Zr-MOF-enabled controllable ion sieving and proton conductivity in flow battery membrane

Membrane with ordered channels is the key to controlling ion sieving and proton conductivity in flow batteries. However, it remains a great challenge for finely controlling the nanochannels of polymeric membranes. Herein, two types of acid-stable Zr-metal organic framework (MOF-801 and MOF-808) with variable pore structures and channel properties are introduced as fillers into a non-fluorinated sulfonated poly (ether ether ketone) (SPEEK). The membrane incorporated with MOF-801 of a smaller triangular window (≈3.5 Å) successfully translates the molecular sieving property into the flow battery membrane, resulting in enhanced coulombic efficiency (98.5-99.2%) at 40-120 mA cm${}^{-2}$ compared with the pristine SPEEK membrane (97.1-98.5%). In contrast, more protophilic internal interconnected channels of MOF-808 yield faster proton highway, leading to a significant increase of voltage efficiency (93.7-84.1%) at 40-120 mA cm${}^{-2}$ compared with the pristine SPEEK membrane (91.7-78.9%). By regulating the ion sieving and proton conductivity, MOF-801/MOF-808 binary composite membrane exhibits synchronously improved performance in the vanadium redox flow battery system. The revealed structure-property relationship in the Zr-MOFs-based membranes provides a general guideline to design new proton exchange membranes with ordered channels for flow battery application. (© 2021 Wiley-VCH GmbH)