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On the Development of Naphthalene-Based Sulfonated Polyimide Membranes for Fuel Cell Applications

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).