Effect of Thermal Annealing on Proton Conduction in Ion Exchange Membranes
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Effect of Thermal Annealing on Proton Conduction in Ion Exchange Membranes Osung Kwon,1 Shijie Wu,2 and Da-Ming Zhu,1,* 1
Department of Physics, University of Missouri - Kansas City, Kansas City, MO 64110
2
Agilent Technologies, Inc., 4330 W. Chandler Blvd., Chandler, AZ 85226
ABSTRACT The configurations of proton channel network on the surface of Nafion® membranes were studied using current sensing atomic force microscopy after the membranes were annealed at elevated temperatures, aimed at understanding the effect of aging process in the membranes. The results reveal that proton conductance of the membranes becomes more uniform and the proton channels become chain-like aligning in parallel to the membrane surface. Accompanied to the configuration changes, the proton conductivity of the membrane shows an increase. As the annealing continues, the chain-like configuration for the proton channels persists but the conductance of the membranes decreases. The time constant of the conductivity decay decreases with increase of the annealing temperature. The observed changes can be attributed to reorientation of proton channels near the membrane surface from perpendicular to parallel to the surface as the annealing temperature approaches the glass transition of the membranes. INTRODUCTION In order to significantly improve the stability and durability of polymer electrolyte membrane (PEM), a challenging task that needs to be accomplished before PEM fuel cells can become commercially viable energy conversion devices in a wide range of applications, it is necessary to understand the mechanisms that govern the stability and durability of these membranes [1]. For this purpose, we have investigated the aging behaviors and effects of thermal annealing to the ionic transport in Nafion® which represents a family of comb-shaped ionomers consisting of polytetrafluoroethylene (PTFE) hydrophobic backbone with perfluorinated pendant terminated by hydrophilic sulphonic acid head groups (-SO3H) [2-10]. The microscopic structure of a Nafion membrane is complex, and has been actively studied, but is still a subject under intensive debate [1-15]. A commonly referenced model describes its structure as PTFE backbone matrix embedded with ionic clusters, which are approximately spherical in shape with an inverted micelle structure formed by aggregated hydrophilic head groups in the membrane, connected by narrow hydrophilic channels allowing ionic conductivity [11,12]. A recent work based on computer simulation and the re-analysis of the currently available small angle X-ray and neutron scattering data finds that the ionic structure of Nafion membranes can be better described as parallel cylindrical nanometer sized hydrophilic channels with directions predominantly perpendicular to the membrane’s surface [14]. Protons transport along the hydrophilic channels, producing excellent proton conductivity of the membranes. These characteristic structural features remain to be experimentally verified. Related to the durability of the membranes, it is i
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