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.