Assessing the Pt-upd Surface Area Stability of Pt1-xMx (M=Re, Nb, Bi) Solid Solutions for Proton Exchange Membrane Fuel Cells

Three Pt1-xMx composition spreads were prepared, characterized, and then tested in 64-electrode proton exchange membrane fuel cells. The spreads were prepared by sputter-depositing a linear gradient of Re, Nb, or Bi with a constant amount of Pt in a manner that allowed for atomic level mixing of the elements. Re and Nb were selected as elements likely to be resistant to corrosion; Bi was selected because it was likely to dissolve. The spreads were deposited onto a range of different substrates including 3M Company's nanostructured thin-film catalyst support. Electron microprobe data showed that the composition spreads contained up to 60 atom % of the intermix element of interest. X-ray diffraction measurements showed that face-centered cubic lattices were retained out to relatively high amounts of the intermix element. Acid exposure testing results showed that essentially all of the Bi initially present dissolved, some Nb was lost from the surface, and almost all of the Re was retained. It is likely that the same changes would have occurred during fuel cell operation. Pt surface areas as measured by cyclic voltammetry increased with increasing atomic fraction of the additive. High potential cycling was used to assess the stability of the Pt-upd (under-potential deposition) surface area; the Pt1-xNbx composition spread was the most stable.