Abstract
The understanding gleaned from studying ion transport within the interaction confinement regime enables the near-frictionless transport of cations (e.g., Na+/K+). However, anion transport (e.g., Cl-) is suppressed under confinement because of the different polarization of water molecules around cations and anions, also known as the charge asymmetry effect. Here we report the rational synthesis of anion-selective framework polymer membranes having similar densities of subnanometer-sized pores with nearly identical micropore size distributions, which overcome the charge asymmetry effect and promote barrierless anion conduction. We find that anion transport within the micropore free volume elements can be dramatically accelerated by regulating the pore chemistry, which lowers the energy barrier for anion transport, leading to an almost twofold increase in Cl- conductivity and barrierless F- diffusion. The resultant membrane enables an aqueous organic redox flow battery that utilizes Cl- ions as charge carriers to operate at extreme current densities and delivers competitive performance to counterparts where K+ ions are charge carriers. These results may benefit broadly electrochemical devices and inspire single-species selectivity with separation membranes that exploit controlled or chemically gated ion/molecule transport.