Adsorption
Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized
Carbon Nanofibers and Their Thermal Stabilities
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Abstract
A systematic
adsorption study of perfluorinated sulfonic acid Nafion ionomer on
ribbon-type highly graphitized carbon nanofibers (CNFs) was carried
out using fluorine-19 nuclear magnetic resonance spectroscopy. On
the basis of the values obtained for the equilibrium constant (<i>K</i><sub>eq </sub>, derived from Langmuir isotherm), the
ionomer has varying affinities for CNFs (<i>K</i><sub>eq </sub>between 5 and 22) as compared to Vulcan (<i>K</i><sub>eq </sub> = 18), depending on surface treatments. However, the interactions
are most likely governed by different adsorption mechanisms depending
on hydrophilicity/hydrophobicity of the adsorbent carbon. The ionomer
is probably adsorbed via the polar sulfonic group on hydrophilic Vulcan,
whereas it is adsorbed primarily via hydrophobic −CF<sub>2</sub>– backbone on the highly hydrophobic pristine CNFs. Ionomer
adsorption behavior is gradually altered from apolar to polar group
adsorption for the acid-modified CNFs of decreasing hydrophobicity.
This is indicated by the initial decrease and then increase in the
value of <i>K</i><sub>eq </sub>with the increasing
strength of the acid treatment. The corresponding carbon–ionomer
composite also showed varying thermal stability depending on Nafion
orientation. The specific maximum surface coverage (Γ<sub>Smax</sub>) of the CNFs is 1 order of magnitude higher than that of Vulcan.
The large discrepancy is due to the fact that the ionomers are inaccessible
to the internal surface area of Vulcan with high microporosity