H-1 Solid-State NMR Study of Nanothin Nafion Films

The unique behaviors of Nafion nanothin films with thicknesses of 10 nm (ultrathin) and 160 nm (thin) were evaluated using variable-temperature and variable-humidity solid-state H-1 NMR spectroscopy. These unprecedented measurements of nanothin films stacked within an NMR rotor represent a remarkable experimental achievement and demonstrate that H-1 NMR spectroscopy of such minute amounts of ionomer might be possible within active catalyst layers in polymer electrolyte fuel-cell electrodes. This study was motivated by the observation, in a separate work, of thickness-dependent and highly suppressed conductivity in nanothin films of Nafion (4-300 nm) compared to counterpart free-standing Nafion membranes. Trends in the line width and, more precisely, the T2 relaxation, as probed using a Hahn echo, showed that the local mobility within the hydrogen-bonded domain is equivalent for 10 and 160 nm films and is governed by the fast exchange limit in terms of NMR time scales. Subtle differences in the chemical shift trends provide insight into the domain structures, where the 10 nm films show no changes whereas the thicker 160 nm films exhibit chemical shift trends that indicate a rearranging hydrogen-bonded network. Thus, it is inferred that domain structure formation is influenced by film thickness and that the interaction with the substrate becomes limiting as the film becomes thinner.