Abstract: Provided are: a practically excellent polymer electrolyte composition having excellent chemical stability of being resistant to strong oxidizing atmosphere during operation of fuel cell, and achieving excellent proton conductivity under low-humidification conditions, excellent mechanical strength and physical durability; a polymer electrolyte membrane, a membrane electrode assembly, and a polymer electrolyte fuel cell each using the same. The polymer electrolyte composition of the present invention comprises at least an ionic group-containing polymer (A) and a phosphorus-containing additive (B), the phosphorus-containing additive (B) being at least one of a phosphine compound and a phosphinite compound. The polymer electrolyte membrane, the membrane electrode assembly, and the polymer electrolyte fuel cell of the present invention are structured by the polymer electrolyte composition.

Abstract: Provided are: a practically excellent polymer electrolyte composition having excellent chemical stability of being resistant to strong oxidizing atmosphere during operation of fuel cell, and achieving excellent proton conductivity under low-humidification conditions, excellent mechanical strength and physical durability; a polymer electrolyte membrane, a membrane electrode assembly, and a polymer electrolyte fuel cell each using the same. The polymer electrolyte composition of the present invention comprises at least an ionic group-containing polymer (A) and a phosphorus-containing additive (B), the phosphorus-containing additive (B) being at least one of a phosphine compound and a phosphinite compound. The polymer electrolyte membrane, the membrane electrode assembly, and the polymer electrolyte fuel cell of the present invention are structured by the polymer electrolyte composition.

Abstract: [Summary] To provide a formed article of polymer electrolyte composition which exhibits excellent proton conductivity even under low-humidification conditions and under low-temperature conditions, which is excellent in chemical stability, mechanical strength, fuel shutoff properties, and which can achieve high output, high energy density, and excellent long-term durability when used in a polymer electrolyte fuel cell; and also to provide a polymer electrolyte fuel cell using thereof. The formed article of polymer electrolyte composition includes: a block copolymer having one or more of each of a hydrophilic segment (A1) containing an ionic group and a hydrophobic segment (A2) not containing an ionic group; and an additive, wherein the formed article forms co-continuous or lamellar phase separation structure, and the additive is hydrophilic.