Hydrogen crossover through microporous anion exchange membranes for fuel cells

Microporous organic polymers have intriguing ion-conducting ability and have potential as electrolyte membranes for fuel cells, but there are latent concerns about deleterious hydrogen crossover caused by their intrinsic micropores. Herein, hydrogen crossover through quaternary ammonium modified polymer of intrinsic microporosity (PIM)-based anion exchange membranes (QPIM-1 AEM), is systemically explored for H2/O2 fuel cell. Hydration state in fuel cells is found to be critical in the hydrogen crossover behavior. The H2 permeability exhibits a remarkable decrease from several hundred barrer in the dry state to only several barrer after exposure to humidified gas, which is ascribed to blockage of the hydrophilic micropores to H2 by adsorbed water. At only 40% relative humidity (RH), near-complete occupation of the QPIM-1 AEM micropores occurs. At higher RH, hydrogen crossover remains very low, without an increasing trend common for commercial dense electrolyte Nafion membrane, owing to low mobility of the rigid PIM structure, which lacks rotational freedom. Consequently, a fuel cell using a 34 μm thick AEM exhibits a high open circuit voltage of 0.925–0.972 V over a practical wide RH range of 40–100%, suggesting sufficient hydrogen crossover suppression.