Arun Venkatnathan

Atomistic simulation of water percolation and proton hopping in Nafion fuel cell membrane.

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H(2)O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (λ) between 5 and 6 H(2)O/SO(3)(-) as the percolation threshold of Nafion. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the λ value is increased from 5 to 15. The proton diffusion coefficient in Nafion at a λ value of 15 is about 1.1 × 10(-5) cm(2)/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity.

Stability and Reactivity of Methane Clathrate Hydrates: Insights from Density Functional Theory

The sI methane clathrate hydrate consists of methane gas molecules encapsulated as dodecahedron (5(12)CH(4)) and tetrakaidecahedron (5(12)6(2)CH(4)) water cages. The characterization of the stability of these cages is crucial to an understanding of the mechanism of their formation. In the present work, we perform calculations using density functional theory to calculate interaction energies, free energies, and reactivity indices of these cages. The contributions from polarization functions to interaction energies is more than diffuse functions from Pople basis sets, though both functions from the correlation-consistent basis sets contribute significantly to interaction energies. The interaction energies and free energies show that the formation of the 5(12)CH(4) cage (from the 5(12) cage) is more favored compared to the 5(12)6(2)CH(4) cage (from the 5(12)6(2) cage). The pressure-dependent study shows a spontaneous formation of the 5(12)CH(4) cage at 273 K (P ≥ 77 bar) and the 5(12)6(2)CH(4) cage (P = 100 bar). The reactivity of the 5(12)CH(4) cage is similar to that of the 5(12) cage, but the 5(12)6(2)CH(4) cage is more reactive than the 5(12)6(2) cage.

Raman spectra of vibrational and librational modes in methane clathrate hydrates using density functional theory

The sI type methane clathrate hydrate lattice is formed during the process of nucleation where methane gas molecules are encapsulated in the form of dodecahedron (5(12)CH(4)) and tetrakaidecahedron (5(12)6(2)CH(4)) water cages. The characterization of change in the vibrational modes which occur on the encapsulation of CH(4) in these cages plays a key role in understanding the formation of these cages and subsequent growth to form the hydrate lattice. In this present work, we have chosen the density functional theory (DFT) using the dispersion corrected B97-D functional to characterize the Raman frequency vibrational modes of CH(4) and surrounding water molecules in these cages. The symmetric and asymmetric C-H stretch in the 5(12)CH(4) cage is found to shift to higher frequency due to dispersion interaction of the encapsulated CH(4) molecule with the water molecules of the cages. However, the symmetric and asymmetric O-H stretch of water molecules in 5(12)CH(4) and 5(12)6(2)CH(4) cages are shifted towards lower frequency due to hydrogen bonding, and interactions with the encapsulated CH(4) molecules. The CH(4) bending modes in the 5(12)CH(4) and 5(12)6(2)CH(4) cages are blueshifted, though the magnitude of the shifts is lower compared to modes in the high frequency region which suggests bending modes are less affected on encapsulation of CH(4). The low frequency librational modes which are collective motion of the water molecules and CH(4) in these cages show a broad range of frequencies which suggests that these modes largely contribute to the formation of the hydrate lattice.

Molecular dynamics simulations of side chain pendants of perfluorosulfonic acid polymer electrolyte membranes

Perfluorosulfonic acid (PFSA) polymer electrolyte membranes like Dow, Aciplex and Nafion have similar backbones but different side chain pendants. The effect of hydration and temperature on the side chain pendant nanostructure, and water and hydronium ion dynamics, are investigated by employing classical molecular dynamics simulations at 300 K and 350 K. The 60% longer side chain pendant length in Aciplex compared to Dow results in phase segregation. The presence of an extra ether oxygen atom in the Nafion side chain pendant provides more flexibility (∼20% chain length contraction caused by flexibility and the hydrophobic force of the pendant CF3 group) where the sulfonate group tends to drift from the hydrophilic–hydrophobic domain, which gives rise to a hydrosphere region at higher hydration. The calculated structure factors and scattering intensities reproduce features of SANS and SAXS profiles for Dow and Nafion, and confirm the existence of spherical water aggregates in the rod shaped pendant nanostructure of Nafion. The effect of hydration on the mobility of hydronium ions at 300 K in Nafion is insignificant at higher hydration (λ ≥ 9), and trends are in agreement with experimental data. The activation energy of the diffusion of hydronium ions and water molecules in Nafion side chain pendant–water mixtures (14–25 kJ mol−1) validate experimental observations (16–22 kJ mol−1).

Molecular dynamics simulations of triflic acid and triflate ion/water mixtures: A proton conducting electrolytic component in fuel cells

Triflic acid is a functional group of perflourosulfonated polymer electrolyte membranes where the sulfonate group is responsible for proton conduction. However, even at extremely low hydration, triflic acid exists as a triflate ion. In this work, we have developed a force‐field for triflic acid and triflate ion by deriving force‐field parameters using ab initio calculations and incorporated these parameters with the Optimized Potentials for Liquid Simulations ‐ All Atom (OPLS‐AA) force‐field. We have employed classical molecular dynamics (MD) simulations with the developed force field to characterize structural and dynamical properties of triflic acid (270–450 K) and triflate ion/water mixtures (300 K). The radial distribution functions (RDFs) show the hydrophobic nature of CF3 group and presence of strong hydrogen bonding in triflic acid and temperature has an insignificant effect. Results from our MD simulations show that the diffusion of triflic acid increases with temperature. The RDFs from triflate ion/water mixtures shows that increasing hydration causes water molecules to orient around the SO3− group of triflate ions, solvate the hydronium ions, and other water molecules. The diffusion of triflate ions, hydronium ion, and water molecules shows an increase with hydration. At λ = 1, the diffusion of triflate ion is 30 times lower than the diffusion of triflic acid due to the formation of stable triflate ion–hydronium ion complex. With increasing hydration, water molecules break the stability of triflate ion–hydronium ion complex leading to enhanced diffusion. The RDFs and diffusion coefficients of triflate ions, hydronium ions and water molecules resemble qualitatively the previous findings using per‐fluorosulfonated membranes.

Application of higher order decouplings of the dilated electron propagator to 2Π CO−, 2Πg N2− and 2Πg C2H2− shape resonances

The full third order (Σ3), quasi-particle third order (Σq3) and outer valence Green’s function (OVGF-A) decouplings of the bi-orthogonal dilated electron propagator have been implemented and results from their application to 2Π CO−, 2Πg N2−, and 2Πg C2H2− shape resonances are presented and compared with energies and widths obtained using the zeroth order (Σ0), quasiparticle second order (Σq2) and second order (Σ2) decouplings. The energies and widths from the various Σ3 decouplings for shape resonances are close to those obtained using the Σ2 approximant but the corresponding Feynman–Dyson amplitudes (FDAs) differ considerably. The differences between FDAs from different decouplings are analyzed to elicit the role of correlation and relaxation in the formation and decay of shape resonances.

Molecular Simulations of Anion and Temperature Dependence on Structure and Dynamics of 1-Hexyl-3-methylimidazolium Ionic Liquids

In this study, we examine the effect of various anions and temperature on structure and dynamics of 1-hexyl-3-methylimidazolium ionic liquids (ILs) from molecular dynamics simulations. The structural properties show that ILs containing smaller anions like Cl(-) and Br(-) are relatively higher cation-anion interactions, compared to ILs containing larger anions like OTf(-) and NTf2(-). In all ILs, the spatial distribution of anions is closer to the acidic hydrogen atom of the cation compared to the two nonacidic hydrogen atoms of the cation. The diffusion coefficients of cations and anions (ionic conductivity) increase with anionic size. At each temperature, the cationic and anionic diffusions and ionic conductivity are lowest in ILs containing anions like Cl(-) and Br(-) and highest in ILs containing anions like BF4(-), OTf(-), and NTf2(-). Consistent with experiments, simulations predict that ILs with an intermediate size BF4(-) anion show the highest cationic and anionic diffusion (and ionic conductivity). At each temperature, the interactions between ion pairs of each IL show that a decrease in ion-pair lifetimes is directly related to the increase in diffusion coefficients and conductivity in ILs, suggesting that characterization of ion-pair lifetimes is sufficient to validate the trends seen in dynamical properties of ILs.

Interplay of Phase Separation, Tail Aggregation, and Micelle Formation in the Nanostructured Organization of Hydrated Imidazolium Ionic Liquid

A molecular investigation on the effect of water on structural properties of imidazolium-based ionic liquids (ILs) is essential due to its various industrial applications. In this work, we employ molecular dynamics simulations to characterize the influence of various water concentrations on nanostructural properties of the 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Hmim][NTf2] IL. An examination of molecular interactions in [Hmim][NTf2] IL-water mixtures shows the following trends: (a) At low water concentration, small regions of water molecules are surrounded by several cation-anion pairs. (b) At medium water concentration, cation tail aggregation starts, and phase separation between the IL and water is observed. (c) At high water concentration, increasing cationic tail aggregation leads to micelle formation. Further aggregates of cations and anions are solvated by large water channels. The radial distribution functions show that cation-anion, cation-cation, and anion-anion interactions decrease and water-water interaction increases with water concentration. The hydrogen bonding interactions occur between the acidic hydrogen of the positively charged imidazolium cation with the nitrogen and oxygen atoms of the anions. However, no hydrogen bonding interactions are seen between water molecules and the hydrophobic anions.

Mechanism of proton transport in ionic-liquid-doped perfluorosulfonic acid membranes.

Ionic-liquid-doped perfluorosulfonic acid membranes (PFSA) are promising electrolytes for intermediate/high-temperature fuel cell applications. In the present study, we examine proton-transport pathways in a triethylammonium-triflate (TEATF) ionic liquid (IL)-doped Nafion membrane using quantum chemistry calculations. The IL-doped membrane matrix contains triflic acid (TFA), triflate anions (TFA(-)), triethylamine (TEA), and triethylammonium cations (TEAH(+)). Results show that proton abstraction from the sulfonic acid end groups in the membrane by TFA(-) facilitates TEAH(+) interaction with the side-chains. In the IL-doped PFSA membrane matrix, proton transfer from TFA to TEA and TFA to TFA(-) occurs. However, proton transfer from a tertiary amine cation (TEAH(+)) to a tertiary amine (TEA) does not occur without an interaction with an anion (TFA(-)). An anion interaction with the amine increases its basicity, and as a consequence, it takes a proton from a cation either instantly (if the cation is freely moving) or with a small activation energy barrier of 2.62 kcal/mol (if the cation is interacting with another anion). The quantum chemistry calculations predict that anions are responsible for proton-exchange between cations and neutral molecules of a tertiary amine. Results from this study can assist the experimental choice of IL to provide enhanced proton conduction in PFSA membrane environments.

Vibrational Raman spectra of hydrogen clathrate hydrates from density functional theory

Hydrogen clathrate hydrates are promising sources of clean energy and are known to exist in a sII hydrate lattice, which consists of H2 molecules in dodecahedron (5(12)) and hexakaidecahedron (5(12)6(4)) water cages. The formation of these hydrates which occur in extreme thermodynamic conditions is known to be considerably reduced by an inclusion of tetrahydrofuran (THF) in cages of these hydrate lattice. In this present work, we employ the density functional theory with a dispersion corrected (B97-D) functional to characterize vibrational Raman modes in the cages of pure and THF doped hydrogen clathrate hydrates. Our calculations show that the symmetric stretch of the H2 molecule in the 5(12)6(4)H2·THF cage is blueshifted compared to the 5(12)6(4)H2 cage. However, all vibrational modes of water molecules are redshifted which suggest reduced interaction between the H2 molecule and water molecules in the 5(12)6(4)H2·THF cage. The symmetric and asymmetric O-H stretch of water molecules in 5(12)H2, 5(12)6(4)H2, and 5(12)6(4)H2·THF cages are redshifted compared with the corresponding guest free cages due to interactions between encapsulated H2 molecules and water molecules of the cages. The low frequency modes contain contributions from contraction and expansion of water cages and vibration of water molecules due to hydrogen bonding and these modes could possibly play an important role in the formation of the hydrate lattice.