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.     

Ionic conductance of composite electrolytes based on network polymer with ceramic powder

Effects of ceramic fillers (α-Al₂O₃, γ-Al₂O₃ and BaTiO₃) have been investigated on the ionic conductance of polymeric complexes consisting of poly(ethylene oxide)-modified poly(methacrylate) (PEO-PMA) and lithium bis(trifluoromethylsulfonyl)imide, Li(CF₃SO₂)₂N, and ceramic powder. The addition of ceramic powder increased the ionic conductivity over an ambient temperature range. Conductivity of 4.9 × 10^− 5 S cm^− 1 at 333 K (60 °C) was obtained for the composite containing 15 wt.% α-Al₂O₃ prepared by photo-polymerization. The optimum content of Al₂O₃ was different among the methods of polymerization. The highest conductivity was obtained for the composite containing 5 wt.% of α-, or γ-Al₂O₃ prepared by thermal polymerization. The addition of the ceramic filler scarcely influenced the thermal properties of the polymer matrix. XRD and NMR experiments showed that the ionic mobility could be enhanced in the composites by addition of α-Al₂O₃. The addition of small amounts of ferroelectric BaTiO₃ also increased the ionic conductivity of the polymeric complex, but its extent was smaller than the case of the Al₂O₃ addition.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Polymer electrolytes based on cross-linked cyclotriphosphazenes

Substituted cyclotriphosphazenes were used to prepare lithium ion conducting polymer networks. Two types of compounds were synthesized starting with the precursor hexachlorocyclotriphosphazene (HCCP): Type I (CVEEP) in which all the chlorine atoms in HCCP were replaced by vinyloxyethoxyethoxy groups (VEE = –OCH₂CH₂OCH₂CH₂OCHCH₂), and type II (CVMEEP) in which half of the chlorine was replaced by VEE and the other half by methoxyethoxyethoxy groups (MEE = –OCH₂CH₂OCH₂CH₂OCH₃). The terminal vinyl groups were used to build up a network by a thermally initiated cross-linking of lithium salt containing membranes. Polymer electrolytes with dissolved LiSO₃CF₃ and LiN(SO₂CF₃)₂ were investigated by impedance measurements. The ionic conductivity of CVMEEP with 10 wt.% LiSO₃CF₃ was 3.2 × 10^− 5 S/cm at 30 °C and 4.1 × 10^− 4 S/cm at 90 °C. Lower conductivity values in the range 10^− 8–10^− 9 S/cm were obtained at 30 °C for the highly crosslinked CVEEP. An interesting polymer electrolyte with good mechanical properties and a good conductivity of 1.3 × 10^− 5 S/cm (30 °C) was obtained from a solution of MEEP (= poly[bis(methoxy–ethoxy–ethoxy)phosphazene]) and LiSO₃CF₃ in CVEEP as an interpenetrating network.     

Synthesis,structural characterization and electrical property of new oxide ion conductors: La3MMo2O12 (M = In,Ga and Al)

New series of oxides, La₃MMo₂O₁₂ (M = In, Ga and Al), have been prepared by the solid-state reaction. The composition and elemental distribution were analyzed by the energy-dispersive X-ray (EDX) analysis. As determined by the X-ray diffraction (XRD), these compounds have similar crystal structures that can be indexed on a monoclinic cell at room temperature. AC impedance spectra and the DC electrical conductivity measurements in various atmospheres indicate that they are oxide ion conductors with ionic conductivities between 10^− 2 and 10^− 3 S/cm at 800 °C. The conductivity decreases in the order of La₃GaMo₂O₁₂ > La₃AlMo₂O₁₂ > La₃InMo₂O₁₂, implying that the effect of cell volume and polarization associated with In³⁺, Ga³⁺ and Al³⁺ play an important role in the anion transport of these materials. The reversible phase transition was observed in all these compounds as confirmed by the differential thermal analysis (DTA) and dilatometric measurements.     

Mixed conductivity and electrocatalytic performance of SrFeO3-δ–SrAl2O4 composite membranes

Oxygen-ionic and electronic transport in dense (SrFe)_1−x(SrAl₂)_xO_z composites, consisting of strontium-deficient Sr(Fe,Al)O_3-δ and SrAl₂O₄ phases, is determined by the properties of perovskite-like solid solution. Increasing the content of SrAl₂O₄, with a total conductivity as low as 5 × 10^− 7 −  10^− 5 S × cm^− 1 at 973–1273 K in air, results in the gradual decrease of the partial conductivities, but also enables the suppression of thermal expansion. Compared to single-phase SrFe_1−xAl_xO_3-δ, (SrFe)_1−x(SrAl₂)_xO_z composites exhibit enhanced thermomechanical properties, while the oxygen permeability of these materials has similar values. The composite membranes exhibit stable performance under air/(H₂–H₂O–N₂) and air/(CH₄–He) gradients at 973–1173 K. The oxidation of dry methane by oxygen permeating through (SrFe)_0.7(SrAl₂)_0.3O_z results in dominant total oxidation, suggesting the necessity to incorporate a reforming catalyst into the ceramic reactors for natural gas conversion.     

Measurement of surface resistivity and surface conductivity of anodised aluminium by optical interferometry techniques

Optical interferometry techniques were used for the first time to measure the surface resistivity and surface conductivity of anodised aluminium samples in aqueous solution, without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out in different sulphuric acid solutions (1.0–2.5% H₂SO₄), by the technique of electrochemical impedance spectroscopy (EIS), at room temperature. In the mean time, the real-time holographic interferometric was carried out to measure the thickness of anodised (oxide) film of the aluminium samples during the anodization process. Then, the alternating current (AC) impedance (resistance) of the anodised aluminium samples was determined by the technique of electrochemical impedance spectroscopy (EIS) in different sulphuric acid solutions (1.0–2.5% H₂SO₄) at room temperature. In addition, a mathematical model was derived in order to correlate between the AC impedance (resistance) and to the surface (orthogonal) displacement of the samples in solutions. In other words, a proportionality constant (surface resistivity or surface conductivity=1/surface resistivity) between the determined AC impedance (by EIS technique) and the orthogonal displacement (by the optical interferometry techniques) was obtained. Consequently the surface resistivity (ρ) and surface conductivity (σ) of the aluminium samples in solutions were obtained. Also, electrical resistivity values (ρ) from other source were used for comparison sake with the calculated values of this investigation. This study revealed that the measured values of the resistivity for the anodised aluminium samples were 2.8×10⁹, 7×10¹², 2.5×10¹³, and 1.4×10¹²  Ω cm in 1.0%, 1.5%, 2.0%, and 2.5% H₂SO₄ solutions, respectively. In fact, the determined value range of the resistivity is in a good agreement with the one found in literature for the aluminium oxide, 85% Al₂O₃ (5×10¹⁰ Ω cm in air at temperature 30 °C), 96% Al₂O₃ (1×10¹⁴  Ω cm in air at temperature 30 °C), and 99.7% Al₂O₃ (>1×10¹⁴ Ω cm in air at temperature 30 °C).     

Preparation of poly(vinyl phosphate-b-styrene) copolymers and its blend with PPO as proton exchange membrane for DMFC applications

Poly(vinyl phosphate-b-styrene) (poly(VPP-b-St)) block copolymers were prepared via consecutive telomerization of vinyl acetate (VAc), atom transfer radical polymerization (ATRP) with styrene, saponification, and phosphorylation with phosphorus oxychloride. The resulting block copolymers were characterized by FT-IR and pH titration. Then, the block copolymers were blended with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) to prepare direct methanol fuel cell (DMFC) membrane. The performance of poly(VPP-b-St)/PPO blend membranes was measured in terms of proton conductivity, methanol permeability, thermal and hydrolytic stability. The proton conductivities were in the range of 10^− 4 to 10^− 2 S/cm (60 °C, RH = 95%); the methanol permeabilities were in the range of 4.14 × 10^− 8 to 9.62 × 10^− 8 cm²/s (25 °C), and quite lower than that of Nafion® 117. Also, the thermal stability of the blend membranes was characterized by TGA, and was stable up to 400 °C; the blend membranes had better hydrolytic stability.     

Phase stability and oxygen non-stoichiometry of SrCo0.8Fe0.2O3−δ measured by in situ neutron diffraction

The phase stability, oxygen stoichiometry and expansion properties of SrCo_0.8Fe_0.2O_3−δ (SCF) were determined by in situ neutron diffraction between 873 and 1173 K and oxygen partial pressures of 5 × 10^− 4 to 1 atm. At a pO₂ of 1 atm, SCF adopts a cubic perovskite structure, space group Pm3¯m, across the whole temperature range investigated. At a pO₂ of 10^− 1 atm, a two-phase region exists below 922 K, where the cubic perovskite phase coexists with a vacancy ordered brownmillerite phase, Sr₂Co_1.6Fe_0.4O₅, space group Icmm. A pure brownmillerite phase is present at pO₂ of 10^− 2 and 5 × 10^− 4 atm below 1020 K. Above 1020 K, the brownmillerite phase transforms to cubic perovskite through a two-phase region with no brownmillerite structure observed above 1064 K. Large distortion of the BO₆ (B = Co, Fe) octahedra is present in the brownmillerite structure with apical bond lengths of 2.2974(4) Å and equatorial bond lengths of 1.9737(3) Å at 1021 K and a pO₂ of 10^− 2 atm. SCF is highly oxygen deficient with a maximum oxygen stoichiometry, 3 − δ, measured in this study of 2.58(2) at 873 K and a pO₂ of 1 atm and a minimum of 2.33(2) at 1173 K and a pO₂ of 5 × 10^− 4 atm. Significant differences in lattice volume and expansion behavior between the brownmillerite and cubic perovskite phases suggest potential difficulties in thermal cycling of SrCo_0.8Fe_0.2O_3−δ membranes.     

Electric property of mixed conductor BaIn1−xCoxO3−δ

We synthesized BaIn_1−xCo_xO_3−δ (x = 0–0.8) with a defective perovskite structure by partly replacing In with Co in Ba₂In₂O₅. Based on XRD measurements, the synthesized compound was found to have cubic perovskite and orthorhombic brownmillerite structures depending on the amount of Co. BaIn_1−xCo_xO_3−δ (x = 0.2 and 0.3) showed high total electrical conductivities without undergoing the structural transformation that the original Ba₂In₂O₅ undergoes. Some of the samples showed both electronic and oxide ionic conductivities. At the same time, the oxide ionic conductivity was comparable with that of Ba₂In₂O₅. For example, the sample with x = 0.1 had a total electrical conductivity of 4.7 × 10^− 1 S cm^− 1 and an oxide ion transport number of 0.52 at 850 °C.     

Oxygen diffusion and surface exchange studies on (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ

Oxygen tracer diffusion coefficient (D^⁎) and surface exchange coefficient (k^⁎) have been measured for (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ using isotopic exchange and depth profiling by secondary ion mass spectrometry technique as a function of temperature (700–1000 °C) in dry oxygen and in a water vapour-forming gas mixture. The typical values of D^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 4 × 10^− 10 cm² s^− 1 and 3 × 10^− 8 cm² s^− 1 respectively, whereas the values of k^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 5 × 10^− 8 cm s^− 1 and 4 × 10^− 8 cm s^− 1 respectively. The apparent activation energies for D^⁎ in oxidising and reducing conditions are 0.8 eV and 1.9 eV respectively.     

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.     

Single ion conductors—polyphosphazenes with sulfonimide functional groups

A novel single ion conductive polymer electrolyte was developed by covalently linking an arylsulfonimide substituent to the polyphosphazene backbone. Polymeric single-ion conductors incorporate the anion of a salt either into the polymer backbone or as a pendent group linked to the polymer backbone. Immobilization of the anion could provide access to electrochemical devices that would be less vulnerable to increased resistance associated with salt concentration gradients at the interfaces during charging and discharging. In this work, an immobilized sulfonimide lithium salt is the source of lithium cations, while a cation-solvating cosubstituent, 2-(2-methoxyethoxy)ethoxy, was used to increase free volume and assist cation transport. The ionic conductivities showed a dependence on the percentage of lithiated sulfonimide substituent present. Increasing amounts of the lithium sulfonimide component increased the charge carrier concentration but decreased the ionic conductivity due to decreased macromolecular motion and possible increased shielding of the nitrogen atoms in the polyphosphazene backbone. Maximum ionic conductivity values of 2.45 × 10^− 6 S/cm at ambient temperature and 4.99 × 10^− 5 S/cm at 80 °C were obtained. Gel polymer electrolytes containing N-methyl-2-pyrrolidone gave ionic conductivities in the 10^− 3 S/cm range. The ion conduction process was investigated through model polymers that contained the non-immobilized sulfonimide — systems that had higher conductivities than their single ion counterparts.     

Mesopore size dependence of the ionic diffusivity in alumina based composite lithium ionic conductors

Ordered-mesoporous Al₂O₃ was synthesized by a sol–gel method using neutral copolymer surfactants as structure-directing agents. The pore size was controlled over the 3–15 nm range by the use of various surfactants. Composites composed of the synthesized mesoporous Al₂O₃ and a lithium ion conductor (LiI) were prepared. The maximum dc electrical conductivity, 2.6 × 10^− 4 S cm^− 1 at 298 K, was observed for 50 LiI·50 Al₂O₃ composite with 4.2 nm average mesopore size, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which conductivity increased with decreasing pore size, except for samples with a pore size of 2.8 nm. The lithium ion diffusion coefficient determined by the ⁷Li pulsed field gradient nuclear magnetic resonance (PFG-NMR) showed excellent agreement with the measured conductivity calculated by the Nernst-Einstein equation. On the other hand, lithium migration activation energies obtained by quasielastic neutron scattering (QENS) and ⁷Li NMR spin-lattice relaxation time (T₁) were considerably smaller than those obtained from electrical conductivity and PFG-NMR. This could be explained by the ion migration mechanism in heterogeneous composites and a possible enhancement of conductivity in mesoscopically confined spaces.     

Kinetic studies of electronically conducting Yb3Al5O12 garnet

Upon reduction, originally fully transparent and insulating ytterbium alumina garnet single crystals, Yb₃Al₅O₁₂, become deeply colored and electrically conducting with a conductivity of the order of 10^− 3 Ω^− 1 cm^− 1 in the temperature range of 550 °C to 1000 °C. The redox kinetics of the material is studied by means of conductivity relaxation experiments performed at oxidising and reducing conditions. Good agreement is obtained with an optical study into the redox kinetics of Yb₃Al₅O₁₂.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Structure and conductivity investigations of alkaline earth substituted uranium oxide,U1 − xMxO2 ± δ (M = Mg,Ca, Sr) for solid oxide fuel cell applications

Alkaline earth substituted UO₂ (U_1 − xM_xO_2 ± δ; M = Mg, Ca, Sr; 0.1 ≤ x ≤ 0.525) with fluorite structure was synthesized in reducing atmosphere. Structure and conductivity properties of U_1 − xM_xO_2 ± δ fluorites were investigated for possible application in solid oxide fuel cells (SOFC). At room temperature and ambient atmosphere the materials are stable; however they decompose at an oxygen partial pressure p_O2 > 10^− 4 atm and temperatures higher than 600 °C. The total conductivity measured for the best conducting U_1 − xM_xO_2 ± δ material with M = Ca and x = 0.177 is as high as 3 S/cm at p_O2 < 10^− 4 atm at 600 °C. The relatively low ionic transference number (t_i∼0.02) is disadvantageous for potential use as electrolyte material for SOFC applications. The high conductivity and possible depolarization effects suggest potential use as anode materials in SOFC.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Synthesis and properties of a new class of fluorine-containing dilithium salts for lithium-ion batteries

A facile synthetic route for the development of a new class of dilithium salts is described. Because of the presence of two lithium ions per molecule, these salts require lower concentrations than commonly used salts to achieve comparable ionic conductivities at ambient temperatures. An ionic conductivity of 3.55 × 10^− 3 S/cm at 30 °C was obtained using 0.5 M salt solution in 1:1 wt/wt ethylene carbonate:dimethyl carbonate. The salts exhibit excellent thermal stabilities to at least 350 °C and are electrochemically stable below 4.2 V versus lithium metal. The best salt was tested with a polymer electrolyte system. Incorporation of a polyethylene glycol-based borate ester plasticizer improved the ionic conductivity of the solid polymer electrolyte film up to 1.36 × 10^− 5 S/cm at 30 °C, which is 10 times higher than that of un-plasticized electrolyte films.     

Conductivity of SnP2O7 and In-doped SnP2O7 prepared by an aqueous solution method

SnP₂O₇ and In-doped SnP₂O₇ have been prepared by an aqueous solution method using (NH₄)₂HPO₄ as phosphorous source. It was found that the solid solution limit in Sn_1 ? xIn_x(P₂O₇)_1 ? δ was at least x = 0.12. All pyrophosphates in the Sn_1 ? xIn_x(P₂O₇)_1 ? δ (x ≤ 0.12) series exhibit 3 × 3 × 3 superlattice structures. The conductivities of Sn_0.92In_0.08(P₂O₇)_1 ? δ in air are 6.5 × 10^? 6 and 8.0 × 10^? 9 S/cm at 900 and 400 °C, respectively, when prepared by an aqueous solution method and annealed at 1000 °C. The conductivity of undoped SnP₂O₇ is slightly lower. However, it was also found that the low-temperature conductivities of pyrophosphates annealed only at 650 °C are several orders of magnitude higher than those annealed at 1000 °C, which could be related to a trace amount of an amorphous secondary phase. The peak conductivity was in this case observed at around 250 °C, which is the same temperature as previously observed in In-doped SnP₂O₇ although the conductivity is still three orders of magnitude lower in the present study. These differences can be related to large differences in particle size and morphology, and all in all, the conductivities of SnP₂O₇-based materials are very sensitive to the synthetic history.