Altering the Mechanical Properties of Scalable, Solvent-Free, Cross-Linked Anion-Exchange Membranes

Anion-exchange membranes (AEMs) are promising polymer electrolytes for electrochemical energy conversion devices such as fuel cells. In this study, AEMs were fabricated by cross-linking a polyelectrolyte, poly(acrylamide- co -diallyldimethylammonium chloride), with glutaraldehyde using a scalable, organic solvent-free approach based on a recently awarded patent. Successful cross-linking was confirmed through Fourier transform infrared spectroscopy, which showed the formation of C═N bonds in the cross-linked AEMs. The effects of cross-linker-to-polyelectrolyte ratio, polyelectrolyte concentration, drying temperature, and drying time on AEM properties were examined. Changing properties, including water uptake, dimensional swelling, ion-exchange capacity (IEC), conductivity, and mechanical strength under wet and dry conditions, were quantified. Overall, AEMs were about 30 μm in thickness, exhibited dry strength at break up to 33 MPa, and showed chloride conductivity around 100 mS/cm at 80 °C. Increases in cross-linker-to-polyelectrolyte ratio led to decreases in water uptake, swelling, IEC, and ionic conductivity. On the one hand, strength at break and Young’s modulus under dry conditions decreased with increasing cross-linking-to-polyelectrolyte ratio. On the other hand, strength at break and Young’s modulus under wet conditions increased with increasing cross-linking-to-polyelectrolyte ratio. Next, variations in polyelectrolyte concentrations before casting did not significantly change mechanical or other properties, which disproved the hypothesis that entanglements would enhance mechanical properties. Finally, increases in drying temperature and time did not significantly alter AEM properties above a minimum value.