Advanced hybrid polybenzimidazole membrane enabled by a “linker” of metal-organic framework for high-performance vanadium flow battery

High current density vanadium flow batteries (VFBs) are important for the wide range of applications of renewable energy. The balance between proton conductivity and vanadium permeability of proton exchange membrane has been a huge challenge for the development of high-power density VFBs. Herein, a post-synthetic modification of Zr-MOFs (PSM) is introduced into PBI matrix to break the trade-off between vanadium permeability and ion selectivity, and remarkably improves the power density of VFBs. As a "linker" between the PBI backbone, the PSM creates more proton transport sites in the hybrid membrane, which significantly increases the proton conductivity. Meanwhile, the "acid-base" pairs are formed between PSM and PBI matrix, further promoting the proton transfer in the hybrid membrane. More importantly, the denser structure of hybrid membrane blocks the vanadium ion permeation based on the Donnan exclusion and size sieving effect, respectively. In consequence, the ion selectivity of the PBI/PSM hybrid membrane is substantially improved, almost four times that of the pure PBI membrane. Benefiting from such rational design, the VFB equipped with the PBI/PSM hybrid membrane exhibits an excellent energy efficiency of 75.4% at 500 mA cm−2, and at 300 mA cm−2, the energy efficiency can reach 81.2%, and the efficiency stability can maintain 600 cycles in charge and discharge. Consequently, the use of "linker" offers a very prospective strategy for the development of high-performance proton exchange membrane.