Proton transport mechanisms in aqueous acids: Insights from ab initio molecular dynamics simulations.

The solvation and transport of protons in aqueous solutions of phosphoric acid (PA), sulfuric acid (SA), and nitric acid (NA) were studied using ab initio molecular dynamics simulations. Systems with acid-to-water ratios of 1:1 and 1:3 were examined to understand the similarities and differences in transport mechanisms. The solvation structure of H3O+ in these systems is similar to that in slightly acidic water, with variations in the strength of hydrogen bonds (H-bonds) accepted by acid molecules.

Effect of Ion Mass on Dynamic Correlations in Ionic Liquids.

Ionic liquids (ILs) are a class of liquid salts with distinct properties such as high ionic conductivity, low volatility, and a broad electrochemical window, making them appealing for use in energy storage applications. The ion-ion correlations are some of the key factors that play a critical role in the ionic conductivity of ILs. In this work, we present the study of the impact of ion mass on ion-ion correlations in ILs, applying a combination of broadband dielectric spectroscopy measurements and molecular dynamics simulations.

Search for a Grotthuss mechanism through the observation of proton transfer.

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between 'chains' of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed.

Perspective: Morphology and ion transport in ion-containing polymers from multiscale modeling and simulations.

Ion-containing polymers are soft materials composed of polymeric chains and mobile ions. Over the past several decades they have been the focus of considerable research and development for their use as the electrolyte in energy conversion and storage devices. Recent and significant results obtained from multiscale simulations and modeling for proton exchange membranes (PEMs), anion exchange membranes (AEMs), and polymerized ionic liquids (polyILs) are reviewed.

Coarse-Grained Modeling of Ion-Containing Polymers.

Ion-containing polymers have continued to be an important research focus for several decades due to their use as an electrolyte in energy storage and conversion devices. Elucidation of connections between the mesoscopic structure and multiscale dynamics of the ions and solvent remains incompletely understood. Coarse-grained modeling provides an efficient approach for exploring the structural and dynamical properties of these soft materials.

Tuning proton conductivity and energy barriers for proton transfer.

Proton transport is critical for many technologies and for a variety of biochemical and biophysical processes. Proton transfer between molecules (via structural diffusion) is considered to be an efficient mechanism in highly proton conducting materials. Yet, the mechanism and what controls energy barriers for this process remain poorly understood.

Quantitative Evidence of Mobile Ion Hopping in Polymerized Ionic Liquids.

Atomistic molecular dynamics simulations were performed, and an extensive set of analyses were undertaken to understand the ion transport mechanism in the polymerized ionic liquid poly(CVIm)TfN. The ion hopping events were investigated at different time scales. Ion hopping was examined by monitoring the instantaneous cation-anion association and dissociation.

Proton Transfer in Phosphoric Acid-Based Protic Ionic Liquids: Effects of the Base.

Electronic structure calculations were performed to understand highly decoupled conductivities recently reported in protic ionic liquids (PILs). To develop a molecular-level understanding of the mechanisms of proton conductivity in PILs, minimum-energy structures of trimethylamine, imidazole, lidocaine, and creatinine (CRT) with the addition of one to three phosphoric acid (PA) molecules were determined at the B3LYP/6-311G** level of theory with the inclusion of an implicit solvation model (SMD with ε = 61). The proton affinity of the bases and zero-point energy corrected binding energies were computed at a similar level of theory.

Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume.

The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and X-ray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with -independent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in -independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations.

Direct Comparison of Atomistic Molecular Dynamics Simulations and X-ray Scattering of Polymerized Ionic Liquids.

The design of solid-state electrolytes for electrochemical applications that utilize polymerized ionic liquids (polyILs) would greatly benefit from a molecular-level understanding of structure-function relationships. We herein use atomistic molecular dynamics simulations to investigate the structural properties of a homologous series of poly(-alkyl-vinylimidzolium bistrifluoromethylsulfonylimide) poly(nVim Tf2N) and present the first direct comparison of the structure factors obtained from X-ray scattering and simulations. Excellent agreement is found in terms of peak position and shape.

Direct calculation of the X-ray structure factor of ionic liquids.

A conceptually simple and computationally efficient direct method to calculate the total X-ray structure factor of ionic liquids from molecular simulations is advocated to be complementary to the popular Fourier transform (FT) method. The validity of the direct method is well formulated and established by comparison with FT results. The effectiveness is demonstrated through versatile partition schemes using tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide P14,666 Tf2N as a model system.

Effect of Sulfuric and Triflic Acids on the Hydration of Vanadium Cations: An ab Initio Study.

Vanadium redox flow batteries (VRFBs) may be a promising solution for large-scale energy storage applications, but the crossover of any of the redox active species V(2+), V(3+), VO(2+), and VO2(+) through the ion exchange membrane will result in self-discharge of the battery. Hence, a molecular level understanding of the states of vanadium cations in the highly acidic environment of a VRFB is needed. We examine the effects of sulfuric and triflic (CF3SO3H) acids on the hydration of vanadium species as they mimic the electrolyte and functional group of perfluorosulfonic acid (PFSA) membranes.

Ab initio molecular dynamics simulations of water and an excess proton in water confined in carbon nanotubes.

Ab initio molecular dynamics simulations were performed to investigate the effects of nanoscale confinement on the structural and dynamical properties of water and slightly acidic water. Single-walled carbon nanotubes (CNTs) of two different diameters (11.0 and 13.

Ab initio molecular dynamics simulations of aqueous triflic acid confined in carbon nanotubes.

Ab initio molecular dynamics simulations were performed to investigate the effects of nanoscale confinement on the structural and dynamical properties of aqueous triflic acid (CF3SO3H). Single-walled carbon nanotubes (CNTs) with diameters ranging from ∼11 to 14 Å were used as confinement vessels, and the inner surface of the CNT were either left bare or fluorinated to probe the influence of the confined environment on structural and dynamical properties of the water and triflic acidic. The systems were simulated at hydration levels of n = 1-3 H2O/CF3SO3H.

Mesoscale modeling of hydrated morphologies of sulfonated polysulfone ionomers.

The hydrated morphologies of sulfonated poly(phenylene) sulfone (sPSO2) ionomers as a function of equivalent weight (EW), molecular weight (MW), and water content were investigated by using mesoscale dissipative particle dynamics (DPD) simulations. The morphological changes were characterized by analyzing the water distribution and plotting the radial distribution functions for the water particles. The results were compared to typical PFSA ionomers (i.

Electron energy loss spectroscopy of polytetrafluoroethylene: experiment and first principles calculations.

We have performed electron energy-loss spectroscopy (EELS) on a 200 kV transmission electron microscope (TEM) equipped with a monochromator to investigate molecular conformation of polytetrafluoroethylene (PTFE). The experimental spectra show several unique features in the low-loss region and the onset of carbon K-edge for PTFE. Density function theory (DFT) methods are employed to calculate the low-loss and core-loss spectra of PTFE with consideration of the effects of phase transitions, chain orientation and polarization.

Side chain flexibility in perfluorosulfonic acid ionomers: an ab initio study.

Side chain flexibility in perfluorosulfonic acid (PFSA) ionomers has been explored through ab initio electronic structure calculations. Three different PFSA side chain fragments were considered with a CF3CFCF3 backbone representation: Nafion (-OCF2CF(CF3)O(CF2)2SO3H), Aquivion or the short side chain (SSC) (-O(CF2)2SO3H), and the 3M PFSA (-O(CF2)4SO3H). Rotational potential energy surfaces for each bond along the length of the side chains were obtained using density functional theory with the B3LYP and the dispersion-corrected B97D functionals with and without the inclusion of a solvation model.

A macroscopic model of proton transport through the membrane-ionomer interface of a polymer electrolyte membrane fuel cell.

The membrane-ionomer interface is the critical interlink of the electrodes and catalyst to the polymer electrolyte membrane (PEM); together forming the membrane electrode assembly in current state-of-the-art PEM fuel cells. In this paper, proton conduction through the interface is investigated to understand its effect on the performance of a PEM fuel cell. The water containing domains at this interface were modeled as cylindrical pores/channels with the anionic groups (i.

Hydration and proton transfer in highly sulfonated poly(phenylene sulfone) ionomers: an ab initio study.

The need to operate proton exchange membrane fuel cells under hot and dry conditions has driven the synthesis and testing of sulfonated poly(phenylene) sulfone (sPSO(2)) ionomers. The primary hydration and energetics associated with the transfer of protons in oligomeric fragments of two sPSO(2)ionomers were evaluated through first-principles electronic structures calculations. Our results indicate that the interaction between neighboring sulfonic acid groups affect both theconformation and stability of the fragments.

The effect of hydrogen bond reorganization and equivalent weight on proton transfer in 3M perfluorosulfonic acid ionomers.

We present an investigation into the energetics associated with proton transfer in ionomeric fragments of the 3M™ perfluorosulfonic acid (PFSA) membrane at different equivalent weights (EW). Electronic structure calculations were performed on two fragments each with two pendant side chains separated along a poly(tetrafluoroethylene) (PTFE) backbone with chemical formula CF(3)CF(-O(CF(2))(4)SO(3)H)(CF(2))(n)CF(-O(CF(2))(4)SO(3)H)CF(3), where n = 5 or 7, corresponding to membrane equivalent weights of 590 and 690 g mol(-1). Potential energy surface (PES) scans were performed for the transfer of a proton in various hydrogen bonds between water molecules, sulfonic acid groups, and charged species.

The mechanism of proton conduction in phosphoric acid.

Neat liquid phosphoric acid (H(3)PO(4)) has the highest intrinsic proton conductivity of any known substance and is a useful model for understanding proton transport in other phosphate-based systems in biology and clean energy technologies. Here, we present an ab initio molecular dynamics study that reveals, for the first time, the microscopic mechanism of this high proton conductivity. Anomalously fast proton transport in hydrogen-bonded systems involves a structural diffusion mechanism in which intramolecular proton transfer is driven by specific hydrogen bond rearrangements in the surrounding environment.

A comparative ab initio study of the primary hydration and proton dissociation of various imide and sulfonic acid ionomers.

We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF(3)SO(2)NHSO(2)CF(3) and CF(3)CF(2)SO(2)NHSO(2)CF(3)), two partially fluorinated sulfonyl imides (CH(3)SO(2)NHSO(2)CF(3) and C(6)H(5)SO(2)NHSO(2)CF(2)CF(3)), and two aromatic sulfonic acid based materials (CH(3)C(6)H(4)SO(3)H and CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G** level of density functional theory.

Ab initio simulations of the effects of nanoscale confinement on proton transfer in hydrophobic environments.

Carbon nanotubes (CNTs) were functionalized with -CF(2)SO(3)H groups and hydrated with 1-3 water molecules per sulfonic acid group to investigate proton dissociation and transport in confined, hydrophobic environments. The distance between sulfonate groups was systematically varied from 6 to 8 Å, and three different CNTs were used to determine the effects of nanoscale confinement. The inner walls of the CNT were either functionalized with fluorine atoms to provide a localized negative charge or left bare to provide a more delocalized charge distribution.

Proton transport in triflic acid pentahydrate studied via ab initio path integral molecular dynamics.

Trifluoromethanesulfonic acid hydrates provide a well-defined system to study proton dissociation and transport in perfluorosulfonic acid membranes, typically used as the electrolyte in hydrogen fuel cells, in the limit of minimal water. The triflic acid pentahydrate crystal (CF(3)SO(3)H·5H(2)O) is sufficiently aqueous that it contains an extended three-dimensional water network. Despite it being extended, however, long-range proton transport along the network is structurally unfavorable and would require considerable rearrangement.