Chemical and Structural Investigation of Pt-Ni Extended Surface Catalyst Electrodes

Abstract

Typical carbon-supported platinum (Pt) nanoparticle catalysts suffer from having only moderate specific activities relative to bulk Pt, as well as activity losses caused by various degradation processes. [1] Extended surface Pt catalysts are a promising alternative to the current state-of-the-art catalysts, particularly if high surface areas are achieved. Recently, we have developed Pt-Ni extended surface catalysts with surface areas >90 m2/gmPt and improvements in mass activity, specific activity, and durability. [2] However, as-synthesized materials are susceptible to dissolution of the Ni from the core of the Pt-Ni nanowire (NW) and therefore exhibit serious performance losses due to Ni ions binding to proton sites in the ionomer. Through a series of post-synthesis treatments involving annealing and subsequent acid leaching to optimize material structure and composition, the NW specific activity and durability were significantly improved. [3] Current efforts focus on performance optimization of the catalyst layer in membrane electrode assemblies (MEAs), which in addition to detailed information on the as-synthesized and treated catalysts, also requires investigation of these materials when part of an electrode. Structure, composition, and morphology were determined with initial 2D characterization of the NWs and electrodes using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) via scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), as well as synchrotron techniques including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Beyond these methods, transmission X-ray microscopy (TXM) allowed for high resolution, non-destructive, element-specific analysis of the electrode structure with 2-D and 3-D visualization. Catalyst composition/structure and electrode fabrication conditions were varied to explore opportunities and limitations of in situ characterization of the electrodes with these low Pt loadings extended surface catalysts. R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.I. Kimijima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chemical Reviews, 107 (2007) 3904-3951.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 Catal 4(2014) 1114-1119.S.M. Alia, C. Ngo, S. Shulda, M.-A. Ha, A.A. Dameron, J.N. Weker, K.C. Neyerlin, S.S. Kocha, S. Pylypenko, B.S. Pivovar, Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum–Nickel Nanowires through Synthesis and Post-Treatment Optimization, ACS Omega, 2 (2017) 1408-1418.