Water uptake and ion mobility in cross-linked bis(terpyridine)ruthenium- based anion exchange membranes

As an alternative to benzyltrimethylammonium (BTMA)-functionalized polymers for use as anion exchange membranes (AEMs), we report here on the properties of cross-linked polymers containing tethered bis(terpyridine)ruthenium(II) complexes as AEMs with chloride, bicarbonate, and hydroxide mobile ions. The maximum conductivity for the Ru(II)-complex-based membranes, measured at 30 C in liquid water, depended on the water uptake and degree of cross-linking more than on the ion exchange capacity (IEC). For membranes with 2:1 cross-linker:monomer ratio, the highest conductivities were 7.9 mS cm ^-1 for the 1.6 mequiv g^-1 IEC membrane in chloride form, with a hydration number of 51, and 6.5 mS cm^-1 for the 1.8 mequiv g^-1 IEC membrane in bicarbonate form, with a hydration number of 124. Additionally, we calculated the chloride and bicarbonate ion diffusion coefficients from conductivity measurements and the samples' hydrated ion concentration, which enabled the membrane ion diffusion coefficients (D) to be related to the dilute solution ion diffusivity (D₀) through the ratio D/D₀. Although membranes with a 1:1 cross-linker:monomer ratio had the lowest barrier to transport, indicated by their high D/D₀ ratio, membranes with a 2:1 cross-linker:monomer ratio demonstrated the highest conductivity due to their balanced water uptake and ion concentration in the hydrated state. At hydration numbers greater than 20, the diffusion coefficient of the mobile chloride or bicarbonate ions was within an order of magnitude of the dilute solution limit. Finally, the properties of the Ru(II)-complex-based AEMs were compared to BTMA-based AEMs by considering the size and charge distribution of the cationic center.