Catalyst for electrodes, composition for forming gas diffusion electrode, gas diffusion electrode, membrane electrode assembly, and fuel cell stack

A catalyst for an electrode comprising a conductive porous carbon support having nanopores each having a pore size of 1 to 20 nm and micropores each having a pore size of less than 1 nm, and a plurality of catalyst particles supported on the support,

• wherein a region made of Pt (0 valence) is formed on at least a part of a surface of each catalyst particle,
• wherein the catalyst particles are supported both inside the nanopores and outside the nanopores of the support,
• wherein a ratio, by number, of the catalyst particles supported inside the nanopores to a total number of the catalyst particles is 50% or more,
• wherein the nanopores comprise at least one interconnected nanopore that is an open, continuous pore extending between two opposite, parallel sides of a square cross-section taken within an interior region of a catalyst aggregate, the square cross-section having a side length of 20 to 50 nm and the catalyst aggregate being accommodated in a rectangular space with one side of 60 to 300 nm, the at least one interconnected nanopore having at least one opening on each of said two opposite, parallel sides and extending continuously between the openings without blocking.