Which mechanism limits catalytic activity in a polymer electrolyte membrane fuel cell (PEMFC) operating at high current density?

💡 Explanation

When current density increases, hydrogen ions (protons) transport through the PEM increases but is limited by saturation of the available transfer sites, which reduces the voltage. Hydrogen ion conductivity saturation leads to decreased catalytic activity, rather than ORR, electrode poisoning, or degradation because alternative choices don’t directly influence ion transport.

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