Magnetic-field-oriented mixed-valence-stabilized ferrocenium anion-exchange membranes for fuel cells

Through-plane (TP) conducting pathways in anion-exchange membranes (AEMs) are desirable for AEM fuel cells as they serve as short and efficient routes for hydroxide ion transport between electrodes, improving power output. Electric and magnetic fields have previously been used to create TP-oriented structures in AEMs, but with modest performance gains. Here we use paramagnetic ferrocenium polymers to prepare TP-oriented AEMs under a magnetic field. The magnetic field induces a mixed-valence state, which effectuates higher anion dissociation and enhanced alkali/redox stability. Our AEMs display a promising TP hydroxide conductivity of ~160 mS cm−1 at 95 °C in water, and no appreciable hydroxide conductivity loss over 4,320 h at 95 °C in alkali. The assembled fuel cells achieve a power output of 737 mW cm−2 at 80 °C and 80% relative humidity, and a durability of 3.9% voltage loss and 2.2% high-frequency resistance increase over 500 h at 500 mA cm−2, 120 °C and 40% relative humidity.