Aliphatic/aromatic sulfonated polyimide membranes with cross-linked structures for vanadium flow batteries

The cross-linked sulfonated polyimide (CSPI-DMDA) membranes with microphase-separated structure are designed and prepared for vanadium flow battery (VFB) application. In main chains of CSPI-DMDA membranes, hydrophobic segment with aliphatic groups as well as hydrophilic segment with aromatic groups promote the formation of microphase-separated structures and thus enhance the ability of proton transfer. Meanwhile, the cross-linked structure in CSPI-DMDA membrane is beneficial to improve the mechanical strength and chemical stability, and decrease the vanadium-ion permeability. The optimized CSPI-DMDA membrane exhibits higher columbic efficiency (CE: 99.2%), voltage efficiency (VE: 76.0%) and energy efficiency (EE: 75.3%) than those of N115 membrane (CE: 96.9%, VE: 73.3%, EE: 71.1%) at a high current density of 200 mA cm(-2). There is no obvious decay in the cycling of the VFB with the CSPI-DMDA (1:1) membrane over 1000 charge-discharge test under 160 mA cm(-2). The VFB also remains stable on a broad temperature (-20 to 60 degrees C). The above results indicate CSPI-DMDA membrane a potential alternative for separator in all-climate VFB applications.