Water vapour solubility and conductivity study of the proton conductor BaCe(0.9 − x)ZrxY0.1O(3 − δ)

The perovskite BaCe_(0.9 ? x)Zr_xY_0.1O_(3 ? δ) has been prepared by solid state reaction at 1400 °C and conventional sintering at 1700 °C. Water uptake experiments performed between 400 and 600 °C, at a water vapour pressure of 0.02 atm, provide data on the concentration of protons incorporated in the sample. The direct current conductivity has been measured as a function of oxygen partial pressure, at a water vapour partial pressure of 0.015 atm. The total conductivity has been resolved into a p-type and an ionic component using a fitting procedure appropriate to the assumed defect model. An estimation of the protonic component was made by assuming a conductivity isotope effect between 1.4 and 1.8. The total conductivity, obtained using impedance spectroscopy has been measured as a function of temperature in the water and heavy water exchanged states. The activation energy has been found to be 0.56 eV to 0.59 eV in the water exchanged state with values 0.03 to 0.04 eV higher in the heavy water exchanged state. Impedance spectra measured at 200 °C showed a reduction in grain boundary resistivity with increasing cerium content. The stability of the compounds to carbon dioxide has been studied by thermogravimetry.     

Ionic and electronic conductivity of nitrogen-doped YSZ single crystals

The ionic and electronic charge transport was studied for single crystals of 9.5 mol% yttria-stabilized zirconia with additional nitrogen doping (YSZ:N) of up to 7.5 at.% (referred to the anion sublattice and formula unit Zr_0.83Y_0.17O_1.91) as a function of temperature and nitrogen content. The total conductivity being almost equivalent to the oxygen ion conductivity has been measured by AC impedance spectroscopy under vacuum conditions in order to prevent re-oxidation and loss of nitrogen. The electronic conductivity has been determined by Hebb–Wagner polarization using ion-blocking Pt microelectrodes in N₂ atmosphere. The ionic conductivity of YSZ:N decreases in the presence of nitrogen at intermediate temperatures up to 1000 °C. The mean activation energy of ionic conduction strongly increases with increasing nitrogen content, from 1.0 eV for nitrogen-free YSZ up to 1.9 eV for YSZ containing 7.3 at.% N. Compared to nitrogen-free YSZ, the electronic conductivity first decreases at nitrogen contents of 2.17 and 5.80 at.%, but then increases again for a sample with 7.53 at.%. At temperatures of 850 °C and above, the presence of the N^3? dopant fixes the electrode potential and thus the oxygen partial pressure at the Pt electrode to very low values. This corresponds to a pinning of the Fermi level at a relatively high energy in the upper half of the band gap. At 7.53 at.% N and 950 °C, the oxygen partial pressure in YSZ:N corresponds to p_O2 = 3 × 10^? 18 bar. At temperatures above 850 °C, even in the presence of a very small oxygen concentration in the surrounding gas phase, the nitrogen ion dopant becomes highly mobile and thus diffuses to the surface where it is oxidized to gaseous N₂. The results are discussed in terms of the ionic and electronic defect structures and the defect mobilities in YSZ:N.     

Ionic and electronic conductivity of Yb2+xTi2−xO7−x/2 materials

Bulk and grain boundary conductivities of Yb_2+xTi_2−xO_7−x/2 (x = 0, 0.1, 0.18 and 0.29) materials were studied by impedance spectroscopy in the range 300–900 °C in air. Ionic and electronic conductivities were separated by both ion blocking Hebb–Wagner measurements and total conductivity measurements as a function of oxygen partial pressure in the temperature range 700–1000 °C. The oxygen partial pressure dependence of the total conductivity shows that these materials are nearly pure ionic conductors in air and that the ionic conductivity decreases for Yb-rich compositions. This was interpreted as a predominant effect of a decrease in mobility of ionic charge carriers, opposing the expected increase in concentration of oxygen vacancies with increasing Yb content. The studied materials become mixed conductors under typical fuel conditions, except possibly at temperatures below about 700 °C. Yb-excess slightly suppresses the electronic conductivity.     

Sintering kinetics and oxide ion conduction in Sr-doped apatite-type lanthanum silicates,La9Sr1Si6O26.5

These last past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10 ? xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at these intermediate temperatures. An apatite powder (x = 1) with a 0.75 µm mean particle size, produced by solid state reaction, was tape cast to obtain green sheets with a thickness of about 260 µm.On one hand, the densification mechanism of the apatite ceramic during the intermediate solid state sintering has been approached. It appeared from the kinetical tests performed under isothermal conditions between 1250 and 1550 °C, that densification could be controlled by the diffusion at grain boundaries of the rare-earth element, La, with an activation energy of 470 kJ/mol.On the other hand, conductivity measurements were performed on apatite samples sintered at 1400 and 1500 °C. The ionic conductivity was mainly sensitive to the presence of secondary phases at 1400 °C. The ionic conductivity of the apatite sintered at 1500 °C (mean grain size = 3.9 µm) is equal to 1.2 × 10^? 2 S/cm at 700 °C.     

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.     

Proton conducting membranes from sulfonated poly[bis(phenoxy)phosphazenes] with an interpenetrating hydrophilic network

Proton conducting membranes have been prepared using sulfonated poly[bis(phenoxy)phosphazene] (s-BPP) trapped in a cross-linked interpenetrating hydrophilic network of hexa(vinyloxyethoxyethoxy)cyclotriphosphazene (CVEEP). Membranes with good mechanical and thermal stabilities were obtained exhibiting high ion exchange capacities in the range of 1.62–1.79 mmol/g. The proton conductivity was measured as a function of water partial pressure in nitrogen (0 mbar to 25 mbar) in the temperature range 25 °C to 75 °C. At 25 mbar water partial pressure and 75 °C, a conductivity of 2.2 × 10^− 4 S/cm was obtained for s-BPP with a network made of 50 wt.% CVEEP. After immersion in water, the conductivity increased up to 0.013 S/cm at 25 °C. The hydrophilic nature of the CVEEP network stabilizes the water content and enhances the proton conductivity at elevated temperatures.     

Acoustic cavitation in 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide based ionic liquid

In this work, a comparison between the temperatures/pressures within acoustic cavitation bubble in an imidazolium-based room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(triflluoromethyl-sulfonyl)imide ([BMIM][NTf₂]), and in water has been made for a wide range of cavitation parameters including frequency (140–1000 kHz), acoustic intensity (0.5–1 W cm⁻²), liquid temperature (20–50 °C) and external static pressure (0.7–1.5 atm). The used cavitation model takes into account the liquid compressibility as well as the surface tension and the viscosity of the medium. It was found that the bubble temperatures and pressures were always much higher in the ionic liquid compared to those predicted in water. The valuable effect of [BMIM][NTf₂] on the bubble temperature was more pronounced at higher acoustic intensity and liquid temperature and lower frequency and external static pressure. However, confrontation between the predicted and the experimental estimated temperatures in ionic liquids showed an opposite trend as the temperatures measured in some pure ionic liquids are of the same order as those observed in water. The injection of liquid droplets into cavitation bubbles, the pyrolysis of ionic liquids at the bubble-solution interface as well as the lower number of collapsing bubbles in the ionic liquid may be the responsible for the lower measured bubble temperatures in ionic liquids, as compared with water.     

Defect structure and electrical conduction behavior of Bi-based pyrochlores

Pyrochlore-type quaternary systems with the generic formula (Bi_1.5Zn_0.5)(Nb_0.5M_1.5)O₇ (BZNM, M=Ti, Sn, Zr, and Ce) have been synthesized by the standard solid-state reaction route. The study of positron annihilation lifetime spectra provides direct evidence for defect characterization for BZNM ceramics with different tetravalent element. The variation of the annihilation lifetime suggests that the defect structure undergoes significant changes. The defect structure is further described by means of the complex-defect model. Electrical conduction in the samples is expected to result mainly from the defects present in the lattice. Conduction measurement shows there are different conduction mechanisms. For temperatures less than 350 °C, electronic conduction dominates, while for temperatures greater than 350 °C, ionic conduction is dominant in the present system. The conduction mechanism includes the formation of charged defect dipoles, causing internal bias fields, creating oxygen vacancies that promote ionic conductivity.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Ion and electron conduction in SrFe1−xScxO3−δ

The oxygen ion and electron transport in SrFe_1−xSc_xO_3−δ  (x = 0.1–0.3) system at 700–950 °C were studied analyzing the total conductivity dependencies on the oxygen partial pressure, pO₂. The conductivity measurements were performed both under reducing conditions (10^− 19 ≤ pO₂ ≤ 10^− 8 atm) comprising the electron-hole equilibrium point, and in oxidizing atmospheres (10^− 5 ≤ pO₂ ≤ 0.5 atm) which are characterized by extensive variations of the oxygen content studied by coulometric titration technique. The incorporation of 10% Sc³⁺ cations into the iron sublattice suppresses transition of the cubic perovskite phase into vacancy-ordered brownmillerite, thus improving ion conduction at temperatures below 850 °C. When scandium content increases, the ion conductivity becomes considerably lower. The hole mobility is thermally-activated and varies in the range of 0.001 to 0.05 cm² V^− 1 s^− 1, increasing with oxygen concentration and decreasing on Sc doping.     

Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure

The electrocatalytic synthesis of ammonia from steam and nitrogen was studied in oxygen ion (O^2?) and proton (H⁺) conducting solid electrolyte cells at 450–700 °C and at atmospheric total pressure. A Ru-based industrial catalyst was used as the working electrode. In the H⁺ cell, steam was electrolyzed at the anode to produce protons and oxygen. Protons, transported to the cathode, reacted with nitrogen to produce ammonia. In the O^2? cell, H₂O and N₂ were fed in together at the cathode. Steam was electrolyzed and the produced hydrogen reacted with nitrogen. Ammonia formation was observed at temperatures between 500 and 700 °C. The conversions with respect to nitrogen or steam were low, primarily because of the poor conductivity of the working electrode. Both cells, however, exhibit promising features that make this alternative approach of ΝΗ₃ synthesis worthy of further investigation.     

Proton conductivity and low temperature structure of In-doped BaZrO3

The proton conductivity and structural features of In³⁺ substituted BaZrO₃ samples, i.e., BaZr_1−xIn_xO_3−δ, were investigated. Rietveld analysis of low temperature (10 K) neutron powder diffraction data collected on as-prepared and deuterated samples confirmed cubic symmetry (space group Pm-3m) for all compositions. The level of oxygen vacancies refined in the as-prepared samples were in good agreement with the values expected to conserve charge neutrality, whilst an increase in oxygen occupancy, reflecting the incorporation of OD⁻ species, was obtained for the deuterated materials. An expansion of the unit cell parameter, a, was observed as a function of In³⁺ doping as well as after the deuteration reaction. The conductivity of pre-hydrated and dry samples was measured using impedance methods. For 25% In-doped BaZrO₃, the low T (300 °C) conductivity of the heating cycle of the dried sample was greater than that of the cooling cycle of the pre-hydrated sample indicating a greater number of protons in the nominally dry sample. In contrast, the conductivity values were similar at higher temperatures e.g. T > 500 °C where proton conduction is not dominant.     

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.     

Synthesis and structural characterization of perovskite type proton conducting BaZr1−xInxO3−δ (0.0 ≤ x ≤ 0.75)

Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr_1−xIn_xO_3−δ (0.0 ≤ x ≤ 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr_1−xIn_xO_3−δ for x = 0.50, and that the maximum water uptake occurs below 300 °C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O–H stretch band, 2500–3500 cm^− 1, in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr_1−xIn_xO_3−δ, x = 0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 °C, whilst these values were similar at higher temperatures i.e. T > 600 °C.     

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.     

Bulk and surface properties of ionic salt CsH5(PO4)2

We report a comparative study of transport and thermodynamic properties of single-crystal and polycrystalline samples of the ionic salt CsH₅(PO₄)₂ possessing a peculiar three-dimensional hydrogen-bond network. The observed potential of electrolyte decomposition ≈ 1.3 V indicates that the main charge carriers in this salt are protons. However, in spite of the high proton concentration, the conductivity appears to be rather low with a high apparent activation energy E_a ≈ 2 eV, implying that protons are strongly bound. The transport anisotropy though is not large, correlates with the crystal structure: the highest conductivity is found in the [001] direction (σ_130 °C ∼ 5.6 × 10^− 6 S cm^− 1) while the minimal conductivity is in the [100] direction (σ_130 °C ∼ 10 ⁻⁶ S cm^− 1). The conductivity of polycrystalline samples appears to exceed the bulk one by 1–3 orders of magnitude with a concomitant decrease of the activation energy (E_a ≈ 1.05 eV), which indicates that a pseudo-liquid layer with a high proton mobility is formed at the surface of grains. Infrared and Raman spectroscopy used to study the dynamics of the hydrogen-bond system in single-crystal and polycrystalline samples have confirmed the formation of such a modified surface layer in the latter. However, no bulk phase transition into the superionic disordered phase is observed in CsH₅(PO₄)₂ up to the melting point T_melt ∼ 151.6 °C, in contrast to its closest relative compound CsH₂PO₄.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Hybrid inorganic–organic proton conducting membranes for fuel cells and gas sensors

We developed new fast proton conducting membranes based on a hybrid inorganic–organic phosphosilicate polymer synthesized from othophosphoric acid, dichlorodimethylsilane, and tetraethoxysilane. The membranes were amorphous, translucent, and flexible. A high concentration of –OH groups and short distances between them promoted fast proton conductivity in dry atmosphere at increased temperatures. The proton conductivity was measured using the electrochemical impedance spectroscopy. Its value increased with rising temperature following the Arrhenius dependence with the activation energy 20 kJ/mol. In dry conditions at 120 °C, the conductivity was 1.6 S/m. The tests in a H₂/O₂ fuel cell confirmed that the membrane was able to operate at temperatures from 100 to 130 °C using dry input gas streams. The cell performance significantly improved with increasing temperature. The membrane was also tested in a potentiometric gas sensor with the TiH_x reference electrode and the Pt sensing electrode. The sensor exhibited fast, stable, and reproducible response to dry H₂ and O₂ gases at temperatures above 100 °C. We expect the application of our membrane in intermediate temperature fuel cells and gas sensors operating in dry conditions.     

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.     

A preliminary study on a new LiBOB/acetamide solid phase transition electrolyte

New solid electrolytes containing acetamide and lithium bioxalato borate (LiBOB) with different molar ratios have been investigated. Their melting points (T_m) are around 42 °C. The ionic conductivities and activation energies vary drastically below and above T_m, indicating a typical feature of phase transition electrolyte. The ionic conductivity of the LiBOB/acetamide electrolyte with a molar ratio of 1:8 is 5 × 10^? 8 S cm^? 1 at 25 °C but increases to 4 × 10^? 3 S cm^? 1 at 60 °C. It was found that anode materials, such as graphite and Li₄Ti₅O₁₂, could not discharge and charge properly in this electrolyte at 60 °C due to the difficulty in forming a stable passivating layer on the anodes. However, a Li/LiFePO₄ cell with this electrolyte can be charged properly after heating to 60 °C, but cannot be charged at room temperature. Although the LiBOB/acetamide electrolytes are not suitable for Li-ion batteries due to poor electrode compatibility, the current results indicate that a solid electrolyte with a slightly higher phase transition temperature than room temperature may find potential application in stationary battery for energy storage where the electrolyte is at high conductive liquid state at elevated temperature and low conductive solid state at low temperature. The interaction between acetamide and LiBOB in the electrolyte is also studied by Raman and FTIR spectroscopy.