Adsorption behavior of perfluorinated sulfonic acid ionomer on highly graphitized carbon nanofibers and their thermal stabilities

Shuang Ma Andersen, Maryam Borghei, Rajnish Dhiman, Virginia Ruiz, Esko Kauppinen, Eivind Morten Skou

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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 19 fluorine nuclear magnetic resonance spectroscopy. Based on 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 one order of magnitude higher than the one of Vulcan. The large discrepancy is due to the fact that the ionomers are inaccessible to the internal surface area of Vulcan with high micro porosity.
Original languageEnglish
JournalThe Journal of Physical Chemistry Part C
Volume118
Issue number20
Pages (from-to)10814–10823
Number of pages10
ISSN1932-7447
DOIs
Publication statusPublished - 28. Apr 2014

Keywords

  • Adsorption
  • carbon nanofibers
  • Nafion ionomer
  • PEM fuel cells
  • thermal stability

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