Proton Hopping in Phosphoric Acid Solvated Nafion Membrane: A Molecular Simulation Study

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• 作者：Liuming Yan ; Suhua Zhu ; Xiaobo Ji ; Wencong Lu
• 刊名：Journal of Physical Chemistry B
• 出版年：2007
• 出版时间：June 14, 2007
• 年：2007
• 卷：111
• 期：23
• 页码：6357 - 6363
• 全文大小：651K
• 年卷期：v.111,no.23(June 14, 2007)
• ISSN：1520-5207

文摘

Molecular simulation studies of the microstructure and of the proton transport properties of phosphoric acidsolvated Nafion membrane are carried out. The ab initio calculations show that the phosphoric acid is a goodsolvent to promote the proton ionization of the sulfonic acid group, and only two phosphoric acid moleculesare necessary for the dissociation of one sulfonic acid group. A mechanism of proton hopping betweenphosphoric acid and protonated phosphoric acid cation in the hydrophilic subphase is also elucidated by abinitio calculations. The molecular dynamics simulations, conducted at a phosphoric acid concentration of25.4% (wt) which is slightly lower than that of phosphoric acid swollen Nafion, show that the phosphoricacid exists in subphases and that it cannot develop into a continuous subphase. Thus, proton-hopping pathwaysare interrupted, and the conductivity is expected to be lower than that for pure phosphoric acid. The moleculardynamics simulations, conducted at a phosphoric acid concentration of 45.1% (wt) which corresponds to anunstable state, show that the hydrophobic poly(tetrafluoroethylene) backbones trend to gather together forminghydrophobic clusters and that the phosphoric acid forms a continuous subphase with the sulfonic acid groupslocated at the hydrophobic/hydrophilic interface. Thus, proton-hopping pathways can develop uninterruptedlylike the pure phosphoric acid, and high conductivity is expected. The molecular dynamics study also showsthat the hydrogen-bonding characteristics of phosphoric acid and sulfonate anion are similar regardless of thefactor that the former can move freely while the latter is attached to Nafion backbone.