Transmission X-Ray Microscopy 3-D Characterization of Pt-Ni Extended Surface Catalyst Electrodes

Abstract

Extended structure Pt catalysts are viewed as a promising alternative to state-of-the art catalysts based on carbon-supported Pt nanoparticles due to improved specific activities and durability. However, these types of catalysts posses lower surface areas and demonstrate lower mass activities than traditional catalysts. We have recently developed high surface area PtNi nanowires that meet the requirements for high mass activity, specific activity, and durability. These extended structure PtNi nanowires were synthesized via galvanic displacement producing extended Pt surfaces with high surface areas (>90 m2/g Pt).1Various post-processing treatments have been applied to alter the chemistry and structure of the nanowires in order to optimize their performance. Annealing the nanowires in hydrogen to alloy the Pt to the Ni, acid leaching to remove unalloyed Ni, and annealing in oxygen was found to significantly enhance performance and durability in a rotating disk electrode (RDE) half-cell. The initial mass activity was found to be 7 times higher than Pt/HSC and after durability testing 97% of performance was retained. Our current efforts focus on optimization of the performance of these extended surface electrode structures in membrane electrode assemblies (MEAs), which requires detailed information about electrode composition and structure. Transmission X-ray microscopy (TXM) allows for non-destructive analysis of the extended surface catalysts electrode structure with 3-D visualization.2,3Another major advantage of the TXM is the ability to selectively image Ni or Pt by tuning the incident X-ray energy to the element’s specific absorption edge and thus analyze their respective distribution throughout the entire electrode structure. A series of MEAs was analyzed to explore the effects of ink formulations (i.e. amount of ionomer, amount and type of carbon, addition of poly(acrylic acid) (PAA)) on electrode structure. MEAs were also analyzed after post-treatments involving acid leaching to remove undesired nickel species. The results showed clear differences in the PtNi nanowire structure with the various ink compositions and post-treatments. Addition of graphitized carbon nanofibers (GCNFs) resulted in significantly less densely packed, but more homogeneous nanowire distribution, as seen in figure 1. Rotation of the MEA confirms the observed structure persists throughout the MEA. Leaching the MEA in acid resulted in a decrease in Ni weight percent and changes in the nickel distribution throughout the MEA. TXM was correlated with analysis from other microscopy techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning TEM (STEM) combined with energy dispersive elemental mapping. The results from these studies provide guidance for further optimization of extended surface catalysts and electrodes. Figure 1: (a) PtNi nanowires with PAA and nafion (b) PtNi nanowires with PAA, nafion, and GCNFs 1. S.M. Alia, B.A. Larsen, S. Pylypenko, D.A. Cullen, D.R. Diercks, D. R.; K.C. Neyerlin, S.S. Kocha, B.S. Pivovar, Acs Catal4(2014) 1114-1119. 2. Y. Singh, O. Luo, F. Orfino, M. Dutta, E. Kjeang, Meeting s 2015, MA2015-02(37), 1355-1355. 3. J. N. Weker, X. Huang, M. F. Toney, Current Opinion in Chemical Engineering 2016, 12, 14-21. Figure 1