Tuning Ion Exchange Capacity in Hydroxide-Stable Poly(arylimidazolium) Ionenes: Increasing the Ionic Content Decreases the Dependence of Conductivity and Hydration on Temperature and Humidity
Philip Overton, Wei Li, Xinzhi Cao, Steven Holdcroft
Abstract
Increasing the ion exchange capacity (IEC, mmol·g–1) of anion exchange membranes (AEMs) decreases the dependence of ionic conductivity (σ) and ion pair hydration on limiting conditions of temperature and humidity. We present novel, hydroxide-stable polycationic ionenes having penta-substituted imidazolium repeating units. Variation of N,N′-dialkyl substituents having 1–4 carbons yields a range in IEC(Cl) (1.56–2.32 mmol·g–1). The trend of σ(Cl) ∼ IEC is in agreement with comparable poly(arylimidazolium)s. This is the first report cross-correlating water uptake (WU) and ionic conductivity of a homologous series of poly(arylimidazolium) ionenes. The AEMs of higher IEC have a lower range in σ and hydration number (λ = WU / IEC) within limits of temperature and humidity. Decreasing IEC correlates to increasing effective activation energy of ion transport (Ea(eff.)), at constant hydration. The AEM of N,N′-dimethyl-2,4,5-arylimidazolium repeating units and the highest IEC provides σ(Cl) = 12 mS·cm–1 (80 °C, 95 RH%) and σ(OH) = 120 mS·cm–1 (40 °C, 90 RH%).