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. We examined three ILs with the same cation but different anions to consider three different cases of cation-anion masses: M+ > M-, M+ ≈ M-, and M+ < M-. We applied the momentum conservation approach to estimate the contribution of distinct ion-ion correlations from experimental data and obtained good agreement with direct calculations of distinct ion-ion correlations from molecular dynamics simulations. Our findings reveal that relative ion mass has a strong effect on the distinct ion-ion correlations, leading to swapping of the relative amplitude of distinct cation-cation and anion-anion correlations.

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. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5-0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

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. The interplay of morphology and ion transport is emphasized. We discuss the influences of polymer architecture, tethered ionic groups, rigidity of the backbone, solvents, and additives on both morphology and ion transport in terms of specific interactions. Novel design strategies are highlighted including precisely controlling molecular conformations to design highly ordered morphologies; tuning the solvation structure of hydronium or hydroxide ions in hydrated ion exchange membranes; turning negative ion-ion correlations to positive correlations to improve ionic conductivity in polyILs; and balancing the strength of noncovalent interactions. The design of single-ion conductors, well-defined supramolecular architectures with enhanced one-dimensional ion transport, and the understanding of the hierarchy of the specific interactions continue as challenges but promising goals for future research.

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. The unique physicochemical properties of such polymers are of broad interest. In this review, we summarize the current development and understanding of the structure-property relationship of ion-containing polymers and provide insights into the design of such materials determined from coarse-grained modeling and simulations accompanying significant advances in experimental strategies. We specifically concentrate on three types of ion-containing polymers: proton exchange membranes (PEMs), anion exchange membranes (AEMs), and polymerized ionic liquids (polyILs). We posit that insight into the similarities and differences in these materials will lead to guidance in the rational design of high-performance novel materials with improved properties for various power source technologies.

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(C2VIm)Tf2N. The ion hopping events were investigated at different time scales. Ion hopping was examined by monitoring the instantaneous cation-anion association and dissociation. Ion diffusion was subsequently evaluated with correlation functions and the calculation of relaxation times at different time scales. Dynamical heterogeneity in the mobility of the ions was observed with only a small portion of the anions classified as fast mobile ions. The mobile ions were characterized as the ones traveling farther than a certain distance during a characteristic period, which was much longer than the time scale of the instant ion pair dissociation. Effective hopping of the mobile ions contributed to the diffusivity which was dominated by interchain hopping and generally facilitated with five associating cations from two different polymer chains. Mobile anions had relatively fewer associating cations from more associating chains than immobile anions. The stringlike cooperative motion was observed in the mobile anions. The string length was determined to decrease with increasing temperature. These findings provided an in-depth understanding of the ion transport in polymerized ionic liquids and important information for the rational design of novel 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. It was shown that mixing phosphoric acid (PA) with lidocaine leads to an increase in proton conductivity at the same liquid viscosity. However, recent simulations of mixtures of PA with various bases, including lidocaine, suggested no decrease in the proton transfer energy barrier. To elucidate this surprising result, we have performed broadband dielectric spectroscopy to verify the predictions of the simulations for mixtures of PA with several bases. Our results reveal that adding bases to PA increases the energy barriers for proton transfer, and the observed increase in proton conductivity at a similar viscosity appears to be related to the increase in the glass transition temperature (Tg) of the mixture. Moreover, the energy barrier seems to increase with Tg of the mixtures, emphasizing the importance of molecular mobility or interactions in the proton transfer mechanism.

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. Proton dissociation from PA occurs in all systems, resulting in the formation of ion pairs due to the relatively strong basicity of the bases (proton acceptors) and the effect of the high dielectric constant solvent in stabilizing the charge separation. The second and third PA molecules preferentially form "ring-like" hydrogen bonds with one another instead of forming hydrogen bonds at the donor and acceptor sites of the bases. Potential energy scans reveal that the bases with stronger proton affinity exert greater influence on the energetics of proton transfer between the individual PA molecules. However, the effects are minimal when shifted into a single-well from a double-well potential. Barrierless proton transfer was observed to occur in the CRT system with several PA molecules present, implying that the CRT may be a promising PA-based PIL.

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 Tg-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 Tg-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity.

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. Three characteristic intermolecular peaks were observed below 2 Å(-1), consistent with experimental X-ray measurements. The prepeak corresponds to the polarity alternation leading to structural heterogeneity and the intermediate shoulder is due to the ubiquitous charge ordering of the ionic liquid. The intense peak is mainly attributed to the adjacent contact of apolar cationic tails. The cationic head-anion correlation function is found to be a unique signature for all three characteristic length scales even if a certain peak is concealed by fortuitous cancellation in the X-ray structure factor. The proposed direct formulation can be readily extended to neutron scattering experiments.

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(n-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. The backbone-to-backbone correlation length increases at a rate of 1 Å/CH2. The longer alkyl chains lead to the longer backbone-to-backbone separation and the larger nonpolar nanodomains. This quantitative comparison of atomistic simulations to X-ray scattering will lead to a fundamental understanding in structure and morphology of polyILs and pave a path forward toward the rational design of future polyILs for electrochemical devices.

A Key concept in Magnesium Secondary Battery Electrolytes.

A critical roadblock toward practical Mg-based energy storage technologies is the lack of reversible electrolytes that are safe and electrochemically stable. Here, we report on high-performance electrolytes based on 1-ethyl-3-methylimidazolium chloride (EMImCl) doped with AlCl3 and highly amorphous δ-MgCl2 . The phase diagram of the electrolytes reveals the presence of four thermal transitions that strongly depend on salt content. High-level density functional theory (DFT)-based electronic structure calculations substantiate the structural and vibrational assignment of the coordination complexes. A 3D chloride-concatenated dynamic network model accounts for the outstanding redox behaviour and the electric and magnetic properties, providing insight into the conduction mechanism of the electrolytes. Mg anode cells assembled using the electrolytes were cyclically discharged at a high rate (35 mA g(-1) ), exhibiting an initial capacity of 80 mA h g(-1) and a steady-state voltage of 2.3 V.

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. Hybrid density functional theory in conjunction with a continuum solvation model was utilized to obtain the local structures of the hydrated vanadium cations in proximity to H2SO4, CF3SO3H, and their conjugate anions. The results indicate that none of these species covalently bond to the vanadium cations. The hydration structure of V(3+) is more distorted than that of V(2+) in an acidic medium. The oxo-group of VO2(+) is protonated by either acid, in contrast to VO(2+) which is not protonated. The atomic partial charge of the four oxidation states of vanadium varies from +1.7 to +2.0. These results provide the local solvation structures of vanadium cations in the VRFBs environment that are directly related to the electrolytes stability and diffusion of vanadium ions into the membrane.

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. Proton dissociation expectedly increased with increasing hydration. Along with the level of hydration, hydrogen bond connectivity between the triflic acid molecules, both directly and via a single water molecule, played a role on proton dissociation. Direct hydrogen bonding between the CF3SO3H molecules, most commonly found in the larger bare CNT, also promoted interactions between water molecules allowing for greater separation of the dissociated protons from the CF3SO3(-) as the hydration level was increased. However, this also resulted in a decrease in the overall proportion of dissociated protons. The confinement dimensions altered both the hydrogen bond network and the distribution of water molecules where the H2O in the fluorinated CNTs tended to form small clusters with less proton dissociation at n = 1 and 2 but the highest at n = 3. In the absence of nearby hydrogen bond accepting sites from H2O or triflic acid SO3H groups, the water molecules formed weak hydrogen bonds with the fluorine atoms. In the bare CNT systems, these involved the CF3 groups of triflic acid and were more frequently observed when direct hydrogen bonding between CF3SO3H hindered potential hydrogen bonding sites. In the fluorinated tubes, interactions with the covalently bound fluorine atoms of the CNT wall dominated which appear to stabilize the hydrogen bond network. Increasing the hydration level increased the frequency of the OH···F (CNT) hydrogen bonding which was highly pronounced in the smaller fluorinated CNT indicating an influence on the confinement dimensions on these interactions.

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 13.3 Å) were used as confinement vessels, and the inner walls of the CNT were either left bare or fluorinated to explore the influence of the confined environment on the determined properties. The water molecules in the fluorinated nanotubes were found to preferentially localize near the CNT surface and exhibit highly ordered structures while those in the bare CNTs were more randomly distributed. Additionally, weak interactions that resembled hydrogen bonds between the water molecules and the fluorine atoms were observed which occurred at a greater frequency in the smaller diameter CNT indicating an influence of the confinement dimensions on these interactions. This was further pronounced when an excess proton was added where, on average, approximately half of the water molecules in the smaller tube were involved in these interactions. This also led to a structured hydrogen bond network with regular defect sites that hindered proton transfer along the channel axis. Addition of the proton in the larger fluorinated CNT, however, disrupted the structural ordering and proton transfer down the nanotube axis near the surface of the CNT wall readily occurred. Proton transfer through the channel was also observed in the smaller nonfluorinated system, however, the proton was located closer to the center of the CNT, while in the larger nonfluorinated CNT proton transfer exhibited less directional preference indicating an impact of the scale of confinement and nature of the surface on proton transfer.

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.e., Nafion and Aquivion) to evaluate the effects of backbone and side chain chemistry. Our results show that water is more likely to be equally distributed within the hydrophilic domains of the sPSO2 ionomers particularly at low water content, which is in contrast to strong phase separation observed in PFSA ionomers at the same level of hydration. As the degree of sulfonation is increased (i.e., decreasing the EW), well-connected water clusters develop in the sPSO2 ionomers even at low water content which are less affected by changes in the MW than observed for PFSA ionomers. The size of the water clusters is estimated to be from 1.2 to 1.5 nm (compared to ∼ 3.5 nm in Nafion) at a water content of 7H2O/SO3H, which is consistent with results determined from previous experiments. This suggests that the high proton conductivity observed in the sPSO2 ionomers is due to the well-connected hydrophilic pathways.

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. The shape and width of the characteristic peaks of the experimental spectra are well reproduced in DFT calculations. By comparing the spectra from experiments and theory, the detailed information about the conformational dependence of EEL spectra for PTFE can be obtained. In the present work, we have demonstrated an application of combining high-resolution EELS and DFT calculations in both low-loss and core-loss regions to discriminate changes of chain conformation and orientation for the polymer with complex phase transition behavior.

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. Solvent effects were found to have minimal effect on bond rotations close to the tetrafluoroethylene backbone but had greater impact near the terminal sulfonic acid group. The carbon-sulfur bond was found to be the most flexible portion of the side chain in each of the fragments which was further enhanced with the inclusion of the solvent. Complete rotation about either the O-CF2 or CF-O bond in the Nafion side chain resulted in fairly high energetic barriers, but significant portions of these rotational surfaces had energetic penalties less than 1.5 kcal/mol indicating substantial conformational freedom. Fully extended and folded conformations of the Nafion side chain exhibit considerable contraction in side chain end-to-end distance and were observed to be nearly isoenergetic using B3LYP, but the folded structures with the ether oxygen atoms in gauche conformations were ~1.5 kcal/mol lower in energy using B97D. Below the second ether linkage of the Nafion side chain, the rotational potential energy profiles were identical to that determined for the SSC side chain. The 3M side chain was generally found to be the most rigid with barriers for complete rotation about the central carbon-carbon bonds of approximately 7 kcal/mol. These results indicate that minor differences in side chain length and chemistry may have a pronounced effect on the rotational potential energy surfaces, particularly those involving rotation about different carbon-carbon bonds with distinctly different character.

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.e., -SO(3)(-)) assumed to be fixed on the pore wall. The interactions of each species with all other species and an applied external field were examined. Molecular-based interaction potential energies were computed in a small test element of the pore and were scaled up in terms of macroscopic variables. Evolution equations of the density and momentum of the species (water molecules and hydronium ions) were derived within a framework of nonequilibrium thermodynamics. The resulting evolution equations for the species were solved analytically using an order-of-magnitude analysis to obtain an expression for the proton conductivity. Results show that the conductivity increases with increasing water content and pore radius, and strongly depends on the separation distance between the sulfonate groups and their distribution on the pore wall. It was also determined that the conductivity of two similar pores of different radii in series is limited by the pore with the smaller radius.

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 number of water molecules required to affect the transfer of a proton in the first hydration shell was observed to be a function of the hydrogen bonding in proximity of the sulfonic acid groups: three H(2)O for the meta- and four H(2)O for the ortho-conformations. Calculations of the rotational energy surfaces indicate that the aromatic backbones of sPSO(2) are much stiffer than the polytetrafluoroethylene (PTFE) backbones in perfluorosulfonic acid (PFSA) ionomers: the largest energy penalty for rotating phenylene rings (i.e., 15.5 kcal/mol for ortho-ortho-sPSO(2)) is nearly twice that computed for the rotation of a CF(2) unit in a PTFE backbone. The energetics for the transfer of various protons in proximity to one or two sulfonate groups (-SO(3)(-)) was also determined. The computed energy barrier for proton transfer when only one sulfonic acid group is present is approximately 1.9 kcal/mol, which is 2.1 kcal/mol lower than similar calculations for PFSA systems. When two sulfonic acid groups are bridged by water molecules, a symmetric bidirectional transfer occurs, which gives a substantially small energy barrier of only 0.7 kcal/mol.

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 scans involved incrementally increasing an O-H bond length in steps of 0.02 Å with geometry optimizations performed at each step at the B3LYP/6-31G** level over all other degrees of freedom. The nature of the hydrogen bond network and the degree of dissociation were found to be critical factors in determining the resulting energetic barrier for proton transfer. The smaller fragment was found to more readily reorient to compensate for the transfer of charge resulting in a lower energetic barrier to proton transfer through stabilization of the new hydrogen bonds. However, when each ionomer had the same degree of dissociation, reprotonation of the sulfonate groups was the most energetically unfavorable in EW 590 when no structural reorganization was observed to occur due to its greater propensity to exist in a dissociated state.