Ab initio molecular dynamics simulation of proton hopping in a model polymer membrane.

We report the results of ab initio molecular dynamics simulations of a model Nafion polymer membrane initially equilibrated using classical molecular dynamics simulations. We studied three hydration levels (λ) of 3, 9, and 15 H2O/SO3(-) corresponding to dry, hydrated, and saturated fuel cell membrane, respectively. The barrier for proton transfer from the SO3(-)-H3O(+) contact ion pair to a solvent-separated ion pair decreased from 2.3 kcal/mol for λ = 3 to 0.8 kcal/mol for λ = 15. The barrier for proton transfer between two water molecules was in the range from 0.7 to 0.8 kcal/mol for the λ values studied. The number of proton shuttling events between a pair of water molecules is an order of magnitude more than the number of proton hops across three distinct water molecules. The proton diffusion coefficient at λ = 15 is about 0.9 × 10(-5) cm(2)/s, which is in good agreement with experiment and our previous quantum hopping molecular dynamics simulations.

Molecular structure and transport dynamics in perfluoro sulfonyl imide membranes.

We report a detailed and comprehensive analysis from classical molecular dynamics simulations of the nanostructure of a model of hydrated perfluoro sulfonyl imide (PFSI) membrane, a polymeric system of interest as a proton conductor in polymer electrolyte membrane fuel cells. We also report on the transport dynamics of water and hydronium ions, and water network percolation in this system. We find that the water network percolation threshold for PFSI, i.e. the threshold at which a consistent spanning water network starts to develop in the membrane, is found to occur between hydration levels (λ) 6 and 7. The higher acidity of the sulfonyl imide acid group of PFSI compared to the sulfonic acid group in Nafion, as computationally characterized in our earlier ab initio study (Idupulapati et al 2010 J. Phys. Chem. A 114 6904-12), results in a larger fraction of 'free' hydronium ions at low hydration levels in PFSI compared to Nafion. However, the calculated diffusion coefficients of the H(3)O(+) ions and H(2)O molecules as a function the hydration level are observed to be almost the same as that of Nafion, indicating similar conductivity and consistent with experimental data.

Atomistic simulations of perfluoro phosphonic and phosphinic acid membranes and comparisons to Nafion.

We used classical molecular dynamics simulations to investigate the morphology and proton transport properties of perfluoro phosphonic (FPA) and phosphinic acid (FPA-I) membranes that have potential applications in low-temperature fuel cells. We systematically investigated these properties as a function of the hydration level. We examined changes in structure, transport dynamics of water and hydronium ions, and water network percolation relative to those in Nafion membrane to examine the effect of functional group acidity on these properties. Phosphonic and phosphinic acid moieties in FPA and FPA-I have lower acidity than sulfonic acid in Nafion, yet the diffusion of water was faster in FPA and FPA-I than in Nafion, particularly at low hydration levels. However this did not give rise to notable differences in hydronium ion diffusion and water network percolation for these membranes over Nafion. These results, along with similar findings from our recent study of perfluoro-sulfonyl imide membranes carrying stronger superacids than the sulfonic acid of Nafion, suggest that there is no strong correlation between the acidity of the functional groups and the dynamics of water and hydronium ions in hydrated polymer electrolyte membranes with similar fluorocarbon backbones and side chains.

Ab initio study of hydration and proton dissociation in ionomer membranes.

We present a comparative study of proton dissociation in various functional acidic units that are promising candidates as building blocks for polymeric electrolyte membranes. Minimum energy structures for four acidic moieties with clusters of 1-6 water molecules were determined using density functional theory at the B3LYP/6-311G** level starting from chemically rational initial configurations. The perfluoro sulfonyl imide acid group (CF(3)CF(2)SO(2)NHSO(2)CF(3)) was observed to be the strongest acid, due to the substantial electron withdrawing effect of both fluorocarbon groups. The hydrophilic functional group (CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H) of sulfonated polyetherether ketone (SPEEK) membrane was found to be the strongest base, with the acidic proton dissociation requiring the addition of six water molecules and the hydrated proton being more tightly bound to the conjugate base. Even though both perfluoro sulfonyl imides and sulfonic acids (hydrophilic functional groups for sulfonyl imide and Nafion ionomers, respectively) required only three water molecules to exhibit spontaneous proton dissociation, the largest possible solvent-separated hydronium ion was attained only for the sulfonyl imide moiety. These results provide a rationale for the enhanced conductivity of perfluorinated sulfonyl imide-based membranes relative to that of the widely used Nafion membrane.

Quantum chemical modeling of methanol oxidation mechanisms by methanol dehydrogenase enzyme: effect of substitution of calcium by barium in the active site.

Previous experimental studies have shown that the activation energy for methanol oxidation by naturally occurring Ca(2+)-containing methanol dehydrogenase (MDH) enzyme is double the methanol activation energy by Ba(2+)-MDH. However, neither the reason for this difference nor the specific transition states and intermediates involved during the methanol oxidation by Ba(2+)-MDH have been clearly stated. Hence, an MDH active site model based on the well-documented X-ray crystallographic structure of Ca(2+)-MDH is selected, where the Ca(2+) is replaced by a Ba(2+) ion at the active site center, and the addition-elimination (A-E) and hydride-transfer (H-T) methanol oxidation mechanisms, already proposed in the literature for Ca(2+)-MDH, are tested for Ba(2+)-MDH at the BLYP/DNP theory level. Changes in the geometries and energy barriers for all the steps are identified, and qualitatively, similar (when compared to Ca(2+)-MDH) intermediates and transition states associated with each step of the mechanisms are found in the case of Ba(2+)-MDH. For both the A-E and H-T mechanisms, almost all the free-energy barriers associated with all of the steps are reduced in the presence of Ba(2+)-MDH, and they are kinetically feasible. The free energy barriers for methanol oxidation by Ba(2+)-MDH, particularly for the rate-limiting steps of both mechanisms, are almost half the corresponding barriers calculated for the case of Ca(2+)-MDH, which is in agreement with experimental observations.