Self-crosslinked Polyethyleneimine-polysulfone Membrane for High Temperature Proton Exchange Membrane

High temperature proton exchange membrane fuel cells (HT-PEMFC) operated at a temperature range from 120 degrees C to 200 degrees C show high reaction kinetics, high tolerance of the Pt catalyst for impurities such as carbon monoxide and simplified water and heat management. HT-PEMFC has attracted great attentions in many applications including portable devices, unmanned vehicles and fuel cell cars. One of the essential components of the HT-PEMFC is high temperature proton exchange membrane (HT-PEM). The state-of-the-art HT-PEM is phosphoric acid (PA) doped polybenzimidazole (PBI) composite membrane. Phosphoric acid acts as the proton conductor while the PBI plays as a skeleton to hold the PA molecules and provides mechanical strength for the composite membrane. Nevertheless, the complex fabrication procedures and expensive cost hinder wide application of PBI in IIT-PFMPC. Alternative polymer skeletons including polyvinylpyrrolidone and amino-functionalized proton exchange membrane have been developed for the HT-PEM. Generally, the high proton conductivity of the HT-PEMs results from high doping level of PA. However, the plasticizer effect of PA molecules reduces the Van der Waals force among the polymer macromolecules. That leads to the low mechanical strength of the HT-PEMs. Cross-linking method significantly increases the mechanical strength of the HT-PEMs. On the other hand, the cross-linking reaction consumes the PA doping site of the HT-PEMs, leading to the low proton conductivity of these HT-PEMs. In this research, a novel self-crosslinked polyethyleneimine-polysulfone (PEI-PSF) HT-PEM with both high mechanical strength and high proton conductivity has been designed. The PEI molecules are anchored to the PSF backbones by chloromethylation and tertiary aminating reactions. That is prone to enhance the mechanical strength of the membrane. In addition, the PEI also acts as PA adsorption sites, which improves the PA doping level and proton conductivity of the HT-PEM. The degree of crosslinking is controlled by the degree of chloromethylation. The H-1 nuclear magnetic resonance characterization shows successfully graft of benzyl chloride onto the PSF backbone to form chloromethylated polysulfone (CMPSF). In addition, the X-ray photoelectron spectra confirm the reaction of PEI with CMPSF to form a self-crosslinked PEI-PSF membrane. With the increase of crosslinking degree, the PA doping level of the PEI-PSF membrane increases whereas its tensile strength decreases. A proton conductivity of 3.4 X10(-2) S.cm(-1) is obtained for a PEI-PSF membrane with a chloromethylation degree of 58%, denoted as PEI-PSF-58, and PA doping level of 122 wt% at 150 degrees C under anhydrous conditions. Meanwhile, the PEI-PSF-58 membrane remains excellent mechanical property with tensile strength of 30 MPa at room temperature. Moreover, HT-PEMFC based on the PEI-PSF-58 membrane exhibits a high peak power density of 200 mW.cm(-2) and outstanding stability under 150 C with a constant cell voltage of 0.4 V. In summary, a series of self-crosslinlced PEI-PSF HT-PEMs with both high proton conductivity and excellent mechanical properties have been synthesized. The self-crosslinking is a promising strategy to cope with trade-off between high proton conductivity and mechanical strength for the conventional PA doped HT-PEMs.