Counterion dynamics and the mechanical properties of fuel cell membranes

1. B A C Counterion dynamics and the mechanical properties of fuel cell membranes DMR-0454672 K. Page1, R. Moore2, J.K. Park2, V. García Sakai3,4 1NIST, 2University of Southern Mississippi, 3University of Maryland, 4CHRNS Perfluorosulfonate ionomers (PFSIs) are of great importance in polymer electrolyte membrane fuel cell (PEMFC) applications. The CHRNS backscattering spectrometer HFBS has been used to study the most widely used PFSI, Nafion, neutralized with tetramethyl (TMA+) and tetrabutyl (TBA+) ammonium ions. Elastic scans (fig. A) unequivocally show that in both cases the onset of counter-ion mobility takes place at the -relaxation temperature, lending support to the notion of a process whereby ions hop among ionic aggregates. Further information has been obtained from quasielastic scans that probe the diffusional motion of the counter-ions. The behavior of the width of the scattering, measured above the -relaxation temperature (fig. B), suggests random jump-diffusion within confined regions. The ratio of the elastic intensity to the elastic plus quasielastic intensity (the “EISF”*), fig. C, affords additional insights into the geometry of the diffusional motion. At low T the dynamics are dominated by local motions of the alkyl “arms” of the counter-ions. At high T the counter-ions move over much larger distances. *EISF  elastic incoherent structure factor K. Page, R. Moore, J.K. Park, and V. García Sakai, paper in preparation