Catalyst for solid polymer fuel cell and method for producing same

Abstract

The present invention is a catalyst for a solid polymer fuel cell including: catalyst particles of platinum, cobalt and manganese; and a carbon powder carrier supporting the catalyst particles, wherein the component ratio (molar ratio) of the platinum, cobalt and manganese of the catalyst particles is of Pt:Co:Mn=1:0.06 to 0.39:0.04 to 0.33, and wherein in an X-ray diffraction analysis of the catalyst particles, the peak intensity ratio of a CoMn alloy appearing around 2=27 is 0.15 or less on the basis of a main peak appearing around 2=40. It is particularly preferred that the catalyst have a peak ratio of a peak of a CoPt.sub.3 alloy and an MnPt.sub.3 alloy appearing around 2=32 of 0.14 or more on the basis of a main peak.

Claims

1. A catalyst for a solid polymer fuel cell comprising: catalyst particles of platinum, cobalt and manganese; and a carbon powder carrier supporting the catalyst particles, wherein the catalyst particles were formed by subjecting a metallic salt solution of said metals to a reduction treatment, a molar ratio of the platinum, cobalt and manganese of the catalyst particles is of Pt:Co:Mn=1:0.06 to 0.13:0.25 to 0.33, and in an X-ray diffraction analysis of the catalyst particles, a peak intensity ratio of a CoMn alloy appearing around 2=27 is 0.15 or less on the basis of a main peak appearing around 2=40, wherein in an X-ray diffraction analysis of the catalyst particles, a peak ratio of a peak of a CoPt.sub.3 alloy and an MnPt.sub.3 alloy appearing around 2=32 is 0.14 or more on the basis of a main peak appearing around 2=40.

2. The catalyst for a solid polymer fuel cell according to claim 1, wherein a cobalt concentration and a manganese concentration at a surface of the catalyst particles are lower than a cobalt concentration and the manganese concentration at a center of the particles.

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

4. The catalyst for a solid polymer fuel cell according to claim 2, wherein a supported density of the catalyst particles is from 30 to 70%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates X-ray diffraction patterns of respective catalysts according to Examples 1 to 4, Comparative Examples 1 and 2, and a conventional example.

(2) FIG. 2A illustrates TEM/EDX concentration distributions of Pt, Co and Mn of catalyst particles according to Example 5.

(3) FIG. 2B is an SEM photograph showing catalyst particles according to Example 5 analyzed with TEM/EDX.

(4) FIG. 3 illustrates X-ray diffraction patterns of respective catalysts for heat treatment temperatures.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(5) A plurality of ternary catalysts of PtCoMn with a different component ratio of catalytic metals were produced, and the property was investigated and the catalytic activity was evaluated. A basic step for producing the catalyst is as follows.

(6) [Supporting of Catalytic Metals]

(7) A commercially-available platinum catalyst was prepared, by which cobalt and manganese were supported. As to the platinum catalyst used, 5 g of the platinum catalyst using a carbon fine powder (a specific surface area of approximately 900 m.sup.2/g) as a carrier with a platinum supporting ratio of 46.5% by mass (2.325 g (11.92 mmol) in terms of platinum) were prepared. This platinum catalyst was immersed and stirred in a metal salt solution, which had been prepared through dissolution of cobalt chloride (CoCl.sub.2.6H.sub.2O) and manganese chloride (MnCl.sub.2.4H.sub.2O) in 100 mL of ion exchanged water, on a magnetic stirrer. To this solution, 500 mL of a sodium borohydride (SBH) solution with a concentration of 1% by mass were added dropwise, followed by stirring and a reduction treatment, so that cobalt and manganese were supported by the platinum catalyst. Subsequently, filtering, washing and drying were performed.

(8) [Supporting of Catalytic Metals]

(9) The catalyst which supported the catalytic metals was subjected to a heat treatment for alloying. In the embodiment, a heat treatment for 30 minutes in 100% hydrogen gas at a heat treating temperature of 900 C. was performed. The heat treatment for alloying produced the ternary catalyst of PtCoMn.

(10) In addition, in the embodiment, as to the metal salt solution of cobalt and manganese in which the platinum catalyst was to be immersed, the added amount of each metal salt was adjusted, thereby the component ratio of the catalytic metals being changed. In the embodiment, as to the component ratio of the individual metals (Pt:Co:Mn), 6 kinds of catalysts were produced, including 1:0.39:0.04 (Example 1), 1:0.26:0.13 (Example 2), 1:0.13:0.25 (Example 3), 1:0.06:0.33 (Example 4), 1:0.38:0 (Mn not added, Comparative Example 1), and 1:0.02:0.39 (Comparative Example 2).

CONVENTIONAL EXAMPLE

(11) Herein, in the catalytic metal-carrying step, platinum, cobalt and manganese were simultaneously carried to produce the ternary catalyst of PtCoMn. Five grams of a carbon carrier (a specific surface area of approximately 900 m.sup.2/g) were prepared. This carrier was immersed and stirred in a predetermined amount of a Pt dinitrodiamine (Pt(NO.sub.2).sub.2(NH.sub.3).sub.2) solution of nitric acid and a metal salt solution, which had been prepared through dissolution of cobalt chloride (CoCl.sub.2.6H.sub.2O) and manganese chloride (MnCl.sub.2.4H.sub.2O) in 100 mL of ion exchanged water, on a magnetic stirrer. To this solution, 500 mL of a sodium borohydride (SBH) solution with a concentration of 1% by mass were added dropwise, followed by stirring and a reduction treatment, so that platinum, cobalt and manganese were supported on the carbon carrier. Subsequently, filtering, washing and drying were performed, and a heat treatment for 30 minutes in hydrogen air flow at 900 C. was carried out for alloying.

(12) Each of the ternary catalysts of PtCoMn in Examples, Comparative Examples and the conventional example, produced as described above, was analyzed by an X-ray diffraction analysis, and the composition of catalyst particles was examined. As an X-ray diffractometer, JDX-8030 made by JEOL was used. A sample was placed in a glass cell in a fine powder form. Cu (a k line) as an X-ray source, a tube voltage of 40 kV, a tube current of 30 mA, 2=20 to 90, a scan speed of 7/min, a step angle of 0.1.

(13) FIG. 1 shows X-ray diffraction patterns of the respective catalysts. In FIG. 1, a peak appearing around 2=40, which is found for all of the catalysts, is a synthetic peak of metals Pt, MnPt.sub.3 and CoPt.sub.3. A peak around 2=32 (32 to 34) in Examples 1 to 4 and Comparative Example 2 is a synthetic peak of MnPt.sub.3 and CoPt.sub.3, which is not influenced by a metal Pt. On the other hand, in the conventional example, a peak that is rarely found in each of the Examples and Comparative Examples is found around 2=27. This is considered to be derived from a CoMn alloy.

(14) Next, a test for the initial performance of each of the ternary catalysts of PtCoMn in Examples, Comparative Examples and the conventional example was performed. This test for the performance was carried out by measuring the Mass Activity. For an experiment, a single cell was used, and a proton conducting polymer electrolyte membrane was held between a cathode electrode and an anode electrode each having an electrode area of 5 cm5 cm=25 cm.sup.2, so that a Membrane Electrode Assembly (MEA) was produced and evaluated. As a pretreatment, a current/voltage curve was drawn in a condition of a hydrogen flow rate=1000 mL/min, oxygen flow rate=1000 mL/min, cell temperature=80 C., anode humidifying temperature=90 C., and cathode humidifying temperature=30 C. Subsequently, as main measurement, the Mass Activity was measured. As to a test method, a current value (A) at 0.9 V was measured, and a current value per 1 g of Pt (A/g-Pt) was obtained based on the Pt weight applied on the electrode, so that the Mass Activity was calculated. The result is indicated in Table 1. In addition, in Table 1, a peak intensity ratio of the CoMn alloy (around 2=27) and peak intensity ratio of the MnPt.sub.3 and CoPt.sub.3 (around 2=32), calculated from the X-ray diffraction pattern of each of the catalysts in FIG. 1, are also indicated.

(15) TABLE-US-00001 TABLE 1 Peak Intensity Ratio*.sup.2 Mass Activity*.sup.1 MnPt.sub.3 + Pt:Co:Mn (A/g-Pt at 0.9 V) CoMn CoPt.sub.3 Example 1 1:0.39:0.04 1.09 0.08 0.14 Example 2 1:0.26:0.13 1.21 0.11 0.21 Example 3 1:0.13:0.25 1.12 0.11 0.14 Example 4 1:0.06:0.33 1.22 0.11 0.22 Comparative 1:0.38:0 1.0 0.11 Example 1 Comparative 1:0.02:0.39 0.93 0.11 0.22 Example 2 Conventional 1:0.25:0.36 0.93 0.33 0.28 Example *.sup.1It is a relative comparison with respect to 1.0 for Comparative Example 1 (a PtCo catalyst) *.sup.2It is an intensity ratio, on the basis of a main peak around 2 = 40

(16) From Table 1, all of the ternary catalysts of PtCoMn in the individual Examples exhibit a good initial activity on the basis of the PtCo catalyst. However, only adding manganese is not always sufficient, because the component ratio with the added amount excessive like Comparative Example 2 has a lower initial activity than the PtCo catalytic activity. In addition, as to the composition of the catalyst particles, the conventional example in which a lot of CoMn phase is generated has a poor initial activity even when the component ratio of Pt, Co and Mn is made appropriate. Accordingly, it has been confirmed that it is insufficient only to make appropriate the component ratio of the individual catalytic metals, and thus it is also necessary to specify the composition of the alloy phase.

Second Embodiment

(17) Herein, as to the ternary catalyst of PtCoMn, a confirmation was done of the durability-enhancement effect by improvement of the surface composition of the catalyst particles through an acid solution treatment. As to the catalyst of Example 2 in the first embodiment (Pt:Co:Mn=1:0.26:0.13), the catalyst that had undergone the heat treatment was treated in 0.2 mol/L sulfuric acid aqueous solution at 80 C. for 2 hours, and subsequently, filtering, washing and drying were performed. The treated catalyst was then treated in 1.0 mol/L nitric acid aqueous solution (a dissolved oxygen content of 0.01 cm.sup.3/cm.sup.3 (in terms of STP)) at 70 C. for 2 hours, and subsequently, filtering, washing and drying were performed (the catalyst that had undergone this acid solution treatment was defined as Example 5).

(18) As to the catalyst that had undergone the treatment, an elementary analysis of the catalyst particles was performed. In the elementary analysis, the cross section of the catalyst particles was line analyzed by TEM/EDX for measurement of the content of Pt, Co and Mn at equal intervals of 0.2 nm, so that the compositions were compared between the particle surface and the particle center. The results of analysis are shown in FIG. 2A and FIG. 2B. As for the catalyst particles that had undergone the acid solution treatment, around the surface (the region at a depth of from 1 to 1.5 nm from the most external surface), the detection intensity of cobalt and manganese is extremely lower. From the analysis, it has been confirmed that this catalyst has an outer layer of the catalyst particles enriched with Pt, and the particle center formed of an alloy including Co and Mn.

(19) Next, a durability test was performed for this catalyst, and the durability was evaluated. In the durability test, a cathode electrode (an air electrode) was produced from the catalyst to compose a fuel cell, for which an accelerated degradation test was performed in which a cell electrical potential of the cathode was swept with a triangular wave, so that the power generation property after degradation was measured. As to the accelerated degradation, sweeping at a sweep rate of 40 mV/s between 200 and 650 mV for 20 hours was performed, the surface of the catalyst particles was cleaned, and subsequently, sweeping at a sweep rate of 100 mV/s between 200 and 650 mV for 24 hours was performed to result in degradation, so that the Mass Activity of the catalyst after the degradation was measured. Evaluation after the accelerated degradation test was also performed for Comparative Example 1 (a PtCo catalyst) (Comparative Example 3). The result is indicated in Table 2.

(20) TABLE-US-00002 TABLE 2 Peak Intensity Ratio*.sup.2 Mass Activity*.sup.1 MnPt.sub.3 + Pt:Co:Mn (A/g-Pt at 0.9 V) CoMn CoPt.sub.3 Example 5 1:0.26:0.13 1.10 0.11 0.21 Comparative 1:0.38:0 0.87 0.11 Example 3 *.sup.1It is a value, in case of 1.0 for a PtCo catalyst without degradation (Comparative Example 1) *.sup.2It is an intensity ratio, on the basis of a main peak around 2 = 40

(21) It is noted from Table 2 that the catalytic activity of the conventional PtCo catalyst decreases in the accelerated degradation test, whereas the ternary catalyst of PtCoMn according to Example 5 exhibits a good activity after the degradation. It has been confirmed that such a catalyst having a difference of platinum concentration between the surface of the catalyst particles and the center thereof has an excellent durability.

Third Embodiment

(22) Herein, in the process for producing the ternary catalyst of PtCoMn, the temperature of the heat treatment for alloying was examined. As to Example 3 (Pt:Co:Mn=1:0.13:0.25), catalysts were produced at heat treatment temperatures of 500 C., 700 C., 900 C. (Example 3) and 1100 C., and the initial activity was evaluated. The result is indicated in Table 3. In addition, in FIG. 3, the X-ray diffraction patterns of the respective catalysts are shown.

(23) TABLE-US-00003 TABLE 3 Heat Peak Intensity Ratio*.sup.2 Treatment Mass Activity*.sup.1 MnPt.sub.3 + Temperature (A/g-Pt at 0.9 V) CoMn CoPt.sub.3 Comparative 500 C. 0.80 0.16 0.19 Example 4 Example 6 700 C. 1.07 0.13 0.23 Example 3 900 C. 1.12 0.11 0.22 Example 7 1100 C. 1.20 0.09 0.23 *.sup.1It is a value, in case of 1.0 for a PtCo catalyst without degradation (Comparative Example 1) *.sup.2It is an intensity ratio, on the basis of a main peak around 2 = 40

(24) From Table 3, it has been confirmed that in order to obtain an effective initial activity, a heat treatment at 700 C. or higher is necessary. In this regard, from the X-ray diffraction patterns in FIG. 3, as the heat treatment temperature is higher, the peak intensity of MnPt.sub.3 and CoPt.sub.3 around 2=32 is sharper and clearer. It may also be confirmed here that it is preferable for the formation of the alloy phases of MnPt.sub.3 and CoPt.sub.3 useful for the catalytic activity to make higher the heat treatment temperature.

INDUSTRIAL APPLICABILITY

(25) According to the present invention, the electrode catalyst for a solid polymer fuel cell can achieve both durability improvement and the initial power generation property improvement. The present invention contributes to popularization of a fuel cell, and eventually becomes a basis for solution to an environmental problem.