(180b) Water and Methanol Vapor Diffusion Rates In Proton Exchange Membranes

Proton exchange membrane fuel cells (PEMFC) and Direct Methanol Fuel Cells (DMFC) continue to be active areas of research for use as low pollution power generators.  In regards to water vapor, the humidity of the feed stream is a critical parameter affecting PEMFC performance.  If the hydration is too low, the fuel cells can exhibit reduced ionic conductivity within the membrane, and proton transport will be insufficient.  If hydration is too high, excess water can flood pores within the Gas Diffusion Layers (GDL) and catalyst layers, eventually leading to blockage of reaction sites.  A potential downfall of DMFC is methanol crossover from anode to cathode due to high methanol permeability through the membranes.  This leads to lower overall fuel cell efficiency.  As a result, the membranes used in DMFC applications should have improved methanol barrier properties.  Therefore, understanding the water and methanol vapor sorption and transport properties in PEMFC and DMFC components are vital in the development of these materials. 

EXPERIMENTAL

The current study investigates water and methanol vapor sorption and transport properties on extruded and dispersion-cast Nafion® based membranes using a gravimetric sorption instrument.  Dynamic gravimetric Vapor Sorption (DVS) is a well-established technique for the measurement of vapor sorption properties on solid materials.  Vapor content and diffusion coefficients of both water and methanol were measured on films of different thicknesses and production processes to study the effects of polymer morphology.  Measurements were carried out between 0 to 90% P/Po vapor concentrations over a range of temperature conditions.  Diffusion rates into the films and across the films (i.e. flux) were determined. 

RESULTS

As expected, increasing temperature expedites the water vapor flux.  This is because at higher temperatures, there is an increased molecular mobility within the film, which facilitates water vapor transport properties across the membrane.  The affect of sample thickness was studied using Nafion® N-117 (183 microns) and N-112 (51 microns), meanwhile the affect of dispersion casting and extruding was studied using N-112 and NR-112 samples, respectively.  Thicker N-117 sample was demonstrated to have a lower flux compared to the N-112 sample.  The dispersion cast NR-112 sample generally exhibited higher water vapor flux values than the extruded N-112 sample. 

Methanol vapor sorption capacity increased with increasing temperature. However, this result is not completely intuitive, as surface thermodynamics would suggest a lower methanol uptake at higher temperatures (driven by heat of sorption).  On the other hand, bulk absorption can increase with increasing temperature as methanol solubility in the polymer increases.  These competing surface and bulk sorption processes result in complex sorption phenomena.  Again, thicker N-117 sample had a lower methanol uptake (based on dry weight) compared to the N-112 sample.  On the contrary, dispersion cast NR-112 sample generally exhibited lower methanol uptake compared to the extruded N-112 sample. 

CONCLUSIONS

Water and methanol vapor sorption and flux measurements were measured on Nafion® based samples over a range of temperatures.  Different morphologies and thicknesses of membranes were also investigated.  Increasing temperatures lead to increasing sorption properties and water vapor flux.  Water and methanol vapor diffusion into the films were maximized at intermediate concentrations.  Similar experiments could be applied to other fuel cell components or with different vapor molecules over a wide range of operating conditions.