Structural and electrochemical investigation of binary mixtures of carbon-supported Pt and Ru catalysts for PEM fuel cells

Commercially available carbon-supported pure Pt and pure Ru catalysts were heat treated in order to modify their individual particle sizes, mixed in a mortar and tested for their suitability as anode catalysts in polymer electrolyte fuel cells (PEMFCs). The procedure chosen for mixing monometallic catalysts should bring more flexibility to the preparation of catalysts, which could thus be easily adjusted to the working conditions. The catalyst mixtures were structurally characterized using a combination of X-ray diffraction (XRD) and transmission electron microscopy (TEM), and indicated an increase in particle size with heat treatment temperature, while the size distribution remained sufficiently narrow. The particle size effect on the resulting activity of the catalyst mixtures was investigated by current-voltage measurements (U/i curves) in single cell fuel cells. A mixture of the as-received platinum and ruthenium gave the best results, which was almost comparable to a commercial Pt-Ru alloy catalyst purchased from E-TEK inc. Only a minor decrease in cell performance was seen during long term operation. Following operation, the X-ray diffraction patterns show reflections of the Pt fcc phase, but not the hcp Ru phase. This may be explained by leaching of the pure ruthenium catalysts and re-decoration on Pt nanoparticles, or by formation of an amorphous Ru oxide or, less likely, Pt-Ru alloy during operation. However, these structural changes do not seem to significantly affect the cell performance