(560g) Zwitterionic Microcapsules As Water Reservoirs and Proton Carriers within Nafion Membrane to Confer High Proton Conductivity Under Low Humidity
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. Herein, Zwitterionic microcapsules (ZMCs) based on sulfobetaine with tunable structures and defined chemical compositions were synthesized via precipitation polymerization, and incorporated into Nafion to prepare composite membranes. The ZMCs, mimicking the water-retention system in plant cells, can act as water reservoir within membrane for stable water environment. The ZMCs exhibited superior water retention properties owing to the ability of the hollow structures to store water, and the superhydrophilic zwitterionic groups to bind with water molecules tightly by strong electrostatic interaction. Moreover, the highly-effective proton conductive sulfonic acid groups in ZMCs can facilitate proton transport. Due to the superior water retention and desirable proton conduction properties of ZMCs, the composite membrane with 15 wt% ZMC-I displayed the highest proton conductivity of 5.8×10-2 S cm-1 at 40 oC and 20% RH after 90 min testing, about 21 times higher than that of the Nafion control membrane. This study may contribute to rational design of water retention materials as well as proton conduction materials, and the ZMCs may find promising applications in diverse energy-relevant fields.