Catalyst for solid polymer fuel cells and method for producing the same

Abstract

The present invention relates to a catalyst for a solid polymer fuel cell that includes catalyst particles supported on a carbon powder carrier, the catalyst particles containing platinum, cobalt, and manganese. In the catalyst particles of the catalyst, the component ratio of platinum, cobalt, and manganese is Pt:Co:Mn=1:0.25 to 0.28:0.07 to 0.10 in a molar ratio, the average particle size is 3.4 to 5.0 nm, and further, in the particle size distribution of the catalyst particles, the proportion of catalyst particles having a particle size of 3.0 nm or less in the entire catalyst particles is 37% or less on a particle number basis. Then, a fluorine compound having a C—F bond is supported at least on the surface of the catalyst particles. The present invention is, with respect to the above ternary alloy catalyst, an invention particularly effective in improving the durability.

Claims

1. A catalyst for a solid polymer fuel cell, comprising catalyst particles supported on a carbon powder carrier, the catalyst particles comprise a catalytic metal containing platinum, cobalt, and manganese, wherein the catalyst particles have a component ratio of platinum, cobalt, and manganese as Pt:Co:Mn=1:0.25 to 0.28:0.07 to 0.10 in a molar ratio, the catalyst particles have an average particle size of 3.4 to 5.0 nm, and further, in a particle size distribution of the catalyst particles, a proportion of catalyst particles having a particle size of 3.0 nm or less in entire catalyst particles is 37% or less on a particle number basis, and a surface of the catalyst particles supports a water-repellent layer made of a fluorine compound having a C—F bond.

2. The catalyst for a solid polymer fuel cell according to claim 1, wherein the fluorine compound constitutes 3 to 20 mass % on a total mass of the catalyst.

3. The catalyst for a solid polymer fuel cell according to claim 1, wherein the fluorine compound is a fluorine resin or a fluorine-based surfactant.

4. The catalyst for a solid polymer fuel cell according to claim 1, wherein a specific surface area of the catalytic metal (S.sub.COMSA) per g of the catalytic metal measured by a CO adsorption method is 130 m.sup.2/g-metal or less.

5. The catalyst for a solid polymer fuel cell according to claim 1, wherein a proportion of 0-valent Pt in Pt present on a catalyst particle surface is 90% or more and 100% or less.

6. The catalyst for a solid polymer fuel cell according to claim 1, wherein when the catalyst particles are subjected to X-ray photoelectron spectrometry to measure valence band spectra in a region of 0 ev or more and 30 ev or less, a d-band center value calculated from a resulting Pt5d orbit-derived spectrum is 4.23 eV or more and 4.30 V or less.

7. The catalyst for a solid polymer fuel cell according to claim 1, wherein when the catalyst particles are subjected to X-ray diffraction analysis, a peak intensity ratio of a Co—Mn alloy that appears near 2θ=27° is 0.25 or less based on a main peak that appears near 2θ=40°.

8. The catalyst for a solid polymer fuel cell according to claim 1, wherein when the catalyst particles are subjected to X-ray diffraction analysis, a ratio of a peak of a CoPt.sub.3 alloy and a peak of a MnPt.sub.3 alloy that appear near 2θ=32° is 0.20 or more based on a main peak that appears near 2θ=40°.

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

10. A method for producing a catalyst for a solid polymer fuel cell according to claim 1, comprising the steps of: supporting cobalt and manganese on a platinum catalyst including platinum particles supported on a carbon powder carrier; heat-treating the platinum catalyst having cobalt and manganese supported thereon in the supporting step at 1,000 to 1,100° C.; and bringing the catalyst after the heat treatment step into contact with a solution containing a fluorine compound and forming a water-repellent layer made of the fluorine compound on surface of the catalyst.

11. The method for producing a catalyst for a solid polymer fuel cell according to claim 10, comprising a step of producing the platinum catalyst, comprising producing a mixed solution by mixing a carbon powder carrier and a platinum compound solution, and adding a reducing agent to the mixed solution to support catalyst particles containing platinum on the carbon powder carrier, and the step of producing a mixed solution includes mixing the platinum compound solution while grinding the carbon powder carrier.

12. The method for producing a catalyst for a solid polymer fuel cell according to claim 10, comprising a step of bringing the heat-treated catalyst into contact with an oxidizing solution at least once to elute cobalt and manganese on the catalyst particle surface.

13. The method for producing a catalyst for a solid polymer fuel cell according to claim 12, wherein the oxidizing solution is sulfuric acid, nitric acid, phosphorous acid, potassium permanganate, hydrogen peroxide, hydrochloric acid, chloric acid, hypochlorous acid, or chromic acid.

14. The catalyst for a solid polymer fuel cell according to claim 2, wherein the fluorine compound is a fluorine resin or a fluorine-based surfactant.

15. The catalyst for a solid polymer fuel cell according to claim 2, wherein a specific surface area of the catalyst metal (S.sub.COMSA) per g of the catalytic metal measured by a CO adsorption method is 130 m.sup.2/g-metal or less.

16. The catalyst for a solid polymer fuel cell according to claim 4, wherein a proportion of 0-valent Pt in Pt present on a catalyst particle surface is 90% or more and 100% or less.

17. The catalyst for a solid polymer fuel cell according to claim 5, wherein when the catalyst particles are subjected to X-ray photoelectron spectrometry to measure valence band spectra in a region of 0 ev or more and 30 ev or less, a d-band center value calculated from a resulting Pt5d orbit-derived spectrum is 4.23 eV or more and 4.30 V or less.

18. The catalyst for a solid polymer fuel cell according to claim 6, wherein when the catalyst particles are subjected to X-ray diffraction analysis, a peak intensity ratio of a Co—Mn alloy that appears near 2θ=27° is 0.25 or less based on a main peak that appears near 2θ=40°.

19. The catalyst for a solid polymer fuel cell according to claim 7, wherein when the catalyst particles are subjected to X-ray diffraction analysis, a ratio of a peak of a CoPt.sub.3 alloy and a peak of a MnPt.sub.3 alloy that appear near 2θ=32° is 0.20 or more based on a main peak that appears near 2θ=40°.

20. The catalyst for a solid polymer fuel cell according to claim 8, wherein a supporting density of the catalyst particles is 30 to 70%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The FIGURE illustrates an X-ray diffraction pattern of each of the catalysts of the present embodiment and Comparative Examples 1 and 2.

DESCRIPTION OF EMBODIMENTS

(2) Hereinafter, preferred embodiments of the present invention will be described.

(3) First Embodiment: In this embodiment, a platinum catalyst was produced. Cobalt and manganese were supported on the platinum catalyst and alloyed, and then treated with a fluorine compound to produce a ternary catalyst.

(4) [Production of Platinum Catalyst]

(5) 603.83 mL of a dinitrodiammine platinum nitric acid solution (platinum content: 30.30 g) and 3,793 mL of pure water were placed in a production vessel. Then, 70.00 g of a fine carbon powder (specific surface area: 800 m.sup.2/g, trade name: KB) serving a carrier was added with grinding. Subsequently, as a reducing agent, 540 mL (10.8 vol %) of denatured alcohol (95% ethanol+5% methanol) was added and mixed. The mixed solution was subjected to a reflux reaction at about 95° C. for 6 hours to reduce platinum, followed by filtration, drying (60° C., 15 hours), and washing. Through the above steps, a platinum catalyst was obtained (platinum supporting: 30 wt %).

(6) [Supporting of Cobalt and Manganese]

(7) Cobalt and manganese were supported on the platinum catalyst produced above. The platinum catalyst was immersed in a metal salt solution prepared by dissolving cobalt chloride (CoCl.sub.2.6H.sub.2O) and manganese chloride (MnCl.sub.2.4H.sub.2O) in 100 mL of ion exchange water, and stirred with a magnetic stirrer. Then, 500 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 manganese on the platinum catalyst, followed by filtration, washing, and drying.

(8) [Alloying Heat Treatment]

(9) The catalyst having supported thereon the catalytic metal 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 1,000° C.

(10) [Treatment with Oxidizing Solution]

(11) The catalyst after the alloying heat treatment was subjected to an oxidizing solution treatment. In this treatment, 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 (dissolved oxygen amount: 0.01 cm.sup.3/cm.sup.3 (in terms of STP)) at 70° C. for 2 hours, followed by filtration, washing, and drying.

(12) [Formation of Water-Repellent Layer]

(13) Then, the produced Pt—Co—Mn ternary catalyst was treated with a fluorine compound solution to form a water-repellent layer. As the fluorine compound solution, 20 mL of a commercially available fluorine resin material (trade name: EGC-1700, manufactured by Sumitomo 3M Limited, fluorine resin content: 1 to 3%) was dissolved in 30 mL of hydrofluoroether (trade name: Novec 7100, manufactured by Sumitomo 3M Limited) as a diluent solvent and used. In this treatment, 5 g of the catalyst was immersed in the above fluorine compound solution, stirred at room temperature for 3 hours, and further stirred at 60° C. for 5 hours. Subsequently, the mixture was maintained at 60° C. in a dryer to cause evaporation until the solvent completely disappeared. As a result of this treatment, a catalyst having a fluorine compound supported thereon and including a water-repellent layer was produced.

(14) Comparative Example 1: The same catalyst as the Pt—Co—Mn ternary catalyst described in Patent Document 1 was produced. In this embodiment, a commercially available platinum catalyst was prepared, and cobalt and manganese were supported in the same manner as in the first embodiment. Then, a heat treatment was performed at a treatment temperature of 900° C. for 30 minutes to produce a Pt—Co—Mn ternary catalyst.

(15) Comparative Example 2: The same catalyst as the Pt—Co—Mn ternary catalyst having a water-repellent layer described in Patent Document 2 was produced. The catalyst produced in Comparative Example 1 was prepared, and, in the same manner as in the first embodiment, the catalyst was treated with a fluorine compound solution to form a water-repellent layer.

(16) Various physical properties of the catalysts of the present embodiment and Comparative Examples 1 and 2 above were evaluated. First, the produced catalysts were subjected to composition analysis to measure the component ratio of platinum, cobalt, and manganese in the catalyst particles. The composition analysis was performed by ICP (high-frequency inductively coupled plasma atomic emission spectrometry). In this analysis by ICP, 20 mg of a catalyst was weighed, calcined, and reduced, and then about 5 ml of aqua regia was added thereto to dissolve the catalyst into a solution. The solution was then diluted about 20-fold and subjected to the analysis.

(17) Next, the average particle size and particle size distribution of catalyst particles of each catalyst were measured. Here, the particle sizes of 300 or more catalyst particles were measured by TEM observation to calculate the particle size distribution. Specifically, each sample was introduced into a TEM device (TEM-STEM device: manufactured by JEOL, JEM-2100F, observation conditions: acceleration voltage: of 200 kV, magnification: ×2500000). Visual fields with sufficient dispersibility were selected, and a plurality of visual fields was photographed in the STEM mode at a constant magnification. Based on the obtained STEM image, the particle size distribution was measured by use of a particle analysis software. At this time, the particle size was determined as a pixel number equivalent-circle diameter. Then, the particle sizes of all the catalyst particles measured were summed up, and the average particle size (D.sub.50) was calculated from the number of measured particles. In addition, from the number of catalyst particles having a particle size of 3 nm or less, their proportion was also calculated.

(18) In addition, the metal specific surface area (S.sub.COMSA) of each catalyst was measured by a CO adsorption method. The metal specific surface area defined herein is a value obtained by calculating a surface area from the CO adsorption amount measured based on the prescribed CO pulse adsorption method, and converting the surface area into a value per gram of metal.

(19) In the measurement of the metal specific surface area (S.sub.COMSA), a metal dispersity measuring device (BEL-METAL-3, manufactured by BEL Japan Inc.) was used. 40.0 mg of a sample was weighed precisely to 0.1 mg and placed in a glass cell. The cell was attached to the measuring device, and automatic measurement was started. As a pretreatment, in a flow of a He gas (50 mL/min), the temperature was raised from room temperature to 100° C. over 20 minutes and maintained for 15 minutes. Subsequently, the gas was changed to H.sub.2 (50 mL/min), and the temperature was maintained at 100° C. for 30 minutes. The gas was then changed to He (50 mL/min), and the temperature was lowered from 100° C. to 30° C., then raised to 40° C., and maintained at 40° C. After the completion of this pretreatment, the amount of CO gas adsorption was measured by the CO pulse adsorption method. From the amount of CO gas adsorption obtained, S.sub.COMSA was determined from the following formula.
S.sub.COMSA (m.sup.2/g)=(26.88×B×σ)/(A×metal content ratio (supporting))  [Equation 2]

(20) A: Weight of the sample (catalyst) placed in the glass cell (g)

(21) B: Amount of CO adsorption (mL)

(22) σ: Adsorption gas molecular cross-sectional area (nm.sup.2/number) (when CO is adopted, 0.163 nm.sup.2/number)

(23) In addition, the catalysts of the first embodiment and Comparative Examples 1 and 2 were subjected to XPS analysis, and the surface platinum state (the proportion of 0-valent platinum) and the d-band center value were evaluated. In the XPS analysis, Quantera SXM manufactured by ULVAC-PHI was used as the analyzer, and the analysis conditions were as follows: X-ray source: monochromatic Al—Kα ray, voltage: 15 kV, output: 25.1 W, beam diameter: 200 μmϕ. In this analysis, the energy of generated photoelectrons was detected, and the wide-range photoelectron spectra (wide spectra) were acquired.

(24) Then, with respect to the Pt4f spectra obtained by XPS, in order to calculate the proportion of 0-valent metal platinum, data analysis was performed by use of a software (MultiPak V8.2C) manufactured by ULVAC-PHI, Inc. In this analysis, as “Pt”, three chemical states (0-valent Pt (0), 2-valent Pt (II), 4-valent Pt (IV)) were assumed. Then, the main peak positions of the respective states were set as 0-valent Pt (0): 71.7 eV, 2-valent Pt (II): 72.7 eV, and 4-valent Pt (IV): 74.4 eV, and the Pt4f spectra measured by the software were subjected to peak separation. After the peak separation, from the area ratio of the peak of each state, the ratio was calculated.

(25) In addition, for the analysis of d-band center values, the valence band spectra were measured by XPS analysis, and, with respect to the peak of the Pt5d orbit-derived spectrum, the background, C- and F-derived components, and the like are subtracted, thereby extracting the d-band. An energy value was then determined from the above calculation formula of Equation 1, and such a value was used in the analysis.

(26) Further, the configuration of the catalyst particles in each catalyst was studied by X-ray diffraction analysis. As the X-ray diffractometer, 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 by use of Cu (kα ray) as an X-ray source under the following conditions: tube voltage: 40 kV, tube current: 30 mA, scan rate: 7°/min up to 2θ=20 to 90°, step angle: 0.1°. In XPS, Al kα ray was applied as the X-ray source, and the analysis was performed under the following conditions: voltage: 15 kV, current: 1.66 mA, beam diameter: 100 μm, measurement range: 250 μm.sup.2.

(27) The FIGURE shows the X-ray diffraction pattern of each of the catalysts of the present embodiment and Comparative Examples 1 and 2. From the FIGURE, the main peak that appears near 2θ=40° observed in all the catalysts is a composite peak of metals Pt, MnPt.sub.3, and CoPt.sub.3. Then, presumably, the peak observed near 2θ=27° is derived from a Co—Mn alloy. Further, the peak near 2θ=32° (32 to 34°) is a composite peak of MnPt.sub.3 and CoPt.sub.3. Based on the FIGURE, regarding each catalyst, the ratio between the peak of the Co—Mn alloy and the main peak and the ratio between the composite peak of MnPt.sub.3 and CoPt.sub.3 and the main peak were measured.

(28) Table 1 shows the composition, configuration (average particle size and particle size distribution), and S.sub.COMSA measurement results of the catalyst particles of the present embodiment and Comparative Examples 1 and 2 obtained above. In addition, Table 2 shows the proportion of 0-valent metal platinum in platinum on the catalyst particle surface and the d-band center value obtained from the XPS analysis, and also the peak ratio obtained from XRD. In these tables, the analysis/measurement results of a platinum catalyst are also shown.

(29) TABLE-US-00001 TABLE 1 Catalyst particle configuration Proportion Average of small- Catalyst particle composition particle size (molar ratio) F size particles S.sub.OOMSA Pt Co Mn (wt %) (nm) (%) (m.sup.2/g-metal) First 1 0.26 0.08 9.7 3.7 29 120 Embodiment Comparative 1 0.19 0.05 — 3.3 40 135 Example 1 Comparative 1 0.22 0.06 9.1 3.2 44 143 Example 2 Platinum 1 — — — — — 200 catalyst

(30) TABLE-US-00002 TABLE 2 Surface Pt state ratio (%) XRD Catalyst particle composition PtO.sub.2, Pt d-Band Peak ratio (molar ratio) F Pt PtO.sub.2 (OH).sub.4 center Peak ratio (MnPt3 + Pt Co Mn (wt %) (Pt.sup.0) (Pt.sup.2+ (Pt.sup.4+) (eV) (CoMn) CoPt3) First 1 0.26 0.08 9.7 90 6 4 4.25 0.21 0.23 Embodiment Comparative 1 0.19 0.05 — 94 4 2 4.28 0.26 0.29 Example 1 Comparative 1 0.22 0.06 9.1 94 4 2 4.24 0.27 0.27 Example 2 Platinum 1 — — — 54 37 9 4.47 — — catalyst
[Initial Activity Test]

(31) The Pt—Co catalysts of the example, comparative examples, and reference example were each subjected to an initial activity test. This performance test was performed by measuring the Mass Activity. In the experiment, a single cell was used. A membrane/electrode assembly (MEA) made of a proton-conducting 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 and evaluated (set utilization efficiency: 40%). As a pretreatment, 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 gram of Pt (A/g-Pt) was determined to calculate the Mass Activity.

(32) [Endurance Test]

(33) Further, in order to evaluate the durability of each catalyst, an endurance test (deterioration test) was performed. In the endurance test, membrane/electrode assemblies (MEA) after the above initial activity test were subjected to an electric potential cycle test. In the electric potential cycle test, sweeping was performed between 650 to 1,050 mV at a sweep rate of 40 mV/s for 20 hours (3,600 cycles) as a pretreatment, and then sweeping was performed between 650 to 1,050 mV at a sweep rate of 100 mV/s (main treatment) for 24 hours (first time: 44 hours in total, 14,400 cycles) and then for 24 hours (second time: 68 hours in total, 25,200 cycles) to deteriorate the catalyst. Then, the Mass Activity of the catalyst that had deteriorated through the cycles of the second time was measured in the same manner as above.

(34) Table 3 shows the results of the above initial activity test and endurance test.

(35) TABLE-US-00003 TABLE 3 Mass Activity (A/g-Pt) Measured value Relative evaluation *.sup.1 Initial After endurance Initial After endurance First Embodiment 69 59 2.2 1.8 Comparative 81 24 2.5 0.8 Example 1 Comparative 90 49 2.8 1.5 Example 2 Platinum catalyst 32 28 1.0 0.9 *.sup.1 A relative value taking the initial activity of the platinum catalyst as 1.0.

(36) It is noted from Table 3 that the catalyst of the present embodiment is slightly inferior to Comparative Examples 1 and 2 in terms of the initial activity, however the drop after the endurance test (after deterioration) is only about 15%. In addition, when evaluated based on the relative value with respect to a platinum catalyst, it can be said that the initial activity initial activity of the catalyst according to the present embodiment is also extremely high. In Comparative Example 1, although the initial activity is excellent, the activity after duration is inferior to that of a platinum catalyst. In addition, in Comparative Example 2, although some improvement in durability can be seen, with reference to the results of the first embodiment, it must be said that there still is room for improvement.

(37) As compared with Comparative Examples 1 and 2, in the catalyst according to the present embodiment, the average particle size is larger, and further the proportion of small-size particles of 3.0 nm or less is lower. Then, in comparison with the initial activity of a platinum catalyst, the catalyst of the present embodiment does not look too bad against Comparative Examples 1 and 2. That is, it can be seen that when the standpoint of durability is considered, the catalyst of the present embodiment is a highly practical catalyst.

(38) Second Embodiment: Here, the treatment conditions for the formation of a water-repellent layer by use of a fluorine compound solution were changed from the catalyst production steps of the first embodiment, and catalysts were thus produced. A ternary catalyst was produced under the same conditions as in the first embodiment, and the amount of fluorine resin in the fluorine compound solution was adjusted to produce a plurality of catalysts. Specifically, in 30 mL of a diluting solvent (trade name: Novec 7100, manufactured by Sumitomo 3M Limited), a fluorine compound solution prepared by dissolving a fluorine resin material (trade name: EGC-1700, manufactured by Sumitomo 3M Limited) in an amount of 2 mL (Example 1), 10 mL (Example 2), 20 mL (Example 3: first embodiment), or 40 mL (Example 4) was used to treat 5 g of a catalyst. Subsequently, the catalytic performance of each catalyst was studied in the same manner as in the first embodiment. The results are shown in Table 4.

(39) TABLE-US-00004 TABLE 4 Catalyst particle configuration Proportion Mass Activity (A/g-Pt) Average of small- Relative Catalyst particle composition particle size Measured value evaluation *.sup.1 (molar ratio) F size particles After After Pt Co Mn (wt %) (nm) (%) Initial endurance Initial endurance Example 1 1 0.27 0.09 6.8 3.8 29 93 46 2.9 1.4 Example 2 1 0.26 0.09 7.5 3.6 30 79 41 2.5 1.3 Example 3 1 0.26 0.08 9.7 3.7 29 69 59 2.2 1.8 Example 4 1 0.27 0.08 12.2 3.9 28 63 47 2.0 1.5 Platinum 1 — — — — — 32 28 1.0 0.9 catalyst *.sup.1 A relative value taking the initial activity of the platinum catalyst as 1.0.

(40) In the catalyst of each example, while the composition of catalyst particles and the amount of fluorine compound are within appropriate ranges, the average particle size of catalyst particles and the proportion of small-size particles are also within suitable ranges. As a result, it can be seen that they have improved initial activity and durability over a platinum catalyst. Incidentally, the catalyst particularly excellent in terms of durability was Example 3.

INDUSTRIAL APPLICABILITY

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