Reconstructing proton channels via Zr-MOFs realizes highly ion-selective and proton-conductive SPEEK-based hybrid membrane for vanadium flow battery

There is an urgent need to break through the trade-off between proton conductivity and ion selectivity of proton exchange membrane (PEM) in vanadium flow battery (VFB). Proton channels in PEM are the key to controlling the ion sieving and proton conductivity in VFB. Herein, two types of proton channels are reconstructed in the hybrid membrane via introducing modified Zr-MOFs (IM-UIO-66-AS) into SPEEK matrix. Internal proton channels in IM-UIO-66-AS and interfacial proton channels between grafted imidazole groups on Zr-MOFs and SPEEK greatly improve the conductivity of the IM-UIO-66-AS/SPEEK hybrid membrane. More importantly, both reconstructed proton channels block the vanadium-ion permeation to realize enhanced ion selectivity according to the size sieving and Donnan exclusion effects, respectively. Moreover, the hybrid membrane exhibits good mechanical property and dimensional stability. Benefiting from such rational design, a VFB loading with the optimized membrane exhibits enhanced voltage efficiency of 79.9% and outstanding energy efficiency of 79.6% at 200 mA cm−2, and keeps a notable cycle stability for 300 cycles in the long-term cycling test. Therefore, this study provides inspiration for preparing next-generation PEMs with high ion selectivity and proton conductivity for VFB application.