JP2007214036A - Operation method for fuel cell, and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an operation method for a fuel cell advantageous to suppress the elution of ruthenium. <P>SOLUTION: A membrane electrode assembly 1 has a cathode 2 formed with a cathode catalyst layer and a cathode gas diffusion layer 21, an anode 3 formed with an anode catalyst layer 30 and an anode gas diffusion layer 31, and an ion conductive membrane 4 interposed between the anode catalyst layer 30 and the cathode catalyst layer 20. An anode catalyst contains platinum and ruthenium. A reference electrode 8 coming in contact with the ion conductive membrane 4 and a counter electrode 7 are installed. Operation for power generation is conducted by supplying fuel to the reference electrode 8 and the counter electrode 7 and oxidant to the cathode 2. At that time, while current is passed between the anode 3 and the counter electrode 7, the potential of the anode 3 vs. the reference electrode is kept at 0 to +0.6 V with a constant-potential maintaining means 9. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method of operating a fuel cell having a membrane electrode assembly containing platinum and ruthenium as an anode catalyst, and a fuel cell.

  Conventionally, a membrane electrode assembly of a fuel cell includes a cathode formed of a cathode catalyst layer including a cathode catalyst and a cathode gas diffusion layer, an anode formed of an anode catalyst layer including an anode catalyst and an anode gas diffusion layer, and an anode. And an ion conductive membrane having ion conductivity sandwiched between the anode catalyst layer and the cathode catalyst layer of the cathode. The fuel gas supplied to the fuel cell may contain carbon monoxide on the order of ppm although it depends on the reforming reaction. Carbon monoxide or the like adsorbs on the reaction site of the catalyst and affects the catalyst performance. In order to suppress the influence of carbon monoxide or the like, an anode catalyst containing platinum and ruthenium is used. Ruthenium is more easily eluted than platinum.

  Patent Document 1 discloses a solid polymer fuel cell electrode structure in which ruthenium is dispersed at the interface between an ion conductive membrane and an anode catalyst layer or at the interface between an anode catalyst layer and an anode gas diffusion layer. It is disclosed. According to this, it is described that ruthenium eluted from the anode catalyst layer is trapped even in a fuel shortage state or in a high potential operation, so that elution of ruthenium is suppressed.

  Patent Document 2 discloses a solid polymer type fuel cell electrode structure in which ruthenium is dispersed in an anode gas diffusion layer. According to this document, it is described that ruthenium eluted from the anode catalyst layer is trapped even in high potential operation, so that elution of ruthenium is suppressed.

  Patent Document 3 discloses a fuel cell electrode in which a first insulator layer is bonded to the anode side of an ion conductive film and a second insulator layer is bonded to the cathode side of the ion conductive film. According to this document, it is described that reduction of the catalyst amount can be prevented.

  Patent Document 4 discloses a fuel cell in which elution of catalyst particles can be suppressed even in a noble potential environment by supporting catalyst particles having different average particle diameters on a conductive carrier.

Patent Document 5 discloses a technique related to a liquid fuel typified by methanol fuel, a container for storing methanol fuel, a third electrode provided in the container, and a fuel electrode and a third electrode. A fuel cell system including an applied voltage controller that applies a voltage is disclosed. According to this, the purpose is to oxidize liquid fuel represented by methanol fuel.
JP 2004-185830 A JP 2004-186050 A JP 2004-327057 A JP-A-2005-141920 JP 2005-293901 A

  As described above, an anode catalyst containing platinum and ruthenium is used in the anode catalyst layer in order to suppress the influence of carbon monoxide or the like contained in the fuel gas supplied to the fuel cell. However, as described above, ruthenium is more easily eluted than platinum. Therefore, further development of technology for suppressing elution of ruthenium is desired in the industry.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fuel cell operating method and a fuel cell that are advantageous for suppressing elution of ruthenium as an anode catalyst.

  The inventor has intensively developed a fuel cell having a membrane electrode assembly having an anode catalyst layer containing platinum and ruthenium. As shown in the following (a) and (b), the inventor paid attention to the fact that ruthenium has a lower oxidation-reduction potential than platinum (c).

Ru ²⁺ +2 ^e- = Ru +0.455 volts (vs. SHE) (a)
Ru ³⁺ +3 ^e− = Ru +0.249 volts (vs. SHE) (b)
Pt ²⁺ +2 ^e- = Pt +1.188 volts (vs. SHE) (c)
Here, SHE means a standard hydrogen electrode.

Actually, it has been confirmed by the present inventor that ruthenium used in the anode catalyst elutes during power generation operation and precipitates at the interface between the cathode and the ion conductive membrane. As a result of investigating the cause of the elution phenomenon of ruthenium based on this fact, it was found that it was based on the potential increase of the anode. Here, it is presumed that the potential increase of the anode is caused by the following three factors, and ruthenium used in the anode catalyst is eluted.
[1] Increase in unreacted area due to fuel gas deficiency [2] Reduction in reaction surface area due to carbon monoxide [3] Reduction in reaction surface area due to flatting

  Therefore, the present inventor provides a reference electrode and a counter electrode that are in contact with the ion conductive membrane, and performs a power generation operation by supplying an oxidant gas to the cathode while supplying a fuel gas to the anode, the reference electrode, and the counter electrode. If the anode potential is maintained within a range of 0 to +0.6 volts by means of constant potential maintaining means, it is possible to suppress the elution of ruthenium used in the anode catalyst, and further, the interface between the cathode and the ion conductive membrane Thus, it was found that ruthenium could be prevented from precipitating and confirmed by a test to complete the method of the present invention. If the potential of the anode with respect to the reference electrode is maintained at 0 to +0.6 volts by the constant potential maintaining means, the cause of the suppression of ruthenium elution is not particularly clear, but the factors [1] to [3] described above This is presumed to be due to the suppression of the potential rise caused by.

  That is, the fuel cell operating method according to the present invention includes a cathode formed of a cathode catalyst layer including a cathode catalyst and a cathode gas diffusion layer, an anode formed of an anode catalyst layer including an anode catalyst and an anode gas diffusion layer, and In a driving method for operating a fuel cell including a membrane electrode assembly having an ion conductive membrane having ion conductivity sandwiched between an anode catalyst layer of an anode and a cathode catalyst layer of a cathode, the anode catalyst contains platinum and ruthenium. In addition, a reference electrode and a counter electrode that are in contact with the ion conductive membrane of the fuel cell are provided. While supplying fuel to the anode, the reference electrode, and the counter electrode, an oxidant is supplied to the cathode to perform a power generation operation. The anode potential is maintained within the range of 0 to +0.6 volts by means of constant potential maintaining means. To.

  According to the operation method of the fuel cell according to the present invention, the increase of the anode potential during the power generation operation is forcibly suppressed, and the elution of ruthenium is suppressed.

  A fuel cell according to the present invention includes a cathode formed of a cathode catalyst layer including a cathode catalyst and a cathode gas diffusion layer, an anode formed of an anode catalyst layer including an anode catalyst and an anode gas diffusion layer, and an anode catalyst for the anode In a fuel cell comprising a membrane electrode assembly having an ion conductive membrane sandwiched between a cathode layer and a cathode catalyst layer of a cathode, the anode catalyst contains platinum and ruthenium, and a reference electrode and a counter electrode in contact with the ion conductive membrane are provided It is characterized by being provided.

  According to the fuel cell of the present invention, the above-described fuel cell operation method can be carried out, the anode potential increase during power generation operation is forcibly suppressed, and elution of ruthenium contained as an anode catalyst is prevented. It is suppressed.

  According to the operation method of the fuel cell and the fuel cell according to the present invention, the increase in the anode potential is forcibly suppressed, and the elution of ruthenium contained as the anode catalyst is suppressed. Furthermore, the precipitation of ruthenium at the interface between the cathode and the ion conductive membrane is suppressed. As a result, the anode catalyst in the anode catalyst layer is well maintained over a long period of time. Therefore, it is advantageous for maintaining good power generation performance of the fuel cell.

  The fuel cell according to the present invention includes a membrane electrode assembly. The membrane electrode assembly includes a cathode formed of a cathode catalyst layer including a cathode catalyst and a cathode gas diffusion layer, an anode formed of an anode catalyst layer including an anode catalyst and an anode gas diffusion layer, and an anode catalyst layer of the anode. It has an ion conductive membrane sandwiched between the cathode catalyst layer of the cathode. Examples of the anode catalyst layer include an anode catalyst, a conductive material, and an ionic conductor. Examples of the cathode catalyst layer include a cathode catalyst (for example, platinum), a conductive material, and an ionic conductor.

  The anode catalyst contains platinum and ruthenium. In general, an anode catalyst containing platinum (Pt) and ruthenium (Ru) is supported on a conductive material such as carbon particles. Ruthenium is effective in reducing the influence of carbon monoxide contained in the fuel gas. Here, the mixing ratio of platinum and ruthenium can be selected as appropriate, and varies depending on the type of fuel cell, the composition of the fuel gas, the operating conditions of the fuel cell, etc. It can be 40% and ruthenium can be 10-40%, especially 15-30%, but is not limited thereto. Examples of platinum and ruthenium include a form in which both are alloyed, or a form in which platinum and ruthenium are present as simple substances. In the case of a simple substance, ruthenium has a lower redox potential than platinum.

  According to the present invention, the reference electrode and the counter electrode that are in contact with the ion conductive membrane of the fuel cell are provided. The reference electrode is also called a reference electrode or a reference electrode, and is used as a reference for the potential of the working electrode. The counter electrode is also called an auxiliary electrode, and makes a pair with the working electrode in order to pass a current.

  If an oxidation reaction (electron emission reaction) has occurred in the anode corresponding to the working electrode, a reduction reaction (electron acceptance reaction) has occurred in the counter electrode (auxiliary electrode) paired with the working electrode. Here, if the area of the counter electrode is small, the reaction in the anode catalyst layer of the anode corresponding to the working electrode tends to be slow. For this reason, it is preferable to ensure the area of a counter electrode. Therefore, when the anode catalyst layer is disposed in the central region of the ion conductive membrane, the counter electrode preferably surrounds the outer periphery of the anode catalyst layer on the surface of the ion conductive membrane by 70% or more, or 80% or more. In particular, as exemplified in the examples described later, it is preferable that the counter electrode is surrounded so as to make one round of the outer periphery of the anode catalyst layer on the surface of the ion conductive membrane.

  However, when the area of the counter electrode increases, the area of the anode catalyst layer relatively decreases and the power generation area of the fuel cell decreases. Therefore, the area of the counter electrode: the area of the anode catalyst layer = 1: (2 to 50). In particular, it can be 1: (5-20) or 1: (8-16).

  According to the present invention, a potentiostat is exemplified as the constant potential maintaining means. A potentiostat is a device that controls a potential difference between a working electrode and a reference electrode to a predetermined set potential while flowing a current between a working electrode, also called a working electrode, and a counter electrode. Here, the working electrode corresponds to the anode of the fuel cell.

  According to the method of the present invention, a power generation operation is performed by supplying an oxidant gas to the cathode while supplying fuel to the anode. At this time, the potential of the anode with respect to the reference electrode is maintained within the range of 0 to +0.6 volts by the constant potential maintaining means while passing a current between the anode functioning as the working electrode and the counter electrode (auxiliary electrode).

  According to the present invention, according to the test example described later, in particular, in order to suppress elution of ruthenium, the potential of the anode with respect to the reference electrode is maintained within the range of 0 to +0.4 volts by the constant potential maintaining means. It is preferable. Furthermore, it is preferable to maintain within the range of 0 to +0.2 volts.

  A mode in which the reference electrode and the counter electrode are provided on the outer peripheral side in the surface direction of the anode catalyst layer on the surface of the ion conductive membrane on which the anode catalyst layer is disposed is exemplified. In order to reduce the ohmic drop, the counter electrode is preferably disposed near the anode catalyst layer of the anode corresponding to the working electrode. Therefore, in the front view, the distance between the counter electrode and the anode catalyst layer (working electrode) is L2, for example, within a range of 0.2 to 10 mm, depending on the type of fuel cell, operating conditions, and the like. It can be set within a range of 0.2 to 3 millimeters. If the distance L2 is excessively small, manufacturing difficulty and sufficient insulation cannot be obtained. If the distance L2 is excessively large, the resistance increases.

  Fuel can be supplied to the reference electrode and the counter electrode simultaneously with the anode catalyst layer. Further, a structure in which fuel is supplied before the anode catalyst layer can be employed for the reference electrode and the counter electrode. The reason is that when the reaction proceeds in the anode catalyst layer, hydrogen as an active material in the fuel is reduced accordingly, and the functions of the reference electrode and the counter electrode may not be sufficiently exhibited. Therefore, it is preferable that the separator has a fuel flow path structure that supplies fuel to the reference electrode and the counter electrode before the anode catalyst layer.

  Hereinafter, Example 1 of the present invention will be described in detail.

(Formation of gas diffusion layer)
First, a commercially available carbon paper (Toray Industries, Inc., TGP-H-60, thickness 190 μm) was impregnated as a carbon substrate. As the paste, a mixture obtained by kneading carbon black (manufactured by Cabot Corporation, VXC-72R) and a dispersion containing polytetrafluoroethylene (PTFE) as a water repellent (Polyflon D-1, Daikin Corporation) was used. After impregnating the paste, the carbon paper was pre-dried at 80 ° C. for 1 hour and then fired at 380 ° C. for 1 hour to form. This was used as an anode gas diffusion layer and a cathode gas diffusion layer.

(Catalyst layer formation)
As a cathode catalyst, 10 g of platinum-supported carbon (Tanaka Kikinzoku Co., Ltd., TEC10E70TPM, 67% by mass of platinum) supporting platinum, 82.5 g of ion exchange resin (Asahi Kasei Corporation, SS-1100 / 05, 5% by mass), 38 g of ion-exchanged water was kneaded for 1 hour in a sand mill (zirconia balls, diameter: 2 millimeters, peripheral speed 15 meters / second) and dispersed to form a catalyst paste. In this case, the mass ratio was 1.25 in ion exchange resin / carbon in platinum-supported carbon = 1.25.

The obtained catalyst paste was applied to a fluororesin sheet (ETFE, thickness 50 μm) by a doctor blade method with a gap of 250 μm (about 1 mg Pt / cm ² ) to form a catalyst sheet for the cathode.

  Further, as an anode catalyst, platinum ruthenium-supported carbon in which platinum ruthenium is supported on carbon (Tanaka Kikinzoku Co., Ltd., TEC62E58, platinum 27.8 mass%, ruthenium 28.8 mass%, platinum: ruthenium atomic ratio = 1: 2. ) 10 g, 108.5 g of ion exchange resin (Asahi Kasei Corporation, SS-1100 / 05, 5 mass%) and 38 g of ion exchange water (condition: zirconia ball, diameter: 2 mm, peripheral speed 15 m / Second) for 1 hour. In this case, the mass ratio was 1.25 in ion exchange resin / carbon in platinum-supported carbon = 1.25.

The obtained catalyst paste was applied to a fluororesin sheet (ETFE, thickness 50 μm) with a gap of 200 μm (about 0.25 g Pt / cm ² ) by a doctor blade method to form an anode catalyst sheet.

(Formation of membrane electrode assembly 1)
An anode catalyst sheet and a cathode catalyst sheet were punched out at a reaction area of 13 cm ² . Two punched catalyst sheets were arranged on both sides of an ion conductive membrane (thickness 30 μm Gore 30-III-B: Japan Gore-Tex) to form a laminate. The laminate was hot-pressed at 150 ° C. and 10 MPa for 1 minute for transfer. Thereafter, the anode gas diffusion layer and the cathode gas diffusion layer punched out to an area of 13 cm ² were arranged on both sides in the thickness direction of the laminate, and hot pressed at 150 ° C. and 8 MPa for 3 minutes to form a membrane electrode assembly.

  FIG. 1 schematically shows a fuel cell according to this embodiment. As shown in FIG. 1, the fuel cell includes a membrane electrode assembly 1. The membrane electrode assembly 1 includes a cathode 2 formed of a cathode catalyst layer 20 including a cathode catalyst and a cathode gas diffusion layer 21, and an anode 3 formed of an anode catalyst layer 30 including an anode catalyst and an anode gas diffusion layer 31. And an ion conductive membrane 4 (proton conductive membrane) having ion conductivity (proton conductivity) sandwiched between the anode catalyst layer 30 of the anode 3 and the cathode catalyst layer 20 of the cathode 2.

  The anode catalyst is formed of a platinum ruthenium alloy. As described above, the mixing ratio of platinum and ruthenium is 27.8% for platinum and 28.8% for ruthenium by mass ratio. The cathode gas diffusion layer 21 and the anode gas diffusion layer 31 are formed of a porous aggregate in which carbon fibers are integrated, and have gas permeability and conductivity.

  Here, as shown in FIG. 1, the anode catalyst layer 30 is joined to one surface 4 c of the ion conductive membrane 4. The cathode catalyst layer 20 is bonded to the other surface 4 d of the ion conductive membrane 4. As shown in FIG. 1, a fuel separator 5 and an oxidant separator 6 are disposed outside the membrane electrode assembly 1 so as to be sandwiched in the thickness direction of the membrane electrode assembly 1. The fuel separator 5 is made of a sintered carbon material and has a fuel flow path 5a. The oxidant separator 6 is made of a sintered carbon material and has an oxidant flow path 6a. An electrical insulating portion 100 is laminated on the back surface 5 c of the fuel separator 5.

According to the membrane electrode assembly 1, the oxidation reaction (1) is generated in the anode catalyst layer 30. In the cathode catalyst layer 20, the reduction reaction (2) is generated.
(1) H ₂ → 2H ⁺ + 2e ⁻
(2) 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O
(Formation of reference electrode 8 and counter electrode 7)
FIG. 2 shows a front view of the surface 4 c of the ion conductive film 4. FIG. 2 shows a front view of the side where the anode catalyst layer 30, the reference electrode 8 and the counter electrode 7 are joined in the ion conductive membrane 4. The anode catalyst layer 30, the reference electrode 8, and the counter electrode 7 are formed of the same material as the anode catalyst layer 30. That is, the reference electrode 8 and the counter electrode 7 are formed of a catalyst sheet of the same type as the anode catalyst sheet, similarly to the anode catalyst layer 30, and carry platinum ruthenium in which a platinum ruthenium alloy is carried on carbon particles (conductive material). Carbon and an ion exchange resin (electrolyte component) having ion conductivity are included as main components.

  As shown in FIG. 2, the reference electrode 8 and the counter electrode 7 are joined to the surface 4c of the ion conductive membrane 4 to which the anode catalyst layer 30 is joined. In this case, a coated sheet is formed by coating a mixture to be the anode catalyst layer 30, the reference electrode 8, and the counter electrode 7 on a release sheet (ETFE). Thereafter, this coated sheet is bonded to the surface 4 c of the ion conductive film 4. Thereafter, the anode catalyst layer 30, the reference electrode 8, and the counter electrode 7 are simultaneously bonded to the surface 4 c of the ion conductive film 4 by peeling the release sheet from the ion conductive film 4. As described above, the anode catalyst layer 30, the reference electrode 8, and the counter electrode 7 are simultaneously bonded to the surface 4c of the ion conductive film 4. However, the present invention is not limited to this, and may be individually bonded to the ion conductive film 4. . By the manufacturing method described above, the reference electrode 8 and the counter electrode 7 are arranged on the same side of the ion conductive membrane 4 as the anode catalyst layer 30.

Further explanation will be added. The ion conductive film 4 has a quadrangular shape when viewed from the front, and has sides 41, 42, 43 and 44. The anode catalyst layer 30 has a reaction area of 13 cm ² . The anode catalyst layer 30 has a quadrangular shape when viewed from the front (FIG. 2) and has four sides 31, 32, 33, and 34. The same catalyst sheet of the anode catalyst sheet is transferred to the surface 4c of the ion conductive film 4 so that the surface 4c of the ion conductive film 4 is positioned on the outer peripheral side in the surface direction of the surface 4c of the anode catalyst layer 30; A counter electrode 7 was formed. As shown in FIG. 2, the counter electrode 7 surrounds the anode catalyst layer 30 so as to make one turn on the outer peripheral side in the surface direction of the anode catalyst layer 30, and has a “mouth” shape.

  According to the present embodiment, as shown in FIG. 2, the counter electrode 7 includes a first side portion 71 that faces the side 31 of the anode catalyst layer 30 via the electric insulation portion 100 a and an anode via the electric insulation portion 100 b. A second side 72 that faces the side 32 of the catalyst layer 30, a third side 73 that faces the side 33 of the anode catalyst layer 30 through the electrical insulating part 100c, and an anode catalyst layer through the electrical insulating part 100d And a fourth side portion 74 opposite to the 30 sides 34.

  As shown in FIG. 2, the reference electrode 8 is disposed between the first side 71 of the counter electrode 7 and the side 31 of the anode catalyst layer 30. Between the reference electrode 8 and the first side portion 71 of the counter electrode 7, an electrical insulating portion 100 a is disposed. Between the reference electrode 8 and the side portion 31 of the anode catalyst layer 30, an electrical insulating portion 100h is disposed. As with the anode catalyst layer 30, the reference electrode 8 and the counter electrode 7 are porous and conductive so that fuel gas (hydrogen-containing gas) can pass therethrough. The above-described electrical insulating portions 100, 100a to 100d, 100h are made of fluororesin (PTFE), which is an electrically insulating material having water repellency, and the reference electrode 8 and the counter electrode 7 are separated by a separator (having conductivity). The anode catalyst layer 30 is prevented from being electrically connected.

  As shown in FIG. 1, the anode gas diffusion layer 31 includes a gas diffusion portion 31 f that faces the back surface 7 r of the counter electrode 7, and a gas diffusion portion 31 s that faces the back surface 8 r of the reference electrode 8. Therefore, the anode gas diffusion layer 31 faces not only the anode catalyst layer 30 but also the counter electrode 7 and the reference electrode 8. As a result, as shown in FIG. 1, the fuel gas separator 5 distributes the fuel gas to the counter electrode 7 and the reference electrode 8 via the gas diffusion part 31f and the gas diffusion part 31s. Reference numeral 150 denotes a support member for supporting the facing surface, which is formed of an electrically insulating material (PTFE or the like), and is disposed outside the separator 6 in the thickness direction.

  As shown in FIGS. 1 and 2, the reference electrode 8 and the counter electrode 7 are positioned on the outer peripheral side in the surface direction of the anode catalyst layer 30 on the surface 4 c of the ion conductive membrane 4 where the anode catalyst layer 30 is disposed. Is provided. As described above, the reference electrode 8 is also called a reference electrode or a reference electrode, and the counter electrode 7 is also called an auxiliary electrode.

  By the way, in order to reduce the ohmic drop, the counter electrode 7 is preferably disposed near the anode catalyst layer 30 corresponding to the working electrode. As shown in FIG. 2, the distance between the counter electrode 7 and the anode catalyst layer 30 in the front view of the surface 4c of the ion conductive film 4 is L2.

  According to this embodiment, a potentiostat 9 (manufactured by Hokuto Denko, potentiostat / galvanostat HA-301) is used as a constant potential maintaining means. The potentiostat 9 is a device that controls the potential difference between the working electrode and the reference electrode (reference electrode) to a predetermined set potential while flowing a current between the working electrode and the counter electrode. According to this embodiment, the working electrode corresponds to the anode catalyst layer 30.

  According to the present embodiment, a power generation operation is performed by supplying an oxidant gas (air) to the cathode 2 while supplying fuel (hydrogen gas) to the anode 3. At this time, the anode catalyst layer 30 functioning as a working electrode is connected to the first terminal 91 of the potentiostat 9, the counter electrode 7 (auxiliary electrode) is connected to the second terminal 92 of the potentiostat 9, and the third electrode of the potentiostat 9 is connected. The reference electrodes 8 are connected to the terminals 93, respectively. In this state, the potentiostat 9 causes the potential of the anode with respect to the reference electrode 8 to be 0 to +0.6 volts by the terminal of the potentiostat 9 while flowing a current between the anode catalyst layer 30 and the counter electrode 7 (auxiliary electrode). maintain. For this reason, according to this embodiment, the increase in the anode potential is forcibly suppressed, and the elution of ruthenium contained as the anode catalyst is suppressed.

  Here, if an oxidation reaction (electron emission reaction) occurs in the anode catalyst layer 30 corresponding to the working electrode, a reduction reaction (electron acceptance reaction) occurs in the counter electrode 7. If the area of the counter electrode 7 is small, the original reaction in the anode catalyst layer 30 corresponding to the working electrode is delayed. For this reason, it is preferable to secure the reaction area of the counter electrode 7 as much as possible. Therefore, as shown in FIG. 2, on the surface 4c of the ion conductive membrane 4, the counter electrode 7 is disposed on the outer peripheral side of the anode catalyst layer 30 disposed in the central region of the ion conductive membrane 4, and the electrically insulating portions 100a and 100b. , 100c, 100d. Thus, since the counter electrode 7 surrounds the anode catalyst layer 30 so as to make one turn, uneven reaction in the anode catalyst layer 30 is reduced.

(Test Example 1)
With the above-described potentiostat, the fuel gas is supplied to the anode 3 while air is supplied to the cathode 2 to perform a power generation operation. In this case, the potential of the anode 3 with respect to the reference electrode 8 was maintained at 0 volts by the potentiostat 9 while a current was passed between the anode 3 and the counter electrode 7. The current density is 0.26 amps / cm ² , the cell temperature is 80 ° C., the anode / cathode humidification dew point temperature is 80 ° C., and the fuel utilization rate / oxidant utilization rate is 80% / 40% (steady state). The fuel utilization rate / oxidant utilization rate was 110% / 40% (gas shortage state), and each 1-minute cycle was subjected to a power generation operation for 1000 hours. The fuel gas supplied to the fuel cell was, by mass ratio, 76% hydrogen, 4% nitrogen, 19% carbon dioxide, 4% methane, and 10 ppm carbon monoxide.

(Test Example 2)
Test Example 2 was basically the same as Test Example 1. However, the potential of the anode with respect to the reference electrode was maintained at 0.2 volts by the potentiostat 9.

(Test Example 3)
Test Example 3 was basically the same as Test Example 1. However, the potential of the anode with respect to the reference electrode was maintained at 0.4 volts by the potentiostat 9.

(Test Example 4)
Comparative Example 1 was basically the same as Test Example 1. However, the potential of the anode with respect to the reference electrode was maintained at 0.6 volts by the potentiostat 9.

(Comparative Example 1)
Comparative Example 1 was basically the same as Test Example 1. However, the power generation operation was performed without executing the voltage control by the potentiostat.

(Evaluation)
FIGS. 3-8 shows the figure which copied the result of having analyzed the membrane electrode assembly 1 which concerns on Test Example 1-Test Example 4 and Comparative Example 1 by EPMA. 3 to 8, the upper diagram shows platinum and the lower diagram shows ruthenium.

  FIG. 3 shows an initial state before the power generation operation. FIG. 4 shows Test Example 1. FIG. 5 shows Test Example 2. FIG. 6 shows Test Example 3. FIG. 7 shows Test Example 4. FIG. 8 shows Comparative Example 1.

  According to the initial state before the power generation operation, as shown in FIG. 3, ruthenium is contained in the anode catalyst layer 30 because it is contained as the anode catalyst, but at the boundary between the ion conductive film 4 and the cathode 2, No precipitation is seen at all.

  According to Test Example 1, as shown in FIG. 4, no ruthenium deposition is observed at the boundary between the ion conductive film 4 and the cathode 2. According to Test Example 2, similarly, as shown in FIG. 5, no ruthenium deposition is observed at the boundary between the ion conductive film 4 and the cathode 2. According to Test Example 3, as shown in FIG. 6, precipitation of ruthenium is slightly observed at the boundary between the ion conductive film 4 and the cathode 2, but the amount is still very small. According to Test Example 4, as shown in FIG. 7, some ruthenium deposition is observed at the boundary between the ion conductive film 4 and the cathode 2, but the amount is still very small. However, the precipitation in Test Example 4 is larger than that in Test Example 3. Thus, the higher the potential of the anode 3, the more the ruthenium elution amount tends to increase.

  On the other hand, according to Comparative Example 1 in which the anode potential was not controlled, as shown in FIG. 8, ruthenium that should have existed in the anode catalyst layer 30 was present at the boundary between the ion conductive membrane 4 and the cathode 2. A considerable amount was deposited. According to Comparative Example 1, it is estimated that the potential of the anode 3 is considerably higher than 0.6 volts.

  If ruthenium is deposited as described above, there is a high possibility that the ion conductive film 4 will conduct electrons in the thickness direction, which is not preferable. Further, since the amount of ruthenium that should have existed in the anode catalyst layer 30 is lowered, the original performance of the anode catalyst layer 30 is lowered. Further, when ruthenium is deposited between the ion conductive film 4 and the cathode 2, the performance at the cathode is lowered.

  According to the test results described above, elution of ruthenium, which functions as an anode catalyst, is suppressed if the power generation operation is performed while the potential of the anode 3 with respect to the reference electrode 8 is maintained at 0 to 0.6 volts or less by the potentiostat 9. It can be seen that the durability of the catalyst layer 30 is improved, and as a result, the durability of the membrane electrode assembly 1 is improved.

  In particular, if the potential of the anode catalyst layer 30 is maintained at 0 to 0.4 volts or less by the potentiostat 9, more preferably, if maintained at 0 to 0.2 volts or less, elution of ruthenium is further suppressed, It can be seen that the durability of the anode catalyst layer 30 and the durability of the membrane electrode assembly 1 are further improved.

(Other examples)
FIG. 9 shows another embodiment. The present embodiment basically has the same configuration and function as the first embodiment. In this case, as shown in FIG. 9, the counter electrode 7 has a first side 71, a second side 72, a third side 73, and a fourth side 74 so as to make one round of the outer periphery of the anode catalyst layer 30. And have. Further, the counter electrode 7 has a plurality of projecting portions 77 projecting toward the anode catalyst layer 30 through the electric insulating portion 100m. The area of the counter electrode 7 is secured by the protrusion 77.

  FIG. 10 shows another embodiment. The present embodiment basically has the same configuration and function as the first embodiment. In this case, as shown in FIG. 10, the counter electrode 7 has a notch 7 x while making almost one circumference of the anode catalyst layer 30.

  FIG. 11 shows still another embodiment. The present embodiment basically has the same configuration and function as the first embodiment. In this case, as shown in FIG. 11, a structure in which fuel gas (hydrogen gas) is preferentially supplied to the reference electrode 8 and the counter electrode 7 before the anode catalyst layer 30 is formed as a fuel separator 300 (fuel separator). 5). The reason is that when the reaction proceeds in the anode catalyst layer 30, hydrogen as an active material in the fuel gas is reduced accordingly, and the functions of the reference electrode 8 and the counter electrode 7 may not be sufficiently exhibited. Therefore, the fuel separator 300 has a structure of the grooved fuel flow path 310 that supplies the fuel gas to the reference electrode 8 and the counter electrode 7 before the anode catalyst layer 30.

  As shown in FIG. 11, the fuel flow path 310 has a spiral shape so as to have a rectangular frame flow path from the outer edge toward the central area 302 toward the central area outlet 302 from the outer edge side inlet 301 of the fuel separator 300. Is formed. The outlet 302 is formed in the central region of the fuel separator 300 so as to penetrate along the cell stacking direction of the fuel cell. On the outer edge side of the fuel separator 300, a recessed channel portion 320 facing the reference electrode 8 and a recessed channel portion 330 facing the counter electrode 7 are formed. Other structures may be used as long as the flow path structure can supply fuel gas (hydrogen gas) to the reference electrode 8 and the counter electrode 7 preferentially over the anode catalyst layer 30.

In addition, the present invention is not limited to the embodiments described above and shown in the drawings, and can be implemented with appropriate modifications within a range not departing from the gist. In the embodiment described above, the anode catalyst layer 30 has a reaction area of 13 cm ² , but it is needless to say that the present invention is not limited to this. In Example 1 described above, the shape of the counter electrode 7 surrounds the anode catalyst layer 30 so as to make one turn, and is a mouth shape in front view, but is not limited thereto, and is a gate shape in front view. It is also good. In the above-described embodiment, the anode catalyst layer 30, the reference electrode 8, and the counter electrode 7 are formed of the same material, but are not limited thereto. A flow path structure that can simultaneously supply fuel gas (hydrogen gas) to the reference electrode 8, the counter electrode 7, and the anode catalyst layer 30 may be used.

  The present invention can be used for, for example, a fuel cell system for vehicles, stationary devices, electric devices, electronic devices, and portable devices.

It is sectional drawing of a fuel cell with a membrane electrode assembly. It is a front view of the side which has arrange | positioned the counter electrode and the reference electrode among ion conductive films. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on the initial state before an electric power generation operation by EPMA. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on the test example 1 by EPMA. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on the test example 2 by EPMA. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on Test Example 3 by EPMA. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on Test Example 4 by EPMA. It is the figure which copied the result of having analyzed the membrane electrode assembly which concerns on the comparative example 1 by EPMA. It is a front view of the side which has arrange | positioned the counter electrode and the reference electrode among ion conductive films concerning another Example. It is a front view of the side which has arrange | positioned the counter electrode and the reference electrode among other ion conductive films concerning another Example. FIG. 10 is a front view of a fuel separator according to still another embodiment.

Explanation of symbols

  1 is a membrane electrode assembly, 2 is a cathode, 3 is an anode, 20 is a cathode catalyst layer, 30 is an anode catalyst layer, 21 is a cathode gas diffusion layer, 31 is an anode gas diffusion layer, 4 is an ion conductive membrane, and 7 is a counter electrode , 8 is a reference electrode, and 9 is a potentiostat (constant potential maintaining means).

Claims (5)

A cathode formed of a cathode catalyst layer containing a cathode catalyst and a cathode gas diffusion layer; an anode formed of an anode catalyst layer containing an anode catalyst; and an anode gas diffusion layer; the anode catalyst layer of the anode; and the cathode of the cathode In an operation method for operating a fuel cell comprising a membrane electrode assembly having an ion conductive membrane having ion conductivity sandwiched between a cathode catalyst layer,
The anode catalyst includes platinum and ruthenium;
A reference electrode and a counter electrode are provided in contact with the ion conductive membrane;
While supplying fuel gas to the anode, the reference electrode, and the counter electrode, an oxidant gas is supplied to the cathode to perform a power generation operation, and the potential of the anode with respect to the reference electrode is set to 0 to +0. A method of operating a fuel cell, characterized in that it is maintained within a range of 6 volts.   2. The method of operating a fuel cell according to claim 1, wherein the power generation operation is performed while maintaining the potential of the anode with respect to the reference electrode at 0 to +0.4 volts. In the front view of the ion conductive membrane according to claim 1 or 2,
The anode catalyst layer is disposed in a central region of the ion conductive membrane;
The fuel cell, wherein the reference electrode and the counter electrode are provided on the outer peripheral side in the surface direction of the anode catalyst layer on the surface of the ion conductive membrane on which the anode catalyst layer is disposed. Driving method.   4. The method of operating a fuel cell according to claim 3, wherein the counter electrode surrounds the outer periphery of the anode catalyst layer once on the surface of the ion conductive membrane. A cathode formed of a cathode catalyst layer containing a cathode catalyst and a cathode gas diffusion layer; an anode formed of an anode catalyst layer containing an anode catalyst; and an anode gas diffusion layer; the anode catalyst layer of the anode; and the cathode of the cathode In a fuel cell comprising a membrane electrode assembly having an ion conductive membrane having ion conductivity sandwiched between a cathode catalyst layer,
The anode catalyst includes platinum and ruthenium, and is provided with a reference electrode and a counter electrode that are in contact with the ion conductive membrane.

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