Electrode catalyst layer for fuel cells, electrode for fuel cells, membrane electrode assembly for fuel cells, and fuel cell

A fuel cell electrode catalyst layer comprising:

• an electrode catalyst having a conductive support and a platinum-containing metal particle that is supported on a surface of the conductive support;
• and
• an ionomer that covers the electrode catalyst, wherein an average thickness of the ionomer is 2.4 nm or less, wherein
• an average particle diameter of the platinum-containing metal particle is in a range from 1 to 30 nm,
• an average particle diameter of the conductive support is in a range from 5 to 200 nm,
• an effective surface area of platinum in the platinum-containing metal particle is is 5 cm2·cm−2 or more to 120 cm2·cm−2 or less, and
• wherein the effective surface area of the platinum is represented by formula:
• Spteff =m×seff,
• where m is an amount of platinum contained in a fuel cell electrode catalyst layer in mgpt·cm−2 and Seff is an electrochemical surface area of the platinum in m2·gpt−1,
• the conductive support has a primary pore having a diameter of 10 nm or less, and
• a ratio of a surface area of the platinum-containing metal particle contained in the primary pore of the conductive support relative to a surface area of the platinum-containing metal particle supported by the conductive support is 50% or more.