Platinum Anti-Dissolution Mechanism of Pt/Nb-SnO2 Cathode Catalyst Layer during Load Cycling in the Presence of Oxygen for Polymer Electrolyte Fuel Cells

The durability of cathode catalyst layers (CLs) using Pt/Nb-SnO2 (niobium-doped tin oxide) was compared with that of Pt/GCB (graphitized carbon black) as a function of the cathode oxygen concentration during galvanostatic/potentiostatic operation under H2/N2, H2/air, and H2/O2 conditions. We found a novel platinum anti-dissolution mechanism of Pt/Nb-SnO2 CLs during load cycling under an oxygen atmosphere, which was not the case for Pt/GCB CLs. The load cycle durability of Pt/Nb-SnO2 CLs was found to increase with increasing cathode oxygen concentration, while that of Pt/GCB CLs decreased. When the Pt nanoparticles (NPs) on the Nb-SnO2 support are oxidized by air and oxygen in the cathode at the open circuit voltage (OCV), the availability of free electrons in the Pt NPs is thought to decrease, and thus they are difficult to supply to the depletion layer on the surface of Nb-SnO2. This effect would be particularly evident for the high cathode oxygen concentration under H2/O2 conditions. The resistance-based performance loss of the Pt/Nb-SnO2 CLs during the oxidation reduction reaction (ORR) with a current load is mitigated, however, because the depletion layer existing on the surface of Nb-SnO2 at OCV would be decreased by the ORR, which involves the consumption of the adsorbed oxygen.