In recent years, a fuel cell has attracted attention as a high-efficiency energy conversion device. Fuel cells are roughly classified, by the type of the electrolyte used, into a low-temperature operating fuel cell such as an alkali type, a polymer electrolyte type and a phosphoric acid type, and a high-temperature operating fuel cell such as a molten carbonate type and a solid oxide type. Among these, a polymer electrolyte fuel cell (PEFC) using, as an electrolyte, a polymer electrolyte membrane having ion conductivity is attracting a lot of attention as a power source for stationary, vehicular, portable and other applications, because a high output density can be obtained with a compact structure and a simple system can be realized thanks to, for example, no use of a liquid for the electrolyte and the capability of operating at low temperatures.
In the polymer electrolyte fuel cell, the basic principle is that one surface of a polymer electrolyte membrane and the opposite surface are exposed to a fuel gas (e.g., hydrogen) and an oxidant gas (e.g., air), respectively, and reaction energy produced in the synthesis of water by a chemical reaction through the polymer electrolyte membrane is electrically extracted.
An assembly obtained by arranging a porous catalyst electrode on both surfaces of a polymer electrolyte membrane and integrally molding these members by means of a hot press, etc., is generally referred to as a membrane electrode assembly (MEA). The polymer electrolyte membrane has ion conductivity but does not have air permeability and electron conductivity and consequently, conducts only ions while physically and electronically insulating a fuel electrode from an oxygen electrode.
The polymer electrolyte membrane greatly affects the resistance and therefore, is required to have a smaller membrane thickness. A polymer electrolyte membrane is a very thin film-like material and difficult to handle, and wrinkles are often generated, for example, when joining it with an electrode or at the time of stacking a plurality of single cells and assembling the cells into a stack. A single cell or stack assembled using a polymer electrolyte membrane with wrinkles being generated therein has a high possibility of causing leakage of a reaction gas from the wrinkled region. In addition, the polymer electrolyte membrane is required to be than, and moreover, this member is the lowest in the mechanical strength among all constituent members constituting the stack and in turn, easily damaged even in the state absolutely free of wrinkles, etc. In the damaged portion of the electrolyte membrane, the electronic insulation or gas impermeability may be impaired. Accordingly, in order to enhance the reliability, maintain ability, etc., of the polymer electrolyte fuel cell, reinforcement of the polymer electrolyte membrane site is necessary.