Phase stability and electrical conductivity of Ca-doped LaNb1−xTaxO4−δ high temperature proton conductors

The electrical conductivity, crystal structure and phase stability of La_0.99Ca_0.01Nb_1−xTa_xO_4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La_0.99Ca_0.01Nb_1−xTa_xO_4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Preparation and characterization of ceramic interconnect La0.8Ca0.2Cr0.9M0.1O3−δ (M = Al, Co, Cu, Fe) for IT-SOFCs

The lattice parameters, electrical conductivity, activation energy, mechanical properties, and microstructure of (La_0.8Ca_0.2)CrO_3−δ-based specimens were investigated systematically in this paper. The tolerance factors for (La_0.8Ca_0.2)CrO_3−δ-based specimens were all greater than 0.9, indicating the perovskite was not distorted with different cations (Al³⁺, Co³⁺, Cu²⁺, Fe³⁺) substitution for B site of (La_0.8Ca_0.2)CrO_3−δ. (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen revealed the maximum electrical conductivity, σ_850 °C = 59.59 S/cm with minimum activation energy, E_a = 11.2 kJ/mol among (La_0.8Ca_0.2)CrO_3−δ-based specimens. The grain size seemed dependent on doping species and the grain sizes were distributed in the range of 2.4-5.6 μm for (La_0.8Ca_0.2)CrO_3−δ-based specimens. The rate of grain growth was proportional to the boundary mobility M_b, which was related to the diffusion coefficient of doping cation. (La_0.8Ca_0.2)CrO_3−δ-based specimens revealed variety in microhardness, in the range of 4.33-9.85 GPa and the fracture toughness were distributed in the range of 3.52-4.33 MPa m^1/2. Based on the results in terms of grain size and mechanical properties, we concluded that the microhardness and fracture toughness were dependent on the dopant ions. The (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen shows high electrical conductivity and mechanical properties Consequently, it is a promising candidate as an interconnect material for intermediate temperature solid oxide fuel cell (IT-SOFC) applications.     

Effect of ionic size of dopants on the lattice structure,electrical and electrochemical properties of La2−xMxNiO4+δ (M = Ba,Sr) cathode materials

La_2−xM_xNiO_4+δ (M = Ba, Sr; x = 0.1, 0.3), with a formula of A₂BO₄, has been prepared and investigated as cathode for solid oxide fuel cells to understand the influence of A-site dopants on the lattice structure, electrical conductivity and electrochemical properties of La₂NiO_4+δ. All the compositions belong to tetragonal I4/mmm space group. La_2−xBa_xNiO_4+δ shows larger lattice parameters than La_2−xSr_xNiO_4+δ due to the large ionic radius of Ba²⁺ compared with Sr²⁺. For both Ba and Sr, the parameters a and b decrease while the c increases with increasing doping level. Rietveld refinement demonstrates that the increase in c parameter is partially originated from the increase in rocksalt layer thickness (La–O2( × 1) bond), which makes the adsorbed oxygen on particle surface much easier to enter the lattice and form interstitials, and thereby promoting the electrode reaction. The electrical conductivity of La_2−xM_xNiO_4+δ increases with doping level but decrease with increasing ionic radius of dopants. Both Ba and Sr doping decrease the electrode polarization and increase the power density of single-cell. La_1.7Ba_0.3NiO_4+δ exhibits superior electrochemical properties than La_1.7Sr_0.3NiO_4+δ. The La_1.7Ba_0.3NiO_4+δ electrode exhibits the best performance with an ASR of 0.13 Ω cm² and a maximum power density of 310 mW cm⁻² at 800 °C under electrolyte (La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ, 300 μm) – supported configuration.     

Electrical conductivity and structural stability of La-doped SrTiO3 with A-site deficiency as anode materials for solid oxide fuel cells

A-site-deficient (La_0.3Sr_0.7)_1−xTiO_3−δ materials were synthesized by conventional solid-state reaction. The A-site deficiency limit in (La_0.3Sr_0.7)_1−xTiO_3−δ was below 10 mol% in 5%H₂/Ar at 1500 °C. A-site deficiency level promoted the sintering process of (La_0.3Sr_0.7)_1−xTiO_3−δ. The ionic conductivity increased but the electronic conductivity decreased with increasing A-site deficiency level. The ionic conductivity of (La_0.3Sr_0.7)_0.93TiO_3−δ sample was as high as 0.2–1.6 × 10⁻² S/cm in 500–950 °C and 1.0 × 10⁻² S/cm at 800 °C, over twice of La_0.3Sr_0.7TiO_3−δ. Its electrical conductivity was in the range of 83–299 S/cm in 50–950 °C and 145 S/cm at 800 °C. A-site deficiency improved the thermal stability of (La_0.3Sr_0.7)_1−xTiO_3−δ and ensured the material with a stable electrical performance in different atmospheres.     

Electrical performance and structural analysis of La1−xBaxGa0.8Mg0.2O3−δ solid electrolyte

The aim of this study was to develop La_1−xBa_xGa_1−yMg_yO_3−δ (x = 0.03–0.1, y = 0.2–0.25) (LBGM) electrolytes for intermediate-temperature solid-oxide fuel cells (SOFCs); these electrolytes were synthesized via a solid-state reaction. In the study, the La_1−xBa_xGa_1−yMg_yO_3−δ samples crystallized in an orthorhombic (Imma) structure, and a BaLaGa₃O₇ phase was detected for x ≥ 0.08 at a fixed y = 0.2. The solubility limit of the Ba ions increased with an increase in the Mg content in the matrix. Two active Raman bands at ca. 677 and 739 cm⁻¹ were observed, and they were attributed to the oxygen vacancies. The La_0.95Ba_0.05Ga_0.75Mg_0.25O_3−δ sample had a higher conductivity ca. 0.1 S/cm at 800 °C, and an activation energy of ca. 0.83–1.27 eV at 500–800 °C. The thermal expansion coefficient (TEC) of the LBGM samples at 200–800 °C was in the range of 10 × 10⁻⁶ to 14 × 10⁻⁶/°C.     

Investigation of compatible anode systems for LaNbO4-based electrolyte in novel proton conducting solid oxide fuel cells

In the current manufacturing process of novel LaNbO₄-based proton conducting fuel cells a thin layer of the electrolyte is deposited by wet ceramic coating on NiO-LaNbO₄ based anode and co-sintered at 1200-1300 °C. The chemical compatibility of NiO with acceptor doped LaNbO₄ material is crucial to ensure viability of the cell, so potential effects of other phases resulting from off-stoichiometry in acceptor doped LaNbO₄ should also be explored. Compatibility of NiO with Ca-doped LaNbO₄ and its typical off-set compositions (La₃NbO₇ and LaNb₃O₉) are investigated in this work. It is shown that while NiO does not react with Ca-doped LaNbO₄, fast reaction occurs with La₃NbO₇ or LaNb₃O₉. La₃NbO₇ and NiO form a mixed conducting perovskite phase LaNi_2/3Nb_1/3O₃, while LaNb₃O₉ and NiO form either NiNb₂O₆ or Ni₄Nb₂O₉ depending on the annealing temperature. This implies that manufacturing LaNbO₄-based proton conducting fuel cells requires a strict control of the stoichiometry of the electrolyte.     

Electrode redox properties of Ba1−xLaxFeO3−δ as cobalt free cathode materials for intermediate-temperature SOFCs

Cobalt-free perovskite oxides Ba_1−xLa_xFeO_3−δ (x = 0.1–0.4) were synthesized by glycine-nitrate combustion method and investigated as a candidate cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). Cubic perovskite structure was obtained when 10–20 mol% La was substituted at Ba-site in Ba_1−xLa_xFeO_3−δ, and the crystal structure was transformed from cubic structure into orthorhombic one at x ≥ 0.2 with an addition of lanthanum doping. The thermal expansion coefficients of Ba_1−xLa_xFeO_3−δ oxides decreased gradually with La content due to increasing electrostatic attraction forces. A gradual increase existed in electrical conductivity tendency with La content due to disproportionation of Fe³⁺ and the larger extent of electron clouds. The electrode redox performance was investigated by electrochemical impedance spectroscopy. Among Ba_1−xLa_xFeO_3−δ series oxides, Ba_0.9La_0.1FeO_3−δ exhibited the best electrochemical performance. The area specific resistance (ASR) of Ba_0.9La_0.1FeO_3−δ was 0.079 Ω cm², 0.37 Ω cm², and 2.15 Ω cm² at 800, 700 and 600 °C under open circuit potential. To investigate electrochemical performances after cathodic polarization, bias potentials were employed on Ba_1−xLa_xFeO_3−δ cathode at 650–800 °C. The results demonstrated the potential applications for Ba_0.9La_0.1FeO_3−δ as cathode materials for IT-SOFCs as a tradeoff between electrochemical and thermal expansion performance.     

Electrical conductivity and performance of doped LaCrO3 perovskite oxides for solid oxide fuel cells

The crystalline structure, redox stability and electrical conductivity of LaCrO₃, (La_1−xM_x)CrO₃ (M = Mg, Ca, Ba for x = 0.3 and M = Sr for x = 0.25), and (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃ (LSCM) perovskites are studied from 500 to 800 °C in both oxidizing and reducing atmospheres. Dopability, redox stability and electrical conductivity are compared and examined. A-site doping with alkaline elements is found to improve significantly the electrical conductivity, particularly if properly doped. The highest conductivity is obtained with Ca- and Sr-doped LaCrO₃. A-site doping also reduces the activation energy of the electrical conductivity, particularly under a reducing environment. Preliminary electrochemical results indicate that Ca-doped LaCrO₃ shows promise as a cathode for solid oxide fuel cells.     

Electrochemical properties of composite cathodes for La0.995Ca0.005NbO4−δ-based proton conducting fuel cells

The electrochemical properties of mixed-conducting ceramic-ceramic (cer-cer) composites for proton-conducting solid oxide fuel cells (PC-SOFCs) based on La_0.995Ca_0.005NbO_4−δ (LCN) have been investigated. Different ratios of La_0.8Sr_0.2MnO_3−δ/La_0.995Ca_0.005NbO_4−δ (LSM/LCN) composites have been tested as cathodes in symmetrical cells based on La_0.995Ca_0.005NbO_4−δ dense electrolytes while two different electrode sintering temperatures (1050 and 1150 °C) have been studied. Additionally, different LCN doped materials (Pr, Ce and Mn), which present a different conduction behavior, have been used as components in composite cathodes (mixtures of LSM/doped-LCN 50/50 vol.%). Electrochemical impedance spectroscopy analysis has been carried out in the temperature range 700-900 °C under moist (2.5%) atmospheres. Different oxygen partial pressures (pO₂) have been employed in order to characterize the processes (surface reaction and charge transport) occurring at the composite electrode under oxidizing conditions. The main outcome of the present study is that the mixture of LSM (electronic phase) and LCN (protonic phase) enables to decrease substantially the electrode polarization resistance. This is ascribed to the increase in the three-phase-boundary length and therefore it allows electrochemical reactions to occur in a larger region (thickness) of the electrode.     

Novel Nd2WO6-type Sm2−xAxM1−yByO6−δ (A = Ca, Sr; M = Mo, W; B = Ce, Ni) mixed conductors

In the present work, we have explored novel Nd₂WO₆-type structure Sm_2−xA_xM_1−yB_yO_6−δ (A = Ca, Sr; M = Mo, W; B = Ce, Ni) as precursor for the development of solid oxide fuel cells (SOFCs) anodes. The formation of single-phase monoclinic structure was confirmed by powder X-ray diffraction (PXRD) for the A- and B-doped Sm₂MO₆ (SMO). Samples after AC measurements under wet H₂ up to 850 °C changed from Nd₂WO₆-type structure into Sm₂MoO₅ due to the reduction of Mo^VI that was confirmed by PXRD and is consistent with literature. The electrical conductivity was determined using 2-probe AC impedance and DC method and was compared with 4-probe DC method. The total electrical conductivity obtained from these two different techniques was found to vary within the experimental error over the investigated temperature of 350-650 °C. Ionic and electronic conductivity were studied using electron-blocking electrodes technique. Among the samples studied, Sm_1.8Ca_0.2MoO_6−δ exhibits total conductivity of 0.12 S cm⁻¹ at 550 °C in wet H₂ with an activation energy of 0.06 eV. Ca-doped SMO appears to be chemically stable against reaction with YSZ electrolyte at 800 °C for 24 h in wet H₂. The ionic transference number (t_i) of Sm_1.9Ca_0.1MoO_6−δ in wet H₂ at 550 °C (pO₂ = 10^−25.5 atm) was found to be about 0.012 after subtraction of electrical lead resistance from the 2-probe AC data and showed predominate electronic conductors.     

La substituted Sr2MnO4 as a possible cathode material in SOFC

Sr_2−xLa_xMnO_4+δ (x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr_2−xLa_xMnO_4+δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm⁻¹ was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr_1.4La_0.6MnO_4+δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm² at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr_1.4La_0.6MnO_4+δ electrode.     

Key trends in the proton conductivity of Ln6−xMoO12−δ (Ln = La,Nd, Sm,Gd -Yb; x = 0, 0.5, 0.6, 0.7, 1) rare-earth molybdates

We have studied the structure and proton conductivity of rhombohedral La_6?xMoO_12?δ (x = 0.5, 0.6, 0.7, 1) lanthanum molybdates prepared via mechanical activation of lanthanum and molybdenum oxides, followed by thermal annealing at 1650 °C. The La_6?xMoO_12?δ (x = 0.5, 0.6) materials were phase-pure and had a complex rhombohedral structure (R1). An increase in the molybdenum concentration leads to a decrease in the degree of rhombohedral distortion and proton conductivity in the La_6?xMoO_12?δ (x = 0.5, 0.6, 0.7, 1) series. The proton conductivity at the optimal composition La_6?xMoO_12?δ (x = 0.5) is ~4.0 × 10^?5 S/cm at 500 °C in wet air.A comparative analysis shows that, in the Ln_6?xMoO_12?δ (Ln = La, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb; x = 0–1) series, proton conductivity decreases with the Ln ionic radii decreasing regardless of the structural type. Because of this, the high proton conductivity is demostrated by the stable La_6?xMoO_12?δ (x = 0.5, 0.6) materials, with an inherently deficient oxygen sublattice, which crystallize in a large-volume, complex rhombohedral cell (R1).     

Preparation and evaluation of Ca3−xBixCo4O9−δ (0< x ≤ 0.5) as novel cathodes for intermediate temperature-solid oxide fuel cells

The misfit compounds Ca_3−xBi_xCo₄O_9−δ (x = 0.1–0.5) were successfully synthesized via conventional solid state reaction and evaluated as cathode materials for intermediate temperature-solid oxide fuel cells. The powders were characterized by X-ray diffraction, scanning emission microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and oxygen-temperature programmed desorption. The monoclinic Ca_3−xBi_xCo₄O_9−δ powders exhibit good thermal stability and chemical compatibility with Ce_0.8Sm_0.2O_2−γ electrolyte. Among the investigated single-phase samples, Ca_2.9Bi_0.1Co₄O_9−δ shows the maximal conductivity of 655.9 S cm⁻¹ and higher catalytic activity compared with other Ca_3−xBi_xCo₄O_9−δ compositions. Ca_2.9Bi_0.1Co₄O_9−δ also shows the best cathodic performance and its cathode polarization resistance can be further decreased by incorporating 30 wt.% Ce_0.8Sm_0.2O_2−γ. The maximal power densities of the NiO/Ce_0.8Sm_0.2O_2−γ anode-supported button cells fabricated with the Ce_0.8Sm_0.2O_2−γ electrolyte and Ca_2.9Bi_0.1Co₄O_9−δ + 30 wt.% Ce_0.8Sm_0.2O_2−γ cathode reach 430 and 320 mW cm⁻² at 700 and 650 °C respectively.     

New SrMo1−xCrxO3−δ perovskites as anodes in solid-oxide fuel cells

Oxides of composition SrMo_1−xCr_xO_3−δ (x = 0.1, 0.2) have been prepared, characterized and tested as anode materials in single solid-oxide fuel cells, yielding output powers higher than 700 mW cm⁻² at 850 °C with pure H₂ as a fuel. All the materials are suggested to present mixed ionic–electronic conductivity (MIEC) from neutron powder diffraction (NPD) experiments, complemented with transport measurements; the presence of a Mo⁴⁺/Mo⁵⁺ mixed valence at room temperature, combined with a huge metal-like electronic conductivity, as high as 340 S cm⁻¹ at T = 50 °C for x = 0.1, could make these oxides good materials for solid-oxide fuel cells. The magnitude of the electronic conductivity decreases with increasing Cr-doping content. The reversibility of the reduction–oxidation between the oxidized Sr(Mo,Cr)O_4−δ scheelite and the reduced Sr(Mo,Cr)O₃ perovskite phases was studied by thermogravimetric analysis, which exhibit the required cyclability for fuel cells. An adequate thermal expansion coefficient, without abrupt changes, and a chemical compatibility with electrolytes make these oxides good candidates for anodes in intermediate-temperature SOFC (IT-SOFCs).     

The microstructures and property analysis of aliovalent cations (Sm,Mg, Ca,Sr, Ba) co-doped ceria-base electrolytes after an aging treatment

Ce_0.8Sm_0.15R_0.05O_2−δ (R = Sm, Mg, Ca, Sr, and Ba) specimens were successfully prepared using a solid-state reaction, and they were used in an intermediate temperate solid oxide fuel cell (IT-SOFC) electrolyte. This study focused on the effects of co-doping and an aging treatment for the conductivities and microstructures of CeO₂-based ceramics and also analyzed the variation of the conductivity in the reducing atmosphere. The study showed that the conductivities of the CeO₂-based materials have a higher conductivity at 500–800 °C by the co-doping aliovalent cations Sm and R. The conductivity increased with the increasing oxygen vacancies that were induced from charge compensation. The XRD and EDS analyses showed that the MgO and BaCeO₃ phases appeared in the Ce_0.8Sm_0.15Mg_0.05O_2−δ and Ce_0.8Sm_0.15Ba_0.05O_2−δ specimens, respectively. The conductivity of the Ce_0.8Sm_0.15Ca_0.05O_2−δ specimens was higher, approximately 0.0837 S/cm at 800 °C in the air. The thermal expansion coefficient (TEC) in all samples was ca. 11–15 × 10⁻⁶/°C at 200–800 °C. After an aging treatment at 700 °C for a holding time of 1000 h, the conductivities of all samples showed almost no change. However, the conductivity in Ce_0.8Sm_0.15Ca_0.05O_2−δ decreased from 0.0837 to 0.0581 S/cm, and the grain size increased. The conductivities of the CeO₂-based specimens were also measured under a 5%H₂–95%N₂ atmosphere, and the conductivity greatly increased in the reducing atmosphere because the Ce⁴⁺ ions reduced to Ce³⁺ ions.     

Electrical conductivity and cell performance of La0.3Sr0.7Ti1−xCrxO3−δ perovskite oxides used as anode and interconnect material for SOFCs

The anode materials La_0.3Sr_0.7Ti_1−xCr_xO_3−δ (LSTC, x = 0, 0.1, 0.2) with cubic structure were prepared via solid state reaction route. The influence of Cr content on the properties of LSTC as anode and interconnect materials for solid oxide fuel cells (SOFCs) was investigated. The Cr-doping decreased the lattice parameter while increased the sinterability of LSTC materials. The total electrical conductivity decreased with Cr doping level, from 230 S cm⁻¹ for x = 0 to 53 S cm⁻¹ for x = 0.2. The total electrical conductivity exhibited good stability and recoverability in alternative atmospheres of air and 5% H₂/Ar, showing excellent redox stability. The cell testing showed that the anode performance of LSTC was enhanced somewhat by Cr doping. The present results indicated that the prepared La_0.3Sr_0.7Ti_1−xCr_xO_3−δ can be potential anode and interconnect materials for SOFCs.     

Preparation and characterization of Nd2−xSrxCoO4+δ cathodes for intermediate-temperature solid oxide fuel cell

K₂NiF₄-type structural Nd_2−xSr_xCoO_4+δ (x = 0.8, 1.0, 1.2) was synthesized and evaluated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion, electrical conductivity and electrochemical properties were investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and polarization techniques. It is found that the electrochemical properties were remarkably improved with the increasing of Sr in the experiment range. Nd_0.8Sr_1.2CoO_4+δ showed the highest electrical conductivity of 212 S cm⁻¹ at 800 °C, the lowest polarization resistance and cathodic overpotential, 0.40 Ωcm² at 700 °C and 35.6 mV at a current density of 0.1 A cm⁻² at 700 °C, respectively. The chemical compatibility experiment revealed that Nd_0.8Sr_1.2CoO_4+δ cathode was chemically stable with the SDC electrolyte. The thermal expansion coefficient also increased with the Sr content.     

Electrical conduction behavior of proton conductor BaCe1-xSmxO3-δ in the intermediate temperature range

Determining the relationship between electrical conductivity and doping level in high temperature proton conductors is of great significance to accelerate their process of practicality and develop new applications. In this work, BaCe_1-xSm_xO_3-δ (x = 0.01-0.50) was synthesized via citric-nitrate method and their electrical conduction behavior in 5% H₂/Ar in the intermediate temperature range was investigated. The solubility of Sm in BaCeO₃ was between 0.30 and 0.40. Both the bulk and the grain boundary conductivity increased with Sm content up to x = 0.20 and then decreased. The infrared spectra results indicated that the degree of hydrogen bonding decreased with Sm content (x = 0.20-0.30), which should be responsible for the descending bulk conductivity of samples with x = 0.25 and 0.30. BaCe_0.80Sm_0.20O_3-δ, with electrical conductivity of 0.017 S cm⁻¹ at 600 °C, was a promising electrolyte for intermediate temperature solid oxide fuel cells.     

Ba1−xCo0.9−yFeyNb0.1O3−δ (x = 0-0.15, y = 0-0.9) as cathode materials for solid oxide fuel cells

Perovskite oxide Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3−δ has been reported as oxygen transport membrane and cathode material for solid oxide fuel cells (SOFCs). In this study, the effects of A-site cation deficiency and B-site iron doping concentration on the crystal structure, thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance of Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.15, y = 0-0.9) have been systematically evaluated. Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.10, y = 0.2 and x = 0.10, y = 0.2-0.6) can be indexed to a cubic structure. Increased electrical conductivity and decreased cathode polarization resistance have been achieved by A-site deficiency. No obvious variation can be observed in TEC by A-site deficiency. The electrical conductivity and TEC of Ba_0.9Co_0.9−yFe_yNb_0.1O_3−δ decrease while the cathode polarization resistance increases with the increase in iron doping concentration. The highest conductivity of 13.9 S cm⁻¹ and the lowest cathode polarization resistance of 0.07 Ω cm² have been achieved at 700 °C for Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ. The composition Ba_0.9Co_0.3Fe_0.6Nb_0.1O_3−δ shows the lowest TEC value of 13.2 × 10⁻⁶ °C⁻¹ at 600 °C and can be a potential cathode material for SOFCs.