Synthesis And Characterization Of Cross-linked Proton Exchange Membranes For Direct Methanol Fuel Cell Applications

The work mechanism of proton exchange membrane fuel cell (PEMFC) is very simple which could convert hydrogen, oxygen, methanol etc to electrical energy directly without Carnot cycle. The perfluorosulfonic acid membranes, such as Nafion, are most widely used as PEM in low temperature fuel cells due to its excellent chemical stability, good mechanical properties and high proton conductivity. However, the high cost and methanol permeability restrict their applications in PEMFC. Therefore, many researchers are always trying to develop kinds of non-fluorinated materials with low cost and high performance, to replace perfluorosulfonic acid membranes. In this paper, we tried to synthesize POSS convalently cross-linked sulfonated polyimides and ionic cross-linked sulfonated poly(ether ether ketone)s to replace the traditional proton exchange membranes. The specific works are shown below:1. Novel cross-linked sulfonated copolyimides containing oligomer silsesquioxanes (POSS) moieties are synthesized by direct polymerization for application in proton exchange membrane fuel cells (PEMFCs). The synthesized copolymers are controlled by various monomers compositions to observe membranes properties with different amount of octa(aminophenyl)silsesquioxane (OAPS). The mechanical and thermal properties are enhanced in POSS cross-linked membranes compared with the non cross-linked membrane. What makes the new is that the water uptake and proton conductivity of POSS cross-linked membranes increase relatively with a suitable cross-linking degree. Meanwhile, the exist of POSS could magnify the hydrophobic volume and thus promote the formation of ionic exchange channel. When the OAPS amount is 0.4 mol%, the membrane proton conductivity could reach 0.111 S cm-1, which is better than Nafion 117.2. Sulfonated polyether ether ketone (SPEEK) membranes groups have been prepared for direct methanol fuel cells (DMFCs). In order to improve dimensional stability and decrease methanol permeability of membranes, the diazafluorene group was introduced to form an ionic cross-linking structure. The ionic cross-linking was realized by interaction between sulfonic acid groups and pyridine ring in diazafluorene. Except for the good mechanical properties, great thermal stability, good oxidative stability, appropriate water uptake and low swelling ratio, the ionic cross-linked membranes still showed lower methanol permeability in the range of 0.56×10-7 cm2 s·1-1.8×10-7 cm2 s-1 which was far lower than Nafion 117(18.6×10-7 cm2 s-1) and higher selectivity in the range of 2.46x 104 Ss cm3-4.82×104 Ss cm-3 which was higher than Nafion 117(1.34x 104 Ss cm-3) at 30 ℃ on the basis of applicable proton conductivity.