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.