(588b) Hydroxyl and Water Transport in Stable Anion Exchange Membranes

Authors: 
Herring, A. M., Colorado School of Mines
Liberatore, M. W., Colorado School of Mines
Knauss, D. M., Colorado School of Mines
Coughlin, E. B., University of Massachusetts
Voth, G. A., Center for Biophysical Modeling and Simulation, University of Utah
Witten, T. A., University of Chicago

The potential of anion exchange membrane (AEM) fuel cells to provide inexpensive compact power from a wider variety of fuels than is possible with a proton exchange membrane (PEM) fuel cell, has continued to drive the research interest in this area.  Alkaline catalysis in fuel cells has been demonstrated with non-precious metal catalysts, and with a variety of fuels beyond H2and methanol. Alkaline fuel cells (AFCs), based on aqueous solutions of KOH, have serious drawbacks associated with system complexity and carbonate formation. Anion exchange membrane (AEMs) fuel cells have a number of advantages over both PEM fuel cells and traditional AFCs; however, although anionic conductivity in AEMs can be comparable to PEMs the chemical stability of membrane attached cations in hydroxide is still not always sufficient for practical applications.  The real issue is water transport; water is both a product and a reactant in these systems and wet cations are much more stable than dry.  Wetted cations are alos much more stable than dry cations.  So an understanding of water in these membranes is essential.

Here we discuss water and anion transport in a series of thin mechanically robust state of the art experimental anion exchange membranes.  The membranes are generally constructed from an isoprene block and a vinyl benzyl bromide block,  either randomly or in di-, tri- or penta- block configurations.  Post quaterniaztion leads to functionalized AEMs.  We use electrochemical impedance spectroscopy to measure anion conductivity, multi-nuclear pulse field gradient spin echo NMR to measure self-diffusion, and broadband electric spectroscopy to measure the relaxation processes in these polymers.  This information is coupled with microscopy and SAXS to explore the polymer morphology.  Putting transport and morphology together allows us to describe a complete picture of water and anion transport in these systems.  Recently, with the realization that AEMs derived from chlorinated polymer backbones are highly stable to hydroxide attack, we have in hand a series of films which are stable enough to study in the hydroxyl form.  This is allowing us for the first time to compare experimental transport studies of hydroxyl with simultions based on chemically accurate theoretical models of hydrated hydroxide.   One surprising outcome of this is that it is becoming apparent that many AEMs exsist in which hydroxyl transport is as facile as proton transport in stte of the art proton exchange membranes but with far less water.  Of course the use of air or hydrocarbon fuels nessecitates understanding carbonate and bicarbonate transport as well, and we are just begining to formulate studies of these important anions in AEMs.