Electrode catalyst, composition for forming gas diffusion electrode, gas diffusion electrode, membrane electrode assembly, and fuel cell stack

An electrode catalyst which includes a conductive hollow carbon carrier having a Nanopore of a pore size of 1 to 20 nm, and a plurality of catalyst particles supported on the carrier, wherein

• the catalyst particle is composed of Pt (0 valent),
• the catalyst particle is supported on both of inside of the Nanopore and outside the Nanopore of the carrier, and
• when an analysis of a particle size distribution of the catalyst particles is performed by using a three-dimensional reconstructed image obtained by an electron beam tomography measurement using an STEM (scanning transmission electron microscopy), the condition of the following equation (S1) is satisfied:
• 100×(N10/N20)≤8.0 (S1)
• in the above equation (S1),
• N10 is the number of non-contact particles (n101+n102), which is the sum of the number of particles (n101) of noble metal particles that are not in contact with pores having a pore diameter of 1 nm or more that can be confirmed by the electron tomography measurement and the number of noble metal particles (n102) that are not in contact with the hollow carbon carrier itself and exist outside the hollow carbon carrier,
• N20 is the number of particles of the catalyst particles supported inside the Nanopores of the carrier.