1. Field of the Invention
The present invention relates to a membrane electrode assembly for solid polymer electrolyte fuel cells. More specifically, it is related to a membrane electrode assembly for solid polymer electrolyte fuel cells which are available for an electrolyte for a primary cell, an electrolyte for a secondary cell, a high polymer solid electrolyte for a fuel cell, a display device, a variety of sensors, a signal transfer medium, a solid condenser, and an ion exchange membrane, and the like.
2. Related Art
A fuel cell is a clean, environment friendly power generating system with high electrical efficiency, and which has been attracting a great deal of attention as earth environmental protection and break away from dependence on fossil fuels in recent years. It is desired that a fuel cell is mounted in a small distribution power generating facility, a power generating device as a driving force of a movable body, such as a vehicle or vessel. Furthermore, the fuel cell is desired to replace a second battery such as a lithium ion battery that mounted in a mobile phone, a mobile personal computer, or the like.
A polymer electrolyte fuel cell provides a pair of electrodes on the opposite sides of a solid polymer electrolyte membrane with proton conductivity, supplies pure hydrogen or reformed hydrogen gas as fuels to one electrode (a fuel electrode) and oxygen gas or air as oxidants to the other electrode, so as to obtain electromotance. In addition, water electrolysis is the reverse action of fuel reaction, which produces hydrogen and oxygen by using the solid polymer electrolyte membrane.
However, in an actual fuel cell or water electrolysis, a secondary reaction other than a primary reaction occurs. The representative example is hydrogen peroxide (H2O2) generated by which radical species cause the deterioration of the solid polymer electrolyte membrane.
Conventionally, a perfluorosulfonic acid membrane, such as Nafion (registered trademark) produced by du Pont Kabushiki Kaisha, Aciplex (registered trademark), produced by Asahi Kasei Corporation, or Flemion (registered trademark) produced in Asahi Glass Co., Ltd. Corporation, is used as a solid polymer electrolyte membrane.
However, the problem with perfluorosulfonic acid membranes, such as Nafion, is that they are very difficult to produce, and therefore can be extremely expensive, which is a large barrier regarding public welfare need to be overcome in order for a fuel cell electric vehicle to become widespread. In addition, perfluorosulfonic acid membranes have a large number of fluorine atoms in the molecule, so that an additional problem exists regarding disposal processing which increases pressure on the environment.
Furthermore, the higher the temperature and the thinner the membrane thickness of the proton conductive membrane which is between the electrodes a fuel cell has, the lower the membrane resistance and the higher power generating output it has. However, the heat distortion temperature of these perfluorosulfonic acid membranes is around 80 to 100 degrees C., which means the high temperature creep resistance is extremely low at high temperature. Therefore, there are problems with limitations in power generating performance, because the power generating temperature must be maintained at below 80 degrees C. In addition, the membrane thickness is unstable when the membrane is used for a long period, so that some membrane thickness (more than 50 μm) is required to prevent short circulation between electrodes; and therefore, it is difficult to thin the proton conductive membrane.
In order to solve these problems with respect to perfluorosulfonic acid membranes, a variety of solid polymer electrolyte membranes having a heat resistant main chain skeleton, which is low cast for being applied to engineer plastics, has been researched recently. Polymers, in which the main chain aromatic ring of a polyarylene system, polyetheretherketone system, polyethersulfone system, polyphenylene sulfide system, polyimide system, or polybenzazol system is sulfonated, have been proposed (see Non-patent Documents 1 to 3).
Non-patent Document 1: Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490-2492 (1993)
Non-patent Document 2: Polymer Preprints, Japan, Vol. 43, No. 3, p. 736 (1994)
Non-patent Document 3: Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)
However, in these polymers in which the main chain aromatic rings exhibit higher water absorption, but lower hot water resistance, the number of hydrophilic groups such as sulfonic acid groups is limited. In addition, the polymers exhibit inferior radical resistance in terms of Fenton reagent resistance (hydroxyl radical resistance characteristics), which determines the power resistance. Furthermore, when these electrolyte membranes are exposed to high temperatures, more than 100 degrees C. for a long period of time, there is a problem in that the sulfonic acid detaches to lower the proton conductivity, or causes a crosslinking reaction with another aromatic ring in which no sulfonic acid group has been introduced, thereby becoming brittle. Once the embrittlement of the membrane progresses, a rupture (pinhole) in the membrane occurs, resulting in a possible shutdown of power generation.
An object of the present invention is to solve problems with respect to a fluorine system electrolyte membrane and an aromatic system electrolyte membrane, improve the hot water resistance of the membranes, provide the solid polymer electrolyte with superior proton conductivity, and a membrane electrode assembly for solid polymer electrolyte fuel cells including the solid polymer electrolyte.