19F NMR studies of Nafion™ ionomer adsorption on PEMFC catalysts and supporting carbons

The adsorption of Nafion™ ionomer on Proton Exchange Membrane Fuel Cell (PEMFC) catalysts and the supporting carbons was studied in aqueous solution by 500 MHz 19Fluorine Nuclear Magnetic Resonance Spectroscopy (NMR). In addition, the materials were characterized with Brunauer-Emmett-Teller (BET) nitrogen adsorption–desorption and X-ray diffraction (XRD). It was observed that Nafion adsorbs strongly on carbon as well as noble metal catalyst: Pt, PtRu. The adsorption was classified into a primary adsorption and a secondary adsorption. At low concentration, the adsorption was found to follow a Langmuir isotherm (primary adsorption), in which important parameters are adsorption equilibrium constant (Keq) and maximum surface coverage (Γmax). The Keq turned out to be around ten, and was found to depend on the specific chemical nature of the surface. The Γmax combining with Nafion two-dimensional projection area estimated from a space-filling model implies that only part of the Nafion molecule adsorbs on the surface.