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Electric field induces temporary polarization on molecular segment. In polymer solution or polymer melt, applying electric field induces self assembly that cannot be established otherwise. This preferentially ordered morphology is preserved after drying out solvent to form polymer membrane. The ordered nano structure delivered favorable ion transport behavior, raised ion conductivity, and improved membrane mechanical properties. In one example given for fuel cell membrane composed of poly(ether sulfones)(PES) with sulfonated poly(ether ether ketone)(sPEEK, degree of sulfonation=50%) prepared with e-field poling, extremely high ion conductivity (7.43x10-2 S/cm) is observed. The hydrophilic dimension is found to be reduced which restricted methanol permeability (3.17x10-7 cm2/s). Due to more densely packed hydrophobic domain, membrane mechanical property is also improved. DMFC power output at 150mA/cm2 (80 oC, 1M Methanol) can be realized. A second example is given for lithium battery membrane: Poly(vinylidenefluoride-co-hexafluoropylene)[PVDF-HFP] blending either with PMMA polymer, or with surface functionalized mineral clay containing ionic liquids. E-field has improved PVDF and PMMA miscibility, gave rise to more organized and more polar beta crystallite formation with denser amorphous region. The clay is homogeneously dispersed and oriented to form ordered and preferentially aligned nanostructure. As a result, the solvent free membrane delivered Li+ conductivity surpassing 10-3 S/cm at room temperature, and shows reduced interface resistance between electrodes. Coin cell (R2032) using this SPEs and LiFePO4cathode displayed 140 mAh/g discharge capacity and stable over 300 cycles, with 0.5C rate. Membrane bearing preferentially ordered morphology induced by E-field is able to balanced high ion conductivity; low fuel/gas permeation and strong mechanical strength, simultaneously. This approach in preparing high performance membrane holds great promises to apply to future energy devices such as Li-S, Li-Air, and thin film battery. |
