Synthesis And Characterization Of Proton Exchange Membranes Based On Sulfonated Poly(Ether Sulfone Ether Ketone Ketone) And Their Nanocomposites

Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), are receiving considerable attention as transport, stationary, and portable future power sources because of their low emissions and high energy conversion efficiency. The proton exchange membrane (PEM), is the key component of the PEMFCs. Nafion, one of the perfluorosulfonic acid membranes, is the current state-of-the-art PEM material. However, several drawbacks of Nafion, such as high cost, high methanol permeability, low humidity and a major reduction in conductivity at high temperatures, have led researchers to investigate promising alternatives.Sulfonated poly(aryl ether ketone)s (SPAEKs), are receiving considerable attention as a kind of promising PEM for their excellent high thermal stability, mechanical properties, oxidation resistance and high proton conductivity. Owing to their unique mechanical properties and excellent high chemical stability, carbon nanotubes (CNTs) are considered to be ideal candidates for polymer reinforcement. Sulfonic acid group functionalized CNTs (sCNTs) are believed to be homogeneously dispersed within the SPAEKs matrix. It is easy to afford sCNTs/SPAEKs composite membranes.by a solution casting method. The composite membranes are expected to exhibit enhanced mechanical properties, improvements in proton conductivity and reduced methanol permeability compared with the neat SPAEKs membrane.The poly(ether sulfone ether ketone ketone) (PESEKK) was synthesized by the low temperature solution polycondensation of 4,4-bis(Phenoxy)diphenyl sulfone (DPODPS) and isophthaloyl chloride (IPC) in 1,2-dichloroethane and in the presence of aluminum chloride (AICl3) and N-methylpyrrolidone (NMP). A series of sulfonated poly(ether sulfone ether ketone ketone) (SPESEKK) with different degree of sulfonation (DS) are prepared by the postsulfonation of PESEKK using chlorosulfonic acid as sulfonating agent and concentrated sulfuric acid as solvent. The chemical structures of the polymers are analyzed by the 1H NMR. The thermal properties of the SPESEKK are studied, showing that they are greatly influenced by the DS value and sulfonation time. The water uptake, proton conductivity and IEC values increase as the sulfonation time increasing. The methanol permeability of the SPESEKK in the range of 7.02×10-8 to 4.48×10-7 cm2·s-1, is one or two orders of magnitude lower than that of Nafion 115. The morphology of the SPESEKK membranes is investigated by scanning electron microscope.A series of composite membranes consisting of sulfonated carbon nanotubes (sCNTs) and SPESEKK (DS=0.58) were successfully fabricated via the solution casting method. The chemical structure, as well as the long-term stability of the sCNTs in different solvents was investigated by FTIR analysis and solubility experiment, respectively. The morphology, tensile strength, proton conductivity and methanol permeability of the composite membranes were also investigated. The SEM observation indicated the good dispersion of the carbon nanotubes in polymer matrix, as well as the strong interfacial bonding between the SPESEKK matrix and sCNTs. The addition of either pristine carbon nanotubes or modified carbon nanotubes significantly enhanced the tensile strength of the SPESEKK membrane. The proton conductivity of the SPESEKK membrane increased while the methanol permeability decreased as the sCNTs content increasing, showing a strong interaction between the modified nanotubes and SPESEKK.