Aggregatively stable suspensions of micrometer powders of doped barium cerate for electrophoretic deposition of thin-film coatings of solid-oxide fuel cells

Potentialities of the method of electrophoretic deposition of thin-film coatings based on micrometer powders of multidoped barium cerate BaCe_0.8Sm_0.19Cu_0.01O_3–δ (BCSCuO) and BaCe_0.89Gd_0.1Cu_0.01O_3–δ (BCGCuO) were considered. Micrometer powders of BCSCuO and BCGCuO were produced by the methods of solid-phase and citrate-nitrate syntheses, respectively. The dispersity, fraction composition, and electrokinetic potential of nonaqueous suspensions of these powders and the electrokinetic parameters of the electrophoretic deposition process were examined. An ultrasonic treatment and ultracentrifugation produced aggregatively stable suspensions of BCGCuO and BCSCuO micrometer particles in a mixed (70/30 vol %) isopropanol–acetyl acetone medium. These suspensions are characterized by high positive values of the zeta potential (+24 and +28 mV, respectively). Thin film coatings of the electrolyte materials BCSCuO and BCGCuO, which are of interest for the technology of medium-temperature solid-oxide fuel cells, were produced by the electrophoretic deposition onto a dense model cathode.     

Formation of Bilayer Thin-Film Electrolyte on Cathode Substrate by Electrophoretic Deposition

Potentialities of the method of bilayer thin-film electrolyte electrophoretic deposition onto cathodic substrate are analyzed. Ce_0.8Sm_0.2O_1.9–δ (SDC) nanopowder and BaCe_0.89Gd_0.1Cu_0.01O_3–δ BCGCuO) micropowder are prepared by the methods of laser evaporation–condensation and pyrolysis, respectively. The effect of ultrasonic treatment on the SDC and BCGCuO particle distribution in suspensions and their electrokinetic properties are studied. The using of the ultrasonic treatment combined with centrifugation allowed obtaining an aggregative-stable suspension of the BaCe_0.89Gd_0.1Cu_0.01O_3–δ micron particles in the isopropanol–acetylacetone mixed medium (70/30 v/v) that is characterized by high zeta potential. Ce_0.8Sm_0.2O_1.9–δ and BaCe_0.89Gd_0.1Cu_0.01O_3–δ thin films are obtained at the La₂NiO_{4 +δ} cathode substrate using electrophoretic deposition; microstructure and electric properties of the prepared thin-film structures are studied. The conductivity and electric properties of the bilayer electrolyte were found to be determined by the Ce_0.8Sm_0.2O_1.9–δ film properties. Despite the sintering high temperature, the grain structure of the BaCe_0.89Gd_0.1Cu_0.01O_3–δ film is underdeveloped; this is determined by the micron powder properties.     

Effect of Solution Characteristics on Morphology of SrZrO<Subscript>3</Subscript> Film Electrolyte in Chemical Solution Deposition

Fundamental aspects of how films of the SrZr_0.95Y_0.05O_3–δ electrolyte are formed from alcoholic-aqueous solutions of salts at various solution characteristics (salt concentration, viscosity, and relative content of water) were studied. It was found that, to obtain dense film electrolytes, it is necessary to use low-viscosity solutions with the minimum content of water. The revealed fundamental aspects will make it possible to obtain dense film membranes based on strontium zirconate for solid-oxide fuel cells by the technologically simple solution method. Solutions with high content of water can be used to form an external porous layer on the surface of a dense membrane, which must favor an increase in the power of fuel cells.     

Mid-temperature solid oxide fuel cells with thin film ZrO<Subscript>2</Subscript>: Y<Subscript>2</Subscript>O<Subscript>3</Subscript> electrolyte

Data on the mid-temperature solid-oxide fuel cells (SOFC) with thin-film ZrO₂-Y₂O₃ (YSZ) electrolyte are shown. Such a fuel cell comprises a carrying Ni-YSZ anode, a YSZ electrolyte 3–5 μm thick formed by vacuum ion-plasma methods, and a LaSrMnO₃ cathode. It is shown that the use of a combined method of YSZ electrolyte deposition, which involves the magnetron deposition of a 0.5–1.5-μm thick sublayer and its pulse electron-beam processing allows a dense nanostructured electrolyte film to be formed and the SOFC working temperature to be lowered down as the result of a decrease in both the solid electrolyte Ohmic resistance and the Faradaic resistance to charge transfer. SOFC are studied by the methods of voltammentry and impedance spectroscopy. The maximum power density of the SOFC under study is 250 and 600 mW/cm⁻² at temperatures of 650 and 800°C, respectively.     

Impedance Study of the Conductivity of Solid Oxide Electrolyte Films SrZr<Subscript>0.95</Subscript>Y<Subscript>0.05</Subscript>O<Subscript>3–δ</Subscript> and CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3–δ</Subscript>

Information on the across-plane conductivity of films of solid-oxide electrolytes SrZr_0.95Y_0.05O_3–δ and CaZr_0.9Y_0.1O_3–δ deposited on ion-conducting supports is acquired by the impedance method. It is shown that the support/film interface and the intergrain boundaries considerably affect the across-plane charge transfer in the film. The effect of the crystallographic orientation of the YSZ support on the microstructure and conductivity of the CaZr_0.9Y_0.1O_3–δ electrolyte film is demonstrated.     

A study of the electrophoretic deposition of thin-film coatings based on barium cerate nanopowder produced by laser evaporation

The method of laser ablation of a target, followed by condensation, was used to obtain a weakly aggregated BCSO nanopowder from barium cerate. The dispersity, fraction composition of the nanopowder, electrokinetic potential of its nonaqueous dispersions, and electrokinetic parameters of the electrophoretic deposition process were determined. An ultrasonic treatment produced a stable suspension of the BCSO nanopowder in a mixed isopropanol–acetyl acetone medium (70/30 vol %). The suspension is characterized by a high and positive ζ-potential of +30 mV. The electrophoretic deposition onto a dense model cathode was used to obtain thin-film BCSO coatings that are of interest for the technology of solid-oxide fuel cells. The phase composition of the coating was examined. It was found that the successive annealings of the nanopowder at temperatures of 800–1400°C make it possible to reduce the content of unidentified crystalline phases in BCSO to trace levels (< 5 vol %).     

Solid-Oxide Electrochemical Cell for Determining the Diffusion of Hydrogen in Nitrogen at High Temperatures

Procedure was developed for measuring the diffusion coefficient of hydrogen in its mixture with nitrogen in the temperature range 450–600°C under atmospheric pressure. This is done in an electrochemical cell based on a solid-oxide electrolyte with proton conductivity of composition BaCe_0.7Zr_0.1Y_0.2O_3–δ. The procedure makes it possible to trace how the hydrogen diffusion coefficient varies with temperature and hydrogen concentration in nitrogen. It is shown that the hydrogen diffusion coefficient grows with increasing hydrogen concentration, its temperature dependence is of power-law type with n = 1.5, and it is in satisfactory agreement with the theoretical temperature dependence     

Synthesis,microstructure, and electric properties of CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3 – δ</Subscript> films obtained on porous SrTi<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3 – δ</Subscript> supports

Compact CaZr_0.9Y_0.1O_3–δ (CZY) film on a porous SrTi_0.8Fe_0.2O_3–δ (STF) support is obtained using the technique of deposition from solutions of inorganic salts in ethanol. According to the data of scanning electron microscopy (SEM), the film has a nanoporous granular structure with the grain size of 0.2 to 1 μm. The thickness of the CZY film on the STF support is about 3 μm after 15-fold solution application. The results of studying the elemental composition showed that elements of the support diffuse into the film in the course of synthesis. Analysis of the data of impedance spectroscopy shows that conductivity of the CZY film is limited the grain bulk. It is assumed that the comparatively low conductivity activation energy of the film (50.3 kJ/mol) is due to diffusion of elements of the STF support that results in variation of the film composition and properties.     

Structure,chemical stability and electrical properties of BaCe0.9Y0.1O3−δ modified with V2O5

Wet vacuum impregnation method was applied in order to evaluate the possibility of the formation of the material in BaCe_0.9Y_0.1O_3?δ–V₂O₅ system. Single-phase BaCe_0.9Y_0.1O_3?δ samples, synthesised by solid-state reaction method, were impregnated with the solution of vanadium(V) oxide precursor. Multi-step, multi-cycle impregnation procedure was applied to enhance the impregnation efficiency. Partial decomposition of Y-doped BaCeO₃ in contact with the solution of the precursor, resulting in the formation of vanadium containing phases (CeVO₄ and BaV₂O₆) on the materials surface, was observed. However, the presence of vanadium was also confirmed for the inner parts of the materials. The synthesised materials were submitted for exposition test to evaluate their chemical stability towards CO₂/H₂O. All BaCe_0.9Y_0.1O₃-based materials modified by impregnation revealed higher chemical stability in comparison with single-phase un-modified BaCe_0.9Y_0.1O_3?δ, since the amount of barium carbonate formed during the exposition was significantly lower. The total electrical conductivity of the received multi-phase materials was generally slightly lower than for the reference BaCe_0.9Y_0.1O_3?δ sample, since the presence of the additional phases had a blocking effect on materials conductivity. The values of BaCeO₃ lattice parameters and the Seebeck coefficient did not show the modification of the defects structure of Y-doped BaCeO₃ during applied synthesis procedure.

     

Chemical solution processing and characterization of Ba(Zr,Ti)O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> layered thin films

Ba(Zr,Ti)O₃/LaNiO₃ layered thin films have been synthesized by chemical solution deposition (CSD) using metal-organic precursor solutions. Ba(Zr,Ti)O₃ thin films with smooth surface morphology and excellent dielectric properties were prepared on Pt/TiO _x /SiO₂/Si substrates by controlling the Zr/Ti ratios in Ba(Zr,Ti)O₃. Chemically derived LaNiO₃ thin films crystallized into the perovskite single phase and their conductivity was sufficiently high as a thin-film electrode. Ba(Zr,Ti)O₃/LaNiO₃ layered thin films of single phase perovskite were fabricated on SiO₂/Si and fused silica substrates. The dielectric constant of a Ba(Zr_0.2Ti_0.8)O₃ thin film prepared at 700°C on a LaNiO₃/fused silica substrate was found to be approximately 830 with a dielectric loss of 5% at 1 kHz and room temperature. Although the Ba(Zr_0.2Ti_0.8)O₃ thin film on the LaNiO₃/fused silica substrate showed a smaller dielectric constant than the Ba(Zr_0.2Ti_0.8)O₃ thin film on Pt/TiO _x /SiO₂/Si, small temperature dependence of dielectric constant was achieved over a wide temperature range. Furthermore, the fabrication of the Ba(Zr,Ti)O₃/LaNiO₃ films in alternate thin layers similar to a multilayer capacitor structure was performed by the same solution deposition process.     

Formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> bilayer using atomic layer deposition and its application to dynamic random access memory

To enhance film conformality together with electrical property suitable for dynamic random access memory (DRAM) capacitor dielectric, the effects of oxidant and post heat treatment were investigated on aluminum and titanium oxide (Al₂O₃–TiO₂) bilayer (ATO) thin film formed by atomic layer deposition method. For the conformal deposition of Al₂O₃ thin film, the O₃ oxidant required a higher deposition temperature, more than 450 °C, while H₂O or combined oxygen sources (H₂O+O₃) needed a wide range of deposition temperatures ranging from 250 to 450 °C. Conformal deposition of the TiO₂ thin film was achieved at around 325 °C regardless of the oxidants. The charge storage capacitance, measured from the ATO bilayer (4 nm Al₂O₃ and 2 nm TiO₂) deposited at 450 °C for Al₂O₃ and 325 °C for TiO₂ with O₃ oxidant on the phosphine-doped poly silicon trench, showed about 15% higher value than that of 5 nm Al₂O₃ single layer thin film without any increase of leakage current. To maintain the improved electrical property of the ATO bilayer for DRAM application, such as enhanced charge capacitance without increase of leakage current, upper electrode materials and post heat treatments after electrode formation must be selected carefully. Dedicated to Professor Su-Il Pyun on the occasion of his 65th birthday.     

Regularities of oxygen and water thermal desorption from barium cerate doped by neodymium,samarium, and gadolinium

Processes of thermal desorption of oxygen molecules and water from BaCe_1–x M _x O_3–δ, where M= Nd, Sm, and Gd, presintered in air at the temperature of 650°C are studied. It is found that oxygen is desorbed only from neodymium–doped barium cerate and is almost not evolved from barium cerate doped by samarium and gadolinium. The amount of desorbed oxygen features a square dependence on cationic doping by neodymium. At similar degrees of cationic doping, the amount of water desorbed from neodymium–doped barium cerate is always lower than that from the cerate doped by samarium and gadolinium. The obtained experimental data on thermal desorption and analysis of literature data served as a basis for the conclusion as to the mixed valency of neodymium Nd(III)–Nd(IV) in BaCe_1–x Nd _x O_3–δ. In this case, at similar doping degrees x, the hydration degree of BaCe_1–x Nd _x O_3–δ is lower and the oxygen index is higher than in BaCe1_–x (Sm,Gd) _x O_3–δ. The differences become more pronounced at high degrees of cationic doping and must decrease at an increase in temperature.     

The Effect of Chromium Oxide on the Conductivity of Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2</Subscript>, a Solid-Oxide Fuel Cell Electrolyte

To study the effect of chromium oxide on the electric properties of Ce_0.9Gd_0.1O₂, a solid-oxide fuel cell electrolyte, two approaches were used: (a) the studying of electrochemical properties of the Ce_0.9Gd_0.1O₂- electrolyte after the spontaneous adsorption of chromium-containing molecules from a gas phase and (b) the analyzing of transport properties of the Ce_0.9Gd_0.1O₂-based chromium-containing compositions obtained by the mixing of solid-oxide electrolyte with chromium(III) oxide. It was found that the chromium reduction at the electrolyte surface dominates when chromium is adsorbed from gas phase. Both approaches allow concluding that the chromium presence in Ce_0.9Gd_0.1O₂ deteriorates the electrolyte transport properties at temperatures above 735°С. This is caused by the chromium incorporation into the electrolyte’s fluorite structure, as well as surface microheterogeneity induced by the chromium presence at the Ce_0.9Gd_0.1O₂ surface and the cerium and gadolinium cation redistribution between the grains’ bulk and surface. At intermediate temperatures (below 735°С) the electric conductivity of the Ce_0.9Gd_0.1O₂-based chromium-containing composition exceeds that of the initial solid-oxide electrolyte, which can be due to changes in transport properties of the chromium-containing phases formed at the Ce_0.9Gd_0.1O₂ surface and grain boundaries.     

Enthalpy of formation of BaCe<Subscript>0.9</Subscript>In<Subscript>0.1</Subscript>O<Subscript>3−δ</Subscript>(s)

Enthalpy of formation of the perovskite-related oxide BaCe_0.9In_0.1O_2.95 has been determined at 298.15 K by solution calorimetry. Solution enthalpies of barium cerate doped with indium and mixture of BaCl₂, CeCl₃, InCl₃ in ratio 1:0.9:0.1 have been measured in 1 M HCl with 0.1 M KI. The standard formation enthalpy of BaCe_0.9In_0.1O_2.95 has been calculated as −1611.7±2.6 kJ mol⁻¹. Room-temperature stability of this compound has been assessed in terms of parent binary oxides. The formation enthalpy of barium cerate doped by indium from the mixture of binary oxides is Δ_ox H ⁰ (298.15 K)=−36.2±3.4 kJ mol⁻¹.     

LSM-infiltrated LSCF cathodes for solid oxide fuel cells

Mixed ionic-electronic conductors in the family of La_xSr_1–xCo_yFe_1–yO_3–δ have been widely studied as cathode materials for solid oxide fuel cells (SOFCs). However, the long-term stability was a concern. Here we report our findings on the effect of a thin film coating of La_0.85Sr_0.15MnO_3–δ (LSM) on the performance of a porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (LSCF) cathode. When the thicknesses of the LSM coatings are appropriate, an LSM-coated LSCF electrode showed better stability and lower polarization (or higher activity) than the blank LSCF cathode without LSM infiltration. An anode-supported cell with an LSM-infiltrated LSCF cathode demonstrated at 825 °C a peak power density of ～1.07 W/cm², about 24% higher than that of the same cell without LSM infiltration (～0.86 W/cm²). Further, the LSM coating enhanced the stability of the electrode; there was little degradation in performance for the cell with an LSM-infiltrated LSCF cathode during 100 h operation.     

Influence of Working Pressure on the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Film Properties in Plasma-Enhanced Atomic Layer Deposition

The effect of working pressure on the properties of Al₂O₃ films was investigated in direct-type plasma-enhanced atomic layer deposition. Increasing pressure yielded a denser Al₂O₃ film and a thinner SiO_x interlayer, but only slightly affected the Al₂O₃ film thickness. The diffusivity of O atoms was evaluated by using time-averaged emission intensities of the He I and O I lines. The consumption rate of O radicals and the production rate of H radicals, as functions of plasma exposure time, were deduced from analyzing temporal evolutions of emission intensities of the O I and H_α lines, respectively. The amounts of C and H impurities in the film were confirmed by using an X-ray photoelectron spectroscopy. Finally, the mechanisms by which the working pressure affected the properties of Al₂O₃ films were discussed based on the experimental results.     

Electrochemical properties of perovskite and A<Subscript>2</Subscript>MO<Subscript>4</Subscript>-type oxides used as cathodes in protonic ceramic half cells

Three selected materials have been prepared and shaped as cathode of half cells using the proton-conducting electrolyte BaCe_0.9Y_0.1O_3 − δ (BCY10): two perovskite compounds, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) and La_0.6Sr_0.4Fe_0.8Co_0.2O_3 − δ (LSFC), and the praseodymium nickelate Pr₂NiO_4 + δ (PRN) having the K₂NiF₄-type structure. The electrochemical properties of these compounds have been studied under zero current conditions (two-electrode cell) and under polarization (three-electrode cell). Their measured area-specific resistances were about 1–2 Ω cm² at 600 °C. Under direct current polarization, it appears that the three compounds show almost similar values of current densities at 625 °C; however, at lower temperatures, BSCF appears to be the most efficient cathode material.     

Fabrication of multilayer ceramic structure for fuel cell based on La(Sr)Ga(Mg)O<Subscript>3</Subscript>–La(Sr)Fe(Ga)O<Subscript>3</Subscript> cathode

Fabrication by co-sintering method of a multilayer pore-free electrode–electrolyte structure promising for use in solid-oxide fuel cell and its characteristics have been studied. A material with high ionic conductivity of La_0.88Sr_0.12Ga_0.82Mg_0.18O_3–δ (LSGM) served as electrolyte. The composite electrode was formed from a 1: 2 mixture of LSGM and LSFG (La_0.7Sr_0.3Fe_0.95Ga_0.05O_3–δ). The maximum temperature of the materials co-sintering ability is 1250°C. It was shown by the impedance spectroscopy that the polarization resistance of the LSGM–LSFG electrode is 0.14 Ω cm² at 800°C.     

Towards high-performance cathode materials for lithium-ion batteries: Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-coated LiNi<Subscript>0.8</Subscript>Co<Subscript>0.15</Subscript>Zn<Subscript>0.05</Subscript>O<Subscript>2</Subscript>

Zn-doped LiNi_0.8Co_0.2O₂ exhibits impressive electrochemical performance but suffers limited cycling stability due to the relative large size of irregular and bare particle which is prepared by conventional solid-state method usually requiring high calcination temperature and prolonged calcination time. Here, submicron LiNi_0.8Co_0.15Zn_0.05O₂ as cathode material for lithium-ion batteries is synthesized by a facile sol-gel method, which followed by coating Al₂O₃ layer of about 15 nm to enhance its electrochemistry performance. The as-prepared Al₂O₃-coated LiNi_0.8Co_0.15Zn_0.05O₂ cathode delivers a highly reversible capacity of 182 mA h g^?1 and 94% capacity retention after 100 cycles at a current rate of 0.5 C, which is much superior to that of bare LiNi_0.8Co_0.15Zn_0.05O₂ cathode. The enhanced electrochemistry performance can be attributed to the Al₂O₃-coated protective layer, which prevents the direct contact between the LiNi_0.8Co_0.15Zn_0.05O₂ and electrolyte. The escalating trend of Li-ion diffusion coefficient estimated form electrochemical impedance spectroscopic (EIS) also indicate the enhanced structural stability of Al₂O₃-coated LiNi_0.8Co_0.15Zn_0.05O₂, which rationally illuminates the protection mechanism of the Al₂O₃-coated layer.     

Influence of manganese ions dissolved from LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode on the degradation of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>-based lithium-ion batteries

A systematic investigation is conducted to evaluate the influence of dissolved manganese ions from LiMn₂O₄ cathode on the degradation of Li₄Ti₅O₁₂-based lithium-ion batteries. Worse capacity fading is found in Li₄Ti₅O₁₂-based full cells with increasing manganese ion addition. The interfacial film covered on Li₄Ti₅O₁₂ anode is affected by the manganese ion contamination during cycling, which becomes thicker but more non-uniform, and is composed by less ratio of compact components and more ratio of loose components compared with that free of contamination. Such flawed passivation film cannot restrain the further penetration of electrolyte and inhibit the contact between electrolyte and Li₄Ti₅O₁₂ anodes efficiently, thus triggering more interfacial reactions and that should be the reason for the more severe capacity degradation. Accordingly, we suggest that in addition to optimizing the chemistry and microstructure of Li₄Ti₅O₁₂ electrode, more attention should also be paid to minimizing the destructive effect imposed on the passivation film of Li₄Ti₅O₁₂ electrode by the transition metal ion contaminations.