ELECTRODE CATALYST LAYER, MEMBRANE ELECTRODE ASSEMBLY, AND SOLID POLYMER FUEL CELL

An electrode catalyst layer includes a catalyst material, a conductive carrier, which supports the catalyst material, a polymer electrolyte, and a fibrous material. The electrode catalyst layer includes a three-dimensional network of a flow path. The flow path includes a plurality of terminal points and a plurality of branches. Each of the branches is an elongated hole connecting a pair of the terminal points. The plurality of branches include a bifurcated branch. One of the pair of the terminal points of the bifurcated branch is connected to another branch. The number of branches per unit volume of the electrode catalyst layer is 8500 or more and 13000 or less. The unit volume is defined as 3.9 µm × 3.9 µm × 4.4 µm.