Electrode catalyst, gas diffusion electrode-forming composition, 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 the support;
• and
• a shell part formed to cover at least a part of a surface of the core part,
• wherein the shell part includes a single-layered structure formed to cover at least a part of the surface of the core part, or a two-layered structure including a first shell part formed to cover at least a part of the surface of the core part, and a second shell part formed to cover at least a part of the surface of the first shell part,
• wherein in a case of the single-layered shell part, the shell part comprises platinum (Pt), and the core part comprises palladium (Pd), whilst in a case of the two-layered shell part, the first shell part comprises palladium (Pd), and the second shell part comprises platinum (Pt),
• wherein a concentration of bromine (Br) species measured by X-ray fluorescence (XRF) spectroscopy, is not greater than 400 ppm, and
• wherein a concentration of chlorine (Cl) species measured by X-ray fluorescence (XRF) spectroscopy, is not greater than 900 ppm.