Aryl‐Tethered Pendant Sterically Shielded Imidazoliums for Durable Anion Exchange Membranes

Conventional quaternary‐ammonium‐based anion‐exchange membranes (AEMs) with insufficient alkaline stability limit the durability of AEM water electrolyzers (AEMWEs). Here we report novel poly(arylimidazolium) membranes (QAI‐x) that overcome this barrier through an aryl‐tethered pendant sterically shielded imidazolium cation. This architecture sterically protects the imidazolium cation while avoiding the introduction of a benzylic cation–polymer linkage. Combined with ether‐free polyaromatic backbones, the QAI‐x membranes simultaneously achieve enhanced chemical stability and dimensional stability, with no measurable loss in hydroxide conductivity after 4000 h in 1 M KOH at 80°C, resistance to Fenton oxidation, and limited dimensional swelling despite relatively high water uptake. In AEMWEs, QAI‐x membranes support voltage decay rates as low as 9 µV h −1 over 5400 h in 1 M KOH and 16 µV h −1 over 2000 h in 5 M KOH at 80°C and 2.5 V, substantially outperforming commercial benchmarks. Cell operation is further demonstrated for more than 1000 h at 3 A cm −2 under a dry‐cathode configuration. These results highlight aryl‐tethered steric shielding as a promising molecular‐design strategy for durable AEMs.