Imidazolium-functionalized polysulfone hydroxide exchange membranes for potential applications in alkaline membrane direct alcohol fuel cells

A series of imidazolium-functionalized polysulfones were successfully synthesized by chloromethylation-Menshutkin two-step method. PSf-ImOHs show the desired selective solubility: insoluble in alcohols (e.g., methanol and ethanol), and soluble in 50 vol.% aqueous solutions of acetone or tetrahydrofuran, implying their potential applications for both the alcohol-resistant membranes themselves and the ionomer solutions in low-boiling-point water-soluble solvents. PSf-ImOH also possesses very high thermal stability (TOD: 258 °C), higher than quaternary ammonium and quaternary phosphonium functionalized polysulfones (TOD: 120 °C and 186 °C, repsectively). Ion exchange capacity (IEC) of PSf-ImOH membranes ranges from 0.78 to 2.19 mmol g−1 with degree of chloromethylation from 42% to 132% of original chloromethylated polysulfone. As expected, water uptake, swelling ratio, and hydroxide conductivity increase with IEC and temperatures. With 2.19 mmol g−1 of IEC, the PSf-ImOH 132% membrane exhibits the highest hydroxide conductivity (53 mS cm−1 at 20 °C), higher than those of all other reported polysulfone-based HEMs (1.6–45 mS cm−1) and other imidazolium-functionalized HEMs (19.6–38.8 mS cm−1). In addition, PSf-ImOH membranes have low methanol permeability of 0.8–4.7 × 10−7 cm2 s−1, one order of magnitude smaller than that of Nafion212 membrane. All these properties indicate imidazolium-functionalized polysulfone is very promising for potential applications in alkaline membrane direct alcohol fuel cells.