Controlling Polymer Electrolyte Interfacial Morphology through Chemical Interactions

Owing to its unique mechanical properties, chemical resistance, and ion conductivity, Nafion is one of the most widely used polymer electrolytes. In hydrogen fuel cells, it constitutes both the macroscopic membrane separating the anode and the cathode, and as a thin film, Nafion appears as a binder in the catalyst layer where conductive ionic pathways must intimately interface with platinum catalyst particles, electrically conductive carbon particles, and porous surfaces that facilitate the transport of gases. Residing at the intersection of this diverse range of materials, the ionomer's interfacial structure influences interfacial impedance and thus device performance. This interface structure has been widely investigated on model surfaces with neutron reflectometry and other techniques, resulting in the discovery of a multilamellar structure at the interface with hydrophilic materials, or a single water-rich layer at the interface with, e.g., metals, favoring tangential vs perpendicular ion transport, respectively. Here we demonstrate that self-assembled monolayers, SAMs, which can coat various surfaces, can control whether single or multiple lamellae occur. These interfacial structures can be further modified through acid-base interactions by protonating the terminal amine group of a SAM at low pH. This establishes a methodology to control the interfacial ionic transport pathways in Nafion and determine the interfacial impedance.