Doping effect on crystal structure and conduction property of fast oxide ion conductor LaGaO3-based perovskite

In order to discuss oxide ion conduction mechanism for LaGaO₃-based perovskite compounds, doping effects were investigated using two kinds of solid solutions whose oxygen vacancy concentrations are the same: one is La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 with A-site and B-site substitutions and the other is LaGa_0.8Mg_0.2O_2.9 with only B-site substitution. Conductivity measurements showed that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had a circumstance whereby oxide ion could more easily diffuse in the perovskite structure than in LaGa_0.8Mg_0.2O_2.9. Structural analyses using neutron diffraction found out the following three differences: the first finding was that the saddle point formed by two A-site cations and one B-site cation in La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 was larger than that in LaGa_0.8Mg_0.2O_2.9 due to larger displacements of A-site and B-site cations; the second was that the doubly doping with Sr and Mg was more effective for reduction of GaO₆ octahedral tilt angles than the doping with Mg; the last was that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had larger oxygen displacement than LaGa_0.8Mg_0.2O_2.9. It was considered that these structurally related parameters dominated the high oxide ion conduction in LaGaO₃-based perovskite compounds.     

Ionic conductivity of ThO2- and ZrO2-based electrolytes between 300 and 2000 K

A conductivity vs temperature relationship is derived which results from simplified geometrical considerations of a polycrystalline solid electrolyte, assuming that oxygen ion diffusion only takes place through the grains or along the grain boundaries. The relationship proves to be capable of describing conductivity data of YDT, CDT, YSZ and CSZ between 300 and 2000 K. The data analysis reveals that the activation enthalpy as well as the activation entropy of grain conductivity are significantly higher in comparison with grain-boundary conductivity. It may be concluded that BAUERLE's electrical equivalent circuit which underlies the interpretation of dispersive type measurements aiming at separate determination of grain and grain-boundary conductivity ought to be revised.     

Protonic conduction in substoichiometric SrZrO3-based ceramics

The transport properties of strontium zirconate based materials were investigated using different characterization techniques. The electrical conductivity is mainly protonic below 610°C, while the contribution of electron holes increases with temperature and oxygen partial pressure. Under extreme reducing conditions, the conductivity is dominated by protons and oxygen ions. Polarization results are consistent with the results obtained. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Recent progress in LaGaO3 based solid electrolyte for intermediate temperature SOFCs

Partial electronic and oxide ionic conduction in LaGaO₃ doped with Sr and Mg, Co for Ga site was studied with the ion blocking method. It was found that doping small amount of Co into Ga site is effective for elevating the oxide ion conductivity. However, it is seen that the partial electronic conduction monotonically increases with increasing Co amount and P_O2 at p–n transition was shifted to lower value. Even at X = 0.1, the oxide ion conductivity in LSGMC is still dominant. Calculation on the theoretical leakage of electrolyte of solid oxide fuel cells suggests that the highest efficiency of the electrolyte was achieved around 100 μm in thickness for La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃ (LSGMC). Preparation of LSGMC film on Ni–Sm_0.2Ce_0.8O₂ porous anode was studied with the colloidal spray method. In order to prevent the reaction between substrate and film, La doped CeO₂ was used for the interlayer film. In accordance with the theoretical calculation, open circuit potential of the cell using LSGMC film electrolyte with 40 μm thickness becomes much smaller than the theoretical value. However, fairly large maximum power density (0.21 W/cm²) can be achieved at 873 K and even at 773 K, the maximum power density of the cell as high as 0.12 W/cm² was exhibited on the SOFC using 40 μm thickness LSGMC electrolyte.     

Optimization of divalent magnesium ion conduction in phosphate based polycrystalline solid electrolytes

A highest Mg²⁺ ion conducting polycrystalline solid electrolyte was successfully realized by improving the characteristics of both grain bulks and grain boundaries simultaneously. The former improvement was achieved by making a solid solution to substitute cation site for higher valent one to create Mg²⁺ ion vacancies in grain bulks. The latter was realized by obtaining a composite in such a manner to microscopically deposit the insulating secondary phase in grain boundaries. By combining above mentioned two effects, the optimization of Mg²⁺ ion conductivity at around 800 °C was effectively achieved to reach the total Mg²⁺ ion conductivity of approximately 10⁻² S·cm⁻¹ which is applicable in a practical range.     

Performance of La-doped strontium titanate (LST) anode on LaGaO3-based SOFC

Ni-containing anode is currently used with many electrolytes of solid oxide fuel cells (SOFCs). However, Ni is easily oxidized and deteriorates the LaGaO₃-based electrolyte. A La-doped SrTiO₃ (LST, La_0.2Sr_0.8TiO₃) is a candidate as an anode material to solve the Ni poisoning problem in LaGaO₃-based SOFC. In this study, a single-phase LST and an LST-Gd_0.2Ce_0.8O_2 ? δ (GDC) composite were tested as the possible anodes on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 ? δ (LSGM) electrolyte. In order to further improve the anodic performance, Ni was impregnated into the LST-GDC composite anode. The performance was examined from 600 °C to 800 °C by measuring impedance of the electrolyte-supported, symmetric (anode/electrolyte/anode) cells. A polarization resistance (R_p) of LST-GDC anode was much reduced from that of LST anode. When Ni was impregnated into LST-GDC composite, the R_p value was further reduced to ~ 10% of the single-phase LST anode, and it was 1 Ωcm² at 800 °C in 97% H₂ + 3% H₂O atmosphere. A single cell with Ni-impregnated LST-GDC as an anode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ (BSCF) as a cathode and LSGM as an electrolyte exhibited the maximum power density of 275 mW/cm² at 800 °C, increased from ~ 60 mW/cm² for the cell using the LST-GDC as an anode. Thus, LST-GDC composite is promising as a component of anode.     

Magnetic properties and electronic structure of the LaGaO3 perovskite doped with nickel

Solid solutions of the composition LaGa_{1 ? x} Ni _x O₃ (0.01 ≤ x ≤ 0.10) are synthesized, and their magnetic and electrical properties are investigated. It is established that the ground state of the Ni(III) atoms is the low-spin state ² E _g ; however, in the temperature range under investigation, there occurs the ² E _g ? ⁴ T _1g spin equilibrium. An increase in the nickel concentration leads to an increase in the electron conduction of the solid solutions. The band structure of the LaGa_0.5Ni_0.5O₃ model compound is calculated using the ab initio full-potential linearized augmented-plane-wave method within the generalized gradient approximation (FLAPWGGA). It is shown that the dominant role in the variations observed in the magnetic and electrical properties of the nonmagnetic semiconductor LaGaO₃ upon doping with nickel is played by the Ni 3d(e _g ↑, ↓) states.     

Potentiometric CO2 - sensors with ion - conducting glasses as solid electrolytes

Alkali-ion conducting glasses/glass ceramics of the system Me₂O-A1₂O₃-SiO₂ (Me=Li, Na) were applied as solid electrolytes in potentiometric gas sensors to detect CO₂ in the presence of O₂ at increased temperatures. The corresponding Me-Carbonates were utilized as auxiliary electrodes. Sensors using the direct Au-glass contact as a kind of reference electrode (type I), as well as symmetrical sensors with carbonate phase at the reference and measuring electrode (type II - for comparative measurements) were manufactured. By applying Au as electrode metal, the theoretically expected EMF difference and the observed EMF difference of both sensor types agree quite well with the expected values according to the Nernst equation between 500 and 600 °C (over four orders of magnitude of CO₂ partial pressure (10⁻⁵ – 10⁻¹ bar) at constant O₂ partial pressure (2.1×10⁻¹ bar)). A long time stability of 120 days for sensors of type I with Li glasses has been observed, although evaporation of carbonate phase (Li₂CO₃) was detected under the conditions of sensor application. Sensors of type I (with Li₂CO₃) show thermodynamically unexpected cross-sensitivities to H₂O. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Sulphur conductive solid electrolytes in MeS–Ln2S3 systems

The principal possibility of sulphide transfer in phases based on BaZrS₃ and MLn₂S₄ has been considered. The regions of solid solutions based on tertiary compounds have been defined. The systematic study of these phases by methods of conductometry, EMF in chemical concentration chains with and without transfer, potentiostatic chronoamperometry has been performed. A possible defect forming mechanism at consecutive alloying of the tertiary sulphides by the binary ones has been suggested.     

Phase transitions in SrTiO3-based solid solutions

Phase transitions in solid solutions of strontium titanate with titanates of the divalent metals Pb, Ba, and Ca are considered. It is shown that the critical concentration x _c in the well-known expression relating the transition temperature to the polar state in SrTiO₃ with the concentration x of divalent metals, T _c=A(x−x _c)^1/2, is practically the same for all impurities. For large values of x, the ferroelectric transition temperature T _c depends linearly on x. The volume and impurity contributions to the T _c(x) relation are determined for this concentration range. Fiz. Tverd. Tela (St. Petersburg) 39, 1645–1651 (September 1997)     

Relation between proton and hole conduction in SrCeO3-based solid electrolytes under water-containing atmospheres at high temperatures

Electrical conduction in the solid proton electrolyte based on SrCeO₃ was studied under water-containing atmospheres at high temperatures. The change of conductivity was measured systematically as a function of the concentration of the dopant or of the partial pressures of water vapor and oxygen. Since the conduction in the oxides was not purely protonic but partially electronic, these conductivities were determined separately using a steam concentration cell. It was observed that the proton conductivity increased in proportion to P^{$\text{1}\text{2}$_H2O} and was independent of P_O2. It was also recognized that the electronic conduction present in the oxides was due to holes and the hole conductivity followed the P^{$\text{1}\text{4}$}_O2 law. A possible model for the proton formation in the oxides is discussed and it is proposed that the protons might be produced from water vapor at the expense of holes.     

Vibrational and impedance spectroscopic analysis of poly(vinyl alcohol)-based solid polymer electrolytes

Solid polymer electrolytes based on poly(vinyl alcohol) (PVA) doped with NH₄Br have been prepared by the solution-casting method. The complex formation between the polymer and the salt has been confirmed by Fourier transform infrared spectroscopy. The highest conductivity at 303 K has been found to be of the order of 10⁻⁴ Scm⁻¹ for 25 mol% NH₄Br-doped PVA system. The ionic transference number of polymer electrolyte has been estimated by Wagner’s polarization method, and the results reveal that the conducting species are predominantly ions. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage (V_cell), applied chemical potential difference (Δμ(O₂)) and cell current (I_t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I_i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 electrolytes with La_0.8Sr_0.16CoO₃ or Pt as the cathode and Ni/Ce_0.9Ca_0.1O_1.9−xor Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.     

Screening possible solid electrolytes by calculating the conduction pathways using Bond Valence method

Inorganic solid electrolytes have distinguished advantages in terms of safety and stability, and are promising to substitute for conventional organic liquid electrolytes. However, low ionic conductivity of typical candidates is the key problem. As connective diffusion path is the prerequisite for high performance, we screen for possible solid electrolytes from the 2004 International Centre for Diffraction Data (ICDD) database by calculating conduction pathways using Bond Valence (BV) method. There are 109846 inorganic crystals in the 2004 ICDD database, and 5295 of them contain lithium. Except for those with toxic, radioactive, rare, or variable valence elements, 1380 materials are candidates for solid electrolytes. The rationality of the BV method is approved by comparing the existing solid electrolytes’ conduction pathways we had calculated with those from experiments or first principle calculations. The implication for doping and substitution, two important ways to improve the conductivity, is also discussed. Among them Li₂CO₃ is selected for a detailed comparison, and the pathway is reproduced well with that based on the density functional studies. To reveal the correlation between connectivity of pathways and conductivity, α/γ-LiAlO₂ and Li₂CO₃ are investigated by the impedance spectrum as an example, and many experimental and theoretical studies are in process to indicate the relationship between property and structure. The BV method can calculate one material within a few minutes, providing an efficient way to lock onto targets from abundant data, and to investigate the structure-property relationship systematically.     

铈基纳米材料水热合成及储放氧性能

" 采用水热方法合成汽车尾气三效催化剂助剂材料NiO-CeO2和Bi2O3-CeO2二元纳米化合物．首先,对NiO-CeO2二元材料进行X射线分析,结果显示颗粒平均尺寸在11~12 nm,这一结果与高分辨电镜结果观察的颗粒尺寸相吻合,同在12 nm左右．同时,对NiO-CeO2二元化合物进行BET比表面积分析,结果显示：水热合成材料的比表面积在54~75 m2/g．另外,对Bi2O3-CeO2二元化合物进行X射线分析,结果显示合成材料的晶粒尺寸在8~11 nm．最后,我们分别对两种材料进行了氧化还原条件下     

Effect of cation doping on ionic and electronic properties for lanthanum silicate-based solid electrolytes

Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La_9.333 + xSi₆O_26 + 1.5x (LSO) were investigated in the oxygen-excess region (x > ca. 0.3). We have found that the oxygen excess-type LSO (OE-LSO), namely La₁₀Si₆O₂₇ on weighted basis, exhibited high conductivity, and substitution of the Si-site of LSO with some dopants (Mⁿ⁺) had a positive effect toward the conducting property. Furthermore, it was also found that addition of a very small amount of iron ions into the M-doped OE-LSO, La₁₀(Si_6-yMⁿ⁺_y)O_27-(2-0.5n)y, improved its conductivity. On the other hand, replacement of the La-site with various ions for La₁₀(Si_6-yMⁿ⁺_y)O_27-(2-0.5n)y did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1-α){La₁₀(Si_5.8Al_0.2)O_26.9}-α(FeO_γ), evaluated with the Hebb-Wagner polarization method were very low: i.e., 1.1 × 10^− 3 and 2.9 × 10^− 3 under P(O₂) = 1.1 × 10⁴ Pa at 1073 K for α = 0.00 and 0.005, respectively. Conductivity for each sample was unchanged under humidified atmosphere at 1073 K sustained for over 50 h, revealing that both compositions were chemically stable. It was concluded that 0.995{La₁₀(Si_5.8Al_0.2)O_26.9}-0.005(FeO_γ) is suitable for the fuel cell electrolytes because of its high and almost pure ionic conductivity, and its good chemical stability under humidified as well as reducing conditions.     

Mechanoelectric effect in solid electrolytes

The mechanoelectric effect in solid electrolytes of the compositions ZrO₂ + 8 mol % Sc₂O₃ and ZrO₂ + 8 mol % Y₂O₃ is investigated experimentally. The mechanical properties of polycrystalline specimens are studied using four-point bending in air. It is shown that a negative charge is induced on the extended side of the bent specimen and that the magnitude of this charge depends on the external mechanical load and the temperature of measurement. The assumption is made that the observed phenomena are associated with the uphill diffusion of vacancies in response to a nonuniform field of mechanical stresses. The theoretical model is compared with the experimental results.     

Ionic polarons in solid electrolytes

Small polaron theory is used to calculate the activation energy of cationic hopping in solid electrolytes. The numerical results show a small activation energy for materials which have forms which are good super ionic conductors, and large for those which have not.     

Triplet correlations in solid electrolytes

The triplet correlations in an AgI-type solid electrolyte have been studied by a two-dimensional molecular-dynamics model. We have found that the interstitial positions are less occupied than the quasi-lattice positions. For distances less than 10 Å the system is predominantly solid-state-like and for distances greater than 16 Å it is predominantly liquid-state-like. The deviations of the exact triplet correlation function from the superposition approximate on is partly large - much larger than those we normally observe in monatomic liquids. These deviations should be strongly reflected in the isothermal pressure derivative of the structure factor and should therefore also lead to large effects in measurements.