A sulfonated diamine monomer, 4,4‘-diaminodiphenyl ether-2,2‘-disulfonic acid (ODADS), was successfully synthesized by direct sulfonation of a commercially available diamine, 4,4‘-diaminodiphenyl ether (ODA), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), ODADS, and common nonsulfonated diamines. The resulting sulfonated polyimides displayed much better stability toward water than those derived from the widely used sulfonated diamine 2,2‘-benzidinedisulfonic acid (BDSA). This is because ODADS-based polyimide membranes have a more flexible structure than the corresponding BDSA-based ones. Fenton's reagent test revealed that ODADS-based polyimide membranes also had fair good stability to oxidation. Polyimide membranes with good water stability as well as high proton conductivity were developed. NTDA−ODADS/BAPB(1/1) copolyimide membrane (BAPB refers to 4,4‘-bis(4-aminophenoxy)biphenyl)), for example, did not lose mechanical properties after being soaked in water at 80 °C for 200 h, while its proton conductivity was still at a high level (comparable to that of Nafion 117).

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A new sulfonated diamine monomer, 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid (BAPFDS), was synthesized by direct sulfonation of the parent diamine, 9,9-bis(4-aminophenyl)fluorene (BAPF), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides with different sulfonation degrees were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPFDS, and common nonsulfonated diamines. The resulting sulfonated polyimides generally showed good solubility in m-cresol and DMSO. Proton conductivities of these polyimide membranes were measured as the functions of relative humidity and temperature. The resulting homopolyimide, NTDA-BAPFDS, displayed proton conductivities quite similar to those of Nafion 117 in the whole humidity range (RH < 100%). At 100% relative humidity, all the BAPFDS-based polyimide membranes showed proton conductivities similar to or higher than those of Nafion 117. In addition, BAPFDS-based polyimide membranes exhibited much better water stability than those derived from a widely used sulfonated diamine, 2,2′-benzidinedisulfonic acid (BDSA), with similar IEC. This is probably because of the higher basicity of BAPFDS, which is favorable for maintaining the stability of imido rings.

Permeability and solubility coefficients for H2, CO2, O2, CO, N2, and CH4 in polyimides prepared from 6FDA and methyl‐substituted phenylenediamines were measured to investigate effects of the substituents on gas permeability and permselectivity. The methyl substituents restrict internal rotation around the bonds between the phenyl rings and the imide rings. The rigidity and nonplanar structure of the polymer chain, and the bulkiness of methyl groups make chain packing inefficient, resulting in increases in both diffusion and solubility coefficients of the gases. Polyimides from tetramethyl‐p‐phenylenediamine and trimethyl‐m‐phenylenediamine display very high permeability coefficients and very low permselectivity due to very high diffusion coefficients and very low diffusivity selectivity, as compared with the other polyimides having a similar fraction of free space. This suggests that these polyimides have high fractions of large‐size free spaces.

Zeolite NaA membranes were prepared reproducibly by a one-time-only hydrothermal synthesis with a short reaction time of 3 h at 373 K using a gel with the composition Al2O3:SiO2:Na2O:H2O = 1:2:2:120 (in moles) and porous α-alumina support tubes seeded with zeolite NaA crystals. A dense intergrown zeolite crystal layer of about 30 μm in thickness was formed on the outer surface. The zeolite NaA membranes were highly permeable to water vapor but impermeable to every gas unless dried completely. The completely dried membranes displayed gas permeation behavior attributed to Knudsen diffusion, indicating the presence of interstitial spaces between the zeolite crystal particles, or nonzeolitic pores. The membranes displayed excellent water-permselective performance in pervaporation (PV) and vapor permeation (VP) toward water/organic liquid mixtures. With an increase in temperature, both the permeation flux Q and the separation factor α increased, and the membrane performance was much better for VP than for PV. For VP at 378 K and 10 wt % of feedwater, Q values were 4.5, 3.5, and 7.8 kg/(m2 h) and α values were >30000, 5700, and >9000 for the water/ethanol, /methanol and /dioxane systems, respectively. A mechanism of PV and VP based on the capillary condensation of water in the zeolitic and nonzeolitic pores and the blocking of other molecules from entering the pores was proposed.

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ABSTRACT:This article reviews the recent progress made over the past years based on naphthalene-based sulfonated polyimides (SPIs) in terms of proton conductivity, membrane swelling behavior, membrane stability toward water, and fuel cell performance in polymer electrolyte fuel cells (PEFCs) or direct methanol fuel cells (DMFCs). The structure-property relationship of SPI membranes is discussed in details with respect to the chemical structure of various sulfonated diamines and morphology of SPI membranes from the viewpoints of viscosity, mechanical strength and proton conductivity. Ion exchange capacity (IEC), basicity of sulfonated diamine, configuration (para-, meta-, or ortho-orientation) and chemical structure of polymer chain (linear or net-work) show great influence on the water stability and mechanical strength of SPI membrane. The SPIs with a branched/crosslinked structure and derived from highly basic sulfonated diamines display reasonably high water stability of more than 200-300 h in water at 130 C, suggesting high potential as PEMs operating at temperatures up to 100 C. The SPI membranes have fairly high proton conductivity at higher relative humidities and low methanol permeability. The water and methanol crossover through membrane under the fuel cell operation conditions is not controlled by electro-osmosis due to proton transport but by diffusion due to activity difference. This is quite different from the case of perfluorosulfonated membranes such as Nafion and results in the advantageous effects on fuel cell performance. SPI membranes displayed high PEFC performances comparable to those of Nafion 112. In addition, SPI membranes displayed higher performances in DMFC systems with higher methanol concentration (20-50 wt %), which is superior to Nafion and have high potential for DMFC applications at mediate temperatures (40-80 C In the past decades, great interest has been focused on the development of polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) as a clean power source of energy for transportation, stationary and portable power applications.1,2 Fuel cells with high performance, high durability and potentially lower cost are greatly required. Polymer electrolyte membrane (PEM) is one of the key components in PEFC and DMFC systems. Perfluorosulfonic acid copolymer membranes, such as DuPont's Nafion membrane, are the state-of-the-art PEMs commercially available due to their high proton conductivity and excellent chemical stability.3,4 However, because of their high cost, low operational temperature below 80 C and large methanol crossover, there has been much interest in alternative PEMs. Many efforts have been done in the development of PEMs based on sulfonated aromatic hydrocarbon polymers. [5][6][7][8][9] The main problem existed in the hydrocarbon PEMs is the membrane stability under fuel cell conditions and low conducting performance at low moisture atmosphere. The balance between ion exchange capacity (IEC), proton conductivity and mechanical stability of a PEM ...