Mechanically robust microporous anion exchange membranes with efficient anion conduction for fuel cells

Abstract

• Brittleness in polymer of intrinsic microporosity (PIM) membranes is alleviated by high molecular weight. • Role of micropores in developing water/ion conduction channels is investigated. • Microporous anion exchange membrane (AEM) has superior ion conduction efficiency. • H 2 crossover through microporous AEM is minimal in the hydrated state. • Feasibility of microporous AEMs for H 2 /O 2 fuel cell is evaluated. Polymers of intrinsic microporosity (PIMs) present an attractive opportunity for developing new types of anion exchange membranes (AEMs) for fuel cell featuring charged subnanometer-sized micropores. But challenges exist to make mechanically robust PIM AEMs due to their high chain rigidity. Imparting more flexibility improves mechanical properties but sacrifices microporosity. Here, a mechanically robust and highly anion conductive PIM AEM (QPIM-1) fabricated by facile animation and quaternization of PIM-1 membrane is reported, and its structure–property relationships are investigated, especially focusing on the microporous structure. High molecular weight alleviates brittleness, as QPIM-1 AEM shows comparable mechanical properties to conventional AEMs, quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO), at a membrane thickness down to ~35 μm and a high ion exchange capacity (IEC) up to ~2.1 mmol g −1 . The micropores situated among the rigid and contorted polymer chains evolve into water/ion conduction channels when the membrane is hydrated. This results in improved morphology over dense polymeric AEMs by less hindered ion pathways. QPIM-1 AEMs exhibit superior ion conduction efficiency, which is 2.6–5.3 times that of dense QPPO AEM at similar ion exchange capacities (IECs). A high hydroxide ion conductivity of 57 mS cm −1 at 20 °C is obtained, which is among the highest reported anion conductive PIM-based AEMs. Even though the AEMs are microporous, only slight H 2 permeation is observed when hydrated and at high open circuit voltage (OCV) of a single fuel cell.