Sulfonated Polyimide Acid-Base Blend Membranes for Vanadium Redox Flow Cell Application

A series of sulfonated polyimides (SPI) which had different degrees of sulfonation were synthesized by high-temperature one-step method with 2, 2'-benzidinedisulfonic (BDSA), oxydiphthalic dianhydride (ODPA) and 4, 4'-diaminodiphenyl methane (MDA) monomer. The molar ratio of BDSA and MDA was controlled to get the different SPIs. The SPIs were characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance analysis and thermogravimetry. The results show that the sulfonated polyimides were successfully synthesized and have good thermal stability with three-step degradation patterns. Then, a series of sulfonated polyimide/polyvinylpyrrolidone (SPI/PVP) acid-base blend membranes were prepared at the mass ratio of 9: 1 and applied in vanadium redox flow battery (VRB). SEM images of the membranes show that the phase separation of the blend membrane is more obvious with the increase of the degree of sulfonation. The parameters of the blend membranes were obtained and reported, such as water uptake, ion exchange capacity (IEC), area resistance and vanadium ion permeability. The blend membranes have lower water uptake and vanadium ion permeability than those of Nafion117. The IEC, water uptake, proton conductivity and the vanadium ion permeability of SPI/PVP acid-base blend membranes increased with increasing the degrees of sulfonation. The selectivity of the membranes is much better than that of Nafion117, Especially SPI/PVP-3, its selectivity is 10 times higher than that of Nafion117 membrane. In the charge-discharge tests the VRB single cell of the SPI/PVP blend membrane shows higher columbic efficiency (CE) and similar energy efficiency (EE) compared with that of Nafion117. And the VE and EE were increased with the increasing of degree of sulfonation, as CE on the contrary. CE and EE of the SPI/PVP-3 membrane are higher than those of Nafion117 membrane. The performance of SPI/PVP-3 in the charge-discharge test of VRB single cell showed good stablilty. All these results demonstrate that the SPI/PVP acid-base blend membrane is a new promising material for VRB application.