Syntyesis And Property Investigation Of Sulfonic Acid Containing Polybenzoxazine For Proton Exchange Membrane

Fuel cell is a clean and efficient energy source for an environmentally responsible planet. Proton exchange membranes (PEMs) are key components of polymer fuel cells because they can provide an ionic pathway for proton transfer while prevent the mixing of reactant gases. Therefore, the property of a proton exchange membrane plays a very important role in the fuel cell performance. The most widely used PEM is Nafion membrane commercially available from DuPont. Nafion is the state-of-the-art material due to its high proton conductivity, good chemical and thermal stability. However, further application of Nafion is hinderedby high methanol crossover rate for direct methanol fuel cells (DMFCs), the strong temperature dependence of its proton conductivity, and the high cost. Alternative proton conducting materials, such as sufonic acid containning polyimides, polybenzimidazoles, and poly (arylene ether sulfone), exhibit excellent thermal stability, but the dimensional stability, mechanical strength, and methanol selectivity was poor with high degree of sulfonation, which also reduces the potential application of these sulfonated aromatic polymers. Crosslinked sulfonated aromatic polymers can effectively improve the overall performance of proton exchange membrane, particularly for the inhibition the excessive welling and reducing the methanol crossover behavior of PEMs. Thus, this paper firstly uses polybenzoxazine with high heat and acid resistance, and low cost propertis as the host network of the PEM. The sulfonic acid groups were intruduced to the benzine ring of polybenzoxazine by simple molecular design. The synthesis and property of sulfonatedPart one:Main chain-type sulfonic acid-containing polybenzoxazine based on sodium4-hydroxybenzenesulfonateThe sulfonic acid-containing benzoxazine monomer, abbreviated as SHS-ddm, was synthesized via Mannich reaction and acidizing reaction using sodium4-hydroxybenzenesulfonate,4,4’-diaminodiphenylmethane, and paraformaldehyde as raw materials. The crosslinked sulfonic acid-containing polybenzoxazine [poly (SHS-ddm)] membrane was prepared through high temperature solution casting of the monomer. Our results showed that poly(SHS-ddm) membrane exhibited a high proton conductivity and a very low methanol permeability of5.8×10-2cm2s-1, which is especially suited for direct methanol fuel cells. Employing this membrane in the membrane-electrode assembly (MEA), high methanol concentration up to7M was used, which is the highest one as reported for the pure PEM.Part two:Main chain-type sulfonic acid-containing polybenzoxazine based on cardanol.Based on the study of part one, we aim to improve the toughness of sulfonic acid-containing polybenzoxazine membrane by using a flexible aliphatic chain cardanol as the phenol source. The long aliphatic chain in each repeat unit of polybenzoxazine structure provide the plasticizing effect on the crosslinked macromolecules, which increased the flexibility of solfonic acid-containning polybenzoxazine membrane while ensuring its proton conductivity and low methanol permeability.Part three:Side chain-type sulfonic acid-containing polybenzoxazine based on hordenine.The hordenine-based benzoxazine monomer was synthesized via Mannich reaction and acidizing reaction using hordenine,4,4’-diaminodiphenylmethane, and paraformaldehyde as raw materials. Then, the sulfonic acid group was introduced via the reaction of1,3-propane sultone and hordenine-based benzoxazine monomer. A new selective ion channels was constructed in the homopolymer by the existence of zwitterionic groups. The physical interaction between the sulfonic acid and quaternary ammonium group can increase the macromolecule interaction so as to better adjust the balance of the membrane parameters. Properties of conductivity under low humidity, the dimensional stability and the methanol permeability will be improved simultaneously.In summary, as a novel non-fluorinated sulfonated polyelectrolyte, sulfonic acid-containning polybenzoxazine has the potential in the proton exchange membrane fuel cells.