Influence of Ionomer Content on Both Cell Performance and Load Cycle Durability for Polymer Electrolyte Fuel Cells Using Pt/Nb-SnO₂ Cathode Catalyst Layers

The steady-state current-voltage performance and load cycle durability of polymer electrolyte fuel cells using Pt catalysts supported on Sn 0.96 Nb 0.04 O 2-δ (Pt/Nb-SnO 2 ) cathode catalyst layers (CLs), without any carbon additive, were evaluated with various ionomer contents. The apparent mass activity at 0.80 V (MA app @0.80 V) of the cell using Pt/Nb-SnO 2 CL improved with decreasing volume ratio of the Nafion ionomer to the support (I/S). At I/S = 0.12, surprisingly, the MA app @0.80 V of the cell using Pt/Nb-SnO 2 CL approached 2 times higher than that using a commercial Pt catalyst supported on GCB (Pt/GCB) CL with the optimized I/S ratio. The current density at 0.60 V of the cell using Pt/Nb-SnO 2 (I/S = 0.12) CL also reached the same value of the cell using Pt/GCB CL. The electrochemically active surface area and MA app @0.80 V of Pt/Nb-SnO 2 CLs during the load cycle durability test maintained higher values than those of Pt/GCB throughout the test, indicating that the Pt/Nb-SnO 2 CL had higher durability than the Pt/GCB CL. Evaluation by means of low acceleration voltage transmission electron microscopy proved that the Nafion ionomer covered the hydrophilic surface of the Pt/Nb-SnO 2 uniformly, whereas the ionomer did not do so on the hydrophobic surface of the Pt/GCB. The thin, uniform coverage of the Nafion ionomer, which was thus easily obtained on the Pt/Nb-SnO 2 surface, was able to lower the overpotential originating from the oxygen diffusion resistance in the Nafion ionomer, thus improving the cell performance while maintaining the high load cycle durability.