Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized Carbon Nanofibers and Their Thermal Stabilities

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 (Keq , derived from Langmuir isotherm), the ionomer has varying affinities for CNFs (Keq between 5 and 22) as compared to Vulcan (Keq = 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 −CF2– 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 Keq 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 (ΓSmax) 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.