JPH0450061B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to electrocatalysts, particularly for use in cathodes in fuel cells.

BACKGROUND OF THE INVENTION Carbon supports for anode catalysts are important elements in fuel cells with regard to cell efficiency and longevity. However, conductive carbon black does not always exhibit sufficient resistance to corrosion caused by electrochemical oxidation in fuel cells. Also, thermodynamically,
Carbon as a material is unstable at the cathode potential in the warm phosphoric acid present in phosphoric acid fuel cells (PAFCs). Corrosion at the anode in a fuel cell can degrade fuel cell performance in various ways, such as increased platinum migration due to undercutting (i.e., corrosion of carbon under the platinum catalyst) and the carbon-Teflon interface. contributes to increased diffusion loss due to rupture. This places a fundamental limit on the lifetime of fuel cell electrodes. Additionally, the recent trend toward higher temperatures and pressures in fuel cells reaching higher anode potentials increases the likelihood of carbon corrosion. Furthermore, in practical working conditions, part-load operations are
It is necessary that the oxygen electrode must be at a potential of 0.8 volts or higher, which promotes carbon corrosion.

Improvements in the electrolyte portend even higher potentials at the oxygen electrode.

Therefore, new and improved support materials and catalysts for oxygen electrodes are needed to operate under the extreme conditions of fuel cells. The phosphoric acid fuel cell (PAFC) described here is based on A.Fickett, “Fuel Cell
Power Plant” Scientific American , 239 , 70~
76 (December 1978).

Titanium carbide (TiC) has been identified as a satisfactory conductor and appears to be stable in warm phosphoric acid at oxygen electrode potentials in fuel cells. However, commercially available titanium carbide has a relatively low surface area (less than 1 m ² /g) and is therefore generally not useful as an electrocatalyst support.

It is therefore an object of the present invention to provide a titanium carbide support having a crystallite size in the range of about 50 to 500 Å and a relatively large surface area of 25 to 125 m ² /g that is useful as an electrocatalyst support in fuel cells. be.

It is further an object of the present invention to provide a new electrocatalyst containing platinum impregnated on a titanium carbide support.

These and other objects of the invention will become apparent from the following description.

SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel electrocatalyst support and electrocatalyst and method for producing the same. The method involves producing an ultrafine titanium carbide catalyst support, which combines titanium tetrachloride in the gas phase with gaseous unsaturated hydrocarbons and hydrogen at temperatures between 500 and 1250 °C.
It consists of a step of contacting in a reaction zone in the range of . The capacity of hydrogen introduced into the reaction zone is such that the capacity ratio of hydrogen to titanium tetrachloride is 6 or more. The volume of unsaturated hydrocarbon introduced into the reaction zone is such that the titanium to carbon ratio is 4 or less.

There are many ways to convert titanium carbide catalyst supports into catalysts by impregnation with platinum. 2
Two methods have been described, one of which (method A) is to impregnate a titanium carbide support composition with a chloroplatinic acid alcohol solution, separate the impregnated particles from the solution, dry the particles, and heat the particles at about 400° C. in a hydrogen atmosphere. The process consists of exposing the particles to a carbon monoxide atmosphere at about 350°C. A second method of producing an electrocatalyst (Method B) consists of contacting the titanium carbide support composition with an aqueous solution of chloroplatinum, an aqueous solution of sodium dithionite, and dilute hydrogen peroxide.

The titanium carbide support composition according to the invention is characterized by a particle size in the range of 50-500 Å with open pores and a chain structure.

The titanium carbide support composition according to the present invention is produced by the following stoichiometric reaction: 2TiCl ₄ +C ₂ H ₂ +3H ₂ →2TiC + 8HCl The above reaction can be carried out using only reactant gases or optionally with an inert gas such as argon. In existence, it takes place in the gas phase. The reaction is carried out at relatively low temperatures in the range of about 500-1250°C, preferably 650-1100°C. The titanium tetrachloride volume concentration should be relatively dilute with respect to the hydrogen gas concentration, such that the ratio of the volume of hydrogen gas introduced into the reaction zone to the volume of titanium tetrachloride should be about 6 or more. The relative concentration of the hydrocarbon gas may be high and therefore the volume of hydrocarbon gas used should be such that the ratio of titanium to carbon in the hydrocarbon gas is 4 or less. Unsaturated hydrocarbons such as ethylene, acetylene, propylene and the like are used, with acetylene being preferred.

The reaction can be carried out in a conventional tube furnace, and the reaction is usually completed in about 2 to 10 hours. Under preferred conditions, the furnace is maintained at a temperature in the range of 650-1100°C, where the titanium tetrachloride vapor, hydrogen and hydrocarbon gases are either separately or premixed before being introduced into the reactor. will be introduced. If introduced separately, the titanium tetrachloride vapor and hydrocarbon gas are optionally introduced with an inert carrier gas such as argon gas. Argon may optionally also be used as carrier gas if all three gases are premixed. A typical production involves combining the three reactant gases in a reaction zone at a preferred temperature.
Total space velocity of less than 10 ^-1 h
This includes holding the sample for about 4 to 8 hours using high velocity. For optimum completion of the reaction, this is usually carried out by treatment of titanium tetrachloride with hydrogen for about 1 hour, followed by treatment with hydrogen for about 1 hour and cooling in an inert atmosphere.

The resulting titanium carbide is finer than fine particles with a grain size in the range of 50-500 Å with open pores and a chain structure that maintains excellent grain-to-grain contact and thereby good electrical conductivity for the composite catalyst layer. Become. Typically, the titanium carbide particles have a surface area of at least 25 m ² /g titanium carbide. Furthermore, such a chain structure can be easily manufactured by a conventional method for gas diffusion electrodes for fuel cells. Although this is an example of such generation, other techniques may be utilized, such as a reactive plasma torch.

To convert the support composition into an electrocatalyst, highly dispersed platinum is deposited onto ultrafine titanium carbide by various techniques to obtain a platinum surface area in the range of 20-90 m ² /g platinum. Ru. The platinum surface area is usually at least about 23 m ² /g, preferably 50 m ² /g.
g platinum or higher. Titanium carbide is impregnated by contacting titanium carbide particles with an alcoholic solution of chloroplatinic acid. The concentration of the chloroplatinic acid solution will vary depending on the amount of platinum desired to be deposited on the support. It is well within the skill of those skilled in the art to determine the appropriate chloroplatinic acid solution concentration according to the required specifications of a particular fuel cell design.

The particles are then separated from the solution, dried to remove the alcohol solvent, and the impregnated platinum salt is dried in a hydrogen atmosphere, e.g.
It is reduced by exposure to a hydrogen atmosphere at 400°C followed by exposure to a carbon monoxide atmosphere at about 350°C. Preferably, the particles should then be rapidly cooled; cooling to about 100°C is usually sufficient, although cooling to room temperature is preferred.

Additionally, titanium carbide particles are suspended in an aqueous chloroplatinic acid solution. As mentioned above, the concentration of chloroplatinic acid, solution and amount of solution used will vary depending on the specific concentration of platinum desired in the catalyst support. This can be easily determined by a person skilled in the art according to the required specifications of the fuel cell. After contacting the particles with the chloroplatinic acid solution, the platinum salt is reduced by contacting with an aqueous sodium dithionite solution and then with dilute hydrogen peroxide to remove excess dithionite. Similar results were obtained by first reacting an aqueous chloroplatinic acid solution with sodium dithionite and hydrogen peroxide to obtain a colloidal platinum suspension, and then adsorbing very fine platinum particles onto titanium carbide particles. It can be obtained by adding titanium carbide particles to. Preferably, the particles are then washed and dried to remove residual moisture.

FIG. 1 shows a platinum-titanium carbide electrocatalyst prepared according to the present invention in which platinum is deposited on a support (0.5 mg platinum/cm ² , 50% phosphoric acid,
20℃, 5mV/S scanning speed, a and b in the figure are method (A)
c is produced by method (B)).
voltammogram). In FIG. 1, the horizontal axis is in units of mV, and the vertical axis is in units of mA/cm ² . The platinum surface areas obtained are shown in drawings a, b, and
It is about 23, 68, 94 m ² /g at c, and the measurement is 23
°C, potential sweep rate of 5 mV/sec
sweep rate) in 50% phosphoric acid solution using a 1 cm ² electrode.

In FIG. 2, the oxygen reduction activity of the platinum-titanium carbide electrical contact according to the invention with a platinum surface area of 23 m ² /g (solid line) is shown.
reduction activity) compared to a conventional platinum-carbon electrocatalyst with a surface area of 120 m ² /g (dotted line). Each catalyst was distributed at 0.5 mg platinum/cm ² and the tests were conducted at 200° C. in a phosphoric acid fuel cell utilizing 100% phosphoric acid as the electrolyte. The vertical axis in FIG. 2 shows the potential measured in mV relative to the reversible hydrogen electrode (RHE). As can be seen from Figure 2, not only is the platinum-titanium carbide catalyst operable up to high current densities (i.e., about 4500 μA/cm ² platinum), but also the platinum-carbon catalyst is operable up to about 18-1000 μ/cm ² . Normal platinum within the range -
It was more viable than a carbon catalyst.

In FIG. 3, the performance of the catalyst preparation of the present invention with a platinum surface area of 68 m ² /g is shown to be further improved over a conventional platinum-carbon catalyst with a platinum surface area of 120 m ² /g. Simple changes in the catalyst support can result in such improved benefits, especially
It is surprising that this occurs under the very strict conditions used in PAFC.

From the above description of the preferred embodiments of the present invention, it will be apparent to those skilled in the art that certain modifications and variations can be made without departing from the spirit of the invention, and such modifications and variations are within the scope of the present invention. means inside.

[Brief explanation of the drawing]

In the accompanying drawings, FIG. 1 shows a cyclic voltammogram of three platinum-titanium carbide catalysts in a phosphoric acid fuel cell (PAFC) according to the invention. FIGS. 2 and 3 are diagrams comparing the oxygen reduction activities of the platinum-titanium carbide according to the present invention (solid line) and a conventional platinum-carbon electrocatalyst (dotted line).

Claims (1)

[Claims] 1. A finely divided platinum catalyst is supported on an ultrafine titanium carbide support having open pores and a chain structure, a particle size of 50 to 500 Å, and a surface area of at least 25 m ² /g, An electrocatalyst for catalytically reducing oxygen, characterized in that it has a platinum surface area ranging from 20 to 95 m ² /g platinum and a titanium carbide surface area of at least 25 m ² /g titanium carbide. 2 Titanium tetrachloride in the gas phase is mixed with gaseous unsaturated hydrocarbons and hydrogen, and the volume of hydrogen introduced into the reaction zone is such that the volume ratio of hydrogen to titanium tetrachloride is about 6 or more, and The volume of hydrocarbon introduced is produced by a contacting process in a reaction zone at a temperature in the range of 500 to 1250°C, such that the titanium to carbon ratio is less than about 4, and has open pores and a chain structure. An ultrafine titanium carbide support characterized in that it has a particle size of 50 to 500 Å and a surface area of at least 25 m ² /g. 3 (a) Has open pores and chain structure, and has 50 to 500
impregnating an ultrafine titanium carbide support with a particle size of Å and a surface area of at least 25 m ² /g with an alcoholic solution of chloroplatinic acid; (b) separating the impregnated particles from said solution; and then drying said particles; c) exposing said particles to a hydrogen atmosphere at about 400°C; and (d) exposing said particles to a carbon monoxide atmosphere at about 350°C, having open pores and a chain structure and having a diameter of 50 to 500 Å. particle size of at least 25
A finely divided platinum catalyst is supported on an ultrafine titanium carbide support having a surface area of m ² /g, a platinum surface area ranging from 20 to 95 m ² /g platinum, and a titanium carbide surface area of at least 25 m ² /g titanium carbide. A method for producing an electrocatalyst for catalytically reducing oxygen, comprising: 4 (a) Has open pores and chain structure, and has 50 to 500
(b) contacting the particles with an aqueous solution of ^{chloroplatinic} acid; It has open pores and a chain structure, and has a diameter of 50 to 500 Å.
A finely divided platinum catalyst is supported on an ultrafine titanium carbide support having a particle size of from 20 to 95 ^{m 2 /} g platinum and a surface area of at least 25 m ² /g titanium. A method for producing an electrocatalyst for catalytically reducing oxygen having a surface area of titanium carbide. 5. A finely divided platinum catalyst supported on an ultrafine titanium carbide support with open pores and chain structure, a particle size of 50 to 500 Å and a surface area of at least 25 m ² /g, and a surface area of 20 to 95 m ² /g. A fuel cell using as an anode an electrocatalyst for catalytically reducing oxygen having a platinum surface area in the range of platinum and a titanium carbide surface area of at least 25 m ² /g titanium carbide.