Anhydrous proton conducting membranes based on crosslinked graft copolymer electrolytes

A comb-like copolymer consisting of a poly(vinylidene fluoride-co-chlorotrifluoroethylene) backbone and poly(hydroxy ethyl acrylate) side chains, i.e. P(VDF-co-CTFE)-g-PHEA, was synthesized through atom transfer radical polymerization (ATRP) using CTFE units as a macroinitiator. Successful synthesis and a microphase-separated structure of the copolymer were confirmed by proton nuclear magnetic resonance (¹H NMR), FT-IR spectroscopy, and transmission electron microscopy (TEM). This comb-like polymer was crosslinked with 4,5-imidazole dicarboxylic acid (IDA) via the esterification of the –OH groups of PHEA and the –COOH groups of IDA. Upon doping with phosphoric acid (H₃PO₄) to form imidazole–H₃PO₄ complexes, the proton conductivity of the membranes continuously increased with increasing H₃PO₄ content. A maximum proton conductivity of 0.015 S/cm was achieved at 120 °C under anhydrous conditions. In addition, these P(VDF-co-CTFE)-g-PHEA/IDA/H₃PO₄ membranes exhibited good mechanical properties (765 MPa of Young's modulus), and high thermal stability up to 250 °C, as determined by a universal testing machine (UTM) and thermal gravimetric analysis (TGA), respectively.     

Anhydrous and water-assisted proton mobility in phosphotungstic acid

Nonlocal gradient-corrected density functional theoretical calculations were used to determine the energetics associated with proton migration in phosphotungstic acid. The activation energy for anhydrous proton hopping between two oxygen atoms on the exterior of the molecular Keggin unit was calculated to be 103.3 kJ mol(-1). The quantum-tunneling effect on the rate of proton movement was determined using semiclassical transition-state theory and was found to be a major contributor to the overall rate of proton movement at temperatures below approximately 350 K. The adsorption of water on an acidic proton decreases the activation barrier for hopping to 11.2 kJ mol(-1) by facilitating proton transfer along hydrogen bonds. The overall rate constant for proton hopping was determined as a function of temperature and water partial pressure. Small amounts of water greatly enhance the overall rate of proton movement.     

Anhydrous proton conducting membranes based on electron-deficient nanoparticles/PBI-OO/PFSA composites for high-temperature PEMFC

PEMFC operating at high temperature (100–200 °C) are expected to have significant advantages but face big challenges in the development of suitable proton exchange membranes. This communication describes novel PBI-OO/PFSA blend membranes, which facilitate proton conduction under anhydrous conditions based on a “proton donor–proton acceptor” concept. The proton conductivity of the blends under anhydrous conditions exceeded that of PFSA by a factor of 50 at ambient temperature and of 2–4 at elevated temperature. Intermolecular interaction between two polymer components was investigated by FT-IR spectroscopy. After incorporation of inorganic electron-deficient compounds (BN nanoparticles), the anhydrous proton conductivity of the composites was higher than that of the bare PFSA by three orders of magnitude at ambient temperature and more than one order of magnitude at 140 °C.     

Anhydrous solid proton conductors based on perfluorosulfonic ionomer with polymeric solvent for polymer electrolyte fuel cell

Novel anhydrous polymeric proton conductors have been prepared from perfluorosulfonic acid ionomer with polymer solvent as supplying proton pathway through the segmental motion of polymer chains for polymer electrolyte fuel cell (PEFC) application. Since the membranes do not contain liquid-state acid or solvent, the membranes may promise more stable performances during the operation of PEFC. The Nafion-based anhydrous proton conductors showed maximum proton conductivity of about 4.0 × 10^?3 S cm^?1 at 130 °C under anhydrous condition. The mechanical properties of the membranes were enhanced by introducing H⁺-doped TiO₂ nanoparticles without the conductivity degradation. In addition, the electrochemical properties of the membrane electrode assembly (MEA) employing the anhydrous membrane as ionomer have been investigated, showing stable open circuit voltages (OCVs) over 0.9 V under non-humidified condition.     

High throughput screening and measurements of proton conductivity of newly developed PEM materials based on proton transport visualization

An electrochemical method for proton transport visualization was developed and applied to the investigation of proton-conducting membrane materials. The method employs the change in the visual appearance of chemo-chromic tungsten oxide WO₃ in the presence of atomic hydrogen. An all-solid electrochemical cell arranged by substituting a fuel cell cathode with a thin film of WO₃-electrode was built and shown to generate both optical and electrical response to hydrogen gas exposure. The design of the cell was extended to a high throughput screening system that was utilized to characterize proton transport properties of samples, including a number of new compounds synthesized in-house by sol–gel wet chemistry. Non-destructive introduction of superacidic groups promoting proton hopping in the membrane materials was achieved by photodecomposition of a photoacid generator just after membrane casting. A model quantitatively describing current–voltage relation in the cell was developed and successfully applied to derive area-specific resistance of proton-conductive membranes from the experimental results. Area-specific resistances of membranes are derived from the slopes of optically reconstructed voltage–current curves. Sensitivity and dynamic range of the screening method are discussed.     

Modeling the hydration and proton transport in solid electrolytes based on phenolsulfonic acids

Geometrical and energetic characteristics of crystal hydrates of individual aromatic sulfonic acids and their complexes with poly(vinyl alcohol) as well as the paths for the proton transport in them are calculated in the framework of the density functional theory (version B3LYP) employing the 6-31G^** basis set. The energy of attachment of water to ortho-substituted aromatic sulfonic acids is demonstrated to diminish from 74.4 to 54.8 kJ mol^?1 in the following series of substituents: -OH,-F,-CH₃,-H,-Cl, and -COOH. For the dimers that comprise individual phenolsulfonic acids, the energy of attachment of one water molecule to the SO₃H group is estimated to be equal to 92–105 kJ mol^?1. In the dimers comprising individual phenolsulfonic acids, the specific energy of intermolecular bonds (bond energy per monomer molecule) is found to be equal to 49.3 and 58.5 kJ mol^?1 for, respectively, phenol-2,4-disulfo and phenol-2-sulfo acids. During the formation of polymer membranes based on poly(vinyl alcohol) and phenolsulfonic acids, it is energetically favorable that at least one water molecule should remain in the vicinity of the SO₃H fragment. According to the calculations, the proton migration along the SO₃H group in anhydrous environment is hampered by a barrier of 125–132 kJ mol^?1. In the presence of water, the proton conductivity is of a relay character, with an activation barrier equal to 21–33 kJ mol^?1. The latter value is close to experimental data (17–25 kJ mol^?1).     

Ion transport mechanisms in salt-doped polymerized zwitterionic electrolytes

Polyzwitterions (polyZIs), macromolecules with repeating ampholytic monomers, are a novel class of materials with attractive properties for battery electrolytes. In this study, we probe the ion transport characteristics and underlying mechanisms in two salt-doped (Li⁺-TFSI⁻) polyZIs of similar composition with contrasting zwitterion (ZI) ionic organization: pendant monomers organized via backbone-anion-cation (B-ZI⁻-ZI⁺, Motif B) and backbone-cation-anion (B-ZI⁺-ZI⁻, Motif C). Within both Motifs B and C, the counterion of the pendant-end ZI moiety shows higher mobility. Similarly, when comparing Li⁺ or TFSI⁻ across motifs, it is seen that the respective pendant-end counterion possesses higher mobility than its backbone-adjacent counterpart. Furthermore, when comparing counterions to same-position ZI moieties, TFSI⁻ is seen to possess higher mobility than Li⁺ in each case, a result rationalized by invoking the lower interaction strength between the TFSI⁻ and ZI⁺. Analysis of ion-transport mechanisms demonstrate that the mobility of countercharges to the pendant-end ZI moiety correlates with the ion-association relaxation timescale, similar to ideas noted in polymerized ionic liquids in past studies. However, the mobility of countercharges to the backbone-adjacent ZI moiety is shown to be correlated with a cage relaxation time, which incorporates the combined effects of frustrated motion due to the presence of the polymer backbone and pendant-end ZI moiety and the higher mobility in a population of lightly ZI-coordinated ions. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 578–588     

Physical properties of proton conducting membranes based on a protic ionic liquid

We have investigated the physical properties of proton conducting polymer membranes based on a protic ionic liquid (IL). Properties such as ionic conductivity, melting point of the polymer phase, and glass transition temperature of the liquid phase are studied as a function of IL/polymer ratio and temperature. We observe an increased thermomechanical stability of the membrane with increasing polymer content. However, there is a concomitant decrease in the conductivity with increasing polymer content. This decrease is larger than what can be expected from the dilution of the conducting IL by the insulating polymer matrix. The origin of this decrease can be caused both by the morphology of the membrane and by interactions between the polymer matrix and the ionic liquid. We find a change in the glass transition temperature and in the temperature dependence of the conductivity with increasing polymer content. Both effects can be related to the physical confinement of the IL in the polymer membrane.     

Development and evaluation of new zwitterionic hydrophilic interaction liquid chromatography stationary phases based on 3-P,P-diphenylphosphonium-propylsulfonate

New zwitterionic stationary phases were synthesized by covalently bonding 3-P,P-diphenylphosphonium-propylsulfonate to silica gel. The resulting materials possess both a negatively charged sulfonate group and a positively charged quaternary phosphonium group, which means that there is no net charge over a wide pH range. The retention mechanism and chromatographic behavior of polar solutes under HILIC conditions were studied on these zwitterionic phases. Compared to the commercial ZIC-HILIC column and a bare silica gel stationary phase, the newly synthesized zwitterionic stationary phases provided greater retention, higher peak efficiency and better peak symmetry in the HILIC mode. The analytes examined included: β-blockers, nucleic acid bases and nucleosides, salicylic acid and its analogues, and water soluble vitamins. Factors, such as the type of organic modifiers, solvent composition, pH and the buffer concentration of the mobile phase, have been considered as potential variables for controlling the chromatographic retention of polar analytes.     

Remarkably stable gold nanoparticles functionalized with a zwitterionic liquid based on imidazolium sulfonate in a high concentration of aqueous electrolyte and ionic liquid

A new type of gold nanoparticle with a zwitterionic liquid function (zwitter-Au) has been prepared using an imidazolium sulfonate-terminated thiol as a capping agent. Zwitter-Au was found to be remarkably stable in high concentrations of aqueous electrolytes, ionic liquids, and protein.     

Smart zwitterionic membranes with on/off behavior for protein transport

Poly(acrylonitrile) (PAN)-based zwitterionic membranes, composed of PAN and poly( N, N-dimethyl- N-methacryloxyethyl- N-(3-sulfopropyl) copolymer, are electrolyte-sensitive smart membranes. The hydrophilicity was increased and protein adsorption was remarkably decreased for the membranes in response to environmental stimuli. FTIR spectroscopic analysis directly provided molecular-level observation of the enhanced dissociation and hydration of zwitterionic sulfobetaine dipoles at higher electrolyte concentrations. The smart PAN-based zwitterionic membranes can close or open channels for protein transport under different NaCl concentrations. The electrolyte-sensitive switch of on/off behavior for protein transport is reversible.     

Special pair dance and partner selection: elementary steps in proton transport in liquid water

Conditional and time-dependent radial distribution functions reveal the details of the water structure surrounding the hydronium during the proton mobility process. Using this methodology for classical multistate empirical valence bond (MS-EVB) and ab initio molecular dynamics trajectories, as well as quantal MS-EVB trajectories, we supply statistical proof that proton hops in liquid water occur by a transition from the H3O+[3H2O] Eigen-complex, via the H5O2+ Zundel-complex, to a H3O+[3H2O] centered on a neighboring water molecule. In the "resting period" before a transition, there is a distorted hydronium with one of its water ligands at a shorter distance and another at a longer distance than average. The identity of this "special partner" interchanges rapidly within the three first-shell water ligands. This is coupled to cleavage of an acceptor-type hydrogen bond. Just before the transition, a partner is selected by an additional translation of the H3O+ moiety in its direction, possibly enabled by loosening of donor-type hydrogen bonds on the opposite side. We monitor the transition in real time, showing how the average structure is converted to a distorted H5O2+ cation constituting the transitional complex for proton hopping between water molecules.     

Fast hydrophilic interaction liquid chromatographic separations on bonded zwitterionic stationary phase

Separation science is an art of obtaining adequate resolution of the desired compounds in minimum time, and with minimum effort in terms of sample preparation and data evaluation. In LC, where selectivity is a main driving force for separation, the availability of different separation modes capable of operating at high flow rates is a way to make combined optimal use of selectivity, efficiency, and speed. The separation of polar and hydrophilic compounds is problematic in RP LC due to the poor retention. Hydrophilic interaction liquid chromatography (HILIC) is a more straightforward separation mode to address this problem. Herein, it is shown that separations in HILIC mode are equally efficient as for RP, providing a potential for very fast separations on short columns. This is not only facilitated by the low viscosity of the mobile phase compositions used, compared to typical RP eluents, but also due to higher column permeability. To exemplify this, baseline separations of uracil and cytosine are shown in less than 4 s and of Tamiflu and its main metabolite in less than 40 s, both under isocratic conditions. HILIC must therefore be considered having potential for high throughput purposes, and being an attractive candidate as the second separation dimension in 2-D HPLC.     

Ion channel gating and proton transport

A model of ion channel gating has been proposed (J. Biomol. Struct. Dyn. 19 (2002) 725). It includes the following. (1) There is a bacterial channel for which an X-ray structure is known (KcsA) that opens (‘gates’) with a drop in pH. In the proposal, a proton gates this channel by adding a charge to the glutamate residues that form the center of the gating region. It is postulated that two water molecules form a strong short hydrogen bond when the glutamates plus the water have a −2 charge. Adding a proton leads to a normal, weak, hydrogen bond, and the groups can separate, opening the channel. 98 calculations support this part of the proposal (J. Phys. Chem. B 105 (2001) 5298). (2) Voltage gated channels contain six transmembrane (TM) segments in each of four domains. We suggest that the additional four TM segments (KcsA has two) act as a voltage-to-proton current transducer. In the model, the first step in gating is proton tunneling (J. Phys. Chem. A 102 (1998) 7181), followed by a proton cascade. Calculations supporting the latter step are presented here. One of the eukaryotic TM segments, S4, is known to be involved in gating. This segment has arginines (occasionally lysine) at every third amino acid. The arginines appear capable of transmitting a proton, or possibly a proton cascade (three per S4 would produce the observed charge movement (‘gating current’) that precedes gating). We have carried out density functional calculations, using 98, on a system that includes: one pair of guanidinium groups, the side chains of arginines responsible for carrying the proton current; a mobile proton; one, two or three water molecules. Several guanidinium spacings have been tried, all in the range of carbon–carbon distances 4–6 Å. The potential energy surface was computed for each, and a minimum path found for the proton, at B3LYP/6-311G** level. It was found that the proton, under some conditions, could follow a path between guanidiniums that had no barriers greater than a few kT (thermal energy), thereby supporting the proposal that protons could move along the chain of guanidiniums.     

Uranyi ion transport through tri-n-octylamine-xylene based supported liquid membranes

Tri-n-octylamine (TOA) dissolved in xylene has been used as carrier, constituting liquid membrane supported in Celgard 2400 polypropylene microporous film for the transport of uranyl ions against their concentration gradient from aqueous acid solutions to an alkaline aqueous phase. Effect of sttrring rate, nitric acid concentration and TOA concentration in the organic membrane phase, on the flux of uranyl ions through the membrane has been studied. Viscosity and density data have been obtained to estimate diffusion coefficients and hence the permeability coefficients to compare the same with experimental values, using distribution coefficient data, measured from solvent extraction experiments and available in the literature. Analysis of the flux data has been performed to study the stoichiometry of the chemical reaction involved in complex formation reaction. The results have been compared with simple liquid-liquid extraction data.     

Lamellar liquid crystals of zwitterionic surfactants with hydrocarbon and polyisobutylene chains

Surfactants from either polyisobutylene or alkylsuccinic anhydrides derivatized with diethanolamine in a 1:1 molar ratio with hydrocarbon and polyisobutylene chains of similar length formed lamellar liquid crystals in situ and also with added water. The repeat distance between layers was determined using low angle X-ray diffraction (LAXD), and the water penetration into the hydrocarbon space in the lamellar structure was calculated.

The results revealed a significantly increased repeat distance for the polyisobutylene chain surfactants compared to the alkyl analogues. The water penetration was significantly greater for a surfactant with a decyl chain compared to the one with a dodecyl chain and was intermediate for the polyisobutylene based surfactant.     

Anhydrous state proton conduction in sulfonated bisphenol a polyetherimide with 1H‐1,2,4 triazole as proton solvent

In this study, bisphenol A polyetherimide was sulfonated to various degrees (22, 48, and 62%) by trimethylsilylchlorosulfonate (TMSCS). Novel anhydrous proton conducting polyelectrolytes were prepared by the incorporation of 1H‐1,2,4‐triazole (Taz) as proton solvent in sulfonated polyetherimide (SPEI) matrix. The conductivity reached about 2 × 10^–3 S/cm at 80 °C and 10^–2 S/cm at 140 °C. The temperature dependence proton conductivity of the polyelectrolytes followed Arrhenius equation. The conductivity improved considerably at a temperature close to the triazole melting temperature in SPEI(X)H matrix. It was proposed that the high mobility of the triazolium ions (vehicle diffusion), in addition to structure diffusion, contribute to the high conductivity of these proton conducting electrolytes above the melting temperature of triazole. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2178–2187, 2009     

Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics

Proton dynamics in polymer electrolyte membranes are multifaceted processes, and the relative contributions of various mechanisms can be difficult to distinguish. Judicious choices of model systems can aid in understanding the critical steps. In this study, we characterize anion dynamics in a series of benzimidazole-alkyl phosphonate salts, and compare those dynamics to a membrane prototype, built on a decane backbone. The series of salts are characterized, using high resolution (1)H solid-state magic angle spinning (MAS) NMR, DQ MAS NMR, and (31)P centreband-only detection of exchange (CODEX) NMR spectroscopy, to determine the influence of the nature of the alkyl group on the rates and geometries of anion dynamics, and overall proton exchange processes. The alkyl group is shown to slow the correlation times for anion reorientation, when compared at ambient temperature. However, it is also apparent that the lowered lattice energy of the salt lowers the activation energy and allows good dynamics at intermediate temperatures in both the benzimidazolium ethylphosphonate and in the HBr adduct of 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa).     

pH-sensitive vesicles based on a biocompatible zwitterionic diblock copolymer

Highly biocompatible pH-sensitive diblock copolymer vesicles were prepared from the self-assembly of a biocompatible zwitterionic copolymer, poly[2-(methacryloyloxy)ethyl phosphorylcholine-block-2-(diisopropylamino)ethyl methacrylate], PMPC-b-PDPA. Vesicle formation occurred spontaneously by adjusting the solution pH from pH 2 to above 6, with the hydrophobic PDPA chains forming the vesicle walls. Transmission electron microscopy (TEM), dynamic laser light scattering (DLS), and UV-visible absorption spectrophotometry were used to characterize these vesicles. Gold nanoparticle-decorated vesicles were also obtained by treating the vesicles with HAuCl4, followed by NaBH4.     

Lamellar liquid crystals of zwitterionic surfactants with hydrocarbon and polyisobutylene chains

Abstract

Surfactants from either polyisobutylene or alkylsuccinic anhydrides derivatized with diethanolamine in a 1:1 molar ratio with hydrocarbon and polyisobutylene chains of similar length formed lamellar liquid crystals in situ and also with added water. The repeat distance between layers was determined using low angle X-ray diffraction (LAXD), and the water penetration into the hydrocarbon space in the lamellar structure was calculated.

The results revealed a significantly increased repeat distance for the polyisobutylene chain surfactants compared to the alkyl analogues. The water penetration was significantly greater for a surfactant with a decyl chain compared to the one with a dodecyl chain and was intermediate for the polyisobutylene based surfactant.