Molecular dynamics studies on the structures of polymer electrolyte membranes and diffusion mechanism of protons and small molecules

We performed a series of molecular dynamics (MD) simulations on Nafion^® membranes containing various quantities of H₂O and CH₃OH. The simulations afforded diverse nanoscale phase-separated structures, such as clusters, channels, and cluster–channels. The calculated cluster–channel structure qualitatively agrees with the experimental results of X-ray diffraction studies. We also investigated the diffusion mechanisms for H₂O, protons, CH₃OH, H₂, and O₂ in these membranes. To reproduce the hopping transfer of protons, we employed a semi-classical MD approach using the empirical valence bond method. The estimated diffusion coefficients of H₂O and proton in the membranes significantly depended on the H₂O content, and these values showed qualitatively good agreement with the experimental results. The diffusion coefficient of proton in H₂O-rich membranes was much larger than that of H₂O, and the proton mainly formed H₅O₂⁺ complex. Furthermore, the simulation results indicate that the majority of CH₃OH permeates through the H₂O clusters, and the majority of H₂ and O₂ permeates through the hydrophobic region of the membrane.     

Agar-based films for application as polymer electrolytes

New types of polymer electrolytes based on agar have been prepared and characterized by impedance spectroscopy, X-ray diffraction measurements, UV-vis spectroscopy and scanning electronic microscopy (SEM). The best ionic conductivity has been obtained for the samples containing a concentration of 50 wt.% of acetic acid. As a function of the temperature the ionic conductivity exhibits an Arrhenius behavior increasing from 1.1 × 10⁻⁴ S/cm at room temperature to 9.6 × 10⁻⁴ S/cm at 80 °C. All the samples showed more than 70% of transparency in the visible region of the electromagnetic spectrum, a very homogeneous surface and a predominantly amorphous structure. All these characteristics imply that these polymer electrolytes can be applied in electrochromic devices.     

Molecular dynamics modeling of polymer crystallization from the melt

Molecular pathways to polymer crystallization and the structures of crystal-melt interfaces are investigated by molecular dynamics simulation. We adopt a simplified molecular model for polymethylene-like chains; the chain is made of CH₂-like beads connected by harmonic springs, and the lowest energy conformation is a linear stretched sequence of the beads with slight bending stiffness being imposed. Two molecular systems are considered, one is made of 640 chains of C100 and the other is made of 64 chains of C1000, both being placed between two parallel substrates that represent the growth surfaces of the lamellae growing toward each other. The initial melt kept at a sufficiently high temperature above the melting point is rapidly cooled down to various crystallization temperatures, and the molecular processes of crystallization that follow are investigated. In both systems, we clearly observe the growth of stacked chain-folded lamellae from the substrates. The growing lamellae have a definite tapered shape, and they show marked thickening growth along the chain axis as well as usual growth perpendicular to it. The overall crystallization rate is found to be very sensitive to the crystallization temperature, showing an apparent maximum around 320-330 K for C100. We find that the lamellae do not grow keeping pace with each other but grow in independent rates especially at higher temperatures. We also examine the structures of the lateral growth surfaces and find that the growth surfaces are locally flat and the Kossel mechanism of crystal growth seems to be operative. In addition, the fold surfaces are found to be covered with relatively short chain-folds; at least about 60-70% of the folds are connecting the nearest or the next nearest neighbor crystalline stems. No appreciable bond orientational order is found in the undercooled melt of C100 and C1000.     

Preparation of thermally stable polymer electrolytes from imidazolium-type ionic liquid derivatives

Thermally stable polymer electrolytes based on ionic liquids were prepared and analyzed. Mono-functional ionic liquid monomers, ionic liquid cross-linkers, and ethylimidazolium-type ionic liquid salts were mixed and polymerized. The ionic liquid-type cross-linkers were effective to prepare thermally stable polymer films. In particular, the copolymerization of cross-linker and ethylimidazolium-type ionic liquid monomers were used to make polymer electrolytes with high ionic conductivities. The copolymerization in ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide gave a transparent film showing no thermal degradation up to 400 °C.     

Complicated phase behavior and ionic conductivities of PVP-co-PMMA-based polymer electrolytes

We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly(vinylpyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) and lithium perchlorate (LiClO₄) as well as these systems' phase behavior and ionic conductivities. The presence of MMA moieties in the PVP-co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature (T_g). In the binary LiClO₄/PVP blends, the presence of a small amount of LiClO₄ reduces the strong dipole-dipole interactions within PVP and leads to a lower T_g. Further addition of LiClO₄ increases T_g as a result of ion-dipole interactions between LiClO₄ and PVP. In LiClO₄/PVP-co-PMMA blend systems, for which the three individual systems—the PVP-co-PMMA copolymer and the LiClO₄/PVP and LiClO₄/PMMA blends—are miscible at all compositional ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO₄, PVP and PMMA units. The PMMA-rich component in the PVP-co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO₄ content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO₄/VP57 blend (i.e., 57 mol% VP units in the PVP-co-PMMA copolymer).     

Modelling of gas transport properties of polymer electrolytes containing various amounts of water

Gas transport properties of three different PEO-based polymer electrolytes containing PEO sulfonic acid dianions, cations and 90 or 35 or 0 wt% of water are studied by atomistic molecular modelling. The gas molecules studied are H₂, O₂, CO₂ and CH₄. The pair correlation functions are calculated to see the distribution of the gas molecules in the systems. The diffusion coefficients of the gas molecules are calculated and found to be dependent of the amount and distribution of the water in the system and of the size of the penetrant. The motion of the gas molecules is explored in each system. The amount and distribution of water has a strong effect on the motion of the gas molecules. The ionic copenetrants seem not to have any direct influence on the diffusion of the gas molecules in the polyelectrolyte materials. However, ions have an effect on the distribution of the water in the polyelectrolyte materials.     

Polymer electrolytes based on poly(ethylene glycol) dimethyl ether and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate: Preparation, physico-chemical characterization, and theoretical study

Polymer electrolytes based on poly(ethylene glycol) dimethyl ether (PEGdME) and the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF₆) have been prepared and characterized by different techniques. Coordination of the IL by the polymer occurs mainly in the amorphous phase. This finding was correlated with previous theoretical investigations of a similar model for polymer electrolytes based on poly(ethylene oxide), PEO, and IL. It has been obtained ionic conductivity σ ∼ 10⁻³ S cm⁻¹ for the polymer electrolyte with 35 wt% of IL at 100 °C. The same order of magnitude for σ was obtained by molecular dynamics simulation of PEO/IL. This work demonstrates consistency between experimental and theoretical results for polymer electrolytes containing ionic liquids.     

Examination of the fundamental relation between ionic transport and segmental relaxation in polymer electrolytes

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes is strongly limited by their poor ionic conductivity. The classical theory predicts that the ionic transport is dictated by the segmental motion of the polymer matrix. As a result, the low mobility of polymer segments is often regarded as the limiting factor for development of polymers with sufficiently high ionic conductivity. Here, we show that the ionic conductivity in many polymers can be strongly decoupled from their segmental dynamics, in terms of both temperature dependence and relative transport rate. Based on this principle, we developed several polymers with “superionic” conductivity. The observed fast ion transport suggests a fundamental difference between the ionic transport mechanisms in polymers and small molecules and provides a new paradigm for design of highly conductive polymer electrolytes.     

Alternating copolymers of carbon dioxide with glycidyl ethers for novel ion-conductive polymer electrolytes

To overcome the low ionic conduction of existing poly(ethylene oxide)-based polymer electrolytes, we consider polycarbonates obtained from the copolymerization of CO₂ and epoxy monomers. We synthesized four types of polycarbonates possessing phenyl, n-butyl, t-butyl and methoxyethyl side groups using zinc glutarate, and measured the ionic conductivity of their electrolytes, including 10 mol% of LiTFSI. The electrolyte possessing methoxyethyl side groups had the highest conductivity, of the order of 10⁻⁶ S cm⁻¹ at room temperature. The activation energy (E_a) for ionic conduction in the polycarbonate electrolytes was estimated from the VTF equation, and the E_a of the electrolyte possessing n-butyl side groups was almost the same with the polyether-based electrolytes. An interesting feature of our study is that the polycarbonate is a unique candidate for ion-conductive polymers because of its flexible and hydrophobic properties.     

Solid polymer electrolytes V: microstructure and ionic conductivity of epoxide-crosslinked polyether networks doped with LiClO4

A crosslinked polyether network was prepared from poly(ethylene glycol) diglycidyl ether (PEGDE) cured with poly(propylene oxide) polyamine. Significant interactions between ions and polymer host have been observed for the crosslinked polyether network in the presence of LiClO₄ by means of FT-IR, DSC, TGA, and ⁷Li MAS solid-state NMR. Thermal stability and ionic conductivity of these complexes were also investigated by TGA and AC impedance measurements. The results of FT-IR, DSC, TGA and ⁷Li MAS solid-state NMR measurements indicate the formation of different types of complexes through the interaction of ions with different coordination sites of polymer electrolyte networks. The dependence of ionic conductivity was investigated as a function of temperature, LiClO₄ concentration and the molecular weight of polyether curing agents. It is observed that the behavior of ion transport follows the empirical Vogel-Tamman-Fulcher (VTF) type relationship for all the samples, implying the diffusion of charge carrier is assisted by the segmental motions of polymer chains. Moreover, the conductivity is also correlated with the interactions between ions and polymer host, and the maximum ionic conductivity occurs at the LiClO₄ concentration of [O]/[Li⁺]=15.     

Theoretical studies of IM-12 zeolite for acidic catalysts

We theoretically investigate the properties of the IM-12 to address a catalyst for acidic conversion reaction of larger organic molecules. The acidic characteristics of the IM-12 are investigated by density functional theory (DFT) considering both the local density and generalized gradient approximations, LDA and GGA, respectively. Based on quantum mechanical (QM) calculation results, we find that the zeolite with Al element prefers the tetrahedral (T) sites, T4 and T6, when replacing Si in IM-12 framework. Isomorphously substituted IM-12 on the T4 and T6 sites by B, Al, and Ga is studied, respectively. Both of the sites give the Brönsted acidity order: B–IM-12 < Ga–IM-12 < Al–IM-12, which is the same as other zeolites. The calculated NH₃ adsorption energies are compared with the calculated and experimental results of H–[Al]MOR [M. Elanany, D.P. Vercauteren, M. Koyama, M. Kubo, P. Selvam, E. Broclawik, A. Miyamoto, J. Mol. Catal. A 243 (2006) 1; C. Lee, D.J. Parrillo, R.J. Gorte, W.E. Farneth, J. Am. Chem. Soc. 118 (1996) 3262]. Molecular dynamics (MD) results show that IM-12 zeolite allows the large molecules such as diisopropylbenzene (DIPB) and triisopropylbenzene (TIPB) to diffuse faster than those in MOR zeolite and IM-12 may have significant selectivity for TIPB over DIPB. We conclude that the IM-12 with Al impurity would be a good candidate for large organic molecule acidic conversion reaction.     

Classical molecular dynamics simulations of gold clusters deposited on rutile TiO2(1 1 0) surface

Classical molecular dynamic simulations of Au clusters supported on the non-defective rutile TiO₂(1 1 0) surface are reported. The oxide surface is represented by a slab obtained by imposing periodic boundary conditions to a 12 × 12 × 1 supercell. The dynamics of the system is accounted for thorough classical pair potentials describing both the metal–metal and the metal–surface interactions, determined from periodic density functional theory model calculations. Deposited particles show a well defined structure and can be described as hexagonal truncated pyramids mainly exhibiting (1 1 1) facets in agreement with scanning tunnel microscopy experiments conducted under atomic resolution.     

Role of entanglements and bond scission in high strain-rate deformation of polymer gels

A key factor that limits the practical implementation of polymer gels is low gel toughness. Here, we present coarse-grained molecular dynamics simulations of the effects of solvent molecular weight on the toughness of entangled and non-entangled polymer gels in the ballistic impact regime. Our results demonstrate that higher molecular weight solvents enhance gel toughness, and that mechanical properties including strength and toughness can be influenced by bond scission. Further, we find a remarkable two-step gel fracture mechanism on the molecular level: network chains undergo scission first (and well before fracture), followed by scission of solvent chains. For strain rates greater than inverse relaxation time of the solvent, long, highly entangled solvent chains provide fracture resistance even after the network chains break by effectively increasing the number of chains that must be broken as a crack propagates.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Electrochemical and NMR characterizations of mixed polymer electrolytes based on oligoether sulfate and imide salts

Polymer electrolytes based on mixtures of lithium trifluoromethylsulfonylimide, LiTFSI and lithium oligoether sulfates dissolved in poly(oxyethylene) were studied. The properties of these mixed electrolytes i.e. thermal stability, ionic conductivities, transference numbers, diffusion coefficients and electrochemical stabilities were established in a wide range of compositions. A satisfactory compromise was found between high cationic transference numbers and high conductivities, while markedly decreasing the total amount of LiTFSI used. Since lithium oligoether sulfates should be considerably less expensive than LiTFSI and easy to recycle, these mixed polymer electrolytes seem to be promising.     

Electrochemical investigation of polymer electrolytes based on lithium 2-(phenylsulfanyl)-1,1,2,2-tetrafluoro-ethansulfonate

The salt PhSCF₂CF₂SO₃Li appears promising for lithium-polymer batteries. Its poly(oxyethylene) complexes, although less conductive than lithium imide complexes, provided cationic transference numbers ranging between 0.45 and 0.5, enabling high cationic conductivities to be obtained. Thanks to its double substitution by aryl and perfluorinated moieties, the thioether function is stable enough to be used with positive electrodes, such as vanadium oxide and perhaps cobalt oxide.     

Effect of imidazole based polymer blend electrolytes for dye-sensitized solar cells in energy harvesting window glass applications

The exploration of polymer electrolyte in the field of dye sensitized solar cell (DSSC) can contribute to increase the invention of renewable energy applications. In the present work, the influence of imidazole on the poly (vinylidene fluoride) (PVDF)-poly (methyl methacrylate) (PMMA)-Ethylene carbonate (EC)-KI-I₂ polymer blend electrolytes has been evaluated. The different weight percentages of imidazole added into polymer blend electrolytes have been prepared by solution casting. The prepared films were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), UV-visible spectra, photoluminescence spectra and impedance spectroscopy. The surface roughness texture of the film was analyzed by atomic force microscopy (AFM). The ionic conductivity of the optimized polymer blend electrolyte was determined by impedance measurement, which is 1.95×10^-3 S·cm^-1 at room temperature. The polymer electrolyte containing 40 wt% of imidazole content exhibits the highest photo-conversion efficiency of 3.04% under the illumination of 100 mW·cm^-2. Moreover, a considerable enhancement in the stability of the DSSC device was demonstrated.     

Structure and conductive properties of poly(ethylene oxide)/layered double hydroxide nanocomposite polymer electrolytes

The oligo(ethylene oxide) modified layered double hydroxide (LDH) prepared by template method was added as a nanoscale nucleating agent into poly(ethylene oxide) (PEO) to form PEO/OLDH nanocomposite electrolytes. The effects of OLDH addition on morphology and conductivities of nanocomposite electrolytes were studied using wide-angle X-ray diffractometer, polarized optical microscopy, differential scanning calorimetry and ionic conductivity measurement. The results show that the exfoliated morphology of nanocomposites is formed due to the surface modification of LDH layers with PEO matrix compatible oligo(ethylene oxide)s. The nanoscale dispersed OLDH layers inhibit the crystal growth of PEO crystallites and result in a plenty amount of intercrystalline grain boundary within PEO/OLDH nanocomposites. The ionic conductivities of nanocomposite electrolytes are enhanced by three orders of magnitude compared to the pure PEO polymer electrolytes at ambient temperature. It can be attributed to the ease transport of Li⁺ along intercrystalline amorphous phase. This novel nanocomposite electrolytes system with high conductivities will be benefited to fabricate the thin-film type of Li-polymer secondary battery.     

Ionic conductivity of polymer electrolytes and future applications

Polymer electrolytes are solvent-free ion-conducting polymers and provide new and attractive materials in both polymer chemistry and electrochemistry. A proper understanding of ion dissociation and ion transport in such polymers is necessary for their application as solid electrolytes in electrochemical devices. Ionic conduction behaviour in polymer electrolytes is described here in relation to the characteristic properties. Of special interest is the ability of polymer electrolytes to include various kinds of electroactive molecules within them. The combination of this ability with their high ionic conductivity has enabled polymer electrolytes to be used as media for electrochemical syntheses and redox reactions.     

New polymer electrolytes based on ether sulfate anions for lithium polymer batteries: Part I. Multifunctional ionomers: Conductivity and electrochemical stability

New lithium conducting ionomers based on commercial polyethers were synthesized by chemical modification in order to incorporate not only the anionic function on the polymer backbone but also polar aprotic and (or) protic groups improving both the salt dissociation and the anion-solvating ability of the multifunctional copolymers. The choice of environmentally friendly rather than perfluorinated anionic functions did not appear to compromise for the ionic conductivity. Lastly, the preliminary results on the use of an electrochemically stable and relatively cheap additive sparteine appear promising and could be generalized to a variety of polymer electrolytes for lithium batteries.