(19d) Platinum Thin-Coated Palladium Nanotubes for the Oxygen Reduction Reaction

Platinum supported on carbon (Pt/C) is commonly the commercial material used as an electrocatalyst in the oxygen reduction reaction (ORR) as a cathode in proton exchange membrane fuel cells (PEMFCs). The small particles are used due to the high surface area and thus high mass activity in the oxygen reduction reaction (ORR). The carbon support provides spacing for the platinum particles and prevents agglomeration. The use of Pt/C, however, creates concerns with respect to catalyst durability and specific activity loss due to alterations in the coordination number and crystal planes of the surface platinum atoms. Platinum nanotubes (PtNTs) with a thickness of 5 nm have previously been synthesized to improve the durability and ORR activity of PEMFC catalysts.[1] The United States Department of Energy (DoE) has set benchmarks for ORR mass activity (0.44 AmgPt-1, at 0.9 V vs. a reversible hydrogen electrode) and ORR specific activity (0.72 mAcmPt-2, at 0.9 V vs. a reversible hydrogen electrode) for 2010-2015. Conventional platinum catalysts and PtNTs in their pure form are not capable of meeting these activity benchmarks.

Pure PtNTs were synthesized by the galvanic replacement of silver nanowires, which were formed by the ethylene glycol reduction of silver nitrate. Palladium has been used as a substitute material for sub-surface platinum due to a lower material cost, high durability, conductivity, and the ORR activity observed in palladium catalysts. Palladium can further be utilized to participate in the galvanic replacement of silver and the galvanic replacement by platinum. In this manner, palladium nanotubes with platinum coatings of varying thickness have been synthesized in an effort to optimize ORR mass activity, ORR specific activity, and durability to meet DoE benchmarks. An increase in ORR platinum mass activity is observed during rotating disk experiments due to a lower platinum loading. An increase in ORR platinum specific activity is observed due to alloying with palladium in the catalyst surface during synthesis and annealing. Compared to conventional electrocatalysts and pure PtNTs, platinum thin-coated palladium nanotubes offer significant activity advantages as PEMFC catalysts.

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