JP2006134752A - Solid polymer fuel cell and vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means of heightening power generating characteristics and durability of a solid polymer fuel cell. <P>SOLUTION: The solid polymer fuel cell is provided with a catalyst layer consisting of catalyst-carrying particles 10, a first electrolyte 20 coating a part or a whole of the surface of the catalyst-carrying particles 10, and a second electrolyte 30 with an EW value larger than that of the first electrolyte coating the catalyst-carrying particles coated with the first electrolyte 20. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polymer electrolyte fuel cell, and more particularly to improvement of a catalyst layer of a polymer electrolyte fuel cell.

  A fuel cell is a clean power generation system in which the product of an electrode reaction is water in principle and has almost no adverse effect on the global environment. Various fuel cells such as a polymer electrolyte fuel cell, a solid oxide fuel cell, a molten carbonate fuel cell, and a phosphoric acid fuel cell have been proposed as fuel cells. Among these, a polymer electrolyte fuel cell (PEFC) can be operated at a relatively low temperature, and thus is expected as a power source for moving bodies such as automobiles, and is being developed.

  The PEFC usually has a polymer electrolyte membrane, a pair of electrodes that sandwich the polymer electrolyte membrane, and a pair of separators arranged outside the electrodes. The electrode includes a catalyst layer in which an electrode catalyst is disposed and a gas diffusion layer. The oxidant and fuel used for power generation are supplied through the separator and supplied to the catalyst layer through the gas diffusion layer. In the catalyst layer, the electrode reaction proceeds to generate power. Water generated by the electrode reaction is discharged to the outside through the gas diffusion layer.

  The electrode reaction in PEFC proceeds in the catalyst layer. Therefore, the battery performance of PEFC greatly depends on the characteristics of the catalyst layer. In order to improve the battery performance, how to devise the components of the catalyst layer, the blending ratio of the components, the structure of the catalyst layer, etc. is important.

In the fuel cell disclosed in Patent Document 1, in order to prevent the occurrence of flooding in the active catalyst layer and improve the power generation characteristics of the fuel cell, the cathode side catalyst layer has a two-layer structure, An ion exchange resin having a high ion exchange capacity is disposed on the electrolyte membrane side, and an ion exchange resin having a low ion exchange capacity is disposed on the gas diffusion layer side.
JP 2001-338654 A

  However, the catalyst layer is a laminated structure of a layer made of an ion exchange resin having a high ion exchange capacity arranged on the polymer electrolyte membrane side and a layer made of an ion exchange resin having a low ion exchange capacity arranged on the gas diffusion layer side. In this case, there is a high possibility that the ion exchange resin on the electrolyte membrane side will deteriorate early. Further, it is expected as a general demand that the polymer electrolyte fuel cell has excellent durability and excellent power generation characteristics.

  Accordingly, an object of the present invention is to provide a means for improving the power generation characteristics and durability of a polymer electrolyte fuel cell.

  The present invention provides a catalyst-supporting particle, a first electrolyte that covers a part or all of the surface of the catalyst-supporting particle, and a catalyst-supporting particle that is coated with the first electrolyte. This is a polymer electrolyte fuel cell having a catalyst layer composed of a second electrolyte having a large value.

  By forming the electrolyte covering the surface of the catalyst-carrying particles into a two-layer structure, a solid polymer fuel cell having high durability and power generation characteristics can be obtained.

  In the polymer electrolyte fuel cell of the present invention, the surface of catalyst-carrying particles such as platinum-carrying carbon particles contained in the catalyst layer is coated with a first electrolyte having a small EW value, that is, an electrolyte having a large ion exchange capacity. The outside of the electrolyte is coated with a second electrolyte having a large EW value, that is, an electrolyte having a small ion exchange capacity. With such a configuration, the stability and proton conductivity of the electrolyte constituting the catalyst layer, and consequently the durability and power generation characteristics of the fuel cell can be improved. In addition, EW value is a value prescribed | regulated as a reciprocal number of an ion exchange capacity, A proton conductivity is so low that an EW value is large, and proton conductivity is so high that an EW value is small.

  Although the battery reaction proceeds in the catalyst layer, proton conductivity in the catalyst layer is one of the factors for an efficient battery reaction. For this reason, it is desired to increase proton conductivity in the catalyst layer. In order to improve proton conductivity in the catalyst layer of the polymer electrolyte fuel cell, it is preferable to increase the ion exchange capacity of the electrolyte contained in the catalyst layer. However, increasing the ion exchange capacity of the electrolyte tends to lower the chemical and mechanical stability of the electrolyte. Moreover, when it is going to improve chemical stability and mechanical stability, the problem that proton conductivity falls will arise. That is, the stability of the electrolyte and the proton conductivity are contradictory factors. Therefore, in general, an electrolyte having appropriate stability and proton conductivity is used as the electrolyte in the catalyst layer.

  In the present invention, the electrolyte covering the catalyst-carrying particles has a two-layer structure, and the first electrolyte on the side close to the catalyst-carrying particles is an electrolyte with high proton conductivity, and the second electrolyte located outside thereof is It is a highly stable electrolyte. By covering the catalyst-carrying particles with the first electrolyte having high proton conductivity, proton conductivity around the catalyst-carrying particles can be secured, and the power generation characteristics are improved. As described above, the first electrolyte having high proton conductivity tends to be less stable. This problem is solved by the second electrolyte covering the first electrolyte. The second electrolyte is a high-strength stable electrolyte and has excellent durability. By adopting such a configuration, it can be expected that both the stability of the electrolyte and the proton conductivity are secured and the power generation performance is improved.

  FIG. 1 is a schematic cross-sectional view showing an embodiment in which a part of the surface of catalyst-carrying particles is coated with a first electrolyte. The catalyst-carrying particles 10 are composed of a catalytic active material 12 having a catalytic action and a conductive carrier 14 that carries the catalytic active material. A part of the surface of the catalyst-carrying particles 10 is covered with the first electrolyte 20. The first electrolyte 20 is covered with a second electrolyte 30 having a large EW value.

  FIG. 2 is a schematic cross-sectional view showing an embodiment in which the entire surface of the catalyst-carrying particles is coated with the first electrolyte. The catalyst-carrying particles 10 are composed of a catalytic active material 12 having a catalytic action and a conductive carrier 14 that carries the catalytic active material. The entire surface of the catalyst-carrying particles 10 is covered with the first electrolyte 20. The first electrolyte 20 is covered with a second electrolyte 30 having a large EW value.

  Whether to cover a part of the surface of the catalyst-carrying particles with the first electrolyte or to cover the whole surface of the catalyst-carrying particles with the first electrolyte can be selected in consideration of various conditions depending on the usage environment and the electrolyte to be used. That's fine. The coating state can be controlled by controlling the amount of the first electrolyte used during manufacture.

The conductive carrier is a substance having a role of supporting the catalyst active material and having conductivity so that electrons can move. The carrier is not particularly limited as long as it can carry a catalyst active material. The catalyst carrier to be actually used may be determined in consideration of the characteristics of the catalyst carrier, compatibility with the electrode active material, and the like. Specific examples of the conductive carrier include carbon blacks such as acetylene black, carbon materials such as activated carbon and graphite, and various metals. Other conductive carriers may be used. Commercially available products such as Ketjen Black ^™ may be used as the conductive carrier.

  The particle size of the conductive carrier is not particularly limited, but is preferably 5 to 100 nm, more preferably 30 to 50 nm.

The specific surface area of the conductive support is not particularly limited, preferably 200~1200m ² / g, more preferably 500 to 1000 m ² / g. If the specific surface area is too small, the dispersibility of the electrode active material in the conductive support may be reduced, and sufficient battery performance may not be exhibited. On the other hand, if the specific surface area is too large, the effective utilization rate of the electrode active material and the electrolyte may decrease, and sufficient battery performance may not be exhibited.

The amount of the catalyst active material supported on the conductive carrier is not particularly limited, but is preferably 0.01 to 5 mg / cm ² , more preferably 0.1 to 1 mg / cm ² . If the supported amount of the catalyst active material is too small, the catalyst function may not be sufficiently exhibited. If the amount of the catalyst active material supported is too large, there may be no particular problem from the viewpoint of the catalyst function, but even if more catalyst active material is supported, an increase in effect commensurate with the increase in production cost can be obtained. It becomes difficult to be.

  The catalytic active material is a material having a function of promoting an electrode reaction at the fuel electrode or the oxygen electrode. A catalyst active material will not be specifically limited if it has a function which accelerates | stimulates the electrode reaction in the electrode arrange | positioned. Specific examples of the catalytic active material include platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and alloys thereof. Is mentioned. Other catalytic active materials may be used.

  The particle size of the catalyst active material is not particularly limited, but is preferably 1 to 30 nm, and more preferably 1.5 to 10 nm. From the viewpoint of improving the catalyst utilization rate, the smaller the particle size, the better. However, when the particle size is too small, it becomes difficult to produce the catalyst active material.

  The first electrolyte and the second electrolyte are made of a polymer having a capability of transferring protons, which is also called a proton conductive polymer. The electrolyte plays a role of transmitting protons generated around the catalyst active material on the fuel electrode side. As the electrolyte, a polymer having the same structure as the polymer constituting the solid polymer electrolyte membrane can be used. For example, perfluorocarbon sulfonic acid polymer, polystyrene sulfonic acid, polytrifluorostyrene sulfonic acid and the like can be used as the electrolyte.

  Although it does not specifically limit about content of a 1st electrolyte, Preferably, the coating amount of a 1st electrolyte is 5-30 mass% of the total electrolytic mass in a catalyst layer. By reducing the coating amount of the low EW value electrolyte as much as possible, the low EW value electrolyte can be suppressed from being exposed to the surface, and the strength can be prevented from decreasing.

  The content of the second electrolyte is not particularly limited, but preferably, the coating amount of the second electrolyte is 70 to 95% by mass of the total electrolytic mass in the catalyst layer. By setting the coating amount of the electrolyte with a high EW value in such a range, a high strength EW value electrolyte can be present on the outermost surface of the particles comprising the catalyst-carrying particles and the electrolyte, thereby improving durability. it can. The content of the first electrolyte and the second electrolyte can be calculated from the blending amount at the time of production.

  The EW value of the first electrolyte is preferably 600 to 1000. Conventionally, the EW value of the electrolyte used in the catalyst layer is about 1000 to 1200, and the strength and proton conductivity are both achieved by using an electrolyte having an EW value in this range. When the EW value is less than this, the proton conductivity is improved, but the strength is lowered. In the present invention, the EW value of the first electrolyte adjacent to the catalyst-carrying particles is lower, but the first electrolyte is covered with the second electrolyte, and exposure to the surface is hindered. Absent. For this reason, an electrolyte having a low EW value and high proton conductivity can be selected.

  The EW value of the second electrolyte is preferably 1000 to 1300. The strength is ensured by coating the surface of the first electrolyte having low EW and low strength with a high strength electrolyte having an EW value of 1000 to 1300.

  By controlling the electrolyte content and the EW value as described above, it is possible to improve the power generation performance of the fuel cell. The EW value is the reciprocal of the ion exchange capacity (eq / g). For example, when the ion exchange capacity is 0.001 (eq / g), the EW value is 1000.

The type of electrolyte used is not particularly limited, and various proton conductive polymers can be used. The solid polymer electrolyte membrane only needs to have high proton conductivity. As a membrane having high proton conductivity, a polymer or copolymer of a monomer having an ion exchange group such as —SO ₃ H group; or a polymer of a monomer having an ion exchange group and another monomer, or the like is used. Can do. For example, a perfluorocarbon sulfonic acid film, an ethylene-tetrafluoroethylene copolymer film, a fluorine-containing resin film containing trifluorostyrene as a base polymer, or the like is used. Specific examples of the polymers of perfluorocarbon sulfonic acid, DuPont Co. NAFION ^TM, Asahi Glass Co., Ltd. FLEMION ^TM, Asahi Kasei Chemicals Corp. Aciplex ^TM, and the like Dow Chemical Company, Ltd. DOWEX ^TM and the like.

  The content ratio of the material constituting the catalyst layer may be determined in consideration of the function as the catalyst layer. In determining the content ratio of each material, it is preferable to consider the balance of the functions of each material. For example, in order to sufficiently advance the electrode reaction, it is necessary to secure an amount of the catalyst carrier containing the catalyst active material. However, when the electrolyte is insufficient, the conductivity of the generated protons is lowered, and the battery performance may be deteriorated. Preferably, the proton conductive electrolyte mass ratio Y (electrolyte mass / conductive carrier mass) to the mass of the conductive carrier is 0.5 ≦ Y ≦ 1.2. If Y is too small, the proton conduction path may decrease, and sufficient battery performance may not be obtained in the initial stage of power generation. On the other hand, when Y is too large, the battery performance at the initial stage of power generation can be satisfied, but there is a possibility that the performance deterioration due to battery operation for a long time becomes large. However, the present invention can be carried out even outside the above range. The mass ratio Y can be controlled by adjusting the raw material ratio when producing the catalyst layer.

  Further, the catalyst layer may contain other materials as necessary. For example, in some cases, the catalyst-supporting particles may be coated with three or more kinds of electrolytes. As an example of the embodiment, there is a mode in which the catalyst-supporting particles are coated in three layers with an electrolyte, and the EW value increases from the inside to the outside. The present invention is not limited to an embodiment in which the electrolyte has a two-layer structure, and such embodiments can also be included in the technical scope of the present invention.

  The solid polymer fuel cell of the present invention is not particularly limited as long as the catalyst layer having the above-described configuration is disposed on at least one of the oxygen electrode and the fuel electrode. For example, the configuration of the solid polymer fuel cell includes a structure in which a catalyst layer, a gas diffusion layer, and a separator are disposed on both sides of the polymer electrolyte membrane. The type of material used for the constituent member of the polymer electrolyte fuel cell is not particularly limited.

  Next, the manufacturing method will be described. Since the present invention is characterized by the structure of the catalyst layer, a method for producing the catalyst layer will be described. In addition, about the material which comprises catalyst layers, such as a catalyst active material, an electroconductive support | carrier, and an electrolyte, since it is the same as that of the said description, description is abbreviate | omitted below.

  First, the catalyst-carrying particles and the first electrolyte are mixed in a solvent, and a part or all of the surface of the catalyst-carrying particles is coated with the first electrolyte. Whether to coat a part or all with the first electrolyte depends on the amount to be mixed. When it is desired to coat the entire surface of the catalyst-carrying particles with the first electrolyte, the blending amount of the first electrolyte may be increased. Depending on the type and particle size of the catalyst-carrying particles, when catalyst-carrying carbon is used, the ratio between the mass of the catalyst-carrying carbon and the mass of the first electrolyte is usually 1: 0.025-1 : About 0.45. Although the solvent used in the case of mixing is not specifically limited, Alcohol, such as water and 1-propanol and 2-propanol, is mentioned.

  When the catalyst-carrying particles whose surfaces are coated with the first electrolyte are obtained, a second electrolyte is further added to coat the first electrolyte with the second electrolyte. As described above, the blending amount of the second electrolyte is preferably set so that the coating amount of the second electrolyte is 70 to 95% by mass of the total electrolytic mass in the catalyst layer.

  The catalyst layer ink thus obtained is applied and the solvent is volatilized to form a film composed of the catalyst-supporting particles, the first electrolyte, and the second electrolyte. When applying the catalyst layer ink, a known method can be used in the preparation of the catalyst layer. For example, a screen printer method, a die coater method, a spray method, or the like is used. The coating amount and coating conditions of the catalyst layer ink may be determined based on the knowledge already obtained, and are not particularly limited in the present invention.

  The catalyst layer ink is applied to, for example, both sides of the proton conductive polymer electrolyte membrane, and a laminate in which the catalyst layer, the polymer electrolyte membrane, and the catalyst layer are laminated in this order is obtained. The catalyst layer may be formed on a base material such as polytetrafluoroethylene and then transferred to the polymer electrolyte membrane.

  A membrane electrode assembly is obtained by hot pressing the obtained laminate and the gas diffusion substrate. However, the manufacturing method of the polymer electrolyte fuel cell is not limited to such a method, and other manufacturing methods may be adopted.

  Hereinafter, the content of the present invention will be described with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1
50% by weight Pt-supported ketjen black ^TM was prepared as catalyst-supported particles and mixed with an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 700, water, 1-propanol, and 2-propanol. As a result, a part of the surface of the catalyst-carrying particles was covered with the ion exchange resin as the first electrolyte. In this catalyst layer ink, the ratio of the mass of the catalyst-carrying particles to the mass of the ion exchange resin was 1: 0.15.

  Next, an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 1000 was added to the catalyst layer ink prepared as described above. At this time, the amount of the newly added ion exchange resin solution is such that the ratio of the catalyst-carrying particles contained in the catalyst layer ink and the dry mass of the ion exchange resin in the ion exchange resin solution is 1: 0.7. It was made to become. As a result of the above operation, a part of the surface of the catalyst-carrying particles is coated with the first electrolyte having an EW value of 700, and the entire surface is further coated with the second electrolyte having an EW value of 1000. A layer ink was obtained.

This catalyst layer ink was applied onto a polytetrafluoroethylene sheet using a screen printer and dried to form a catalyst layer. The catalyst layer formed on the polytetrafluoroethylene sheet was hot-pressed to transfer the catalyst layer onto the proton conductive polymer electrolyte membrane to form a membrane-electrode assembly. As the polymer electrolyte membrane, a DuPont Nafion ^™ membrane (25 μm thick) was used.

(Example 2)
50% by weight Pt-supported ketjen black ^TM was prepared as catalyst-supported particles and mixed with an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 700, water, 1-propanol, and 2-propanol. As a result, a part of the surface of the catalyst-carrying particles was covered with the ion exchange resin as the first electrolyte. In this catalyst layer ink, the ratio of the mass of the catalyst-carrying particles to the mass of the ion exchange resin was 1: 0.30.

  Next, an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 1000 was added to the catalyst layer ink prepared as described above. At this time, the amount of the newly added ion exchange resin solution is such that the ratio of the catalyst-carrying particles contained in the catalyst layer ink and the dry mass of the ion exchange resin in the ion exchange resin solution is 1: 0.55. It was made to become. Through the above operation, a part of the surface of the catalyst-carrying particles is covered with the first electrolyte having an EW value of 700 very thinly, and the entire surface is further covered with the second electrolyte having an EW value of 1000. A catalyst layer ink containing was obtained.

This catalyst layer ink was applied onto a polytetrafluoroethylene sheet using a screen printer and dried to form a catalyst layer. The catalyst layer formed on the polytetrafluoroethylene sheet was hot-pressed to transfer the catalyst layer onto the proton conductive polymer electrolyte membrane to form a membrane-electrode assembly. As the polymer electrolyte membrane, a DuPont Nafion ^™ membrane (25 μm thick) was used.

(Comparative Example 1)
50% by weight Pt-supported ketjen black ^TM was prepared as catalyst-supported particles and mixed with an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 700, water, 1-propanol, and 2-propanol. As a result, a part of the surface of the catalyst-carrying particles was covered with the ion exchange resin as the first electrolyte. In this catalyst layer ink, the ratio of the mass of the catalyst-carrying particles to the mass of the ion exchange resin was 1: 0.85.

This catalyst layer ink was applied onto a polytetrafluoroethylene sheet using a screen printer and dried to form a catalyst layer. The catalyst layer formed on the polytetrafluoroethylene sheet was hot-pressed to transfer the catalyst layer onto the proton conductive polymer electrolyte membrane to form a membrane-electrode assembly. As the polymer electrolyte membrane, a DuPont Nafion ^™ membrane (25 μm thick) was used.

(Comparative Example 2)
50 wt% Pt-supported ketjen black ^TM was prepared as catalyst-supported particles, and mixed with an ion exchange resin solution (solvent: water + 1-propanol) having an EW value of about 1000, water, 1-propanol, and 2-propanol. As a result, a part of the surface of the catalyst-carrying particles was covered with the ion exchange resin as the first electrolyte. In this catalyst layer ink, the ratio of the mass of the catalyst-carrying particles to the mass of the ion exchange resin was 1: 0.85.

This catalyst layer ink was applied onto a polytetrafluoroethylene sheet using a screen printer and dried to form a catalyst layer. The catalyst layer formed on the polytetrafluoroethylene sheet was hot-pressed to transfer the catalyst layer onto the proton conductive polymer electrolyte membrane to form a membrane-electrode assembly. As the polymer electrolyte membrane, a DuPont Nafion ^™ membrane (25 μm thick) was used.

(Calculation method of EW value)
The sample was immersed in a saturated aqueous sodium chloride solution and reacted in a water bath at 60 ° C. for 3 hours. After cooling to room temperature, the sample was thoroughly washed with ion exchange water, titrated with an aqueous sodium hydroxide solution using a phenolphthalein solution as an indicator, and the ion exchange capacity was calculated. The EW value was calculated as the reciprocal of the ion exchange capacity.

(Initial power generation performance)
A single cell for evaluation was manufactured using the manufactured membrane-electrode assembly. Hydrogen was supplied as a fuel to the anode side of the single cell, and air was supplied as an oxidant to the cathode side. The supply pressure of both gases was atmospheric pressure, hydrogen was set at a temperature of 58.6 ° C. and relative humidity was 60%, air was set at a temperature of 54.8 ° C. and a relative humidity was 50%, and the cell temperature was set at 70 ° C. The hydrogen utilization rate was 67% and the air utilization rate was 40%. Under this condition, the cell voltage when power was generated at a current density of 1.0 A / cm ² was measured as the initial cell voltage.

(durability)
The supply gas and set temperature were the same as the measurement conditions for the initial power generation performance. Under the above conditions, power generation was performed for 3 minutes, and then power generation was stopped for 3 minutes. During the power generation period, it took 1 minute to increase the current from 0 to 1 A / cm ² . During the stop period, no gas was supplied to the cell for the first 2 minutes, hydrogen was supplied to the anode side for the remaining 1 minute, and no gas was supplied to the cathode side. The supply of hydrogen to the anode side was the same as that during 1 A / cm ² power generation, and the relative humidity was 60%. This power generation and stop (6 minutes in total) was taken as one cycle, and the number of cycles that could be repeated until the cell voltage during 1 A / cm ² power generation became 0.5 V or less was used as an index of durability.

  The evaluation results for the examples and comparative examples are shown in Table 1.

  As shown in Table 1, a fuel cell excellent in both power generation characteristics and durability can be obtained by coating the catalyst-supporting particles with the first electrolyte having a small EW and the second electrolyte having a large EW value. I understand that.

It is a cross-sectional schematic diagram which shows the aspect by which a part of surface of the catalyst carrying particle was coat | covered with the 1st electrolyte. It is a cross-sectional schematic diagram which shows the aspect by which the whole surface of the catalyst support particle | grains was coat | covered with the 1st electrolyte.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Catalyst carrying particle, 12 ... Catalyst active material, 14 ... Conductive support | carrier, 20 ... 1st electrolyte, 30 ... 2nd electrolyte.

Claims (6)

Catalyst-carrying particles;
A first electrolyte covering a part or all of the surface of the catalyst-carrying particles;
A second electrolyte having a larger EW value than the first electrolyte, covering the catalyst-carrying particles coated with the first electrolyte;
A polymer electrolyte fuel cell having a catalyst layer comprising:   2. The polymer electrolyte fuel cell according to claim 1, wherein the coating amount of the first electrolyte is 5 to 30 mass% of the total electrolytic mass in the catalyst layer.   3. The polymer electrolyte fuel cell according to claim 1, wherein the coating amount of the second electrolyte is 70 to 95% by mass of the total electrolytic mass in the catalyst layer.   The polymer electrolyte fuel cell according to any one of claims 1 to 3, wherein the EW value of the first electrolyte is 600 to 1000.   The solid polymer fuel cell according to any one of claims 1 to 4, wherein the EW value of the second electrolyte is 1000 to 1300.   A vehicle equipped with the polymer electrolyte fuel cell according to any one of claims 1 to 5.