(254c) Platinum Thin-Coated Palladium Nanotubes for Oxygen Reduction

Platinum supported by amorphous carbon (Pt/C) is often the commercial material used as an electrocatalyst in the oxygen reduction reaction (ORR) as a cathode in proton exchange membrane fuel cells (PEMFCs). Small platinum particles are used due to the high surface area to volume ratio, and the carbon support spaces the particles to prevent agglomeration and increase specific surface area and mass activity. The use of this catalyst, however, creates durability concerns and yields a drop in catalyst specific activity due to alterations in the coordination number and crystal planes of the surface platinum atoms. Platinum nanotubes of 5 nm thickness have previously been synthesized to improve the activity and durability of conventional PEMFC catalysts.[1] Neither platinum nanoparticles nor nanotubes are capable of meeting United States Department of Energy (DOE) benchmarks for mass activity (0.44 Amg-1, at 0.9 V vs reversible hydrogen electrode) or specific activity (0.72 mAcm-2, at 0.9 V vs reversible hydrogen electrode) in their present form.

Solid platinum nanotubes were synthesized by the galvanic replacement of silver nanowires, which were formed by the ethylene glycol reduction of silver nitrate. Palladium can be used as a substitute material for sub-surface platinum as it does not corrode as quickly as carbon, participates in the galvanic replacement of silver, and benefits catalytic activity of ORR. Solid palladium nanotubes can be partially coated with a sub-stoichiometric amount of platinum to form platinum thin-coated palladium nanotubes. This material reduces the amount of platinum required to complete ORR and increases the mass activity of the catalyst. Further annealing of this material, typically used to improve catalyst durability, is intended to introduce palladium alloying into the catalyst surface layer. Through palladium alloying, the catalyst specific activity can be increased to meet current DOE benchmarks in ORR. The use of platinum thin-coated palladium nanotubes in ORR is intended to improve mass and specific activity without sacrificing the improved durability of platinum nanotubes to conventional catalysts.

[1] Z. W. Chen, M. Waje, W. Z. Li, Y. S. Yan, Angewandte Chemie-International Edition 2007, 46, 4060.