(203h) Glass Transition and Ion Transport in Hydrogen Bonded Thin Film Layer-by-Layer Assemblies
Understanding the mechanism of ion conduction in ultrathin polymer electrolytes allows for design of next-generation electrolytes for electrochemical energy devices like fuel cells, batteries, and photovoltaics. Of note, the mode of ion conduction in polymer electrolytes differs with humidity. In dry conditions, as in a Li-ion battery, lithium-ion mobility depends upon the segmental motion of its polymer ?host?, meaning that the glass transition of that polymer plays an important role. Conversely in humid or saturated conditions, as in a proton-exchange membrane fuel cell, proton conduction relies upon the Grotthus mechanism, where membrane hydration aids the transport of protons along absorbed water clusters.
Here, we report the fabrication of hydrogen-bonded layer-by-layer assemblies as electrolytes for dry or saturated states. Proton-donating polyacrylic acid (PAA) and proton-accepting polyethylene oxide (PEO) are alternately adsorbed from solution to a substrate via the layer-by-layer technique, yielding ultrathin architectures of fine control and tunability. We manipulate the glass transition of the assembly by altering the adsorption solution pH, a parameter intimately tied to PAA intramolecular bonding. Of note, the layer-by-layer assembly of PEO and PAA demonstrates one glass transition between its pure components, like a blend. In the dry state, Li-ion conductivity in the ultrathin (PEO/PAA) film reaches that of neat PEO doped with lithium salt.
Additionally, we explore humid PEO/PAA assemblies as proton-exchange membranes. Since pH controls (PEO/PAA) composition, the conductivity of the membrane is closely tied to the content of its hygroscopic constituents. In saturated humidity at room temperature, PEO/PAA exhibits a conductivity of 10-4 S/cm, whereas Dupont's Nafion has an ionic conductivity ~ 0.1 S/cm in these conditions. As an advantage, the thickness of the PEO/PAA membrane can be controlled on the nanometer scale to yield ultra-thin films with conductances comparable to thicker Nafion membranes.