Fast and selective ion transport in ultrahigh-charge-density membranes

Ion-selective membranes are central to electrochemical technologies due to their ability to regulate ion transport and differentiate between cations and anions. However, a major obstacle to their effective implementation is the inherent trade-off between ionic conductivity and cation/anion selectivity, a consequence of the interdependence between membrane charge and water content. Here we introduce a membrane design strategy that not only achieves high charge densities but also nearly decouples charge from water content. Our strategy involves the copolymerization of low-molecular-weight charged monomers and charged cross-linkers, ensuring that every repeat unit of the polymer backbone contains a charged group. Anion-exchange membranes synthesized using this strategy exhibit ultrahigh charge densities, substantially advancing the conductivity/selectivity upper bound. We further demonstrate the practical implications of these ultrahigh-charge-density membranes for electrodialytic brine concentration, achieving a lower specific energy consumption than the state-of-the-art benchmark. This advancement in membrane design can impact the development and deployment of electrochemical systems across a spectrum of energy and environmental applications. This study reports positively charged membranes with ultrahigh charge densities and tunable water content. These membranes exhibit enhanced ionic conductivity and counter-ion/co-ion selectivity compared with commercially available alternatives, enabling energy-efficient brine concentration via electrodialysis.