1 research outputs found
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 bisÂ(terpyridine)ÂrutheniumÂ(II)
complexes as AEMs with chloride, bicarbonate, and hydroxide mobile
ions. The maximum conductivity for the RuÂ(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<sup>–1</sup> for the 1.6 mequiv
g<sup>–1</sup> IEC membrane in chloride form, with a hydration
number of 51, and 6.5 mS cm<sup>–1</sup> for the 1.8 mequiv
g<sup>–1</sup> 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 (<i>D</i>) to be related to the
dilute solution ion diffusivity (<i>D</i><sub>0</sub>) through
the ratio <i>D</i>/<i>D</i><sub>0</sub>. Although
membranes with a 1:1 cross-linker:monomer ratio had the lowest barrier
to transport, indicated by their high <i>D</i>/<i>D</i><sub>0</sub> 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 RuÂ(II)-complex-based
AEMs were compared to BTMA-based AEMs by considering the size and
charge distribution of the cationic center