(290d) Water Transport In Nafion® Using Time-Resolved Ftir-Atr Spectroscopy

The hydrogen polymer electrolyte membrane (PEM) fuel cell is an innovative, environmentally-benign alternative power source with potential for large-market applications, such as transportation. Its success depends on increasing its efficiency, which in large part is limited by properties of the PEM. Current PEMs, such as Nafion^®, require high water contents to maintain conductivities above 10 mS/cm. However, other factors necessitate hydrogen fuel cell operation at temperatures above 120°C and humidities below 20% relative humidity (RH). A PEM with high conductivity at these hot, dry external conditions will require either decoupling proton conductivity from membrane water content or enhancing the ability of the membrane to retain water. To achieve either of these goals, a fundamental understanding of multicomponent transport of molecules and ions in PEMs is necessary.

In this study, the transport of water in Nafion^® was investigated using time-resolved Fourier-transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy. This technique not only provides molecular-level contrast between diffusants and polymers in real time, but can also determine chemical interactions via shifts in the infrared spectra. In other words, the diffusion of molecularly distinct species in the polymer membrane can be independently and simultaneously measured in the presence of a concentration gradient, while at the same time monitoring changes in intermolecular interactions.

In these experiments, water diffusion in Nafion^® was examined as a function of external relative humidity using time-resolved FTIR-ATR spectroscopy. Regions of the infrared spectrum associated with the stretching and bending vibrations of the hydroxyl bond (O-H) were deconvoluted. Infrared bands associated with different states of water (hydronium ions and several states of bulk water) and anhydrous sulfonic acid in Nafion^® were identified and their rates quantified in real time. The ability to monitor separate molecular diffusing species reveals new underlying transport mechanisms. Specifically, the dynamic and equilibrium state of each species and its relationship to water sorption, water diffusivity, and proton conductivity will be discussed. These results not only provide new insights into understanding water management in current PEMs, but also could have a significant impact on the development of new PEMs for high temperature, low humidity fuel cell operation.