Electrochemical reduction of NO and O2 on La2−x Sr x CuO4-based electrodes

A series of La_2 − x Sr _x CuO₄ (x = 0.0, 0.05, 0.15, 0.25 and 0.35) compounds was investigated for the use of direct electrochemical reduction of NO in an all-solid-state electrochemical cell. The materials were investigated using cyclic voltammetry in 1% NO in Ar and 10% O₂ in Ar. The most selective electrode material was La₂CuO₄, which had an activity of NO reduction that was 6.8 times higher than that of O₂ at 400 °C. With increasing temperature, activity increased while selectivity decreased. Additionally, conductivity measurements were carried out, and the materials show metallic conductivity behavior which follows an adiabatic small polaron hopping mechanism.     

Glycolate Ti1 − x Fe x (OCH2CH2O)2 − x/2 as a precursor for the preparation of quasi-one-dimensional (1D) solid solutions Ti1 − x Fe x O2 − 2x/2 (0 ≤ x ≤ 0.1)

Quasi-one-dimensional (1D) solid solutions Ti_{1 ? x} Fe _x (OCH₂CH₂O)_{2 ? x/2} (0 < x ≤ 0.1) with the structure of anatase were prepared by heating the glycolate Ti_{1 ? x} Fe _x (OCH₂CH₂O)_{2 ? x/2} in an atmosphere of air at a temperature of >450°C. The conditions of formation and the properties of the new glycolate Ti₃Fe₂(OCH₂CH₂O)₉ were described. It was found that the synthesized Ti_{1 ? x} Fe _x O_{2 ? 2x/2} solid solutions exhibit photocatalytic activity in the reaction of hydroquinone oxidation in an aqueous solution on irradiation with UV light. A correlation between the rate of oxidation of hydroquinone and the concentration of iron in the catalyst was established. A procedure for the preparation of titanium dioxide with the structure of anatase doped with iron and carbon (Ti_{1 ? x} Fe _x O_{(2 ? x/2) ? yCy)} and also composites on its basis, which contain an excess amount of carbon, was proposed.     

Oxygen evolution on Ru1 − x Ni x O2 − y nanocrystalline electrodes

Nanocrystalline Ru_1 − x Ni _x O_2 − y with 0.02 < x < 0.30 were prepared by a sol–gel approach at temperatures between 300 and 600 °C. XRD patterns of the prepared materials indicate a single-phase character conforming to a tetragonal syngony. All prepared materials are sufficiently stable in acid media and show activity towards oxygen evolution. The activity towards oxygen evolution reaction of the materials with constant chemical composition decreases with increasing particle size. The increasing Ni content enhances the electrocatalytic activity in a stepwise manner with abrupt changes for materials containing approximately 5% or 15% of the cationic sites substituted by Ni.     

Crystal structure and properties of the Na1−x Ru2O4 phase

Sodium ruthenium(III,IV) oxide Na_1−x Ru₂O₄ was synthesized by the solid state reaction of Na₂CO₃ and RuO₂ in inert atmosphere and characterized by X-ray powder diffraction, electron diffraction, and high-resolution transmission electron microscopy. The compound crystallizes in the CaFe₂O₄-type structure (space group Pnma, Z = 4, a = 9.2641(7) Å, b = 2.8249(3) Å, c = 11.1496(7) Å). Double rutile-like chains of the RuO₆ octahedra form a three-dimensional framework, whose tunnels contain sodium cations. The structure contains two crystallographically independent sites of ruthenium atoms randomly occupied by the Ru^III and Ru^IV cations. The superstructure with the doubled b parameter found for one of the samples under study using electron diffraction is caused, probably, by ordering of the Ru cations in the rutile-like chains. The Na_{1− x} Ru₂O₄ compound exhibits temperature-independent paramagnetism with χ₀ = 1.9·10⁻⁴ cm³ (mole of Ru⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1655–1660, October, 2006.     

Sol–gel synthesis and catalytic property of Ce1−x Ti x O2 nanocomposites supported on attapulgite clay

Ce_1?x Ti _x O₂ nanocomposites supported on attapulgite clay (Ce_1?x Ti _x O₂/ATP) were prepared by a facile sol–gel route. The textural and structural properties of the prepared products were characterized by thermogravimetric-differential scanning calormetry analysis, X-ray diffraction, transmission electron microscopy, energy-dispersive spectrometry, Fourier transform infrared spectroscopy and Nitrogen adsorption-desorption measurements. The catalytic activity of the prepared Ce_1?x Ti _x O₂/ATP catalysts for rhodamine B degradation was investigated. Results indicate that the particle size of Ce_1?x Ti _x O₂ nanoparticles evenly attached onto the surface of ATP is approximately 10 nm. The Ti⁴⁺ doping ratios exhibit considerable impact on the redox ability and catalytic activity of Ce_1?x Ti _x O₂/ATP composites. The introduction of an optimal amount of Ti⁴⁺ contributes to forming structure defects and electronic defects in the oxide lattice, increasing concentration of oxygen vacancies, consequently improving low-temperature redox ability of Ce⁴⁺ and enhancing catalytic activity of the composites. Ce_1?x Ti _x O₂/ATP (x = 0.5) catalyst has the best catalytic degradation efficiency, which can reach as high as 97 % after reaction for 240 min. It is also found that attapulgite clay exhibit a positive synergistic effect to the Ce_1?x Ti _x O₂ nanoparticles.     

Electrochemical study of spinel oxide systems with nominal compositions Ni1− x Cu x Co2O4 and NiCo2− y Cu y O4

Studies on the electrochemical behaviour of Ni_{1− x} Cu _x Co₂O₄ (x ≤ 0.75) and NiCo_{2− y} Cu _y O₄ (y ≤ 0.30) electrodes in 5 mol dm⁻³ KOH aqueous solutions are presented. The oxide layers have been prepared by thermal decomposition of aqueous nitrate solutions on nickel supports at 623 K. Powder samples were also prepared by thermal decomposition under the same conditions. The powder samples and the oxide layers were characterised by X-ray powder diffraction. The influence of the copper content on the voltammetric response of the electrodes and activity towards oxygen evolution reaction is analysed and correlated with the surface composition of the electrodes by means of X-ray photoelectron spectroscopy data. The analysis of the results reveals that the presence of Cu affects the electrode behaviour and its influence depends on which cation has been replaced. Received: 22 February 1999 / Accepted: 26 October 1999     

Electrochemical properties of solid solutions in the Li8Zr1 − x Ce x O6 system

Li₈Zr_{1 ? x} Ce _x O₆ solid solutions based on lithium zirconate Li₈ZrO₆ were obtained by solid-state synthesis in an inert atmosphere. Their transport properties (the electron component of the total conductivity, the temperature and concentration dependences of conductivity, and the activation energies) were studied by impedance spectroscopy. The practical stability of the ceramic samples of Li₈Zr_{1 ? x} Ce _x O₆ solid solutions (x = 0–0.07) at 473–573 K against melted lithium was studied.     

Study of ceramic pigments based on Er2Ce2?x Mo x O7

This contribution is focused on the synthesis, characterization and optical properties of new inorganic pigments which are environment friendly and can substitute some toxic metals in interesting colour compounds. Pyrochlores belong to the group of high-temperature pigments, and are a variety of actual and potential applications for several materials. Examples include catalysts, thermal barrier coatings, solid electrolytes, nuclear waste forms and host materials for luminescence centers. The pigments were prepared by the solid state reaction and also by method of suspension mixing of materials in the series with increasing content of molybdenum. The pigments were applied into organic matrix and ceramic glaze. The colour properties of these applications were investigated depending on content of Mo, method of preparation and temperature of calcination (1350?C1550?°C after step 50?°C). The optimum conditions for their synthesis were determined. The pigments were evaluated from standpoint of their structure, colour and particle sizes. Characterization of Er₂Ce_2?x Mo _x O₇ pigments (x?=?0.1, 0.3, 0.5 and 0.7) suggests that they have a potential to be alternative yellow or orange colourants for paints, plastics, ceramics and building materials.     

Solid potassium-conducting electrolytes in the K2 − 2x Ga2 − x V x O4 system

New potassium-conducting solid electrolytes based on potassium monogallate in the K_2?2x Ga_2?x V _x O₄ system are synthesized and studied. It is found that an introduction of V⁵⁺ ions leads to a considerable increase in the KGaO₂ conductivity due to the formation of vacancies in the potassium sublattice. The conductivity for optimal compositions is approximately 10^?3 S cm^?1 at 400°C and above 10^?2 S cm^?1 at 700°C. The results are compared with early obtained data for potassium monogallate dopped with four-charged cations.     

Raman spectroscopy investigation on (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramic system

A Raman spectroscopy study at room temperature was carried out on (Pb_1−xLa_x)(Zr_0.90Ti_0.10)_1−x/4O₃ ceramics (x = 2, 3, 4 at%). The results were analyzed considering the x-ray patterns at room temperature showing a mixture of two phases: a rombohedral-ferroelectric phase and an orthorhombic-antiferroelectric, increasing the% of the second one with the lanthanum concentration. For x = 3 at%, the analysis was also carried out in a wide temperature range. Two anomalies were evaluated, one around 363 K, which has been associated to a ferroelectric-antiferroelectric phase transition; the second one around 430 K, which has been associated to a transition from an incommensurable state to a ferroelectric phase.     

Characterization of superparamagnetic Mg x Zn1−x Fe2O4 powders

Structural and magnetic properties of Mg _x Zn_1−x Fe₂O₄ powders have been studied with respect to the application for thermal cancer therapy (magnetic hyperthermia). Mg _x Zn_1−x Fe₂O₄ (x=0.1–0.5) powders with particle sizes between 5 and 8 nm were produced by citrate method. The X-ray diffraction patterns of the samples correspond to a spinel phase. The lattice constant and the volume of the elementary cell increase when x changes from 0.1 to 0.5. The FTIR-spectra ascertain the spinel phase formation. The Mossbauer studies reveal the presence of extremely small particles, which undergo superparamagnetic relaxation at room temperature. The core-shell model has been applied to explain quadruple doublets. The quadruple splitting at “shells” is bigger than those at “cores” whereas the isomer shifts remain close. Magnetic studies confirm the presence of extremely small particles that behave as superparamagnetic ones.      

The promotion effect of catalytic activity by Ru substitution at the B site of La1−x Sr x Cr1−y Ru y O3−z for propane steam reforming

A series of La_1?x Sr _x Cr_1?y Ru _y O_3?δ (0.1 ≤ x ≤ 0.5, 0.05 ≤ y ≤ 0.15) materials was prepared by the sol–gel method to develop alternative catalysts for propane steam reforming. Catalyst characteristics were evaluated using physicochemical methods including X-ray diffraction, Brunauer–Emmett–Teller methods, H₂ temperature-programmed reduction, and thermogravimetry analysis (TGA). Effects of the amount of ruthenium (Ru) and strontium and the steam-to-carbon ratio (S/C) were investigated. An increase in Ru content led to increased propane conversion and H₂ yield, especially below 700 °C. Dramatic enhancement of catalytic activity was observed with La_0.8Sr_0.2Cr_0.85Ru_0.15O₃ under 600 °C, achieving propane conversion over 79% between 600 and 800 °C with maximum propane conversion and H₂ yield of 98.3% and 63.3%, respectively. Also, good resistance to carbon formation for the La_0.8Sr_0.2Cr_0.85Ru_0.15O₃ catalyst was confirmed by long-term testing and TGA analysis.

     

Electrochemical preparation and magnetic properties of submicron Co x Pb1−x dendrites

Magnetic dendrites of Co _x Pb_1−x were fabricated through potentiostatic electrochemical deposition on Cu substrates in boric acid solution at room temperature. The as-deposited dendrites were determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), electrodeposition (ED), and energy dispersive X-ray spectroscopy (EDS). SEM results indicate that the Co _x Pb_1−x dendrites are highly symmetrical in structures. The diameters of the branches are about 50 ~ 200 nm, and the backbones are continuous with lengths up to about 10 μm. XRD patterns show that the as-deposited dendrites are solid solutions. The annealing treatment can result in the recrystallization of these metastable alloys into two separate phases. TEM, ED, and EDS results also reveal that the backbones and the branches of the dendrites are composed of different amounts of cobalt. Magnetic measurements confirm that the as-deposited Co _x Pb_1−x dendrites have a softly ferromagnetic behavior, and a small coercive force (about 80 Oe). Also the saturation magnetizations of the Co _x Pb_1−x dendrites decrease rapidly with the temperature increasing.     

Anodic electrodeposition of Ag1− x Cu x O microcrystals

We demonstrate the anodic electrodeposition of copper-doped AgO at high pH using a silver counter-electrode. Precipitates from a mixture of nitrates and NaOH provided source material for the deposition, and application of a moderate anodic voltage (0.9 V) to the substrate led to deposition of crystalline nanoparticles with incorporated copper. Further increase of the NaOH concentration reduced the amount of copper in the crystals, and higher voltages degraded the crystal quality. XRD confirms the underlying structure to be that of AgO, and Auger and energy dispersive x-ray analyses confirm copper concentrations of approximately 3 % in the crystals.     

Synthesis and properties of fuel cell anodes based on (La0.5 + x Sr0.5 − x )1 − y Mn0.5Ti0.5O3 − δ (x = 0–0.25, y = 0–0.03)

Results are presented of studying electrochemical properties of perovskite-like solid solutions (La_{0.5 + x} Sr_{0.5 ? x} )_{1 ? y} Mn_0.5Ti_0.5O_{3 ? δ} (x = 0–0.25, y = 0–0.03) synthesized using the citrate technique and studied as oxide anodic materials for solid oxide fuel cells (SOFC). X-ray diffraction (XRD) analysis is used to establish that the materials are stable in a wide range of oxygen chemical potential, stable in the presence of 5 ppm H₂S in the range of intermediate temperatures, and also chemically compatible with the solid electrolyte of La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O_{3 ? δ} (LSGMC). It is shown that transition to a reducing atmosphere results in a decrease in electron conductivity that produced a significant effect on the electrochemical activity of porous electrodes. Model cells of planar SOFC on a supporting solid-electrolyte membrane (LSGMC) with anodes based on (La_0.6Sr_0.4)_0.97Mn_0.5Ti_0.5O_{3 ? δ} and (La_0.75Sr_0.25)_0.97Mn_0.5Ti_0.5O_{3 ? δ} and a cathode of Sm_0.5Sr_0.5CoO_{3 ? δ} are manufactured and tested using the voltammetry technique.     

High-voltage aqueous symmetric electrochemical capacitor based on Ru0.7Sn0.3O2·nH2O electrodes in 1 M KOH

A mild hydrothermal process is applied to synthesize hydrous ruthenium–tin binary oxides (Ru_0.7Sn_0.3O₂·nH₂O) with good capacitive performance in alkaline system. Then, a symmetric electrochemical capacitor (EC) is fabricated based on the as-synthesized Ru_0.7Sn_0.3O₂·nH₂O material and 1 M KOH aqueous electrolyte. Electrochemical performance of the symmetric EC is investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy tests. Electrochemical tests demonstrate that the symmetric EC surprisingly can operate with a high upper cell voltage limit of 1.45 V in 1 M KOH electrolyte. Maximum specific capacitance and energy density of the symmetric aqueous EC are approximately 160 F/g and 21 Wh/kg, respectively, delivered at a current density of 1.25 A/g. And the specific energy density decreases to approximately 15 Wh/kg when the specific power density increases up to approximately 1,770 W/kg. The promising specific energy and power densities are obtained simultaneously for the unwonted symmetric EC due to its larger operating potential range. Moreover, the symmetric EC exhibits electrochemical stability with 85.2% of the initial capacitance over consecutive 1,000 cycle numbers.     

Effect of electronic state of ions on the electrochemical properties of layered cathode materials LiNi1−2x Co x Mn x O2

The cathode materials of the composition LiNi_{1 − 2x} Co _x Mn _x O₂ (x = 0.1, 0.2. 0.33) synthesized from the Ni, Co, Mn mixed hydroxides and LiOH by using mechanical activation method are studied. It is shown that all synthesized compounds have layered structure described by the space group R-3m. With the decreasing of the nickel content the cell volume and the degree of structure disordering decrease. According to XPS data, the electronic main state of d-ions at the prepared samples’ surfaces corresponds to Ni²⁺, Co³⁺, and Mn⁴⁺. An increase in the nickel content leads to the increase of the Ni2p _3/2 and Co2p _3/2 binding energy, which points to the change in the Me-O bond covalence. According to magnetic susceptibility measurements data, the nickel ions in LiNi_0.6Co_0.2Mn_0.2O₂ exist in the two oxidation states: Ni²⁺ and Ni³⁺. It is shown that this sample has the highest specific discharge capacity (∼170 mAh/g). The positions of redox peaks in the differential capacitance curves depend on the sample composition: with the increasing of nickel content they are shifted toward lower voltages. Based on the paper presented in the IX International Conference “Basic Problems of Energy Conversion in Lithium Electrochemical Systems” (Ufa, 2006).