| dc.description.abstract | Enhancing ion conductivity is the primary goal in membrane development in fuel cell, lithium battery, vanadium redox flow battery, or energy storage devices involving ion exchange. The challenge grows when requirements are also given to preserve high mechanical strength while improving ion transport property. A common approach proposed to make the break through is by forming inorganic–organic hybrid. Composite with inorganic nanoparticles is found to retain water/moisture through inorganic surface such that it maintains high conductivity and conserved high energy out-put at elevated temperature and at low environment humidity.
Though highly important, the study of membrane morphology has not been an issue of focus to improve ion conductivity. Membrane morphology is known to be a primary structure factor responsible for fluid transportation in semi-permeable membrane. Since ion conductivity relies heavily on fluid transport; change on channel morphology will lead to change of ion conduction. In present study, a novel approach is reported to prepare ion conducting membrane under high electric field poling condition. One dimension metal oxide (ZrO2, TiO2) nanorods and nanotubes are impregnate first in Nafion, and the whole drying and membrane forming process is carried out under electric field polled condition. The field induced dipole orients the low dimensional nanotube/nanorod thus created aligned hydrophilic morphology that was fixed during membrane formation. The ordered and oriented nano-structures formed in the direction of the applied electric field, provided a direct and continuous ion path. Proton conductivity has reached 7.5x10-2 S/cm in 100% RH condition when 5 wt% of sulfonate functionalized ZrO2 or TiO2 nanotube are composited with Nafion. Upon applying a DC voltage over 1000V, the conductivity is raised to 8.35x10-2 S/cm. With continue increasing of the electric field to 7000 V/cm, the conductivity in the composite film raised to a record value of 11.6x10-2 S/cm. This is substantially improved over that of commercially available Nafion membrane N117 (5.84x10-2 S/cm) or the locally recast Nafion (5.2x10-2 S/cm) membrane.
Diffusion tensor mapping derived from NMR micro-imagine of these membrane confirmed (1) faster water diffusion as reflected in the stronger diffusion tensor, (2) more ordered tensor orientation (narrowing of Euler angle distribution) along the Z-direction (cross-channel director), and (3) more homogeneously distributed diffusion tensor in the electric field poled membrane. These results confirmed ordered diffusion in the e-field poled ionomer is indeed responsible for the high proton conductivity. Due to the more ordered morphology originated from the e-field poling, membrane mechanical property is also enhanced. However, water uptake is gradually reduced from 24% (without poling) to 21% (with poling at 7000 V/cm). Small angle x-ray diffraction shows the tubular flow channel dimension shrunk after electric field poling. This corroborates with the results that swelling ratio is reduced from 23% (without poling) to 18% (with poling at 7000 V/cm).
The fact that electric field poling produces membrane with lower water uptake and smaller swelling ratio and yet displayed high proton conductivity is a surprising find. The results alluded to the possibility that higher proton conductivity can be achieved by more effective use of water molecules through synergistic cooperation of direct water permeation channel and well distributed and connected sulfonate groups. In the present case, the amount of water required to deliver an optimized proton conductivity can be reduced to nearly ½, provided that the membrane morphology is optimized. Enhanced fuel cell performance is also realized by employing the e-poled membrane which shows superbly high ion conductivity.
| en_US |