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 ...