Catalyst for solid polymer fuel cells and method for producing same
11145874 · 2021-10-12
Assignee
Inventors
Cpc classification
H01M4/8878
ELECTRICITY
International classification
Abstract
The present invention relates to a catalyst for solid polymer fuel cells in which catalyst particles including platinum and a transition metal M are supported on a carbon powder carrier. The catalyst of the present invention is a catalyst for solid polymer fuel cells in which a molar ratio (Pt/M) of platinum to the transition metal M that form catalyst particles is 2.5 or more, and a ratio (S.sub.COMSA/S.sub.BET) of a platinum specific surface area (S.sub.COMSA) measured by a CO adsorption method to a catalyst specific surface area (S.sub.BET) measured by a BET method is 0.26 or more and 0.32 or less. The catalyst can be produced by preparing an alloy catalyst, then washing the alloy catalyst with a platinum compound solution, and additionally supplying platinum to the surfaces of catalyst particles.
Claims
1. A catalyst for solid polymer fuel cells in which catalyst particles comprising platinum and a transition metal M are supported on a carbon powder carrier, wherein a molar ratio (Pt/M) of the platinum to the transition metal M in the catalyst particles is 2.5 or more, and a ratio (S.sub.COMSA/S.sub.BET) of a platinum specific surface area (S.sub.COMSA) measured by a CO adsorption method to a catalyst specific surface area (S.sub.BET) measured by a BET method is 0.26 or more and 0.32 or less.
2. The catalyst for solid polymer fuel cells according to claim 1, wherein in X-ray diffraction analysis of catalyst particles, a ratio (I.sub.Pt/I.sub.cat) of a Pt-derived peak intensity (I.sub.Pt) near 2θ=67.4° to a peak intensity (I.sub.cat) of a main peak appearing in a range of 2θ=69° to 71° is 0.35 or less.
3. The catalyst for solid polymer fuel cells according to claim 1, wherein the transition metal M is at least one transition metal selected from cobalt, nickel, manganese, iron, titanium, vanadium, chromium, copper, zinc and zirconium.
4. The catalyst for solid polymer fuel cells according to claim 1, wherein a supporting ratio of catalyst particles to an entire catalyst is 30 to 70% on a mass basis.
5. The catalyst for solid polymer fuel cells according to claim 2, wherein the transition metal M is at least one transition metal selected from cobalt, nickel, manganese, iron, titanium, vanadium, chromium, copper, zinc and zirconium.
6. The catalyst for solid polymer fuel cells according to claim 2, wherein a supporting ratio of catalyst particles to an entire catalyst is 30 to 70% on a mass basis.
7. The catalyst for solid polymer fuel cells according to claim 3, wherein a supporting ratio of catalyst particles to an entire catalyst is 30 to 70% on a mass basis.
8. A method for producing the catalyst for solid polymer fuel cells as set forth in claim 1, comprising the steps of: supporting a transition metal M on a platinum catalyst in which platinum particles are supported on a carbon powder carrier; subjecting the platinum catalyst, on which the transition metal M is supported, to a heat treatment at 700 to 1100° C.; bringing the catalyst after the heat treatment into contact with an oxidizing solution at least once; and bringing a platinum compound solution into contact with the catalyst treated with the oxidizing solution.
9. The method for producing the catalyst for solid polymer fuel cells according to claim 8, wherein the catalyst is brought into contact with at least one oxidizing solution selected from a group including solutions of sulfuric acid, nitric acid, phosphorous acid, potassium permanganate, hydrogen peroxide, hydrochloric acid, chloric acid, hypochlorous acid and chromic acid.
10. The method for producing the catalyst for solid polymer fuel cells according to claim 8, wherein at least one platinum compound solution selected from a group including a platinic chloride solution, a dinitrodianmine platinum nitric acid solution and a potassium chloroplatinate aqueous solution is brought into contact with the catalyst.
11. A method for producing the catalyst for solid polymer fuel cells as set forth in claim 2, comprising the steps of: supporting a transition metal M on a platinum catalyst in which platinum particles are supported on a carbon powder carrier; subjecting the platinum catalyst, on which the transition metal M is supported, to a heat treatment at 700 to 1100° C.; bringing the catalyst after the heat treatment into contact with an oxidizing solution at least once; and bringing a platinum compound solution into contact with the catalyst treated with the oxidizing solution.
12. A method for producing the catalyst for solid polymer fuel cells as set forth in claim 3, comprising the steps of: supporting a transition metal M on a platinum catalyst in which platinum particles are supported on a carbon powder carrier; subjecting the platinum catalyst, on which the transition metal M is supported, to a heat treatment at 700 to 1100° C.; bringing the catalyst after the heat treatment into contact with an oxidizing solution at least once; and bringing a platinum compound solution into contact with the catalyst treated with the oxidizing solution.
13. A method for producing the catalyst for solid polymer fuel cells as set forth in claim 4, comprising the steps of: supporting a transition metal M on a platinum catalyst in which platinum particles are supported on a carbon powder carrier; subjecting the platinum catalyst, on which the transition metal M is supported, to a heat treatment at 700 to 1100° C.; bringing the catalyst after the heat treatment into contact with an oxidizing solution at least once; and bringing a platinum compound solution into contact with the catalyst treated with the oxidizing solution.
14. The method for producing the catalyst for solid polymer fuel cells according to claim 9, wherein at least one platinum compound solution selected from a group including a platinic chloride solution, a dinitrodianmine platinum nitric acid solution and a potassium chloroplatinate aqueous solution is brought into contact with the catalyst.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
DESCRIPTION OF EMBODIMENTS
(3) Hereinafter, a preferred embodiment of the present invention will be described. In the embodiments, a platinum alloy catalyst including cobalt or nickel as a transition metal M was produced, the properties of the catalyst were examined, and the catalytic activity was evaluated.
(4) Example 1: In this example, an alloy catalyst of platinum and cobalt (Pt—Co catalyst) was produced. In this example, a platinum catalyst was produced, cobalt was supported, and alloy formation heat treatment and oxidizing solution treatment were performed to produce a platinum alloy catalyst. Further, platinum compound solution treatment was performed to produce a Pt—Co catalyst of Example 1. A detailed method for producing the catalyst is as follows.
(5) [Supporting Catalyst Metals (Platinum and Transition Metal M)]
(6) For producing a platinum catalyst, carbon fine powder (specific surface area: 850 m.sup.2/g, trade name: OSAB) as a carrier was prepared. 1000 g of a dinitrodianmine platinum nitric acid solution having a platinum concentration of 4.6% by mass (platinum content: 46 g) as a platinum solution and 46 g of carbon fine powder were added into a producing vessel, and mixed while being ground. Thereafter, 540 mL of 100% ethanol was added as a reducing agent, and the mixture was mixed. The mixed solution was refluxed and reacted at about 85° C. for 4 hours to reduce the platinum. Thereafter, filtration, drying and washing were performed. A platinum catalyst was obtained by the above steps. The result of X-ray diffraction showed that the platinum particle size was 2.1 nm.
(7) Cobalt was supported on the platinum catalyst produced as described above. 100 g of cobalt chloride hexahydrate (CoCl.sub.2.6H.sub.2O) was dissolved in 500 mL of water to prepare a metal salt solution as a metal solution, and the platinum catalyst was immersed in and mixed with the solution. To this solution was added dropwise 10 L of a sodium borohydride (SBH) solution having a concentration of 1% by mass, the mixture was stirred, and subjected to reduction treatment, and cobalt was supported on the platinum catalyst. Thereafter, filtration, washing and drying were performed. By the above operations, a catalyst having platinum and cobalt supported on a carbon fine powder carrier was obtained.
(8) [Alloy Formation Heat Treatment]
(9) The catalyst in which a catalyst metal was supported was subjected to heat treatment for alloy formation. In this embodiment, heat treatment was performed in 100% hydrogen gas at a heat treatment temperature of 900° C. for 0.5 hours.
(10) [Oxidizing Solution Treatment]
(11) The catalyst after the heat treatment was treated with an oxidizing solution. First, the catalyst after heat treatment was treated in a 0.5 mol/L sulfuric acid aqueous solution at 80° C. for 2 hours, and then filtered, washed and dried. The catalyst was immersed in a 1.0 mol/L nitric acid aqueous solution at 70° C. for 2 hours, and then filtered, washed and dried. The treatment with the nitric acid aqueous solution was performed twice.
(12) [Platinum Compound Solution Treatment]
(13) The catalyst after the oxidizing solution treatment was treated with a platinum compound solution. As the platinum compound solution, a platinic chloride aqueous solution (H.sub.2PtCl.sub.6) was used. The platinum specific surface area (S.sub.COMSA-PRE) of the catalyst to be treated was measured by a method as described later, and the result showed that the platinum specific surface area (S.sub.COMSA-PRE) was 88.9 (m.sup.2/g-Pt). Further, the platinum content (R.sub.Pt) of the Pt—Co catalyst after the oxidizing solution treatment is calculated to be 0.472. In this example, 10 g of the catalyst after the oxidizing solution treatment was subjected to platinum compound solution treatment. Thus, the amount of platinum in the platinum compound solution was derived in accordance with Formula 2 above, and set to 1.78 g.
(14) In the platinum compound solution treatment, first 11.9 g of platinic chloride (platinum: 1.78 g) was diluted by 2.5 times. On the other hand, 1 L of a 1 M hydrochloric acid aqueous solution was prepared per 10 g of the catalyst, and the catalyst was dispersed in the hydrochloric acid solution to form a slurry. To the catalyst slurry was added dropwise the platinic chloride aqueous solution (10 mL/min). After the dropwise addition, the mixture was stirred for 24 hours, and filtration and washing were repeated three times. Finally, drying was performed at 60° C. for 24 hours. A Pt—Co catalyst was obtained by the above steps.
(15) Example 2: In this example, a Pt—Co catalyst was produced in the same manner as in Example 1 except that the supporting ratio of catalyst particles (platinum alloy) was changed. At the time of supporting the catalyst metal of Example 1, 600 g of a dinitrodianmine platinum nitric acid solution having a platinum concentration of 4.6% by mass (platinum content: 27.6 g) as a platinum solution was supported on 64.4 g of the same carbon fine powder as in Example 1 to produce a platinum catalyst. A metal salt solution obtained by 60 g of cobalt chloride hexahydrate (CoCl.sub.2.6H.sub.2O) was dissolved in 500 mL of water was adsorbed to the platinum catalyst to support cobalt on the platinum catalyst.
(16) As described above, at the time of supporting the catalyst metal (platinum and cobalt), the adsorption amount of the platinum solution and the content of cobalt chloride in the cobalt solution were made smaller than those in Example 1 to decrease the supporting amounts of platinum and cobalt as catalyst metals, so that the supporting ratio of catalyst particles was reduced. Except for these points, the same steps and conditions as in Example 1 were applied to obtain a Pt—Co catalyst.
(17) Example 3: In this example, a Pt—Co catalyst was produced in the same manner as in Example 1 except that the molar ratio (Pt/M) of platinum to the transition metal M (cobalt) was higher than that in Example 1. At the time of supporting the catalyst metal of Example 1, 600 g of a dinitrodianmine platinum nitric acid solution having a platinum concentration of 4.6% by mass (platinum content: 27.6 g) as a platinum solution was supported on 64.4 g of the same carbon fine powder as in Example 1 to produce a platinum catalyst. A metal salt solution obtained by 40 g of cobalt chloride hexahydrate (CoCl.sub.2.6H.sub.2O) was dissolved in 500 mL of water was adsorbed to the platinum catalyst to support cobalt on the platinum catalyst.
(18) As described above, at the time of supporting the catalyst metal (platinum and cobalt), the ratio of the supporting amount of platinum to the supporting amount of cobalt was made higher than that in Example 1, so that a catalyst having a Pt/M ratio higher than that in Example 1 was produced. Except for these points, the same steps and conditions as in Example 1 were applied to obtain a Pt—Co catalyst.
(19) Example 4: In this example, a Pt—Ni catalyst having nickel supported as a transition metal M was produced. At the time of supporting the catalyst metal of Example 1, 600 g of a dinitrodianmine platinum nitric acid solution having a platinum concentration of 4.6% by mass (platinum content: 27.6 g) as a platinum solution was supported on 64.4 g of the same carbon fine powder as in Example 1 to produce a platinum catalyst. A metal salt solution obtained by 60 g of nickel chloride hexahydrate (NiCl.sub.2 s.6H.sub.2O) was dissolved in 500 mL of water was adsorbed to the platinum catalyst to support nickel on the platinum catalyst.
(20) As described above, a nickel compound solution was adsorbed to the same platinum catalyst as in Examples 2 and 3, the same steps and conditions as in Example 1 were applied to perform alloy formation and acidic solution treatment, and platinum compound solution treatment was performed to obtain a Pt—Ni catalyst.
(21) Comparative Example 1: As a comparative example against the Pt—Co catalyst of Example 1, the Pt—Co catalyst after the oxidizing solution treatment in Example 1 was prepared as Comparative Example 1.
(22) Comparative Example 2: As a comparative example against the Pt—Co catalyst of Example 2, the Pt—Co catalyst after the oxidizing solution treatment in Example 2 was prepared as Comparative Example 2.
(23) Comparative Example 3: The catalysts of Comparative Examples 1 and 2 are the same as in Examples 1 and 2 except that the catalysts are obtained without performing platinum compound solution treatment. That is, in Comparative Examples 1 and 2, platinum is not added to the Pt—Co catalyst after oxidizing solution treatment. In the catalyst of Comparative Example 3, platinum is added to the Pt—Co catalyst after oxidizing solution treatment by a method different from that in Examples 1 and 2.
(24) In Comparative Example 3, platinic chloride in the same amount as in Example 1 (11.9 g) was diluted with water to 3 L, the mixture was adjusted to have a pH of 10, and stirred at 70° C. for 2 hours. This was then cooled to 50° C., 10 g of the Pt—Co catalyst (platinum weight ratio: 0.472) after the oxidizing solution treatment and 10 ml of a reducing agent were then added, the mixture was stirred at 75° C. for 2 hours, and filtration and washing were three times. Finally, drying was performed at 60° C. for 24 hours. A Pt—Co catalyst was obtained by the above steps.
(25) Various physical properties were evaluated for the catalysts of Examples 1 to 4 and Comparative Examples 1 to 3. First, the produced catalyst was subjected to composition analysis to measure the composition ratio of platinum and the transition metal M (cobalt and nickel) forming catalyst particles and the supporting ratio of catalyst particles. The composition analysis was performed by ICP (high-frequency inductive coupling plasma emission analysis). In the analysis by ICP, a solution obtained by weighing 20 mg of a catalyst, firing and reducing the catalyst, adding about 5 ml of aqua regia, dissolving the catalyst to form a solution, and diluting the solution by about 20 times was analyzed.
(26) Next, for each catalyst, the platinum specific surface area (S.sub.COMSA) was measured by a CO adsorption method. The platinum specific surface area specified here is a value obtained by calculating a surface area from a CO adsorption amount measured in accordance with the specified CO pulse adsorption method, and converting the surface area into a surface area per 1 g of platinum in the sample.
(27) The platinum specific surface area (S.sub.COMSA) was measured by use of a metal dispersion degree measuring apparatus (BEL-METAL-3 manufactured by Nippon BEL Inc.). 40.0 mg of a sample was precisely weighed to the order of 0.1 mg, and added into a glass cell. A cell was attached to the measuring apparatus, and automatic measurement was started. While a He gas (50 mL/min) was kept flowing, the sample was heated to 100° C. from room temperature over 20 minutes, and held for 15 minutes. The gas was changed to H.sub.2 (50 mL/min), and the sample was held at 100° C. for 30 minutes. Next, the gas was changed to He (50 mL/min), and the sample was cooled to 30° C. from 100° C., then heated to 40° C., and held at 40° C. After the above pretreatment was performed, the CO gas adsorption amount was measured by a CO pulse adsorption method. From the obtained CO gas adsorption amount, S.sub.COMSA was determined in accordance with the following method.
S.sub.COMSA(m.sup.2/g−Pt)=(26.88×B×σ)/(A×R.sub.Pt) [Formula 3]
(28) (A: weight (g) of sample added into glass cell), B: CO adsorption amount (ml), σ: adsorption gas molecular cross-sectional area (nm.sup.2 per cell) (0.163 nm.sup.2 per cell for CO), R.sub.Pt: content (% by mass) of platinum in catalyst to be measured)
(29) Next, for each catalyst, the catalyst specific surface area (S.sub.BET) was measured by a BET method. The catalyst specific surface area specified here is a value obtained by calculating a surface area from a monomolecular layer adsorption N.sub.2 gas amount measured in accordance with the specified N.sub.2 BET multipoint method (constant-volume method), and converting the surface area into a surface area per 1 g of the sample.
(30) The catalyst specific surface area (S.sub.BET) was measured by use of a chemical/physical adsorption measuring apparatus (NOVA-4200e manufactured by Yuasa Ionics Co., Ltd.) 50.0 mg of a sample was precisely weighed to the order of 0.1 mg, and added into a glass cell. The sample was held in a vacuum state at 100° C. for 30 minutes, and then cooled to room temperature, and a sample-containing cell was precisely weighed to the order of 0.1 mg. The cell was removed and attached to a measurement station, and the amount of N.sub.2 gas adsorbed by a N.sub.2 BET multipoint method (constant-volume method) was measured. From the obtained N.sub.2 gas adsorption method, S.sub.BET was determined in accordance with the following method.
S.sub.BET(m.sup.2/g)=(214.85×B×σ)/(C−A) [Formula 4]
(31) (A: weight (g) of glass cell), B: monomolecular layer adsorption N.sub.2 gas amount (g), C: weight (g) of sample-containing cell after pretreatment, σ: adsorption gas molecular cross-sectional area (nm.sup.2 per cell) (0.162 nm.sup.2 per cell for CO)
(32) Further, for each cell, the configuration of X-ray diffraction analysis catalyst particles was examined. As an X-ray diffractometer, JDX-8030 manufactured by JEOL Ltd. The sample was formed into fine powder, and added into a glass cell, and subjected to X-ray diffraction analysis at a tube voltage of 40 kV, a tube current of 30 mA, and a scan speed of 7°/min and a step angle of 0.1° over a range of 2θ=20 to 90° with a Cu (kα ray) as a X-ray source. XPS was performed at a voltage of 15 kV, a current of 1.66 mA, a beam diameter of 100 μm over a measurement range of 250 μm.sup.2 with an Al kα ray applied as an X-ray source.
(33)
(34) First, the peak intensity (I.sub.cat) of the main peak was evaluated in each catalyst. Here, I.sub.cat was determined by approximating the main peak in the vicinity of 2θ=69° to 71° by changing I.sub.cat, u and w by use of Solver commands in Excel in such a manner that the square of a remainder was the minimum in accordance with the Lorentz equation (Formula 1) shown below.
[Formula 5]
f(x)=I.sub.cat/(1+(x−u)2/w.sub.cat2)+I.sub.base Formula 1
(35) (I.sub.cat: peak height from base line, u: peak position (degrees), w.sub.cat: half-width/2 (degrees), I.sub.base: base line height: (intensity at 2θ=64° in original XRD spectrum))
(36) Next, for evaluating the peak intensity (I.sub.Pt) associated with crystalline atomic platinum, the remainder obtained by subtracting the value approximated by Formula 1 from the original spectrum was approximated by the following Lorentz equation 2 to determine I.sub.Pt.
[Formula 6]
f(x)=I.sub.Pt/(1+(x−67.4)2/w.sub.Pt2)+I.sub.base Formula 2
(37) (I.sub.Pt: peak height from base line, w.sub.Pt: half-width/2 (degrees), I.sub.base: base line height: (intensity at 2θ=64 in original XRD spectrum))
(38) Peak separation results obtained by the above analysis method are shown in
(39) Various physical property values for the catalysts of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1.
(40) TABLE-US-00001 TABLE 1 Catalyst Configuration Property Values Metal Addition of Supporting S.sub.COMSA S.sub.BET S.sub.COMSA/ XRD M Pt/M Platinum Ratio (m.sup.2/g-Pt) (m.sub.2/g) S.sub.BET I.sub.Pt/I.sub.oat Example 1 Co 2.92 Dropwise Addition 52.0 96.1 337.2 0.285 0.217 Example 2 3.39 of H.sub.2PtCl.sub.6 32.0 105.5 391.6 0.269 0.079 Example 3 5.01 115.5 418.6 0.276 0.233 Example 4 Ni 2.73 114.4 436.9 0.262 0.036 Comparative Co 2.26 None 52.0 88.9 326.9 0.272 0.038 Example 1 Comparative 3.00 32.0 125.5 484.9 0.259 0.054 Example 2 Comparative 3.67 Dropwise Addition 52.0 90.3 279.0 0.324 0.443 Example 3 of H.sub.2PtCl.sub.6 + Reducing Agent
(41) Measurement results in examples and comparative examples will be discussed with reference to Table 1. First, the catalysts of Example 1 and Comparative Examples 1 and 3 which have the same supporting ratio of catalyst particles will be compared. The measurement results of these catalysts show that in each of the catalysts of Example 1 and Comparative Example 3 in which platinum was additionally supported, the molar ratio of platinum to cobalt (transition metal M) (Pt/M(Co)) is 2.5 or more, and higher than that of the catalyst of Comparative Example 1. This is considered ascribable to that platinum is added after acidic solution treatment.
(42) Concerning the ratio (S.sub.COMSA/S.sub.BET) of a platinum specific surface area (S.sub.COMSA) measured by a CO adsorption method to a catalyst specific surface area (S.sub.BET) measured by a BET method, the S.sub.COMSA/S.sub.BET ratio of the catalyst of Example 1 (0.285, Pt/M=2.92) is slightly larger than the S.sub.COMSA/S.sub.BET ratio of the catalyst of Comparative Example 1 (0.272, Pt/M=2.26). This may be because platinum is additionally supported. The reason why such results are obtained in Example 1 is that in the catalyst, platinum is additionally supported on the surfaces of catalyst particles, and the carrier is not influenced.
(43) On the other hand, for the catalyst of Comparative Example 3, a high-temperature and short-time reaction is produced by use of a reducing agent at the time of additionally supporting platinum. Such additional support causes precipitation of platinum on the surface of the carrier, and excessive precipitation of platinum of the surfaces of catalyst particles. Thus, S.sub.COMSA significantly increases, and the specific surface area (S.sub.BET) decreases. As a result, the S.sub.COMSA/S.sub.BET ratio of the catalyst of Comparative Example 3 (0.324) was higher than the S.sub.COMSA/S.sub.BET ratio of the catalyst of Example 1 (0.285).
(44) Further, concerning the results of XRD analysis, Table 1 shows that the ratio of the peak intensity (I.sub.Pt) of atomic platinum to the peak intensity (I.sub.cat) of the main peak (I.sub.Pt/I.sub.cat) in the catalyst of Example 1 is 0.217, whereas the I.sub.Pt/I.sub.cat ratio in Comparative Example 3 is as high as 0.443. This is considered ascribable to that the presence state of platinum additionally supported in the catalyst of Comparative Example 3 is such that the ratio of atomic platinum present alone is high. Concerning the results of XRD analysis in Comparative Example 1, the I.sub.Pt/I.sub.cat ratio is 0.038, and lower than that in Example 1. This may be because platinum in the catalyst of Comparative Example 1 is present as platinum alloy, and forms catalyst particles, platinum is not added, and thus there is substantially no atomic platinum.
(45) In the catalyst of Example 1, added platinum is present, but the intensity (I.sub.Pt) of crystalline atomic platinum is not high in the XRD profile. This is supposed to be because in Example 1, added platinum is in a state of being precipitated in a layered form on catalyst particles which have been present since before the addition of the platinum. It is considered that when the platinum was precipitated in a layered form, the peak intensity (I.sub.Pt) of atomic platinum was small because of low crystallinity.
(46) Further, comparison between Example 2 and Comparative Example 2 with equal supporting ratios showed that an effect was obtained by additionally supporting platinum. That is, for the ratio (S.sub.COMSA/S.sub.BET) of a platinum specific surface area (S.sub.COMSA) measured by a CO adsorption method to a catalyst specific surface area (S.sub.BET) measured by a BET method, the S.sub.COMSA/S.sub.BET ratio of the catalyst of Example 2 (0.269, Pt/M=3.39) is larger than the S.sub.COMSA/S.sub.BET ratio of the catalyst of Comparative Example 2 (0.259, Pt/M=3.00).
(47) [Initial Activity Test]
(48) The catalysts of examples and comparative examples (Pt—Co catalyst and Pt—Ni catalyst) 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 (A/g-Pt) per 1 g of Pt was determined from the weight of Pt applied onto an electrode, and the mass activity was calculated.
(49) [Durability Test]
(50) 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 to clean the surfaces of catalyst particles. Thereafter, sweeping was performed between 650 mV and 1050 mV at a sweeping speed of 100 mV/s over 3600 cycles (first cycles), 10800 cycles (second cycles) and 10800 cycles (third cycles) to degrade the catalyst. For catalysts degraded by the third cycles, the mass activity was measured.
(51) The results of the initial activity test and the durability test are shown in Table 2.
(52) TABLE-US-00002 TABLE 2 Mass Activity (A/g-Pt) Pro- Dura- Main- perty bility tenance Catalyst Value Test Ratio Configuration S.sub.COMSA/ Initial (After De- After De- Metal M Pt/M S.sub.BET Activity gradation) gradation Example 1 Co 2.92 0.285 89.0 36.4 40.9% Example 2 3.39 0.269 39.9 14.4 36.1% Example 3 5.01 0.276 31.7 28.1 88.6% Example 4 Ni 2.73 0.262 49.2 14.1 28.7% Comparative Co 2.26 0.272 90.0 24.7 27.4% Example 1 Comparative 3.00 0.259 82.6 13.2 15.9% Example 2 Comparative 3.67 0.324 74.1 14.0 18.8% Example 3
(53) As is apparent from Table 2, there is almost no difference in performance between the catalysts of Example 1 and Comparative Example 1 in evaluation performed in terms of initial activity. Concerning durability, however, the catalyst of Example 1 has a smaller reduction in activity after degradation as compared to Comparative Example 1, and has a high maintenance ratio of activity. Thus, the catalyst of Example 1 in which platinum was additionally supported by a suitable method to optimize the ratio (S.sub.COMSA/S.sub.BET) of a platinum specific surface area (S.sub.COMSA) measured by a CO adsorption method to a catalyst specific surface area (S.sub.BET) measured by a BET method is shown to have excellent durability.
(54) Concerning durability in Comparative Example 3, the catalyst has larger reduction in activity after degradation as compared to Example 1. The catalyst of Comparative Example 3 is a catalyst in which platinum was additionally supported. It has been shown that whether a catalyst is excellent or poor in durability depends on a method for adding platinum.
(55) From Examples 2 to 4, it can be confirmed that when the S.sub.COMSA/S.sub.BET ratio is 0.262 or more, durability is improved. In Example 3, the ratio of the supporting amount of Pt was increased, and resultantly, a catalyst having a Pt/M ratio (Pt/M=5.01) higher than that in Example 1 was obtained. As a result of passing through a suitable process, even such a catalyst has a S.sub.COMSA/S.sub.BET ratio of 2.62 or more, and a high maintenance ratio after degradation. Further, it was shown that even when nickel is used as the transition metal M as in Example 4, it was possible to produce a platinum alloy catalyst having high durability.
INDUSTRIAL APPLICABILITY
(56) The present invention allows to improve the durability of an electrode catalyst for solid polymer fuel cells. The present invention contributes to popularization of fuel cells, and hence provides a foundation for environmental problem solution.