Unconventional Dual Ion Selectivity Determined by the Forward Side of a Bipolar Channel toward Ion Flux

Ion selectivity is an essential property of ion-selective membranes (ISMs). To date, all of the artificial ISMs have been reported to exhibit sole ion selectivity (SIS), either cation or anion selectivity. Here, we first demonstrate unconventional dual ion selectivity (DIS) in a bipolar channel membrane determined by the forward side toward ion flux. When the bipolar membrane meets the conditions of opposite ion selectivities and comparable resistance for both constructive layers, no matter which layer faces the ion flux, it functions as a selective layer and determines the selectivity of the whole membrane. The exploration of the unconventional DIS property inspires us to fabricate a new generation of ISMs, as well as other membranes for separation.