A fundamental study of high temperature proton exchange membranes for fuel cells.

Abstract

Proton exchange membrane fuel cells (PEMFC) have been considered as an attractive energy source for both portable and automobile applications because of their high energy conversion efficiency, high power density, quiet operation and low greenhouse gas emission. The efficiency and performance of PEMFCs can be further enhanced by increasing the operating temperature to 120-250 oC due to the improved reaction kinetics and significantly reduced catalyst poisoning by CO. Water and heat management of fuel cells systems becomes easier at high temperatures. However the grand challenge is to develop proton exchange membranes with high proton conductivity and stability at elevated temperatures and low humidity. For instance, the state-of-the-art PEM is the perfluorosulfonic acid (PFSA) polymers membrane due to their good mechanical properties, excellent chemical stability and relatively high proton conductivity under highly hydrated conditions. But the conductivity of PFSA-based membranes decreases significantly at elevated temperatures due to the dehydration and degradation of the membranes under elevated temperature (over 100 oC) and low relative humidity (RH) environment.

It has been well known that heteropolyacid (HPA) is a superionic conductor in its fully hydrated states. Among various HPAs, the highest stability and strongest acidity are observed for phosphotungstic acid (H3PW12O40, abbreviated as HPW or PWA). Despite their high acidity, stability and high proton conductivity, the application of HPAs in proton exchange membranes of fuel cells is limited due to the sensitivity oftheir conductivity to the relative humidity of the surrounding environment in addition to their solubility in water. The risk of the leakage of HPAs during cell operation is thus high. In this thesis, a novel inorganic progon exchange membrane using HPAs as a proton carrier and mesoporous silica as framework materials is successfully developed as proton exchange membranes for fuel cells. Mesoporous silica is used as the host material to stablize HPAs which are introduced by a vacuum impregnation method. The threshold for a good proton conductivity of the HPW-meso-silica nanocomposite is ~10 wt%. The best proton conductivity is 0.07 Scm-1 at 25oC under 100 %RH with activation energy of ~13.5 kJ mol-1, obtained on the 67-83% HPW-meso-silica nanocomposites. The high stability of the Keggin anions of HPW within the meso-silica host most likely due to the formation of (≡SiOH2+) (H2P4W12O40-) species. The proton conductivity and performance of HPW-meso-silica nanocomposites also increase with RH, but are far less sensitive to RH changes as compared to conventional PFSA polymers such as Nafion. These properties make the the HPW-meso-silica nanocomposite a promising proton conductor under high temperature and low RH environment.