A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to an anode and cathode, respectively, and an electromotive force is generated by an electrochemical reaction occurring at both poles is generally constituted by sequentially laminating a bipolar plate, a gas diffusion electrode substrate, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode substrate, and a bipolar plate. The gas diffusion electrode substrate is required to have high gas diffusivity for allowing a gas supplied from the bipolar plate to be diffused into the catalyst layer and high water removal performance for discharging liquid water generated by the electrochemical reaction to the bipolar plate, as well as high electrical conductivity for extracting generated electric current, and gas diffusion electrode substrates composed of carbon fibers and the like are widely used.
However, the following problems are known: (1) when the polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the electrode substrate by liquid water generated in a large amount and shortage in the fuel gas supply, the fuel cell performance is impaired (this problem is hereinafter referred to as “flooding”) ; and (2) when the polymer electrolyte fuel cell is operated at a relatively high temperature of 80° C. or higher, as a result of drying of the electrolyte membrane due to water vapor diffusion and a reduction in the protonic conductivity, the fuel cell performance is impaired (this problem is hereinafter referred to as “dry-out”). In order to solve these problems of (1) to (2), various efforts have been made. A method of improving gas diffusivity and water removal performance by forming a microporous part on the surface of the gas diffusion electrode substrate, and forming pores in the microporous part is the basic solution to these problems.
Patent Document 1 discloses that stable fuel cell performance can be obtained in a low humidity condition and high humidity condition by having a structure in which a carbon porous material, i.e., a microporous part, is impregnated in an electrode substrate, and the density of the impregnated layer is set to a predetermined range. However, by the structure in which a microporous part is impregnated in an electrode substrate, obtained by the above method, high gas diffusivity and high water removal performance cannot be simultaneously satisfied, and particularly, fuel cell performance has been insufficient at low temperatures.
Patent Document 2 discloses a technology to form a through hole by putting a large quantity of pore-forming particles into the inside of the microporous part, and obtaining high performance in the drying conditions and humidified conditions by separating the paths of water and gas. However, while water removal performance is improved by the microporous part in the method disclosed in Patent Document 2, there is a problem that discharged water accumulates in carbon paper and inhibits diffusion of gas, and sufficient properties could not be obtained.
As described above, a variety of efforts have been made; however, one that can be satisfied as a gas diffusion electrode substrate which has excellent anti-flooding characteristic particularly at low temperatures without deteriorating anti-dry-out characteristic is yet to be discovered.