Proton-Blocking Anion-Exchange Membranes for Efficient Lithium Hydroxide Recovery by Bipolar Membrane Electrodialysis

In bipolar membrane electrodialysis (BPED), proton transport through the anion-exchange membrane (AEM) is a major factor that reduces overall process efficiency. In this study, we propose a composite AEM incorporating a proton-blocking layer that combines strongly basic and weakly basic functional groups on top of a strongly basic AEM, providing proton-blocking capability while minimizing degradation of membrane conductivity. The proton-blocking layer is prepared by reacting brominated poly(phenylene oxide) (BPPO) with diamines having different alkyl chain lengths, namely N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA), N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA), and N,N,N',N'-tetramethylethylenediamine (TMEDA). When TMHDA, which has the longest alkyl chain, is introduced into PPO, the resulting membrane exhibits high conductivity but low proton-blocking performance. In contrast, when TMEDA, which has the shortest alkyl chain, is introduced, the membrane shows low conductivity and high proton-blocking performance. Therefore, the balance between membrane conductivity and proton-blocking performance can be optimized by adjusting the molar ratio of the two diamines. The composite AEM prepared with the optimal composition simultaneously demonstrates superior conductivity and proton-blocking performance compared to the commercial proton-blocking membrane (ACM, Astom Corp., Tokyo, Japan). Furthermore, the application of this membrane has been shown to effectively improve both the energy efficiency and current efficiency of the BPED process for lithium hydroxide recovery.