Electric Field Poling Induces Ordered Membrane Morphology Giving Exceedingly High Proton Conductivity

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 issue of membrane morphology has not been addressed sufficiently in order to improve ion conductivity. Membrane morphology is known to be a primary structure factor responsible for fluid transport in semi-permeable membrane.  Since ion transportation relies heavily on fluid transport behavior; charge conduction would be also deeply dependant on channel morphology.  In present study, a novel approach is proposed to prepare ion conducting membrane by applying electric field poling on Nafion ionomer where one dimension metal oxide (ZrO₂, MnO₂, TiO₂) nanorods and nanotubes are impregnate first. The field induced dipole oriented the low dimensional nanotube/nanorod thus created aligned hydrophilic morphological texture that is fixed after 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 sulfonated group surface functionalized ZrO2 and TiO2 nanotube is 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 further to a record value of 11.6x10^-2S/cm.  This is substantially improved over that of commercially available Nafion membrane N117 (5.84x10^-2S/cm) or the locally recast Nafion(5.2x10^-2S/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 membranes 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. Most interestingly, 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 shrinks after electric field poling.  This corroborates with the fact that water swelling ratio is reduced from 30% (without poling) to 11% (with poling at 7000 V/cm).  

The fact that electric field poling produces membrane with lower water uptake and smaller swelling ratio but displayed high proton conductivity is a surprising find.  The results asserted that high proton conductivity can be achieved by effective use of water through synergistic cooperation of direct water permeation channel and well distributed and connected sulfonate groups. The amount of water required to conduct proton can actaully be reduced with membrane morphology optimized.  Enhanced fuel cell performance is realized by employing the e-poled membrane with superbly high ion conductivity.