Covalent organic framework (COF) constructed proton permselective membranes for acid supporting redox flow batteries

Covalent organic framework (COF) based proton permselective membranes are for the first time designed to break through the trade-off between proton permeability and selectivity for improved efficiencies of acid supporting redox flow batteries (the representative one is all vanadium redox flow battery). The precise hydrated proton (<0.24 nm)/vanadium (>0.6 nm) sieving is achieved originating from the appropriate pore size (0.48 nm) of COF prepared here. As a result, no vanadium permeation can be detected for the membrane. While, the presences of micropores and the amino groups in COF lead to a low area resistance. The cell assembled with this membrane presents very high coulombic efficiency (CE: 98.7–99.6%) and energy efficiency (EE: 93.5–76.4%) at 40–180 mA cm−2. In addition, the membrane is free of ion exchange groups, leading to excellent chemical stability. The membrane structure shows no changes after the cell steadily runs over 500 cycles (>1300 h) at 80 mA cm−2. This work provides a new orientation to design a membrane with an ultrahigh proton permselectivity for redox flow batteries.