A fuel cell produces electrical energy by electrochemically oxidizing a fuel such as hydrogen and methanol in the cell to directly convert the chemical energy of the fuel into electrical energy. Fuel cells have recently drawn attention as a clean supply source for electrical energy.
Fuel cells are classified into a phosphoric acid type, a molten salt of a carbonic acid type, a solid oxide type and a solid polymer electrolyte type. Of these, the solid polymer electrolyte type fuel cell using a cation exchange mambrane as an electrolyte is called a proton exchange membrane type fuel cell. The proton exchange membrane type fuel cell is expected as a portable electric source such as an electric source for an electric car and a simple auxiliary electric source because it has high energy density even at a low operating temperature of 100.degree. C. or less.
A proton exchange membrane type fuel cell comprises an ion exchange membrane and a pair of gas diffusion electrodes bonded to both sides of the ion exchange membrane. Each gas diffusion electrode has a catalyst at least on a side thereof facing the ion exchange membrane. The cell is operated by feeding a fuel such as hydrogen to one gas diffusion electrode and feeding an oxidizing agent such as oxygen and air to the other gas diffusion electrode respectively, and connecting an external load circuit to both gas diffusion electrodes.
That is, a proton (a hydrogen ion) and an electron are generated due to the oxidization of fuel at one gas diffusion electrode. The proton is transferred to the other gas diffusion electrode through the membrane by conduction and there, water is produced by the reaction of the proton with oxygen contained in the oxidizing agent. At this time, the electron generated at one gas diffusion electrode is transferred to the other through the external load circuit to obtain electrical energy.
As mentioned above, in a proton exchange membrane type fuel cell, an ion exchange membrane operates as an electrolyte to conduct the proton. Further, the ion exchange membrane substantially forms one body structure with the gas diffusion electrodes due to a bonding of the electrodes to both sides of the membrane. Therefore, the ion exchange membrane also plays a part of a diaphragm by not allowing fuel to mix directly with the oxidizing agent.
The ion exchange membrane used for the proton exchange membrane type fuel cell requires low electrical resistance, quick movement of water through the ion exchange membrane, high water retention characteristics to maintain low electrical resistance and permeability to gases, which allows oxygen gas and hydrogen gas to be fed to the electrodes at a high enough speed. In addition, the ion exchange membrane requires an appropriate permeability to gases, excellent chemical stability during prolonged use and strong physical strength in view of its role as a diaphragm.
As a conventional ion exchange membrane used for the proton exchange membrane type fuel cell, for example, NAFION (registered trademark) manufactured by E.I. du Pont de Nemours and Co. having a fluororesin as a main chain of a polymer and a sulfonic acid group as an ion exchange group is used.
However, the conventional ion exchange membrane used for the proton exchange membrane type fuel cell can not respond to the increased request these days for a proton exchange membrane type fuel cell having high performance. The conventional ion exchange membrane is excellent in chemical permanence properties and stability. However, it has high electrical resistance. Further, it easily becomes dry due to low water retention characteristics so that proton conductivity is reduced or the reaction of fuel gas or oxidizing agent gas is inhibited at the electrode having a catalyst.
International Unexamined Patent Publication No. Wo86/06879 discloses a diaphragm used for a fuel cell having an equivalent weight of less than 1000 g/eq and strong physical strength. This ion exchange membrane has relatively high strength even at a high temperature of 110.degree. C. or more. However, when it is used for fuel cell at a temperature of 100.degree. C. or less, its performance is not sufficient.
European Patent Unexamined Publication No. 0498076 discloses an ion exchange membrane having an equivalent weight of 700 to 1000 g/eq and a water content of 35 to 100% by weight. When it is used for a fuel cell, which is operated at a low pressure of about 1 atm., or uses air as an oxygen resource, its performance is not sufficient.
Neither of the above-mentioned ion exchange membranes has necessary performance as a diaphragm and an electrolyte when used for a fuel cell.
The present invention has been completed to overcome the above-described problems of the prior art. That is, the present invention provides an ion exchange membrane used for a proton exchange membrane type fuel cell having excellent performance as a diaphragm and an electrolyte by specifying the molecular structure of the ion exchange membrane and limiting its electrical conductivity, permeability to gases and water content to appropriate ranges. The proton exchange membrane type fuel cell comprising the ion exchange membrane of the present invention can maintain high output performance for a long time.