Effect of phase separation and adsorbed water on power consumption and current efficiency of terpolymer conetwork-based anion exchange membrane†
We report the preparation of a new type of anion exchange membrane (AEM) from a random terpolymer of poly n-butyl acrylate (PnBA), polyacrylonitrile (PAN) and poly(2-dimethyl aminoethyl)methacrylate (PDMA) through quaternization and cross-linking. Moderate amounts of hydrophobic and soft PnBA (ca. 18 wt%) in the terpolymer considerably influenced the electrochemical properties and desalination performance of the membrane. Standardization experiments revealed that a membrane (AEM-2) prepared from a terpolymer containing 18 wt% PnBA, 50 wt% PAN and 32 wt% PDMA exhibited low power consumption (W) (0.66–0.95 kW h kg−1) and high current efficiency (CE) (94–96%) at an applied potential of 1.5–2 volts per cell pair during the desalination of water (containing 2000 mg L−1 NaCl) via electrodialysis (ED). On the other hand, membranes prepared from the bi-component PAN-co-PDMA with a similar PDMA content (28–34 wt%) exhibited a relatively higher W (0.94–1.32 kW h kg−1) and lower CE (68–80%) under similar experimental conditions compared to AEM-2. The remarkable effect of hydrophobic PnBA on AEM-2 performance is attributed to the nanophase separation of quaternized PAN-co-PDMA domains, low degree of water uptake (12%) along with the absence of freezing bound water in the membrane. Rapid desalination (low W) with high CE by five pairs of alternative AEM-2 and standard cation exchange membrane in the ED unit (13 × 5 cm2) shows promise for the development of en energy efficient ED unit.