Poly(2,6-dimethyl-1,4-phenylene oxide) containing imidazolium-terminated long side chains as hydroxide exchange membranes with improved conductivity

•A long side chain was introduced onto PPO in a moderate reaction condition.•The continuous hydroxide conducting channels were formed in the membrane.•The conductivity was highly enhanced without sacrificing dimensional stability.

Hydroxide exchange membrane fuel cells (HEMFCs) receive growing interest due to the usability of non-precious metal catalysts in the basic operating environment. The applications of HEMFCs are hindered by the trade-off between high hydroxide conductivity and good dimensional stability of the HEMs. Here, a novel poly(2,6-dimethyl-1,4-phenylene oxide) with imidazolium-terminated long side chains (PPO-COC5H10-Im) was synthesized by simple and controllable acetylation reaction of PPO and 6-bromohexanoyl chloride in a moderate condition (at room temperature), followed by the Menshutkin reaction with 1,2-dimethylimidazole. The introduction of long chains between imidazolium groups and polymer main chains facilitates the formation of good hydrophilic/hydrophobic micro-phase separation structure, which is illustrated by TEM. The hydroxide effective mobility in PPO-COC5H10-Im membrane is almost 2-fold that in short side chain imidazolium functionalized PPO (PPO-CH2-Im) membrane at a certain IEC. Given similar swelling ratios (25% vs. 27%), PPO-COC5H10-Im membrane shows much higher hydroxide conductivity than PPO-CH2-Im membrane (62 vs. 42 mS cm−1) at 60 °C. It indicates that the PPO-COC5H10-Im membranes prepared here exhibits highly enhanced conductivity without sacrificing the dimensional stability.