This invention relates to a process for producing polymer ion-exchange membranes which are solid polymer electrolyte membranes suitable for use in fuel cells.
The invention also relates to a process for producing polymer ion-exchange membranes that are solid polymer electrolyte membranes suitable for use in fuel cells and which have not only high oxidation and heat resistance as well as high dimensional stability but also high electrical conductivity while permitting the ion exchange capacity to be controlled over a wide range.
Fuel cells that employ solid polymer electrolyte ion-exchange membranes have high energy density and hence hold promise for use as power supplies to electric vehicles or as simplified auxiliary power sources. For fuel cells, the development of polymer membranes having satisfactory characteristics is one of the most important steps to take.
In polymer ion-exchange membrane fuel cells, the ion-exchange membrane not only acts as a proton conducting electrolyte but also has the role of a diaphragm that prevents the fuel hydrogen or methanol from directly mixing with the oxidant air (oxygen). Working as the electrolyte, the ion-exchange membrane must satisfy the following requirements: large ion-exchange capacity; high enough chemical stability to withstand prolonged current impression, in particular, high resistance (oxidation resistance) against hydroxyl radicals, etc. that are a major cause of membrane deterioration; heat resistance to at least 80° C. which is the cell operating temperature; and consistently high enough water-retaining ability of the membrane to keep low levels of electrical resistance. The membrane which also has the role of a diaphragm must satisfy other requirements including high mechanical strength and good dimensional stability, as well as the absence of excessive permeability to hydrogen gas, methanol and oxygen gas.
Early polymer ion-exchange membrane fuel cells employed hydrocarbon-based polymer ion-exchange membranes as produced by copolymerizing styrene with divinylbenzene. However, those ion-exchange membranes did not have high practical feasibility since they were very low in durability on account of poor oxidation resistance; hence, they were later replaced by NafionRT and other fluorine-containing polymer ion-exchange membranes. NafionRT is the fluorinated sulfonic acid polymer membrane developed by Du Pont.
The conventional fluorine-containing polymer ion-exchange membranes including NafionRT are satisfactory in terms of chemical durability and stability; on the other hand, their ion-exchange capacity is small, only about 1 meq/g, and on account of insufficient water retention, the ion-exchange membrane dries and its proton conductivity is lowered, or in the case where methanol is used as fuel, the membrane will swell or “cross-over” of methanol or hydrogen gas will occur.
If, with a view to increasing the ion-exchange capacity, an attempt is made to introduce more sulfonic acid groups, the membrane, having no crosslinked structure in the polymer chains, swells and its strength decreases so markedly that it may break easily. Therefore, with the conventional fluorine-containing polymer ion-exchange membranes, it has been necessary to reduce the amount of sulfonic acid groups to levels that can retain the membrane strength and the only products that could be obtained had no greater ion-exchange capacity than about 1 meq/g.
Other problems with NafionRT and other conventional fluorine-containing polymer ion-exchange membranes are that monomer synthesis is difficult and complicated and that the process of polymerizing the monomers to produce a polymer membrane is also complicated; the resulting prohibitive price of the product membrane has been a great obstacle to the effort in commercialization by installing the proton-exchange membrane fuel cell on vehicles, etc. Under the circumstances, massive efforts have been made to develop low-cost yet high-performance electrolyte membranes that can be substituted for NafionRT and other conventional fluorine-containing polymer ion-exchange membranes.
In radiation-induced graft polymerization which is closely related to the present invention, attempts have been made to prepare solid polymer electrolyte membranes by grafting monomers that can introduce sulfonic acid groups into polymer membranes. The present inventors made intensive studies in order to develop such new solid polymer electrolyte membranes and by first introducing a styrene monomer into a poly(tetrafluoroethylene) film having a crosslinked structure through radiation-induced graft reaction and then sulfonating the introduced grafts, they invented a solid polymer electrolyte membrane characterized by ion-exchange capacity that was high and could be controlled over a wide range. The membrane and the process for its production were applied for patent (JP 2001-348439 A). However, since the styrene graft chains in this ion-exchange membrane were composed of hydrocarbons, prolonged application of an electric current to the membrane caused oxidation in part of the graft chains, resulting in a lower ion-exchange capacity of the membrane.
The present inventors continued their study and by first performing radiation-induced grafting of a fluorine-containing monomer or co-grafting of fluorine-containing monomers to a poly(tetrafluoroethylene) film having a crosslinked structure and then introducing sulfone groups into the graft chains, they invented a solid polymer electrolyte membrane characterized by a broader range of high ion-exchange capacity and satisfactory oxidation resistance. The membrane and the process for its production were applied for patent (JP 2002-348389 A). However, as it turned out with the ordinary fluorinated polymer membranes, the graft reaction of the fluorine-containing monomer or monomers did not progress efficiently to the inside of the membrane and depending on the reaction conditions, the graft reaction was only limited to the surface of the film and it was difficult to produce an electrolyte membrane having improved characteristics.