Abstract
The conductivity of fuel cell membranes as well as their mechanical properties at the nanometer scale were characterized using advanced tapping mode atomic force microscopy (AFM) techniques. AFM produces high-resolution images under continuous current flow of the conductive structure at the membrane surface and provides some insight into the bulk conducting network in Nafion membranes. The correlation of conductivity with other mechanical properties, such as adhesion force, deformation and stiffness, were simultaneously measured with the current and provided an indication of subsurface phase separations and phase distribution at the surface of the membrane. The distribution of conductive pores at the surface was identified by the formation of water droplets. A comparison of nanostructure models with high-resolution current images is discussed in detail.
Highlights
Proton-conducting membranes are one of the key components in polymer-electrolyte fuel cells
At a protonated and water-soaked but not activated Nafion® sample, the conductivity measured by conductive atomic force microscopy (AFM) is barely detectable
Based on the present measurements, the model proposed by Gebel and Diat is consistent with almost all ionic current density distributions and mechanical properties observed on Nafion®, whereas the model with cylindrical inverted micelles is unable to explain the majority of our observations
Summary
Proton-conducting membranes are one of the key components in polymer-electrolyte fuel cells. The detailed phase-separated structure of Nafion® is still a matter for continuous study, it is well known that the microstructure of Nafion® strongly depends upon the hydration state, the temperature, and the current flow These morphological changes can be fast, such as the restructuring of the surface in contact with liquid water within seconds, or slow, such that achieving a steady state in the bulk can require days or even months [1]. Based on contact-angle measurements taken at different interfaces, the fraction of hydrophilic surface area has been calculated to change from 0.05 in the dry polymer to greater than 0.11 in water vapor and up to 0.95 in liquid water [5,6]. In the AFM measurements, the contact time between the Nafion® surface and the platinum tip is short and may not provide sufficient time to evolve the equilibrium structure of a membrane-platinum interface according to the interfacial energy of this configuration. Membranes were analyzed by AFM after being stored in water without an initial current flow and after an activation of the membrane in an electrolysis arrangement
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