Current Sensing Atomic Force Microscopy Study of Thermal Aging of Nafion Membranes

Abstract

We have investigated thermal aging in the ionic conductivity of Nafion membranes being annealed at elevated temperatures using current sensing atomic force microscopy (CSAFM), a unique technique that allows simultaneous imaging surface morphology and surface conductance of the materials. The local current flows on the membranes show an increase after initial annealing, followed by a gradual decrease as the annealing continues. The current distribution extracted from CSAFM images shows that the decrease in the conductance follows approximately an exponential decay which occurs uniformly across the membrane. Least-square fits to the measured conductance versus time t yield time constants at the annealing temperature. Considering aging is a thermally activated process, we used an Arrhenius equation to describe the relationship between the time constant and the annealing temperature, which gives activation energies for the membranes. The activation energy yields following this process are 54 and 83 kJ/mol for Nafion 115 and Nafion NR212, respectively. These results provide guidance for employing the materials for device applications.

Direct Observation and Semiquantitative Analysis of Hierarchical Structures in Graft-Type Polymer Electrolyte Membranes Using the AFM Technique

“…AFM is often used in systems that have an ideal composition and a high degree of regularity, and its molecular-level measurements have been useful in understanding many physical phenomena. AFM has been used to analyze the structure of polyelectrolyte membranes in recent years by detecting structure, 16 viscoelasticity, 17 and electrical conduction, 18 allowing structural measurements to be made under conditions like those in use. To design new materials, it is necessary to determine the structural factors that contribute to the above-mentioned required multifunctions, which are related to power generation efficiency and durability in actual fuel cell operation conditions.…”

Three-Dimensional Fine Structure of Nanometer-Scale Nafion Thin Films

“…Although Nafion has been used and studied extensively over the decades, its structure–property relationships, especially when utilized as nanometer-scale thin films, are still not unambiguously known. A variety of study methods have included swelling measurements, ,, small-angle x-ray scattering (SAXS), − ,,− grazing-incidence small-angle X-ray scattering (GISAXS), , atomic force microscopy, − electrostatic force microscopy, impedance spectroscopy, contact angle measurements, transmission electron microscopy (TEM), − cryo-electron microscopy (cryo-EM), diffraction, energy-dispersive X-ray spectroscopy, neutron scattering, − neutron reflectometry, infrared spectroscopy, , and ionic diffusion. , To summarize the extensive literature on Nafion, these numerous studies have produced a variety of simulation results, reconstructions, and models, − which often comprise round and/or cylindrical shapes, that have been used to predict the film transport coefficient, proton transport , and structure. However, there exists no consensus as to which of the several models best describes clustering in Nafion or whether the assumed channels in Nafion are connected and precisely in which way.…”