RESUMEN

Alternative solid electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft solid electrolyte, (Adpn)2LiPF6 (Adpn, adiponitrile), is synthesized and characterized that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A substantially high ion conductivity (~10-4 S cm-1) and lithium-ion transference number (0.54) are obtained due to weak interactions between 'hard' (charge dense) Li+ ions and the 'soft' (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li+ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy Ea and within the interstitial regions between the co-crystals with higher Ea values, where the bulk conductivity is a smaller but extant contribution. These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF6 by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.

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In this work, the possible mechanisms for the reactions of CO2 with various positional isomers of methylpiperidines (MPs) (N-MP, 2-MP, 3-MP, and 4-MP) including the effect of aqueous solvation have been explored using quantum chemical methods. The major pathways investigated for CO2 capture in aqueous amines are carbamate formation, its hydrolysis, and the bicarbonate formation (CO2 + H2O + MP) reaction. The calculations indicate that an axial orientation for the methyl group and an equatorial for the COO- group could be energetically ideal in the carbamate product of MPs. The proton abstraction step in the carbamate pathway is almost barrierless for the zwitterion-amine route, while a much higher energy barrier is observed for the zwitterion-H2O route. During carbamate hydrolysis, the addition of even two explicit water molecules does not exhibit any notable effect on the already high energy barrier associated with this reaction. This indicates that bicarbonate formation is less likely to occur via carbamate hydrolysis. The calculations suggest that, although the carbamate pathway is kinetically favored, the MP carbamate could still be a minor product, especially for sterically hindered conformations, and the bicarbonate pathway should be predominant in aqueous MPs.

RESUMEN

Glyme-based sodium electrolytes show excellent electrochemical properties and good chemical and thermal stability compared with existing carbonate-based battery electrolytes. In this investigation, we perform classical molecular dynamics (MD) simulations to examine the effect of concentration and temperature on ion-ion interactions and ion-solvent interactions via radial distribution functions (RDFs), mean residence time, ion cluster analysis, diffusion coefficients, and ionic conductivity in sodium hexafluorophosphate (NaPF6) salt in diglyme mixtures. The results from MD simulations show the following trends with concentration and temperature: The Na+---O(diglyme) interactions increase with concentration and decrease with temperature, while the Na+---F(PF6-) interactions increase with concentration and temperature. The mean residence time suggests that Na+---O(diglyme) are significantly longer lived compared with that of Na+---F(PF6-) and H (diglyme)---F(PF6-), which shows the affinity of diglyme to the Na+ ions. The ion cluster analysis suggests that the Na+ ions largely exist as solvated ions (coordinated to diglyme molecules), whereas some fractions exist as contact-ion pairs, and negligible fractions as aggregated ion pairs, with the latter two increasing slightly with temperature and more with ion concentration. The magnitude of the diffusion coefficients of Na+ and PF6- ions decreases with concentration and increases with temperature, where the Na+ ion has slightly lower mobility compared with the PF6- anion. The simulated total ionic conductivities show qualitative trends comparable to experimental data and highlight the need for the inclusion of ion-ion correlations in the Nernst-Einstein equation, especially at higher concentrations and lower temperatures.

RESUMEN

A new type of crystalline solid, termed "solvate sponge crystal", is presented, and the chemical basis of its properties are explained for a melt- and press-castable solid sodium ion conductor. X-ray crystallography and atomistic simulations reveal details of atomic interactions and clustering in (DMF)3NaClO4 and (DMF)2NaClO4 (DMF = N-N'-dimethylformamide). External pressure or heating results in reversible expulsion of liquid DMF from (DMF)3NaClO4 to generate (DMF)2NaClO4. The process reverses upon the release of pressure or cooling. Simulations reveal the mechanism of crystal "juicing," as well as melting. In particular, cation-solvent clusters form a chain of octahedrally coordinated Na+-DMF networks, which have perchlorate ions present in a separate sublattice space in 3 : 1 stoichiometry. Upon heating and/or pressing, the Na+⋯DMF chains break and the replacement of a DMF molecule with a ClO4 - anion per Na+ ion leads to the conversion of the 3 : 1 stoichiometry to a 2 : 1 stoichiometry. The simulations reveal the anisotropic nature of pressure induced stoichiometric conversion. The results provide molecular level understanding of a solvate sponge crystal with novel and desirable physical castability properties for device fabrication.

RESUMEN

In this investigation, we examine the effect of water concentration and temperature on the dynamical properties of [Hmim][Cl] and [Hmim][NTf2] ionic liquids (ILs). The dynamical properties such as translational diffusion coefficients, ion-pair lifetimes, and rotational correlation times are calculated using molecular dynamics simulations. The simulations predict that water concentration also significantly impacts the magnitude of dynamical properties. At low, intermediate and high water concentrations, the following trend in diffusion coefficients is seen: Cl- > Hmim+; Cl- > NTf2-; Hmim+ ([Hmim][Cl]) > Hmim+ ([Hmim] [NTf2]). At ultra-low water concentrations of [Hmim][Cl] IL, several bridge like configurations form between water molecules and Cl- anions, which are supported by a complex distribution of water clusters. The effect of an increase in the water concentration leads to a decrease in ion-pair lifetimes between the Hmim+ cations and Cl-/NTf2- anions, which strongly correlates with the trends observed from the diffusion coefficients. A biexponential function was found to be the best fit for the RACF at neat/ultra-low water concentrations of [Hmim][Cl] and [Hmim][NTf2] ILs, whereas a single exponential function was sufficient to fit the RACF at low, intermediate and high water concentrations. The rotational relaxation time of the Hmim+ cations is larger in neat [Hmim][Cl] compared to that in neat [Hmim][NTf2] with an opposite trend seen with hydration. The rotational correlation time of water molecules is larger in [Hmim][Cl] compared to that in [Hmim][NTf2] at low and intermediate water concentrations, with similar correlation times observed at high water concentrations.

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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.

Asunto(s)

RESUMEN

Hydrogen bonding in imidazole plays a key role in proton conduction and rotation of an imidazole molecule in the process results in the cleavage of hydrogen bonds between molecules. In the present work, we characterize proton transport and rotation energy barriers in imidazole chains by density functional theory. Our calculations show that propagation of an excess proton along the chain requires crossing of energy barriers, lower than 1 kcal/mol. The presence of the proton has stronger effect on the immediate neighboring imidazole molecules, and the effect is negligible after two molecules. The subsequent rotation of all imidazole molecules after the transfer of first proton is essential to allow the transfer the second proton. The presence of an excess proton in the chain leads to cleavage of hydrogen bonds and the rotation of neighboring imidazole molecule. Further, rotation of one imidazole molecule results in rotation of all molecules in the chain. The calculated rotational energy barriers in two-, three-, and four-imidazole-molecule chains are 8.0, 17.1, and 20.0 kcal, respectively, and are equivalent to the number of hydrogen bonds broken in the process. The rotational barrier is higher than the proton transport barrier along the hydrogen bond and, thus, is the rate-determining step of proton conduction.

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RESUMEN

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.

RESUMEN

Ammonium-based benzyl-NX3 (X = methyl, ethyl) trifluoromethanesulfonate (TFA) ionic liquids (ILs) are low cost, nontoxic, thermally stable ion-conducting electrolytes in fuel cells and batteries. In the present study, we have characterized the structure and dynamics of these ILs using molecular dynamics (MD) simulations and ionic conductivity using electro-chemical impedance spectroscopy (EIS) at varying temperature and relative humidity (RH). Results from MD simulations predict that cation-cation and cation-anion interactions are stronger in benzyltrimethylammonium (BzTMA) compared to benzyltriethylammonium (BzTEA) that diminish with increase in RH. Further, the BzTMA cations show both C-H/Ph (center of mass of phenyl ring) and cation-Ph interactions whereas BzTEA cations show only strong cation-Ph interactions. The C-H/Ph interactions (ψ ≥ 90°, d(H-Ph) ≤ 4 Å, θ < 50° and d(C-Ph) ≤ 4.3 Å) in BzTMA cations increase with RH and are highest at RH = 90%. The cumulative impact of electrostatic, cation/Ph, and C-H/Ph interactions results in lower conductivity of BzTMA-TFA IL compared to BzTEA-TFA IL. The EIS measurements show that the trends in ionic conductivity of ILs at RH = 30 and 90% are qualitatively similar to the Nernst-Einstein conductivity from MD simulations. The ionic conductivity of BzTEA-TFA IL is ~3 times higher than BzTMA-TFA IL at 353 K and RH = 90%.

RESUMEN

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.

RESUMEN

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.

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RESUMEN

Benzimidazole-based polymer membranes like poly(2,5-benzimidazole) (ABPBI) doped with phosphoric acid (PA) are electrolytes that exhibit high proton conductivity in fuel cells at elevated temperatures. The benzimidazole (BI) moiety is an important constituent of these membranes, so the present work was performed in order to achieve a molecular understanding of the BI-PA interactions in the presence of varying levels of the PA dopant, using classical molecular dynamics (MD) simulations. The various hydrogen-bonding interactions, as characterized based on structural properties and hydrogen-bond lifetime calculations, show that both BI and PA molecules exhibit dual proton-acceptor/donor functionality. An examination of diffusion coefficients showed that the diffusion of BI decreases with increasing PA uptake, whereas the diffusion of PA slightly increases. The hydrogen-bond lifetime calculations pointed to the existence of competitive hydrogen bonding between various sites in BI and PA.

RESUMEN

Calculations with density functional theory (DFT) and MP2 have been done to investigate the potential of recently synthesized durable zero-dimensional (OD) nitrogen-based cage structures to perform as efficient proton-exchange membranes (PEMs) in fuel cells. Our calculations suggest that the hydrogenated 0-D cages, in combination with hydrogen-bonding 1,2,3- and 1,2,4-triazole molecules, would perform as highly efficient PEMs. The results are important in the context of the need for efficient PEMs for fuel cells, especially at higher temperatures (greater than 120 °C) where conventional water-based PEMs such as Nafion have been found to be ineffective.

RESUMEN

Phosphoric acid doped polybenzimidazole is promising electrolyte membranes for high temperature (100 °C and above) fuel cells. Proton conduction is governed by the amount of phosphoric acid content in the polymer membrane. In this present work, we perform molecular dynamics simulations on phosphoric acid doped 2-phenyl-1H,1'H-5,5'-bibenzo[d]imidazole (monomer unit of polybenzimidazole) to characterize the structural and dynamical properties at varying phosphoric acid content and temperature. From the structural analysis, we have predicted the arrangement of the phosphoric acids, formation of H-bonds in the system, and the contribution of different atoms toward H-bonding. We have also examined the stacking of 2-phenyl-1H,1'H-5,5'-bibenzo[d]imidazole molecules and how their arrangement changes with the increasing amount of PA in the system with the help of cluster analysis. From the molecular dynamics simulation conducted at different temperatures and phosphoric acid doping level, we have predicted the diffusion of phosphoric acid and monomer. As a dynamic quantity, we have also calculated ring flipping of the imidazole ring of the monomer.

RESUMEN

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.

RESUMEN

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.

RESUMEN

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 CF(3) 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 SO(3)(-) 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.

RESUMEN

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.

Asunto(s)

RESUMEN

The effects of hydration level and temperature on the nanostructure of an atomistic model of a Nafion (DuPont) membrane and the vehicular transport of hydronium ions and water molecules were examined using classical molecular dynamics simulations. Through the determination and analysis of structural and dynamical parameters such as density, radial distribution functions, coordination numbers, mean square deviations, and diffusion coefficients, we identify that hydronium ions play an important role in modifying the hydration structure near the sulfonate groups. In the regime of low level of hydration, short hydrogen bonded linkages made of water molecules and sometimes hydronium ions alone give a more constrained structure among the sulfonate side chains. The diffusion coefficient for water was found to be in good accord with experimental data. The diffusion coefficient for the hydronium ions was determined to be much smaller (6-10 times) than that for water. Temperature was found to have a significant effect on the absolute value of the diffusion coefficients for both water and hydronium ions.

RESUMEN

This paper details the multiscale methodology developed to analyze the formation of nanoparticles in a manner that makes it possible to follow the evolution of the structures in a chemically specific way. The atomistic model for particle inception code that combines the strengths of kinetic Monte Carlo and molecular dynamics is used to study the chemical and physical properties of nanoparticles generated in a premixed fuel-rich benzene flame, providing atomistic scale structures (bonds, bond angles, dihedral angles) as soot precursors evolve into a three-dimensional structure. Morphology, density, porosity, and other physical properties are computed. Two heights corresponding to two different times in the benzene flame, experimentally studied by Bittner and Howard [Proc. Combust. Inst. 18, 1105 (1981)], were chosen to examine the influence of different environments on structural properties of the particles formed.