(606a) Understanding Ion Diffusion in Charged Polymer Membranes

Ion-exchange membranes (IEMs) are key components of electrochemical technologies for water desalination, energy generation, and energy storage due to their ability to regulate ion transport. Conventional applications of IEMs, such as water desalination via electrodialysis, require membranes that can effectively separate oppositely charged counter-ions from similarly charged co-ions. There is growing interest in expanding the utility of IEMs to applications that require the separation of different counter-ions. For example, IEMs that exhibit selectivity for lithium ions over other cations could enable recovery of this valuable resource from brines and industrial wastewater. Likewise, valence-selective IEMs that could separate divalent cations such as calcium and magnesium from monovalent cations would be beneficial for applications such as water softening and prevention of scale formation in desalination. These applications require membranes with the ability to differentiate between ions of similar charge. However, gaps in the fundamental understanding of ion transport in charged polymer membranes are a barrier to the development of membranes tailored for these specific needs.

In this presentation, I will discuss our latest progress in improving the fundamental understanding of ion diffusion in IEMs. To this end, we synthesized homogeneous cation- and anion-exchange membranes with controlled levels of water and charge content, providing ideal models for examining ion transport. We measured the diffusion coefficients of a variety of different ions in these membranes and interpreted the results within the framework of transition state theory. The ion activation energies of diffusion were analyzed and correlated with intrinsic membrane and ionic properties, like the ionic size, ionic hydration energy, bond vibration energies (probed via Fourier transformation infrared spectroscopy), states of water in the membrane (probed via differential scanning calorimetry), water contents, and charge densities. To complement our experimental work, we also conducted molecular dynamics simulations of ion diffusion in polymer membranes having similar compositions to those investigated experimentally. These simulations offered additional insights into the mechanisms of ion diffusion within the membranes, enhancing our overall understanding of this phenomenon.