Catalyst for solid polymer fuel cell and method for producing same

Abstract

The present invention relates to a catalyst for a solid polymer fuel cell, including platinum, cobalt, and zirconium supported as a catalytic metal on a carbon powder carrier, in which the supporting ratio of platinum, cobalt, and zirconium on the carbon powder carrier is Pt:Co:Zr=3:0.5 to 1.5:0.1 to 3.0 by molar ratio. In the present invention, it is preferable that the peak position of Pt.sub.3Co seen in the X-ray diffraction pattern of catalyst particles is 2θ=41.10° or more and 42.00° or less, and moderate alloying has occurred in the catalytic metal.

Claims

1. A catalyst for a solid polymer fuel cell, comprising a catalytic metal dispersed and supported on a surface of a carbon powder carrier, wherein the catalytic metal is an alloy of platinum, cobalt, and zirconium and is in the form of catalyst particles having an average particle size of 2 to 20 nm, wherein a supported ratio of the platinum, cobalt, and zirconium constituting the catalytic metal is Pt:Co:Zr=3:0.5 to 1.5:0.1 to 3.0 by molar ratio, and wherein a supporting density of the catalytic metal is 30 mass % or more and 70 mass % or less.

2. The catalyst for a solid polymer fuel cell according to claim 1, wherein the supported ratio of platinum, cobalt, and zirconium on the carbon powder carrier is Pt:Co:Zr=3:0.5 to 1.5:0.2 to 1.8 by molar ratio.

3. The catalyst for a solid polymer fuel cell according to claim 1, wherein in a diffraction pattern obtained from the X-ray diffraction analysis of catalyst particles, the peak position of Pt.sub.3Co that appears in a region of 2θ=40.0° or more and 42.0° or less is 2θ=41.10° or more and 42.00° or less.

4. The catalyst for a solid polymer fuel cell according to claim 1, wherein in the diffraction pattern obtained from the X-ray diffraction analysis of catalyst particles, the ratio (I.sub.o/I.sub.a) of the peak intensity of ZrO.sub.2 (I.sub.o) that appears in a region of 2θ=28.0° or more and 28.4° or less to the peak intensity of Pt.sub.3Co (I.sub.a) that appears in a region of 2θ=40.0° or more and 42.0° or less is 1.3 or less.

5. The catalyst for a solid polymer fuel cell according to claim 1, wherein the catalyst particles have a surface and a core, and wherein the catalyst particles are configured such that the concentrations of cobalt and zirconium at the catalyst particle surface are lower than the concentrations of cobalt and zirconium at the catalyst particle core.

6. A method for producing the catalyst for a solid polymer fuel cell defined in claim 1, comprising: a step of supporting cobalt and zirconium on a platinum catalyst including platinum particles supported on a carbon powder carrier; a step of heat-treating the platinum catalyst having cobalt and zirconium supported thereon in the supporting step at 900° C. or more and 1,200° C. or less; and a step of bringing the heat-treated catalyst into contact with an oxidizing solution at least once to elute at least part of the supported cobalt and zirconium.

7. The method for producing a catalyst for a solid polymer fuel cell according to claim 6, wherein the oxidizing solution is a solution of sulfuric acid, nitric acid, phosphorous acid, potassium perzirconate, hydrogen peroxide, hydrochloric acid, chloric acid, hypochlorous acid, or chromic acid.

8. The method for producing a catalyst for a solid polymer fuel cell according to claim 6, wherein the contact treatment with the oxidizing solution is such that the treatment temperature is 40° C. or more and 90° C. or less, and the contact time is 1 hour or more and 10 hours or less.

9. The catalyst for a solid polymer fuel cell according to claim 2, wherein in a diffraction pattern obtained from the X-ray diffraction analysis of catalyst particles, the peak position of Pt.sub.3Co that appears in a region of 2θ=40.0° or more and 42.0° or less is 2θ=41.10° or more and 42.00° or less.

10. The catalyst for a solid polymer fuel cell according to claim 2, wherein in the diffraction pattern obtained from the X-ray diffraction analysis of catalyst particles, the ratio (I.sub.o/I.sub.a) of the peak intensity of ZrO.sub.2 (I.sub.o) that appears in a region of 2θ=28.0° or more and 28.4° or less to the peak intensity of Pt.sub.3Co (I.sub.a) that appears in a region of 2θ=40.0° or more and 42.0° or less is 1.3 or less.

11. The catalyst for a solid polymer fuel cell according to claim 3, wherein in the diffraction pattern obtained from the X-ray diffraction analysis of catalyst particles, the ratio (I.sub.o/I.sub.a) of the peak intensity of ZrO.sub.2 (I.sub.o) that appears in a region of 2θ=28.0° or more and 28.4° or less to the peak intensity of Pt.sub.3Co (I.sub.a) that appears in a region of 2θ=40.0° or more and 42.0° or less is 1.3 or less.

12. The catalyst for a solid polymer fuel cell according to claim 2, wherein the catalyst particles are configured such that the concentrations of cobalt and zirconium at the catalyst particle surface are lower than the concentrations of cobalt and zirconium at the catalyst particle core.

13. The catalyst for a solid polymer fuel cell according to claim 3, wherein the catalyst particles are configured such that the concentrations of cobalt and zirconium at the catalyst particle surface are lower than the concentrations of cobalt and zirconium at the catalyst particle core.

14. The catalyst for a solid polymer fuel cell according to claim 4, wherein the catalyst particles are configured such that the-concentrations of cobalt and zirconium at the catalyst particle surface are lower than the concentrations of cobalt and zirconium at the catalyst particle core.

15. A method for producing the catalyst for a solid polymer fuel cell defined in claim 2, comprising: a step of supporting cobalt and zirconium on a platinum catalyst including platinum particles supported on a carbon powder carrier; a step of heat-treating the platinum catalyst having cobalt and zirconium supported thereon in the supporting step at 900° C. or more and 1,200° C. or less; and a step of bringing the heat-treated catalyst into contact with an oxidizing solution at least once to elute at least part of the supported cobalt and zirconium.

16. A method for producing the catalyst for a solid polymer fuel cell defined in claim 3, comprising: a step of supporting cobalt and zirconium on a platinum catalyst including platinum particles supported on a carbon powder carrier; a step of heat-treating the platinum catalyst having cobalt and zirconium supported thereon in the supporting step at 900° C. or more and 1,200° C. or less; and a step of bringing the heat-treated catalyst into contact with an oxidizing solution at least once to elute at least part of the supported cobalt and zirconium.

17. The method for producing a catalyst for a solid polymer fuel cell according to claim 7, wherein the contact treatment with the oxidizing solution is such that the treatment temperature is 40° C. or more and 90° C. or less, and the contact time is 1 hour or more and 10 hours or less.

18. The method for producing a catalyst for a solid polymer fuel cell according to claim 6, wherein the particulate catalyst metal is formed by being subjected to reduction and precipitation from a metal salt solution of platinum, cobalt, and zirconium and then subjected to alloying by heat treatment.

19. The catalyst for solid polymer fuel cell according to claim 1, wherein the carbon powder carrier is a carbon powder having a specific surface area of 50 to 1,200 m.sup.2/g.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram showing the X-ray diffraction pattern of a Pt—Co—Zr alloy catalyst produced in the second embodiment.

(2) FIG. 2 is a diagram showing the X-ray diffraction pattern of a Pt—Co—Zr alloy catalyst produced in the third embodiment.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(3) Here, on a Pt catalyst, Co was used as a second metal, and further Zr or the like was used as a third metal (M). The metals were supported and alloyed to give a ternary catalyst (Pt—Co-M catalyst). Then, the activity of each catalyst was evaluated to check suitable added metals. The basic steps of catalyst production are as follows.

(4) [Supporting of Platinum Catalyst]

(5) First, a platinum catalyst to serve as a precursor was produced. A dinitrodiammine platinum nitric acid solution and pure water were placed in a grinding container, and a carbon fine powder (specific surface area: 900 m.sup.2/g) to serve a carrier was added to the container with grinding. Subsequently, a denatured alcohol (95% methanol+5% ethanol) was added as a reducing agent, and the mixed solution was allowed to react under reflux at about 95° C. for 6 hours to reduce platinum, followed by filtration, drying (125° C., 15 hours), and washing, thereby giving a platinum catalyst. This platinum catalyst has a platinum supporting of 46.5 mass %.

(6) [Supporting of Co and Metal M]

(7) Cobalt and a third metal M were supported on the platinum catalyst prepared as discussed above. A metal salt solution was produced by dissolving cobalt chloride and a chloride or sulfate of the metal M in 100 mL of ion exchange water, and 10 g of the above platinum catalyst was immersed in the solution and stirred with a magnetic stirrer. Then, 400 mL of a sodium borohydride (SBH) solution having a concentration of 1 mass % was added dropwise to this solution to perform a reduction treatment, thereby supporting cobalt and the metal M on the platinum catalyst, followed by filtration, washing, and drying. In this embodiment, the supporting ratio of the metals was Pt:Co:M=3:2:1.

(8) [Heat Treatment]

(9) The catalyst having supported thereon the catalytic metal composed of Pt, Co, and the metal M was heat-treated for alloying. In this embodiment, the heat treatment was performed in 100% hydrogen gas for 30 minutes at a heat treatment temperature of 900° C.

(10) [Acidic Solution Treatment]

(11) The heat-treated catalyst was subjected to an acidic solution treatment. In the acidic solution treatment, first, the heat-treated catalyst was treated in a 0.2 mol/L aqueous sulfuric acid solution at 80° C. for 2 hours, followed by filtration, washing, and drying. Then, the catalyst was treated in a 1.0 mol/L aqueous nitric acid solution at 70° C. for 2 hours, followed by filtration, washing, and drying. Further, the catalyst was treated again in an aqueous nitric acid solution and washed, thereby giving a Pt—Co-M ternary catalyst. Incidentally, in this embodiment, a Pt—Co catalyst, in which only cobalt was added to the platinum catalyst produced above and alloyed, was also produced (supported at Pt:Co=3:2).

(12) Then, each of the Pt—Co-M ternary catalysts produced was subjected to a performance test for evaluating the initial activity after production and the activity after accelerated aging (durability). In the performance test, a cathode electrode (air electrode) was produced from the catalyst to produce a fuel cell (single cell), and the Mass Activity of the catalyst was measured. Based on the results of measurement, evaluation was performed. In the production of a fuel cell, a membrane/electrode assembly (MEA) containing a proton-conductive polymer electrolyte membrane sandwiched between cathode and anode electrodes each having an electrode area of 5 cm×5 cm=25 cm.sup.2 was produced. As a pretreatment before the activity measurement, a current/voltage curve was drawn under the following conditions: hydrogen flow rate: 1,000 mL/min, oxygen flow rate: 1,000 mL/min, cell temperature: 80° C., anode humidification temperature: 90° C., cathode humidification temperature: 30° C. Subsequently, as the main measurement, the Mass Activity was measured. The test method was as follows. The current value (A) at 0.9 V was measured, and, from the weight of Pt applied onto the electrodes, the current value per 1 g of Pt (A/g-Pt) was determined to calculate the Mass Activity.

(13) After the initial activity was measured as above, the catalyst was subjected to a durability test to evaluate the subsequent activity resistance. As the durability test, an accelerated deterioration test, in which the cell potential of the cathode of the produced fuel cell was swept with a triangular wave, and the activity (Mass Activity) after deterioration was measured. In the accelerated deterioration test, the catalytic metal surface was cleaned by sweeping between 200 to 650 mV at a sweep rate of 40 mV/s for 20 hours, followed by sweeping between 200 to 650 mV at a sweep rate of 100 mV/s for 24 hours to cause deterioration. Then, the Mass Activity of the catalyst after deterioration was measured in the same manner as above.

(14) All the Pt—Co-M ternary catalysts, Pt—Co catalyst, and platinum catalyst produced in this embodiment were evaluated for the initial activity and the activity after a durability test. Then, based on the initial activity (Mass Activity) value of the platinum catalyst as 100, the activity of each catalyst was evaluated. The results are shown in Table 1.

(15) TABLE-US-00001 TABLE 1 Mass Activity (A/g-pt at 0.9 V)*.sup.1 Catalyst Added Initial After Durability Test Composition Metal (M) Activity 20 h 44 h 66 h*.sup.2 Present Pt—Co—M Zr 181 110 99 88 Invention (Example 1) Comparative Ti 171 87 46 — Examples Hf 170 106 46 — Cr 134 73 47 — Mo 92 37 — — W 149 56 37 — Ag 82 27 20 — Au 24 19 — — Ru 79 72 65 57 La 76 44 — — Ce 103 33 — — Conventional Mn 151 88 57 47 Examples Pt—Co — 130 81 70 49 Pt — 100 65 55 47 *.sup.1Values relative to the initial activity value of the Pt catalyst as 100 *.sup.2Measured when “50” is exceeded in 44 h

(16) From Table 1, with reference only to the initial activity, it can be said that the addition of metals of Group 4 elements, such as Ti and Hf, is also effective in addition to Zr. However, in the case of Ti and Hf, the activity decreases after a durability test (44-hours durability). It can be seen that considering durability, the addition of Zr is preferable. In this embodiment, other transition metals such as Cr and Mo, precious metals such as Ag and Au, rare earth elements such as La and Ce were also evaluated. However, most of them were inferior to the conventional platinum catalyst or the Pt—Co catalyst. From the above test results, it was confirmed that a Pt—Co—Zr catalyst obtained by adding Zr to a Pt—Co catalyst exerts better initial activity and durability over conventional catalysts.

Second Embodiment

(17) In this embodiment, the composition ratio of the catalytic metal of a Pt—Co—Zr catalyst was changed, and the resulting characteristics were evaluated. Using the same platinum catalyst as in the first embodiment as a precursor, cobalt chloride and zirconium sulfate were adsorbed. Here, the molar ratio of the metals was set as follows: Pt:Co:Zr=3:2:0.5 (Example 2), Pt:Co:Zr=3:2:1 (Example 3), Pt:Co:Zr=3:2:3 (Example 4). Then, a heat treatment heat was performed at a treatment temperature set at 1,050° C., followed by an acidic solution treatment in the same manner as in the first embodiment, thereby producing a Pt—Co—Zr catalyst.

(18) Incidentally, in the present invention, after the catalytic metal is supported and heat-treated, Co and Zr are partially dissolved by an acidic solution treatment. Therefore, the metal contents after catalyst production are different from the ratio in the supporting stage (preparation stage). Thus, with respect to each Pt—Co—Zr catalyst produced in this embodiment, the contents of the constituent metals and their molar ratio were measured. In this measurement, the catalyst was subjected to an ICP analysis to measure the content (mass %) of each metal, and the composition molar ratio was calculated from the results.

(19) Then, the Pt—Co—Zr catalysts produced in this embodiment were evaluated for the initial activity and the activity after a durability test. The contents of the evaluation test are the same as in the first embodiment. The results are shown in Table 2.

(20) TABLE-US-00002 TABLE 2 Mass Activity (A/g-pt at 0.9 V)*.sup.1 After Catalyst Catalytic metal Initial Durability Test Composition Molar Ratio Activity 44 h 66 h Example 2 3.0:1.1:0.3 149 115 100 Example 3 Pt—Co—Zr 3.0:1.1:0.8 188 112 104 Example 4 3.0:1.0:2.7 158 84 65 Conventional Pt—Co 3.0:1.0 130 70 49 Examples Pt — 100 55 47 *.sup.1Values relative to the initial activity value of the Pt catalyst as 100

(21) In the Pt—Co—Zr catalysts of Example 2 to Example 4, depending on the supporting amount of the catalytic metal at the time of preparation, the ratio of the metals (Pt:Co:Zr) is different. It was confirmed that in all the catalysts, excellent characteristics in terms of both initial activity and durability are exerted over the Pt catalyst and the Pt—Co catalyst.

(22) Next, the three kinds of Pt—Co—Zr catalysts produced in the second embodiment were subjected to an XRD analysis to examine the phase composition. In this analysis, an X-ray diffractometer JDX-8030 manufactured by JEOL was used. The sample was formed into a fine powder and placed in a glass cell, and analyzed using Cu (ka ray) as an X-ray source under the following conditions: tube voltage: 40 kV, tube current: 30 mA, up to 2θ=20 to 90°, scan rate: 7°/min, step angle: 0.1°.

(23) FIG. 1 shows the X-ray diffraction pattern of each catalyst. In this XRD profile, the peak position of Pt.sub.3Co and the ratio (I.sub.o/I.sub.a) of the peak intensity of ZrO.sub.2 (I.sub.o) to the peak intensity of Pt.sub.3Co (I.sub.a) was calculated. The results were as follows.

(24) TABLE-US-00003 TABLE 3 XRD Io/Ia Peak Catalytic metal PtCo.sub.3 Peak Intensity Composition Molar Ratio Position Ratio Example 2 Pt—Co—Zr 3.0:1.1:0.3 41.18° 0 Example 3 3.0:1.1:0.8 41.42° 0 Example 4 3.0:1.0:2.7 41.70° 1.28 Conventional Pt—Co 3.0:1.0 40.80° — Example

(25) From FIG. 1 and Table 3, it can be seen that when Zr is alloyed with a Pt—Co catalyst, the peak position of Pt.sub.3Co shifts to the higher-angle side. The shift amount of the peak position of Pt.sub.3Co tends to increase with an increase in the proportion of Zr. In this embodiment of the present application, it was confirmed that Examples 2, 3, and 4 all have excellent catalytic activity and durability. From this, it is estimated that the suitable range of the peak position of Pt.sub.3Co is a region of 2θ=41.10° or more and 42.00° or less.

(26) In addition, with respect to the ZrO.sub.2 phase formation, it can be seen that the peak of the ZrO.sub.2 phase is clearly developed as a result of an increase in the Zr proportion. Then, from the catalytic activity of Example 4, it is considered that as the peak intensity of the ZrO.sub.2 phase, the ratio (I.sub.o/I.sub.a) to the peak intensity of Pt.sub.3Co (la) that appears in a region of 2θ=40.0° or more and 42.0° or less is preferably 1.3 or less.

Third Embodiment

(27) Here, with respect to Pt—Co—Zr catalysts, the range of the heat treatment temperature after catalytic metal supporting was examined. In the same manner as in the first embodiment, Co and Zr were supported on a platinum catalyst (the ratio in the preparation stage: 3:2:1) and then heat-treated at three temperatures: 900° C. (Example 1), 1,050° C. (Example 3), 1,200° C. (Example 5). Then, the catalysts after the respective heat treatments were subjected to a heat treatment and an acidic solution treatment in the same manner as in the first embodiment, thereby producing Pt—Co—Zr catalysts. For the three kinds of Pt—Co—Zr catalysts produced, the composition ratio of the catalytic metal was measured, and then the initial activity and the activity after a durability test were evaluated. The contents of the evaluation test are the same as in the first embodiment. The results are shown in Table 4.

(28) TABLE-US-00004 TABLE 4 Mass Activity (A/g-pt at 0.9 V)*.sup.1 Heat Catalytic After Treatment metal Durability Tem- Molar Initial Test Composition perature Ratio Activity 44 h 66 h Example 1 Pt—Co—Zr  900° C. 3.0:1.2:0.9 181 99 88 Example 3 1050° C. 3.0:1.1:0.8 188 112 104 Example 5 1200° C. 3.0:0.9:0.7 174 83 67 Con- Pt—Co  900° C. 3.0:1.0 130 70 49 ventional Pt — — 100 55 47 Examples *.sup.1Values relative to the initial activity value of the Pt catalyst as 100

(29) From the test results, the catalysts heat-treated at 900° C. or more and 1,200° C. or less all exert excellent initial activity and durability over the conventional Pt—Co catalyst. Then, it turned out that a heat treatment temperature of 1,050° C. is optimal in terms of durability.

(30) Then, the three kinds of Pt—Co—Zr catalysts produced in the third embodiment were subjected to an XRD analysis under the same conditions as in the second embodiment to examine the phase composition. FIG. 2 shows the X-ray diffraction pattern of each catalyst. In this XRD profile, the peak position of Pt.sub.3Co was calculated. The results were as follows.

(31) TABLE-US-00005 TABLE 5 Heat XRD Treatment Catalytic metal PtCo.sub.3 Peak Composition Temperature Molar Ratio Position Example 1 Pt—Co—Zr  900° C. 3.0:1.2:0.9 41.20° Example 3 1050° C. 3.0:1.1:0.8 41.42° Example 5 1200° C. 3.0:0.9:0.7 41.70° Conventional Pt—Co  900° C. 3.0:1.0 40.80° Example

(32) When Zr is alloyed with a Pt—Co catalyst, the peak position of Pt.sub.3Co shifts to the higher-angle side. However, from FIG. 2 and the results shown in Table 5, it can be seen that the shift amount of the peak position of Pt.sub.3Co tends to increase with an increase in the heat treatment temperature. In this embodiment, it was confirmed that Examples 1, 3, and 5 all have excellent catalytic activity and durability. From the results of this embodiment, it can be seen that the suitable range of the peak position of Pt.sub.3Co is a region of 2θ=41.10° or more and 42.00° or less.

INDUSTRIAL APPLICABILITY

(33) The present invention, as an electrode catalyst of a solid polymer fuel cell, is capable of achieving both improvement in durability and improvement in initial power generation characteristics. The present invention contributes to the spread of fuel cells, and eventually forms the basis for the solution to environmental problems.