Zwitterionic interface engineering enables ultrathin composite membrane for high-rate vanadium flow battery

Thin membranes with high conductivity are of great importance for high power density vanadium flow batteries (VFBs), however, the trade-off between conductivity and ion selectivity limits their further development. Herein, an ultrathin composite membrane (∼10 μm) enabled by a zwitterionic interface between conductive polybenzimidazole (PBI) and porous polyethylene (PE) substrate is proposed to break the trade-off, and greatly enhance the power density of a VFB. The zwitterionic interface is engineered by the covalent reaction between polydopamine and zwitterionic sulfonated 3-dimethylaminopropylamine (DMAPAPS). The as-fabricated composite membrane features excellent mechanical properties and high dimensional stability. Furthermore, the zwitterionic interface facilitates proton transportation while blocking vanadium-ion permeation . More importantly, the ion selectivity of the ultrathin membrane is nearly three times higher than the pure PBI membrane (i.e., 2.71 × 10 4 vs. 1.09 × 10 4  S min cm −3 ). As a result, a VFB assembled with the ultrathin membrane demonstrates a superior energy efficiency of 78.7% at 300 mA cm −2 , and achieves an excellent cycle stability over 500 cycles at 300 mA cm −2 . Therefore, the engineering of zwitterionic interface provides a promising strategy in the development of ultrathin composite membrane for high power density VFBs.