Adsorption Behavior of Nafion Ionomers on Au and Pt Surfaces
Nafion did not adsorb on the gold surface at 0 V (vs. Ag/AgCl) and started to adsorb at 0.45 V in a lying-down configuration. The adsorbed amount increased as the potential was made more positive. In the potential range between 0.75 and 1.0 V, Nafion aggregated at the surface. Those aggregated Nafion desorbed from the surface by keeping the potential at 0 V.
Nafion also desorbed from the surface upon the gold oxide formation. When the gold oxide was reduced, Nafion re-adsorbed on the surface. Thus, adsorption behavior of Nafion is similar to that of the sulfate anion, implying that sulfonate groups of Nafion play an important role in adsorption of Nafion on gold surfaces.
In contrast, Nafion adsorbed on the platinum surface without any significant morphology change in the potential range between 0.1 and 0.8 V despite the fact that the adsorbed amount of the sulfate anion increases as the potential goes more positive. When the potential was kept at 1.1 V where the place-exchange oxide is formed, Nafion desorbed from the surface. Once the platinum oxide was reduced, Nafion re-adsorbed on the surface.
The interaction of fluorocarbon chains on a bare platinum surface and the desorption of Nafion upon the oxide formation were confirmed by surface x-ray scattering in an MEA-like configuration.
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