Relative Humidity Dependence of Pt Utilization in Polymer Electrolyte Fuel Cell Electrodes: Effects of Electrode Thickness, Ionomer-to-Carbon Ratio, Ionomer Equivalent Weight, and Carbon Support

Pt utilization, i.e., the ratio of the electrochemically active Pt surface area of an MEA to the total Pt surface area of the catalyst, was analyzed at 20–100% RH, and 40–80°C by a newly developed technique using CO as a probe. The Pt utilization significantly decreased with the decreasing relative humidity (RH) for a Pt/C catalyst in which the Pt particles are supported not only on the outer surface, but also within the pores of the carbon particles, while it remained high even at a low RH for the samples with Pt particles supported only on the outer surface of the carbon particles. We presume that this is because the Pt particles within the pores are not in contact with the ionomer therefore, the protons cannot access them at a low RH.