Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells *

Enhanced electrocatalysis of the oxygen reduction reaction (ORR) on carbon-supported binary and ternary alloys of Pt in phosphoric acid fuel cells has been reported previously. This investigation focuses on the electrocatalysis of the ORR on some binary alloys of Pt (Pt+Ni, Pt+Cr and Pt+Co) at interfaces with proton exchange membranes (Dow perfluorinated sulfonic acids). Comparison of the results of these studies with those on carbon-supported Pt electrocatalysts (electrodes containing same Pt loading of 0.3 mg/cm 2) revealed enhanced activities, lower activation energies and different reaction orders for all the alloys. X-ray powder diffraction showed lattice contractions for the alloys, the predominant phase being Pt3M (LI 2) f.c.c, crystallite. X-ray photoelectron spectroscopy studies on the constituent elements of the electrocatalyst showed no chemical energy shifts owing to alloying and/or the presence of oxides on the surface. Lifetime evaluations of proton exchange membrane fuel cells, using both electrochemical as well as scanning electron microscopy/energy-dispersive X-ray analysis techniques, revealed only small amounts of dissolution of the more oxidizable component during the testing periods, which ranged from 400 to 1200 h. Therefore, the enhanced electrocatalysis exhibited by the binary Pt alloys appears to originate primarily as a result of changes in the lattice structure owing to alloying and the unique environment of the supported catalyst in the particle size range 35-75 ,~.