Solid polymer electrolyte having high-durability

Details Solid polymer electrolyte having high-durability Solid polymer electrolyte having high-durability

2000-11-29

2003-08-19

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06607856

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid polymer electrolyte having high-durability, and more particularly, to the solid polymer electrolyte having high-durability, which is excellent in oxidation resistance and preferably employed as a solid polymer electrolyte membrane for use in a polymer electrolyte fuel cell, a water electrolysis cell and the like.
2. Description of Related Art
Solid polymer electrolyte is the solid polymer material, in which the polymer chains have electrolyte groups, such as sulfonic acid groups, carboxylic acid groups and the like. The solid polymer electrolyte forms a strong bond with a specific ion and causes cations or anions to pass through selectively. The solid polymer electrolyte, therefore, is formed into particles, fibers, films or the like, and then is utilized for various purposes, such as electrodialysis, diffuse dialysis, and a battery.
In such backgrounds, for example, the above mentioned solid polymer electrolyte acts as a polymer electrolyte membrane for use in a polymer electrolyte fuel cell and a water electrolysis cell. In this case, the polymer electrolyte fuel cell comprises a proton-conducting solid polymer electrolyte membrane, the both surfaces of which are provided with a pair of electrodes. With such structure, the polymer electrolyte fuel cell produces electromotive force in a manner of supplying pure H
2
or reformate H
2
gases to one electrode (an anode) as fuel gases, and supplying O
2
gases or air to another electrode as oxidant. Water electrolysis is a process for producing H
2
and O
2
by electrolyzing water by using a solid polymer electrolyte membrane.
On the other hand, in case of the above-mentioned polymer electrolyte fuel cell and water electrolysis cell, peroxide is produced by the electrode reaction, at a catalyst layer formed on a boundary face between a solid polymer electrolyte membrane and an electrode. Then peroxide diffuses, and then, the radical reaction occurs to form peroxide radicals, thereby causing the electrolyte to be degraded disadvantageously. Forming peroxide radicals is promoted by metal ions (Fe
2+
and Cu
2+
and the like) effluent from a tube for supplying mist, blended with supplied-gases for keeping electrolyte wetting.
To solve such problems, various electrolytes have been developed that are excellent in oxidation resistance. Particularly, perfluorosulfonic acid polymers known under the trade name of “Nafion” available from E.I. du Pont de Nemours and Company, have extremely-high chemical stability and are hardly oxidized in the presence of peroxide (hydrogen peroxide H
2
O
2
), because they are the perfluorinated electrolyte materials having C—F bond. Thus perfluorosulfonic acid polymers are excellent in property.
However, there is such problem that it is difficult to manufacture the perfluorinated perfluorosulfonic acid polymers, and it costs too much to obtain the raw materials because of its difficulty in mass production. Accordingly, use of the electrolyte membrane made of perfluorosulfonic acid polymer materials is limited to the special destination, such as a solid polymer fuel cell for space or military and the like, so it makes difficult to apply perfluorosulfonic acid polymer materials to unmilitary demands, such as a solid polymer fuel cell for a low-pollution drive source for automobile.
While, another materials have been examined and researched, instead of wholly fluorinated perfluorosulfonic, acid polymers. For example, following are listed up as a polymer electrolyte other than a fluorinated electrolyte: a crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in Swiss patent application No. 02 636/93-6, a polyethersulfone resin membrane introducing sulfonic acid groups disclosed in Japanese patent publication laid-open No. Hei 10-45913, and the like. In addition, Japanese patent publication laid-open No. Hei 9-102322 discloses a sulfonic acid type ethylene-tetrafluoroethylene (ETFE) copolymer-graft-polystyrene membrane, which comprises the main chains formed by copolymerization of a fluorocarbon-based vinyl monomer and a hydrocarbon vinyl monomer and the hydrocarbon-based side chains containing sulfonic acid groups.
Furthermore, U.S. Pat. No. 4,012,303 and U.S. Pat. No. 4,605,685 disclose a sulfonic acid type ETFE-graft-poly(trifluorostyrene) membrane, which is prepared by graft polymerization of &agr;,&bgr;,&bgr;-trifluorostyrene and the membrane prepared by copolymerization of a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer, then introducing sulfonic acid groups into the resulting membrane to prepare the desired solid polymer electrolyte membrane. In this process, &agr;,&bgr;,&bgr;-trifluorostyrene, produced by styrene fluorination, is employed instead of styrene on the assumption that chemical stability of the side chains introducing sulfonic acid groups in polystyrene is insufficient.
These electrolyte materials, however, such as a non-fluorinated electrolyte membrane, for example, the crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in Swiss patent application No. 02 636/93-6, the polyether sulfone resin membrane introducing sulfonic acid groups disclosed in Japanese patent publication laid-open No. Hei 10-45913 and the like, can advantageously be manufactured easier at lower cost than the wholly fluorinated electrolyte membrane represented by Nafion, but the non-fluorinated electrolyte membrane is easily degraded by peroxide produced by the electrode reaction, thus the oxidation resistance of which has been controlled to be low disadvantageously. Because the non-fluorinated compounds have the hydrocarbon structure susceptible to the oxidation reaction caused by peroxide radicals.
Furthermore, the sulfonic acid type ETFE-graft-polystyrene membrane disclosed in Japanese patent publication Laid-open No. Hei 9-102322 can be obtained at a low price and robust enough to function as a solid polymer electrolyte membrane for use in a fuel cell, in addition to this, a conductivity of which can be improved by increasing an introducing amount of sulfonic acid groups. Furthermore, an oxidation resistance of the main chains produced by copolymerization between a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer is sufficiently high, but the side chains introducing sulfonic acid groups is a hydrocarbon-based polymer susceptible to oxidation and degradation. Accordingly, application of the sulfonic acid type ETFE-graft-polystyrene membrane to a fuel cell causes the oxidation resistance of whole membrane to be insufficient, thus resulting in poor durability disadvantageously.
Furthermore, in case of utilizing a sulfonic acid type ETFE-graft-poly (trifluorostyrene) membrane disclosed in U.S. Pat. No. 4,012,303 and the like, it is considered that the above-mentioned problem is solved because the side chains thereof are composed of fluorine-based polymers. However, the base material of the side chains, &agr;,&bgr;,&bgr;-trifluorostyrene, is difficult to synthesize, thus it will be costly to apply the same to a solid polymer electrolyte membrane for use in a fuel cell, as similar to the above-mentioned Nafion. In addition, &agr;,&bgr;, &bgr;-trifluorostyrene is susceptible to degradation, thus it is difficult to deal with &agr;,&bgr;,&bgr;-trifluorostyrene, and it is not excellent in polymerization reactivity. Accordingly, an amount of &agr;,&bgr;,&bgr;-trifluorostyrene, which should be introduced as the graft side chains, is small, as a result, the resulting membrane has a low conductivity.
Furthermore, durability of the crosslinked polystyrene-grafted resin membrane introducing sulfonic acid groups disclosed in above-mentioned Swiss patent application is higher than that one disclosed in the above-mentioned US Patent. However, the above-mentioned problem cannot solved essentially by the above-mentioned technique in the point of improvement of polymeric oxidation resistance, because the technique pre

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