A Novel Sulfonated Polyimide Composite Membrane Containing a Sulfonated Porous Material for All-Vanadium Redox Flow Batteries

To improve the battery efficiency and cycling stability of sulfonated polyimide (SPI), a polyphosphazene with built-in -SO₃H moieties (PP-SO₃H), 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-SO₃H composite membrane has an excellent conductivity (up to 114.8 mS cm^-1); the ion selectivity of the SPI/2% PP-SO₃H membrane is 11.69 × 10⁴ S min cm^-3, which is 2.18 times higher than that of the SPI base membrane. PP-SO₃H also improves the SPI membrane's mechanical strength, and the effect of PP-SO₃H on SPI intermolecular interactions is analyzed by surface electrostatic potential (ESP) theoretical calculations. The Coulombic efficiency (CE) of the SPI/2% PP-SO₃H 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-SO₃H membrane is 3.5 times compared with the SPI base membrane. To measure the cycling stability of the composite membrane, the SPI/2% PP-SO₃H 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-SO₃H composite membranes are viable for VRFB applications.