S. I. Bredikhin

NO decomposition by an electrochemical cell with mixed oxide working electrode

A new family of electrochemical cells for decomposition of NO gas in the presence of excess oxygen were designed and investigated. It was shown that covering of the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the properties of the electrochemical cell for nitrogen monoxide decomposition. The mechanism of the process of NO decomposition was proposed and the theoretical value of the current efficiency was calculated. A good numerical correlation between the calculated and measured characteristics of the electrochemical cell were observed. For the first time an electrochemical cell for NO decomposition with a value of current efficiency exactly equal to the theoretical value has been designed. The correlation between the microstructure of the working electrode and conversion rate of NO gas was studied. The phenomenon of self-organizing of the microstructure of the NiO-YSZ working electrode during the process of cell operation was observed and investigated. It was shown that the specific microstructure of the NiO-YSZ mixed oxide enhanced the properties of the electrochemical cell for NO decomposition and increased the value of the current efficiency.

Novel low voltage electrochemical cell for NO decomposition

In recent years, an intense research effort has been focused on electrochemical cells for reduction of gases of nitrogen oxide radicals (NOx) due to the need to design an effective method for the purification of the exhaust gas from lean burn and diesel engines. A new type of solid-state electrochemical cell with composite Pt–YSZ cathode and nano-porous NiO–YSZ working electrode for decomposition of NOx gas in the presence of excess oxygen was proposed and investigated. It was shown that the dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nano-porous NiO– YSZ working electrode was linear and that the value of current efficiency is dependent on the NO and O2 gas concentrations only. For the first time, an electrochemical cell for NO decomposition operating at dc voltage lower than 1.7 V in the temperature range of 550–700 jC was designed. The combination of the new type of composite cathode with a nano-porous working electrode provides the possibility of large reductions in the electrical power required for NO decomposition in the presence of excess oxygen. D 2002 Elsevier Science B.V. All rights reserved.

Low current density electrochemical cell for NO decomposition

Abstract Solid-state electrochemical cells with two cathode layers were designed for effective NO x decomposition. It was found that covering of the cathode by a mixed oxide electrode increased NO decomposition even in the presence of excess oxygen. A strong correlation between the microstructure of the working electrode and conversion rate of NO x gas was observed and studied. It was shown that there was a linear dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nano-porous working electrode and that the value of current efficiency depended on the NO and O 2 gas concentrations only. A mechanism for the process of NO decomposition on nano-porous ceramics electrode was proposed and the theoretical value of the current efficiency was calculated. A good numerical correlation between the calculated and measured characteristics of the electrochemical cell were observed.

Nonstoichiometry and electrocoloration due to injection of Li+ and O2− ions into lithium niobate crystals

The high oxygen and lithium ion conductivity in LiNbO3 was investigated and interpreted in terms of lithium and oxygen vacancies being intrinsically present in congruently grown single crystals. As a result of this, it was found that the stoichiometry of lithium niobate crystals may be changed with respect to lithium and oxygen. The optical and electrical properties of electrically colored LiNbO3 crystals were studied and it was shown that the absorption spectra of thermally reduced and electrocolored samples are identical. Therefore, the origin of the absorption processes is considered to be the same in both cases. The formation of regions with different stoichiometry due to the injection of additional lithium or oxygen into the LiNbO3 crystals was also observed and investigated. The motion of these stoichiometric domains through a LiNbO3 crystal from one electrode to the other was studied and described in terms of electrodiffusion of the ions and electrons. A model is proposed which considers the injection of oxygen and lithium vacancies for the generation of concentration profiles in the originally homogeneous material. The numerical calculation of the concentration profiles shows good agreement with the experimental results.The high oxygen and lithium ion conductivity in LiNbO3 was investigated and interpreted in terms of lithium and oxygen vacancies being intrinsically present in congruently grown single crystals. As a result of this, it was found that the stoichiometry of lithium niobate crystals may be changed with respect to lithium and oxygen. The optical and electrical properties of electrically colored LiNbO3 crystals were studied and it was shown that the absorption spectra of thermally reduced and electrocolored samples are identical. Therefore, the origin of the absorption processes is considered to be the same in both cases. The formation of regions with different stoichiometry due to the injection of additional lithium or oxygen into the LiNbO3 crystals was also observed and investigated. The motion of these stoichiometric domains through a LiNbO3 crystal from one electrode to the other was studied and described in terms of electrodiffusion of the ions and electrons. A model is proposed which considers the inject...

Optimization of an electrochemical cell for NO decomposition by compositional control of the electro-catalytic electrode

Abstract A new family of electrochemical cells ((Electro-catalytic electrode)|Pt(Cathode)|YSZ|Pt(Anode)) for NO decomposition in the presence of excess oxygen have been designed and investigated. It was shown that by control of the composition of the (NiO) x –(YSZ) 1− x electro-catalytic electrode, it was possible to minimize the values of the cell operating voltage. The electrochemical properties of an electrochemical cell for NO decomposition were investigated and correlated with the compositions of the (NiO) x –(YSZ) 1− x electro-catalytic electrode using the effective medium percolation theory (EMPT). The optimum NiO addition (35% by volume) to the electro-catalytic electrode resulted in a decrease of the cell operating voltage and in a decrease in the electrical power required for NO decomposition. A mechanism of NO decomposition by this new type of the electrochemical cell with composite electro-catalytic electrode was proposed and investigated.

Electrochemical cell with two layers cathode for NO decomposition

An electrochemical cell composed of an yttria-stabilized zirconia disk and two layers cathode was used for nitrogen monoxide decomposition. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) leads to enhancement of the performance of the electrochemical cell for NOx decomposition in the presence of excess oxygen. The decomposition activity was measured for the one-compartment cell oxide|(cathode)|YSZ|(anode) by applying a DC voltage lower than 3.7 V in the temperature range 550–700 °C. The microstructure of the YSZ-NiO mixed oxide electrodes was investigated in dependence of the cell operating condition and the working electrode sintering temperature. The correlation between the microstructure of the mixed oxide electrode and conversion rate of NO was studied. The phenomenon of self-optimization of the microstructure of the NiO-YSZ working electrode during the cell operation was observed and investigated.

Sr0.75Y0.25Co0.5Mn0.5O3 − y Perovskite Cathode for Solid Oxide Fuel Cells

The Sr 0.75 Y 0 . 25 Co 0.5 Mn 0.5 O 3-y , (SYCM) oxide with a cubic perovskite structure was examined as a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity, thermal expansion coefficient (TEC), and reactivity with gadolinia-doped ceria (GDC) or yttria-stabilized zirconia (YSZ) were studied. Reflections from SrZrO 3 (~6 wt %) after heat-treatment of the SYCM:YSZ mixture at 900°C for 48 h and no reaction after heat-treatment with GDC were observed. In the low and intermediate temperature region (473-873 K), the TEC value is 13.3 ppm K -1 and most closely matches the common IT-SOFC electrolyte material GDC, but the TEC value increases to 19.6 ppm K -1 at 873-1073 K. The investigations of the electrochemical properties of the model SOFCs at 900°C with the SYCM cathode show that substituting the SYCM cathode for the standard (La,Sr)Mn03 cathode improves the cell performance.

Analysis of electric properties of ZrO2-Y2O3 single crystals using teraherz IR and impedance spectroscopy techniques

The data obtained by impedance spectroscopy (1 Hz to 32 MHz) and broad-band dielectric spectroscopy (30 GHz-150 THz) are presented for crystals based on zirconia doped by 1.5–30 mol % Y2O3 or 10 mol % Sc2O3 and 1 mol % Y2O3. The maximum of ionic conductivity is confirmed for the latter composition in the working temperature range of solid oxide fuel cells where the doping by scandium and yttrium oxides makes it possible to obtain isotropic single crystals. Dependences of dielectric permeability and high-frequency conductivity of materials on the composition of crystals and temperature are presented.

Microstructural and Electrochemical Study of Charge Transport and Reaction Mechanisms in Ni/YSZ Anode

The microstructure and chemical changes in the Ni-YSZ heterojunctions were studied by scanning, transmission, and highresolution electron microscopy and X-ray diffraction. It was shown that new nano-size Ni grains are formed at Ni-YSZ interface at the cell operation. The following two stage reaction mechanism was proposed for hydrogen oxidation on the nano-size Ni grains: 1. NiO + Н2 ⇒ Н2О + Ni; 2. O(YSZ) + Ni ⇒ NiO + 2e. This mechanism implies that the SOFC will operate during some time even after switching off the hydrogen flow. We have observed and investigated current-voltage characteristics of SOFC after cutting off the hydrogen pass through anode chamber. Electrochemical properties of SOFC were investigated as a function of Ni volume content for two different geometry of anode current collector. Good correlations of experimentally measured SOFC characteristics with results of numeric calculations were found.

Photoinduced phenomena in RbAg4I5 superionic crystals

Abstract We have observed a new phenomena of photoinduced transformation of Raman scattering and photoluminescence spectra in RbAg 4 I 5 superionic crystal. A reversible change of the fine structure in Raman spectra at low temperature and reversible change of the concentration of luminescence centers due to irradiation of sample have been studied. Both of these processes are due to the change of local concentration of mobile silver ions in irradiated region of RbAg 4 I 5 . The mechanism of interaction between nonequilibrium electrons and silver disordered ionic sublattice has been proposed. The excess concentration of electrons gives rise to diffusion flow of electrons and ions from irradiated region. A creation of hole trapped centers in irradiated region of γ-RbAg 4 I 5 puts additional constraints on Ag site occupancy. Therefore, for irradiated γ-RbAg 4 I 5 , a “random tetrahedral” model has been used to describe a reversible change of the fine structure in Raman spectra at low temperature.