Novel Anti-Oxidative Membranes Based On Sulfide-Containing Polybenzimidazole For High Temperature Proton Exchange Membrane Fuel Cells

The anti-oxidative capacity of proton exchange membrane is crucial to its lifetime. As a fully aromatic heterocyclic polymer used as a promising membrane material, polybenzimidazole contains multiple reactive sites, which are electron rich and easy to be attacked by oxidants. Up to now, a large number of measures has been carried out to overcome the shortcomings. Among the various reports, sulfide-containing compounds are widely used for their antioxidant activity, especially in the field of cell biology, pharmacology and organic chemistry; however, the introduction of sulfide units to enhance the anti-oxidative capacity of polybenzimidazole has not been studied. Therefore, the application of sulfide in modifying the rigid matrix of polybenzimidazole might be a promising trial.In this study, three sulfide-containing monomers were synthesized via facile routes, and were added as anti-oxidative additives in microwave-assisted tetracopolymerizations with 3,3’,4,4’-tetraaminobiphenyl,2,6-pyridinedicarboxylic acid, hexanedioic acid to obtained sulfide-containing copolymers. The membranes from the synthesized copolymers were prepared by solution casting, and were further characterized about the corresponding fuel cell properties including phosphoric acid (PA) doping level, swelling ratio, thermal gravity analysis, and proton conductivity. Some beneficial results are as follows.(1) In this study, sulfide-containing diester 3,3’-((thiobis(4,1-phenylene)) bis(sulfanediyl)) dipropionate (DTMA for short), of which the structure was rigid and aromatic, has been synthesized through facile Michael addition and acts as a promising monomer in the PBI polycondensation. The oxidative stability of the membranes was studied by enhanced Fenton test. The experimental results show that PBI-DTMA-5% membrane suffers less than 3% weight loss after 24 h treatment with enhanced Fenton reagent. Typically, after treated in enhanced Fenton reagent, the oxidized sulfide-containing membranes still retain considerable thermal stability and mechanical strength.(2) In order to better understand the influence brought by sulfide structure, sulfide-containing diester methyl 3-((2-methoxy-2-oxo-ethyl)thio) propanoate (MTMA for short), of which the structure is similar to hexanedioic acid, has been synthesized through facile Michael addition and acts as a promising monomer in the PBI polycondensation. At 160℃, the proton conductivity of PBI-MTMA-7% can reach up to 29.25 mS·cm-1. Additional tests show that sulfide-containing polybenzimidazoles are more likely to dissolve in dipole solvent (even non-proton presented) than pristin ones, which is a great improvement.(3) Phenothiazine-based polybenzimidazoles were prepared to further study about the influences of vertical thioether structure in the matrix of copolymer. These vertical-structure-containing copolymers exhibit much better solubility, and the corresponding membranes show improved mechanical properties, thermal stabilities, proton conductivities than that of unmodified polybenzimidazoles. At 160℃, the proton conductivity of PBI-PTZA-5% can reach up to 38.63 mS·cm-1, which is qualified for PEMs.In this research, the sulfide-containing additives were synthesized and used in a facile way, with no more obvious defects concerned. This study provides new insights for novel membrane design as high temperature PEMs.