Mechanism of High Proton Mobility in the Two-Dimensional Nanospace at the Interlayer of a Multilayer Polymer Nanosheet Film

Highly proton-conductive materials are important for creating efficient energy-harvesting devices, such as fuel cells. Here, we demonstrated that protons exhibit mobility in the 2D nanospace at the interlayer of a multilayer polymer lamellar film 100 times greater than that in bulk water. The multilayer film was prepared by the successive deposition of poly(N-dodecyl acrylamide-co-acrylic acid) monolayers on a solid substrate using the Langmuir-Blodgett technique. The proton mobility was 0.43 ± 0.04 cm² V^-1 s^-1, as calculated using the proton conductivity obtained by impedance measurements and the proton concentration obtained by surface pressure-area isotherm and X-ray diffraction measurements. Analyses by X-ray reflectivity and p-polarized multiple-angle incidence resolution spectrometry showed that the 2D nanospace was created by the separation (by 4.7 Å) of uniform polymer nanosheets, which were formed by the physical cross-linking of polymer chains through well-ordered hydrogen bonding between amide groups and the dimerization of carboxylic acid groups. Furthermore, in situ Fourier transform infrared measurements under humid conditions indicated that the high proton mobility in the 2D nanospace originated from the collective hydrogen-bonding network, which maintained the active librational motion of water at the interlayer. Finally, the importance of the 2D nanospace to a high proton conductivity was confirmed by comparison with the proton conductivity of a polymer lamellar film without a 2D nanospace. These results suggest that the creation of a 2D nanospace is an effective strategy for obtaining highly conductive polymer electrolytes for fuel cells.