Abstract: An electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell, which are capable of improving material transport properties and proton conductivity in the electrode catalyst layer, and which are capable of exhibiting high power generation performance over the long term and have good durability. The electrode catalyst layer for use in a polymer electrolyte fuel cell, and includes: a catalyst material; a conductive carrier that supports the catalyst material; a polymer electrolyte; and a fibrous material, wherein the fibrous material contains a material having nitrogen atoms, and the content of the fibrous material in the electrode catalyst layer is 1 wt % or more and less than 12 wt %.

Abstract: A cathode catalyst layer and an anode catalyst layer used for a membrane-electrode assembly in a polymer electrolyte fuel cell, wherein the cathode catalyst layer and the anode catalyst layer each include catalyst particles, a conductive carrier, a polymer electrolyte, and a fibrous material, the fibrous material includes at least one of an electron conductor and a proton conductor, and the fibrous material has a specific surface area in a range of 40 m2/g or more and 80 m2/g or less.