CATALYST FOR SOLID POLYMER FUEL CELL AND METHOD FOR SELECTING CATALYST FOR SOLID POLYMER FUEL CELL

Abstract

The present invention relates to a catalyst for solid polymer fuel cells in which catalyst particles containing Pt as an essential catalyst metal are supported on a carbon powder carrier. The catalyst has good initial activity and good durability. When the catalyst is analyzed by X-ray photoelectron spectroscopy after potential holding at 1.2 V (vs. RHE) for 10 minutes in a perchloric acid solution, a ratio of zero-valent Pt to total Pt is 75% or more and 95% or less. The present inventive catalyst metal is preferably one obtained by alloying Pt with one of Co, Ni and Fe, and further with one of Mn, Ti, Zr and Sn. In addition, it is preferable that a fluorine compound having a C—F bond is supported on at least the surfaces of catalyst particles in an amount of 3 to 20 mass % based on the total mass of the catalyst.

Claims

1. A catalyst for solid polymer fuel cells in which catalyst particles containing Pt as an essential catalyst metal are supported on a carbon powder carrier, wherein when the catalyst for solid polymer fuel cells is analyzed by X-ray photoelectron spectroscopy after potential holding at 1.2 V (vs. RHE) for 10 minutes in a perchloric acid solution, a ratio of zero-valent Pt to total Pt as measured by the analysis is 75% or more and 95% or less.

2. The catalyst for solid polymer fuel cells according to claim 1, wherein the ratio of tetravalent Pt to total Pt is 1.5% or less as measured by the analysis.

3. The catalyst for solid polymer fuel cells according to claim 1, wherein the catalyst particles comprise Pt and metal M1 as catalyst metals, and the metal M1 is one of Co, Ni and Fe.

4. The catalyst for solid polymer fuel cells according to claim 3, wherein the catalyst particles comprise Pt, metal M1 and metal M2 as catalyst metals, and the metal M2 is one of Ni, Fe, Mn, Ti, Zr and Sn.

5. The catalyst for solid polymer fuel cells according to claim 1, wherein a fluorine compound having a C—F bond is supported on at least surfaces of the catalyst particles, and a supporting amount of the fluorine compound is 3 to 20 mass % based on a total mass of the catalyst.

6. The catalyst for solid polymer fuel cells according to claim 5, wherein the fluorine compound is a fluororesin or a fluorine-based surfactant.

7. The catalyst for solid polymer fuel cells according to claim 1, wherein a supporting density of the catalyst particles is 30 to 70%.

8. A method for selecting a catalyst which is manufactured by any method and which is used for an electrode of a solid polymer fuel cell, comprising the steps of: subjecting the catalyst to potential holding treatment for 10 minutes at 1.2 V (vs. RHE) in a perchloric acid solution; analyzing the catalyst after the potential holding treatment by X-ray photoelectron spectroscopy to measure a Pt spectrum of surfaces of catalyst particles of the catalyst; and calculating a ratio of zero-valent Pt to total Pt by the analysis, and it is determined that the catalyst is suitable for use when the ratio is 75% or more.

9. The catalyst for solid polymer fuel cells according to claim 2, wherein the catalyst particles comprise Pt and metal M1 as catalyst metals, and the metal M1 is one of Co, Ni and Fe.

10. The catalyst for solid polymer fuel cells according to claim 2, wherein a fluorine compound having a C—F bond is supported on at least surfaces of the catalyst particles, and a supporting amount of the fluorine compound is 3 to 20 mass % based on a total mass of the catalyst.

11. The catalyst for solid polymer fuel cells according to claim 3, wherein a fluorine compound having a C—F bond is supported on at least surfaces of the catalyst particles, and a supporting amount of the fluorine compound is 3 to 20 mass % based on a total mass of the catalyst.

12. The catalyst for solid polymer fuel cells according to claim 4, wherein a fluorine compound having a C—F bond is supported on at least surfaces of the catalyst particles, and a supporting amount of the fluorine compound is 3 to 20 mass % based on a total mass of the catalyst.

13. The catalyst for solid polymer fuel cells according to claim 2, wherein a supporting density of the catalyst particles is 30 to 70%.

14. The catalyst for solid polymer fuel cells according to claim 3, wherein a supporting density of the catalyst particles is 30 to 70%.

15. The catalyst for solid polymer fuel cells according to claim 4, wherein a supporting density of the catalyst particles is 30 to 70%.

16. The catalyst for solid polymer fuel cells according to claim 5, wherein a supporting density of the catalyst particles is 30 to 70%.

17. The catalyst for solid polymer fuel cells according to claim 6, wherein a supporting density of the catalyst particles is 30 to 70%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG. 1 is a diagram illustrating a configuration of a compounding apparatus for electrolysis treatment/XPS analysis, which is used in an embodiment.

DESCRIPTION OF EMBODIMENTS

[0066] Hereinafter, a preferred embodiment of the present invention will be described. In this embodiment, three catalysts: a Pt catalyst, a Pt—Co alloy catalyst and a Pt—Co—Mn alloy catalyst were manufactured as catalysts for solid polymer fuel cells, and dynamic characteristics and catalytic properties were measured and evaluated.

Example 1 (Pt Catalyst)

[Manufacturing of Pt Catalyst]

[0067] 996.42 mL of a dinitrodiammine Pt nitric acid solution (Pt content: 50.00 g) and 3793 mL of pure water were put into a manufacturing vessel. 50.00 g of carbon fine powder (specific surface area: 800 m.sup.2/g, trade name: KB) to be used as a carrier was added while being ground. Thereafter, 540 mL (10.8 vol %) of a denatured alcohol (95% ethanol+5% methanol) as a reducing agent was added and mixed. The mixed solution was refluxed and reacted at about 95° C. for 6 hours to reduce the Pt. Thereafter, filtration, drying (60° C. for 15 hours) and washing were performed.

[Heat Treatment]

[0068] The Pt catalyst was subjected to heat treatment. The heat treatment was performed in a 100% hydrogen gas at a heat treatment temperature of 1050° C. for 2 hours. A Pt catalyst as Example 1 was obtained by the heat treatment. The supporting density of the platinum catalyst was 52%. The average particle size of catalyst particles was 4.2 nm.

Example 2 (Pt—Co Alloy Catalyst)

[0069] A Pt—Co alloy catalyst was manufactured by having Co supported on the Pt catalyst as a precursor before heat treatment, which had been obtained in the step of manufacturing a Pt catalyst in Example 1, to alloy the Pt catalyst into an alloy.

[Supporting of Co]

[0070] The Pt catalyst as a precursor was immersed in a metal salt solution obtained by dissolving 1.6 g of cobalt chloride (CoCl.sub.2.6H.sub.2O) in 100 mL of ion-exchange water, and was stirred with a magnetic stirrer. To this solution was added dropwise 500 mL of a sodium borohydride (SBH) solution having a concentration of 1% by mass, the mixture was stirred, and subjected to reduction treatment, and Co was supported on the Pt catalyst. Thereafter, filtration, washing and drying were performed.

[Alloy Formation Heat Treatment]

[0071] The Pt catalyst in which Co was supported was subjected to heat treatment for alloy formation. This heat treatment was performed in 100% hydrogen gas at a heat treatment temperature of 1000° C. for 30 minutes.

[Treatment with Oxidizing Solution]

[0072] The catalyst after the heat treatment was treated with an oxidizing solution. In this treatment, the catalyst after heat treatment was immersed in a 0.2 mol/L sulfuric acid aqueous solution at 80° C. for 2 hours, and then filtered, washed and dried. Thereafter, the catalyst was immersed in a 1.0 mol/L nitric acid aqueous solution (dissolved oxygen amount: 0.01 cm.sup.3/cm.sup.3 (in terms of STP) at 70° C. for 2 hours, and then filtered, washed and dried. A Pt—Co alloy catalyst was obtained by the above steps (loading ratio of catalyst metal: 50%).

[0073] The Pt—Co catalyst of Example 2 was subjected to composition analysis based on weight analysis and fluorescent X-ray analysis. In the weight analysis, 50 mg of the catalyst was weighed, and heated in air to burn and remove the carbon carrier, the remaining Pt metal component and Co metal component were reduced with hydrogen, and the weights of the reduced products were then measured to calculate the content of metal components in the catalyst. In the fluorescent X-ray analysis, the amount of Co (mass %) in the Pt—Co catalyst was analyzed. Thus, the amount of Co (mass %) determined in the fluorescent X-ray analysis was subtracted from the PtCo metal component amount obtained in the weight analysis to calculate the Pt amount (mass %). The result showed that the composition of catalyst particles of Example 2 was Pt:Co=about 1:0.33. The average particle size of catalyst particles was 4.5 nm.

Example 3 (Pt—Co—Mn Alloy Catalyst)

[0074] A Pt—Co—Mn alloy catalyst was manufactured by supporting Co and Mn on the Pt catalyst precursor of Example 1 to form an alloy, and then treating the alloy with a fluorine compound to form a water-repellent layer.

[Supporting of Co and Mn]

[0075] The Pt catalyst as a precursor was immersed in a metal salt solution obtained by dissolving 1.6 g of cobalt chloride (CoCl.sub.2.6H.sub.2O) and 0.8 g of manganese chloride (MnCl.sub.2.4H.sub.2O) in 100 mL of ion-exchange water, and was stirred with a magnetic stirrer. To this solution was added dropwise 500 mL of a sodium borohydride (SBH) solution having a concentration of 1% by mass, the mixture was stirred, and subjected to reduction treatment, and Co and Mn were supported on the Pt catalyst. Thereafter, filtration, washing and drying were performed.

[0076] The Pt catalyst in which Co and Mn were supported was subjected to heat treatment for alloy formation under the same conditions as in Example 2. Further, oxidizing solution treatment was performed under the same conditions as in Example 2.

[Formation of Water-Repellent Layer]

[0077] The Pt—Co—Mn ternary catalyst manufactured as described above was treated with a fluoro-compound solution to form a water-repellent layer. In this embodiment, a commercially available fluororesin material (trade name: EGC-1700 manufactured by Sumitomo 3M Limited, fluororesin content: 1 to 3%) was used as the fluorine compound. As a solvent, hydrofluoroether as a commercially available solvent (trade name: Novec7100 manufactured by Sumitomo 3M Limited).

[0078] In water repellency imparting treatment, first, 5 g of the catalyst was immersed in 100 mL of the solvent, and the resulting dispersion liquid was stirred at room temperature for 1 hour. A fluoro-compound solution obtained by dissolving 20 mL of the fluorine compound in 200 mL of a solvent was added dropwise to the dispersion liquid after stirring. After the fluoro-compound solution was added dropwise, the mixed solution was heated to 60° C., and stirred at this temperature for 1 hour. Thereafter, the solution was held at 60° C. in a dryer to evaporate the solvent completely. Through this treatment, a catalyst which had a water-repellent layer with a fluorine compound supported on the catalyst was manufactured. The supporting amount of the fluorine compound in the catalyst was 8.6 mass % based on the total mass of the catalyst.

[0079] The Pt—Co—Mn ternary catalyst of Example 3 was subjected to composition analysis in the same manner as in Example 2, and the result showed that the composition of the Pt alloy was Pt:Co:Mn=1:0.33:0.07. The average particle size of catalyst particles was 3.3 nm.

Comparative Example 1 (Pt Catalyst)

[0080] Here, a Pt catalyst to be compared with Example 1 was manufactured. In Example 1, a carbon fine powder carrier was introduced into a dinitrodiammine platinum nitric acid solution, and the mixture was stirred to prepare a slurry without pulverizing treatment. Reduction treatment was performed in the same manner as in Example 1 to form a platinum catalyst without performing heat treatment. The supporting density in the Pt catalyst was 50%, and the average particle size of the catalyst particles was 2.5 nm.

Comparative Example 2 (Pt—Co—Mn Alloy Catalyst)

[0081] Next, a Pt—Co—Mn ternary catalyst to be compared with Example 3 was manufactured. In the same manner as in Example 3, Co and Mn were supported on the Pt catalyst, heat treatment and oxidizing solution treatment were performed, and water repellency imparting treatment was performed.

[0082] In the water repellency imparting treatment in Comparative Example 2, a fluoro-compound solution was prepared from 20 mL of the same fluorine compound as in Example 3 and 200 mL of a solvent, and 5 g of the catalyst was immersed in this solution, immediately heated to 60° C., and stirred for 1 hour. Thereafter, the solvent was removed at 60° C. in a dryer to manufacture a catalyst. The composition of the Pt alloy of the catalyst of Comparative Example 2 was Pt:Co:Mn=1:0.33:0.07. The average particle size of catalyst particles was 3.3 nm.

[Evaluation of Dynamic Characteristics (Potential Holding Treatment—XPS Analysis)]

[0083] Physical properties for the state of Pt on the surfaces of catalyst particles after potential holding treatment were evaluated for the catalysts of Examples 1 to 3 and Comparative Examples 1 and 2. The potential holding treatment and XPS analysis were performed with a compounding apparatus as shown in FIG. 1. The compounding apparatus of FIG. 1 has a structure in which a potential holding treatment chamber for the catalyst sample is connected to an XPS analysis chamber through an intermediate chamber. First, the catalyst sample adjusted in advance is subjected to potential holding treatment in a potential holding treatment chamber. In the potential holding chamber, the catalyst sample is held at a predetermined potential while a degassed 0.1 M perchloric acid solution is supplied to the surface of the catalyst sample. The catalyst sample after potential holding treatment is transferred into a vacuum analysis chamber through an evacuated intermediate chamber, and analyzed with an XPS analysis apparatus.

[0084] In this embodiment, potential holding treatment was performed with a 0.1 M perchloric acid solution. The catalyst sample was subjected to analysis after potential holding with the potential set to 1.2 V (vs. RHE) (counter electrode:platinum electrode) and the potential holding time set to 10 minutes. The XPS analysis was performed with a monochromatic Al-Kα ray (1486.6 eV) as an X-ray source and a power of 300 W over a measurement range of 2 mm×0.8 mm. In this analysis, generated photoelectric energy was detected to acquire a wide-area photoelectron spectrum (wide spectrum).

[0085] For calculating the ratio of zero-valent Pt on the surfaces of catalyst particles after potential holding, the data of the Pt4f spectrum obtained by XPS was analyzed by use of software (MultiPak) manufactured by ULVAC-PHI, Inc. In this analysis, “Pt” was associated with three chemical states (zero-valent Pt (0), divalent Pt (II) and tetravalent Pt (IV)). The main peak positions for the states were set at 71.6 eV for zero-valent Pt (0), 74.0 eV for divalent Pt (II) and 75.2 eV for tetravalent Pt (IV), and separation of peaks in the Pt4f spectrum measured by the software was performed. After the separation of peaks was performed, the ratio of each Pt was calculated from the area ratio of the peak for each state.

[0086] Next, catalyst properties were evaluated for each catalyst. In this embodiment, initial activity was measured, and the activity of the catalyst degraded by a potential cycle test was measured to evaluate durability.

[Initial Activity Test]

[0087] The catalysts of examples and the comparative example were subjected to an initial activity test. This performance test was conducted by measuring the mass activity. In the experiment, a single cell was used, and a membrane electrode assembly (MEA) obtained by sandwiching a proton conductive polymer electrolyte membrane between cathode and anode electrodes having an electrode area of 25 cm.sup.2 (5 cm×5 cm) was prepared, and evaluated (set utilization efficiency: 40%). As pretreatment, a current-voltage curve was prepared under the conditions of a hydrogen flow rate of 1000 mL/min, an oxygen flow rate of 1000 mL/min, a cell temperature of 80° C., an anode humidified temperature of 90° C. and a cathode humidified temperature of 30° C. Thereafter, the mass activity was measured as main measurement. In the test method, a current value (A) was measured at 0.9 V, a current value (Ng-Pt) per 1 g of Pt was determined from the weight of Pt applied onto an electrode, and the mass activity was calculated.

[Durability Test]

[0088] Further, each catalyst was subjected to a durability test (degradation test) for evaluating durability. The durability test was conducted by subjecting the membrane electrode assembly (MEA) after the initial activity test to a potential cycle test. In the potential cycle test, sweeping was performed between 650 mV and 1050 mV at a sweeping speed of 40 mV/s for 20 hours (3600 cycles) to pretreat the catalyst. Thereafter, the catalyst was subjected to main treatment in which sweeping was performed between 650 mV and 1050 mV at a sweeping speed of 100 mV/s. This main treatment was performed for 24 hours (10800 cycles), and sweeping was further performed for 24 hours (21600 cycles) to degrade the catalyst. For the degraded catalyst (after 21600 cycles), mass activity was measured.

[0089] Table 1 shows the results of the property evaluation, the initial activity test and the durability test.

TABLE-US-00001 TABLE 1 Ratio of each Pt after potential holding Mass Activity (A/g-Pt at 0.9 V) Water- at 1.2 V (%) After Catalyst repellent Zero-valent PtO.sub.ad Divalent Pt Tetravalent Initial durability Maintenance particles layer Pt (Pt.sup.0) PtOH.sub.ad (Pt.sup.2+) Pt (Pt.sup.4+) activity*.sup.1 test*.sup.1 ratio*.sup.2 Example 1 Pt — 85.2 8.5 3.3 3.0 1.00 0.60 60.0% Example 2 Pt-Co — 85.0 8.4 5.3 1.3 1.65 0.80 48.5% Example 3 Pt-Co-Mn Present 81.1 12.0 6.0 0.9 1.95 1.10 56.4% Comparative Pt — 67.0 13.0 12.0 8.0 0.95 0.40 42.1% Example 1 Comparative Pt-Co-Mn Present 74.3 14.3 9.9 1.5 1.90 0.50 26.3% Example 2 *.sup.1Relative value against initial activity value in Example 1 which is defined as “1.0” *.sup.2(activity after durability test)/(initial activity)

[0090] The effect of defining the state of the surfaces of catalyst particles (ratio of zero-valent Pt) after potential holding treatment as examined in this embodiment can be determined by comparison of catalysts having basically the same composition. In this respect, from comparison between Example 1 and Comparative Example 1 each using a Pt catalyst and comparison between Example 3 and Comparative Example 2 each using a Pt—Co—Mn ternary catalyst, it can be confirmed that a catalyst in which the ratio of zero-valent Pt after potential holding treatment is 75% or more has high activity maintenance ratio after the durability test, leading to improvement of durability. Comparison between Example 3 and Comparative Example 2 indicates that even catalysts of the same composition have different ratios of zero-valent Pt after potential holding treatment depending on whether water repellency imparting treatment is optimized. In addition, the result of comparison of the example to the comparative example indicates that setting the ratio of zero-valent Pt after potential holding treatment to 75% has little initial-activity improving action.

[0091] With regard to initial activity alone, the Pt alloys (Pt—Co of Example 2 and Pt—Co—Mn of Example 3) have higher activity over the Pt catalyst of Example 1. However, the Pt catalyst does not have so high initial activity, but has a relatively high activity maintenance ratio after the durability test even in Comparative Example 1. That is, the Pt catalyst can be considered to intrinsically have high durability.

[0092] The technical significance of definition by the state of the surfaces catalyst particles after electrolysis treatment, which is the main subject of the present invention, is associated with a Pt alloy catalyst, particularly a ternary alloy catalyst such as a Pt—Co—Mn catalyst. The Pt—Co—Mn catalyst of Comparative Example 2 has high initial activity, but has an extremely low activity maintenance ratio after the durability test (26.3%). This means that the Pt—Co—Mn catalyst tends to have low durability. For the Pt—Co—Mn catalyst, the activity maintenance ratio becomes 56.4% or more when the ratio of zero-valent Pt after potential holding treatment is increased (Example 3). The catalyst of Example 3 has the highest initial activity and the highest activity after the durability test. Thus, it is apparent that both initial activity and durability can be made suitable by setting the composition of catalyst particles and the water-repellent layer, and optimizing the surface state hardly recognizable directly from these conditions.

INDUSTRIAL APPLICABILITY

[0093] The present invention enables the durability of an electrode catalyst for solid polymer fuel cells to be improved while maintaining good initial activity. The present invention contributes to popularization of fuel cells, and hence provides a foundation for environmental problem solution.