(5ao) Transport in Polymer Electrolyte Membranes for Energy Applications

Polymer electrolyte membranes (PEMs) hold potential to improve performance in fuel cells and batteries, electrochemical devices that can generate electricity efficiently. In particular, hydrogen fuel cells are promising for clean, renewable transportation. Direct methanol fuel cells (DMFCs) and lithium ion batteries can operate at room temperate and have high energy density, making them ideal for portable power applications. The electrolyte is an integral part of any electrochemical cell because it serves to conduct ions while separating the electrodes. Useful hydrogen fuel cell performance requires high water sorption and dynamics (diffusion) to maintain sufficient ion conductivity in Nafion (most frequently used PEM to date), which conflicts with the goal of high temperature and low humidity operation. DMFC power output suffers from methanol crossover (when methanol permeates across Nafion from the anode to the cathode). Therefore, understanding water and methanol diffusion in Nafion is critical. Although many investigators have reproduced water sorption isotherms in Nafion, reported diffusion coefficients varied considerably. Also, many investigators have focused on reducing methanol diffusion in PEM development for the DMFC, despite no clear evidence for that approach.

Water and methanol diffusion in Nafion was studied using time-resolved Fourier transform infrared ? attenuated total reflectance (FTIR-ATR) spectroscopy. Critical assessment of water transport phenomena in Nafion identified vapor-phase mass transfer resistance and two non-Fickian regimes, explaining some of the variation in reported diffusion coefficients and improving the fundamental understanding of the transport mechanisms. Methanol crossover (flux) was explicitly shown to increase with increasing methanol concentration. More importantly, the increase was found to be more strongly dependent on methanol sorption or partitioning (chemical interaction) rather than methanol diffusion. The implication is that an effective PEM for the DMFC must be chemically incompatible with methanol.