Proton-conducting properties of the membranes based on poly(vinyl phosphonic acid) grafted poly(glycidyl methacrylate)

Proton-conducting properties of the graft copolymer electrolytes were examined throughout this work. The homopolymers poly(glycidyl methacrylate), PGMA and poly(vinyl phosphonic acid), PVPA were synthesized by free-radical polymerizations of the monomers glycidyl methacrylate, GMA, and vinyl phosphonic acid, VPA, respectively. The graft copolymers were produced by grafting of PVPA onto PGMA via ring opening of ethylene oxide groups. To examine the influence of the concentration of VPA on the proton conductivity, several graft copolymers were produced at various stoichiometric ratios with respect to monomer repeat units. The materials were characterized by FT-IR and ¹H NMR spectroscopy and the thermal properties were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The TGA results demonstrated that the samples are thermally stable up to at least 150 °C. The proton conductivities of humidified and dry samples were studied via impedance spectroscopy. In the anhydrous state, the proton conductivity of P(GMA)-graft-P(VPA)₁₀ was 5 × 10^? 5 S/cm at 150 °C. The proton conductivity of the same material increased with the humidity content and reached to 0.03 S/cm at 80 °C under 50% of RH, which approached to that of Nafion^® at the same humidification level.     

Polymer electrolytes based on copolymer of poly(ethylene glycol) dimethacrylate and imidazolium ionic liquid

Polymer electrolytes based on the copolymer of N-vinylimidazolium tetrafluoroborate (VyImBF₄) and poly(ethylene glycol) dimethacrylate (PEGDMA) have been prepared. Ethylene carbonate (EC) and LiClO₄ are added to form gel polymer electrolytes. The chemical structure of the samples and the interactions between the various constituents are studied by FT-IR. TGA results show that these polymer electrolytes have acceptable thermal stability, are stable up to 155 °C. Measurements of conductivity are carried out as a function of temperature, VyImBF₄ content in poly(VyImBF₄-co-PEGDMA), and the concentration of EC and LiClO₄. The conductivity increases with PEGDMA and EC content. The highest conductivity is obtained with a value of 2.90 × 10^? 6 S cm^? 1 at room temperature for VP1/EC(25 wt.%)–LiClO₄ system, corresponding to the LiClO₄ concentration of 0.70 mol kg^? 1 polymer.     

Conductivity and thermal studies of blend polymer electrolytes based on PVAc–PMMA

The polymer electrolytes comprising blend of poly(vinyl acetate) (PVAc) and poly(methylmethacrylate) (PMMA) as a host polymer and LiClO₄ as a dopant are prepared by solution casting technique. The amorphous nature of the polymer–salt complex has been confirmed by XRD analysis. The DSC thermograms show two T_g's for PVAc–PMMA blend. A decrease in T_g with the LiClO₄ content reveals the increase of segmental motion. Conductance spectra results are found to obey the Jonscher's power law and the maximum dc conductivity value is found to be 1.76 × 10^− 3 S cm^− 1 at 303 K for the blend polymer complex with 20 wt.% LiClO₄, which is suitable for the Li rechargeable batteries. The conductivity–temperature plots are found to follow an Arrhenius nature. The dc conductivity is found to increase with increase of salt concentration in the blend polymer complexes.     

Polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) with crosslinked poly(ethylene glycol) for lithium batteries

The combination of a poly(ethylene glycol) (PEG) network and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) copolymer chains is one of the most efficient means for modifying PVDF-HFP gel electrolytes. Previous preparations tend to introduce contamination into the polymer gel electrolyte because of irradiation, high temperature or the initiator needed for crosslinking which might result in the electrochemical degradation. In order to overcome the above disadvantages, a new method has been developed to successfully prepare the semi-interpenetrating polymer networks of PVDF-HFP based electrolytes with crosslinked diepoxy polyethylene glycol (DIEPEG). In this process, impurities are avoided because of a moderate reaction temperature at 50 °C and poly(ethylenimine) (PEI) as the crosslinking agent. Microporous films with various compositions are prepared and characterized. Thermal, mechanical, swelling and electrochemical properties, as well as microstructures of the prepared polymer electrolytes have been investigated using thermogravimetric analysis, electrochemical impedance spectroscopy, linear sweep voltammetry, and scanning electron microscopy. The results show that the blend polymer electrolyte with PVDF-HFP/PEI + DIEPEG (60:40 w/w) has an ionic conductivity of 2.3 mS cm^? 1 at room temperature in the presence of 1 M LiPF₆ in EC and DMC (1:1 w/w). All the blend electrolytes are electrochemically stable up to 4.8 V versus Li/Li⁺. The results reveal that this new method may be very promising for improving PVDF-HFP based electrolytes.     

Ionic conductivity of polymer electrolyte membranes based on polyphosphazene with oligo(propylene oxide) side chains

A polyphosphazene [NP(NHR)₂]_n with oligo[propylene oxide] side chains − R = –[CH(CH₃)–CH₂O]_m–CH₃ (m = 6 – 10) was synthesized by living cationic polymerisation and polymer-analogue substitution of chlorine from the intermediate precursor [NPCl₂]_n using the corresponding primary amine RNH₂. The polymer had an average molecular weight of 3.3 × 10⁵ D. Polymer electrolytes with different concentrations of dissolved lithium triflate (LiCF₃SO₃) were prepared. Mechanically stable polymer electrolyte membranes were formed using UV radiation induced crosslinking of the polymer salt mixture in the presence of benzophenone as photoinitiator. The glass transition temperature of the parent polymer was found to be − 75 °C before cross linking. It increases after crosslinking and with increasing amounts of salt to a maximum of − 55 °C for 20 wt.% LiCF₃SO₃. The ionic conductivity was determined by impedance spectroscopy in the temperature range 0–80 °C. The highest conductivity was found for a salt concentration of 20 wt.% LiCF₃SO₃: 6.5 × 10^− 6 S·cm^− 1 at 20 °C and 2.8 × 10^− 4 S cm^− 1 at 80 °C. The temperature dependence of the conductivities was well described by the MIGRATION concept.     

Structural,vibrational study and superprotonic behavior of a new mixed dipotassium hydrogenselenate dihydrogenphosphate K2(HSeO4)1.5(H2PO4)0.5

Ongoing studies of the KHSeO₄–KH₂PO₄ system aiming at developing novel proton conducting solids resulted in the new compound K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (dipotassium hydrogenselenate dihydrogenphosphate). The crystals were prepared by a slow evaporation of an aqueous solution at room temperature. The structural properties of the crystals were characterized by single-crystal X-ray analysis: K₂(HSeO₄)_1.5(H₂PO₄)_0.5 (denoted KHSeP) crystallizes in the space group P 1¯ with the lattice parameters: a = 7.417(3) Å, b = 7.668(2) Å, c = 7.744(5) Å, α = 71.59(3)°, β = 87.71(4)° and γ = 86.04(6)°. This structure is characterized by HSeO₄⁻ and disordered (H_xSe/P)O₄⁻ tetrahedra connected to dimers via hydrogen bridges. These dimers are linked and stabilized by additional hydrogen bonds (O–H–O) and hydrogen bridges (O–H…O) to build chains of dimers which are parallel to the [0, 1, 0] direction at the position x = 0.5.The differential scanning calorimetry diagram showed two anomalies at 493 and 563 K. These transitions were also characterized by optical birefringence, impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of the proton conductors in this compound were determined in a wide temperature range. The transport properties in this material are assumed to be due to H⁺ protons hopping mechanism.     

Study on ionic conductivity of polymer electrolyte plasticized with PEG–aluminate ester for rechargeable lithium ion battery

We have synthesized poly(ethylene glycol) (PEG)-aluminate ester as a plasticizer for solid polymer electrolytes. The thermal stability, ionic conductivity and electrochemical stability of the polymer electrolyte which consist of poly(ethylene oxide) (PEO)-based copolymer, PEG–aluminate ester and lithium bis-trifluoromethanesulfonimide (LiTFSI) were investigated. Addition of PEG–aluminate ester increased the ionic conductivity of the polymer electrolyte, showing greater than 10^− 4 S cm^− 1 at 30 °C. The polymer electrolyte containing PEG–aluminate ester retained thermal stability of the non-additive polymer electrolyte and exhibited electrochemical stability up to 4.5 V vs. Li⁺/Li at 30 °C.     

Ionic and electronic conductivities,stability and thermal expansion of La10 − x(Si,Al)6O26 ± δ solid electrolytes

Apatite-type La_10 − xSi_6 − yAl_yO_{27 − 3x/2 − y/2} (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10^− 6 K^− 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.     

Synthesis and characterization of Ge and Sn(II) iodide-doped thioborate anhydrous proton conductors

Metal iodide-doped anhydrous proton conductors in the series xMI₂ + (1 − x)(HBS₂)₃, where M = Ge and Sn, have been prepared. These samples improve upon the anhydrous proton conductivity shown previously in the H₂S + B₂S₃ + GS_y series, where G = Si, Ge, and As, through a displacement reaction to incorporate HI into the materials. This is analogous to doping a silver halide salt into fast ion conducting chalcogenide glasses, such as AgI + Ag₂S + B₂S₃ + SiS₂, which results in a one to two orders of magnitude improvement in the ionic conductivity. The structural modification of the boroxol ring units in the thioboric acid is discussed based on the infrared and Raman spectroscopy. The DC conductivity, estimated from AC impedance spectra, of the metal iodide-doped (HBS₂)₃ samples is reported as a function of temperature and related back to the underlying structural chemistry of these materials. The static solid state proton NMR spectra were also used to identify the proton environment and proton dynamics. These materials represent an improvement upon previous anhydrous proton-conducting materials and represent an important step in finding intermediate temperature proton conductors.     

Tuneability of Sm(1 − x)CexFeO3 ± λ perovskites: Thermal stability and electrical conductivity

Trimetallic perovskite oxides, Sm_(1 ? x)Ce_xFeO_3 ± λ (x = 0–0.05), were prepared by thermal decomposition of amorphous citrate precursors followed by calcinations. The material properties of the substituted perovskites were characterized by X-ray diffraction (XRD), X-ray florescence spectroscopy (XRF), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The doped materials exhibited a single perovskite phase in air up to 1350 °C and have specific surface areas in the range of 2.696–8.665 m²/g. In reducing atmosphere (5%v/vH₂/N₂), the unsubstituted perovskite (x = 0) decomposed into two phases while the ceria stabilized materials (x = 0.01, x = 0.03, x = 0.05) remained in a single phase as revealed by XRD analysis. Their conductivities were measured by the four point probe method in air and in dilute hydrogen (5%v/vH₂/N₂) separately. The ceria substituted materials show increased stability versus reduction and phase separation for a wide temperature range (up to 1000 °C). Although undoped SmFeO₃ has higher conductivity under oxidizing conditions than ceria doped SmFeO₃ due its p-type nature, the situation is reversed under reducing conditions. The ceria substituted perovskites (Sm_(1 ? x)Ce_xFeO_3 ± λ, x = 0–0.05) showed higher conductivity in reducing than in oxidizing conditions, suggesting that ceria doping at the A-site has changed the SmFeO₃ from p-type to n-type semi-conducting behavior.     

Synthesis and ionic conductivity of polymeric ion gel containing room temperature ionic liquid and phosphotungstic acid

Composite electrolyte comprising phosphotungstic acid (PWA) filler and 1-butyl-3-methyl-imidazolium-tetrafluoroborate (BMImBF₄) room temperature ionic liquid (RTIL) in poly(2-hydroxyethyl methacrylate) (PHEMA) matrix has been prepared. The polymer matrix was formed by free radical polymerization of 2-hydroxyethyl methacrylate (HEMA) monomers. BMImBF₄ was used as both ionic source and plasticizer, and PWA filler provided the proton conductivity in this system. The interactions and structure changes of the PHEMA-RTIL-PWA composites were investigated by Fourier transform infrared spectra, differential scanning calorimetry, and X-ray diffraction. PWA fillers maintained their Keggin structure within a limited range and enhanced the ionic conductivity of the composite electrolyte. The electrolyte with PWA at the 2 wt.% showed the highest ionic conductivity of 8 × 10^− 4 S cm^− 1 at room temperature and 96% relative humidity.     

Conductivities of electrolytes based on PEI-b-PEO-b-PEI triblock copolymers with lithium and copper TFSI salts

Tri block-copolymer poly(iminoethylene)-b-poly(oxyethylene)-b-poly(iminoethylene) with a poly(oxyethylene) central block (PEI-b-PEO-b-PEI) were used as a “dual” matrix for polymer electrolytes having selectivity for hard cations (Li⁺/PEO) in one phase and for soft cations (Cu²⁺/PEI) in the other. Conductivity measurements were recorded for 20:1, 12:1 and 8:1 coordinating atom (O or/and N) to cation (Li⁺, Cu²⁺) ratios, for each of the three complexes studied: PEI-b-PEO-LiTFSI-b-PEI, PEI-Cu(TFSI)₂-b-PEO-b-PEI-Cu(TFSI)₂ and PEI-Cu(TFSI)₂-b-PEO-LiTFSI-b-PEI-Cu(TFSI)₂. For either low (20 °C) or high temperature (80 °C) the highest conductivity was given by the polymer electrolyte based on Cu(TFSI)₂ with N/Cu²⁺ = 20:1 (10^− 6, respectively 2 × 10^− 4 S cm^− 1). In the present paper, the conductivity evolution is discussed in relation with the polymer structure, the type and the concentration of the salt and the thermal behavior of our systems.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Novel clays: Solid-state synthesis,characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na₂Si₆Al₂Mg₆O₂₀F₄ · xH₂O (Na-2-mica), Na₃Si₅Al₃Mg₆O₂₀F₄ · xH₂O (Na-3-mica) and Na₄Si₄Al₄Mg₆O₂₀F₄ · xH₂O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water swollen micas with c-axis spacing of ∼1.2 nm except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state ²⁷Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state ²⁹Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr²⁺ or 8.12 ppm of La³⁺. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.     

Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1−xCoxO3−δ materials

Oxygen deficiency, thermal and chemical expansion of La_0.5Sr_0.5Fe_1−xCo_xO_3−δ (x = 0, 0.5, 1) have been measured by thermogravimetry, dilatometry and high temperature X-ray diffraction. The rhombohedral perovskite materials transformed to a cubic structure at 350 ± 50 °C. The thermal expansion of the materials up to the onset of thermal reduction was 14–18 × 10^− 6 K^− 1. Above 500 °C in air (400 °C in N₂), chemical expansion contributed to the thermal expansion and the linear thermal expansion coefficients were significantly higher, 16–35 × 10^− 6 K^− 1. The chemical expansion, ε_c, showed a maximum of 0.0045 for x = 0.5 and 0.0041 for x = 1 at 800–900 °C. The normalized chemical expansion, ε_c/Δδ, was 0.036 for x = 0.5 and 0.035 for x = 1 at 800 °C. The chemical expansion can be correlated with an increasing ionic radius of the transition metals with decreasing valence state.     

A co-doping approach towards enhanced ionic conductivity in fluorite-based electrolytes

A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r_c), different compositions in the Lu_xNd_yCe_1−x−yO_2−δ (x + y = 0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r_c for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 °C to 700 °C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in Lu_xNd_yCe_1−x−yO_2−δ system is found to be negligible when compared to Lu_xCe_1−xO_2−δ (negative) and Nd_xCe_1−xO_2−δ (positive) systems. Grain ionic conductivity of Lu_xNd_yCe_1−x−yO_2−δ (where x + y = 0.05) at 500 °C is observed to be 1.9 × 10^− 3 S/cm which is twice as high as that of Lu_0.05Ce_0.95O_2−δ. These results extend the validity of the r_c concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity.     

La0.8Sr0.2Cr1 − xRuxO3 − δ–Gd0.1Ce0.9O1.95 solid oxide fuel cell anodes: Ru precipitation and electrochemical performance

Composites containing La_0.8Sr_0.2Cr_1 ? xRu_xO_3 ? δ (LSCrRu) with x = 0–0.25 and Gd_0.1Ce_0.9O_1.95 (GDC) were studied as anodes in solid oxide fuel cells (SOFCs) with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 ? δ (LSGM) electrolytes. Electrode polarization resistance R_P decreased during initial SOFC operation before reaching a minimum. The decrease was more rapid, and the ultimate R_P value reached was generally lower, with increasing temperature and Ru content x. R_P was stable at longer times except for x = 0.25 where it increased slightly. SOFCs with x = 0.18 anodes at 800 °C yielded power densities as high as 0.53 W/cm² with an R_P value, including the (La,Sr)(Co,Fe)O₃–GDC cathode, of < 0.15 Ω cm². Transmission electron microscopy revealed Ru nano-particles on LSCrRu surfaces; their size increased and their density decreased with increasing temperature. Increasing the Ru content increased the density of Ru surface particles at a given time and temperature. Measured early-stage Ru surface coverage values were consistent with a model where Ru supply to the LSCrRu surface was limited by Ru bulk out-diffusion, but the coverage saturated at longer times. There was surprisingly little Ru particle coarsening over times up to 1000 h at 800 °C, with Ru particles sizes remaining < 10 nm. The cell R_P values generally decreased with increasing Ru nano-particle surface area.     

Preparation and characterization of solid polymer electrolytes based on PHEMO and PVDF-HFP

Polymer electrolyte membranes consisting of a novel hyperbranched polyether PHEMO (poly(3-{2-[2-(2-hydroxyethoxy) ethoxy] ethoxy}methyl-3′-methyloxetane)), PVDF-HFP (poly(vinylidene fluoride-hexafluoropropylene)) and LiTFSI have been prepared by solution casting technique. X-ray diffraction of the PHEMO/PVDF-HFP polymer matrix and pure PVDF-HFP revealed the difference in crystallinity between them. The effect of different amounts of PVDF-HFP and lithium salts on the conductivity of the polymer electrolytes was studied. The ionic conductivity of the prepared polymer electrolytes can reach 1.64 × 10^? 4 S·cm^? 1 at 30 °C and 1.75 × 10^? 3 S·cm^? 1 at 80 °C. Thermogravimetric analysis informed that the PHEMO/PVDF-HFP matrix exhibited good thermal stability with a decomposition temperature higher than 400 °C. The electrochemical experiments showed that the electrochemical window of the polymer electrolyte was around 4.2 V vs. Li⁺/Li. The PHEMO/PVDF-HFP polymer electrolyte, which has good electrochemical stability and thermal stability, could be a promising solid polymer electrolyte for polymer lithium ion batteries.     

Polyphosphazene based composite polymer electrolytes

Composite salt-in-polymer electrolyte membranes were prepared from poly[(bis(2-methoxyethyl)amino)_1−x(n-propylamino)_x-phosphazene] (BMEAP) with dissolved LiCF₃SO₃ and dispersed Al₂O₃ nanoparticles (40 nm). Membranes with good mechanical stability were obtained. Low ionic conductivities were found in particle free membranes with maximum conductivities at 10 wt.% LiCF₃SO₃ ranging from 3.1 × 10^− 7 S/cm at 30 °C to 1.8 × 10^− 5 S/cm at 90 °C. For the composite membranes, addition of 2 wt.% Al₂O₃ nanoparticles leads to a steep increase of the conductivity by almost two orders of magnitude as compared to the homogeneous membranes. The highest room temperature conductivity for the investigated BMEAP–LiCF₃SO₃–Al₂O₃ composite systems was 10^− 5 S/cm.