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.     

High temperature properties of the n = 2 Ruddlesden–Popper phases (La,Sr)3(Fe,Ni)2O7−δ

The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden–Popper (R–P) phases Sr_3−xLa_xFe_2−yNi_yO_7−δ with 0 ≤ x ≤ 0.3 and 0 ≤ y ≤ 1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO₂ (10^− 5 ≤ pO₂ ≤ 1 atm) in the temperature range 400 ≤ T ≤ 1000 °C indicate that the Sr₃FeNiO_7−δ phase is able to accommodate a large oxygen non-stoichiometry (δ ∼ 1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550 ≤ T ≤ 1000 °C. The electrical transport of the Sr₃FeNiO_7−δ phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

Ionic conductivity of Ce1−xNdxO2−x / 2

The electrical conductivity of sintered samples of Ce_1−xNd_xO_2−x / 2 (0.01 ≤ x ≤ 0.2) was investigated in air as a function of temperature between 150 and 600 °C using AC impedance spectroscopy. The individual contribution of the bulk and grain boundary conductivities has been discussed in detail. In the low temperature range (< 350 °C), the activation enthalpy for bulk conductivity exhibited a shallow minimum at 3 mol% Nd, with a value of 0.68 eV. The activation enthalpy also produced a shallow minimum at 5 mol% Nd in the high temperature range (> 350 °C), with a value of 0.56 eV. It was shown that Ce_1−xNd_xO_2−x / 2 is an electrolyte that obeys the Meyer Neldel rule. The bulk conductivity data measured by others for the same system has also been recalculated and re-evaluated to facilitate easier comparison with our own data.     

Synthesis and characterization of layered Li1−x(Ni0.5Co0.5)1−yFeyO2(0 ≤ (1 − x) ≤ 1 and 0 ≤ y ≤ 0.2) cathodes

Layered Li(Ni_0.5Co_0.5)_1−yFe_yO₂ cathodes with 0 ≤ y ≤ 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 °C and characterized as cathode materials for lithium ion batteries to various cutoff charge voltages (up to 4.5 V). While the y = 0.05 sample shows an improvement in capacity, cyclability, and rate capability, those with y = 0.1 and 0.2 exhibit a decline in electrochemical performance compared to the y = 0 sample. Structural characterization of the chemically delithiated Li_1−x(Ni_0.5Co_0.5)_1−yFe_yO₂ samples indicates that the initial O3 structure is maintained down to a lithium content (1 − x) ≈ 0.3. For (1 − x) < 0.3, while a P3 type phase is formed for the y = 0 sample, an O1 type phase is formed for the y = 0.05, 0.1 and 0.2 samples. Monitoring the average oxidation state of the transition metal ions with lithium contents (1 − x) reveals that the system is chemically more stable down to a lower lithium content (1 − x) ≈ 0.3 compared to the Li_1−xCoO₂ system. The improved structural and chemical stabilities appear to lead to better cyclability to higher cutoff charge voltages compared to that found before with the LiCoO₂ system.     

Defect equilibria and partial molar properties of (La,Sr)(Co,Fe)O3−δ

The oxygen nonstoichiometry δ of La_1−xSr_xCo_1−yFe_yO_3−δ (x = 0.6 and y = 0.2, 0.4) was investigated by thermogravimetry in the range 703 ≤ T/°C ≤ 903 and 1E−5 < pO₂/atm < 1. The oxygen deficit increases with increasing T and decreasing pO₂. Electronic conductivities σ were measured as a function of pO₂ in the range 1E−5 < pO₂/atm < 1 at 700 ≤ T/°C ≤ 900. At constant T, a p-type pO₂-dependence of σ is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation ΔH_ox^θ and ΔS_ox^θ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO₂^θ = 1 atm), (h_O − H_O^θ) and (s_O − S_O^θ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (h_O − H_O^θ) decreases with increasing δ, while (s_O − S_O^θ) is constant within the limits of error. Defect chemical modelling was performed by an ideal solution model under consideration of three different valence states for B-site ions (Co or Fe). The dependence of σ on δ is modelled, using calculated defect concentrations as functions of δ. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies.     

Electrode reaction of Sr1−xLaxCo0.8Fe0.2O3−δ with x = 0.1 and 0.6 on Ce0.9Gd0.1O1.95 at 600 ≤ T ≤ 800 °C

The electrode reaction of the perovskite phases Sr_1−xLa_xCo_0.8Fe_0.2O_3−δ (x = 0.1 and 0.6) on Ce_0.9Gd_0.1O_1.95 has been investigated by impedance spectroscopy in the temperature range 600 ≤ T ≤ 800 °C. Thick porous electrodes (t ∼ 20 μm) were sprayed on Ce_0.9Gd_0.1O_1.95 and ac impedance spectra were recorded on symmetrical cells at the equilibrium. The analysis of the complex impedance diagrams clearly indicates the presence of two contributions. The low frequency one was assigned to the gas phase oxygen diffusion through the porous electrode and a finite length diffusion (Warburg) impedance was used to describe the high frequency (HF) data. The polarization resistance of the HF impedance contribution (R_w) is higher for x = 0.1 while the activation energy of R_w is higher for x = 0.6. The variations of R_w versus the La content, temperature and thickness indicate that the Warburg-type impedance contains information of both bulk oxygen diffusion and surface processes.     

Spectroscopic studies of the structural properties of Ni substituted spinel LiMn2O4

Microstructure and local structure of spinel LiNi_xMn_2 − xO₄ (x = 0, 0.1 and 0.2) were studied using X-ray diffraction (XRD) and a combination of X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and Raman scattering with the aim of getting a clear picture of the local structure of the materials responsible for the structural stability of LiNi_xMn_2 − xO₄. XRD study showed that Ni substitution caused the changes of the materials’ microstructure from the view of the lattice parameter, mean crystallite size, and microstrain. XPS and XANES studies showed the Ni oxidation state in LiNi_xMn_2 − xO₄ was larger than + 2, and the Mn oxidation state increased with Ni substitution. The decrease of the intensity of the 1s → 4p_z shakedown transition on the XANES spectra indicated that Ni substitution suppressed the tetragonal distortion of the [MnO₆] octahedron. The Mn(Ni)–O bond in LiNi_xMn_2 − xO₄, which is stronger than the Mn–O bond in LiMn₂O₄ was responsible for the blue shift of the A_1g Raman mode and could enhance the structural stability of the [Mn(Ni)O₆] octahedron.     

Structure,optical, and magnetic properties of rutile Sn1−xMnxO2 thin films

Sn_1?xMn_xO₂ (x ≤ 0.11) thin films were fabricated by sol–gel and spin-coated method on Si (1 1 1) substrate. X-ray diffraction revealed that single-phase rutile polycrystalline structure was obtained for x up to about 0.078. Evolution of the lattice parameters and X-ray photoelectron spectroscopy studies confirmed the incorporation of Mn³⁺ cations into rutile SnO₂ lattice. Optical transmission studies show that the band gap energy (E_g) broadens with the increasing of Mn content. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. Interestingly, the magnetic moment per Mn atom decreases with the increasing Mn content. The origin of RTFM can be interpreted in terms of the bound magnetic polaron model.     

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La_0.9Sr_0.1)[(Ga_1−xM_x)_0.8Mg_0.2]O_3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La_0.2Sr_1.8)[Ga(Fe_1−xMg_x)]O_5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.     

Hydrothermal phase formation of orthorhombic LiMnO2 and its derivatives as lithium intercalation compounds

Different from a conventional solid state reaction, a hydrothermal reaction mechanism is very difficult to illuminate and sometimes it remains undisclosed. Making an attempt to understand the hydrothermal phase formation process of o-LiMnO₂ obtained between the reaction of spinel type Mn₃O₄ precursor and LiOH aqueous solution, the possible reaction route was postulated and experimentally testified. Firstly, the selective dissolution of Mn²⁺ from the tetrahedral site of [Mn^II]^tet_4a[Mn^III₂]^oct_8dO₄, which is considered as to be an ionic exchange reaction with Li⁺, and an additional Li⁺ intercalation into the host structure of precursor would give rise to the formation of meta-stable Li₂Mn₂O₄ ([Li^I]^tet_4a[Li^I]^oct_8c[Mn^III₂]^oct_8dO₄). Secondly, the phase would be simultaneously transformed to thermodynamically stable o-LiMnO₂ phase under hydrothermal state during hydrothermal reaction. Through the above reaction process, the solid solution range of o-LiCo_xMn_1−xO₂ was as large as x ≤ 0.14, and that of o-LiFe_xMn_1−xO₂ was x ≤ 0.05. Co doped o-LiMnO₂ has higher capacity and good cyclability upon cycling, being substantially more stable to cycle than the unsubstituted and Fe doped materials.     

Structural and electrochemical properties of LiNi0.5Mn0.5 − xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_0.5 ? xAl_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge–discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi_0.5Mn_0.5 ? xAl_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_0.5 ? xAl_xO₂ materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219, 169, 155, and 129 mAh g^? 1 at 10, 100, 200, and 400 mA g^? 1 current density respectively. Cycled under 40 mA g^? 1 in 2.8–4.6 V, LiNi_{0. 5}Mn_0.45Al_0.05O₂ shows the highest discharge capacity of about 199 mAh g^? 1 for the first cycle, and 179 mAh g^? 1 after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.     

Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ

Electronic conductivity in the potential SOFC anode material La_1−xSr_xCr_0.5Mn_0.5O_3±δ has been investigated in the range 0.2 < x < 0.3. log(σT) vs. 1/T plots indicate conduction via thermally activated polaron hopping. At 900 °C, conductivity in air increases with Sr²⁺ via an increase in [B_B] holes (B—transition metal). X-ray absorption spectroscopy (XAS) studies indicate that compensation for A-site Sr substitution and oxygen vacancy formation is via the Mn cation only; Cr maintains a 3+ oxidation state and 6-fold oxygen coordination. Electronic transport occurs by percolation between Mn cations in a disordered B-site sub-lattice. Conductivity decreases with p(O₂), which is indicative of p-type conduction behaviour, but the relationship cannot be explained by a simple redox equilibrium involving Mn³⁺, Mn⁴⁺ and oxygen, possibly due to co-existence of Mn²⁺, Mn³⁺ and Mn⁴⁺ via disproportionation as with La_1−xSr_xMnO_3±δ.     

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.     

The effect of cation nonstoichiometry on the electrical conductivity of acceptor-doped CaZrO3

The electrical properties of acceptor-doped Ca_1−xZr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems were investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The characterization was carried out using the impedance spectroscopy between 550 °C and 1100 °C in dry air. The contributions of the grain and grain boundary conductivity to the total conductivity were obtained from the impedance data. When the Ca deficiency (x) increased, the total conductivity rapidly decreased with the corresponding increase in activation energy. Although the grain conductivity increased slightly with increasing x, the total conductivity is mostly determined by the highly resistive grain boundary. With varying x, the activation energy of total conductivity showed the percolation behavior. The percolation threshold values vary according to the doped species. It may be due to the difference in concentration of oxygen vacancies of the specimens.     

A study on the Raman spectra of Al-doped and Ga-doped ZnO ceramics

Al_xZn_1−xO and Ga_yZn_1−yO ceramics were synthesized through a solid-state reaction technique. The crystal phase of the samples was identified by an X-ray diffraction experiment. For each sample, the electrical resistivity was determined. The Al 2-mol%-doped and Ga 0.5-mol%-doped ZnO ceramics had the lowest resistivity. Raman measurement was performed to study the doping effects in the ZnO ceramics including ZnO single crystal as a reference. The line-shape parameters, q₁ and Γ₁, at the same certain doping rate and the solubility limit of Al (2 mol%) and Ga (0.5 mol%) in ZnO ceramics, are strongly related to the each other, and that the solubility limit plays an important role. The second-order Raman peak at 1162 cm⁻¹ of the ZnO ceramics was fitted by Fano formalism. The Fano’s fitting parameters, such as the lifetime of phonon and the degree of asymmetry degree of the second-order Raman peak changed as the amounts of dopants were varied.     

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 characterization of La4Ni3−xCoxO10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2) for solid oxide fuel cell cathodes

The cobalt-doped lanthanum–nickel oxide system, La₄Ni_(3−x)Co_xO_10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2), was investigated as possible cathode materials for intermediate-temperature solid-oxide fuel cells. X-ray diffraction shows the presence of two structural phases in the series belonging to Bmab for 0.0 ≤ x ≤ 0.2, 0.8 ≤ x ≤ 2.0 and 2.6 ≤ x ≤ 3.0 and Fmmm for 0.4 ≤ x ≤ 0.6 and 2.2 ≤ x ≤ 2.4. All compositions are oxygen-deficient (δ < 0). Electrical conductivity measurements show a systematic decrease in the conductivity as cobalt content increases from x = 0.0 to 2.0, and reverses for x > 2.0. AC impedance measurements of the x = 0.4 composition in symmetrical cells with LSGM as an electrolyte show improved electrode performance over the parent nickelate La₄Ni₃O_9.78. Long-term thermal stability studies show the x = 0.4 composition to be more stable than the x = 3.0 phase after annealing at 1173 K in air for 1 week making this material a viable candidate for cathodes in solid oxide fuel cells.     

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.