Electrical conductivity and chemical stability of perovskite-type BaCe0.8-xTixY0.2O3-δ

Here we report the synthesis, chemical stability, and electrical conductivity of Ti-doped perovskite-type BaCe_0.8-x Ti _x Y_0.2O_3-δ (x = 0.05, 0.1, 0.2, and 0.3; BCTY). Samples were synthesized by conventional solid state (ceramic) reaction from corresponding metal salts and oxides at elevated temperature of 1,300–1,500 °C in air. The powder X-ray diffraction confirmed the formation of a simple cubic perovskite-type structure with a lattice constant of a = 4.374(1), 4.377(1), and 4.332(1) ? for x = 0.05, 0.1, and 0.2 members of BCTY, respectively. Like BaCe_0.8Y_0.2O_3-δ (BCY), Ti substituted BCTY was found to be chemically not stable in 100% CO₂ and form BaCO₃ at elevated temperature. The bulk electrical conductivity of BCTY decreased with increasing Ti content and the x = 0.05 member exhibited the highest conductivity of 2.3 × 10⁻³ S cm⁻¹ at 650 °C in air, while a slight increase in the conductivity, especially at low temperatures (below 600 °C), was observed in humidified atmospheres.     

A stable BaCeO3-based proton conductor for solid oxide fuel cells

In order to improve the chemical stability of BaCeO₃, Ti⁴⁺ was introduced into B site of BaCeO₃ to modify the chemical stability. XRD test demonstrates that \textBaC\texte_0.6\textT\texti_0.2\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.6}}{\text{T}}{{\text{i}}_{0.2}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCTY) keeps its original pervoskite-type structure at a high doping level of 20%. After exposure in 94% N₂ + 3% CO₂ + 3% H₂O at 700 °C for 10 h, BCTY exhibited adequate chemical stability while decomposition was found in \textBaC\texte_0.8\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.8}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCY). Accordingly, the conductivity of BCTY reaches 0.0072 S/cm at 700 °C in humidified hydrogen which is a little lower than BCY (0.0085). Besides, BCTY displayed better sintering characteristics than BCY at high temperatures and the relative density reaches 96.4% and 94.8%, respectively. The two samples also exhibited similar thermal expansion behavior from 30 to 1,000 °C. A fuel cell with BCTY as electrolyte exhibited 244 mW/cm² at 700 °C and the stable short-term performance further proved the stability of BCTY.     

Chemical and redox stabilities of a solid oxide fuel cell with BaCe0.8Y0.2O3−α functioning as an electrolyte and as an anode

The operation of a solid oxide fuel cell (SOFC) based on BaCe_0.8Y_0.2O_3−α (BCY20) at 800 °C was studied without using an anode material. A porous, Ce-rich phase with a fluorite structure was formed at a depth of approximately 10 μm from the BCY20 surface by heat treatment at 1700 °C. This was due to the vaporization of BaO from the BCY20 surface. This treatment improved the cell performance and chemical stability to CO₂ because the Ce-rich phase functioned as an electrically conducting and protective layer. The heat-treated BCY20 also had better chemical and redox stabilities over a Ni–Ce_0.8Sm_0.2O_1.9 (SDC) cermet anode attached to the SDC electrolyte. The cell with the heat-treated BCY20 operated well on unhumidified methane, ethane, propane, and butane without carbon deposition, while the cell with the Ni–SDC cermet anode degraded within a few hours. Moreover, the BCY20 showed higher tolerance to 10 ppm H₂S and stability over 20 times redox cycling in comparison to the Ni–SDC cermet anode.     

Structure and impedance spectroscopy of Pr1? x Sr x Fe0.8Co0.2O3? δ ( x =0.1, 0.2, 0.3) thin films grown by laser ablation

Polycrystalline samples of Pr_1−x Sr _x Fe_0.8Co_0.2 O_3−δ (x=0.1, 0.2, 0.3) (PSFC) were prepared by the combustion synthesis route at 1200°C. The structure of the polycrystalline powders was analysed with X-ray powder diffraction data. The X-ray diffraction (XRD) patterns were indexed as the orthoferrite similar to that of PrFeO₃ having a single-phase orthorhombic perovskite structure (Pbnm). Pr_1−x Sr _x Fe_0.8Co_0.2O_3−δ (x=0.1, 0.2, 0.3) films have been deposited on yttria-stabilized zirconia (YSZ) single-crystal substrates at 700°C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM). All films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distributions. Electrochemical impedance spectroscopy (EIS) measurements of PSFC/YSZ single crystal/PSFC test cells were conducted. The Pr_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ film at 850°C presents a lower area specific resistance (ASR) value, 1.65 Ω cm², followed by the Pr_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (2.29 Ω cm² at 850°C) and the Pr_0.9Sr_0.1Fe_0.8Co_0.2O_3−δ films (5.45 Ω cm² at 850°C).     

Thermal degradation of proton conductors BayM1–xYx O3–δ (M=Zr, Ce)(M=Zr, Ce)

We optimize the preparation of BZY20 and BCY10 by a wet chemistry route (the polyacrylamide gel process) and mixed oxide route, respectively. For both materials, the purity of powders drastically depends on the annealing conditions of the raw materials. Pure BZY20 powder can be prepared at 1250 °C while, for pure BCY10, completion of the reaction is achieved if the raw powder is pressed. After polishing the surface and crushing the bulk of the pellet annealed at 1425 °C, pure powder of BCY10 is obtained. Water uptake measurement is leading to values corresponding to an almost complete filling of the oxygen vacancies. Furthermore, we check the sample degradation during sintering of pellets from pure BCY10 and BZY20 powder. Dense ceramic of pure material can be prepared after sintering at 1500 °C for 10 h. Above this temperature, a degradation of the pellets both in the surface and the bulk occurs. This paper points out the difficulties in preparing pure Ba_yM_1−xY_x O_3–δ (M=Zr, Ce) for use in electrical characterization or functional for fuel cell technology studies.     

A wet-chemical route for the preparation of Ni–BaCe0.9Y0.1O3 − δ cermet anodes for IT-SOFCs

Nano-sized BaCe_0.9Y_0.1O_3 ? δ (BCY10) protonic conductor powders were used to prepare Ni-BCY10 cermets for anode-supported intermediate temperature solid oxide fuel cells. A new wet-chemical route was developed starting from Ni nitrates as precursors for NiO. BCY10 powders were suspended in a Ni nitrate aqueous solution that was evaporated to allow NiO precipitation on the BCY grains, obtaining NiO-BCY10 cermets. To obtain the final Ni-BCY10 anodes, pellets were reduced in dry H₂ at 700 °C. The structural and microstructural properties of the pellets were investigated using X-ray diffraction analysis and field emission scanning electron microscopy. A homogeneous dispersion of perovskite and nickel phases was observed. The chemical stability of the anodes was evaluated under wet H₂ and CO₂ atmosphere at 700 °C. The electrical properties of the Ni-BCY10 pellets were evaluated using electrochemical impedance spectroscopy measurements. The Ni-BCY10 cermet electrodes showed large electronic conductivity, demonstrating percolation through the Ni particles, and low area specific resistance at the BCY10 interface. These characteristics make the cermet suitable for application in BCY-based protonic fuel cells. The developed chemical route offers a simple and low-cost procedure to obtain promising high performance anodes.     

Preparation and electrochemical properties of nano Al<Subscript>0.2</Subscript>V<Subscript>2</Subscript>O<Subscript>5.3−<Emphasis Type="BoldItalic">δ</Emphasis></Subscript> cathode materials for rechargeable lithium batteries

Nano-sized Al³⁺-doped V₂O₅ cathode materials, Al_0.2V₂O_5.3−δ , were prepared by an oxalic acid assisted sol–gel method at 350 °C (sample A) and 400 °C (sample B). X-ray diffraction confirmed that samples A and B were pure phase Al_0.2V₂O_5.3−δ with an orthorhombic structure close to that of V₂O₅. Scanning electron microscopy showed that sample A was in nanoscale with a mean particle size about 50 nm. Cyclic voltammetry showed the good electrochemical and structural reversibility of the Al_0.2V₂O_5.3−δ nanoparticles during the Li⁺ insertion/extraction process. The Al_0.2V₂O_5.3−δ nanoparticles exhibited excellent charge–discharge cycling performance and rate capability compared to that of bulky V₂O₅ electrodes. For instance, the materials delivered a reversible specific capacity about 180 mAh g⁻¹ (sample A) and 150 mAh g⁻¹ (sample B), in the potential window of 4.0–2.0 V at the current density of 150 mA g⁻¹. The Al_0.2V₂O_5.3−δ nanoparticles in particular showed almost no capacity fading for at least 50 cycles.     

On the use of NiO as sintering additive for BaCe0,9Y0,1O3 − α

The effect of nickel oxide addition on the densification behaviour and electrical conductivity of BaCe_0.9Y_0.1O_3 ? α (BCY10) is investigated. Small addition (4 mol%) of this sintering aid reduces the sintering temperature by 200 °C and promotes the densification. 95% of the theoretical density was reached after sintering at 1250 °C for 10 h in air. Addition of NiO has no detrimental effect on the total conductivity of BCY10 in wet hydrogen. Nevertheless, the activation energy of the recorded blocking effect is higher than that of the bulk contribution, indicating different electrical properties between grains and internal interfaces in pure and NiO doped BCY10.     

Microstructure and electrode properties of La0.6Sr0.4Co0.2Fe0.8O3-δ spin-coated on Ce0.8Sm0.2O2?δ electrolyte

Fine and uniform La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ powder was synthesized by a glycine–nitrate combustion process. La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ electrodes were prepared on dense Ce_0.8Sm_0.2O_2−δ electrolyte substrates using a spin-coating technique by sintering at 900–1,000 °C. The electrode properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ were investigated by electrochemical impedance spectroscopy and chronopotentiometry techniques with respect to preparation conditions and the resulting microstructures. The results indicate a significant effect of the microstructure on the electrode processes and polarization characteristics. The oxygen adsorption and dissociation process acted as a larger contribution to the overall electrode polarization R _p in magnitude compared with the charge transfer process due to relatively low porosity levels of the electrodes. It was detected that the grain size of the electrodes exhibited a crucial role on the electrocatalytic reactivity. At 800 °C, the electrode sintered at 950 °C attained a polarization resistance of 0.18 Ω cm², an overpotential of 27 mV at a current density of 200 mA cm⁻², and an exchange current density of 308 mA cm⁻².     

Structural,thermal and electrical properties of Bi and Y co-doped barium zirconium cerates

Bismuth- and yttrium-co-doped barium cerates were successfully synthesised by solid-state reactions followed by sintering between 1,400 and 1,500 °C for 1 to 6 h allowing densification above 98 % to be obtained. All samples were found to retain their original orthorhombic structure after treatment in either oxidising or reducing atmospheres (dry and wet). Mechanical strength was affected by structure upon reduction due in part to strains and stresses induced by bismuth ionic size variations. Conductivity values as high as 0.055 S/cm were obtained for sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ and of 0.0094 S/cm for the Zr-free compound BaCe_0.7Y_0.2Bi_0.1O_3?δ at 700 °C in air. In all the investigated materials, sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ exhibits the highest conductivity in both air and wet 5 % H₂/Ar with good mechanical strength. BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ is a promising mixed H⁺/e^? conductor, a potential component of composite anode for solid oxide fuel cells.