(535a) Some Features of Ion Exchange Membranes in Complex Environments for Electrochemical Conversions

Ion exchange membranes are frequently used in redox flow batteries and electrochemical flow cells in general. However, there are few general guidelines on how to select membranes. Uptake and transport of ions and the factors underlying them are critical determinants of the behavior of conducting membranes. Substantial work was carried out on issues such as partitioning of ions into membranes in the early studies of Nafion. The heavy focus on fuel cell applications narrowed the scope of many studies to an emphasis on proton conduction. However, with the increased emphasis on redox flow batteries and other, similar electrochemical devices, there is a need to build a stronger base of studies of ions other than protons or hydroxide in various membrane situations. Furthermore, the operating conditions for practial applications typically entails exposure of membranes to concentrated electrolytes.

We will discuss recent studies of the uptake, conduction and transport of ions in various membranes that are exposed to both aqueous and non-aqueous environments. In this presentation, we will describe the separation of factors determining permeation through ion exchange membranes and the molecular basis of these parameters based on our recent work. Several highlights of the work include:

• Comparisons of the measured uptake and transport, in many different membrane types and with different treatment protocols, for materials exposed to typical VRB electrolytes.
• Interpretation of those data in the context of Donnan potentials, with implications for use of the Donnan potential itself as a descriptor.
• Studies of the conductivity and other transport (e.g. NMR diffusion) in membranes ion exchanged with different cations (aqueous) to probe the Nernst-Einstein equation and its interpretation in light of various concentration references.
• Studies of conductivity and other transport (e.g. NMR diffusion) in membranes ion exchanged with different cations in non-aqueous electrolytes, showing some surprising results for conductivity as a function of cation type. This is further investigated in detail using various spectroscopic methods, culminating in a description of a simple membrane modification that will lead to decreased permeation while maintaining conductivity.

By comparing these behaviors across different membrane and electrolyte types, we will begin to systematize our understanding of the interactions leading to observed phenomena. In addition, we will outline our recent work in developing a description of the factors influencing uptake of ions into membranes.

Acknowledgements

We gratefully acknowledge the current support of this work by the Office of Naval Research and the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk).