JP5298303B2 - Membrane electrode assembly and fuel cell - Google Patents

Description

  The present invention relates to a fuel cell that generates electricity by an electrochemical reaction between hydrogen and oxygen.

  In recent years, fuel cells that have high energy conversion efficiency and do not generate harmful substances due to power generation reactions have attracted attention. As one of such fuel cells, a polymer electrolyte fuel cell that operates at a low temperature of 100 ° C. or lower is known.

  A polymer electrolyte fuel cell has a basic structure in which a polymer electrolyte membrane, which is an electrolyte membrane, is disposed between a fuel electrode and an air electrode. The fuel electrode contains hydrogen and the air electrode contains oxygen. It is a device that supplies the agent gas and generates power by the following electrochemical reaction.

Fuel electrode: H ₂ → 2H ⁺ + 2e ⁻ (1)
^{_{Cathode: 1 / 2O 2 + 2H +}} + 2e - → H 2 O ··· (2)
The anode and cathode each have a structure in which a catalyst layer and a gas diffusion layer are laminated. The catalyst layers of the electrodes are arranged opposite to each other with the solid polymer film interposed therebetween, thereby constituting a fuel cell. The catalyst layer is a layer formed by binding carbon particles carrying a catalyst with an ion exchange resin. The gas diffusion layer becomes a passage for the oxidant gas and the fuel gas.

  At the anode, hydrogen contained in the supplied fuel is decomposed into hydrogen ions and electrons as shown in the above formula (1). Among these, hydrogen ions move inside the solid polymer electrolyte membrane toward the air electrode, and electrons move to the air electrode through an external circuit. On the other hand, in the cathode, oxygen contained in the oxidant gas supplied to the cathode reacts with hydrogen ions and electrons that have moved from the fuel electrode, and water is generated as shown in the above formula (2). Thus, in the external circuit, electrons move from the fuel electrode toward the air electrode, so that electric power is taken out (see Patent Document 1).

  As the fuel supplied to the anode, a reformed gas obtained by steam reforming a hydrocarbon gas such as natural gas or LPG using a reformer is used. The reformed gas generated in the reformer contains CO that degrades the performance of the catalyst used for the electrode. For this reason, the reformed gas generated in the reformer is sequentially processed in stages by the CO converter and the CO remover. However, since CO is not completely removed, CO having a low concentration of about 10 ppm flows to the anode.

For this reason, platinum ruthenium alloy catalysts containing ruthenium having excellent CO resistance are widely used as anode catalysts.
Japanese Patent Laid-Open No. 2006-140087

  When an alloy of ruthenium and a noble metal such as platinum is used as the catalyst metal constituting the anode catalyst layer of the fuel cell, part of the ruthenium is eluted when the fuel cell is started or stopped, and the ruthenium is further absorbed into the electrolyte membrane and further to the cathode. A moving phenomenon was found. When ruthenium or ruthenium ions are present in the cathode, the oxygen reduction reaction, which is a cathode reaction, is hindered, leading to a decrease in cathode voltage, which may cause a decrease in power generation performance of the fuel cell.

  The present invention has been made in view of these problems, and its purpose is to move ruthenium to the cathode in the electrolyte membrane and further to the cathode when an alloy catalyst of noble metal such as platinum and ruthenium is used for the anode catalyst layer of the fuel cell. And providing a technique for suppressing the deterioration of the cathode.

  One embodiment of the present invention is a membrane electrode assembly. The membrane electrode assembly includes an electrolyte membrane, an anode provided on one surface of the electrolyte membrane, and a cathode provided on the other surface of the electrolyte membrane. The anode includes an ion conductor, a noble metal, and It includes a first alloy catalyst made of ruthenium and a second alloy catalyst made of a noble metal and a metal having a higher ionization tendency than ruthenium.

  According to this aspect, ruthenium ions contained in the catalyst layer constituting the anode are reduced to ruthenium by a substitution reaction with a metal having a higher ionization tendency than ruthenium, so that the ruthenium ions move to the cathode further in the electrolyte membrane, It can suppress that a cathode deteriorates. As a result, the power generation performance of the membrane electrode assembly can be improved.

  In the above aspect, the second alloy catalyst may be unevenly distributed on the electrolyte membrane side as compared with the first alloy catalyst.

  In the above aspect, the anode includes a first catalyst layer, and a second catalyst layer interposed between the first catalyst layer and the electrolyte membrane, and the first catalyst layer includes the first catalyst layer. An ion conductor and a first alloy catalyst may be included, and the second catalyst layer may include a second ion conductor and a second alloy catalyst.

  Moreover, in the said aspect, the moisture content of a 2nd ion conductor may be low compared with the moisture content of a 1st ion conductor. Further, the moisture content of the ion conductor contained in the second catalyst layer may be lower than the moisture content of the electrolyte membrane.

  Another embodiment of the present invention is a fuel cell. The fuel cell has any of the membrane electrode assemblies described above.

  A combination of the above-described elements as appropriate can also be included in the scope of the invention for which patent protection is sought by this patent application.

  According to the present invention, when an alloy catalyst of noble metal such as platinum and ruthenium is used for the anode catalyst layer of the fuel cell, it is possible to prevent ruthenium from moving into the electrolyte membrane and further to the cathode and deteriorating the cathode. it can.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate.

(Embodiment 1)
FIG. 1 is a perspective view schematically showing the structure of a fuel cell 10 according to the first embodiment. FIG. 2 is a cross-sectional view taken along the line AA of FIG. The fuel cell 10 includes a flat membrane electrode assembly 50, and a separator 34 and a separator 36 are provided on both sides of the membrane electrode assembly 50. In this example, only one membrane electrode assembly 50 is shown, but a plurality of membrane electrode assemblies 50 may be stacked via the separator 34 or the separator 36 to constitute a fuel cell stack. The membrane electrode assembly 50 includes a solid polymer electrolyte membrane 20, an anode 22, and a cathode 24.

  The anode 22 has a laminate composed of a first catalyst layer 26, a second catalyst layer 27, and a gas diffusion layer 28. That is, in the present embodiment, the second catalyst layer 27 is disposed in a layered manner between the first catalyst layer 26 and the solid polymer electrolyte membrane 20. On the other hand, the cathode 24 has a laminate composed of a catalyst layer 30 and a gas diffusion layer 32. The first catalyst layer 26 and the second catalyst layer 27 of the anode 22 and the catalyst layer 30 of the cathode 24 are provided so as to face each other with the solid polymer electrolyte membrane 20 interposed therebetween.

  A gas flow path 38 is provided in the separator 34 provided on the anode 22 side. Fuel gas is distributed to a gas flow path 38 from a fuel supply manifold (not shown), and the fuel gas is supplied to the membrane electrode assembly 50 through the gas flow path 38. Similarly, a gas flow path 40 is provided in the separator 36 provided on the cathode 24 side.

  The oxidant gas is distributed to the gas flow path 40 from the oxidant supply manifold (not shown), and the oxidant gas is supplied to the membrane electrode assembly 50 through the gas flow path 40. Specifically, when the fuel cell 10 is operated, a reformed gas containing a fuel gas, for example, hydrogen gas, flows through the gas flow path 38 along the surface of the gas diffusion layer 28 from the upper side to the lower side. The fuel gas is supplied to 22.

  On the other hand, during operation of the fuel cell 10, an oxidant gas, for example, air flows through the gas flow path 40 from the upper side to the lower side along the surface of the gas diffusion layer 32, whereby the oxidant gas is supplied to the cathode 24. The Thereby, a reaction occurs in the membrane electrode assembly 50. When hydrogen gas is supplied to the first catalyst layer 26 and the second catalyst layer 27 via the gas diffusion layer 28, hydrogen in the gas becomes protons, and these protons pass through the solid polymer electrolyte membrane 20 into the cathode 24. Move to the side. At this time, the emitted electrons move to the external circuit and flow into the cathode 24 from the external circuit. On the other hand, when air is supplied to the catalyst layer 30 through the gas diffusion layer 32, oxygen is combined with protons to become water. As a result, electrons flow from the anode 22 toward the cathode 24 in the external circuit, and power can be taken out.

  The solid polymer electrolyte membrane 20 exhibits good ion conductivity in a wet state, and functions as an ion exchange membrane that moves protons between the anode 22 and the cathode 24. The solid polymer electrolyte membrane 20 is formed of a solid polymer material such as a fluorine-containing polymer or a non-fluorine polymer. For example, the polymer electrolyte membrane 20 is a sulfonic acid type perfluorocarbon polymer, a polysulfone resin, a phosphonic acid group or a carboxylic acid group. A fluorocarbon polymer or the like can be used. Examples of the sulfonic acid type perfluorocarbon polymer include Nafion (manufactured by DuPont: registered trademark) 112. Examples of non-fluorine polymers include sulfonated aromatic polyetheretherketone and polysulfone. A typical film thickness of the solid polymer electrolyte membrane 20 is 50 μm.

  The first catalyst layer 26 constituting the anode 22 is composed of a first ion conductor (ion exchange resin) and carbon particles supporting the first alloy catalyst, that is, catalyst-supporting carbon particles. A typical film thickness of the first catalyst layer 26 is 10 μm. The first ionic conductor connects the carbon particles carrying the first alloy catalyst and the second catalyst layer 27 and has a role of transmitting protons between the two. The first ionic conductor may be formed from the same polymer material as the solid polymer electrolyte membrane 20.

  The first alloy catalyst is composed of a noble metal and ruthenium. Examples of the noble metal used for the first alloy catalyst include platinum and palladium. Examples of the carbon particles supporting the first alloy catalyst include acetylene black, ketjen black, carbon nanotube, and carbon nano-onion.

  The second catalyst layer 27 is composed of carbon particles carrying a second ion conductor and a second alloy catalyst, that is, catalyst-carrying carbon particles. A typical film thickness of the second catalyst layer 27 is 1 μm. The second ion conductor connects the carbon particles carrying the second alloy catalyst and the solid polymer electrolyte membrane 20, and has a role of transmitting protons between the two. The second ionic conductor may be formed of the same polymer material as the solid polymer electrolyte membrane 20.

  The second alloy catalyst is composed of a noble metal and a metal having a higher ionization tendency than ruthenium. Examples of the noble metal used for the second alloy catalyst include platinum and palladium. Examples of metals having a higher ionization tendency than ruthenium include cobalt, nickel, iron, tantalum, molybdenum, tungsten, silver, tin, copper, and manganese.

  For example, when cobalt is used as a metal having a higher ionization tendency than ruthenium, ruthenium ions eluted from the first catalyst layer 26 are reduced to ruthenium due to an ionization tendency, in other words, a difference in standard electrode potential, as described below. Is oxidized to cobalt ions.

Ru ²⁺ + 2e ⁻ → Ru
Co ²⁺ + 2e ⁻ ← Co
(Ruthenium standard electrode potential E ⁰ = 0.455 V, cobalt standard electrode potential E ⁰ = −0.28 V)

  Moreover, it is preferable that the moisture content of the second ionic conductor is lower than the moisture content of the first ionic conductor. In other words, the ion exchange group concentration Ew of the second ion conductor is preferably higher than the Ew of the first ion conductor. For example, the moisture content may be changed by applying heat treatment to the ionic conductor. According to this, since the diffusion rate of ruthenium ions from the first catalyst layer 26 to the second catalyst layer 27 is decreased, the above-described substitution reaction is likely to occur. Moreover, it can suppress that the water | moisture content of the 1st catalyst layer 26 runs short in a low humidification state by making the moisture content of a 1st ion conductor relatively high. In addition, proton conductivity becomes high, so that Ew is small, and hydrophilicity and water content become high.

  The water content of the second ionic conductor is preferably lower than the water content of the solid polymer electrolyte membrane 20. In other words, the Ew of the second ionic conductor is preferably higher than the Ew of the solid polymer electrolyte membrane 20. According to this, since the diffusion rate of ruthenium ions from the second catalyst layer 27 to the solid polymer electrolyte membrane 20 is reduced, the above substitution reaction is likely to occur.

  The gas diffusion layer 28 constituting the anode 22 has an anode gas diffusion base material and a microporous layer applied to the anode gas diffusion base material. The anode gas diffusion base material is preferably composed of a porous body having electron conductivity, and for example, carbon paper, carbon woven fabric or nonwoven fabric can be used.

  The microporous layer applied to the anode gas diffusion base material is a paste-like kneaded product obtained by kneading a conductive powder and a water repellent. For example, carbon black can be used as the conductive powder. As the water repellent, a fluorine resin such as tetrafluoroethylene resin (PTFE) can be used. It is preferable that the water repellent has a binding property. Here, the binding property refers to a property that can be made sticky (state) by joining things that are less sticky or those that tend to break apart. Since the water repellent has binding properties, a paste can be obtained by kneading the conductive powder and the water repellent.

  The catalyst layer 30 constituting the cathode 24 is composed of an ion conductor (ion exchange resin) and carbon particles carrying a catalyst, that is, catalyst-carrying carbon particles. The ionic conductor connects the carbon particles carrying the catalyst and the solid polymer electrolyte membrane 20, and has a role of transmitting protons between the two. The ionic conductor may be formed from the same polymer material as the solid polymer electrolyte membrane 20. The supported catalyst includes, for example, one of platinum, cobalt, rhodium, palladium and the like, or an alloy of these two. Examples of the carbon particles supporting the catalyst include acetylene black, ketjen black, carbon nanotube, and carbon nano-onion.

  The gas diffusion layer 32 constituting the cathode 24 has a cathode gas diffusion base material and a microporous layer applied to the cathode gas diffusion base material. The cathode gas diffusion base material is preferably composed of a porous body having electron conductivity, and for example, carbon paper, carbon woven fabric or nonwoven fabric can be used.

  The microporous layer applied to the cathode gas diffusion substrate is a paste-like kneaded product obtained by kneading a conductive powder and a water repellent. For example, carbon black can be used as the conductive powder. Further, as the water repellent, a fluorine resin such as tetrafluoroethylene resin can be used. It is preferable that the water repellent has a binding property. Since the water repellent has binding properties, a paste can be obtained by kneading the conductive powder and the water repellent.

  According to the fuel cell 10 described above, ruthenium ions contained in the first catalyst layer 26 constituting the anode 22 are reduced to ruthenium by a substitution reaction with a metal having a higher ionization tendency than ruthenium. It is possible to suppress the deterioration of the cathode 24 by moving to the cathode 24 in the polymer electrolyte membrane 20. As a result, the power generation performance of the fuel cell 10 can be improved and the life can be extended.

(Production method of membrane electrode assembly)
Here, a manufacturing method of the membrane electrode assembly of the present embodiment will be described.

<Cathode catalyst slurry preparation>
Platinum supported carbon (TEC10E50E, Tanaka Kikinzoku Kogyo Co., Ltd.) is used as the cathode catalyst, SS700C solution (20%, Ew = 780, moisture content = 36 wt% (25 ° C)), ionomer solution Aciplex made by Asahi Kasei Chemicals (Registered trademark) SS700C (hereinafter abbreviated as SS700) was used. To 5 g of platinum-supporting carbon, 10 mL of ultrapure water was added and stirred, and then 15 mL of ethanol was added. The catalyst dispersion solution was subjected to ultrasonic stirring and dispersion for 1 hour using an ultrasonic stirrer. A predetermined SS700 solution was diluted with an equal amount of ultrapure water and stirred with a glass rod for 3 minutes. Thereafter, ultrasonic dispersion was performed for 1 hour using an ultrasonic cleaner to obtain an SS700 aqueous solution. Thereafter, the SS700 aqueous solution was slowly dropped into the catalyst dispersion. During the dropping, stirring was continuously performed using an ultrasonic stirrer. After completion of the dropwise addition of the SS700 aqueous solution, 10 g of a mixed solution of 1-propanol and 1-butanol (weight ratio 1: 1) was dropped, and the resulting solution was used as a catalyst slurry. During mixing, the water temperature was all adjusted to about 60 ° C., and ethanol was evaporated and removed.

<Production of cathode electrode>
The catalyst slurry prepared by the above method is applied by screen printing (150 mesh) to a gas diffusion layer with a microporous layer prepared by Vulcan XC72, followed by drying at 80 ° C for 3 hours and heat treatment at 180 ° C for 45 minutes. went.

<Preparation of catalyst slurry for anode first catalyst layer>
The preparation method of the catalyst slurry for the anode first catalyst layer is the same as the preparation method of the cathode catalyst slurry, except that platinum ruthenium-supported carbon (TEC61E54, Tanaka Kikinzoku Kogyo Co., Ltd.) is used as the catalyst. SS700 was used as the ion conductor.

<Preparation of catalyst slurry for anode second catalyst layer>
The preparation method of the catalyst slurry for the anode second catalyst layer is the same as the preparation method of the cathode catalyst slurry, except that platinum cobalt-supported carbon (TEC36F52, Tanaka Kikinzoku Kogyo Co., Ltd.) is used as the catalyst. As an ionic conductor, an SS1100C solution (20%, Ew = 1050, moisture content = 20 wt% (25 ° C.), ionomer solution Aciplex SS1100C, hereinafter abbreviated as SS1100, manufactured by Asahi Kasei Chemicals) was used.

<Anode fabrication>
A gas diffusion layer with a microporous layer produced by Vulcan XC72 by screen printing (150 mesh) in order of the catalyst slurry for the anode first catalyst layer and the catalyst slurry for the anode second catalyst layer produced by the above method And then heat-treated at 80 ° C. for 3 hours and at 180 ° C. for 45 minutes.

<Preparation of membrane electrode assembly>
Hot pressing is performed with the solid polymer electrolyte membrane sandwiched between the anode and cathode produced by the above method. Aciplex (registered trademark) (SF7201x, manufactured by Asahi Kasei Chemicals) was used as the solid polymer electrolyte membrane. A membrane electrode assembly was produced by hot pressing the anode, the solid polymer electrolyte membrane, and the cathode under the bonding conditions of 170 ° C. and 200 seconds.

(Embodiment 2)
FIG. 3 is a cross-sectional view showing the structure of the fuel cell 10 according to the second embodiment. In the fuel cell 10 of the present embodiment, one catalyst layer 29 is provided between the gas diffusion layer 28 and the solid polymer electrolyte membrane 20.

  The catalyst layer 29 includes an ion conductor (ion exchange resin), carbon particles supporting the first alloy catalyst, that is, catalyst-supporting carbon particles, and carbon particles supporting the second alloy catalyst, that is, catalyst-supporting carbon particles. Is done.

  The ionic conductor connects the carbon particles supporting the first alloy catalyst or the carbon particles supporting the second alloy catalyst and the solid polymer electrolyte membrane 20, and has a role of transmitting protons between the two. The ionic conductor may be formed from the same polymer material as the solid polymer electrolyte membrane 20.

  The first alloy catalyst is composed of a noble metal and ruthenium. Examples of the noble metal used for the first alloy catalyst include platinum and palladium. Examples of the carbon particles supporting the first alloy catalyst include acetylene black, ketjen black, carbon nanotube, and carbon nano-onion.

  The second alloy catalyst is composed of a noble metal and a metal having a higher ionization tendency than ruthenium. Examples of the noble metal used for the second alloy catalyst include platinum and palladium. Examples of metals having a higher ionization tendency than ruthenium include cobalt, nickel, iron, tantalum, and molybdenum.

  According to this, ruthenium ions eluted in the catalyst layer 29 are reduced to ruthenium due to an ionization tendency, in other words, a difference in standard electrode potential, and cobalt is oxidized to cobalt ions. Since ruthenium ions contained in the catalyst layer 29 constituting the anode 22 are reduced to ruthenium by a substitution reaction with a metal having a higher ionization tendency than ruthenium, the ruthenium ions move to the cathode 24 in the solid polymer electrolyte membrane 20. The cathode 24 can be prevented from deteriorating. As a result, the power generation performance of the fuel cell 10 can be improved and the life can be extended.

  Regarding the catalyst layer 29, it is preferable that the second metal catalyst is unevenly distributed on the solid polymer electrolyte membrane 20 side as compared with the first metal catalyst. According to this, since the ruthenium ions are easily reduced to the ruthenium ions on the solid polymer electrolyte membrane 20 side, the ruthenium ions are more effectively suppressed from moving from the catalyst layer 29 to the solid polymer electrolyte membrane 20. can do.

  The fact that the second metal catalyst is unevenly distributed as compared with the first metal catalyst means that, for example, in the region of the catalyst layer 29 having a thickness of 1 μm on the solid polymer electrolyte membrane 20 side, the catalyst layer having a thickness of 1 μm. The weight percentage of the second metal catalyst relative to the weight of 29 is greater than the weight percentage of the first metal catalyst.

  The water content of the ionic conductor is preferably lower than the water content of the solid polymer electrolyte membrane 20. In other words, it is preferable that Ew of the ionic conductor is higher than Ew of the solid polymer electrolyte membrane 20. According to this, since the diffusion rate of ruthenium ions from the catalyst layer 29 to the solid polymer electrolyte membrane 20 is reduced, the above-described substitution reaction is likely to occur.

  The present invention is not limited to the above-described embodiments, and various modifications such as design changes can be added based on the knowledge of those skilled in the art. The form can also be included in the scope of the present invention.

1 is a perspective view schematically showing the structure of a fuel cell according to Embodiment 1. FIG. It is sectional drawing on the AA line of FIG. 4 is a cross-sectional view showing a structure of a fuel cell 10 according to Embodiment 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel cell, 20 Solid polymer electrolyte membrane, 22 Anode, 24 Cathode, 26 1st catalyst layer, 27 2nd catalyst layer, 29, 30 Catalyst layer, 28, 32 Gas diffusion layer, 34, 36 Separator, 50 Membrane electrode assembly.

Claims (2)

An electrolyte membrane;
An anode provided on one surface of the electrolyte membrane;
A cathode provided on the other surface of the electrolyte membrane;
With
The anode is
An ionic conductor;
A first alloy catalyst comprising a noble metal and ruthenium;
A second alloy catalyst comprising a noble metal and a metal having a higher ionization tendency than ruthenium;
Including
The membrane electrode assembly, wherein the second alloy catalyst is unevenly distributed on the electrolyte membrane side as compared with the first alloy catalyst . A fuel cell comprising the membrane electrode assembly according to claim 1 .

Images