JP2006236631A - Polymer electrolyte fuel cell and vehicle mounting the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for preventing deterioration of a catalyst layer for achieving a PEFC maintaining excellent power generating property for a long period. <P>SOLUTION: A buffer layer 150 restraining deterioration of carbon carrier apt to be generated at neighboring area of a boundary between a cathode (air electrode) catalyst layer 110 and a solid polymer electrolyte film 100 and elution of catalyst from the cathode catalyst layer into the solid polymer electrolyte film, is formed between the cathode catalyst layer 110 and the solid polymer electrolyte film 100. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polymer electrolyte fuel cell. More specifically, the present invention relates to a technique for preventing deterioration that is likely to occur near the interface between a solid polymer electrolyte membrane and a catalyst layer.

  In recent years, fuel cells have attracted attention as vehicle drive sources and stationary power sources in response to social demands and trends against the background of energy and environmental problems. Fuel cells are classified into various types depending on the type of electrolyte, the type of electrode, and the like, and representative types include alkaline type, phosphoric acid type, molten carbonate type, solid electrolyte type, and solid polymer type. Among them, a polymer electrolyte fuel cell (PEFC) that can operate at a low temperature (usually 100 ° C. or less) attracts attention, and in recent years, development and practical application as a low-pollution power source for automobiles is progressing. As a PEFC application, a vehicle drive source and a stationary power source have been studied, but in order to be applied to these applications, durability over a long period of time is required.

  The PEFC generally has a structure in which a membrane-electrode assembly (hereinafter also referred to as “MEA”) is sandwiched between separators. The MEA generally has a structure in which a gas diffusion layer, a cathode catalyst layer, a solid polymer electrolyte membrane, an anode catalyst layer, and a gas diffusion layer are laminated. The cell reaction proceeds in a catalyst layer comprising a catalyst, a carrier supporting the catalyst, and an ionomer (ion conductive polymer). For this reason, preventing deterioration of the catalyst layer is an important issue in enhancing the durability of PEFC.

  However, the three-phase structure composed of a catalyst, a carrier, and an ionomer tends to deform the electrode structure due to carbon corrosion and ionomer degradation degradation. When the electrode structure is changed, gas diffusibility and generated water discharge properties are lowered, and power generation performance is also lowered. It also causes a decrease in battery durability. In particular, in the vicinity of the interface between the solid polymer electrolyte membrane and the cathode catalyst layer, carbon oxidation or elution of the catalyst metal into the solid polymer electrolyte membrane is likely to occur due to the oxidation-reduction reaction, which causes the power generation characteristics of PEFC to deteriorate. Become. Therefore, development of a technique for preventing deterioration of the catalyst layer is desired.

As a technique for improving the durability of the catalyst layer, for example, the catalyst layer has a two-layer structure, and the gas movement resistance of the catalyst layer on the solid polymer electrolyte membrane side is higher than the gas movement resistance of the catalyst layer on the gas diffusion layer side. A large PEFC is disclosed (see Patent Document 1). Such a configuration is intended to suppress the decomposition of the ionomer in the cathode catalyst layer.
JP 2004-349037 A

  An object of the present invention is to provide a means for preventing the deterioration of the catalyst layer, thereby contributing to the realization of PEFC in which excellent power generation characteristics are sustained over a long period of time.

  The present invention provides a polymer electrolyte fuel cell having an anode catalyst layer, a solid polymer electrolyte membrane, and a cathode catalyst layer, wherein the cathode catalyst layer is interposed between the cathode catalyst layer and the solid polymer electrolyte membrane. A solid polymer type in which a buffer layer is formed to suppress the deterioration of the carbon support that is likely to occur near the interface between the cathode polymer layer and the solid polymer electrolyte membrane, and the elution of the catalyst from the cathode catalyst layer to the solid polymer electrolyte membrane It is a fuel cell.

  The present invention also provides a polymer electrolyte fuel cell having an anode catalyst layer, a solid polymer electrolyte membrane, and a cathode catalyst layer, wherein the catalyst, A solid polymer fuel cell comprising a carbon support carrying a catalyst and an ionomer, wherein a buffer layer having a higher ionomer content ratio than the cathode catalyst layer is formed.

  By disposing a buffer layer between the cathode catalyst layer and the solid polymer electrolyte membrane, it is possible to improve the durability of the PEFC and maintain the power generation characteristics over a long period of time.

  The oxygen reduction reaction that occurs in the cathode catalyst layer significantly occurs near the interface between the solid polymer electrolyte membrane having a high proton concentration and the cathode catalyst layer. For this reason, deterioration such as oxidative corrosion of the carbon support and elution of platinum caused by start / stop and potential cycle occurs remarkably in the vicinity of the interface. Therefore, power generation characteristics and durability can be improved if the reaction in the vicinity of the interface can be suppressed and the reaction can be progressed in the entire catalyst layer.

  Therefore, in the present invention, the deterioration of the carbon carrier that easily occurs near the interface between the cathode catalyst layer and the solid polymer electrolyte membrane between the cathode catalyst layer and the solid polymer electrolyte membrane, and the cathode catalyst layer to the solid polymer electrolyte membrane. A buffer layer is formed to suppress the elution of the catalyst. FIG. 1 is a schematic cross-sectional view of one embodiment of the PEFC of the present invention. As shown in the figure, PEFC 10 has a cathode (air electrode) supplied with an oxidant and an anode (fuel electrode) supplied with hydrogen gas on both sides of solid polymer electrolyte membrane 100. The cathode and anode have a catalyst layer 110 and a gas diffusion layer 120, respectively. A separator 130 and a current collector 140 are disposed outside the gas diffusion layer 120. A flow path 135 is formed in the separator 130. An oxidant such as air is supplied to the channel on the cathode side, and hydrogen gas is supplied to the channel on the anode side. In the PEFC of the present invention, the buffer layer 150 is formed between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100.

  The buffer layer 150 alleviates the concentration of reaction near the interface between the solid polymer electrolyte membrane 100 and the cathode catalyst layer 110, and the deterioration of the carbon support contained in the cathode catalyst layer 110 and the solid polymer polymer from the cathode catalyst layer 110. It functions to suppress elution of the catalyst to the electrolyte membrane 100. The buffer layer 150 having such a function improves the durability of the catalyst layer, ensures high gas diffusibility from the initial operation of the PEFC to after a long period of time, and generates high power in the range of low current density to high current density. Performance is obtained.

  The configuration of the PEFC is not limited to the embodiment shown in FIG. For example, a water repellent layer for preventing a flooding phenomenon is disposed between the catalyst layer 110 and the gas diffusion layer 120. Further, the buffer layer 150 may exist in the entire range between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 as shown in FIG. 1, and as described later, the buffer layer 150 and the solid polymer electrolyte membrane 100 The buffer layer 150 may be formed only at a site where undesired reactions tend to concentrate at the interface with the cathode catalyst layer 110.

  By making the composition of the buffer layer 150 different from that of the cathode catalyst layer 110, the oxidation-reduction reaction can be mitigated. Preferably, the buffer layer 150 formed between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 is made of a catalyst, a carbon carrier supporting the catalyst, and an ionomer, and contains an ionomer more than the cathode catalyst layer 110. The ratio is high. By controlling the composition in this way, it is possible to suppress the deterioration of the carbon support in the catalyst layer and the elution of the catalyst metal. Since the buffer layer 150 includes a catalyst, the oxidation-reduction reaction at the cathode proceeds to some extent. However, since the content ratio of the ionomer is high, the oxidation-reduction reaction is gentle compared to the vicinity of the interface when the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 are in direct contact. Therefore, corrosion of the carbon support and elution of the catalyst metal from the cathode catalyst layer 110 to the solid polymer electrolyte membrane 100 that tend to proceed at the interface between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 are suppressed.

  More specifically, the mass ratio of ionomer (I) / carbon support (C) in the buffer layer 150 is preferably I / C = 1.0 to 4.0, more preferably I / C = 1. 0-2.0. By setting it as this range, oxidation-reduction reaction activity can be made mild. On the other hand, the mass ratio of ionomer (I) / carbon support (C) in the cathode catalyst layer is preferably I / C = 0.7-0.95, more preferably I / C = 0.7-0. 90. By controlling the ionomer amount, it is possible to form a water-repellent gradient in the direction of the solid polymer electrolyte membrane-buffer layer-catalyst layer-gas diffusion layer, and promote the movement of water. The calculation method of the ratio of the ionomer and the carbon support in the buffer layer is not particularly limited. For example, it can be calculated from the amount of ionomer and carbon support used in preparing the buffer layer.

  If the amount of the metal catalyst in the buffer layer 150 is small, the metal catalyst is easily trapped in the buffer layer 150 even if the metal catalyst is eluted from the cathode catalyst layer 110. With this mechanism, it is possible to suppress the adverse effect of the metal catalyst eluting and depositing on the solid polymer electrolyte membrane 100 and degrading the power generation characteristics.

  Preferably, the content of the catalyst in the buffer layer 150 is 5 to 20% by mass with respect to the total amount of the catalyst and the support. Usually, the content of the catalyst in the cathode catalyst layer 110 is 30 to 70% by mass with respect to the total amount of the catalyst and the carrier. By reducing the amount of catalyst supported in the buffer layer 150, the metal eluted from the cathode catalyst layer 110 is easily trapped in the buffer layer 150.

  For the same reason, it is preferable to use a carrier having a high specific surface area as the carrier in the buffer layer 150. When the specific surface area of the support in the buffer layer 150 is large, the metal catalyst is easily trapped in the buffer layer 150 even if the metal catalyst is eluted from the cathode catalyst layer 110. With this mechanism, it is possible to suppress the adverse effect of the metal catalyst eluting and depositing on the solid polymer electrolyte membrane 100 and degrading the power generation characteristics.

Specifically, the specific surface area of the carrier in the buffer layer 150 is preferably 300 to 3000 m ² / g, more preferably 300 to 1200 m ² / g. The specific surface area of the carrier can be calculated by a known measurement method. For example, the specific surface area is calculated by a conventional nitrogen gas adsorption method (BET specific surface area measurement method). The material of the carrier is not particularly limited, and examples thereof include carbon black.

On the other hand, the support in the cathode catalyst layer 110 is preferably graphitized carbon black having a specific surface area of 80 to 300 m ² / g. By using graphitized carbon black, it is possible to form a water-repellent gradient in the direction of the solid polymer electrolyte membrane-buffer layer-catalyst layer-gas diffusion layer, and promote the movement of water.

  The thickness of the buffer layer 150 is preferably 0.5 to 2.0 μm. Since the buffer layer 150 has a higher ionomer ratio than the cathode catalyst layer 110, the water retention is high and the gas permeability tends to decrease. Therefore, it is preferable to reduce the thickness of the buffer layer 150 to 0.5 to 2.0 μm to ensure sufficient water mobility and gas permeability. Here, the “buffer layer thickness” means the average thickness of the buffer layer.

  The type of the catalyst contained in the buffer layer 150 is not particularly limited, and examples thereof include platinum, a platinum-iridium alloy, and a platinum-iridium-cobalt alloy. Preferably, an alloy containing platinum and iridium such as a platinum-iridium alloy and a platinum-iridium-cobalt alloy is used. The amount of water in the buffer layer 150 having a high ionomer amount tends to be higher than that of the cathode catalyst layer 110 due to the influence of the water moving from the solid polymer electrolyte membrane side and the reaction water generated by the oxidation-reduction reaction. However, carbon corrosion and catalyst metal elution in the catalyst layer are both greatly affected by water. For this reason, it is preferable to reduce the amount of water in the buffer layer 150. By using an alloy containing platinum and iridium, water can be electrolyzed at a low potential, and the water content of the cathode catalyst layer 110 can be reduced.

  The region where the buffer layer 150 is disposed may be the entire region between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 or a part thereof. When the buffer layer 150 is partially disposed, the buffer layer is a gas flow path between the cathode catalyst layer 110 and the solid polymer electrolyte membrane 100 through which the gas supplied to the cathode catalyst layer 110 passes. It is preferable that it exists in the range of 1/3 to 3/4 from the exit side. That is, when viewed from the separator 130 side to the cathode catalyst layer 110 side, the buffer layer is formed in a region corresponding to the outlet side of the gas flow path.

  The PEFC of the present invention is excellent in durability, and even when PEFC is used for a long period of time, the power generation characteristics are hardly deteriorated. Such characteristics are particularly beneficial in applications that require durability over a long period of time. Such applications include vehicles. Since the power generation characteristics of the PEFC of the present invention can be maintained over a long period of time, the life of a vehicle equipped with the PEFC of the present invention can be prolonged and the vehicle value can be improved.

  Hereinafter, the present invention will be described more specifically with reference to examples. In addition, this invention is not limited to the following Example.

Example 1
1. Preparation of Anode Catalyst Layer Platinum-supported carbon black (Ketjen Black ^TM EC manufactured by Ketjen Black International Co., Ltd., BET surface area = 800 m ² / g) was prepared as a catalyst-supported carrier disposed in the anode catalyst layer. The amount of the catalyst supported was 50% by mass with respect to the total amount of the catalyst and the carrier.

  Five times the amount of purified water was added to the mass of the anode electrode catalyst (a1), and a vacuum degassing operation was added for 5 minutes. To this, 0.5 times the amount of n-propyl alcohol was added, and a solution containing a proton conductive polymer electrolyte (containing 5 wt% Nafion (registered trademark) manufactured by DuPont) was added. For the content of the polymer electrolyte in the solution, both were used such that the mass ratio of the solid content to the carbon mass of the electrode catalyst (a1) was I / C = 0.9.

The obtained mixed slurry was well dispersed with an ultrasonic homogenizer, and a catalyst slurry was prepared by applying a vacuum degassing operation. An amount of the catalyst slurry corresponding to the desired thickness was printed on one side of the polytetrafluoroethylene sheet by screen printing and dried at 60 ° C. for 24 hours. The size of the formed anode catalyst layer was 5 cm × 5 cm. The coating layer on the polytetrafluoroethylene sheet was adjusted so that the Pt amount was 0.25 mg / cm ² .

2. Preparation of Cathode Catalyst Layer Platinum-supported carbon black (Ketjen Black ^TM EC manufactured by Ketjen Black International Co., Ltd.) is graphitized at 2700 ° C. for 10 hours as a catalyst-supported carrier disposed in the cathode catalyst layer. Treated carbon black (graphitized ketjen black, BET surface area = 125 m ² / g) was prepared. The amount of the catalyst supported was 50% by mass with respect to the total amount of the catalyst and the carrier. Moreover, as ionomer, 20 wt% Nafion (registered trademark) made by DuPont was prepared.

  Five times the amount of purified water was added to the mass of the cathode electrode catalyst (c1), and a vacuum degassing operation was added for 5 minutes. To this, 0.5 times the amount of n-propyl alcohol was added, and a solution containing a proton conductive polymer electrolyte (containing 20 wt% Nafion (registered trademark) manufactured by DuPont) was added. For the content of the polymer electrolyte in the solution, both were used so that the mass ratio of the solid content to the carbon mass of the electrode catalyst (c1) was I / C = 0.9.

The obtained mixed slurry was well dispersed with an ultrasonic homogenizer, and a catalyst slurry was prepared by applying a vacuum degassing operation. An amount of the catalyst slurry corresponding to the desired thickness was printed on one side of the polytetrafluoroethylene sheet by screen printing and dried at 60 ° C. for 24 hours. The size of the formed cathode catalyst layer was 5 cm × 5 cm. Further, the coating layer on the polytetrafluoroethylene sheet was adjusted so that the amount of Pt was 0.4 mg / cm ² .

3. Preparation of Cathode Buffer Layer Platinum-supported carbon black (Ketjenblack ^™ EC, BET surface area = 800 m ² / g manufactured by Ketjen Black International Co., Ltd.) was prepared as a catalyst-supported carrier disposed in the cathode buffer layer. The supported amount of the catalyst was 20% by mass with respect to the total amount of the catalyst and the support. Moreover, as ionomer, 5 wt% Nafion (trademark) containing DuPont was prepared. Five times the amount of purified water was added to the mass of the cathode buffer layer catalyst (b1), and a vacuum degassing operation was added for 5 minutes. To this, 0.5 times the amount of n-propyl alcohol was added, and a solution containing a proton conductive polymer electrolyte (containing 20 wt% Nafion (registered trademark) manufactured by DuPont) was added. For the content of the polymer electrolyte in the solution, both were used such that the mass ratio of the solid content to the carbon mass of the electrode catalyst (c1) was I / C = 2.0.

  The obtained mixed slurry was well dispersed with an ultrasonic homogenizer, and a catalyst slurry was prepared by applying a vacuum degassing operation. An amount of catalyst slurry corresponding to a desired thickness was printed on the polytetrafluoroethylene sheet having the cathode electrode catalyst layer formed thereon by screen printing, and dried at 60 ° C. for 24 hours. The size of the formed cathode buffer layer was 5 cm × 5 cm.

4). Bonding with a solid polymer electrolyte membrane As a solid polymer electrolyte membrane, Nafion ^™ 111 (film thickness 25 µm) and an electrode catalyst layer formed on the previously prepared polytetrafluoroethylene sheet were superposed. At that time, an anode catalyst layer, a solid polymer electrolyte membrane, a cathode buffer layer, and a cathode catalyst layer were laminated in this order. Thereafter, hot pressing was performed at 130 ° C. and 2.0 MPa for 10 minutes.

The cathode catalyst layer has a thickness of about 10 μm, the amount of Pt supported is 0.4 mg per 1 cm ^{2 of the} apparent electrode area, and the cathode buffer layer has a thickness of about 1 μm and the amount of Pt supported is about 1 cm ^{2 of the} apparent electrode area. 0.05 mg and the electrode area were 25 cm ² . The anode catalyst layer had a thickness of about 6 μm, the amount of Pt supported was 0.25 mg per 1 cm ^{2 of} apparent electrode area, and the electrode area was 25 cm ² .

5. PEFC performance evaluation A gas separator with a gas flow path is placed on both sides of the laminate on which the cathode catalyst layer, cathode buffer layer, and anode catalyst layer are formed, and sandwiched between gold-plated stainless steel current collector plates for evaluation. A single cell was used. In the evaluation unit cell, hydrogen was supplied as a fuel to the anode side, and air was supplied as an oxidant to the cathode side. The supply pressure of both gases was atmospheric pressure, hydrogen was set to 58.6 ° C. and relative humidity 60%, air was set to 54.8 ° C. and relative humidity 50%, and the cell temperature was set to 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.

Subsequently, after generating power for 60 seconds, power generation was stopped. After stopping, the supply of hydrogen and air was stopped, the single cell was replaced with air, and the system was on standby for 50 seconds. Thereafter, hydrogen gas was supplied to the anode side for 10 seconds at 1/5 of the above utilization rate. Thereafter, hydrogen gas was supplied to the anode side and air was supplied to the cathode side under the same conditions as described above, and power was generated again at a current density of 1.0 A / cm ² for 60 seconds. The load current at this time was increased from 0 A / cm ² to 1 A / cm ² in 30 seconds. After this power generation / stop operation was repeated 5000 times, the cell voltage was measured for the output at 1.0 A. The configuration and results of PEFC are shown in Table 1.

(Example 2)
A PEFC was produced in the same manner as in Example 1 except that the thickness of the cathode buffer layer was 0.5 μm. As with Example 1, the power generation performance after 5000 potential cycle tests were evaluated on the PEFC for evaluation was 0.620V. The configuration and results of PEFC are shown in Table 1.

(Example 3)
A PEFC was produced in the same manner as in Example 1 except that the thickness of the cathode buffer layer was 2.0 μm. For this evaluation PEFC, the power generation performance after conducting the potential cycle test 5000 times in the same manner as in Example 1 was 0.613 V. The configuration and results of PEFC are shown in Table 1.

(Comparative Example 1)
A PEFC was produced in the same manner as in Example 1 except that the cathode buffer layer was not formed. As with Example 1, the power generation performance after 5000 potential cycle tests were evaluated on this evaluation PEFC was 0.575V. The configuration and results of PEFC are shown in Table 1.

  Table 1 shows that by forming a buffer layer between the cathode catalyst layer and the solid polymer electrolyte membrane, the durability of the fuel cell is improved and excellent power generation characteristics are maintained over a long period of time. .

  The PEFC of the present invention is used as various power sources. The PEFC of the present invention that is excellent in durability is used as a power source of a vehicle, for example.

It is a cross-sectional schematic diagram of one Embodiment of PEFC of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... PEFC, 100 ... Solid polymer electrolyte membrane, 110 ... Catalyst layer, 120 ... Gas diffusion layer, 130 ... Separator, 135 ... Channel, 140 ... Current collector, 150 ... Buffer layer.

Claims (14)

A polymer electrolyte fuel cell having an anode catalyst layer, a solid polymer electrolyte membrane, and a cathode catalyst layer,
Deterioration of the carbon carrier that tends to occur near the interface between the cathode catalyst layer and the solid polymer electrolyte membrane between the cathode catalyst layer and the solid polymer electrolyte membrane, and from the cathode catalyst layer to the solid polymer electrolyte membrane A polymer electrolyte fuel cell, in which a buffer layer for suppressing elution of the catalyst is formed. A polymer electrolyte fuel cell having an anode catalyst layer, a solid polymer electrolyte membrane, and a cathode catalyst layer,
Between the cathode catalyst layer and the solid polymer electrolyte membrane, a buffer layer comprising a catalyst, a carbon carrier supporting the catalyst, and an ionomer and having a higher ionomer content than the cathode catalyst layer is formed. , Solid polymer fuel cell.   The mass ratio of ionomer (I) / carbon support (C) in the buffer layer is I / C = 1.0 to 4.0, and the mass of ionomer (I) / carbon support (C) in the cathode catalyst layer. The polymer electrolyte fuel cell according to claim 2, wherein the ratio is I / C = 0.7 to 0.95.   The mass ratio of ionomer (I) / carbon support (C) in the buffer layer is I / C = 1.0 to 2.0, and the mass of ionomer (I) / carbon support (C) in the cathode catalyst layer. The polymer electrolyte fuel cell according to claim 3, wherein the ratio is I / C = 0.7 to 0.90.   5. The polymer electrolyte fuel cell according to claim 2, wherein the amount of the catalyst supported in the buffer layer is 5 to 20 mass% with respect to the total amount of the catalyst and the carrier.   6. The polymer electrolyte fuel cell according to claim 5, wherein the amount of the catalyst supported in the cathode catalyst layer is 30 to 70% by mass with respect to the total amount of the catalyst and the carrier. The solid polymer fuel cell according to any one of claims 2 to 6, wherein the carrier in the buffer layer is carbon black having a specific surface area of 300 to 3000 m ² / g. The polymer electrolyte fuel cell according to claim 7, wherein the carrier in the buffer layer is carbon black having a specific surface area of 300 to 1200 m ² / g. 9. The polymer electrolyte fuel cell according to claim 7, wherein the support in the cathode catalyst layer is graphitized carbon black having a specific surface area of 80 to 300 m ² / g.   The polymer electrolyte fuel cell according to claim 2, wherein the buffer layer has a thickness of 0.5 to 2.0 μm.   The solid polymer fuel cell according to any one of claims 2 to 10, wherein the catalyst in the buffer layer is an alloy containing platinum and iridium.   The solid polymer fuel cell according to any one of claims 2 to 11, wherein the buffer layer is present in the entire range between the cathode catalyst layer and the solid polymer electrolyte membrane.   The buffer layer is in the range of 1/3 to 3/4 from the outlet side of the gas flow path through which the gas supplied to the cathode catalyst layer passes between the cathode catalyst layer and the solid polymer electrolyte membrane. The polymer electrolyte fuel cell according to any one of claims 2 to 11, which is present.   A vehicle on which the polymer electrolyte fuel cell according to any one of claims 1 to 13 is mounted.

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