A sulfonated polyimide containing flexible aliphatic segments with high chemical stability for vanadium redox flow battery application

To improve sulfonated polyimide (SPI) chemical stability, the SPI containing flexible aliphatic segments is successfully designed and synthesized. The highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy and their energy gaps (HOMO-LUMO gap (Delta E)) can quantitatively compare the molecular reactivity, which are calculated and used to compare firstly in the SPIs for vanadium redox flow battery (VRFB) application. Furthermore, the natural bond orbital (NBO) charge of SPIs is also used to calculate and compare, respectively. Theoretical calculation shows SPI(DAD) has higher chemical stability. Obtained by experimental results that the retention time in water of SPI(DAD)-60 membrane is 23.58 h at 80 degrees C, which is nearly four times than conventional SPI under the same conditions. The SPI(DAD)-60 membrane at a current density of 100 mA cm-2 shows higher columbic efficiency (CE: 96.83%), and energy efficiency (EE: 79.47%) than that of N212 membrane (CE: 93.14%, EE: 74.13%). Besides, the SPI(DAD)-60 membrane has 479.1 mA h more charging capacity after 100 cycles compared to N212 membrane. The theoretical calculation of the LUMO energy, HOMO-LUMO gap may be a potential tool that is used to analyze and evaluate the chemical stability of membranes for VRFB application.