Self-Assembly and TransportLimitations in ConfinedNafion Films
2016
Ion-conducting polymers are important materials for a
variety of
electrochemical applications. Perfluorinated ionomers, such as Nafion,
are the benchmark materials for proton conduction and are widely used
in fuel cells and other electrochemical devices including solar-fuel
generators, chlor-alkali cells, and redox flow batteries. While the
behavior of Nafion in bulk membranes (10 to 100s μm thick) has
been studied extensively, understanding its properties under thin-film
confinement is limited. Elucidating the behavior of thin Nafion films
is particularly important for the optimization of fuel-cell catalyst
layers or vapor-operated solar-fuel generators, where a thin film
of ionomer is responsible for the transport of ions to and from the
active electrocatalytic centers. Using a combination of transport-property
measurements and structural characterization, this work demonstrates
that confinement of Nafion in thin films induced thickness-dependent
proton conductivity and ionic-domain structure. Confining Nafion films
to thicknesses below 50 nm on a silicon substrate results in a loss
of microphase separation of the hydrophilic and hydrophobic domains,
which drastically increases the material’s water uptake while
in turn decreasing its ionic conductivity.
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