Effect of Y substitution for Nb in Li5La3Nb2O12 on Li ion conductivity of garnet-type solid electrolytes

We report the effect of Y substitution for Nb on Li ion conductivity in the well-known garnet-type Li₅La₃Nb₂O₁₂. Garnet-type Li₅La₃Nb_2−xY_xO_12−δ (0 ≤ x ≤ 1) was prepared by ceramic method using the high purity metal oxides and salts. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), ⁷Li nuclear magnetic resonance (Li NMR) and AC impedance spectroscopy were employed for characterization. PXRD showed formation of single-phase cubic garnet-like structure for x up to 0.25 and above x = 0.25 showed impurity in addition to the garnet-type phases. The cubic lattice constant increases with increasing Y content up to x = 0.25 in Li₅La₃Nb₂−_xY_xO12−δ and is consistent with expected ionic radius trend. ⁷Li MAS NMR showed single peak, which could be attributed to fast migration of ions between various sites in the garnet structure, close to chemical shift 0 ppm with respect to solid LiCl and which confirmed that Li ions are distributed at an average octahedral coordination in Li₅La₃Nb₂−_xY_xO₁₂−_δ. Y-doped compounds showed comparable electrical conductivity to that of the parent compound Li₅La₃Nb₂O₁₂. The x = 0.1 member of Li₅La₃Nb₂−_xY_xO₁₂−_δ showed total (bulk + grain-boundary) ionic conductivity of 1.44 × 10⁻⁵ Scm⁻¹ at 23 °C in air.     

La0.9−xCaxCe0.1CrO3−δ as potential anode materials for solid oxide fuel cells

LaCrO₃ doped with calcium and cerium on the A-site in the series of La_0.9−xCa_xCe_0.1CrO_3−δ (LCCC3060, LCCC4050, LCCC5040, LCCC6030 corresponding to x = 0.6, 0.5, 0.4, and 0.3 respectively), is synthesized by a sol–gel combustion method and evaluated as anode material for solid oxide fuel cells (SOFCs). Relatively higher Ca-doping on La in LaCrO₃ is found to improve both electronic and ionic conductivity. LCCC compositions have demonstrated good chemical stability in reducing atmospheres. Evaluation of the LCCC material as anode in symmetrical cell configuration shows that the highest Ca-doping composition results in the lowest activation energy and the lowest polarization resistance. La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) electrolyte-supported single cells with LCCC3060 as the anode and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) as the cathode show that LCCC3060 can be a potential anode material for H₂, but not for CH₄.     

Assessment of perovskite-type La0.8Sr0.2ScxMn1−xO3-δ oxides as anodes for intermediate-temperature solid oxide fuel cells using hydrocarbon fuels

Composites formed by the infiltration of 40 wt% La_0.8Sr_0.2Sc_xMn_1−xO_3-δ (LSSM) oxides (x = 0.1, 0.2, 0.3) into 65% porous yttria-stabilized zirconia (YSZ) are investigated as anode materials for intermediate-temperature solid oxide fuel cells for hydrocarbon oxidation. The oxygen non-stoichiometry and electrical conductivity of each LSSM-YSZ composite are determined by coulometric titration. As the concentration of Sc increases, the composites show higher phase stability and the electrical conductivity of LSSM is significantly affected by the Sc doping, the non-stoichiometric oxygen content, and oxygen partial pressure (p(O₂)). To achieve better electrochemical performance, it is necessary to add ceria-supported palladium catalyst for operation with humidified CH₄. Anode polarization resistance increases with Sc doping due to a decrease in electrical conductivity. An electrolyte-supported cell with a LSSM-YSZ composite anode delivers peak power densities of 395 and 340 mW cm⁻² at 923 K in humidified (3% H₂O) H₂ and CH₄, respectively, at a flow rate of 20 mL min⁻¹.     

Synthesis and performance of Sr1.5LaxMnO4 as cathode materials for intermediate temperature solid oxide fuel cell

A-site non-stoichiometric materials Sr_1.5La_xMnO₄ (x = 0.35, 0.40, 0.45) are prepared via solid state reaction. The structure of these materials is determined to be tetragonal. Both the lattice volume and the thermal expansion coefficient reduce with the decrease of lanthanum content. On the contrary, the conductivity increases and the maximum value of 13.9 S cm⁻¹ is found for Sr_1.5La_0.35MnO₄ at 750 °C in air. AC impedance spectroscopy and DC polarization measurements are used to study the electrode performance. The optimum composition of Sr_1.5La_0.35MnO₄ results in 0.25 Ω cm² area specific resistance (ASR) at 750 °C in air. The oxygen partial pressure measurement indicates that the charge transfer process is the rate-limiting step of the electrode reactions.     

Preparation and characterization of Pr1−xSrxFeO3 cathode material for intermediate temperature solid oxide fuel cells

A kind of cathode material of Pr_1−xSr_x FeO₃ (x = 0–0.5) for intermediate temperature solid oxide fuel cells (IT-SOFCs) was prepared by the coprecipitation method. Crystal structure, thermal expansion, electrical conductivity and electrochemical performance of the Pr_1−xSr_xFeO₃ perovskite oxide cathodes were studied by different methods. The results revealed that Pr_l−xSr_xFeO₃ exhibited similar orthorhombic structure from x = 0.1 to 0.3 and took cubic structure when x = 0.4–0.5. The unit cell volume decreased and the thermal expansion coefficient (TEC) of the materials increased as the strontium content increased. When 0 < x ≤ 0.3, the samples exhibited good thermal expansion compatibility with YSZ electrolyte. The electrical conductivity increased with the increasing of doped strontium content. When x = 0.3–0.5, the electrical conductivities were higher than 100 S cm⁻¹. The conductivity of Pr_0.8Sr_0.2FeO₃ was 78 S cm⁻¹ at 800 °C. Compared with the La_0.8Sr_0.2MnO₃ cathode, Pr_0.8Sr_0.2FeO₃ showed higher polarization current density and lower polarization resistance (0.2038 Ω cm²). The value of I₀ for Pr_0.8Sr_0.2FeO₃ at 800 °C is 123.6 mA cm⁻². It is higher than that of La_0.8Sr_0.2MnO₃. Therefore, Pr_1−xSr_xFeO₃ can be considered as a candidate cathode material for IT-SOFCs.     

Development of anode material based on La-substituted SrTiO3 perovskites doped with manganese and/or gallium for SOFC

Materials based on La-substituted SrTiO₃ perovskites doped with manganese and/or gallium for SOFC have been studied as novel anodes for solid oxide fuel cell. La₄Sr₈Ti₁₁Mn_1−xGa_xO_38−δ (0 ≤ x ≤ 1) oxides were synthesized by solid state reaction and the influences of the manganese and/or gallium content on the structure, morphology, thermal properties and electrical conductivity of these materials has been investigated. All compounds show cubic structure with a space group Pm-3m. These compounds presented high electrical conductivity values under reducing atmosphere between 7.9 and 6.8 S cm⁻¹ at 900 °C. For the composition x ≥ 0.5, the thermal expansion coefficient in both reducing and oxidizing atmosphere are close to that of SOFC electrolytes (8YSZ, CGD). In general, the substitution of Ga by Mn causes a slight reduction in each of the following, lattice parameter, degree of oxygen loss on reduction, thermal expansion coefficient, and electrical conductivity.     

Evaluation of A-site cation-deficient (Ba0.5Sr0.5)1−xCo0.8Fe0.2O3−δ (x > 0) perovskite as a solid-oxide fuel cell cathode

A-site cation-deficient (Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ ((BS)_1−xCF) oxides were synthesized and evaluated as cathode materials for intermediate-temperature solid-oxide fuel cells (ITSOFCs). The material's thermal expansion coefficient, electrical conductivity, oxygen desorption property, and electrocatalytic activity were measured. A decrease in both the electronic conductivity and the thermal expansion coefficient was observed for increasing values of the stoichiometric coefficient, x. This effect was attributed to the creation of additional oxygen vacancies, the suppression of variation in the oxidation states of cobalt and iron, and the suppression of the spin-state transitions of cobalt ions. The increase in A-site cation deficiency resulted in a steady increase in cathode polarization resistance, because impurities formed at the cathode/electrolyte interface, reducing the electronic conductivity. A single SOFC equipped with a BS_0.97CF cathode exhibited peak power densities of 694 and 893 mW cm⁻² at 600 and 650 °C, respectively, and these results were comparable with those obtained with a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode. Slightly A-site cation-deficient (BS)_1−xCF oxides were still highly promising cathodes for reduced temperature SOFCs.     

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.     

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.     

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.     

Electrical properties of Al-doped oxyapatites at intermediate temperature

In order to obtain acceptable performances in SOFC, relatively high operating temperatures (∼850–1000 °C) are required. That is mainly due to the low ionic conductivity of YSZ electrolyte. These temperature conditions are at the origin of the cell degradation, which justify the search for alternative electrolytes presenting the same performances but at lower temperature (∼800 °C). It is in this context that the oxyapatites of general formula La_10−xSi_6−yAl_yO_{27−3x/2−y/2} (x = 0, 0.67; y = 0.25, 0.50, 0.75) are studied in this work. The solid solutions are prepared by sintering the oxide powders at 1600 °C. All samples have shown a hexagonal structure with an increase in the cell parameters with the aluminium content. Electrical properties are determined by impedance spectroscopy between 169 and 790 °C. The results are treated by separating the bulk and grain boundary conductivities between 169 and 500 °C and in the form of total (bulk + grain boundary) conductivity between 500 and 790 °C. The respective influences of the activation energy and the pre-exponential factor on conductivity are analyzed. Activation energies are about 0.65 eV suggesting an interstitial mechanism. No variation of conductivity is observed with the oxygen partial pressure. Finally, La₁₀Si_5.5Al_0.5O_26.75 shows conductivity higher than that of YSZ at intermediate temperature.     

Electrical conduction behavior of La,Co co-doped SrTiO3 perovskite as anode material for solid oxide fuel cells

The effects of La- and Co-doping into SrTiO₃ perovskite oxides on their phase structure, electrical conductivity, ionic conductivity and oxygen vacancy concentration have been investigated. The solid solution limits of La in La_xSr_1 − xTiO_3 − δ and Co in La_0.3Sr_0.7Co_yTi_1 − yO_3 − δ are about 40 mol% and 7 mol%, respectively, at 1500 °C. The incorporation of La decreases the band gap and thus increases the electrical conductivity of SrTiO₃ remarkably. La_0.3Sr_0.7TiO_3 − δ shows an electrical conductivity of 247 S/cm at 700 °C. Co-doping into La_0.3Sr_0.7TiO_3 − δ increases the oxygen vacancy concentration and decreases the migration energy for oxygen ions, leading to a significant increase in ionic conductivity but at the expense of some electrical conductivity. The electrical and ionic conductivities of La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ are 63 S/cm and 6 × 10⁻³ S/cm, respectively, at 700 °C. Both La_0.3Sr_0.7TiO_3 − δ and La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ show relatively stable electrical conductivities under oxygen partial pressure of 10⁻¹⁴–10⁻¹⁹ atm at 800 °C. These properties make La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ a promising anode candidate for solid oxide fuel cells.     

Characterization of perovskite-type cathode,La0.75Sr0.25 Mn0.95−xCox Ni0.05O3+δ (0.1 ≤ x ≤ 0.3), for intermediate-temperature solid oxide fuel cells

Phase evolution, structure, thermal property, morphology, electrical property and reactivity of a perovskite-type cathode system, La_0.75Sr_0.25 Mn_0.95−xCo_xNi_0.05O_3+δ (0.1 ≤ x ≤ 0.3), are reported. The samples are synthesized using metal acetates by the Pechini method. A perovskite-type phase is formed after calcination at ∼700 °C and a rhombohedral symmetry of R – 3c space group is stabilized at ∼1100 °C. An increase in x decreases the unit cell volume linearly, accompanying with a linear decrease in bond lengths and tilt angle. The differential thermal analysis suggests the phase stabilization for a temperature range, 50–1100 °C. The thermo-gravimetric, thermal expansion, and electrical and ionic conductivities studies suggest presence of a Jahn–Teller transition at ∼260–290 °C. The samples with x = 0.1 mol exhibit electrical conductivity of ∼55 S cm⁻¹ at ∼600 °C, activation energy of ∼0.13 eV, coefficient of thermal expansion of ∼12 × 10⁶ °C⁻¹, crystallite size of ∼45 nm, Brunauer–Emmett–Teller (BET) surface area of 1.26 m² g⁻¹ and average particle size of ∼0.9 μm. A fairly high ionic conductivity, 5–9 × 10⁻² S cm⁻¹ makes the sample with x = 0.1 mole suitable for intermediate-temperature solid oxide fuel cell cathode applications. The experimental results are discussed with the help of the defect models proposed for La_1−xSr_xMnO_3+δ.     

Effect of Fe substitution on the structure and properties of LnBaCo2−xFexO5+δ (Ln = Nd and Gd) cathodes

The effect of Fe substitution for Co on the crystal chemistry, thermal and electrical properties, and catalytic activity for oxygen reduction reaction of the layered LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) perovskite has been investigated. The air-synthesized LnBaCo_2−xFe_xO_5+δ samples exhibit structural change with increasing Fe content from tetragonal (0 ≤ x ≤ 1) to cubic (1.5 ≤ x ≤ 2) for the Ln = Nd system and from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) for the Ln = Gd system. The thermal expansion coefficient (TEC) and electrical conductivity decrease with increasing Fe content in LnBaCo_2−xFe_xO_5+δ. While the substitution of a small amount of Fe (x = 0.5) for Co leads to slightly improved performance in solid oxide fuel cells (SOFC), larger Fe contents (x ≥ 1.0) deteriorate the fuel cell performance. In the Ln = Gd system, the better performance of the x = 0.5 sample is partly due to the improved chemical stability with the LSGM electrolyte at high temperatures. With an acceptable electrical conductivity of >100 S cm⁻¹ at 800 °C, the x = 0.5 sample in the LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) system offers promising mixed oxide-ion and electronic conducting (MIEC) properties.     

Advanced ceramic interconnect material for solid oxide fuel cells: Electrical and thermal properties of calcium- and nickel-doped yttrium chromites

The structural, thermal and electrical characteristics of calcium- and nickel-doped yttrium chromites were studied for potential use as the interconnect material in high temperature solid oxide fuel cells (SOFCs) and other high temperature electrochemical and thermoelectric devices. The Y_0.8Ca_0.2Cr_1−xNi_xO_3±δ compositions with x = 0-0.15 showed single phase orthorhombic perovskite structures between 25 and 1200 °C over a wide range of oxygen partial pressures. Nickel doping remarkably enhanced sintering behavior of otherwise refractory chromites, and densities 94% of theoretical density were obtained after sintering at 1400 °C in air with 15 at.% Ni. The thermal expansion coefficient (TEC) was increased with nickel content to closely match that of an 8 mol% yttria-stabilized zirconia (YSZ) electrolyte for 0.05 ≤ x ≤ 0.15. Nickel doping significantly improved the electrical conductivity in both oxidizing and reducing atmospheres. Undesirable oxygen ion “leakage” current was insignificant in dual atmosphere conditions. No interfacial interactions with YSZ were detected after firing at 1400 °C.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Ln0.6Sr0.4Co1−yFeyO3−δ (Ln = La and Nd; y = 0 and 0.5) cathodes with thin yttria-stabilized zirconia electrolytes for intermediate temperature solid oxide fuel cells

The electrochemical performances of the solid oxide fuel cells (SOFC) fabricated with Ln_0.6Sr_0.4Co_1−yFe_yO_3−δ (Ln = La, Nd; y = 0, 0.5) perovskite cathodes, thin yttria-stabilized zirconia (YSZ) electrolytes, and YSZ–Ni anodes by tape casting, co-firing, and screen printing are evaluated at 600–800 °C. Peak power densities of ∼550 mW cm⁻² are achieved at 800 °C with a La_0.6Sr_0.4CoO_3−δ (LSC) cathode that is known to have high electrical conductivity. Substitution of La by Nd (Nd_0.6Sr_0.4CoO_3−δ) to reduce the thermal expansion coefficient (TEC) results in only a slight decrease in power density despite a lower electrical conductivity. Conversely, substitution of Fe for Co (La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ or Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ) to reduce the TEC further reduces the cell performance greatly due to a significant decrease in electrical conductivity. However, infiltration of the Fe-substituted cathodes with Ag to increase the electrical conductivity increases the cell performance while preserving the low TEC.     

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.     

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.     

Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO4

The sintering properties, crystal structure and electrical conductivity of La_1−xCa_xNbO_4−δ (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025), prepared by a solid-state reaction, have been investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and electrochemical impedance spectroscopy (EIS). In 2.5% Ca-doped samples, a small amount of impurities Ca₂Nb₂O₇ were observed from the XRD patterns. Impedance spectra show that the grain boundary resistance increases with increasing Ca content, while the bulk resistance remains essentially constant below 550 °C. Despite the higher degree of grain growth observed for higher Ca doping levels, the total conductivity of the La_1−xCa_xNbO_4−δ series decreases with increasing Ca content from 0.5 to 2.0 mol%. The activation energy for the total conductivity decreases with increasing Ca content from 0.71 eV (x = 0) to 0.54 eV (x = 0.01) for the high temperature tetragonal phase, then it increases to 0.60 eV for x = 0.02. For the monoclinic phase, the activation energy exhibits similar trend except La_0.995Ca_0.005NbO_4−δ shows the lowest value of 1.26 eV. The Ca and Nb content present at the grain boundaries for La_0.99Ca_0.01NbO_4−δ are much higher than that on the grain surface, as determined from the EDS analysis. These results imply that the solubility of CaO in LaNbO₄ is in the range from 0.5 to 1.0 mol%. By increasing the sintering temperature from 1500 °C to 1550 °C, the proton conductivity of the Ca-doped LaNbO₄ was improved with enlarged grain size due to a reduction in the resistive grain boundary contribution.