Reduction of electrocatalyst costs is crucial to the commercialization of proton exchange membrane fuel cells (PEMFCs). Platinum nanoparticles on high surface area carbon (Pt/HSC) substantially increase the Pt surface area and thus decrease the required amount of Pt, but suffer from loss of activity from various degradation processes. Extended Pt nanostructures are promising candidates to replace traditional electrocatalysts, as they have been shown to improve specific activity and durability; however, these materials have typically exhibited limited surface areas.

In this work, extended surface Pt-nickel (Ni) nanowires with 7 and 17 wt% Pt have been synthesized via spontaneous galvanic displacement and achieved exceptionally high surface areas (up to >90m²/g Pt). Various post-processing treatments have been applied to modify the composition and structure of these nanowires, with the goal of further improving their activity and durability. Extensive spectroscopy and microscopy studies were carried out to elucidate relationships between electrochemical performance and material composition, morphology, and structure both in bulk and at the surface.

Annealing the Pt-Ni nanowires in hydrogen continuously increased the specific activity with a peak in mass activity at 250°C. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) show that this activity increase is a result of Pt-Ni alloying. An additional cause of the increased activity is likely the formation of a more homogeneous Pt layer, which was induced by hydrogen annealing and clearly visible in elemental maps aquired with energy dispersive x-ray spectroscopy (EDS) via transmission electron microscopy (TEM). Further modification of the hydrogen annealed samples discussed in this work include oxygen annealing, acid leaching, and combinations of acid leaching with further heat treatments. XPS, TEM and EDS were used to investigate changes in the nanowires resulting from different strengths of acid treatments, which were utilized to remove undesired Ni species while maintaining high activity and durability.

[1]. S.M. Alia, Y. Yan, B.S. Pivovar, “Galvanic Displacement as a Route to Highly Active and Durable, Extended Surface Electrocatalysts” Catalysis Science and Technology, 2014, 4 (10), 3589 - 3600. DOI: 10.1039/C4CY00736KG.

Figure 1. EDS maps of Pt-Ni nanowire annealed in hydrogen at 250°C.