CATALYST PARTICLES, CATALYST INK, ELECTRODE CATALYST LAYER FOR FUEL CELLS, MEMBRANE ELECTRODE ASSEMBLY, SOLID POLYMER FUEL CELL, METHOD FOR PRODUCING CATALYST PARTICLES, AND METHOD FOR PRODUCING CATALYST INK

Provided is a method for producing catalyst particles (P3), which are used in electrode catalyst layers (52A, 52F) for the purpose of increasing the output voltage of a polymer electrolyte fuel cell (50). Before dispersing the catalyst particles (P3), which are carbon particles (P1) supporting a catalyst material (P2), and a polymer electrolyte (PE) in a solvent (S) for a catalyst ink, the carbon particles (P1) are exposed to a humidified atmosphere (M). The moisture content of the catalyst particles (P3) is from 4.8 mass% to 20 mas%, inclusive.