Hydroxide Transport in Next Generation Anion Exchange Membranes

The potential of anion exchange membrane (AEM) fuel cells/electrolyzers and other devices 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. AEM 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 though, is water transport; water is both a product and a reactant in these systems, and wet cations are much more stable than dry. So an understanding of water in these membranes is essential.

Recenetly we have fabricated a  series of membrnes with sufficent stability that we are able to measure the hydroxide conductivity in them at elevted temperatures while being humidified.  Here we discuss water and hydroxide transport in a series of thin mechanically robust state of the art experimental AEMs.  The membranes are generally constructed from an isoprene or ethylene block and a vinyl benzyl bromide block, either randomly or in di-, tri- or penta- block configurations.  Post quaternization 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.