Electrode catalyst for fuel cell, method for producing the same, and polymer electrolyte fuel cell using the same

Abstract

A method for producing an electrode catalyst for a fuel cell is provided. The electrode catalyst includes a carbon support and a catalyst supported on the carbon support. The catalyst is one of platinum and a platinum-alloy. The method includes supporting the catalyst on the carbon support; and treating the carbon support carrying the catalyst with a nitric acid and cleaning the treated carbon support, such that an amount of an acid present on the carbon support becomes in a range from 0.7 mmol to 1.31 mmol of the acid per gram of the electrode catalyst.

Claims

1. A method for producing an electrode catalyst for a fuel cell, the electrode catalyst including a carbon support; and a catalyst supported on the carbon support, wherein the catalyst is one of platinum and a platinum-alloy, the method comprising: supporting the catalyst on the carbon support; and treating the carbon support carrying the catalyst with a nitric acid and cleaning the treated carbon support, such that an amount of the nitric acid present on the carbon support becomes in a range from 0.7 mmol of the acid per gram of the electrode catalyst to 1.31 mmol of the acid per gram of the electrode catalyst when the catalyst is a platinum-alloy, or such that an amount of the nitric acid present on the carbon support becomes in a range of 1.0 mmol of the acid per grain of the electrode catalyst to 1.31 mmol of the acid per grain of the electrode catalyst when the catalyst is platinum.

2. The method according to claim 1, wherein the platinum-alloy comprises an alloy of platinum and at least one metal selected from ruthenium, molybdenum, osmium, cobalt, rhodium, iridium, iron nickel, titanium, tungsten, palladium, rhenium, chromium, manganese, niobium, and tantalum.

3. The method of claim 1, wherein the carbon support on which the nitric acid is present and the catalyst is supported is hydrophilic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein:

(2) FIG. 1 is a graph showing the relationship between the catalyst acid amount and the low-humidity efficiency point performance (at 0.2 A/cm.sup.2) with regard to Examples 1, 2 and Comparative Examples 1-3;

(3) FIG. 2 is a graph showing the relationship between the catalyst acid amount and the low-humidity output point performance (at 1.02 A/cm.sup.2) with regard to Examples 1, 2 and Comparative Examples 1-3;

(4) FIG. 3 is a graph showing the relationship between the catalyst acid amount and the low-humidity efficiency point performance (at 0.2 A/cm.sup.2) with regard to Examples 3-7 and Comparative Examples 4-14; and

(5) FIG. 4 is a graph showing the relationship between the catalyst acid amount and the low-humidity output point performance (at 1.02 A/cm.sup.2) with regard to Examples 3-7 and Comparative Examples 4-14.

DETAILED DESCRIPTION OF EMBODIMENTS

(6) The inventors reached the present invention by subjecting carbon carrying a platinum catalyst or platinum-alloy catalyst to a particular treatment so as to bring it into a particular carbon state.

(7) In the following, some examples of this invention and comparative examples will be described in detail. A process for producing a single cell used for evaluation, a method of evaluating the performance of a catalyst of each example, and a method of determining the amount of acid in the catalyst will be described below.

(8) The process for producing a single cell used for evaluation will be described. A single cell for use in a polymer electrolyte fuel cell was formed in the following manner, using a catalyst powder obtained in each example or comparative example. The catalyst powder was dispersed in an organic solvent, and the resulting dispersion liquid was applied by coating to a Teflon sheet to form catalyst layers (i.e., electrodes). The amount of Pt catalyst per 1 cm.sup.2 of electrode area was 0.4 mg. The electrodes formed from the catalyst powder were attached to each other via a polymer electrolyte membrane by hot press, to provide a membrane-electrode assembly, and diffusion layers were mounted on the opposite sides of the membrane-electrode assembly, to form a single-cell electrode.

(9) The method of evaluating the catalyst performance will be described. To evaluate the catalyst performance, the initial voltage measurement was conducted in the following manner. The temperature of the single cell was set to 80 C., and moisturized air that passed a bubbler heated to 60 C. was supplied to the cathode-side electrode at a rate of 2.0 L/min, while moisturized hydrogen that passed a bubbler heated to 60 C. was supplied to the anode-side electrode at a rate of 0.5 L/min. In this condition, current voltage characteristics were measured. Comparisons of the performance among the catalysts of the respective examples were made through measurements of voltage values at current densities of 0.2 A/cm.sup.2 and 1.0 A/cm.sup.2.

(10) The method of determining the acid amount in the catalyst will be described. After 0.5 g of catalyst was added to 20 ml of 0.1N sodium hydroxide, which was then ultrasonically stirred for 20 min., the resulting liquid was subjected to filtration. Then, 0.05 ml of Methyl Orange as an indicator was added to 5 ml of filtrate while it was being stirred, and titration was conducted with 0.05N hydrochloric acid.

(11) Example 1 will be described. Initially, 4.2 g of Ketjen EC (manufactured by Ketjen Black International Company, JAPAN), which is commercially available, and 5.0 g of platinum were added to and dispersed in 0.5 L of pure water. About 100 mL of 0.1N ammonia was then added to the resulting liquid to make PH equal to about 10, so that a hydroxide was formed and deposited on carbon. The resulting dispersion liquid was subjected to filtration, and the obtained powder was dried at 100 C. in a vacuum for 10 hours. Then, the powder was held at 400 C. for 2 hours in hydrogen gas so as to be reduced, and then held at 1000 C. for 10 hours in nitrogen gas so as to provide a catalyst powder. The obtained catalyst was thrown into 1 L of 0.5N nitric acid, heated to 80 C., and was stirred for 30 min. Then, the catalyst was isolated by filtration, and was dried in a blowing drier at 80 C. for 15 hours or longer, to provide a catalyst powder. The acid amount in the catalyst was measured, and the result of the measurement was 1.020 mmol/g-cat.

(12) Example 2 will be described. A catalyst powder as Example 2 was obtained by preparing a catalyst in the same manner as in Example 1, except that, after the catalyst powder was treated with the acid, it was dried in a vacuum drier at 60 C. for 15 hours or longer. The amount of acid in the catalyst was 1.156 mmol/g-cat.

(13) Comparative Example 1 will be described. A catalyst powder as Comparative Example 1 was obtained by preparing a catalyst in the same manner as in Example 1, except that the acid treatment (i.e., a process of treating the catalyst powder with an acid) was not conducted. The amount of acid in the catalyst was 0.52 mmol/g-cat.

(14) Comparative Example 2 will be described. A catalyst powder as Comparative Example 2 was obtained by preparing a catalyst in the same manner as in Example 1, except that, after the acid treatment was conducted, the catalyst was filtered and cleaned with 1 L of pure water, and the filtration and cleaning were repeated until the conductivity of drainage or waste liquid became equal to or lower than 20 S/cm. The amount of acid in the catalyst was 0.628 mmol/g-cat.

(15) Comparative Example 3 will be described. A catalyst power as Comparative Example 3 was obtained by preparing a catalyst in the same manner as in Example 1, except that, after the acid treatment was conducted, the catalyst was filtered and cleaned with 1 L of pure water only once. The amount of acid in the catalyst was 0.996 mmol/g-cat.

(16) FIG. 1 shows the relationship between the catalyst acid amount and the low-humidity efficiency point performance (at 0.2 A/cm.sup.2) with regard to Examples 1, 2 and Comparative Examples 1-3 as described above. FIG. 2 shows the relationship between the catalyst acid amount and the low-humidity output point performance (at 1.02 A/cm.sup.2) with regard to Examples 1, 2 and Comparative Examples 1-3.

(17) As is understood from FIG. 1 and FIG. 2, Examples of the invention showed high voltage values at both of the current densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2, since the catalysts of these Examples had an acid that can be hydrophilic. On the other hand, Comparative Examples showed low voltage values at both of the current densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2. It is concluded from these results that the catalyst becomes hydrophilic when it contains an acid that can be hydrophilic, and the water-hold property around the catalyst is improved, resulting in a reduction in the resistance to proton shift in the catalyst layer.

(18) Comparative Example 4 will be described. Initially, 4.71 g of a commercially available carbon powder having a high specific surface area was added to and dispersed in 0.5 L of pure water, to provide a dispersion liquid. A hexahydroxo platinum nitric acid solution containing 4.71 g of platinum and an aqueous solution of cobalt nitrate containing 0.592 g of cobalt were dropped in this order into the dispersion liquid, to be sufficiently brought into contact with carbon. Then, about 5 mL of 0.01N ammonia was added to the resulting liquid to make PH equal to about 9, so that a hydroxide was formed and deposited on the carbon. The resulting dispersion liquid was repeatedly filtered and cleaned until the conductivity of filtration drainage became equal to or lower than 50 S/cm, and the obtained powder was dried in a vacuum at 100 C. for 10 hours. Then, after the dried powder was held in hydrogen gas at 500 C. for 2 hours so as to be reduced, it was held in nitrogen gas at 700 C. for 0.5 hour and held in the same gas at 600 C. for 6 hours, to provide an alloy of platinum and cobalt.

(19) Furthermore, the catalyst powder was thrown into 0.5 L of 0.5N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, the catalyst was filtered and cleaned with 1 L of pure water, and the filtration and cleaning were repeated until the conductivity of the cleaning drainage became equal to or lower than 20 S/cm. Then, the catalyst was isolated by filtration, and was dried in a vacuum drier at 100 C. for 12 hours or longer, to provide a catalyst powder as Comparative Example 4.

(20) Comparative Example 5 will be described. A catalyst powder as Comparative Example 5 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour and held in the same gas at 600 C. for 12 hours, to provide an alloy of platinum and cobalt.

(21) Comparative Example 6 will be described. A catalyst powder as Comparative Example 6 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour and held in the same gas at 600 C. for 18 hours, to provide an alloy of platinum and cobalt.

(22) Comparative Example 7 will be described. A catalyst powder as Comparative Example 7 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 6.5 hours, to provide an alloy of platinum and cobalt.

(23) Comparative Example 8 will be described. A catalyst powder as Comparative Example 8 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 800 C. for 6.5 hours, to provide an alloy of platinum and cobalt.

(24) Comparative Example 9 will be described. A catalyst powder as Comparative Example 9 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour, to provide an alloy of platinum and cobalt.

(25) Comparative Example 10 will be described. A catalyst powder as Comparative Example 10 was obtained in the same manner as in Comparative Example 4, except for the following steps. In Comparative Example 10, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour, to provide an alloy of platinum and cobalt. Furthermore, the catalyst powder was thrown into 0.5 L of 0.5N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, in Comparative Example 10, the catalyst was isolated by filtration but not cleaned with pure water, and was dried in a vacuum drier at 100 C. for 12 hours or longer.

(26) Comparative Example 11 will be described. A catalyst powder as Comparative Example 11 was obtained in the same manner as in Comparative Example 4, except for the following steps. In Comparative Example 11, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour, to provide an alloy of platinum and cobalt. Furthermore, the catalyst powder was thrown into 0.5 L of 0.5N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, in Comparative Example 11, the catalyst was isolated by filtration but not cleaned with pure water, and was dried in a blowing drier at 80 C. for 12 hours or longer.

(27) Example 3 will be described. A catalyst powder as Example 3 was obtained in the same manner as in Comparative Example 4, except that, after the alloying process, the catalyst powder was thrown into 0.5 L of 2N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, in Example 3, the catalyst was isolated by filtration but not cleaned with pure water, and was dried in a vacuum drier at 100 C. for 12 hours or longer.

(28) Example 4 will be described. A catalyst powder as Example 4 was obtained in the same manner as in Comparative Example 4, except for the following steps. In Example 4, after the alloying process, the catalyst powder was thrown into 0.5 L of 2N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, the catalyst was filtered and cleaned with 1 L of pure water, and the filtration and cleaning were repeatedly conducted until the conductivity of the cleaning drainage became equal to or lower than 20 S/cm. The catalyst was isolated by filtration, and was further thrown into 0.5 L of 0.5N nitric acid and stirred for 30 min at room temperature. Thereafter, the catalyst was isolated by filtration but not cleaned with pure water, and was dried in a vacuum drier at 100 C. for 12 hours or longer.

(29) Example 5 will be described. A catalyst powder as Example 5 was obtained in the same manner as in Comparative Example 4, except for the following steps. In Example 5, after the alloying process, the catalyst powder was thrown into 0.5 L of 2N nitric acid, heated to 80 C., and was stirred for 30 min, so that cobalt that had not been alloyed was removed by acid cleaning. Then, the catalyst was filtered and cleaned with 1 L of pure water, and the filtration and cleaning were repeatedly conducted until the conductivity of the cleaning drainage became equal to or lower than 20 S/cm. The catalyst was isolated by filtration, and was further thrown into 0.5 L of 0.5N nitric acid and stirred for 30 min at room temperature. Thereafter, the catalyst was isolated by filtration but not cleaned with pure water, and was dried in a blowing drier at 80 C. for 12 hours or longer.

(30) Example 6 will be described. A catalyst powder as Example 6 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.5 hour, to provide an alloy of platinum and cobalt, and the catalyst powder was thrown into 0.5 L of 0.5N nitric acid, heated to 80 C., and was stirred for 48 hours, so that cobalt that had not been alloyed was removed by acid cleaning.

(31) Comparative Example 12 will be described. A catalyst powder as Comparative Example 12 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 800 C. for 0.5 hour, to provide an alloy of platinum and cobalt.

(32) Comparative Example 13 will be described. A catalyst powder as Comparative Example 13 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 700 C. for 0.05 hour, to provide an alloy of platinum and cobalt.

(33) Example 7 will be described. A catalyst powder as Example 7 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 800 C. for 0.5 hour, to provide an alloy of platinum and cobalt, and that the catalyst powder was thrown into 0.5 L of 0.5N nitric acid, heated to 80 C., and was stirred for 48 hours, so that cobalt that had not been alloyed was removed by acid cleaning.

(34) Comparative Example 14 will be described. A catalyst powder as Comparative Example 14 was obtained in the same manner as in Comparative Example 4, except that, after the reduction process, the catalyst powder was held in nitrogen gas at 800 C. for 0.5 hour, to provide an alloy of platinum and cobalt, and the catalyst powder was thrown into 0.5 L of 0.05N nitric acid, heated to 80 C., and was stirred for 48 hours, so that cobalt that had not been alloyed was removed by acid cleaning.

(35) TABLE 1 below shows, in list form, the remaining acid amount and the power generation performance with regard to Examples 3-7 and Comparative Examples 4-14. FIG. 3 shows the relationship between the amount of acid in the catalyst and the low-humidity efficiency point performance (at 0.2 A/cm.sup.2), with regard to Examples 3-7 and Comparative Examples 4-14. FIG. 4 shows the relationship between the amount of acid in the catalyst and the low-humidity output point performance (at 1.02 A/cm.sup.2), with regard to Examples 3-7 and Comparative Examples 4-14.

(36) TABLE-US-00001 TABLE 1 Initial Cell Performance Low Humidity Catalyst Powder (Both Electrodes RH = 40) Acid Amount by Efficiency-point Output-point Back Titration Voltage at Voltage at mmol/g 0.2 A/cm.sup.2(V) 1.0 A/cm.sup.2(V) Com. Ex. 4 0.40 0.708 0.476 Com. Ex. 5 0.56 0.716 0.402 Com. Ex. 6 0.52 0.713 0.408 Com. Ex. 7 0.44 0.724 0.421 Com. Ex. 8 0.44 0.690 0.378 Com. Ex. 9 0.47 0.729 0.436 Com. Ex. 10 0.56 0.735 0.444 Com. Ex. 11 0.63 0.761 0.508 Example 3 1.01 0.765 0.514 Example 4 1.05 0.764 0.528 Example 5 1.31 0.758 0.525 Example 6 0.80 0.772 0.548 Com. Ex. 12 0.43 0.730 0.457 Com. Ex. 13 0.46 0.729 0.438 Example 7 0.73 0.763 0.532 Com. Ex. 14 0.55 0.760 0.521

(37) As is understood from FIG. 3 and FIG. 4, the platinum-alloy catalysts (Examples 3-7) according to the invention showed high voltage values at both of the current densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2, since the catalysts of these examples had an acid that can be hydrophilic. On the other hand, Comparative Examples showed low voltage values at both of the current densities, 0.2 A/cm.sup.2 and 1.02 A/cm.sup.2. It is concluded from these results that the catalyst becomes hydrophilic when it contains an acid that can be hydrophilic, and the water-hold property around the catalyst is improved, resulting in a reduction in the resistance to proton shift in the catalyst layer.

(38) In practicing the present invention, carbon carrying a known platinum catalyst or known platinum-alloy catalyst may be used. Also, in practicing the present invention, various types of acids may be used in an acid treatment performed on the platinum or platinum-alloy carrying carbon, and nitric acid may be preferably used.

(39) While some embodiments of the invention have been illustrated above, it is to be understood that the invention is not limited to details of the illustrated embodiments, but may be embodied with various changes, modifications or improvements, which may occur to those skilled in the art, without departing from the scope of the invention.

(40) The electrode catalyst for fuel cells according to the invention has a higher activity than the platinum catalyst or platinum-alloy catalyst of the related art, thus making it possible to reduce the amount of expensive platinum used in the catalyst.