The water stability of sulfonated copolyimides (SPIs) derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), sulfonated diamines of 4,4‘-bis(4-aminophenoxy)biphenyl-3,3‘-disulfonic acid (pBAPBDS), and 2,2‘- or 3,3‘-bis(3-sulfopropoxy)benzidine (2,2‘- or 3,3‘-BSPB) and nonsulfonated diamines was investigated in detail from viewpoints of viscosity, mechanical strength, proton conductivity, weight loss, and hydrolysis products eluted into the soaking water. With the aging in water or 100% relative humidity vapor at 130 °C, the polymer chain scission took place mainly in the early stage but thereafter slightly, and as a result the SPI membranes kept the reasonably high mechanical properties even after 192 h. The branched/cross-linked SPI membranes prepared by incorporating a flexible triamine kept better mechanical properties. The weight loss and sulfur loss of <10% were observed for the pBAPBDS-based SPIs. This was due mainly to the elution of hydrolysis product, the oligomer of NTDA, and sulfonated diamine, which did not contain the nonsulfonated diamine moieties. With the aging at 130 °C, the proton conductivity did not change for the pBAPBDS-based SPIs, but for the BSPB-based SPIs it decreased 20% in water and much more at the lower relative humidities because of the cleavage of the sulfopropoxy group. The accelerated water stability tests reveal that the water stability of the present SPI membranes is not sufficiently high at 130 °C but is high enough for PEFC and DMFC applications at least at 80 °C.

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 ...

A series of sulfonated copolyimides (co‐SPIs) bearing pendant sulfonic acid groups were synthesized from 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTDA), bis(3‐sulfopropoxy) benzidines (BSPBs), and common nonsulfonated diamines via statistical or sequenced polycondensation reactions. Membranes were prepared by casting their m‐cresol solutions. The co‐SPI membrane had a microphase‐separated structure composed of hydrophilic and hydrophobic domains, but the connecting behavior of hydrophilic domains was different from that of the homo‐SPIs. The co‐SPI membranes displayed clear anisotropic membrane swelling in water with negligibly small dimensional changes in the plane direction of the membrane. With water uptake values of 39–94 wt %, they showed dimensional changes in membrane thickness of about 0.11–0.58, which were much lower than those of homo‐SPIs. The proton conductivity σ values of co‐SPI membranes with ion exchange capacity values ranging from 1.95–2.32 meq/g increased sigmoidally with increasing relative humidity. They displayed σ values of 0.05–0.16 S/cm at 50 °C in liquid water. Increasing temperature up to 120 °C resulted in further increase in proton conductivity. The co‐SPI membranes showed relatively good conductivity stability during the aging treatment in water at 100 °C for 300 h. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1545–1553, 2005

Summary: Branched/crosslinked sulfonated polyimide membranes incorporating superior mechanical properties, high proton conductivity, and excellent fuel cell performance were successfully developed. The resulting polymer electrolytes displayed conductivity values of about 0.2 S · cm−1 at 120 °C and 100% relative humidity. In a single H2/O2 fuel cell system at 90 °C, they exhibited reasonably high fuel cell performances comparable to that of Nafion 112.The structure of the branched/crosslinked sulfonated polyimide membranes studied here.imageThe structure of the branched/crosslinked sulfonated polyimide membranes studied here.

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Novel sulfonated diamines bearing aromatic pendant groups, namely, 3,5diamino-3 0 -sulfo-4 0 -(4-sulfophenoxy) benzophenone (DASSPB) and 3,5-diamino-3 0 -sulfo-4 0 -(2,4-disulfophenoxy) benzophenone (DASDSPB), were successfully synthesized. Novel side-chain-type sulfonated (co)polyimides (SPIs) were synthesized from these two diamines, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and nonsulfonated diamines such as 4,4 0 -bis(3-aminophenoxy) phenyl sulfone (BAPPS). Tough and transparent membranes of SPIs with ion exchange capacity of 1.5-2.9 meq g À1 were prepared. They showed good solubility and high thermal stability up to 300 8C. They showed isotropic membrane swelling in water, which was different from the mainchain-type and sulfoalkoxy-based side-chain-type SPIs. The relative humidity (RH) and temperature dependence of proton conductivity were examined. At low RH, the novel SPI membranes showed much higher conductivity than the sulfoalkoxy-based SPIs. They showed comparable or even higher proton conductivity than Nafion 112 in water at 60 8C (>0.10 S cm À1 ). The membrane of NTDA-DASDSPB/BAPPS (1/1)-s displayed reasonably high proton conductivities of 0.05 and 0.30 S cm À1 at 50 and 100% RH, respectively, at 120 8C.

A series of branched/crosslinked sulfonated polyimide (B/C‐SPI) membranes were prepared and evaluated as proton‐conducting ionomers based on the new concept of in situ crosslinking from sulfonated polyimide (SPI) oligomers and triamine monomers. Chemical branching and crosslinking in SPI oligomers with 1,3,5‐tris(4‐aminophenoxy)benzene as a crosslinker gave the polymer membranes very good water stability and mechanical properties under an accelerated aging treatment in water at 130 °C, despite their high ion‐exchange capacity (2.2–2.6 mequiv g−1). The resulting polymer electrolytes displayed high proton conductivities of 0.2–0.3 S cm−1 at 120 °C in water and reasonably high conductivities of 0.02–0.03 S cm−1 at 50% relative humidity. In a single H2/O2 fuel‐cell system at 90 °C, they exhibited high fuel‐cell performances comparable to those of Nafion 112. The B/C‐SPI membranes also displayed good performances in a direct methanol fuel cell with methanol concentrations as high as 50 wt % that were superior to those of Nafion 112. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3751–3762, 2006