Enhancement of Proton Conduction Under Low Humidity By Incorporating Polymeric Microcapsules Into Polymer Electrolyte Membranes

Learning from nature is an eternal motif for designing hierarchically structured nanocomposite membranes due to the existence of abundant composite biomaterials fabricated in a breathtakingly facile way. Recently, a series of polymeric microcapsules(PMCs), mimicking the water-storage mechanisms in plant cells, have been synthesized and incorporated into polymer electrolyte membranes(PEM). The hierarchical structures of PMCs are manupulated by using facile and controlled distillation–precipitation polymerization technique. The PMCs with controlled lumen size and shell thickness possess a high density of hydrophilic and proton conductive functional groups such as carboxylic acid, sulfonic acid, phosphoric acid and imidazole. Double-shelled PMCs bearing a carboxylic acid inner shell and imidazole outer shell is developed to independently control water retention and acid–base pairing of the membrane. Carboxylic acid functionalized PMCs with controlled hydrophilicity is prepared to tune the hydrophilic/hydrophobic balance of the composite membrane. All the PMCs dramatically enhance water-retention properties of the membranes and provided additional proton-transport pathway, and consequently increase the proton conductivity notably under low relative humidity of 20%. These studies unlock a gate towards better understanding and fine tuning of the water retention properties of fuel cell membranes. This review summarizes the recent work about PMCs-based composite membrane.