Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-09-15
2004-01-20
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S047000, C429S047000
Reexamination Certificate
active
06680138
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a proton-conducting membrane, method for producing the same, and fuel cell using the same, more particularly the proton-conducting membrane, excellent in resistance to heat and durability and showing excellent proton conductivity at high temperature, method for producing the same, and fuel cell using the same.
Recently, fuel cell has been attracting attention as a power generating device of the next generation, which can contribute to solution of the problems related to environments and energy, now having been increasingly becoming serious social problems, because of its high power generation efficiency and compatibility with the environments.
Fuel cells fall into several categories by electrolyte type. Of these, a polymer electrolyte fuel cell (PEFC), being more compact and generating higher output than any other type, is considered to be a leading fuel cell type in the future for various purposes, e.g., small-size on-site facilities, and as movable (i.e., power source of vehicles) and portable cells.
However, PEFCs are still in the development or testing stages and not yet commercialized so far, in spite of their inherent advantages in principle, because of lack of the practical electrolytic membrane which satisfies all of the requirements, e.g., resistance to heat, durability and proton conductivity. The electrolytic membranes for the current PEFCs are mainly of fluorine-based ones, with a perfluoroalkylene as the main skeleton, and partly with ion-exchangeable groups, e.g., sulfonic and carboxylic acid groups, at the terminal of the perfluorovinyl ether side chains. Several types of these fluorine-based membranes have been proposed, e.g., Nafion membrane (Du Pont, U.S. Pat. No. 4,330,654), Dow membrane (Dow Chemical, Japanese Patent Application Laid-Open No.4-366137), Aciplex membrane (Asahi Chemical Industry, Japanese Patent Application Laid-Open No.6-342665), and Flemion membrane (Asahi Glass).
The current PEFCs using the above fluorine-based membranes as the electrolyte are normally operated in a relatively low temperature range, e.g., room temperature to around 80° C., because the fluorine-based membrane itself has a glass transition temperature (Tg) of around 130° C., above which its ion channel structure responsible for the ion conductivity will be destroyed. It is not desirable for a fuel cell to operate in a low temperature range, because of some serious problems, e.g., low power generation efficiency and notable poisoning of the catalyst with carbon monoxide.
Fuel cells have been continuously developed to operate in a higher temperature range, in order to avoid the problems resulting from operation in a low temperature range. Operability at higher temperature brings about several advantages. For example, when operated at 100° C. or higher, power generation efficiency should increase and, at the same time, heat can be utilized to improve energy efficiency. When operating temperature can be increased to 140° C., still other advantages, in addition to the above, can be expected, e.g., increased choices for the catalyst material, thus helping reduce fuel cell cost.
A variety of electrolyte membranes (e.g., proton-conducting membranes) have been proposed so far to increase operating temperature of PEFCs.
Some of more representative ones are heat-resistant aromatic-based polymers to replace the conventional fluorine-based membranes. These include polybenzimidazole (Japanese Patent Application Laid-Open No.9-110982), polyether sulfone (Japanese Patent Application Laid-Open Nos.10-21943 and 10-45913), and polyetheretherketone (Japanese Patent Application Laid-Open No.9-87510). However, each of these aromatic-based polymers is highly rigid, possibly causing damages while the membrane-electrode assembly (MEA) is formed.
They have other types of disadvantages. For example, they are modified with an acidic group (e.g., sulfonic or phosphoric acid group) to have proton conductivity necessary for the electrolytic membrane, with the result that they are water-soluble or swelling in the presence of water. The water-soluble ones are not applicable to fuel cells, because water is produced therein. On the other hand, those swelling in the presence of water may cause problems, because the swelling can generate a sufficient stress in the membrane to damage the electrode, or deteriorate membrane strength leading to its destruction.
On the other hand, the following inorganic materials have been proposed as the proton-conducting materials. For example, Minami et al. incorporate a variety of acids in hydrolysable silyl compounds to prepare inorganic proton-conducting materials (Solid State Ionics, 74 (1994), pp.105). They stably show proton conductivity at high temperature, but involve several problems; e.g., they tend to be cracked when made into a thin film, and difficult to handle and make them into MEAs. Several methods have been proposed to overcome these problems. For example, the proton-conducting material is crushed to be mixed with an elastomer (Japanese Patent Application Laid-Open No.8-249923) or with a polymer containing sulfone group (Japanese Patent Application Laid-Open No.10-69817). However, these methods have their own problems. For example, the polymer as the binder for each of these methods has no bond or the like with an inorganic crosslinked compound and has basic thermal properties not much different from those of the polymer itself, with the result that it undergoes structural changes in a high temperature range, failing to stably exhibit proton conductivity.
A number of R & D efforts have been made for various electrolyte membranes to solve these problems involved in the conventional PEFCs. None of them, however, have succeeded in developing proton-conducting membranes showing sufficient durability at high temperature (e.g., 100° C. or higher) and satisfying the mechanical requirements.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a proton-conducting membrane excellent in resistance to heat and durability and showing excellent proton conductivity at high temperature, which can solve the problems involved in the conventional PEFCs, and a method for producing the same and fuel cell using the same.
The inventors of the present invention have found, after having extensively studied a variety of electrolyte membranes to solve the above problems, that an innovative organic/inorganic composite membrane can be obtained by including, as the essential components, a selected combination of specific organic material, three-dimensionally crossliiked structure containing a specific metal-oxygen bond, agent for imparting proton conductivity and specific proton-conducting material, reaching the present invention. It shows much higher resistance to heat and durability, and proton conductivity at high temperature than the conventional one, because of the covalent bond formed between the organic material and three-dimensionally crosslinked structure to disperse them very finely at the molecular level (nano-dispersion).
The first invention is a proton-conducting membrane, comprising (A) an organic material, (B) a three-dimensionally crosslinked structure containing a specific metal-oxygen bond, (C) an agent for imparting proton conductivity, and (D) water, wherein
(i) the organic material (A) has a number-average molecular weight of 56 to 30,000, and at least 4 carbon atoms connected in series in the main chain, and
(ii) the organic material (A) and three-dimensionally crosslinked structure (B) are bound to each other via a covalent bond.
The second invention is the proton-conducting membrane of the first invention, wherein the organic material (A) is a polyether.
The third invention is the proton-conducting membrane of the second invention, wherein the organic material (A) is a polytetramethylene oxide.
The fourth invention is the proton-conducting membrane of the first invention, wherein the organic material (A) is a polymethylene.
The fifth invention is the proton-conducting membrane of the fourt
Honma Itaru
Nishikawa Osamu
Nomura Shigeki
Sugimoto Toshiya
Armstrong Kratz Quintos Hanson & Brooks, LLP
Electrotechnical Laboratory (Agency of Industrial Science and Te
Mercado Julian
Ryan Patrick
LandOfFree
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