] The test cell as fabricated above was evaluated in the same manner as in Examples 1 to 4. FIG. 2 shows the current density vs. power density characteristics of Examples 1, 2, and 3 and Comparative Examples 1 and 2.

[0047] As seen from FIG. 2, the test confirmed that the fuel cells according toexamples 1 to 3, which include the electrode-hydrophilic gel (filter impregnation type)-separating membrane assembly exhibited an improved power density compared to Comparative Examples 1 and 2.

[0048] FIG. 3 shows the current density vs. voltage characteristics of Examples 1, 2, and 3 and Comparative Examples 1 and 2, that is, in the case where a 2M aqueous sodium ascorbate solution was used as an anode reactant solution.

[0049] As seen from FIG. 3, the test confirmed that the fuel cells according toexamples 1 to 3, which have the electrode-hydrophilic gel (filter impregnation type)-separating membrane assembly exhibited a current vs. voltage characteristic line with a small slope compared to Comparative Examples 1 and 2. Because the slope of a current vs. voltage characteristic line represents the internal resistance according to_. Ohm's law, the results suggest that the improved power of Examples 1 to 3 may be attributed to a reduction in internal resistance.

[0050] FIG. 4 shows the current density vs. power density characteristics of Example 4 and Comparative Example 3 when 1M NADH and lOOmM mPMS are used as the anode reactant.

[0051] As seen from FIG. 4, the fuel cell according to Example 4, which uses the hydrophilic gel/filter assembly exhibited improved power density compared to the fuel cell according to Comparative Example 3, in which an appropriate amount of a hydrophilic gel that has the same composition was applied to an electrode.

[0052] FIG. 5 shows the current density vs. voltage characteristics of Example 4 and Comparative Example 3 when 1M NADH and lOOmM rriPMS are used as the anode reactant.

[0053] As seen from FIG. 5, the test confirmed that the fuel cell according to Example 4, which has the electrode-hydrophilic gel (filter impregnation type)-separating membrane assembly exhibited a current vs. voltage characteristic line with a small slope compared to the fuel cell according to Comparative Example 3, in which an appropriate amount of a hydrophilic gel that has the same composition was applied to an electrode. Since the slope of a current vs. voltage characteristic line represents an internal resistance according to Ohm's law, it is suggested that the improved power of the fuel cells which have the gel/filter assembly should be attributed to a reduction in internal resistance.

Claims

1. A fuel cell comprising:

an anode;

a cathode that is provided on the opposite side of an ion-conductive membrane from the anode; and

a hydrophilic gel membrane that is provided between at least one of the anode and the ion-conductive membrane and between the cathode and the ion-conductive membrane.

2. The fuel cell according to claim 1, wherein the hydrophilic gel membrane is formed by impregnating a carrier membrane with a hydrophilic gel.

3. The fuel cell according to claim 2, wherein the carrier membrane has a thickness of 1 μπι to 10 μπι.

4. The fuel cell according to claim 2 or 3, wherein the carrier membrane is a porous ion-permeable membrane.

5. The fuel cell according to any one of claims 1 to 4, wherein at least one of the anode and the cathode contains an enzyme as a catalyst.

6. A method of producing a fuel cell that includes an anode and a cathode that are provided on opposite sides of an ion-conductive membrane, the method comprising: forming a hydrophilic gel into a thin membrane;

disposing the formed hydrophilic gel membrane between at least one of the anode and the ion-conductive membrane and between the cathode and the ion-conductive membrane; and

bringing the hydrophilic gel membrane, the anode, the ion-conductive membrane, and the cathode into pressure contact with each other in normal temperature circumstance

7. The method of producing a fuel cell according to claim 6, wherein the hydrophilic gel is formed into a thin membrane by impregnating a carrier membrane with a hydrophilic gel.

8. The method of producing a fuel cell according to claim 7, wherein the carrier membrane has a thickness of 1 μηι to 10 μπι.

9. The method of producing a fuel cell according to claim 7 or 8, wherein the carrier membrane is a porous ion-permeable membrane.

10. The method of producing a fuel cell according to any one of claims 6 to 9, wherein at least one of the anode and the cathode contains an enzyme as a catalyst.