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
2020
Overton, Philip | Li, Wei | Cao, Xinzhi | Holdcroft, Steven
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%).
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