Magnetic field-oriented ferroferric oxide/poly(2,6-dimethyl-1,4-phenylene oxide) hybrid membranes for anion exchange membrane applications

Abstract

Concentrating on the ion conductivity of anion exchange membranes (AEMs), we present a magnetic-field-oriented strategy to address the insufficient ion conductivity and the lifetime problem of AEMs used in alkali membrane fuel cells (AMFCs). Magnetic ferroferric oxide (Fe₃O₄) is functionalized with quaternary ammonium (QA) groups to endow the QA-Fe₃O₄ with ion-exchange ability. A series of aligned QA-Fe₃O₄/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) hybrid membranes were fabricated by doping QA-Fe₃O₄ in a triple-ammonium-functionalized PPO (TA-PPO) solution in an applied magnetic field. The structure of aligned QA-Fe₃O₄ in the TA-PPO membrane is clearly observed by using a scanning electron microscope (SEM). More importantly, the aligned QA-Fe₃O₄ constructs successive and effective ion-transport channels in the QA-Fe₃O₄/TA-PPO membrane, which dramatically improves the ion conductivity of the membranes. Notably, the magnetic-field-induced ion channels (MICs) are different from microscopic phase-induced ion channels (PICs). These MICs display much shorter ion transport distances and broader water channels than traditional PICs in AEMs. The aligned QA-Fe₃O₄/TA-PPO hybrid membrane displays a further 55% increase in ion conductivity after magnetic-field orientation compared to the normal QA-Fe₃O₄/TA-PPO membrane. Surprisingly, the aligned QA-Fe₃O₄ also improves the alkali stability and fuel cell performance of the hybrid membrane. The aligned 6%-QA-Fe₃O₄/TA-PPO hybrid membrane realizes a maximal power density of 224 mW cm^-2. In summary, this work provides a novel and effective method to prepare high-performance AEMs.