Assessment of Nanofiber Electrode MEA Durability By Analytical Electron Microscopy

Abstract

Carbons typically used as the support for Platinum catalysts in proton-exchange membrane fuel cells (PEMFCs) are vulnerable to electrochemical corrosion, especially during potential excursions generated by startup/shutdown cycling and local anode starvation [1]. Pintauro and coworkers have shown that an electrospun nanofiber cathode, composed of Pt/C particles and a binder of NafionÒ+ poly(acrylic acid), performs remarkably well in a hydrogen/air PEMFC, with high power output at low Pt loading and excellent durability, as determined from end-of-life polarization curves after an accelerated start-stop voltage cycling (carbon corrosion) test. For example, after 1,000 simulated start-stop cycles (voltage cycling between 1.0 and 1.5 V), a nanofiber membrane electrode assembly (MEA) with a Johnson Matthey Pt/C nanofiber catalyst maintained 53% of its initial power at 0.65 V and 85% of its maximum power, as compared to a 28% power retention at 0.65 V and 58% maximum power for a sprayed electrode MEA [2]. In another study [3], constant power output, or even and increase in power, was observed after a carbon corrosion voltage cycling test with a nanofiber electrode MEA, where the cathode Pt/C catalyst binder was a blend of Nafion + poly(vinylidene fluoride) (PVDF). For a Nafion-poly(acrylic acid) binder, the superior end-of-life performance of the nanofiber MEA after a carbon corrosion test was tentatively attributed to the combined effects of a high initial electrochemical cathode surface area, the retention of the nanofiber structure after testing (no collapse of the cathode), and the rapid/effective expulsion of product water from the cathode, which minimizes/eliminates flooding. Nafion-PVDF binders appear to suppress corrosion by increasing the nanofiber hydrophobicity, thus decreasing the concentration of water at the catalyst surface. The unusual but desirable behavior of nanofiber cathodes during a carbon corrosion voltage cycling test warranted further studies, in particular imaging and chemical analysis using scanning transmission electron microscopy (STEM), of cathodes at beginning-of-life (BOL) vs. end-of-life (EOL) (after 1,000 voltage cycles, triangular wave at 1.0 V to 1.5 V at a scan rate of 500 mV/s). Characterization experiments were performed at Oak Ridge National Laboratory using nanofiber MEAs prepared and tested for power output at Vanderbilt University. All MEAs were made with Johnson Matthey HiSpec® 4000 (40% Pt on Vulcan carbon) catalyst at an electrode loading (anode and cathode) of 0.1 mgPt/cm2. The anode binder was always Nafion + poly(acrylic acid) whereas the cathode nanofiber binder was either Nafion + poly(acrylic acid) or Nafion + poly(vinylidene fluoride) (PVDF), where the Nafion/PVDF weight ratio was varied from 80/20 to 20/80. We were particularly interested in understanding how the nanofiber electrode morphology and/or binder composition (e.g., PVDF content and binder hydrophobicity) affected cathode carbon corrosion and power output. In this presentation, results of STEM imaging and compositional analysis of nanofiber MEAs at BOL and EOL will be presented, along with fuel cell polarization curves for the same MEAs. References C. A. Reiser, et al. Electrochemical and Solid State Letters, 8(6): A273-A276 (2005)M. Brodt, T. Han, N. Dale, E. Niangar, R. Wycisk, and P. Pintauro, , J. Electrochem. Soc., 162, F84-F91 (2015).M. Brodt, R. Wycisk, J. Slack, and P. N. Pintauro, “New Developments in Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells” paper #1250, Electrochemical Society Meeting, Cancun, Mexico (2014).