Effect of Polymer Architecture on the Nanophase Segregation, Ionic Conductivity, and Electro-Osmotic Drag of Anion Exchange Membranes

Anion exchange membranes (AEMs) are considered an attractive alternative to proton exchange membranes in fuel cell applications because they can operate with nonprecious metal electrodes. However, widespread adoption of AEMs has been hampered by their insufficient ionic conductivity. Much of the growing body of research on AEMs focuses on designing new polymer chemistries and architectures that would increase their conductivity, while controlling the swelling of the membrane. It is, however, challenging to assess the separate effects of water content and polymer architecture on the phase segregation and molecular transport of the ions and water in the membrane because changes in the chemistry of the polymer also impact the equilibrium water uptake. Here, we use large-scale molecular simulations to study the distinct effects of water content and ionomer architecture on the nanophase segregation, anion and water diffusivity, ion conductivity, and water electro-osmotic drag coefficient of anion exchange membranes based on tetraalkylammonium-functionalized polyphenylene oxide (PPO/TMA). We find that the transport properties of the AEM are very sensitive to the water content but quite robust against changes in the architecture of the polymer electrolyte. Our analysis indicates that this insensitivity stems from the similarity in the structure of the hydrophilic domains for a given ratio of water to anions in the membrane. This is belied by the extreme differences in the structure factor of the different polymer electrolytes and indicates that the structure factor alone is not appropriate to characterize the size of the hydrophilic channels that control the transport of ions and water in the AEM. Our results suggest that future efforts to design AEMs with improved conductivity should focus on elucidating the relationship between the polymer chemistry and the equilibrium water uptake of the membrane.