Proton Transfer in Perfluorosulfonic Acid Functionalized Carbon Nanotubes
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Proton Transfer in Perfluorosulfonic Acid Functionalized Carbon Nanotubes Bradley F. Habenicht,1 Stephen J. Paddison,1 and Mark E. Tuckerman2 1
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, 2
Department of Chemistry, Courant Institute of Mathematical Sciences, New York University, New York, NY, U.S.A. 10003
ABSTRACT Proton dissociation and transfer are investigated with ab initio molecular dynamics (AIMD) simulations of carbon nanotubes (CNT) functionalized with perfluorosulfonic acid (-CF2SO3H) groups with 3 H2O/–SO3H. The CNT systems were constructed both with and without fluorine atoms covalently bound to the inner walls to determine the effects of the presence of fluorine on proton dissociation, hydration, and stabilization. The results of the AIMD trajectories show that decreasing the separation of sulfonic acid groups increases the propensity for proton dissociation. The simulations also revealed that the dissociated proton was preferentially stabilized as a hydrated hydronium (H3O+) cation in the CNT systems with the fluorine. This feature is attributed to the fluorine atoms providing a localized negative charge that promotes hydrogen bonding of the water molecules coordinated to the central hydronium ion. The hydrated H3O+ ion differed from a traditional Eigen cation (H9O4+) as it donated hydrogen bonds to sulfonate oxygen atoms, as well as water molecules.
INTRODUCTION Proton exchange membrane (PEM) fuel cells utilize an electrolyte material to separate the electrodes of the device, isolate the reactant and product gases, and provide a medium for proton conduction. The most common PEMs are based on perfluorosulfonic acid (PFSA) ionomers which consist of a hydrophobic polytetrafluoroethylene (PTFE) backbone functionalized with hydrophilic sulfonic acid (–SO3H) terminated side chains. These materials phase separate into hydrophobic and hydrophilic domains when hydrated. The characteristic dimensions of these domains are only a few nanometers, with the long range transport of protons occurring in the hydrophilic domains.1 PFSA ionomers require high water contents for sufficient proton conductivity and hence PEM fuel cells must operate under high humidity conditions, with water being introduced at the anode. Due to difficulties with water management and catalysis of the fuels at the electrodes, there is substantial need to operate the device at temperatures greater than 100 °C and under low humidity conditions.2 At very low levels of hydration, however, the protons that dissociated from the SO3H groups are not fully solvated by the water. This minimal hydration of the dissociated protons complicates proton transport as there is substantial
interaction between the acidic protons and the tethered sulfonate ions (–CF2SO3–), which are the conjugate bases to the dissociated protons.3 The structure PFSA ionomers and the transport of species within them have been simulated using a variety of methods including: phenomological,4-6 classical
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