JPS6343260A - Manufacture of fuel cell - Google Patents

Abstract

PURPOSE:To keep activity for a long time, by forming a electrode catalyst by holding and dispersing fine metal powder to carbon powder subjected to an oxidizing process, mixing it with Teflon dispersion, applying the mixture to a side of a porous base member and baking it. CONSTITUTION:Carbon powder is subjected to an oxidizing process by use of a catalyst chiefly composed of iron oxide and platinum fine powder held by and dispersed in the carbon powder oxidized to form an electrode catalyst. The electrode catalyst is mixed with Teflon dispersion as a binding agent and the mixed paste is applied to one side of a porous base member 1 and baked for forming a cathode 3 and an anode 4. The electrodes 3, 4 are disposed in such a manner that an electrolytic layer 5 is interposed between their catalyst layers and grooves defined on the back surfaces of the electrodes 3, 4 are orthogonal each other. Fuel gas 6 and oxidation agent gas 7 are supplied to the electrodes 3, 4, respectively. With the arrangement, excellent activity can be maintained for a long time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a gas diffusion electrode used in a fuel cell, and more particularly to a method for manufacturing a fuel cell in which an electrode catalyst is improved.

(Prior Art) Fuel cells are conventionally known as devices that directly convert chemical energy contained in fuel into electrical energy. This fuel cell usually consists of an electrolyte layer (
A pair of gas diffusion electrodes consisting of a fuel electrode and an oxidizing agent 1 each having a catalyst layer formed on one side are placed facing each other with a matrix layer) in between, and a fuel gas such as hydrogen is brought into contact with the back surface of the fuel electrode. Oxidizing gas such as oxygen is brought into contact with the back side of the oxidizing electrode, and the electrochemical reaction that occurs is utilized to extract electrical energy from between the two electrodes. Electrical energy can be extracted with high conversion efficiency as long as it is supplied.

Incidentally, a unit cell of a fuel cell usually has a pair of electrodes, as shown in FIG. An electrolyte layer 5 is sandwiched between an anode (fuel electrode) 3 and a cathode (oxidation electrode) 4, which are pressed into one piece.

The picture electrodes 3 and 4 are arranged so that grooves formed on their back surfaces are perpendicular to each other, and a fuel gas 6 and an oxidizing gas 7 are supplied from each side to start an electromotive reaction. , obtaining direct current electricity. A plurality of these unit batteries are stacked with separators 8 in between and used for power generation.

Therefore, in order to obtain an efficient fuel cell, it is necessary to increase the speed of the electrochemical reaction occurring on the picture electrodes 3 and 4 of the cell, and an electrocatalyst is used for this purpose. As this electrode catalyst uses a white metal such as platinum, in order to obtain a high performance product in a small amount, carbon powder with a relatively large specific surface area is used to disperse and support fine particles of the platinum metal. . In order to obtain even better electrode catalysts, measures have been taken to maximize the surface area of the catalyst, which is the site of electrode reactions, and to increase the activity of the metal fine particles themselves, such as investigating methods of supporting catalysts and alloying platinum metals. There is.

On the other hand, voltage loss in electromotive reactions is particularly large in cathode reactions (oxidation-reduction reactions), so much effort has been made to develop cathode catalysts.

In other words, in recent years Giner's Jalan (J
a 1an) investigated the correlation between the catalytic activity for redox reactions and the distance between metal atoms in alloy catalysts (platinum-based), and formed an alloy with the shortest interatomic distance with platinum among the alloy catalysts investigated. It has been found that a catalyst in which a Pt-Cr alloy is supported on heat-treated carbon has the highest activity. (
J, Electrschem, Soc, 130 229
9c1983) Based on the criterion of the correlation between the distance between alloy metal atoms and the catalytic activity, roughly ternary alloy catalyst Pt-C
It has been found that o-Ni, Pt-Cr-C, Pt-Cr-Ce, etc. exhibit high activity. (Unexamined Japanese Patent Publication No. 61-885
In addition, the authors claim that by using such an electrode catalyst, the catalytic activity can be increased, and at the same time, the electrode catalyst can be resistant to a decrease in surface area due to sintering of the catalyst. (USP 4373014, 43169 (Problems to be Solved by the Invention)) Using the above-mentioned electrode catalyst, phosphoric acid, which has been developed for use in power generation plants, is used as an electrolyte in a fuel cell having the configuration shown in Figure 2. In order to promote the electromotive reaction and utilize waste heat, the batteries are operated at around 200°C.For this reason, the catalyst, which showed high activity at the beginning of the battery operation, sintered as the operation time progressed and the catalyst formed on the carrier. Since the catalyst dispersed and supported on the catalyst agglomerates and its surface area decreases, there is a frequent problem that the initial performance cannot be maintained over a long period of time.

By the way, in order to suppress sintering of a carbon-supported platinum metal catalyst, it is first necessary to understand the sintering mechanism.

The sintering mechanism has been investigated by many people and has been broadly classified into the following three types.

(1), agglomeration of platinum due to oxidative depletion of carrier carbon (2)
), aggregation due to heat-induced diffusion of platinum atoms or crystallites on the surface of the carrier (3), aggregation due to dissolution-re-precipitation of precious metal fine particles in electrolytic snow.The present inventors will investigate the sintering of carbon-supported platinum catalysts. As a result of conducting half-cell tests on catalysts placed in various conditions, it was found that the above-mentioned (2) effect of movement and aggregation of platinum fine particles due to heat was dominant.

Therefore, it was determined that as a means to suppress sintering, it is necessary to firmly adhere the platinum fine particles precipitated on the carrier onto the carrier surface. 'Proposals to suppress syringing based on this idea have been made before, for example in the aforementioned VTC Jalan.
) etc. heat-treat a carbon-supported platinum catalyst at 500 to 1200°F in a CO or hydrocarbon gas atmosphere to precipitate carbon on or around the supported platinum crystallites, thereby inhibiting platinum migration and aggregation. I suggested that it be suppressed. (υSP, 4137372) Although this method certainly suppressed platinum aggregation,
There was a point where the catalytic activity changed slightly depending on the gradient of the CO or hydrocarbon gas introduced.

1' Therefore, based on the idea that the movement and aggregation of platinum fine particles on the carrier can be suppressed by modifying the carrier itself, the present inventor carried out an oxidation treatment on the carrier, thereby modifying the carrier surface and increasing the platinum content. An attempt was made to suppress sintering by forming a barrier to diffusion.

The present invention has been made based on the above-mentioned attempts, and has an object to enable a fuel cell electrode catalyst to have a low degree of sintering under cell operating conditions, and to enable an electrode using the same to maintain high activity over a long period of time. The purpose is to

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention oxidizes carbon powder using a catalyst containing iron oxide as a main component. Platinum metal particles are dispersed and supported on carbon powder to form an electrode catalyst, and this electrode catalyst is kneaded with Teflon-based dispersion as a binder to form a kneaded paste. It is characterized in that a fuel cell is manufactured using an electrode obtained by coating one side of a base and then firing it.

(Function) As mentioned above, by subjecting the carrier to oxidation treatment in advance, the surface of carbon is modified, and the fine metal particles of platinum metal attached thereto diffuse and aggregate on the surface in the operating environment of the fuel cell. The process is suppressed.

(Example) 35 g of carbon powder Vxc-72R (manufactured by Cabot Corporation, specific surface area 2507 rt/g, average particle diameter 30 mμ) was collected, 3 g of iron oxide was mixed therein, this was placed in a graphite crucible, and the mixture was placed in a nitrogen gas atmosphere. The weight of the mixture at 1500℃ is 33
It was heated until it reached about C1. Thereafter, iron oxide was removed, and fine platinum particles were precipitated and supported on the carbon by a known method (wet reduction of chloroplatinic acid with sodium formate).

A Teflon dispersion (Mitsui Fluorochemical Co., Ltd.) was added to this carbon-supported platinum catalyst so that the Teflon content was 30-t% as a binder based on the total catalyst weight.
on30-J) to create a kneaded paste, filtered this kneaded paste, divided into 10 equal parts, and divided into 2
After coating on a porous substrate at 00'C, it is pressed at a constant pressure.

Then, dry it for 330 days in a nitrogen gas atmosphere.
It was baked for 20 minutes at °C.

A test piece with an apparent area of 1 crA was cut out from this electrode and incorporated into a floating half cell.

This test electrode is placed at a constant potential (0,
7VVS, RHE) was applied, and changes in surface area over time were investigated at fixed time intervals using a potential scanning method.

For comparison, test specimens of the catalyst were prepared by supporting platinum on a conventionally produced carbon carrier without oxidation treatment, and changes in surface area over time at the same potential were measured at regular intervals.

Figure 1 shows the above experimental results. The electrode (A>) manufactured by the method according to the present invention using a catalyst in which fine platinum particles were deposited on a carrier subjected to oxidation treatment was Compared to the electrode (B) manufactured by a conventional method using a catalyst in which platinum was deposited on a carrier without any treatment, both the speed and rate of decrease in surface area were lower.

In addition, in order to investigate the activity of both voltages at the initial stage of this test, a 1-7 curve of the air reduction reaction was taken, and the activity of the electrode whose carrier was oxidized by this method was 0.0% compared to that of the untreated electrode. At 9V (vsRHE), the current value was 30% larger, and no decrease in catalytic activity was observed due to the oxidation treatment.

It was also found through experiments that a similar effect can be obtained by using copper oxide instead of iron oxide.

Other Embodiments In the oxidation treatment of the carrier in the above embodiments, instead of using only iron oxide as an oxidation catalyst, a trace amount of iron oxide was added.
When this treatment was performed with the addition of Al2O2, the reduction rate of the surface area of the electrode was significantly reduced compared to the untreated one. This is carbon basa
It is presumed that this is because the effect of generating e-plane chpit was greatly increased by adding a cocatalyst such as Kana 20.

[Effects of the Invention] As described above, the electrode for fuel cells manufactured by the method according to the present invention has great durability against sintering and the activity of the electrode itself is excellent, and it can be used to manufacture single cells. The constructed fuel cell exhibits long life and high performance characteristics, an effect that cannot be obtained with conventional manufacturing methods.

[Brief Description of the Drawings] Figure 1 is a graph showing changes over time in the platinum surface area of electrodes manufactured by the method according to the present invention and electrodes manufactured by the conventional method (however, the vertical axis is the graph after 2.5 hrs). Figure 2 is a perspective view of a unit cell of a fuel cell. 1...Porous substrate 2...Catalyst layer 3...Anode 4...Cathode 5...Electrolyte layer 6...Fuel gas 7...Oxidant gas 8...Separator representative Patent attorney Noriyuki Chika Yudo Hirofumi Mitsumata

Claims (4)

[Claims] (1) A pair of electrodes, a fuel electrode and an oxidation electrode each having a catalyst layer formed on one side, are arranged so that the surfaces on which the catalyst layer is formed sandwich an electrolyte layer, and fuel is supplied to the fuel electrode, and In a fuel cell manufacturing method in which an oxidizing agent is supplied to an oxidizing electrode and electrical energy is extracted from between the two electrodes through the electrochemical reaction that occurs, carbon powder is oxidized using a catalyst containing iron oxide as a main component. The oxidized carbon powder is dispersed and supported with platinum metal fine particles to form an electrode catalyst, and this electrode catalyst is kneaded with a Teflon dispersion as a binder to form a kneaded paste. A method for manufacturing a fuel cell, characterized in that the kneaded paste is applied to one side of a porous base, and then the electrode obtained by firing is used to manufacture a fuel cell. (2) The method for manufacturing a fuel cell according to claim 1, characterized in that k_2O or Al_2O_3 is added to the iron oxide. (3) The method for manufacturing a fuel cell according to claim 1, characterized in that copper oxide is used in place of the iron oxide. (4) The method for manufacturing a fuel cell according to claim 1, wherein the firing is performed in a nitrogen gas atmosphere.