Communication: Nanoscale ion fluctuations in Nafion polymer electrolyte

Abstract

Ion conduction mechanisms and the nanostructure of ion conduction networks remain poorly understood in polymer electrolytes which are used as proton-exchange-membranes (PEM) in fuel cell applications. Here we study nanoscale surface-potential fluctuations produced by Brownian ion dynamics in thin films of low-hydration Nafion™, the prototype PEM. Images and power spectra of the fluctuations are used to derive the local conductivity-relaxation spectrum, in order to compare with bulk behavior and hopping-conductivity models. Conductivity relaxation-times ranged from hours to milliseconds, depending on hydration and temperature, demonstrating that the observed fluctuations are produced by water-facilitated hydrogen-ion hopping within the ion-channel network. Due to the small number of ions probed, non-Gaussian statistics of the fluctuations can be used to constrain ion conduction parameters and mechanisms.

Excited‐State Proton Transfer and Conformational Relaxation of 2‐(4′‐Pyridyl)benzimidazole in Nafion Films

“…Furthermore, the extent of variation in lifetimes upon decrease in hydration level is less in annealed film than in nonannealed films. This is in agreement with a report in which annealing was found to generate completely dried regions having a crystalline fluorocarbon backbone that is difficult to hydrate . This makes water uptake of annealed films difficult.…”

dcProton conductivity at low-frequency in Nafion conductivity spectrum probed by time-resolved SAXS measurements and impedance spectroscopy

“…Previous dielectric spectroscopy studies reported that Nafion membranes exhibits two distinct dc conduction processes, one occurring at high frequencies ( f ∼ 10 6 Hz) and another at lower frequencies ( f < 10 −1 Hz) . Evidences of two dc conductivities have been also provided by different techniques, such as electric force microscopy (EFM), NMR, quasielastic neutron scattering, and radiotracer studies of Nafion membranes . Such reports revealed ion conduction process occurring at long time scales t ∼ 1 – 100 s ( f ∼ 10 0 – 10 −2 Hz), in agreement with the low‐frequency data of Figure .…”

Interplay between α-relaxation and morphology transition of perfluorosulfonate ionomer membranes

“…The depth of the contrasted regions is~5 nm in agreement with previous AFM analysis [16,30]. In these images, the topography displays a rather irregular structure.…”

“…The crossover frequency from ac to dc conductivity, identified in earlier reports at f~10 À3 Hz [17], is in good agreement with the onset frequency for dc conductivity, observed at f < 10 À3 Hz [15,17,19]. Therefore, the relaxation observed at f~10 À3 Hz in Nafion dielectric spectrum can be attributed to a conductive dispersion (s relaxation) [16,21]. Fig.…”

“…Small angle Xray scattering (SAXS), fluorescence, and infrared spectroscopy studies of hydrated Nafion membranes revealed that a considerable fraction of protons is condensed in sulfonic acid groups [10,11]. Atomic force microscopy measurements of Nafion detected nanoscale surface-potential fluctuations, which were mainly derived from the ac ion-hopping conductivity, usually observed for disordered ion conductors [16]. Moreover, recent dielectric spectroscopy measurements of Nafion solutions revealed that a and b relaxations are mostly associated with the fluctuations of condensed counterions along the longitudinal and radial directions of Nafion polymer backbone, respectively [15].…”