| AVS 63rd International Symposium & Exhibition | |
| Novel Trends in Synchrotron and FEL-Based Analysis Focus Topic | Tuesday Sessions |
| Session SA-TuP |
| Session: | Novel Trends in Synchrotron and FEL-Based Analysis Poster Session |
| Presenter: | Sarah Shulda, Colorado School of Mines |
| Authors: | S. Shulda, Colorado School of Mines J. Nelson Weker, SLAC National Accelerator Laboratory C. Ngo, Colorado School of Mines S. Mauger, National Renewable Energy Laboratory K.C. Neyerlin, National Renewable Energy Laboratory S. Alia, National Renewable Energy Laboratory B. Pivovar, National Renewable Energy Laboratory S. Pylypenko, Colorado School of Mines |
| Correspondent: | Click to Email |
Proton exchange membrane fuel cells (PEMFCs) have a high power-to-weight ratio making them well suited for transportation applications. Platinum (Pt) nanoparticles on high surface area carbon is the current state of the art catalyst for the oxygen reduction reaction at the cathode. However, the high cost and inherent durability issues of this catalyst significantly limit the commercialization potential of PEMFCs in automobiles. Pt nanowires are a promising alternative to the carbon-supported Pt nanoparticles. High surface area platinum nickel (PtNi) nanowires have been synthesized and demonstrated exceptionally high activity and durability in electrochemical studies using rotating disk electrodes (RDEs). The incorporation of nanowire catalysts into full membrane electrode assemblies (MEAs) is not straightforward due to significant differences in the morphology of these materials as compared to traditional catalysts based on carbon-supported nanoparticles, and requires optimization of electrode composition and structure. Factors effecting the performance of the electrodes include catalyst content, amount of ionomer, amount and type of carbon additive, the three dimensional morphology of the nanowires, and nanowire contact with each other and with the other constituents of the MEA. In the specific case of NiPt nanowires, preventing Ni leaching is also imperative as Ni will poison the fuel cell and inevitably cause a significant drop in performance. Optimization of these parameters requires detailed understanding of the electrode structure, preferably using non-destructive techniques.
Transmission x-ray microscopy (TXM) allows for non-destructive three-dimensional analysis of full MEAs providing detailed information on electrode composition and structure. Ni and Pt are imaged separately through selective tuning of the incident x-ray energy, making their relative distribution throughout the MEA readily discernible. A series of MEAs with varying ink compositions was analyzed with TXM to study the effects of ionomer content, amount and type of carbon, and addition of poly(acrylic acid) (PAA) on electrode structure. MEAs pre-leached with acid to remove Ni were also imaged. Results demonstrated that ink formulations and acid leaching significantly impacted the nanowire morphology within the MEA. The addition of graphitized carbon nanofibers (GCNFs) resulted in more homogeneous and less densely packed nanowire distribution. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) with energy dispersive elemental mapping complemented the TXM studies.