The impedance caused by interfacial current constriction: a case study on BaF2/YSZ bicrystals

Abstract  

BaF₂/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF₂, and the low frequency arc is caused by an additional resistance in BaF₂ due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF₂/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F⁻ ions being pumped into YSZ and acting as a counter dopant to Y³⁺.     

The impedance caused by interfacial current constriction: a case study on BaF<Subscript>2</Subscript>/YSZ bicrystals

Abstract  BaF₂/YSZ (yttria stabilized zirconia) bicrystals are investigated by means of impedance spectroscopy. The spectra show two semicircles in the complex impedance plane, and both arcs exhibit very similar temperature dependences. The high frequency semicircle can be attributed to the ideal (quasi one-dimensional) bulk resistance of BaF₂, and the low frequency arc is caused by an additional resistance in BaF₂ due to the current constriction taking place close to the contact spots at the imperfect bicrystal interface. This interpretation is supported by finite element calculations revealing that bicrystals with imperfect contacts indeed exhibit an additional semicircle in the complex impedance plane even without any ion transfer resistance between the two crystals. The low frequency arc of the BaF₂/YSZ bicrystal drastically increases upon bias voltage, but relaxes to its original value after removing the bias. This phenomenon can be associated with a strong decrease of the vacancy concentration in YSZ (close to the interfacial contact spots), which is caused by F⁻ ions being pumped into YSZ and acting as a counter dopant to Y³⁺. Graphical Abstract        

The impedance of the Ag-CeF<Subscript>3</Subscript>/CeF<Subscript>3</Subscript> bicrystal-Ag system

The impedance spectra of CeF₃/CeF₃ bicrystal (two single crystals separated by a single intercrystalline boundary) between Ag-electrodes are studied over a 135 to 410 K temperature interval (including temperatures below room temperature). The bicrystal was prepared by thermal-diffusion welding under a pressure of 1.5 × 10⁷ Pa at 1473 K in vacuum (∼10⁻² Pa). It is shown that the intercrystalline boundary affects but insignificantly the bicrystal bulk impedance. The CeF₃/CeF₃ ionic conductivity is 3 × 10⁻⁶ S/cm at 293 K; it is mainly determined by transfer processes in the single crystal bulk.     

Oxygen isotope exchange between the gas-phase and the electrochemical cell O<Subscript>2</Subscript>, Pt | YSZ | Pt,O<Subscript>2</Subscript> under conditions of applied potential difference

The kinetics of oxygen isotope exchange between gas-phase oxygen and the electrochemical cell O₂, Pt | ZrO₂ + 10 mol % Y₂O₃ (YSZ) | Pt, O₂ with applied potential difference (ΔU = ±1.2 V) is studied in the temperature range of 600–800°С and the oxygen pressure interval of 3–13 kPa. An original design of a vacuum electrochemical cell with the separated gas space is put forward for studying how the potential difference on the electrochemical cell influences the kinetics of interaction of gas-phase oxygen with the gas electrode O₂, Pt | YSZ in the electrochemical cell. It is shown that the oxygen interphase exchange rate is the higher the more negative the charge on the electrode studied; moreover, the mechanism of gas-phase oxygen exchange with the gas electrode O₂, Pt | YSZ in the electrochemical cell depends fundamentally on the electrode charge sign. The possible reasons for the revealed differences are discussed; the corresponding models are proposed.     

Quantitative analysis of thin films of RBa2Cu3O7−x (R-rare earth elements) and ZrO2 (Y2O3) buffer layers by SNMS

It is shown that RBa₂Cu₃O_7-X single crystals may be used as reference samples for the quantitative analysis of RBa₂Cu₃O_7-X thin films by SNMS. RSF-values for Y and Cu (relative to Ba) determined for ceramic RBa₂Cu₃O_7-X samples are higher than those for single crystals. This difference may be caused by Ba segregation on grain boundaries. The depth profile analysis of YSZ/Al₂O₃ samples was performed by DBM using a Ni grid to prevent sample charging. The reproducibility of analysis was better than 10%.     

O_2存在下NO在固体电解质电池上分解机理

研究了在O_2存在条件下,NO在Pd |YSZ| Pd固体电解质电池和RuO_2 |Pd|YSZ| Pd固体电解质电池上的分解性质,在O_2存在条件下650 ～ 700 ℃之间 ,在0 ～ 4.4 V直流电压作用下,NO在Pd |YSZ| Pd电池和RuO_2|Pd|YSZ| Pd电池 上的分解不以电解机制进行,而以电催化机理进行的。即在直流电压下,阴极催化 剂上的O~(2-)被直流电压通过YSZ固体电解质转移到阳极,以O_2的形式放出,以此 保持催化剂的活性状态。在Pd|YSZ|Pd 固体电解质电池上,Pd金属表面是催化NO分 解的主要活性位。RuO_2 |Pd|YSZ| Pd固体电解质电池上,某特定还原态的RuO_x (0 < x < 2)是NO分解的主要活性位。在O_2存在下,该电池在1 ～ 4 V间合适的电 压下,在650 ～ 700 ℃能选择性地对NO进行电催化分解。     

The effect of TiO2 additives on the structural stability and thermal properties of yttria fully-stabilized zirconia

TiO₂(0–20 mol%)-8 mol% YSZ (8YSZ) ceramics were synthesized by a traditional solid-state reaction method. A cubic single phase was observed for 8YSZ, 4 mol% TiO₂-8YSZ and 8 mol% TiO₂-8YSZ. Tetragonal and cubic mixed phases were observed for 12–20 mol% TiO₂-8YSZ ceramics. The sintering temperature was 1,700 °C for 8YSZ and 4 mol% TiO₂-8YSZ ceramics, whereas it was 1,500 °C for 8–20 mol% TiO₂-8YSZ. The thermal conductivity at room temperature decreased in proportion to increasing TiO₂ content, from 3.0 to 2.3 W/m K. The specific heat of TiO₂-8YSZ ceramics was unaltered as the TiO₂ content changed.     

Lanthanum-strontium cuprate: A promising cathodic material for solid oxide fuel cells

Samples of lanthanum-strontium cuprate LaSrCuO_3.61 are synthesized by a solid-phase method at 1473 K, in air. Electron diffraction patterns reveal low-intensity satellites whose position is described in the reciprocal space by the (3 + 2)-dimensional space group I4/mmm(αα0, α-α0)00mg. Using a five-layer cell LaSrCuO_4-δ|YSZ|LaSrCuO_4-δ|YSZ|LaCuO_4-δ prepared by isostatic hot pressing, the ionic component of conductivity of LaSrCuO_4-δ is determined (σ_{1179 K} = 3.8 × 10^?3 S cm^?1), which is commensurate with other mixed conductors based on complex oxides of cobalt and iron.     

在RuO2|Pd|YSZ|PddeNOx电解池中O2-传导的速控步骤

研究了500～750℃富氧条件下RuO₂|M|YSZ|Pd(M=Ag,Pd,Pt,Au)固体电解质电解池在0～4V直流电压下对NO的分解性质.600℃下,O^2-在Pd|YSZ阴极界面处的传导是O^2-在RuO₂|Pd|YSZ|Pd固体电解质电解池中传导过程的速控步骤.反应温度越低,RuO₂|Pd|YSZ|Pd电解池上NO电催化分解相对于氧分解的选择性因子α越大.600℃下O^2-在M|YSZ阴极界面处的传导阻力按Ag2|Ag|YSZ|Pd电解池上分解率为15.3%,NO选择性因子达到13.4.     

Characteristics of Pt Electrode Activated by Tb1 − xCexO2 − α Films in Contact with ZrO2 + 10 mol % Y2O3 Electrolyte

Porous platinum electrodes on ZrO₂ + 10 mol % Y₂O₃ solid electrolyte (YSZ) are activated by Tb_{1 ? x}Ce_xO_{2 ? α} (x = 0; 0.15; 0.33; 0.5; 1.0) mixed oxides by impregnation, and their polarization characteristics are studied. The activation is carried out under the conditions that an oxide activator nanofilm forms on the electrolyte surface as a result of heat treatment of the electrode. The activation is performed by impregnating the electrodes with low-concentrated alcohol solution of terbium and cerium nitrates (1.5% as recalculated to the oxides) and subsequent slow heating (≤50°C/h) to 850°C. An average thickness of the film on the electrolyte after a single activation (≈0.1 mg oxides/cm²) is estimated at 10–20 nm. The electrodes of Pt|YSZ|Pt cell activated by Tb_{1 ? x}Ce_xO_{2 ? α} films are studied by the impedance method in the oxidative and reductive atmospheres in the range of 700 to 500°C. The polarization conductivities of the activated electrodes increase by 2–3 orders of magnitude. The studied electrodes are discussed within the model of compact oxide electrodes, where platinum plays the role of collector. The advantage of these electrodes is that they can work both in the oxidative and reductive conditions. According to the aggregate of the properties, Tb_{1 ? x}Ce_xO_{2 ? α} compounds at x = 0.3–0.5 are recommended for activation.

     

Kinetics of Sorption,Desorption, and Diffusion in Zeolites

Sorption and desorption in zeolites (molecular sieves) have to be considered as complex processes, involving simultaneous diffusion in zeolite crystals, mass transfer in the intercrystalline void of a pellet, and heat transfer between the zeolitic sorbent and its surroundings. The kinetics of sorption and desorption, respectively, of n-C₄H₁₀ in zeolite X and of CO₂ in zeolite A have been investigated: only the initial rates of uptake or release of the sorbet are controlled by mass transfer alone whereas ultimately they also depend on the rate of heat transfer from the sorbent to its surroundings or vice versa. Diffusivities of the sorbate in the zeolite crystals can be obtained from the kinetics of mass transfer, provided the resistance due to viscous or Knudsen flow between the crystals of the zeolitic sorbent can be eliminated. A sample consisting of a monolayer of single crystals had to be used for this purpose in the n-butane/zeolite X system; the intracrystalline diffusivity obtained in this way is not in conflict with data obtained by NMR spectroscopy. The intracrystalline diffusivities obtained in this way—taking into account the coupling of several processes during sorption — are higher than values reported in the literature.     

Thermal electromotive force of hot carriers in γ-irradiated single crystals of cadmium mercury tellurides

The effect of γ-irradiation on the thermal electromotive force (emf) of hot carriers in Cd _x Hg_{1 ? x} Te (0.25 ≤ x ≤ 0.95) single crystals has been investigated in weak and strong electric fields. It has been shown that in relatively low fields in which the charge carriers are still warm, the relationship |U _T | ～ E ² is observed because of the dominance of scattering by acoustic vibrations of the lattice with the increasing electric field strength, whereas the dependence of |U _T | on E turns to be linear with a further increase of the electric field when the charge carriers become hot. The thermal emf retains its sign in the γ-irradiated crystals. However, there are significant changes in pattern of the field and temperature dependences of |U _T |, with the dependence of |U _T on E being nonmonotonic at high doses.     

Direct Synthesis of H2O2 by a H2/O2 Fuel Cell

Direct synthesis of H₂O₂ solutions by a fuel cell method was reviewed. The fuel cell reactor of [O₂, gas-diffusion cathode electrolyte solutions Nafion membrane electrolyte solutions gas-diffusion anode, H₂] is very effective for formation of H₂O₂. The three-phase boundary (O₂(g)–electrode(s)–electrolyte(l)) in the gas-diffusion cathode is essential for efficient formation of H₂O₂. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk electrolyte solutions are essential for H₂O₂ accumulation. The maxima H₂O₂ concentrations of 1.2 M (3.5 wt%) and 2.4 M (7.0 wt%) were accomplished by the heat-treated Mn-OEP/AC electrocatalyst with H₂SO₄ electrolyte and by the VGCF electrocatalyst with NaOH electrolyte, respectively, under short circuit conditions.     

Sr2InF7-Sr2[InF7] oder Sr2F[InF6]? [1]

The new compound Sr₂InF₇ single crystals has been prepared from a melt using single crystals, the strucutre of Sr₂InF₇ is isotypic with K₂NbF₇ [2]. Details of the structure are discussed, using the Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN.     

Study of anionic conductors R1 ? y Sr y F3 ? y (R = Ce, Pr, Gd) and M1 ? x Y x O2 ? x /2 (M = Zr, Hf) using four-electrode method

The anionic conductivity parameters are determined for R_{1 − y} Sr _y F_{3 − y} (R = Ce, Pr, Gd), Zr_0.88Y_0.12O_1.94 single crystals and Zr_0.88Y_0.12O_1.94|M_0.88Y_0.12O_1.94 (M = Zr, Hf) bicrystals using an ac (5 Hz to 100 kHz) four-electrode method. The obtained conductometric data are compared with the results of studying these crystals using a two-electrode method. It is shown that physical parameters of R_{1 − y} Sr _y F_{3 − y} fluoride crystals with the tysonite (LaF₃) structure are stable at heating in air up to the temperatures of 200–300°C, which is important for their practical application. Original Russian Text ? N.I. Sorokin, B.P. Sobolev, 2008, published in Elektrokhimiya, 2008, Vol. 44, No. 9, pp. 1111–1115.     

Magnetron formation of Ni/YSZ anodes of solid oxide fuel cells

Physico-chemical and structural properties of nanocomposite NiO/ZrO₂:Y₂O₃ (NiO/YSZ) films applied using the reactive magnetron deposition technique are studied for application as anodes of solid oxide fuel cells. The effect of oxygen consumption and magnetron power on the discharge parameters is determined to find the optimum conditions of reactive deposition. The conditions for deposition of NiO/YSZ films, under which the deposition rate is maximum (12 μm/h), are found and the volume content of Ni is within the range of 40–50%. Ni-YSZ films reduced in a hydrogen atmosphere at the temperature of 800°C have a nanoporous structure. However, massive nickel agglomerates are formed in the course of reduction on the film surface; their amount grows at an increase in Ni content in the film. Solid oxide fuel cells with YSZ supporting electrolyte and a LaSrMnO₃ cathode are manufactured to study electrochemical properties of NiO/YSZ films. It is shown that fuel cells with a nanocomposite NiO/YSZ anode applied using a magnetron sputtering technique have the maximum power density twice higher than in the case of fuel cells with an anode formed using the high-temperature sintering technique owing to a more developed gas-anode-electrolyte three-phase boundary.     

Oxydation intermetallischer Phasen: CsK2[AuO2] aus CsAu + K2O2

Oxidation of Intermetallic Phases CsK₂[AuO₂] from CsAu+K₂O₂ We prepared the hitherto unknown CsK₂[AuO₂] [Heating mixtures of CsAu and K₂O_2,2; 1:1; gives single crystals (Ag-cylinder, 430°C, 6d)]. The single crystals are light blue, nearly colourless, and transparent. A new type of structure is found. The single crystal data are: Pnma; a = 1256.5(5), b = 727.3(2), c = 627.9(2)pm, Z = 4; four-circle diffractometer PW 1100, MoKα;849 out of 871 I₀(hkl), R = 7.3% and R_w = 6.3%. The Madelung Part of Lattice Energy, MAPLE, is calculated.     

Nitrieren von Eisen mit Ammoniak: Wachstum und Mikrostruktur der Nitride und Porenbildung in ζ‐Fe2N

Nitridation of Iron with Ammonia: Growth and Microstructure of Nitrides and Pore Formation in ζ‐Fe₂N Layers of γ′‐Fe₄N, ϵ‐Fe₃N, and ζ‐Fe₂N on ferrite were produced by nitridation of single crystals and rolled sheet of iron in flowing ammonia at 430 and 475 °C. The nitride layers on ferrite were prepared in cross section and were characterised using X‐ray diffraction, light microscopy as well as scanning and transmission electron microscopy. The growth rate of the nitride layers on the single crystals were found to be similar as that on the rolled sheet. The size of the nitride grains on the single crystal are significantly larger, which is identified as the reason for the different pore structure in the two materials. Pores only occur in the outer ζ‐Fe₂N layer. The formation of pores at the interface ϵ‐Fe₃N/ζ‐Fe₂N and the crystallographic orientation relationship between ϵ‐Fe₃N and ζ‐Fe₂N were studied and determined using electron microscopy and electron diffraction. The pore formation is explained by the existence of a two‐phase region at the phase boundary ϵ‐Fe₃N/ζ‐Fe₂N and by an indifferent equilibrium which is caused by local instabilities in the nitrogen flux.     

梯度Cu-CeO2-Ni-YSZ复合阳极直接甲烷SOFC的制备与性能

采用三层共压-共烧结法, 并涂覆La_0.8Sr_0.2MnO₃ (LSM)阴极, 制备了梯度Ni-YSZ阳极结构的固体氧化物燃料电池(SOFC)(大孔Ni-YSZ|微孔Ni-YSZ|YSZ|LSM) (YSZ: Y₂O₃稳定的ZrO₂; LSM: Sr 掺杂的LaMnO₃).通过浸渍法在大孔Ni-YSZ 基底中沉积占总重量约1%的Cu-CeO₂抗积碳催化剂, 形成梯度Cu-CeO₂-Ni-YSZ复合阳极. 分别以CH₄和H₂为燃料, 空气为氧化剂, 测定了构造的SOFC输出电性能和长期稳定性. 结果表明,三层共压-共烧结法制备的梯度阳极SOFC, 层间结合紧密无缺陷, 阳极梯度孔结构明显, YSZ膜致密无缺陷.在850℃下操作, 以梯度Ni-YSZ 阳极制备的SOFC, 燃料由H₂切换为甲烷时, 最大功率密度由284 mW·cm^-2下降到143 mW·cm^-2; 而以Cu-CeO₂-Ni-YSZ 复合阳极构造的SOFC出现相反趋势, H₂切换为甲烷后最大输出由176 mW·cm^-2增加到196 mW·cm^-2. 在250 mA·cm^-2负荷下, 梯度Ni-YSZ阳极支撑的直接甲烷SOFC仅稳定运转10 h 便出现明显衰减, 阳极中积碳严重; 但Cu-CeO₂-Ni-YSZ 复合阳极支撑SOFC连续运转50 h, 输出电压与输出功率密度基本不变, 电镜观察不到积碳.     

Conductivity of Lanthanum-Strontium Cuprate LaSrCuO<Subscript>4−δ</Subscript> and Ionic Reversibility of Relevant Electrodes

A single-phase sample of the LaSrCuO_3.58 composition is prepared by solid-state synthesis at 1473 K in air. The conductivity of LaSrCuO_3.58 is measured by a four-probe method at direct current in the temperature range from 298 to 1173 K at oxygen partial pressures from 28 to 2.1 × 10⁴ Pa. Heating samples above ∼670 K is shown to result in a changeover of the conduction type from semiconducting (p-type) to metallic. Exchange currents at the LSCuO/YSZ interface are measured by impedance spectroscopy. The measurements are carried out in high-density symmetrical cells LSCuO|YSZ|LSCuO, fabricated by hot pressing of powders under a pressure of 3.5 × 10⁹ Pa at 773 K. Experimental exchange currents i ₀ (varying from 10⁻³ to 10⁻⁴ A/cm² in the temperature interval from 800 to 1173 K) are comparable with such of materials based of lanthanum-strontium manganite.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 516–522.Original Russian Text Copyright © 2005 by Mazo, Savvin, Mychka, Dobrovol’skii, Leonova.