Catalyst for solid polymer fuel cells and method for producing same

Abstract

The present invention aims to provide a catalyst that makes it possible to reduce an amount of solid electrolyte mixed and improve initial performance of a fuel cell, and also a method for producing the catalyst. The present invention relates to a catalyst for a solid polymer fuel cell, which has sulfo groups (SO.sub.3H) on catalyst particles. In TEM-EDX analysis, a ratio (I.sub.S/I.sub.Pt) of a sulfur peak intensity (I.sub.S) to a platinum peak intensity (I.sub.Pt) on the catalyst particles is within a range of 0.0044 or more and 0.0090 or less. The catalyst makes it possible to reduce the amount of solid electrolyte added and also a fuel cell with excellent initial performance, and thus contributes to a practical use of a fuel cell.

Claims

1. A catalyst for a solid polymer fuel cell, comprising catalyst particles containing platinum and cobalt supported on a carbon powder carrier, the catalyst having sulfo groups (SO.sub.3H) directly on the catalyst particles and directly on the carbon powder carrier, wherein an amount of the sulfo groups on the catalyst particles exceeds an amount of the sulfo groups on the carbon powder carrier.

2. A method for producing the catalyst for the solid polymer fuel cell defined in claim 1, wherein the method comprises a step of immersing the catalyst including the catalyst particles containing the platinum and the cobalt supported on the carbon powder carrier in a mixed solution containing concentrated sulfuric acid and fuming sulfuric acid, a mixing ratio between the concentrated sulfuric acid and the fuming sulfuric acid in the mixed solution in the form of the concentrated sulfuric acid/the fuming sulfuric acid is 1.0 or more and 2.0 or less, a temperature of the mixed solution is 40 C. or higher and 90 C. or lower, and a treatment time by the mixed solution is 8 h or more and 24 h or less.

3. The method for producing the catalyst for the solid polymer fuel cell defined in claim 2, wherein an amount of the mixed solution is 10 ml or more and 20 ml or less per gram of the catalyst.

4. The catalyst for the solid polymer fuel cell according to claim 1, wherein in a TEM EDX analysis, a ratio (I.sub.S/I.sub.Pt) of a sulfur peak intensity (I.sub.S) to a platinum peak intensity (I.sub.Pt) on the catalyst particles being within a range of 0.0044 or more and 0.0090 or less, and on the catalyst particles and on the carbon powder carrier, appearance rates of sulfur peaks based on results of TEM-EDX analysis at five or more measurement points (a number of sulfur peaks appeared/a total number of measurements 100) are 55% or more on the catalyst particles (X.sub.PtCo) and 30% or less on the carbon powder carrier (X.sub.C), respectively.

5. The catalyst for the solid polymer fuel cell according to claim 4, wherein a ratio (X.sub.PtCo/X.sub.C) of the appearance rates of sulfur peaks on the catalyst particles (X.sub.PtCo) to the appearance rates of sulfur peaks on the carbon powder carrier (X.sub.C) is 2.5 or more.

Description

DESCRIPTION OF EMBODIMENTS

(1) Hereinafter, best embodiments of the present invention will be described.

(2) First Embodiment: Sulfo groups were introduced into a catalyst (sulfonation) under various conditions, and the obtained sulfonated catalyst was subjected to functional group analysis, sulfur analysis, and an initial performance test.

(3) [Sulfonation of Catalyst]

(4) 3 g of a commercially available PtCo/Carbon catalyst (TEC36F52HT2, manufactured by Tanaka Kikinzoku Kogyo K.K.) was immersed in 30 ml of concentrated sulfuric acid having a purity of 96 wt % and 30 ml of fuming sulfuric acid having an SO.sub.3 content of 25 vol %, and stirred for 8 hours at a liquid temperature of 40 C. to cause sulfonation. In this PtCo/Carbon catalyst, Pt:Co=2.2:1, the average particle size of the platinum-cobalt particles is 4.2 nm, and the specific surface area of the carbon powder is 800 m.sup.2/g. After sulfonation, the catalyst was filtered, immersed in 4 L of ion exchange water at 70 C., stirred for 30 minutes, filtered again to perform a washing step, and unreacted sulfuric acid and fuming sulfuric acid were removed. The washing step was repeatedly performed until the washing water became neutral. After washing, the mixture was dried in air at 60 C. overnight and then ground in a mortar to give a sulfonated catalyst. Such a sulfonated catalyst was subjected to the following various analyses. Additionally, catalysts sulfonated with chemical liquids and heating under the conditions shown in Table 1 were also analyzed in the same manner.

(5) [Strongly Acidic Functional Groups]

(6) The obtained sulfonated catalysts were subjected to the quantification of strongly acidic functional groups, such as sulfo groups (SO.sub.3H) and carboxyl groups (COOH), by a titration method. 55 ml of ion exchange water was added to 0.25 g of each sulfonated catalyst in terms of carbon (about 0.5 g), stirred for 10 minutes, and then ultrasonically dispersed for 2 minutes. The catalyst dispersion liquid was filtered, then the filtrate was transferred to a glove box purged with nitrogen gas, and the filtrate was bubbled with nitrogen gas for 10 minutes. For titration, after adding an excess of a 0.1 mol/L aqueous sodium hydrogen carbonate solution, neutralization titration was performed with 0.1 mol/L hydrochloric acid, and the amount of functional groups was quantified from the neutralization point. This is because the acid dissociation constant (pKa) of carbonic acid is 6, and sodium hydrogen carbonate reacts with a strongly acidic functional group having a pKa of less than 6. From the amount of bases added in this titration and the amount of hydrochloric acid consumed, the amount of strongly acidic functional groups on the catalyst surface was calculated. The neutralization point was checked with a pH meter, and pH 4.5 was defined as the neutralization point.

(7) [Sulfur Analysis]

(8) The amount of sulfur (ppm) in a sulfonated catalyst was measured with an automatic halogen-sulfur analysis system (SQ-10 electric furnace and HSU-35 absorption unit, manufactured by Yanaco LID Co., LTD) and ion chromatography (manufactured by DKK-TOA). In an electric furnace, while air is circulated at a flow rate of 2.2 l/min, 0.05 g of a sulfonated catalyst was retained for 5 minutes at normal pressure and a temperature raised from 450 C. to 750 C., and then retained for 5 minutes at a temperature raised to 900 C. Combustion decomposition gas containing a sulfur component (sulfur dioxide, SO.sub.2) generated during the combustion process was dissolved and collected in a hydrogen peroxide solution, and sulfate ions (SO.sub.4.sup.2) were separated and quantified by ion chromatography. From the value of sulfur concentration (ppm) measured, the molar amount of sulfo groups in terms of sulfo groups (SO.sub.3H) per 1 g of the catalyst was calculated (mmol/g-catalyst).

(9) [Initial Performance Test]

(10) The catalysts of examples and comparative examples were subjected to a fuel cell initial performance test. This performance test was performed by measuring the mass activity. In the experiment, a single cell was used, and a membrane/electrode assembly (MEA) containing a proton-conducting polymer electrolyte membrane sandwiched between cathode and anode electrodes each having an electrode area of 5 cm5 cm=25 cm.sup.2 was produced and evaluated. As a pretreatment, a current/voltage curve was prepared at a hydrogen flow rate of 1000 mL/min, a cell temperature of 80 C., an anode humidification temperature of 90 C., and a cathode humidification temperature of 30 C. Subsequently, the mass activity was measured as the main measurement. The test method was as follows: the current value (A) at 0.9 V was measured, and, from the weight of Pt applied onto the electrodes, the current value per 1 g of Pt (A/g-Pt) was determined to calculate the mass activity. The initial performance shown in Table 1 is the mass activity calculated above relative to that of a non-sulfonated catalyst (test No. 1-6) (=1.00).

(11) TABLE-US-00001 TABLE 1 Chemical liquid Strongly acidic Concentrated Fuming Heating conditions functional Molar amount Initial sulfuric acid sulfuric acid Temperature Time groups of sulfo groups performance Test No. mL mL C. h mmol/g-carbon mmol/g-cat A/g-Pt 1-1 30 30 40 8 0.49 0.124 1.36 1-2 90 0.76 0.141 1.28 1-3 110 0.64 0.177 1.03 1-4 60 0 90 8 0.54 0.069 1.18 1-5 0 60 0.74 0.214 1.02 1-6 Not sulfonated 0.36 0.004 1.00 1-7 30 30 90 24 0.63 0.112 1.26

(12) From Table 1, as compared with the non-sulfonated catalyst (test No. 1-6), in the catalysts sulfonated at 40 C. or more and 90 C. or less by use of concentrated sulfuric acid and fuming sulfuric acid as a chemical liquid, the initial performance improved. In contrast, in the catalyst sulfonated with only fuming sulfuric acid (test No. 1-5), although the amounts of strongly acidic functional groups and sulfur increased as a result of sulfonation, the initial performance hardly increased as compared with that before sulfonation.

(13) Second Embodiment: The catalysts sulfonated in the first embodiment were subjected to TEM-EDX analysis to check the position and amount of sulfo groups introduced.

(14) [TEM-EDX Analysis]

(15) The sulfonated catalysts were each observed under TEM (transmission electron microscope, Cs-corrected STEM device, Model No. JEM-ARM 200F, manufactured by JEOL Ltd.) under the following conditions: accelerating voltage: 80 kV, STEM beam diameter: less than 0.2 nm, analysis area: a circular area of about 2 nm . For arbitrary seven points on the catalyst particles (PtCo), the peak intensity was measured for an integration time of 60 seconds with an SDD detector manufactured by JEOL Ltd. and an EDX (energy dispersive X-ray analysis) device of a system analyzer Noran System 7 manufactured by Thermo Fisher Scientific Inc.

(16) Of the measured EDX data, sulfur peak intensities (near 2.307 keV) were subjected to the following analyses (1) and (2) to eliminate the Pt-derived overlapping portion contained in the measured values.

(17) (1) Seven points on catalyst particles of a non-sulfonated catalyst (Pt/carbon catalyst, manufactured by Tanaka Kikinzoku Kogyo K.K., trade name: TEC10E50E) were subjected to EDX analysis, and the obtained spectra were each defined as a Pt standard spectrum. For the measurement of the Pt standard spectra, a catalyst in which catalyst particles have the same level of average particle size as in sulfonated catalysts was used.

(18) (2) Seven points on catalyst particles of a sulfonated catalyst were subjected to EDX analysis. For each obtained spectrum, the difference in waveform from the Pt standard spectrum of (1) was calculated for every measurement point to determine the sulfur peak intensity (SK intensity).

(19) For the calculated sulfur peak intensities (SK intensities) and also the platinum peak intensities (near 2.0485 keV) actually measured by EDX, the averages (I.sub.S, I.sub.Pt) of the seven measurement points were calculated. Additionally, the sulfur peak intensity at each measurement point was divided by the average platinum peak intensity (I.sub.Pt) to calculate the peak intensity ratio at each measurement point. For the intensity ratios, the average of the seven points was calculated to determine the peak intensity ratio (I.sub.S/I.sub.Pt).

(20) TABLE-US-00002 TABLE 2 Molar amount Strongly acidic of sulfo Initial Peak intensity Intensity ratio functional groups groups performance Test No. I.sub.s I.sub.Pt I.sub.s/I.sub.Pt mmol/g-carbon mmol/g-cat A/g-Pt 1-1 179 37620 0.0056 0.49 0.124 1.36 1-2 184 47923 0.0067 0.76 0.141 1.28 1-3 224 45794 0.0098 0.64 0.177 1.03 1-4 85 49764 0.0040 0.54 0.069 1.18 1-5 339 35828 0.0095 0.74 0.214 1.02 1-6 62 34655 0.0042 0.36 0.004 1.00 1-7 129 51899 0.0044 0.63 0.112 1.26

(21) Table 2 shows that when the ratio (I.sub.S/I.sub.Pt) of the sulfur peak intensity (I.sub.S) to the platinum peak intensity (I.sub.Pt) on the catalyst particles is 0.0044 or more, such a catalyst provides a fuel cell with high initial performance, while when the ratio is more than 0.009, the initial performance tends to decrease.

(22) Additionally, the appearance rate of sulfur peaks was calculated from the EDX analysis. To determine the appearance rates of sulfur peaks (X.sub.PtCo, X.sub.C), sulfur peak intensities on catalyst particles (arbitrary six or seven points) were measured in the same manner as above, and also sulfur peak intensities at arbitrary six or seven points on a carbon powder carrier 10 nm or more away from the end of the catalyst particles were measured. When the measured sulfur peak intensity value was 100 or more, such a sample was defined as having a sulfur peak, and the proportion (%) of the number of samples having a sulfur peak based on the total number of measurements (six or seven points) was calculated (Table 3).

(23) TABLE-US-00003 TABLE 3 No. 1-1 No. 1-2 No. 1-3 No. 1-4 No. 1-5 No. 1-6 No. 1-7 PtCo C PtCo C PtCo C PtCo C PtCo C PtCo C PtCo C Peak 211 40 0 18 631 32 0 6 375 224 0 18 0 0 intensity 352 517 0 170 0 0 0 14 544 260 0 70 70 0 191 61 256 16 447 41 0 32 471 123 214 27 0 45 182 20 433 0 0 24 0 0 86 56 0 11 281 0 113 79 288 97 0 62 181 0 303 32 193 0 146 62 0 34 308 59 267 31 120 80 406 248 26 14 180 113 204 303 0 48 292 22 186 270 0 0 228 55 Sulfur peak X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c X.sub.ptCo X.sub.c appearance 85.7 28.6 57.1 14.3 50.0 0 42.9 0 85.7 71.4 28.6 0 57.1 14.3 rate (%) X.sub.ptCo/X.sub.c 3.0 4.0 0 1.2 4.0 Initial 1.36 1.28 1.03 1.18 1.02 1.00 1.26 performance (A/g-Pt)

(24) From Table 3, when the appearance rate of sulfur peaks on the catalyst particles (X.sub.PtCo) was 55% or more, and that on the carrier (X.sub.C) was 30% or less, the initial performance was high. Additionally, the initial activity was high when X.sub.PtCo/X.sub.C was 2.5 or more.

(25) Third Embodiment: For sulfonated catalysts obtained in the first embodiment, a solid electrolyte was added at various mixing ratios, and the fuel cell initial performance was measured.

(26) As the solid electrolyte, a powder of Nafion (manufactured by Du Pont: registered trademark) was added to a sulfonated catalyst (test No. 1-1) and a non-sulfonated catalyst (test No. 1-6) of the first embodiment such that the Nafion/Carbon weight ratio would be as shown in Table 4, and the initial performance test was evaluated. The initial performance test was performed in the same manner as in the first embodiment.

(27) TABLE-US-00004 TABLE 4 Performance Mixing ratio Initial performance improvement Test Nafion/ A/g-Pt rate(No. 1-1)/ No. Carbon No. 1-6 No. 1-1 (No. 1-6) 2-1 0.6 29 33 1.14 2-2 0.8 46 56 1.22 2-3 1.0 61 83 1.36 2-4 1.2 67 83 1.24

(28) Table 4 shows that the initial performance of the sulfonated catalyst (test No. 1-1) was improved as compared with the non-sulfonated catalyst (test No. 1-6), and high initial performance can be achieved even when the mixing ratio of a solid electrolyte is reduced.

INDUSTRIAL APPLICABILITY

(29) The catalyst for a solid polymer fuel cell of the present invention makes it possible to reduce of the amount of solid electrolyte added to reduce the cost of fuel cell electrodes, provides a fuel cell with excellent initial performance, and contributes to the practical use of a fuel cell.