Facile preparation of high-performance sulfonated polyimide proton exchange membrane by doping nano carbon sulfonic acid

Sulfonated polyimide (SPI) is a potential material for proton exchange membranes (PEMs) under some harsh conditions for its good mechanical properties and excellent thermal stability. However, the complicated preparation of monomers required for SPI and limited proton conductivity restrict its practical application. In this work, we report the significant improvements on proton conductivity of SPI membrane, by directly doping nano carbon sulfonic acid (NCSA) in casting process. The SPI are synthesized using commercial monomers and the NCSA is prepared by chemically bonding benzenesulfonic acid groups to nano carbon particles (NCPs). The proton conductivity of the composite membrane reaches similar to 140 mS/cm, 2.3 times higher than the base SPI membrane and similar to 50 % higher than the commercial PFSA membrane at same conditions. Atomic force microscope (AFM) reveals that the "proton pool" structure around NCSA is the main reason for the improvement of proton conductivity of SPI composite membrane. A hydrogen fuel cell with the membrane electrode assembly (MEA) using the composite membrane achieves a power density of 1.58 W/cm(2) at ambient pressure with platinum loading as low as 0.1 mg/cm(2) at anode. We envision the facile method of doping NCSA and reinforcement to SPI composite membrane can promote its scale applications.