A Novel Sulfonated Polyimide Composite Membrane Containing a Sulfonated Porous Material for All-Vanadium Redox Flow Batteries
Xuesong Li, Gang Wang, Shuwen Zhang, Shiguo Wei, Yan Yu, Bing Wang, Yangtian Jing, Ji‐Jun Chen, Jie Zhang, Yufeng Zhou, Jinwei Chen, Ruilin Wang
Abstract
To improve the battery efficiency and cycling stability of sulfonated polyimide (SPI), a polyphosphazene with built-in −SO 3 H moieties (PP-SO 3 H), which is a porous covalent organic framework (COF) material, is facilely synthesized by the polymeric combination of hexachlorocyclotriphosphazene (HCCP) and p -diaminobenzenesulfonic acid. Due to its tunable pore size and flexible molecular design, the COF material can address the trade-off between the conductivity and the ion permeability of ion exchange membranes well, thereby improving the ion selectivity of membranes. The experimental results show that the SPI/PP-SO 3 H composite membrane has an excellent conductivity (up to 114.8 mS cm –1 ); the ion selectivity of the SPI/2% PP-SO 3 H membrane is 11.69 × 10 4 S min cm –3, which is 2.18 times higher than that of the SPI base membrane. PP-SO 3 H also improves the SPI membrane’s mechanical strength, and the effect of PP-SO 3 H on SPI intermolecular interactions is analyzed by surface electrostatic potential (ESP) theoretical calculations. The Coulombic efficiency (CE) of the SPI/2% PP-SO 3 H membrane is 98.92%, the energy efficiency (EE) is 84.1% at a current density of 100 mA cm –2, and the self-discharge time of the SPI/2% PP-SO 3 H membrane is 3.5 times compared with the SPI base membrane. To measure the cycling stability of the composite membrane, the SPI/2% PP-SO 3 H membrane is cycled in the VRFB for more than 400 cycles, which is more stable than that of the SPI base membrane. These results show that SPI/2% PP-SO 3 H composite membranes are viable for VRFB applications.