ELECTRODE CATALYST, COMPOSITION FOR FORMING GAS DIFFUSION ELECTRODE, GAS DIFFUSION ELECTRODE, MEMBRANE ELECTRODE ASSEMBLY, AND FUEL CELL STACK

The present invention provides an electrode catalyst which has excellent catalytic activity, and which can contribute to reducing the cost of a polymer electrolyte fuel cell (PEFC). According to the present invention, an electrode catalyst includes a hollow carrier including nanopores having a pore size of 1 to 20 nm, and a plurality of catalyst particles. The catalyst particles are supported both inside and outside the nanopores of the carrier, and comprise (zero-valent) Pt, and when a particle size distribution analysis of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron beam tomography measurement employing STEM, the conditions of formula (S1): 100×(N10/N20)≤8.0 are satisfied (in the formula, N10 is the number of noble metal particles not in contact with a pore having a pore size of 1 nm or more, and N20 is the number of catalyst particles supported inside the nanopores of the carrier).