To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78-2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 degrees C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm(-)(1) was obtained for 1 at 140 degrees C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1-2 orders of magnitude lower than that of Nafion under both dry and wet conditions. Fuel cell was fabricated with 1 membrane and operated at 80 degrees C and 0.2 A/cm(2) supplying H(2) and air both at 60% or 90% RH. Ionomer 1 membrane showed comparable performance to Nafion and was durable for 5000 h without distinct degradation.

Solute segregation at grain boundaries has been correlated with grain‐boundary conductivity in high‐purity 15‐mol%‐CaO‐stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels <80 ppm) is grain‐size dependent. The boundary coverage of silicon can be systematically varied by varying grain size at concentrations low enough that a discrete siliceous film does not form. The cosegregation of calcium and silicon is observed. The grain‐boundary solute coverage (Tsi+Ca) has been correlated with the specific grain‐boundary conductivity (σspgb) determined using impedance spectroscopy. At monolayer segregation levels, the specific boundary conductivity is less than the bulk conductivity by a factor >103 at 500°C. At the lowest levels of segregation achieved, <0.1 monolayer, σspgb remains ∼102 less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3‐doped ZrO2 suggests similar behavior in this system. The control of grain‐boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain‐boundary impedance of polycrystalline ionic conductors.