Novel sulfonated polyimide membrane blended with flexible poly[bis(4-methylphenoxy) phosphazene] chains for all vanadium redox flow battery

Abstract To improve SPI stability, poly[bis(4-methylphenoxy) phosphazene] (PMPP) was designed and synthesized using polyphosphazene's unique and strong flexibility. A novel sulfonated polyimide (SPI) composite membrane embedded with flexible PMPP chains was prepared for all vanadium redox flow battery applications. SPI/PMPP composite membrane characterizations, such as proton conductivity, vanadium ion permeability, stability, and single cell performance, were thoroughly investigated. PMPP's unique properties not only allow the membrane to exhibit excellent anti-hydrolysis and anti-oxidation stability, but also maintain high proton conductivity and low vanadium ion permeability (as low as 4.81 × 10−7 cm2 min−1). It can also form a rigidity-flexibility network with SPI to improve membrane mechanical properties. It can be deduced that introducing PMPP has achieved many things in one stroke. Results show that the hydrolysis and oxidation stabilities of SPI/PMPP membrane were significantly improved. The SPI/2% PMPP membrane exhibits higher columbic efficiency (CE: 97.52%), voltage efficiency (VE: 84.88%), and energy efficiency (EE: 82.21%) than that of the N212 membrane (CE: 94.97%, VE: 82.04%, EE: 76.99%) at a high current density of 100 mA cm−2. In addition, the charging capacity loss of a battery with a SPI/2% PMPP membrane has also significantly decreased by about 206.70 mAh after 110 cycles at 100 mA cm−2 with the same electrolyte volume compared to a pristine SPI. Meanwhile, the high temperature resistance of SPI/PMPP membrane also shows the possibility of being applied with fuel cells.