Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells

Abstract

The impact of polyvinylidene fluoride (PVDF) as a binder component on the durability of Pt/C cathodes in a proton exchange membrane fuel cell membrane-electrode-assembly (MEA) during a carbon corrosion accelerated stress test (AST) was examined using electrochemical fuel cell data and visual inspection/analysis of the cathode morphology via electron-microscopy. Electrospun nanofiber cathode mat MEAs with a Nafion ® /PVDF or Nafion/poly(acrylic acid) (PAA) binder or a slurry cathode MEA with neat Nafion or a Nafion/PVDF binder were investigated. The presence of PVDF had profound effects on the structure and chemical/electrochemical properties of a fuel cell cathode; its hydrophobic property slowed the rate of carbon loss and its robust mechanical properties added strength to the binder. Thus, the extent of carbon loss during an AST was inversely proportional to the PVDF content of the binder and there was no observable cathode thinning nor any change in cathode porosity after the AST, when the cathode binder contained at least 50 wt% PVDF. In terms of long-term durability, these beneficial structural effects outweighed the lower Nafion/PVDF binder conductivity and the associated lower initial power output of a Nafion/PVDF cathode MEA. For hydrophilic slurry and nanofiber cathodes with neat Nafion or Nafion/PAA fibers, low power after the carbon corrosion AST was due to greater carbon losses, cathode thinning and the collapse of cathode pores, which dominated MEA performance even though the initial cathode ECSA and mass activity were high for these two MEAs.