Proton Conductive Polyimide Electrolytes Containing Trifluoromethyl Groups:  Synthesis, Properties, and DMFC Performance

Abstract

A series of sulfonated polyimide copolymers (FSPIH-X; X refers to molar percentage of bis(trifluoromethyl)biphenylene content) with X from 0 to 60 mol % were synthesized, of which electrolyte properties were investigated and compared to those of the perfluorinated ionomer (Nafion 112). FSPIH-X membranes are thermally stable with no glass transition temperature observed below the decomposition temperature (280 °C). Oxidative stability of the membranes is improved with an increase in the content of trifluoromethyl substituents in the copolymer structure. FSPIH-60 endured for more than 9 h in Fenton's reagent at 80 °C. Bis(trifluoromethyl)biphenylene groups with the molecular size of 6.1 Å make each polymer chain separate and produce space to hold water molecules despite their hydrophobic property so that the maximum water uptake was observed for FSPIH-20. Unlike the fluorene groups containing polyimides (SPIH-X), a strong water confinement effect was not obtained for FSPIH-X. The optimum composition of bis(trifluoromethyl)biphenylene groups was 30 mol %, and the FSPIH-30 membrane showed higher proton conductivity than 0.2 S cm-1 at 30−140 °C. A direct methanol fuel cell (DMFC) using FSPIH-30 membrane has revealed that the methanol crossover through the membrane equivalent to the current density of methanol oxidation at cathode (j(CH3OH)) is 64 mA/cm2 and merely 30% of that of Nafion 112 at open-circuit potential. A terminal voltage of 0.38 V was obtained at 200 mA/cm2 by the operation at 80 and 90 °C with supplying dry and humidified oxygen.

Electrochemical and structural characterization of sulfonated polysulfone

“…Polysulfone (Mn 26,000 g/mol), chlorosulfonic acid (ClSO 3 H, 98%), trimethylsilyl chlorosulfonate (ClSO 3 Si(CH 3 ) 3 , TMSCS, 99%) N,N dimethylacetamide (DMAc), 1,2 dichlorethane (DCE), DMF d 7 and DMSO d 6 were supplied by SigmaeAldrich (Sant Louis, USA). The solvents were high grade reagents and used as received.…”

“…Therefore, extensive studies have been focused on developing alternatives to these per fluorosulfonated membranes [3]. In particular, different polymers that adopt the sulfonated aromatic hydrocarbon structure have been tested, including poly(arylene ether sulfone)s [4], poly(ether ether ketone)s [5], poly(sulfide sulfone)s [6], polyimides [7], poly phosphazenes [8] and polybenzimidazoles [9].…”

Polymer Electrolyte Membranes Derived from New Sulfone Monomers with Pendent Sulfonic Acid Groups

“…[4][5][6] Consequently, much progress has been made to develop novel hydrocarbon PEMs based on sulfonated aromatic polymers, which have good physical properties and are inexpensive. The most widely investigated PEMs include sulfonated derivatives of poly(arylene ether sulfone)s (SPAES), 7,8 poly-(arylene ether ether ketone)s (SPEEK), [9][10][11] poly(arylene sulfide sulfone)s (SPSS), 12,13 poly(arylene ether nitrile)s (SPAEEN), [14][15][16] polyimides (SPI), [17][18][19][20][21] and polyphenylenes (PP). [22][23][24] Many hydrocarbon polymers show sufficiently high conductivities only at high ion-exchange capacities (IEC)s, which causes extensive water uptake above a critical temperature (percolation threshold), or a dramatic loss of mechanical properties due to dimensional swelling that render them unsuitable for practical PEM applications.…”

Phase Separation and Water Channel Formation in Sulfonated Block Copolyimide

“…Therefore, structural information on phase separation behaviors of hydrocarbon PEMs obtained using molecular simulation is rare. 32 In the present study, sulfonated block copolyimides (SPIs) were chosen as representative hydrocarbon PEMs [33][34][35][36][37][38][39] to explore the relationship among microscopic phase-separated structures, water channel formation, and macroscopic physical properties. We prepared SPIs with different compositions of hydrophilic and hydrophobic segments and characterized their densities, proton conductivities, and water uptakes.…”