Doped Germanate‐Based Apatites as Electrolyte for Use in Solid Oxide Fuel Cells

Apatite ceramics, known for their good electrical conductivities, have garnered substantial attention as an alternative electrolyte for solid oxide fuel cells (SOFCs). However, studies focusing on the electrochemical performances of SOFCs with apatities as electrolytes remain rare, partly due to their high sintering temperature. In this study, the effects of Mg²⁺, Al³⁺, Ga³⁺, and Sn⁴⁺ dopants on the characteristics of La_9.5Ge₆O_{26 ± δ} are examined and their potential for use as SOFC electrolytes evaluated. The results indicate that La_9.5Ge_5.5Al_0.5O₂₆ is stabilized into a hexagonal structure, while the La_9.5Ge_5.5Sn_0.5O_26.25, La_9.5Ge_5.5Ga_0.5O₂₆, and La_9.5Ge_5.5Mg_0.5O_25.75 ceramics reveal triclinic cells accompanied with the second phase La₂Sn₂O₇ or La₂GeO₅. The study further demonstrates that a high sintering temperature is needed for both the La_9.5Ge_5.5Mg_0.5O_25.75 and the La_9.5Ge_5.5Sn_0.5O_26.25 ceramics, and the worst electrical conductivity among the examined systems appears in the La_9.5Ge_5.5Ga_0.5O₂₆ ceramic. The La_9.5Ge_5.5Al_0.5O₂₆ ceramic is accordingly selected for cell evaluation due to its ability to reach densification at 1,350 °C, its good electrical conductivity of 0.026 S cm^–1 at 800 °C, and its acceptable thermal expansion coefficient of 10.1 × 10^–6 K^–1. The maximum power densities of the NiO‐SDC/La_9.5Ge_5.5Al_0.5O₂₆/LSCF‐SDC single cell are found to be respectively 0.22, 0.16, 0.11, and 0.07 W cm^–2 at 950, 900, 850, and 800 °C.     

Complex Effect of Sm3+/W6+ Codoping on α‐β Phase Transformation and Phonon Scattering of Oxygen‐Deficient La2Mo2O9

Lanthanum molybdate, La₂Mo₂O₉, has been attracted considerable attention owing to its high concentration of intrinsic oxygen vacancies, which could be reflected by enhanced phonon scattering and low thermal conductivity. A new series of La₂Mo₂O₉‐based oxides of the general formula La_2?xSm_xMo_2?xW_xO₉, where x ≤ 0.2, were synthesized by citric acid sol–gel process. The variation in thermal conductivity with Sm³⁺and W⁶⁺ fractions was analyzed based on structure information provided by X‐ray diffraction and Raman spectroscopy. The fully dense La_2?xSm_xMo_2?xW_xO₉ ceramics showed a minimum thermal conductivity value [κ = 0.84 W·(m·K)^?1,T = 1073 K] at the composition of La_1.8Sm_0.2Mo_1.8W_0.2O₉, which stems from the multiple enhanced phonon scatterings due to mass and strain fluctuations at the La³⁺ and Mo⁶⁺ sites as well as the high concentration of intrinsic oxygen vacancies embedded in the crystal lattice. The thermal conductivities present an abrupt decrease at the structural transition, which is due to the phase transformation from a low‐temperature ordered form (monoclinic α‐La₂Mo₂O₉) to a high‐temperature disordered form (cubic β‐La₂Mo₂O₉₎.     

Tests for the Use of La2Mo2O9‐based Oxides as Multipurpose SOFC Core Materials

The mixed ionic–electronic conductivity under dilute hydrogen, the stability and the catalytic activity under propane:air type mixtures of a series of LAMOX oxide‐ion conductors have been studied. The effect of exposure to dilute hydrogen on the conductivity of the β‐La₂(Mo_{2 – y}W_y)O₉ series at 600 °C depends on tungsten content: almost negligible for the highest (y = 1.4), it is important for La₂Mo₂O₉ (y = 0). In propane:air, all tested LAMOX electrolytes are stable at 600–700 °C, but get reduced when water vapour is present. La₂Mo₂O₉ is the best oxidation catalyst of the series, with an activity comparable to that of nickel. The catalytic activity of other tested LAMOX compounds is much lower, (La_1.9Y_0.1)Mo₂O₉ showing a deactivation phenomenon. These results suggest that depending on composition, La₂(Mo_{2 – y}W_y)O₉ compounds could be either electrolytes in single‐chamber SOFC and dual‐chamber micro‐SOFC (y = 1.4) or anode materials in dual‐chamber SOFC (low y) or oxidation catalysts in SOFCs operating with propane (y = 0).     

Sintering and Electrical Characterization of La and Nb Co‐doped SrTiO3 Electrode Materials for Solid Oxide Cell Applications

Single‐phase lanthanum and niobium co‐doped strontium titanate (Sr_1–3x/2La_xTi_0.9Nb_0.1O₃; x = 0–0.02) ceramics were prepared. Dilatometry in reducing atmosphere showed an increase in the sintering rate and sintered density with an increase in La amount. Microscopy of fractured surfaces of sintered samples showed that the average grain size increased drastically in reducing conditions with increasing La content (and associated A‐site vacancies). By incorporating 2 mol.% La, the electronic conductivity significantly improved from 80 to 135 S cm⁻¹ at 1,000 °C, and even larger improvements were observed at lower temperatures. These observations demonstrate the flexibility in tailoring the microstructure and electronic transport properties by doping small amounts of La into the Nb‐doped SrTiO₃ and show that Sr_1–3x/2La_xTi_0.9Nb_0.1O₃ is a potential electrode material for solid oxide cells.     

Development of Mn and Mo double-substituted La5.5WO11.25-δ based membranes with enhanced hydrogen permeation flux

Mixed protonic-electronic ceramic lanthanum tungstate membranes of La_5.5W_1-xMn_xO_11.25-δ (LWMn_x, x = 0.1, 0.15 and 0.2) and La_5.5W_0.8-yMn_0.2Mo_yO_11.25-δ (LWMn_0.2Mo_y, y = 0.1, 0.2, 0.3 and 0.4) were developed in present work. The H₂ permeation flux through lanthanum tungstate membrane was increased by substitution of Mn ions into W sites. However, the impurity of La₂O₃ formed in LWMn_0.15 and LWMn_0.2 membranes. Introduction of a moderate amount of Mo ions in LWMn_0.2 was beneficial to single phase formation and H₂ permeation. LWMn_0.2Mo_0.2 exhibited a pure cubic phase structure without any impurities. LWMn_0.2Mo_0.2 membrane had a H₂ permeation flux about 0.12 mL/min?cm² at 1000 °C, which was 1.7 times higher than that through LWMn_0.2 membranes.     

Synthesis,Structure and Electrical Properties of Mo‐doped CeO2–Materials for SOFCs

In this paper, we report the synthesis, structure and electrical conductivity of Mo‐doped compounds with a nominal chemical formula of Ce_1–xMo_xO_2+δ (x = 0.05, 0.07, 0.1) (CMO). The formation of fluorite‐like structure with a small amount of Ce₈Mo₁₂O₄₉ impurity (JCPDS Card No. 31‐0330) was confirmed using a powder X‐ray diffraction (PXRD). The fluoride‐type structure was retained under wet H₂ and CH₄ atmospheres at 700 and 800 °C, while diffraction peaks due to metal Mo were observed in dry H₂ under the same condition. AC impedance measurements showed that the total conductivity increases with increasing Mo content in CMO, and among the investigated samples, Ce_0.9Mo_0.1O_2+δ exhibited the highest electrical conductivity with a value of 2.8 × 10^–4 and 5.08 × 10^–2 S cm^–1 at 550 °C in air and wet H₂, respectively. The electrical conductivity was found to be nearly the same, especially at high temperatures, in air, O₂ and N₂. Chemical compatibility of Ce_0.9Mo_0.1O_2+δ with 10 mol‐% Y₂O₃ stabilised ZrO₂ (YSZ) and Ce_0.9Gd_0.1O_1.95 (CGO) oxide ion electrolytes in wet H₂ was evaluated at 800–1,000 °C, using PXRD and EDX analyses. PXRD showed that CMO was found to react with YSZ electrolyte at 1,000 °C. The area specific polarisation resistance (ASPR) of Ce_0.9Mo_0.1O_2+δ on YSZ was found to be 8.58 ohm cm² at 800 °C in wet H₂.     

Enhanced sinterability of mechanical alloyed La9.33Si2Ge4O26 oxyapatite powders for IT-SOFC electrolytes

Dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared by electrical sintering at 1400 °C in static air for 1 h from dry milling La₂O₃, SiO₂ and GeO₂ powders, in adequate atomic proportions, at 350 rpm for 15 h, under controlled environmental conditions, in a planetary ball mill. The densification behaviour of apatite-type phase La_9.33Si₂Ge₄O₂₆ powders synthesized by mechanical alloying was investigated through microstructural evolution with sintering temperature by means of XRD and SEM/EDS analyses. The content of germanium in the sintered samples remained almost constant, suggesting that its incorporation in the apatite phase hinders the high temperature (>1250 °C) volatilization process.     

Structure,Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)Ba0.6Sr0.4La4Ti4O15–xBa5Nb4O15 Solid Solutions

(1 ? x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xBa₅Nb₄O₁₅ (x = 0.05, 0.1, 0.15 and 0.2, BSLT–BN) ceramic samples were prepared by co‐firing the mixtures of Ba_0.6Sr_0.4La₄Ti₄O₁₅ and Ba₅Nb₄O₁₅ powders. Crystal structure, microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of the BSLT–BN series ceramics were investigated. X‐ray diffraction patterns reveal that all the samples exhibit a hexagonal perovskite structure, which implies that the BSLT–BN mixtures form solid solutions. With increasing Ba₅Nb₄O₁₅ content, the diffraction peaks shift to low angles and the sintering temperature of BSLT–BN decreases. Raman spectra analysis reveals the shifting and splitting of the vibration modes. The microwave dielectric properties of the well‐sintered (1 ? x)BSLT–xBN ceramics vary with Ba₅Nb₄O₁₅ content. The dielectric permittivity of the ceramics exhibits a slight decreasing trend. The quality factor varies in the range of 45 000–11 200 GHz, whereas near‐zero temperature coefficients of the resonant frequency may be achieved by changing the Ba₅Nb₄O₁₅ content. TSDC was utilized to explore the extrinsic loss mechanism associated with defects. TSDC relaxation peaks are mainly generated by oxygen vacancies, and the Ba₅Nb₄O₁₅ content has a significant influence on the TSDC spectra.     

Microwave dielectric properties and vibrational spectra of (Na0.5xLa1–0.5x)(Nb1−xMox)O4 ceramics with scheelite structures

In this study, (Na_0.5xLa_1–0.5x)(Nb_1?xMo_x)O₄ (0.0 ≤ x ≤ 1.0) ceramics were synthesized by the traditional solid-state reaction method. All the (Na_0.5xLa_1–0.5x)(Nb_1?xMo_x)O₄ samples could be densified well at 1120–1200 °C. Solid solutions with tetragonal scheelite structures were obtained in the (Na_0.5xLa_1–0.5x)(Nb_1?xMo_x)O₄ ceramics when 0.4 ≤ x < 1.0. A temperature-stable microwave dielectric ceramic with a near-zero temperature coefficient of resonant frequency (TCF) of approximately 2.4 ppm/°C was obtained for the (Na_0.25La_0.75)(Nb_0.5Mo_0.5)O₄ sample, along with a high Q×f value of approximately 45,600 GHz and a low permittivity of approximately 14.5. Introducing the fergusonite phase into the scheelite phase proved effective in obtaining a near-zero TCF.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.     

Structural and transport properties of neodymium tungstates prepared via mechanochemical activation

Nd_5.5WO_11.25-δ, (Nd_5/6La_1/6)_5.5WO_11.25-δ and Nd_5.5W_0.5Mo_0.5O_11.25-δ mixed oxides have been prepared using mechanochemical activation (MA) of Nd₂O₃-WO₃, Nd₂O₃-La₂O₃-WO₃, Nd₂O₃-MoO₃-WO₃ mixtures in a high-power planetary ball mill. Genesis of these tungstates structural properties was studied by XRD, SEM, TEM with EDX analysis, ¹H NMR, IR and Raman spectroscopy. X-ray diffraction has revealed that MA results in formation of a pure fluorite structure already after milling. High-density ceramic pellets have been obtained after sintering at 1350?°C. Transport properties of Nd_5.5WO_11.25-δ mixed oxide were improved by the partial substitution of Nd with La in (Nd_5/6La_1/6)_5.5WO_11.25-δ and W with Mo in Nd_5.5W_0.5Mo_0.5O_11.25-δ samples. For all samples the electrical conductivity values measured in a humid atmosphere (up to ~ 10^?3 S/cm at 550?°C) exceed those measured in a dry atmosphere indicating the proton character of conductivity.     

Electronic Conductivity Enhancement of (La,Sr)TiO3 with Nb‐Doping on B‐Site

B‐site doped, A‐site deficient strontium titanates with general formula La_xSr_1–3x/2+γxNb_yTi_1–yO_3–δ (x = 0.2, 0.25, 0.3, 0.35, 0.4; y = 0, 0.02, 0.03; γ = 0.03) were produced by flame spray synthesis and the electrical conductivity of dense ceramic bars sintered under reducing conditions measured over broad temperature and oxygen concentration ranges. As‐synthesized powders were nano‐scale and slightly reduced, while it was observed that sinterability was promoted by both La substitution and Nb doping. Electrical conductivity measurements on dense bars under reducing conditions revealed that stoichiometric substitution of Ti⁴⁺ by Nb⁵⁺ raised conductivity by ∼ 40%, with a maximum value of 226 S cm⁻¹ observed at 900 °C. This was attributed to a higher concentration of defects, due to concurrent and enhanced oxygen vacancy ( ) formation under reducing conditions. The electronic properties of a slightly porous bar sample of the La_0.3Sr_0.559Nb_0.02Ti_0.98O_3–δ composition were found to be stable over one redox cycle.     

High lithium ionic conductivity in the garnet‐type oxide Li7−2xLa3Zr2−xMoxO12 (x=0‐0.3) ceramics by sol‐gel method

Lithium garnet‐type oxides Li_7?2xLa₃Zr_2?xMo_xO₁₂ (x=0, 0.1, 0.2, 0.3) ceramics were prepared by a sol‐gel method. The influence of molybdenum on the structure, microstructure and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X‐ray diffraction, scanning electron microscopy, and impedance spectroscopy. The cubic phase Li₇La₃Zr₂O₁₂ has been stabilized by partial substitution of Mo for Zr at low temperature. The introduction of Mo (x≥0.1) can accelerate densification. Li_6.6La₃Zr_1.8Mo_0.2O₁₂ sintered at lower temperature 1100°C for 3 hours exhibits highest total ionic conductivity of 5.09 × 10^?4 S/cm. Results indicate that the Mo doping LLZO synthesized by sol‐gel method effectively lowers its sintering temperature and improves the ionic conductivity.     

Effect of Nb Doping on Hydration and Conductivity of La27W5O55.5−δ

Hydration properties and electrical characteristics of the high‐temperature proton conductor La₂₇(W_0.85Nb_0.15)₅O_55.5?δ are investigated by means of thermogravimetry, impedance spectroscopy, and the electromotive force (EMF) method as a function of temperature, water vapor, and oxygen partial pressures, as well as isotope exchange measurements in order to elucidate the mechanism and thermodynamics of protons formation and transport. The highest proton conductivity, 1.3 × 10^‐3 S/cm, is achieved at 700°C in wet O₂. Proton self‐diffusion coefficients are estimated from thermogravimetric measurements of hydration and conductivity data. Comparison of the conductivity characteristics between nominally pure and Nb‐substituted materials reveals that the ionic conductivity increases and the activation energy decreases with Nb doping. These differences are discussed to reflect changes in the structure promoting ionic transport rather than changing the concentration of defects to any large extent.     

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.     

On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCs

Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La_9.33(RO₄)₆O₂ with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La₂O₃, SiO₂ and GeO₂ powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO₂ used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La_9.33Si₂Ge₄O₂₆ integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO₂ do promote densification of these electrolytes thereby improving their structural integrity.     

BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High‐Temperature Capacitor Applications

Solid solutions of (1?x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (0 ≤ x ≤ 0.6) were prepared via a standard mixed‐oxide solid‐state sintering route and investigated for potential use in high‐temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (ε_r). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative ε_r ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg_2/3Nb_1/3)O₃ concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high‐temperature multilayer ceramic capacitors for automotive applications.     

A CO2‐stable hollow‐fiber membrane with high hydrogen permeation flux

A Mo‐substituted lanthanum tungstate mixed proton‐electron conductor, La_5.5W_0.6Mo_0.4O_11.25?δ (LWM04), was synthesized using solid state reactions. Dense U‐shaped LWM04 hollow‐fiber membranes were successfully prepared using wet‐spinning phase‐inversion and sintering. The stability of LWM04 in a CO₂‐containing atmosphere and the permeation of hydrogen through the LWM04 hollow‐fiber membrane were investigated in detail. A high hydrogen permeation flux of 1.36 mL/min cm² was obtained for the U‐shaped LWM04 hollow‐fiber membranes at 975°C when a mixture of 80% H₂?20% He was used as the feed gas and the sweep side was humidified. Moreover, the hydrogen permeation flux did not significantly decrease over 70 h of operation when fed with a mixture containing 25% CO₂, 50% H₂, and 25% He, indicating that the LWM04 hollow‐fiber membrane has good stability under a CO₂‐containing atmosphere. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1997–2007, 2015     

Stability of High‐Temperature Dielectric Properties for (1 − x)Ba0.8Ca0.2TiO3–xBi(Mg0.5Ti0.5)O3 Ceramics

Ceramics in the solid solution system, (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃, were prepared by a conventional mixed oxide route. Single‐phase perovskite‐type X‐ray diffraction patterns were observed for compositions x < 0.6. A change from tetragonal to single‐phase cubic X‐ray patterns occurred at x ≥ 0.1. Dielectric measurements indicated relaxor behavior for x ≥ 0.1. Increasing the Bi(Mg_0.5Ti_0.5)O₃ content improved the temperature sensitivity of relative permittivity ?_r at high temperatures. At x = 0.5, a near‐plateau relative permittivity, 835 ± 40, extended across the temperature range, 65°C–550°C; the permittivity increased at x = 0.6 to 2170 ± 100 for temperatures 160°C–400°C (1 kHz). The corresponding loss tangent, tanδ, was ≤0.025 for temperatures between 100°C and 430°C for composition x = 0.5; at x = 0.6, losses increased sharply at >300°C. Comparisons of dielectric properties with other materials proposed for high‐temperature capacitor applications suggest that (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃ ceramics are a promising base material for further development.     

Glass‐Like Thermal Conductivity of (010)‐Textured Lanthanum‐Doped Strontium Niobate Synthesized with Wet Chemical Deposition

We have measured the cross‐plane thermal conductivity (κ) of (010)‐textured, undoped, and lanthanum‐doped strontium niobate (Sr_2?xLa_xNb₂O_7?δ) thin films via time‐domain thermoreflectance. The thin films were deposited on (001)‐oriented SrTiO₃ substrates via the highly‐scalable technique of chemical solution deposition. We find that both film thickness and lanthanum doping have little effect on κ, suggesting that there is a more dominant phonon scattering mechanism present in the system; namely the weak interlayer‐bonding along the b‐axis in the Sr₂Nb₂O₇ parent structure. Furthermore, we compare our experimental results with two variations of the minimum‐limit model for κ and discuss the nature of transport in material systems with weakly‐bonded layers. The low cross‐plane κ of these scalably‐fabricated films is comparable to that of similarly layered niobate structures grown epitaxially.