Pt-Ru catalysts prepared by high energy ball-milling for PEMFC and DMFC: Influence of the synthesis
High energy ball-milling was used to prepare several unsupported Pt-Ru anode catalysts for PEM- and direct methanol fuel cells. Pt and Ru with a 50:50 nominal Pt/Ru ratio were ball-milled at various ball-to-powder weight ratios (from 4/1 to 12/1) and with various Pt:Ru:MgH2 proportions (from 1:1:2 to 1:1:10), where MgH2 is a leacheable dispersive agent. The presence of MgH2 is necessary to obtain unsupported catalysts with a specific surface area of between 50 and 75 m2 g−1. The ball-milling parameters greatly affected the relative proportions of the three phases constituting the catalysts. These phases are: Pt(Ru) alloy nanocrystallites, unalloyed Ru crystallites and nanocrystallites. The best CO tolerant catalyst is obtained by using a 12/1 ball-to-powder ratio and a 1:1:8 Pt:Ru:MgH2 proportion of dispersive agent. It is made of 57 at.% of a nanocrystalline (3 nm) Pt80Ru20 alloy, 42 at.% of a nanocrystalline (3 nm) Ru phase and 1 at.% of a crystalline (∼40 nm) Ru phase. This catalyst has the lowest Pt/Ru surface ratio (0.9), the highest content in nanocrystalline Ru, and the highest ratio of oxidized/metallic Ru (3.3). Both Pt-Ru alloy and nanocrystalline Ru participate to the CO tolerance. The best CO tolerant catalyst is, however, not the best catalyst in DMFC. The latter is obtained by using a 4/1 ball-to-powder ratio and a 1:1:6 Pt:Ru:MgH2 proportion. Within the starting 50:50 Pt-Ru nominal atomic ratio, no specific correlation was found between catalyst performance in DMFC and atomic surface Pt/Ru ratio, nor nanocrystalline Ru content, nor oxidized/metallic Ru ratio. Performances of the best ball-milled catalysts are compared to those of commercial unsupported catalysts in PEMFC and DMFC.