JP2008098006A - Fuel cell membrane-electrode junction material and solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell which can remove from a cell any harmful peroxide such as hydrogen peroxide arising during the operation of the fuel cell and which reduces deterioration of electrolyte present in an electrolyte membrane or electrode catalyst layer due to peroxide such as hydrogen peroxide for improving the durability performance. <P>SOLUTION: The fuel cell membrane-electrode junction material includes a pair of electrodes consisting of a fuel electrode to which fuel gas is supplied and an oxygen electrode to which oxidizer gas is supplied, and a polymer electrolyte membrane which is interposed between the pair of electrodes. The electrodes comprise a catalyst layer and a diffusion layer which are joined to the polymer electrolyte membrane, wherein the concentration of peroxide decomposition agent in the diffusion layer is greater in the vicinity of an end portion in the direction of the cell surface than in any area excepting the end portion. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a membrane-electrode assembly for a fuel cell that suppresses deterioration of an electrolyte in an electrolyte membrane or an electrode catalyst layer and has improved durability, and a polymer electrolyte fuel cell including the same.

  A fuel cell that generates electricity by an electrochemical reaction of hydrogen gas has high power generation efficiency, clean gas discharged, and extremely little influence on the environment. Therefore, in recent years, various uses such as power generation and low-pollution automobile power supplies are expected. Fuel cells can be classified according to their electrolytes. For example, solid polymer fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and the like are known.

  The polymer electrolyte fuel cell can be operated at a low temperature of about 80 ° C. and has a large output density. In general, a polymer electrolyte fuel cell uses a proton conductive polymer membrane as an electrolyte. A pair of electrodes each serving as a fuel electrode and an oxygen electrode are provided on both sides of the polymer film serving as an electrolyte to form an electrode assembly. A single cell in which this electrode assembly is sandwiched between separators serves as a power generation unit. Then, hydrogen or a fuel gas containing hydrogen is supplied to the fuel electrode, and an oxidant gas such as oxygen or air is supplied to the oxygen electrode, and power is generated by an electrochemical reaction at the three-phase interface between the gas, the electrolyte, and the electrode. The polymer membrane serving as an electrolyte has proton conductivity in a state containing water. In order to maintain the proton conductivity of the polymer membrane, the fuel gas and the oxidant gas are usually humidified by a humidifier and then supplied to each electrode.

By the way, in the polymer electrolyte fuel cell, peroxide is generated in the catalyst layer formed at the interface between the solid polymer electrolyte membrane and the electrode by the cell reaction, and the peroxide is generated while diffusing the generated peroxide. This degrades the electrolyte. For example, in a fuel cell, fuel is oxidized at the fuel electrode and oxygen is reduced at the oxygen electrode. The ideal reaction when hydrogen is used as the fuel and an acidic electrolyte is used is expressed by the following equations (1) and (2). It is expressed as shown in the equation.
Anode (hydrogen electrode): H ₂ → 2H ⁺ + 2e ⁻ (1)
Cathode (oxygen electrode): 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)

Hydrogen ions generated by the reaction of the formula (1) at the anode permeate (diffuse) the solid polymer electrolyte membrane in the hydrated state of H ⁺ (XH ₂ O), and the hydrogen ions that permeate the membrane are expressed at the cathode. It is used for the reaction (2). The electrode reaction at the anode and the cathode proceeds at the interface between the catalyst and the solid polymer electrolyte membrane in the electrode catalyst layer with the electrode catalyst layer in close contact with the solid polymer electrolyte membrane as a reaction site.

However, in an actual fuel cell, side reactions occur in addition to these main reactions. A typical example is the generation of hydrogen peroxide (H ₂ O ₂ ). The mechanism of its generation is not necessarily fully understood, but possible mechanisms are: That is, the generation of hydrogen peroxide can occur at either the hydrogen electrode or the oxygen electrode. For example, at the oxygen electrode, it is considered that hydrogen peroxide is generated by the following equation due to incomplete reduction of oxygen. .
O ₂ + 2H ⁺ + 2e ⁻ → 2H ₂ O ₂ (3)

In addition, at the hydrogen electrode, it is considered that oxygen contained as an impurity in the gas or intentionally mixed, or oxygen diffused into the electrolyte at the oxygen electrode and diffused into the hydrogen electrode is involved in the reaction. It is considered to be the same as the above formula (3) or represented by the following formula.
_{^{2M-H + O 2- → 2M}} + H 2 O 2 ... (4)
Here, M represents a catalyst metal used for the hydrogen electrode, and MH represents a state in which hydrogen is adsorbed on the catalyst metal. Usually, a noble metal such as platinum (Pt) is used as the catalyst metal.

Hydrogen peroxide generated on these electrodes moves away from the electrodes due to diffusion or the like and moves into the electrolyte. This hydrogen peroxide is a substance having a strong oxidizing power and oxidizes many organic substances constituting the electrolyte. Although the detailed mechanism is not necessarily clear, in many cases, it is considered that hydrogen peroxide is radicalized, and the generated hydrogen peroxide radical is a direct reactant of the oxidation reaction. That is, it is considered that radicals generated by the reaction represented by the following formula draw hydrogen from the organic substance of the electrolyte or cut other bonds. The cause of radicalization is not necessarily clear, but contact with heavy metal ions is considered to have a catalytic action. In addition, it is considered that the radical is also formed by heat, light or the like.
H ₂ O ₂ → 2.OH
Or H ₂ O ₂ → · H + · OOH
As described above, the electrolyte membrane of the fuel cell is required to have both durability (reduction of fluorine emission and prevention of increase of cross leak) and improvement of output (decrease in proton conductivity).

  It has been found that the addition of Ce (cerium) or a compound thereof to the electrolyte membrane or MEA suppresses chemical deterioration of the electrolyte membrane due to radicals derived from hydrogen peroxide, and greatly improves durability.

  For example, in Patent Document 1 below, an electrolyte that constitutes an electrolyte membrane electrode assembly for a polymer electrolyte fuel cell for the purpose of efficiently decomposing peroxide generated in the battery and suppressing deterioration of the electrode and the electrolyte membrane It is disclosed that a peroxide decomposition catalyst for decomposing peroxide is arranged with a difference in concentration on at least one of a pair of electrodes provided on both sides of the membrane.

  Specifically, the peroxide decomposition catalyst is arranged with a concentration difference in the electrode, and the concentration difference may be in the thickness direction of the electrode or in the surface direction of the electrode. It is described that it is desirable to dispose the oxide decomposition catalyst with a concentration difference in the thickness direction of the electrode (paragraphs 0021 and 0022).

  Further, in Patent Document 2 below, the electrolyte membrane electrode assembly is sandwiched from both sides for the purpose of suppressing the deterioration of the sealing member for sealing between the electrolyte membrane and the separator and the electrolyte membrane and improving the durability. It is disclosed that a peroxide decomposition catalyst for decomposing peroxide is disposed on a seal member that seals between the separator and the electrolyte membrane.

  As a contradiction to using a peroxide decomposition catalyst for decomposing such a peroxide, it has also been found that the initial output is reduced by addition of Ce as a peroxide decomposition catalyst. This cause is considered to be caused by a decrease in proton conductivity due to ion exchange of Ce ions with a part of the sulfonic acid group of the electrolyte membrane. For example, it has been found that the greater the Ce addition amount, the greater the initial output drop. Therefore, it is considered necessary to devise such as reducing the amount of Ce added (or using a poorly soluble compound) or adding only to the necessary site.

JP 2005-235437 A JP 2005-267904 A

  Then, an object of this invention is to search the application method of the peroxide decomposition agent which is calculated | required by the membrane-electrode assembly (MEA) for fuel cells, and is making high durability and high performance compatible. Accordingly, an object of the present invention is to provide a fuel cell in which the deterioration of the electrolyte in the electrolyte membrane or the electrode catalyst layer is suppressed and the durability is improved.

  The present inventor has found that the peroxide is efficiently trapped by allowing the peroxide decomposing agent to be unevenly distributed at a specific site, and has reached the present invention.

  That is, first, the present invention is an invention of a fuel cell membrane-electrode assembly, comprising a pair of electrodes each comprising a fuel electrode supplied with a fuel gas and an oxygen electrode supplied with an oxidant gas, A membrane-electrode assembly for a fuel cell including a polymer electrolyte membrane sandwiched between a pair of electrodes, the electrode comprising a catalyst layer and a diffusion layer joined to the polymer electrolyte membrane, and the diffusion It is characterized in that the peroxide decomposing agent is present in the layer at a higher concentration in the vicinity of the end in the cell surface direction than at the end.

  Peroxides such as hydrogen peroxide generated during operation of the fuel cell are captured by the radical scavenger. As a result, harmful peroxides such as hydrogen peroxide are removed from the cell. It has been found that the deterioration of the electrolyte membrane occurs notably in the vicinity of the MEA, but in the vicinity of the periphery. One possible reason for this is that chemical degradation of the electrolyte membrane may be promoted due to poor drainage and contamination with impurities from the adhesive.

  In the present invention, in particular, the peroxide decomposer is present at a higher concentration near the end in the cell surface direction in the diffusion layer than at the end, so that harmful hydrogen peroxide and the like can be obtained without reducing the power generation performance. Peroxide can be efficiently removed from the cell. Adding Ce to the end of the diffusion layer increases the amount of cerium added to the periphery of the MEA, which is considered to cause significant deterioration of the electrolyte membrane and contribute little to power generation. Reduce the electrolyte membrane deterioration.

As the peroxide decomposer added unevenly in the diffusion layer, it is known by the names of peroxide decomposer, peroxide decomposition catalyst, radical scavenger, etc. having the function of decomposing the peroxide of fuel cells. These compounds can be widely used. Among these, selected from CeO ₂ , Ru, Ag, RuO ₂ , WO ₃ , FeO ₄ , CePO ₄ , CrPO ₄ , AlPO ₄ , FePO ₄ , CeF ₃ , FeF ₃ , Fe-polyfin, and Co-polyfin. One or more of these are preferably exemplified.

  Secondly, the present invention is a polymer electrolyte fuel cell comprising the fuel cell membrane-electrode assembly.

  According to the present invention, harmful peroxides such as hydrogen peroxide generated during fuel cell operation can be removed from the cell, and electrolytes in the electrolyte membrane and electrode catalyst layer due to peroxides such as hydrogen peroxide can be removed. It is possible to obtain a fuel cell in which deterioration is suppressed and durability is improved.

  In FIG. 1, the cross-sectional schematic diagram of the fuel cell of this invention is shown. A membrane-electrode junction for a fuel cell comprising a pair of electrodes comprising a fuel electrode supplied with fuel gas and an oxygen electrode supplied with oxidant gas, and a polymer electrolyte membrane sandwiched between the pair of electrodes In the body, the electrode comprises a catalyst layer and a diffusion layer joined to the polymer electrolyte membrane, and the peroxide decomposition agent is present in the diffusion layer near the end in the cell surface direction at a higher concentration than other than the end. Yes. In FIG. 1, a separator is disposed outside the diffusion layer. Here, the peroxide decomposition agent is present in the diffusion layer at a higher concentration near the end in the cell surface direction than at the end, so that hydrogen peroxide and the like generated in the electrolyte membrane during fuel cell operation Peroxide can be removed efficiently. That is, by increasing the amount of Ce added only to the peripheral part of the diffusion layer (or adding Ce only to the peripheral part), the MEA can reduce initial output decrease as much as possible and suppress deterioration of the electrolyte membrane. Can provide.

  Specifically, after applying and drying a cerium solution (such as a cerium nitrate solution) on the end of the GDL, MEA is formed. The surface to be applied is preferably on the side in contact with the catalyst layer during MEA conversion (the amount of Ce on the side close to the electrolyte membrane is preferably large). In the conventional Ce addition method, there are a method in which ion exchange is performed on the membrane and then it is deposited in the membrane by reduction treatment, or a MEA is formed by adding it as a solid content in the catalyst layer solution or diffusion layer water paste. However, since these methods basically add Ce uniformly in the plane, it is considered that the effect of improving the durability due to the addition is reduced although the output is greatly reduced.

  Further, by increasing the amount of Ce added in the peripheral portion, Ce is ion-exchanged before impurities from the adhesive are mixed into the electrolyte membrane in the peripheral portion, so that the electrolyte of impurities from the adhesive is consequently obtained. It is also expected to prevent the film from entering.

  Hereinafter, the present invention will be described with reference to examples.

In the fuel cell shown in FIG. 1, a cerium nitrate solution was applied to the periphery of the diffusion layer to a width of about 1 cm and dried. The amount of cerium after coating and drying was set to about 300 μg / cm ² . Using this diffusion layer for both electrodes, an MEA was fabricated and performance evaluation was performed. The results are shown in Table 1 below.

  From the results in Table 1, it was found that the addition of cerium did not show an initial decrease in output, and showed good IV characteristics. Thus, the presence of a peroxide decomposing agent near the end in the cell surface direction of the diffusion layer at a higher concentration than other than the end can prevent harmful peroxides such as hydrogen peroxide generated during operation of the fuel cell. It is effective for removing from the cell, and it is understood that the deterioration of the electrolyte in the electrolyte membrane and the electrode catalyst layer due to peroxides such as hydrogen peroxide is suppressed and the durability of the fuel cell is improved.

  ADVANTAGE OF THE INVENTION According to this invention, deterioration by the peroxide of a fuel cell can be suppressed and durability can be improved. This contributes to the practical application and spread of fuel cells.

The cross-sectional schematic diagram of the fuel cell of this invention is shown.

Claims (3)

  A membrane-electrode junction for a fuel cell comprising a pair of electrodes comprising a fuel electrode supplied with fuel gas and an oxygen electrode supplied with oxidant gas, and a polymer electrolyte membrane sandwiched between the pair of electrodes The electrode comprises a catalyst layer joined to the polymer electrolyte membrane and a diffusion layer, and in the diffusion layer, the peroxide decomposing agent has a higher concentration near the end in the cell surface direction than at the end. A membrane-electrode assembly for a fuel cell, characterized in that it exists. The peroxide decomposer is CeO ₂ , Ru, Ag, RuO ₂ , WO ₃ , FeO ₄ , CePO ₄ , CrPO ₄ , AlPO ₄ , FePO ₄ , CeF ₃ , FeF ₃ , Fe-polyfin, and Co-polyfin. The membrane-electrode assembly for a fuel cell according to claim 1, wherein the membrane-electrode assembly is one or more selected.   A polymer electrolyte fuel cell comprising the fuel cell membrane-electrode assembly according to claim 1.

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