(486a) How Do Water and Protons Move Through Ionomers?

Benziger, J., Princeton University

Ionomer membranes are the heart of Polymer Electrolyte Membrane Fuel Cells.  The ionomer membrane transports protons from the anode to the cathode.  Proton conductivity is a strong function of water activity.  In Nafion, the archetypal PEM for fuel cells the proton conductivity increases by more than six orders of magnitude as water activity increases from 0 (0% humidity) to 1 (saturated vapor).  Water transport in Nafion also increases by more than two orders of magnitude as water activity increases. 

We have examined in detail the transport and properties of Nafion, sulfonated polyether etherketone (SPEEK) and sulfonated polystyrene (SPS)  as functions of temperature and water activity.  Ionomers separate into hydrophilic domains of sulfonic acid and water dispersed in a hydrophobic matrix.  Water and protons are transported through the hydrophilic domains. The volume fraction of the hydrophilic phase increases with water activity, which also changes the structure of the hydrophilic domains.   A percolation threshold of 0.1 vol fraction for proton and water transport is found for 1100 EW Nafion; above the percolation threshold water diffusion and proton mobility increase quadratically with water volume fraction. The percolation thresholds for SPEEK and SPS are larger, indicating different shapes of hydrophilic domains for different ionomers.  We will show how ionomer structure is related to the shape and percolation of hydrophilic domains. 

Water transport in PEMs involves the coupling of interfacial mass transport, diffusion and polymer swelling.  We will show that interfacial transport resistance at the vapor/membrane interface is the limiting transport resistance at high water activity.  The interfacial transport resistance is a key reason why Nafion is such a good PEM for fuel cells.  We will show how the coupling of internal diffusional resistance and external interfacial transport resistances are couple in ionomers and how it affects the coupled water and proton transport.