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

An electrode catalyst having a core-shell structure comprising:

• a support;
• a core part formed on said support;
• and
• a shell part formed to cover at least a part of a surface of said core part, said shell part having a two-layered structure composed of a first shell part formed to cover at least a part of said surface of said core part and a second shell part formed to cover at least a part of a surface of said first shell part,
• wherein a concentration of bromine (Br) species of the electrode catalyst is not higher than 500 ppm when measured by X-ray fluorescence (XRF) spectroscopy, and a concentration of chlorine (Cl) species of the electrode catalyst is greater than 5,000 ppm and not higher than 8,500 ppm when measured by X-ray fluorescence (XRF) spectroscopy, and
• wherein said core part contains at least one metal other than a noble metal as a main component(s), said first shell part contains at least one of palladium (Pd), a palladium (Pd) alloy, a platinum (Pt) alloy, gold (Au), nickel (Ni) and a nickel (Ni) alloy, and said second shell part contains at least one of platinum (Pt) and a platinum (Pt) alloy.