JP2010009761A - Solid polymer fuel battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode structure for improving electromotive force of a solid polymer fuel battery. <P>SOLUTION: In the electrode structure for a solid polymer fuel battery cell 11 with an anode electrode 7 and a cathode electrode 6 arranged on both faces of a solid polymer electrolyte film 1, ionomers as aromatic hydrocarbon system polymer sulfide contained in each a three-phase interface of an anode-electrode side catalyst layer 3 and a cathode-electrode side catalyst layer 2, are to be of different chemical species, PH values of the both electrodes are controlled by changing density of functional groups governing each ionomer PH value, and a theoretical electromotive force is to be larger than 1.23 V. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a solid polymer fuel cell including a solid polymer electrolyte membrane and a catalyst layer containing an ionomer, and more particularly to improvement of an electromotive voltage.

  Conventionally, polymer electrolyte fuel cells (PEFC) have been researched and developed as power sources for electric vehicles, stationary cogeneration systems, and portable devices, for example. This polymer electrolyte fuel cell is configured by stacking a plurality of polymer electrolyte fuel cells (electrically connected in series), and generates power using a redox reaction via a catalyst.

  Prior art documents relating to such a polymer electrolyte fuel cell include the following.

Japanese Patent No. 3954793 JP 2007-165005 A JP 2001-118579 A JP 2006-155965 A

  Patent Document 1 describes a technique related to a conventional polymer electrolyte fuel cell. FIG. 3 is a fragmentary block diagram showing an example of a conventional polymer electrolyte fuel cell. In FIG. 3, in a conventional polymer electrolyte fuel cell, a cathode electrode side catalyst layer 2 containing a noble metal (for example, platinum) and an anode electrode side catalyst layer 3 are joined to both surfaces of a solid polymer electrolyte membrane 1. A membrane electrode assembly is used.

  The cathode electrode side gas diffusion layer 4 is disposed so as to face the solid polymer electrolyte membrane 1 with the cathode electrode side catalyst layer 2 interposed therebetween, and the anode electrode side catalyst layer 3 is solid with the anode electrode side gas diffusion layer 5 interposed therebetween. It arrange | positions so that the polymer electrolyte membrane 1 may be opposed.

  The cathode electrode side catalyst layer 2 and the cathode electrode side gas diffusion layer 4 constitute a cathode electrode 6, and the anode electrode side catalyst layer 3 and the anode electrode side gas diffusion layer 5 constitute an anode electrode 7.

  The cathode electrode side gas diffusion layer 4 and the anode electrode side gas diffusion layer 5 function to transmit an oxidant gas (mainly oxygen or air, etc.) and a fuel gas (mainly hydrogen, etc.) to transmit an electric current to the outside. To do.

  Separator 10a having a plurality of grooves 8a and grooves 9a formed on both sides is joined to cathode electrode side catalyst layer 2 or cathode electrode side gas diffusion layer 4, and separator 10b having a plurality of grooves 8a and grooves 9a formed on both sides. Is joined to the anode electrode side catalyst layer 3 or the anode electrode side diffusion layer 5. That is, the pair of separators are arranged so as to sandwich the membrane electrode assembly.

  The separator 10a is joined to the cathode electrode side catalyst layer 2 or the cathode electrode side gas diffusion layer 4, so that the opening portion of the groove 8a is covered with the cathode electrode 6 and the gas flow path FP1 for reaction gas circulation is configured. Is done.

  Further, when the separator 10b is joined to the anode electrode side catalyst layer 3 or the anode electrode side gas diffusion layer 5, the opening portion of the groove 8b is covered with the anode electrode 7 to form a gas flow path FP2 for reaction gas circulation. The

  Thus, the polymer electrolyte fuel cell 11 is configured. A plurality of such polymer electrolyte fuel cells 11 can be stacked (electrically connected in series) to form a fuel cell stack. In this fuel cell stack, a voltage higher than that obtained in each individual cell can be taken out.

  The operation of the conventional polymer electrolyte fuel cell will be described. Oxidizing gas (for example, oxygen, air, etc.) is supplied to the gas flow path FP1 through a supply port (not shown). The fuel gas (for example, hydrogen) is supplied to the gas flow path FP2 through a supply port (not shown).

At the anode electrode, an anode electrode reaction is performed to decompose hydrogen molecules into hydrogen ions (H ⁺ ) and electrons. In the cathode electrode, an electrochemical reaction is performed in which hydrogen ions (H ⁺ ) and oxygen molecules propagating through the solid polymer electrolyte membrane 1 generate water (H ₂ O) from electrons (e ⁻ ). In other words, the following electrochemical reactions are performed.

The anode electrode _{^{side: H 2 → 2H + + 2e}} - ··· (1)
Cathode side: 2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O (2)
Overall: H ₂ + (1/2) O ₂ → H ₂ O (3)

At this time, an electron (e ⁻ ) generated on the anode electrode side can be taken out by connecting an external load between the cathode electrode side and the anode electrode side (specifically, separators 10a and 10b). In other words, current can be taken out.

The theoretical voltage of the fuel cell is obtained from the theoretical potential difference E _r between the cathode side and the anode side, that is, Gibbs free energy of water generation reaction, and the theoretical electromotive force (open circuit voltage) is about 1.23V. is there. This is obtained from the following Nernst equation (equation (4)). Here, E ₀ is a standard potential, R is a gas constant, T is an absolute temperature, and F is a Faraday constant.
E _r = E ₀ + (2.3RT / nF) log (P _H2 · P _{O 2} ^0.5 / P _{H 2 O} ) (4)

  On the other hand, Patent Document 2 describes a conventional example of a platinum-supported carbon catalyst electrode (Pt / C catalyst electrode) as an electrode catalyst used in a polymer electrolyte fuel cell. The platinum-supported carbon catalyst electrode has a structure in which an active metal mainly composed of platinum is supported on a carrier made of conductive fine particles such as carbon black.

  For the purpose of improving the power generation capacity, an ionomer (hydrogen ion conductive polymer material soluble in an organic solvent) is added to the platinum-supported carbon catalyst. A Nafion membrane is typically used for the solid polymer electrolyte membrane, and Nafion particles are typically used for the ionomer in the catalyst. In other words, the solid polymer electrolyte membrane and the ionomer of both electrodes are the same chemical species, and one ionomer is provided as a membrane and the other ionomer is provided as a dispersed particle.

  In other words, the platinum-supported carbon catalyst electrode is composed of three different phases: a gas phase such as hydrogen or oxygen as a reaction gas, a catalyst electrode phase in which electrons move, and an ionomer phase in which ions move. It proceeds at the three-phase interface where these different phases come into contact.

  Patent Document 3 describes a method for producing an electrode catalyst in a polymer electrolyte fuel cell.

However, in a conventional polymer electrolyte fuel cell, a potential loss occurs inside the fuel cell during actual power generation, so the voltage E that can be taken out of the battery is lower than the theoretical voltage _Er. (See equation (4)).

The battery voltage obtained by power generation decreases as the current increases, and the difference between this voltage drop and the theoretical value of 1.23 V at the open circuit voltage and the measured value of the cell is hereinafter referred to as “overvoltage”. There are two types of overvoltages: activation overvoltage η _act corresponding to reaction activity and concentration overvoltage η _r corresponding to resistance voltage drop of electrons and ions, and these relations are expressed by equation (5). .
E = E _r −η _act −η _r (5)

  Patent Document 4 describes Equation (5) representing such overvoltage, the above-mentioned Nernst equation (Equation (4)), and the like.

  Thus, the battery voltage value E that is actually output is a value obtained by subtracting the overvoltage from the theoretical electromotive voltage. Specifically, an open circuit voltage of about 0.95 to 1 V can be obtained with a conventional polymer electrolyte fuel cell. For this reason, the conventional fuel cell is required to improve the cell voltage value.

  For such a problem, various methods for suppressing the overvoltage value in order to improve the battery voltage have been studied, but it is still not sufficient.

Further, based on the formula (5), there is a method of increasing the theoretical voltage value _Er as a method of improving the battery voltage value E, but this kind of examination has not been made.

  The present invention solves the above-described problems, and an object thereof is to improve the electromotive force of the polymer electrolyte fuel cell.

  Further, the inventor of the present application focuses on the relationship between the pH (hydrogen ion index) values of the anode electrode and the cathode electrode and the respective electrode potentials, and aims to improve the electromotive voltage by making a difference between the pH values of both electrodes. We have newly found the characteristics of fuel cells that can be used.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In a polymer electrolyte fuel cell in which an anode electrode and a cathode electrode are arranged on both sides of a polymer electrolyte membrane,
A polymer electrolyte fuel cell, wherein the anode electrode and the cathode electrode have different pH values.

The invention according to claim 2
2. The polymer electrolyte fuel cell according to claim 1, wherein the ionomer contained in the three-phase interface between the anode electrode and the cathode electrode is a hydrogen ion conductive polymer of a combination of different chemical species.

The invention described in claim 3
The polymer electrolyte fuel cell according to claim 1 or 2,
The ionomer contained in the three-phase interface between the anode electrode and the cathode electrode is a hydrogen ion conductive polymer having a combination in which the density of functional groups governing the pH value is different.

The invention according to claim 4
The polymer electrolyte fuel cell according to claim 1 or 2,
A cathode electrode side catalyst layer containing the ionomer and disposed on one surface of the solid polymer electrolyte membrane;
An anode electrode side catalyst layer containing the ionomer and disposed on the other surface of the solid polymer electrolyte membrane,
The ionomer contained in the cathode catalyst layer and the ionomer contained in the anode catalyst layer are different chemical species.

The invention according to claim 5
The polymer electrolyte fuel cell according to any one of claims 1 to 3,
A cathode electrode side catalyst layer containing the ionomer and disposed on one surface of the solid polymer electrolyte membrane;
An anode electrode side catalyst layer containing the ionomer and disposed on the other surface of the solid polymer electrolyte membrane,
The ionomer contained in the cathode catalyst layer and the functional group density controlling the ionomer pH value contained in the anode catalyst layer are different.

The invention described in claim 6
The polymer electrolyte fuel cell according to any one of claims 1 to 5,
The pH values are controlled, and the theoretical electromotive force is greater than 1.23V.

The invention described in claim 7
The polymer electrolyte fuel cell according to any one of claims 1 to 6,
The ionomer is an aromatic hydrocarbon polymer sulfonated product.

  According to the polymer electrolyte fuel cell of the present invention, the electromotive force can be improved.

  The inventor of the present application pays attention to the relationship between the electromotive force expressed by the above formula (4) (Nernst equation) and the potentials of both electrodes, that is, the relationship between the pH values of the anode and cathode and the respective electrode potentials. The inventors have found that the electromotive force can be improved by providing a difference in pH value between the two electrodes.

  The inventor of the present application newly found that the pH value of both electrodes can be controlled easily and stably by using different materials for the ionomer in consideration of this point. Hereinafter, examples of the polymer electrolyte fuel cell capable of such pH control will be described.

  FIG. 1 is a block diagram showing an embodiment of a polymer electrolyte fuel cell according to the present invention. The same reference numerals are given to the same parts as those in FIG. The difference between FIG. 1 and FIG. 3 is that, in FIG. 1, ionomers added to the cathode electrode 6 and the anode electrode 7 are the same chemical species having different materials or different functional group densities. In addition, it demonstrates below as a general term also including the ionomer of the chemical species different from a dissimilar material.

Further, the density of functional groups in the present application refers to the number of functional groups in one molecule (number of functional groups / number of molecules) or the number of functional groups in a unit volume (number of functional groups / mm ³ ). Specifically, when Nafion (registered trademark) is used for an ionomer material, a sulfate group (SO ₃ H) becomes a functional group (see the following chemical formula (1)). The density of the functional groups in this case is, for example, Y (X / functional number / number of molecules) when the number of Nafion molecules covered on the cathode electrode side catalyst layer 2 is X and the number of functional groups is Y. On the other hand, if the volume of Nafion coated on the cathode electrode side catalyst layer 2 is Zmm ³ , (Y / Z (number of functional groups / mm ³ )).

  In FIG. 1, ionomer IO200 (not shown) is coated on the surface of cathode electrode side catalyst layer 2 in the form of dispersed particles, and ionomer IO300 is coated on the surface of anode electrode side catalyst layer 3 in the form of dispersed particles. The ionomer IO200 and the ionomer IO300 are made of different materials.

  These ionomers IO200 and IO300 are made of different materials such that the pH value on the cathode electrode (oxygen electrode) side is a and the pH value on the anode electrode (hydrogen electrode) side is b (where a ≠ b).

  The operation of the polymer electrolyte fuel cell according to the present invention will be described. An oxidizing gas (for example, oxygen or air) is supplied to the gas flow path FP1, and a fuel gas (for example, r hydrogen) is supplied to the gas flow path FP2.

  The anode electrode 6 and the cathode electrode 7 perform the electrochemical reaction represented by the above formulas (1) and (2), and the fuel cell as a whole performs the electrochemical reaction represented by the formula (3). . Hereinafter, formulas (1), (2), and (3) will be described again.

The anode electrode _{^{side: H 2 → 2H + + 2e}} - ··· (1)
Cathode side: 2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O (2)
Overall: H ₂ + (1/2) O ₂ → H ₂ O (3)

At this time, by connecting an external load between the cathode electrode 6 and the anode electrode 7, electrons (e ⁻ ) generated on the anode electrode side can be taken out.

Here, the electromotive voltage of the fuel cell is a potential difference between the cathode electrode 6 and the anode electrode 7. In both electrodes, the equilibrium potential E _H2 of the anode electrode (hydrogen electrode) and the equilibrium potential E _O2 of the cathode electrode (oxygen electrode) are generated by the electrochemical reaction represented by the following formulas (6) and (7). R is gas constant, T is absolute temperature, F is Faraday constant, aH is hydrogen ion activity, E _H is hydrogen electrode potential (0V) when aH = 1, E _O is oxygen when aH = 1 The electrode potential is 1.23V. In addition, although the definitions of “hydrogen ion activity” and “hydrogen ion concentration” are strictly different, in this specification, both terms are used as synonymous words.

E _H2 = E _H + (RT / F) × ln (aH) = E _H − (RT / F) × pH (6)
E _O2 = E _O + (RT / F) × ln (aH) = E _O − (RT / F) × pH (7)

FIG. 2 is a relationship diagram between the equilibrium potentials E _O2 and E _H2 of the cathode electrode (oxygen electrode) and the anode electrode (hydrogen electrode) and the pH value, and illustrates the above formulas (6) and (7). is there. The dotted line indicates the equilibrium potential of the cathode electrode 6, and the two-dot chain line indicates the equilibrium potential of the anode electrode 7.

  According to FIG. 2, as the pH value increases, in other words, the potential of each electrode decreases as the hydrogen ion concentration decreases. Further, the equilibrium potential line of the cathode electrode 6 and the equilibrium potential line of the anode electrode 7 are in parallel relation, and the potential difference between both electrodes at each pH value, that is, the electromotive force is constant at 1.23V.

At this time, when the pH value on the anode electrode (hydrogen electrode) side is b and the pH value on the cathode electrode (oxygen electrode) side is a (where a ≠ b), the electromotive force β = V _oa −V _hb > _1.23V It becomes.

  Thus, the polymer electrolyte fuel cell according to the present invention obtains a high electromotive force by providing a difference in pH value between the anode electrode (hydrogen electrode) and the cathode electrode (oxygen electrode). Can do.

  In other words, by changing the type of ionomer at the three-phase interface to control the pH and making the hydrogen ion concentrations of the hydrogen electrode and the oxygen electrode different values, the electromotive voltage can be improved due to the difference.

  Further, if the solid polymer fuel cell according to the present invention is used as a single cell to constitute a fuel cell stack, the number of single cells required for outputting the required voltage can be reduced, The effect can be expected to reduce the size of the power supply device and the stack manufacturing cost.

  The difference in electromotive voltage between the conventional fuel cell and the fuel cell of the present invention will be compared and examined. In the conventional polymer electrolyte fuel cell, as a matter of course, since the same polymer material is used for the electrolyte membrane, the ionomer of the anode electrode, and the ionomer of the cathode electrode, there is a difference in pH value between both electrodes. It was difficult to control the pH so that the electromotive force could be improved.

  For example, in a conventional fuel cell, a perfluoro resin represented by Nafion (registered trademark) is used for both an ionomer material and an electrolyte material. Since the pH of Nafion (registered trademark) is about pH 1, the pH of the region between electrodes of the conventional fuel cell is considered to be 1.

3, when the pH is 1, the cathode electrode (oxygen electrode) potential is V _o , the anode electrode (hydrogen electrode) potential is V _h , and the electromotive force α is α = V _o −V _h = 1. 23.

  Further, FIG. 3 shows that the theoretical value of the electromotive voltage is 1.23 V if the pH value is uniform in the region between the anode electrode (hydrogen electrode) and the cathode electrode (oxygen electrode). That is, even if the pH of Nafion is not 1, the electromotive force α is 1.23V.

On the other hand, the polymer electrolyte fuel cell according to the present invention uses different materials for the ionomers of both electrodes, and has a difference in pH value between the cathode electrode (oxygen electrode) and the anode electrode (hydrogen electrode). Thus, the electromotive force β = V _oa −V _hb > α (1.23 V).

  For this reason, the polymer electrolyte fuel cell according to the present invention can obtain an electromotive voltage higher than that of a conventional polymer electrolyte fuel cell.

  In addition, as long as the material used as ionomer IO200 and IO300 is the combination of the material from which pH differs, what kind of thing may be sufficient as it. For example, perfluoro proton conductive polymers (trade names include Nafion membrane, Flemion, Aciplex, etc.), polyetheretherketone (PEEK) sulfonated products, polyethersulfone (PES) sulfonated products, polyetherimide (PEI) Sulfonated products, polyimide (PI) sulfonated products, polyphenylene oxide (PPO) sulfonated products, polyphenylene sulfide (PPS) sulfonated products, aromatic hydrocarbon polymer sulfonated products such as sulfonated polysulfone (PSF) sulfonated products, sulfonated polysulfone, fluorocarbon Of sulfonic acid type polystyrene-graft-ethylene-tetrafluoroethylene copolymer of styrene vinyl monomer and hydrocarbon vinyl monomer, and aromatic hydrocarbon having sulfonic acid group , Sulfonic acid type polystyrene-graft-ethylene-tetrafluoroethylene copolymer (ETFE-g-PSt), styrene- (ethylene-butylene) -styrene sulfonated product, copolymer of fluoro vinyl monomer and hydrocarbon vinyl monomer, Cation-exchange group-introduced styrene-divinylbenzene copolymer, anion-exchange-group-introduced styrene-divinylbenzene copolymer, polyolefin polymer, hyperbranched polymer having terminal sulfonic acid group (HBPES) and linear aromatic A solid polymer fuel cell provided by the present invention can be constructed by using any combination of a copolymer with a polymer (PEEEES) and another hydrocarbon polymer having hydrogen ion conductivity. .

  In the above embodiment, the proposed fuel cell can be realized by using different ionomer materials for the cathode electrode (oxygen electrode) 6 and the anode electrode (hydrogen electrode) 7. However, if the pH of both electrodes can be changed. Alternatively, the same chemical species may be used as the bipolar ionomer.

  Specifically, when using the same species as an ionomer, the density of functional groups (for example, sulfate groups in Nafion) that dominate the pH value is made different at each electrode, so that the pH of both electrodes It may be a value that makes a difference.

  For example, Nafion (perfluoro-type proton conductive polymer) having different sulfate group densities may be used for the ionomer of both electrodes, so that the pH values of both electrodes are different.

  In the above-described embodiment, the ionomer species of both electrodes are different as a method for controlling the pH of both electrodes. However, the present invention is not particularly limited to this, and the cathode 6 and anode 7 of the solid polymer electrolyte membrane 1 are not limited thereto. The pH may be controlled by changing the chemical structure of the film surface in contact with the surface and the vicinity thereof.

  For example, the solid polymer electrolyte membrane 1 may be realized by using Nafion (registered trademark) and changing the sulfate group density on the side surface of the cathode electrode 6 (oxygen electrode) and the side surface of the anode electrode 7 (hydrogen electrode).

  Here, the thickness of the solid polymer electrolyte membrane 1 is 10 to 50 μm. In view of stably producing electrolyte membranes having different sulfate group densities on both surfaces, a method of controlling the pH by changing the type of ionomer at the three-phase interface is particularly preferred as a method of controlling the pH of both electrodes.

  As described above, the polymer electrolyte fuel cell according to the present invention can improve the electromotive voltage by controlling the pH by changing the type of ionomer in the anode and cathode, It can be expected to contribute as a stationary cogeneration system and power source for portable devices.

It is a block diagram which shows one Example of the polymer electrolyte fuel cell which concerns on this invention. FIG. 6 is a relationship diagram between an equilibrium potential of a cathode electrode (oxygen electrode) and an anode electrode (hydrogen electrode) and a pH value. It is a fragmentation block diagram which shows an example of the conventional polymer electrolyte fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solid polymer electrolysis chamber membrane 2 Cathode electrode side catalyst layer 3 Anode electrode side catalyst layer 4 Cathode electrode side gas diffusion layer 5 Anode electrode side gas diffusion layer 6 Cathode electrode (oxygen electrode)
7 Anode electrode (hydrogen electrode)
8a, 8b groove (flow path FP1, FP2)
9a, 9b Groove 10a, 10b Separator 11 Polymer electrolyte fuel cell

Claims (7)

In a polymer electrolyte fuel cell in which an anode electrode and a cathode electrode are arranged on both sides of a polymer electrolyte membrane,
A polymer electrolyte fuel cell, wherein the anode electrode and the cathode electrode have different pH values. 2. The polymer electrolyte fuel cell according to claim 1, wherein the ionomer contained in the three-phase interface between the anode electrode and the cathode electrode is a hydrogen ion conductive polymer of a combination of different chemical species. The ionomer contained in the three-phase interface between the anode electrode and the cathode electrode is a hydrogen ion conductive polymer having a combination of different functional group densities that control the pH value. The solid polymer fuel cell as described. A cathode electrode side catalyst layer containing the ionomer and disposed on one surface of the solid polymer electrolyte membrane;
An anode electrode side catalyst layer containing the ionomer and disposed on the other surface of the solid polymer electrolyte membrane,
3. The polymer electrolyte fuel cell according to claim 1, wherein the ionomer contained in the cathode catalyst layer and the ionomer contained in the anode catalyst layer are different chemical species. A cathode electrode side catalyst layer containing the ionomer and disposed on one surface of the solid polymer electrolyte membrane;
An anode electrode side catalyst layer containing the ionomer and disposed on the other surface of the solid polymer electrolyte membrane,
4. The solid according to claim 1, wherein the ionomer contained in the cathode-side catalyst layer and the functional group density controlling the ionomer pH value contained in the anode-side catalyst layer are different. Polymer fuel cell. 6. The polymer electrolyte fuel cell according to claim 1, wherein each pH value is controlled, and a theoretical electromotive force is larger than 1.23V. 7. The polymer electrolyte fuel cell according to claim 1, wherein the ionomer is an aromatic hydrocarbon polymer sulfonated product.

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