JPH01246765A - Electrode of fuel cell - Google Patents

Abstract

PURPOSE:To suppress dissolution of a catalyst by electrolyte and provide a cell characteristic with stability for a long time by mixing a small amount of iridium with platinum as active substance of catalyst. CONSTITUTION:Chloroplatinic acid and iridium trichloride are added to a solution of carbon powder and ion exchange water which are well mixed and agitated An aqueous solution of sodium phosphorate is added to it followed by ultrasonic agitation, and chlorine ions are removed by suction and filtration, that is followed by heating to 100 deg.C to produce a platinum-iridium mixed catalyst. This is dispersed in ion exchange water, and polytetrafluoroethylene dispersed liquid is added to provide a catalytic black mix. An electrode using this catalytic black mix is highly active and hard to dissolve in electrolyte, so that a cell characteristic with stability for a long time is obtained.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to electrodes for fuel cells.

(Prior art) A fuel cell is a device that generates electric power by directly converting the chemical energy of the fuel into electrical energy by electrochemically reacting the fuel and an oxidizing agent. In principle, it causes the reverse reaction of water electrolysis. The electrode used for this is called a gas diffusion electrode, and has a catalyst layer coated on a conductive porous substrate, and the back side is made of polytetrafluoroethylene (hereinafter referred to as PTFE).
Apply a dispersion liquid etc. to give it appropriate water repellency,
The electrode is formed so that the electrode reaction in the three-phase zone of gas phase, liquid phase, and solid phase can be carried out efficiently.

A matrix layer holding an electrolyte such as phosphoric acid is interposed between a pair of gas diffusion electrodes of this type, and a superelectric reaction is performed by flowing fuel gas and oxidizing gas to the back surface of each electrode. ing.

There is no doubt that the performance of this battery is primarily controlled by the performance of the above-mentioned gas diffusion electrode, and various electrode catalysts have been studied in the past. Precious metals are generally used as catalysts,
From a cost perspective, in order to obtain high performance with a small amount of catalyst used, precious metal fine particles are dispersed and supported on a conductive carrier such as durable carbon powder, and the amount of precious metal catalyst used is at least as large as the active surface area. Large electrodes can be obtained.

In addition, each of the other electrode reactions in the battery is, for example, in the case of a phosphoric acid fuel cell, an oxygen reduction reaction at the cathode,
Although it is a hydrogen oxidation reaction at the cathode, the oxygen reduction reaction is more irreversible than the hydrogenation reaction, so more catalysts are required. Recently, a method of obtaining a high-performance electrode by reducing the oxygen overvoltage by using an alloy catalyst of platinum and other base metal elements has become mainstream.

That is, in order to improve performance, an alloy catalyst such as an alloy catalyst (carbon powder (manufactured by Cabot: Vulcan XC-72) made by Cabot Corporation) containing 10 wt.
%pt and 2wt%Cr) and a conventional non-alloyed catalyst (carbon powder added with 10vt%pt), the terminal voltage is approximately 30V at a constant polarization flow density of 220A/aJ. It was recognized that there was an improvement. In addition, alloyed catalysts that use not only chromium but also other base metals (metals with standard oxidation ring potential lower than platinum: Ti, V, Si, Mn, etc.) have higher performance than non-alloyed catalysts that use platinum alone. It has been shown that there is (USP 4,373,014)
.

(Problem to be solved by the invention) However, when a fuel cell is operated for a long period of time with one alloying catalyst incorporated, the base metal on the cathode side is dissolved by the electrolyte.
RB S (Rutherford Back-Sc
attering) method, P I X E (Parti
cle InducedXrey Emission)
Confirmed by law. For this reason, as the operating time elapses, for example, in the case of a Pt/V catalyst, the characteristics deteriorate after about 300 hours, and it has been desired to develop an alloying catalyst that does not dissolve out with the electrolyte.

Therefore, an object of the present invention is to provide an electrode for a fuel cell that can exhibit stable performance over a long period of time.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention forms a catalyst layer by applying a catalyst mixture in which an active ingredient is dispersed and supported on carbon or carbide on an electrostatic porous basic surface. The fuel cell electrode is characterized in that the active component is composed of a mixed powder of platinum and a small amount of iridium.

(Function) As mentioned above, since the electrode of the fuel cell of the present invention has a small amount of isodium mixed with platinum as an active component of the catalyst, it is possible to suppress the dissolution of the catalyst by the electrolyte, and it is stable for a long period of time. It is possible to obtain the desired battery characteristics.

(Example) An embodiment of the present invention will be described below.

That is, the electrode catalyst is based on platinum and has iridium added thereto.

Next, specific examples will be described in detail.

First, carbon powder (manufactured by Cabot: Vulcan
XC-72) Add chloroplatinic acid (H, PtCQ
,・6)1.0)6.5 method and iridium trichloride (IrC
ffa)・l'8 to 20)0.49 was added and stirred. Next, in another container, add 20g of sodium phosphate to 99g of water.
After adding the solution dissolved in 0 mQ to the above solution, the solution was ultrasonically stirred for about 5 hours, and then filtered by suction and washed with water until chlorine ions ((j2-) were no longer observed in the furnace solution.

Then, the mixture is heated to 100°C to obtain a platinum-iridium mixed catalyst.

Next, this mixed catalyst 2 method is dispersed in ion-exchanged water and a PTFE dispersion is added to make a catalyst mixture. After spraying this catalyst mixture on carbon paper, it is dried and the back side is made waterproof. A test electrode was prepared by processing and then baking at 350° C. for 30 minutes in a nitrogen atmosphere. At this time, the content of PTFH was 55 wt%.

Next, the platinum-iridium alloy catalyst of this example (8w
t%Pt-1wt%Ir, test electrode: A), conventional platinum catalyst (10wt%Pt:B) and platinum-chromium catalyst (
An electrode made of 10wt%Pt-2tzt%Cr:C) is used as the cathode side of the fuel cell, and a platinum catalyst (
7wt%) electrode was incorporated, and the initial voltage characteristics and life characteristics of the single cell were tested. Test conditions were normal pressure 205℃,
A constant polarization current density of 220 mA/a#, fuel and oxidant utilization were each 60%.

As a result, the voltage characteristics of test electrode A to which iridium was added were only about 5 to 10 mV higher than the conventional test electrode made of platinum alone.

On the other hand, as for the life characteristics, as shown in FIG.
After 0 to 350 hours have elapsed, the terminal voltage is found to be lower than test electrode B of the platinum single catalyst. This is because the iridium-added electrode has high activity and is difficult to dissolve in the electrolyte, so it can maintain stable performance over a long period of time.

In addition, we investigated the relationship between the content of iridium in platinum and the cathode electrode potential (Volts Vs RHE) using a half-cell device and at normal pressure and 205°C.
Figure 2 shows the results of testing under conditions of a current density of 220 +++ A/d and a large excess of air. From FIG. 2, it is clear that when 10 wt % or more of iridium is added, the battery characteristics gradually deteriorate, while if it is less than 1 wt %, there is no effect of adding iridium, so it is preferable to add 1 to low wt %.

Therefore, by adding 1 to 10 vj% iridium to platinum in the electrode catalyst, it has become possible to obtain an electrode with high performance and long life.

In the above example, the electrode catalyst was obtained by firing at 350°C for 30 minutes in a nitrogen gas atmosphere, but it may also be fired at 1200°C for 1 hour in a nitrogen or helium gas atmosphere. Pretreatment for firing in a helium gas atmosphere is at 350°C in a carbon monoxide gas atmosphere for 30 minutes, then at 1200°C in a nitrogen gas atmosphere.
for 1 hour at -350°C in a carbon oxide gas atmosphere.
0 minutes may be used, and the effect is the same as or better than that of the above embodiment.

〔Effect of the invention〕

As explained above, according to the present invention, since electrolyte-resistant iridium is added to platinum, stable battery characteristics can be maintained for a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a diagram comparing the terminal voltage with respect to the operating time of the fuel cell with that of a conventional electrode catalyst, and FIG. 2 is a diagram showing the relationship between the iridium content of platinum and the cathode electrode potential. Agent Patent Attorney Noriyuki Chika Yudo Ken Daishimaru

Claims (1)

[Claims] In fuel cell electrodes, the catalyst layer is formed by coating the surface of a conductive porous substrate with a catalyst mixture in which the active ingredient is dispersed and supported on carbon or carbide.The active ingredient is a mixed powder of platinum and a small amount of iridium. An electrode for a fuel cell, characterized in that it is configured by: