Anion-exchange membrane with ion-nanochannels to beat trade-off between membrane conductivity and acid blocking performance for waste acid reclamation

The bottleneck for the reclamation of low concentration waste acid by electrodialysis (ED) is the proton leakage through anion exchange membranes (AEMs). In order to reject acid without compromising conductivity, we have designed and prepared a series of weak base AEMs with ion-nanochannels through introducing weak base groups. Various weak base groups of different carbon chain lengths and grafting densities were linked onto the Polysulfone skeleton as side chains. Fourier transform infrared spectra provided proof of the successful synthesis of chloromethylated polysulfone (CMPS) and the corresponding AEMs with designated chemical compositions. Thermo-gravimetric analysis showed their thermal stabilities were high enough for ED applications. AFM phase images and a two-phase model confirmed the formation of a microphase-separated structure and the corresponding ion-nanochannels because of the polarity difference between hydrophilic side chains and hydrophobic skeletons. Subsequently, the effects of side chain lengths and ion exchange capacity on the properties of prepared AEMs, including conductivity, water uptake, permselectivity and acid blocking performance, were explored in detail. Lastly, the acid concentration behavior and electrodialytic transport properties of a typical AEM sample were investigated and compared to those of commercial AEMs. Experimental results have shown that the as-prepared AEM exhibited an outstanding performance in membrane conductivity, surface homogeneity, and the rejection of acid. Accordingly, it is reasonable to believe that the confinement effect in the formed ion-nanochannels simultaneously contributes to the reduction of membrane water uptake, the enhancement of the Donnan exclusion for protons, and the facilitated transport for counter-ions. Therefore, the synthesized membranes are promising for the applications in waste acid reclamation by ED.