ELECTRODE CATALYST LAYER FOR FUEL CELLS, ELECTRODE FOR FUEL CELLS, MEMBRANE ELECTRODE ASSEMBLY FOR FUEL CELLS, AND FUEL CELL

The fuel cell electrode catalyst layer of the present invention includes an electrode catalyst having a conductive support and a platinum-containing metal particle supported on the surface of the conductive support, and an ionomer covering the electrode catalyst. Then, it is characterized in that the average thickness of the ionomer is 2.4 nm or less. The fuel cell electrode catalyst layer can achieve simultaneously a transportability of gas such as oxidant gas and fuel gas and a proton transportability even when the platinum supporting amount is reduced. Further, by applying the fuel cell electrode catalyst layer, it is possible to obtain a fuel cell electrode, a fuel cell membrane electrode assembly and a fuel cell that exert good current-voltage characteristics.