Construction of hierarchical proton sieving-conductive channels in sulfated UIO-66 grafted polybenzimidazole ion conductive membrane for vanadium redox flow battery

Sieving of H+/Vn + ions is essential to vanadium redox flow battery (VRFB). Herein, the interconnected hierarchical proton sieving-conductive channels are constructed through the graft of sulfated UIO-66 (UIO–66OSO3) along the polybenimidazole (PBI) backbone with the highly reactive and acidic sulfate ester functional group. Highly porous UIO-66OSO3 provides numerous angstrom scale sieving channels (about 5 Å) to transport proton and repel vanadium ion, and the sulfated groups ended flexible side chains are densely bonded to UIO-66 and aggregate into broad nanoscale ionic clusters (∼8 nm) for fast proton conduction between UIO-66OSO3 nanoparticles. The as-prepared membrane exhibits low vanadium permeability (7.88 × 10−9 cm2s−1) and area resistance (0.23 Ω cm−2, lower than that of Nafion 212). High energy efficiency (86.1%) and low discharge capacity decay (0.15% per cycle) are achieved even after 100 charge-discharge cycles at 100 mA cm−2, and the battery keeps running for 2000 cycles without morphological and chemical changes. The VRFB proprieties are far superior to Nafion 212 membrane VRFB proprieties (energy efficiency 74.9%, decay rate 0.71% per cycle), and also surpasses most recently reported porous, dense and hybrid ion conductive membranes.