CA1120099A

CA1120099A - Borides of molybdenum as catalyst in fuel cell electrodes

Info

Publication number: CA1120099A
Application number: CA000329715A
Authority: CA
Inventors: Hidehito Obayashi; Tetsuichi Kudo
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1978-06-19
Filing date: 1979-06-14
Publication date: 1982-03-16
Also published as: DE2924678C2; FR2429500B1; GB2023445A; GB2023445B; JPS551818A; JPS5731454B2; DE2924678A1; NL7904745A; US4242421A; FR2429500A1

Abstract

ELECTRODE CATALYST

Abstract of the Disclosure The specification discloses an electrode catalyst for a fuel cell comprising at least one boride of molybdenum.
The electrode catalyst is better than conventional catalysts, such as platinum black, in that the resistance to acids is very high, there is little polarization and the price is low, etc. It is particularly suitable as an electrode catalyst for a fuel cell emloying an acidic electrolyte.

Description

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This invention relates to an electrode catalyst for a fuel cell. More particularly, it relates to an electrode catalyst which is especially suitable for a fuel cell using an acidic electrolyte.
Fuel cells employing an acidic solution as an electro-lyte (in this specification, such fuel cells are referred to as "acidic electrolyte type fuel cells"~ show no tendency for the electrolyte to degenerate because neither the carbonic acid - gas produced by the electromotive reaction nor carbonic acid gas contained in the air is absorbed by the electrolyte. This is an important advantage over fuel cells employing alkaline electrolytes. On the other hand, however, effective electrode catalysts for acidic electrolytes are not available, which is a serious problem~
An electrode catalyst for acidic electrolyte type fuel cells needs not only a high catalytic activity, but also a sufficiently high acid resistance. Therefore, suitable materials are inevitably limited in the extreme.
Heretofore, the precious metals platinum, palladium etc. have usually been employed as electrode catalysts for acidic electrolyte type fuel cells. However, they are expensive and the catalytic activities are unsatisfactory when uslng methanol etc. as fuels.
As substitutes for the precious metals, electrode catalysts have recently been proposed ln which acid-proof com-pounds, such as tungsten carbide (WC) [H. Bohm, F. A. Paul, Wissenschaft. Ber. AEG Telefunken, 41 (1968), 46] and cobalt phosphide (CoP3) [G. Luft et al, Siemens Forsch. u. Entwickl.-Ber. Bd. 3 (1974) Nr~ 3,177], are used in an aqueous solution of sulfuric acid or phosphoric acid.

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- ~2~)099 These electrode catalysts can achieve comparatively favorable results when employed as hydrogen electrode catalysts. However, they exhibit very low catalytic activities when usin~ methanol etc. as fuels, and they are difficult to put into practical use.
An object o this invention is to provide an elec-trode catalyst for an acidic electrolyte type fuel cell which is inexpensive and which has good characteristics.
According to the invention there is provided an electrode catalyst for a fuel cell employing an acidic electrolyte and utilizing methanol as a fuel, comprising at least one boride of molybdenum.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:-Figures 1 and 4 are schematic views showing sectional structures of different embodiments of fuel cells; and Figures 2 and 3 are graphs explaining the effects of the present invention.
Unlike the various catalysts of the prior art, elec-trode catalysts which are made of borides of molybdenum present unexpected and remarkable catalytic activities as electrode catalysts for the electrolytic oxidation of various fuels, e.g., hydrogen, formalin, ormic acid, etc.
in acidic aqueous solutions~ ;
In particular, the activities of the molybdenum borides for the electrolytic oxidation reaction of methanol: ~ -3 2 CO2 6 H ~ 6 e are much higher than those of the precious metals such as platinum etc. When platinum is employed as the electrode catalyst for the electrolytic oxidation of methanol, the '13 catalyst is soon poisoned by intermediate products, such as CHO and CO, and hence the polarization increases. In contrast, the -:, ~ .
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l~Z~)~)99 electrode catalysts made of the molybdenum borides according to this invention scarcely undergo such poisoning, so that the open circuit voltages exhibit values close to the theoretical voltage ~0.04 V, versus the potential of the reference hydrogen electrode)of the electrolytic oxidation of methanol. Even when current is taken out of the cell, the polarization increases by very little.
Examples of the borides of molybdenum which may be used in the invention are Mo2B, a-MoB, ~-MoB, MoB2 and Mo2B5 In this invention, any one of these molybdenum borides can be used alone or a mixture consisting of two or more of the borides can be employed. Favorable results are obtained in any case.
Even when Mo3B2 and MoB4 are used in the aforementioned com-pounds,favorable results can be similarly obtained.
The borides of molybdenum presently known are the seven compounds mentioned above. It is firmly believed that other compounds, ~hich are generally expressed as MoBx, will also have catalytic activities in the electrolytic oxidation , reactions of fuels.
The acid resistance of the borides of molybdenum increases as the ratio of the boron content to the molybdenum content increases. Thus MoB2 and Mo2B5 are not dissolved even when boiled in sulfuric acid or phosphoric acid at a con-centration of 6 mols, and they are particularly suitable as electrode catalysts for fuel cells employing acidic electrolytes.
When the ratio of the boron content to the molybdenum content is small, the acid resistance is reduced. However, even when Mo2B or MoB are put in sulfuric acid or phosphoric acid at a concentration of 1 mol and allowed to stand at normal temperature for 100 hours, no dissolution is noted, and it is .

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' l~LZ()099 recognized that any of the molybdenum boride can be used with-out difficulty as electrode catalysts for acidic electrolyte type fuel cells.
Example 1 Figure 1 shows an example of a sectional structure of a methanol - air fuel cell. In Figure 1, an air electrode 1 is an electrode which is usually called the Teflon (trademark, polytetrafluoroethylene) bonded electrode. It is formed in such a way that a mixture of active carbon and graphite grains are made into a paste like substance by a small quantity of polytetrafluoroethylene dispersion and applied to a wire net made of tantalum.
A fuel electrode 2 was formed as follows.
Commercially available Mo2B5 powder was thoroughly washed with a hot aqueous solution of phosphoric acid at 3 mols, to dissolve and remove impurities. When the powder thus obtained was examined by X ray diffraction, only the diffraction spectrum of Mo2B5 was noted. Active carbon (10 g.) was added to 1 g. of this powder,and they were thoroughly mixed. The mixture was molded after adding a small quantity of pitch thereto. The molded compact was heat-treated and sintered to form a porous carbon plate containing Mo2B5 and~which was about 2mm thick.
` ` In this Example, the electrode was formed by mixing the powder of the boride of molybdenum with another material in advance and molding the mixture as described above. The fuel electrode in this invention, however, can be formed by various methods other than the foregoing.
By way of example, it is possible that a porous molded compact of active carbon or the like may be formed in advance and that the boride of molybdenum provided in the porous molded compact by chemical vapor deposition. The porous , Z~O9~ -, - molded compact may also be immersed in a solution containing molybdenum and boron and then dried and heat treated. By this `
means, the boride of molybdenum is carried as a thin layer on 'the walls of the pores of the porous molded compact. Therefore, the required amount of molybdenum and boron may be much smaller then in the method of the present Example.
A phosphoric acid electrolyte 3 is an aqueous solution of phosphoric acid which is dissolved in methanol used as the ' -fuel. Both the concentrations of methanol and phosphoric acid are 2 mol/Q. The air electrode 1 and the fuel electrode 2 are electrically connected to terminals 4 and 5, respectively: The phosphoric acid electrolyte 3 is received in a cell container 5 made of an acrylic resin. Further, a gas exhaust port 7, which also serves as an electrolyte injection port, is provided. The `~ single-electrode potential of the fuel electrode 2 can be evaluated by means of a referen~e hydrogen'electrode 8 which is immersed in the common electrolyte 3.
The relationship between the voltage and the current , density at 50 C of a fuel cell having such~a structure is ' illustrated in Figure 2.
In Figure 2, a curve 9`indicates the relation obtained when employing a fuel electrode containing the electrode , catalyst according to this invention formed by the method déscribed above. A curve 10 indicates the relation obtained when platinum black was used instead of, Mo2B5 as the electrode catalyst and the other conditions were,made the same.
It can be seen from the ,two'~curves 9 and lO`that when Mo2B5 is employed as the electrode catalyst, far better results are obtained than when using platinum black. In order to make' - , .
it more definite that the difference is based on the different fuel electrodes, the relationships between the single-electrodè

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potential and the current density were investigated for both the fuel electrodes. The results indicated in Figure 3 were thus obtained. In the figure, a curve ll illustrates the characteristics at 50 C of the electrode employing Mo2B5 as the catalyst. It is nobler by only `about 0.07 V than the theoretical potential of the following reaction in the open-circuit state:
CH30H + H2O -~ CO2 + 6 H + 6 e On the other hand, when employing platinum black as the catalyst, the open-circuit voltage is nobler as much as 0.25 V with respect to the theoretical value as seen from curve 12.
Obviously, this is the most important cause for the reduction of operating voltage of the fuel cell when employing platinum black as the catalyst.
Example 2 Methanol - air fuel cells were fabricated by the method of Example 1 respectively provided with three sorts of fuel electrodes whose catalysts were MoB2, a mixture consisting of MoB2 and Mo2B5 (at a weight ratio of 1 : l)j and a mixture consisting of -MoB and Mo2B5 (at a weight ratio of 1 : 2).
The current density - voltage characteristics of the fuel cells at 50 C were measured, and were compared with that in the case of employing platinum as the cataIyst. The results indicated in Table 1 were obtained.
As is apparent from Table l, the terminal voltages at 20 mA/cm in all cases when employing the borides of molybdenum as the electrode catalysts were higher than in the case of using platinum.

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Weight ratio of Terminal voltage (V) Catalyst to catalystat 20 mA/cm ~; ~ .
MoB2 1.10 0.78 MoB2 ~ Mo2B5 .
(weight ratio 1.120.76 a-MoB ~ Mo2B5 10(weight ratio 1.10 0.75 ~of l : 2) platinum -_ Example 3 Figure 4 shows an example of a sectional structure of a ~;~ hydrogen - air fuel cell. In Figure 4, a porous hydrogen electrode 21 was fabricated as follows~. ~Active carbon powder ~(10 g.) was added to 1 g. of MoB2 powder and~they were thoroughly mixed. Thereafter, a suspension of polytetrafluoroethylene was added to the mixture to form a pasty substance. The paste ~was applied onto a grid made of tantalum, and it was heated at 300 C for 30 minutes, thus forming the hydrogen electrode. ~ -.
An air electrode 22 was formed by the same method as described in Example 1. ~ ~
An aqueous solution of phosphoric acid at a concentration of 5 mol /~ was employed as an electrolyte 23. Numerals 24 and 25 dé~ignate the terminals of the hydrogen electrode and the air electrode respectively, numerals 27 and 28 designate inlets for hydrogen and air respectively, and numerals 29 and 30 designate outlets for exhaust hydrogen and exhaust air, respectively.

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, .Z~099 The voltage of the hydrogen - air fuel cell at a temperature of 60 C and a current density of 80 mA/cm was 0.84 V, which was much better than a voltage of 0.75 V obtained under the same conditions by employing platinum as the catalyst.
. Example 4 Ion-exchanged water (5 mQ) was added to 1 g. of`Mo2B5 powder of under 300 mesh size. While agitating well, 0.1 mQ of a polytetrafluoroethylene suspension (content of polytetra-fluoroethylene: 0.4 gr/mQ) was dropped into the ion-exchanged water Agitation was continued for 5 minutes after the com-pletion of the dropping whereupon 100 mQ of acetone was added.
Thèn, a slimy sediment with Mo2~5 and polytetrafluoroethylene mixed therein was obtained. It was applied onto a tantalum-coated Ni wire net of 60 mesh at a density of about 20 mg/cm2, and the resultant Ni wire net was pressed under 1 ton/cm2. The net was heat-treated in N2 at 230 C for 1 hour. Thus, an electrode was obtained. The characteristics of this electrode at 50 C were measured by the same method as that used in obtaining Figure 3 in Example 1. Then, a favorable result of -0.24 V (potential versus a hydrogen reference electrode)`wasobtained as the single-electrode potential at a current density of 50 mA/cm2.
Although the above examples have illustrated those cases .. employing methanol and hydrogen as fuels, this invention is of course applicable to fuel cells employing other various fuels, such as ethyl alcohol, hi.therto known. This invention, however, can produce its most favorable result when using methanol or hydrogen as the fuel.
Although various acids can be employed as acidic electrolytes, phosphoric acid and sulfuric acid are favoured as their boiling points are high and they do not generate any harmful gases.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrode catalyst for a fuel cell employing an acidic electrolyte and utilizing methanol as a fuel, comprising at least one boride of molybdenum.

2. A catalyst according to claim 1, wherein the boride of molybdenum is selected from the group consisting of Mo2B, .alpha.-MoB, .beta.-MoB, MoB2 and Mo2B3.

3. A catalyst according to claim 2, wherein the boride of molybdenum further includes Mo3B2 and/or MoB4.

4. A catalyst according to claim 1, 2 or 3, wherein said acidic electrolyte is sulfuric acid or phosphoric acid.