Effect of sintering aids on the conductivity of BaCe0.9Ln0.1O3−δ

Polycrystalline powders of BaCe_0.9Ln_0.1O_3−δ (Ln = La, Nd, Sm, Gd, Yb, Tb and Y) have been prepared using a freeze-drying precursor route at 1000 °C. In order to decrease the densification temperature, different sintering aids (e.g. Co, Zn, Ni, Fe and Cu) were added by mixing the polycrystalline powders with a nitrate solution containing these metals. This allows obtaining dense ceramics at temperatures as low as 1000 °C, when compared to samples without sintering aids at 1400 °C. The effect of aid content and sintering temperature on the microstructure and electrical conductivity were investigated by scanning electron microscopy and impedance spectroscopy. The addition of small amounts of transition metals does not produce observable structural changes by conventional X-ray powder diffraction. However, the bulk and total conductivities decrease when compared to samples without transition metals. Zn seems to be the most effective sintering element for BaCe_0.9Ln_0.1O_3−δ electrolytes because it does not cause significant changes in the ionic and electronic conductivities.     

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.     

Structural and conductivity study of the proton conductor BaCe(0.9−x)ZrxY0.1O(3−δ) at intermediate temperatures

The perovskite BaCe_(0.9−x)Zr_xY_0.1O_(3−δ) is prepared by solid-state reaction at 1400 °C and sintering at 1700 °C. It is characterised using X-ray diffraction, Raman spectroscopy and electrical measurements. A distortion from the cubic structure at room temperature is noticeable in the Raman spectra for 0.2 < x < 0.8, but not in the X-ray diffraction patterns. This work points out the rhombohedral nature of this distortion. Phase transitions are studied up to 600 °C. The direct current conductivity is measured as a function of oxygen partial pressure, and at a water vapour partial pressure of 0.015 atm. The total conductivity is resolved into an ionic and a p-type component using a fitting procedure appropriate to the assumed defect model. The first contribution is useful for estimating the proton transport number, while the value of the second one should not be too high not to deteriorate the electrodes performance.     

Electrical conductivity of CaZr0.9Y0.1O3−δ films deposited from liquid solutions

CaZr_0.9Y_0.1O_3−δ (CZY) films were fabricated by chemical solution deposition (CSD) from ethanol-based liquid solutions. The films were deposited on single crystals of yttria-stabilized zirconia (YSZ) in (100), (110) and (111) crystallographic orientations. The structural, mechanical and electrical properties of the films have been studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation and impedance spectroscopy methods. XRD and SEM results have shown that the films were polycrystalline. Effect of the substrate orientation on the microstructural, mechanical and electrical properties of CZY films has been examined. The films deposited on (110)-substrates consist of the largest grains; show the highest values of hardness and electrical conductivity. Conductivity of CZY film on (110)-substrates increases with rise in humidity which proves proton transport in the films.     

Structure, chemical stability and mixed proton-electron conductivity in BaZr0.9−xPrxGd0.1O3−δ

BaZr_0.9−xPr_xGd_0.1O_3−δ (x = 0.3 and 0.6) was prepared by combustion synthesis and characterised with respect to conductivity and stability in an attempt to combine the desirable properties of the end members. The polycrystalline materials exhibit a cubic or pseudo-cubic structure as determined by X-ray synchrotron radiation and transmission electron microscopy. The chemical stability of the compositions is strongly dependent on the praseodymium content, the materials with more Pr present lower stability. Electron holes dominate the conductivity under oxidising atmospheres in BaZr_0.3Pr_0.6Gd_0.1O_3−δ, while BaZr_0.6Pr_0.3Gd_0.1O_3−δ exhibits a mixed electron hole-proton conducting behaviour. Substitution of Zr by Pr in acceptor-doped BaZrO₃ decreases the sintering temperature and increases the grain growth rate.     

A simple and effective process for Sr0.995Ce0.95Y0.05O3−δ and BaCe0.9Nd0.1O3−δ solid electrolyte ceramics

A simple and effective reaction-sintering process for Sr_0.995Ce_0.95Y_0.05O_3−δ and BaCe_0.9Nd_0.1O_3−δ solid electrolyte ceramics was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Sr_0.995Ce_0.95Y_0.05O_3−δ ceramics with 98.4% of the theoretical density were obtained after being sintered at 1350 °C for 2 h. A total conductivity 1.42 mS cm⁻¹ at 900 °C could be obtained in Sr_0.995Ce_0.95Y_0.05O_3−δ sintered at 1500 °C for 4 h. BaCe_0.9Nd_0.1O_3−δ ceramics with 91.7% of the theoretical density were obtained after being sintered at 1500 °C for 2 h. A total conductivity 11.54 mS cm⁻¹ at 900 °C could be obtained in BaCe_0.9Nd_0.1O_3−δ sintered at 1350 °C for 6 h. The reaction-sintering process has proven a simple and effective method to obtain useful Sr_0.995Ce_0.95Y_0.05O_3−δ and BaCe_0.9Nd_0.1O_3−δ ceramics for solid electrolyte applications in solid oxide fuel cells.     

Electrical conductivity,thermal expansion and electrochemical performances of Ba-doped SrCo0.9Nb0.1O3−δ cathodes for IT-SOFCs

Perovskite-type oxides Ba_xSr_1−xCo_0.9Nb_0.1O_3−δ (BSCNx, x = 0.0–0.8) were synthesized and investigated as cathodes for IT-SOFCs. Ba doping improves chemical compatibility between BSCNx oxides and Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte. Effects of Ba doping on electrical conductivity, thermal expansion and electrochemical performances were systematically elucidated and discussed. Both thermal expansion coefficient (TEC) and polarization resistance (R_p) decrease with increasing Ba doping level up to x = 0.6, attain a minimum at x = 0.6 and then increase with further increasing x > 0.6. The decrease of TEC with the incorporation of Ba can be attributed to the weakened chemical expansion and the decrease of R_p with Ba is due to the increase of oxygen vacancy concentration and oxygen vacancy diffusion coefficient. With a 300 μm-thick GDC as electrolyte and BSCN0.6 as the cathode, the maximum power density of a single-cell achieves 778 mW cm⁻² at 800 °C. All these results indicate that the BSCN0.6 oxide is a promising cathode material for IT-SOFC.     

A cobalt-free SrFe0.9Sb0.1O3−δ cathode material for proton-conducting solid oxide fuel cells with stable BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte

A cobalt-free cubic perovskite oxide SrFe_0.9Sb_0.1O_3−δ (SFSb) is investigated as a novel cathode for proton-conducting solid oxide fuel cells (H-SOFCs). XRD results show that SFSb cathode is chemically compatible with the electrolyte BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) for temperatures up to 1000 °C. Thin proton-conducting BZCYYb electrolyte and NiO-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (NiO-BZCYYb) anode functional layer are prepared over porous anode substrates composed of NiO-BZCYYb by a one-step dry-pressing/co-firing process. Laboratory-sized quad-layer cells of NiO-BZCYYb/NiO-BZCYYb/BZCYYb/SFSb are operated from 550 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. An open-circuit potential of 0.996 V, maximum power density of 428 mW cm⁻², and a low electrode polarization resistance of 0.154 Ω cm² are achieved at 700 °C. The experimental results indicate that the cobalt-free SFSb is a promising candidate for cathode material for H-SOFCs.     

The effect of Cr deposition and poisoning on BaZr0.1Ce0.7Y0.2O3−δ proton conducting electrolyte

With the development of chromium tolerant electrode materials, the evaluation of the chromium deposition and poisoning on electrolyte is critical significance for the commercial and widespread application of solid oxide fuel cell stacks (SOFCs). The Cr deposition and poisoning on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) proton conducting electrolyte are initially studied, in order to understand and develop the compatibility for proton conducting SOFC (H-SOFCs). The XRD results imply that Cr₂O₃ is not chemically compatible with BZCY and BaCrO₄ is formed at high temperature above 600 °C. To simulate the Cr volatilization from interconnect and poisoning on BZCY surface, the BZCY bar sample is heat-treated in the presence of Cr₂O₃ at 600 °C, 700 °C, and 800 °C for 50 h. It is clear that Cr deposition occurs even at 600 °C by SEM examination. The XPS results indicate the chemical deposition of BaCrO₄ and physical deposition of Cr₂O₃ on BZCY surface at 600 °C but only chemical deposition at 700 °C and 800 °C. The content of Cr deposition increases with the increase of poisoning temperature. Moreover, the proton conductivity of BZCY after Cr deposition reduces after Cr deposition, indicating the Cr poisoning effect of the electrochemical performance of BZCY electrolyte.     

Fe doping effects on phase stability and conductivity of La0.75Sr0.25Ga0.8Mg0.2O3−δ

The effects of Fe doping on phase stability and electrical conductivity were investigated for the 25 mol% Sr and 20 mol% Mg-doped lanthanum gallates (LSGM-2520). While secondary phases, in addition to perovskite phase, were formed for an undoped LSGM-2520 in accordance with previous reports, a single phase perovskite phase was obtained by addition of a small amount (∼4 mol%) of Fe to LSGM-2520. The conductivity of 4 mol% Fe-doped LSGM-2520 was as high as 0.17 S cm⁻¹ at 800 °C and it was confirmed to be ionic based on its dependency on oxygen partial pressures. Crystallographic analysis using neutron diffraction and electron diffraction determined the crystal structure and lattice parameters for 4 mol% Fe-doped LSGM-2520.     

Effects of iron content on the structural evolution,electrical properties and thermochemical stability of BaCo0.9−xFexNb0.1O3−δ ceramic membrane

Cubic perovskite oxygen permeation materials BaCo_0.9−xFe_xNb_0.1O_3−δ (BCFN, x = 0.1–0.7) are prepared by the conventional solid state reaction process. The crystal structure development, structural stability, electrical conductivity and oxygen permeation flux are investigated. The introduction of iron makes the formation of cubic perovskite structure for BCFN materials much easier. BCFN exhibits a p-type semiconductor and obeys the thermally activated small polarons hopping mechanism. The electrical conductivity of BCFN increases with increasing temperature and decreases with the Fe-doping concentration. The incorporation of Fe decreases slightly the oxygen permeability of BCFN membranes, but enhances significantly the structure stability of the oxygen permeation membrane in reducing atmosphere. A high oxygen permeation flux of 1.7 ml cm⁻² min⁻¹ at 900 °C through 1 mm densified membrane under air/helium condition is obtained for the composition of BaCo_0.6Fe_0.3Nb_0.1O_3−δ.     

Pd-substituted (La,Sr)CrO3−δ-Ce0.9Gd0.1O2−δ solid oxide fuel cell anodes exhibiting regenerative behavior

Composite anodes consisting of Pd-substituted (La,Sr)CrO_3−δ mixed with 50 wt% Ce_0.9Gd_0.1O_2−δ were tested in La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ-electrolyte supported fuel cells at 800 °C with humidified H₂ fuel. Low anode polarization resistance was observed during the first several hours of operation, explained by the nucleation of Pd nano-particles on perovskite particle surfaces. Anode performance then degraded gradually before stabilizing. Redox cycling repeatedly restored the anodes to their initial peak performance, followed again by degradation. This regenerative behavior was explained by the observation that the Pd nano-particles were removed by oxidation, and then re-nucleated upon reduction.     

Phase stability and conductivity of Ba1−ySryCe1−xYxO3−δ solid oxide fuel cell electrolyte

The structure, phase stability, and electrical properties of BaCe_1−xY_xO_3−δ (x = 0-0.4) in humidity air and CO₂ atmosphere are investigated. XRD results indicate that the BaCe_0.9Y_0.1O_3−δ sample has a symmetric cubic structure, and its phase changes to tetragonal as the Y³⁺ doping amount increases to 20 mol%. The conductivity of BaCe_1−xY_xO_3−δ increases with temperature, and it depends on the amount of yttrium doping and the atmosphere. BaCe_0.8Y_0.2O_3−δ exhibits the highest conductivity of 0.026 S cm⁻¹ at 750 °C. The activation energy for conductivity depends on yttrium doping amount and temperature. The conductivity of BaCe_0.8Y_0.2O_3−δ is 0.025 S cm⁻¹ in CO₂ atmosphere at 750 °C which is 3.8% lower than that in air due to reactions with CO₂ and BaCO₃ and the CeO₂ impure phases formed. The structure of BaCe_0.8Y_0.2O_3−δ is unstable in water and decomposes to Ba(OH)₂ and CeO₂ phases. It is found that the activation energy of samples in CO₂ atmosphere is higher than that of sample in air. Sr-doped Ba_1−ySr_yCe_0.8Y_0.2O_3−δ (y = 0-0.2) is prepared to improve the phase stability of BaCe_0.8Y_0.2O_3−δ in water. The conductivity of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is 0.023 S cm⁻¹ at 750 °C which was 11% lower than that of BaCe_0.8Y_0.2O_3−δ, however, the phase stability of Ba_0.9Sr_0.1Ce_0.8Y_0.2O_3−δ is much better than that of BaCe_0.8Y_0.2O_3−δ in water.     

Novel BaCe0.7In0.2Yb0.1O3−δ proton conductor as electrolyte for intermediate temperature solid oxide fuel cells

Novel proton conductor BaCe_0.7In_0.2Yb_0.1O_3−δ (BCIYb) has been successfully synthesized by a modified Pechini method and characterized as electrolyte for intermediate temperature solid oxide fuel cells. Acceptable tolerance to wet CO₂ environment was found during chemical stability tests. No interaction between the BCIYb electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode was observed during the cathode fabrication process. Further, no detectable impurity phase was found when the BCIYb-LSCF mixed powders were calcined at 700 °C for 50 h. BCIYb dense samples sintered at 1450 °C for 5 h showed acceptable conductivities of 7.2 × 10⁻³, 8 × 10⁻³, 4.5 × 10⁻³ and 3.1 × 10⁻³ S cm⁻¹ at 800 °C in dry air, wet air, wet H₂ and wet N₂, respectively. The maximum cell power outputs of single cells with the configuration of Ni-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)|BCIYb|BZCY-LSCF were 0.15, 0.218 and 0.28 W cm⁻² at 600, 650 and 700 °C, respectively. No cell degradation was observed for cells operated at a constant voltage of 0.7 V in the 25 h short-term durability test.     

Synthesis and characterization of BaIn0.3−xYxCe0.7O3−δ (x = 0, 0.1, 0.2, 0.3) proton conductors

The morphological and electrical properties of yttrium (Y) and indium (In) doped barium cerate perovskites of the form BaIn_0.3−xY_xCe_0.7O_3−δ (with x = 0–0.3) prepared by a modified Pechini method were investigated as potential high temperature proton conductors with improved chemical stability and conductivity. The sinterability increased with the increase of In-doping, and the perovskite phase was found in the BaIn_0.3−xY_xCe_0.7O_3−δ solid solutions over the range 0 ≤ x ≤ 0.3. The conductivities decreased from x = 0.3 to 0 while the tolerance to wet CO₂ improved for BaIn_0.3−xY_xCe_0.7O_3−δ samples with an increase of In-doping. BaIn_0.1Y_0.2Ce_0.7O_3−δ was found to have relatively high conductivity as well as acceptable wet CO₂ stability.     

The effect of doping in the electrochemical performance of (Ln1−xMx)FeO3−δ SOFC cathodes

A family of iron perovskites with the general formula AFeO_3−δ (A = Ln_1−xM_x; Ln = La, Nd and/or Pr; M = Sr or/and Ca) has been prepared keeping fixed the A cation radius 〈r_A〉 and cation size mismatch to isolate the effect of divalent dopant concentration from the A-cation steric effects. The electrochemical behaviour of these compounds for their application as SOFC cathodes was evaluated by using I-V curve measurements and ac impedance spectroscopy over three electrodes electrolyte supported cells processed under identical conditions. In contrast with the bulk behaviour, trends are more difficult to observe due to microstructural effects, but results seem to indicate that the doping level, x, does not influence in a significant way the electrochemical performance of iron perovskites with identical 〈r_A〉 and σ²(r_A).     

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.     

BaCe0.85Tb0.05Co0.1O3−δ perovskite hollow fibre membranes for hydrogen/oxygen permeation

Co-doped BaCe_0.85Tb_0.05Co_0.1O_3−δ (BCTCo) nanopowder was synthesized via a sol–gel method using ethylenediaminetetraacetic acid (EDTA) and citric acid as the chelating agents. Using the resultant powder, BCTCo perovskite hollow fibre membranes were then fabricated by the combined phase inversion and sintering technique. Properties of the BCTCo powder and the hollow fibre membranes in terms of crystalline phase, morphology, electrical conductivity, porosity, mechanical strength and hydrogen/oxygen permeation were investigated by a variety of characterization methods. The results indicated that doping of cobalt in the BCTb oxide led to a higher electrical conductivity and lower calcination temperature for the powder precursor to a perovskite structure as well as sintering temperature for the hollow fibre precursors to gastight membranes. In order to obtain gastight and robust hollow fibre membranes, the sintering temperature should be controlled between 1300 and 1450 °C. The maximum hydrogen flux through the BCTCo hollow fibre membranes reached up to 0.385 mL cm⁻² min⁻¹ at 1000 °C under 50% H₂–He/N₂ gradient, which is higher than that of the un-doped BCTb hollow fibre membranes with the same effective thickness, and especially much higher than that obtained from other proton conductors due to the asymmetric structure of the membrane designed. Moreover, the BCTCo hollow fibre membrane also exhibited noticeable oxygen permeation fluxes, i.e. 0.122 mL cm⁻² min⁻¹ at 1000 °C under the air/He gradient. However, doping of cobalt might damage the mechanical stability of the perovskite membranes in the hydrogen-containing atmosphere.     

Nd1.8Sr0.2NiO4−δ:Ce0.9Gd0.1O2−δ composite cathode for intermediate temperature solid oxide fuel cells

The (100 − x)Nd_1.8Sr_0.2NiO_4−δ:(x)Ce_0.9Gd_0.1O_2−δ (x = 00, 10, 20, 30, 40 and 50 vol%) composites are obtained by ball milling requisite mixture at 200 rotations per minute for 2 h under acetone followed by sintering at 1000 °C for 4 h. The increase in concentration of Ce_0.9Gd_0.1O_2−δ in composite reduces the crystallite size of host Nd_1.8Sr_0.2NiO_4−δ from 378 ± 0.7 to 210 ± 0.8 nm. The dc (electronic) conductivity of composite decreases moderately with an increase in Ce_0.9Gd_0.1O_2−δ content in composite up to 30 vol%, and it decreases abruptly, thereafter at x > 30. A minimum polarization resistance value of 0.24 Ω cm² (at 700 °C) is obtained for a (70)Nd_1.8Sr_0.2NiO_4−δ:(30)Ce_0.9Gd_0.1O_2−δ composite cathode, and this value is attributed to the optimal dispersion of Ce_0.9Gd_0.1O_2−δ into Nd_1.8Sr_0.2CuO_4−δ matrix. The oxygen partial pressure dependent polarization resistance study suggests that the charge transfer and the non-charge transfer oxygen adsorption–desorption along with diffusion are the major rate limiting steps of overall oxygen reduction reaction process.