Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. II. Perovskite-related oxides

The review is devoted to the analysis of experimental results on electrochemical and physicochemical properties of the perovskite-related oxide phases obtained at scientific centers of the former Soviet Union. The main attention is focused on oxides with high electronic conductivity, which are potentially useful as electrodes for high-temperature electrochemical cells with oxygen-ion conducting solid electrolytes and interconnectors of solid oxide fuel cells, and on mixed ionic-electronic conductors for oxygen separation membranes. Along with perovskite-like solid solutions based on LnMO_3−δ (Ln is a rare-earth element, M = Cr, Mn, Fe, Co, Ni) and SrCoO_3−δ, properties of the oxide phases Ln₂MO_4±δ (M = Cu, Ni, Co) with the K₂NiF₄-type structure are briefly reviewed. Received: 5 November 1998 / Accepted: 26 November 1998     

Solvothermal Syntheses,Crystal Structures,and Properties of New [Mn(<Emphasis Type="BoldItalic">dien</Emphasis>)<Subscript>2</Subscript>]<Superscript>2+</Superscript> Complexes

Summary.  The two new compounds Mn(dien)₂[MoS₄] (1) and Mn(dien)₂[Mo₂O₂S₆] (2) (dien = diethylenetriamine) were prepared under solvothermal conditions. Both compounds were obtained as phase-pure products. The structures consist of new [Mn(dien)₂]²⁺ cations and isolated tetrahedral [MoS₄]²⁻ (1) or [Mo₂O₂S₆]²⁻ (2) anions. Between the anions and the cations, hydrogen bonding is observed. Compound 1 crystallizes in the tetragonal space group I (a = 10.219(2), c = 9.259(2) ?, Z = 2), whereas 2 crystallizes in the monoclinic space group P2₁/c (a = 8.703(2), b = 18.390(4), c = 14.603(3) ?, β = 103.18(3)°, Z = 4). The thermal behaviour of the thiomolybdates was investigated using difference thermoanalysis (DTA) and thermogravimetry (TG). Both compounds decompose under argon with a single endothermic signal in the DTA curve (peak maximum: 252 (1) and 242°C (2)). Received November 5, 2001. Accepted December 27, 2001     

Solvothermal Syntheses, Crystal Structures, and Properties of New [Mn( dien )2]2+ Complexes

 The two new compounds Mn(dien)₂[MoS₄] (1) and Mn(dien)₂[Mo₂O₂S₆] (2) (dien = diethylenetriamine) were prepared under solvothermal conditions. Both compounds were obtained as phase-pure products. The structures consist of new [Mn(dien)₂]²⁺ cations and isolated tetrahedral [MoS₄]²⁻ (1) or [Mo₂O₂S₆]²⁻ (2) anions. Between the anions and the cations, hydrogen bonding is observed. Compound 1 crystallizes in the tetragonal space group I (a = 10.219(2), c = 9.259(2) ?, Z = 2), whereas 2 crystallizes in the monoclinic space group P2₁/c (a = 8.703(2), b = 18.390(4), c = 14.603(3) ?, β = 103.18(3)°, Z = 4). The thermal behaviour of the thiomolybdates was investigated using difference thermoanalysis (DTA) and thermogravimetry (TG). Both compounds decompose under argon with a single endothermic signal in the DTA curve (peak maximum: 252 (1) and 242°C (2)).     

Uniform particles of manganese compounds obtained by forced hydrolysis of manganese(II) acetate

 Procedures for the preparation at low temperature (80 °C) of uniform colloids consisting of Mn₃O₄ nanoparticles (about 20 nm) or elongated α-MnOOH particles with length less than 2 μm and width 0.4 μm or less, based on the forced hydrolysis of aqueous manganese(II) acetate solutions in the absence (Mn₃O₄) or the presence (α-MnOOH) of HCl are described. These solids are only produced under a very restrictive range of reagent concentrations involving solutions of 0.2–0.4 mol dm⁻³ manganese(II) acetate for Mn₃O₄ and of 1.6–2 mol dm⁻³ Mn(II) and 0.2–0.3 mol dm⁻³ HCl for α-MnOOH. The role that the acetate anions play in the precipitation of these solids is analyzed. It seems that these anions promote the oxidation of Mn(II) to Mn(III), which readily hydrolyze causing precipitation. The evolution of the characteristics of the powders with temperature up to 900 °C is also reported. Thus, Mn₃O₄ particles transform to Mn₂O₃ upon calcination at 800 °C; this is accompained by a sintering process. The α-MnOOH sample also experiences several phase transformations on heating. First, it is oxidized at low temperatures (250–450 °C) giving MnO₂ (pyrolusite), which is further reduced to Mn₂O₃ at 800 °C. After this process the particles still retain their elongated shape. Received: 19 October 1999 Accepted: 24 November 1999     

Dissolution behavior of anodic oxide films formed in sulfanic acid on aluminum

Chemical dissolution of the barrier layer of porous oxide films formed on an aluminum foil (99.5% purity) in 1.5 M sulfanic acid after immersion in a 2 mol dm⁻³ sulphuric acid at 50 °C was studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. Re-anodizing was conducted in 0.5 mol dm⁻³ H³BO³/0.05 mol dm⁻³ Na²B⁴O⁷ solution. We found that the change in the porous oxide growth mechanism was observed at the anodizing voltage of 30 V. Taking into account this result chemical dissolution behaviour of the barrier layer of porous films formed at 20 V and 36 V and also the influence of annealing of oxide films at 200 °C were studied. We showed the interplay between the dissolution rates and charge distribution across the barrier layer. We conclude that the outer and middle layers have negative space charges and the inner layer has positive space charges.     

Complexation Studies of N,N′-ethylenedi-L-cysteine with Some Metal Ions

As part of a search for environmentally friendly metal chelating ligands, the stability constants of N, N′-ethylenedi-L-cysteine (EC) complexes with Ca(II), Cu(II), Mg(II) and Mn(II) were determined by potentiometry with a glass electrode in aqueous solutions containing 0.1 mol⋅L⁻¹ KCl at 25 °C. Final models are proposed. For the Ca(II)–EC system, the overall stability constants are log ₁₀ β _CaHL=14.53±0.03, log ₁₀ β _CaL=4.79±0.01 and log ₁₀ β _CaL2=8.38±0.04. For the M(II)–EC systems, where M=Cu(II) or Mg(II), the overall stability constants are log₁₀ β _CuHL=31.19±0.02 and log ₁₀ β _CuL=27.02±0.06 for Cu(II), and are log ₁₀ β _MgHL=14.84±0.02 and log ₁₀ β _MgL=6.164±0.008 for Mg(II). For the Mn(II)–EC system, the overall stability constant is log ₁₀ β _MnL=10.12±0.01. Metal–chelate speciations simulations showed that EC is an efficient chelating agent for Cd(II), Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) for pH≥7.     

Nanometer-sized zirconium dioxide microcolumn separation/preconcentration of trace metals and their determination by ICP-OES in environmental and biological samples

A new method is proposed using a microcolumn (20 mm × 2.0 mm) packed with nanometer-sized zirconia as solid-phase extractor for the separation/preconcentration of Mn, Cu, Cr, Zn, Ni and Co prior to their determination by inductively coupled plasma optical emission spectrometer (ICP-OES) in environmental samples. The factors affecting the separation and preconcentration of analytes such as pH, sample flow rate and volume, eluent concentration and volume were determined, interfering ions were studied, and the optimal experimental conditions were established. The adsorption capacity of nanometer-sized ZrO₂ for Mn, Cu, Cr, Zn, Ni and Co was found to be 1.3, 1.3, 1.7, 2.0, 3.9 and 1.5 mg g⁻¹, respectively. The detection limits of the method were 12, 58, 24, 2, 7 and 36 ng L⁻¹, respectively, with a preconcentration factor of 25. The precision of this method was 1.7% (Mn), 2.9% (Cu), 5.9% (Mn), 3.8% (Mn), 6.2% (Mn) and 4.3% (Mn) with 9 determinations of 10 ng mL⁻¹ of target analytes, respectively. The method was successfully applied to the determination of trace metals in lake water, dried fish samples, certified reference materials of human hair and milk, and provided satisfactory results.     

Application of rapid electrothermal atomic absorption spectrometric methods to the determination of Ag, Al, Cd and Mn in cocaine and heroin samples

 Rapid methods were developed for the direct determination of Ag, Al, Cd and Mn in cocaine and heroin by ETAAS using programmes omitting the charring step . Sample pretreatment was simple: dissolution in ultrapure water or in 35.0% (v/v) HNO₃ for heroin or cocaine, respectively. Optimum drying temperatures were 250 °C for Ag, Al and Mn, and 300 °C for Cd. The run cycles were 35 and 37 s, for Ag and Al respectively, and 36 s for Cd and Mn. The best results were obtained with Pd, Mg(NO₃)₂ and (NH₄)₂HPO₄, as chemical modifiers. The limits of detection were 8.6, 55.9, 2.2 and 12.4 μg kg^-1 for Ag, Al, Cd and Mn, respectively. Received: 14 November 1996/Revised: 14 January 1997/Accepted: 18 January 1997     

Influence of surface contaminations on the hydrogen storage behaviour of metal hydride alloys

Hydrogen storage in metal hydrides is a promising alternative to common storage methods. The surface of a metal hydride plays an important part in the absorption of hydrogen, since important partial reaction steps take place here. The development of surface contaminations and their influence on hydrogen absorption is examined by means of absorption experiments and surface analysis, using X-ray photoelectron spectroscopy (XPS), thermal desorption mass spectrometry (TDMS) and secondary neutral mass spectrometry (SNMS), in this work. All investigations were carried out on a modern AB₂ metal hydride alloy, namely Ti_0.96Zr_0.04Mn_1.43V_0.45Fe_0.08. Surface analysis (SNMS, XPS) shows that long-term air storage (several months) leads to oxide layers about 15 nm thick, with complete oxidation of all main alloy components. By means of in situ oxygen exposure at room temperature and XPS analysis, it can be shown that an oxygen dose of about 100 Langmuirs produces an oxide layer comparable to that after air storage. Manganese enrichment (segregation) is also clearly observed and is theoretically described here. This oxide layer hinders hydrogen absorption, so an activation procedure is necessary in order to use the full capacity of the metal hydride. This procedure consists of heating (T = 120° C) in vacuum and hydrogen flushing at pressures like p = 18 bar. During the activation process the alloy is pulverized to particles of ∼20 μm through lattice stretches. It is shown that this pulverization of the metal hydride (creating clean surface) during hydrogen flushing is essential for complete activation of the material. Re-activation of powder contaminated by small doses of air (p ≈ 0.1 bar) does not lead to full absorption capacity. In ultrahigh vacuum, hydrogen is only taken up by the alloy after sputtering of the surface (which is done in order to remove oxide layers from it), thus creating adsorption sites for the hydrogen. This is shown by TDMS measurements with and without sputtering and oxygen exposure. Figure Investigated metal hydride before and after activation     

Microstructural evolution of protective La–Cr–O films studied by transmission electron microscopy

Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were performed to study the microstructural evolution of La–Cr–O thin films deposited by radio frequency (RF)-magnetron sputtering on stainless steel substrates. Chromium L edges and oxygen K edges are analyzed to determine the valence states of the chromium and elucidate the phase evolution of the thin film. The as-deposited amorphous thin film crystallized to LaCrO₄ and finally transformed to the LaCrO₃ stable phase during annealing at 800°C. An intermediate Cr/Mn oxide layer was formed in all annealed samples. The thickness of this oxide layer stabilizes after 700°C, which indicates that the LaCrO₃ thin film plays a role in inhibiting the growth of an oxide layer on the metal surface.     

Thermal decomposition of some linear trinuclear schiff base complexes with acetate bridges

Ni(II)–M(II)–Ni(II) nuclear structured complexes were prepared from N,N’-bis(salicylidene)-1,3-propanediamine (LH2) and its derivatives N,N’-bis(salicylidene)-2,2’-dimethyl-1,3-propanediamine (LDMH₂) and N,N’-bis(salicylidene)-2-hydroxy-1,3-propanediamine (LOH3), where M represents one of the following metal ions; Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). Two different μ-bridges are found between the metal nucleus of the complexes. The phenolic oxygens and acetate ions tend to form μ-bridges between the terminal Ni(II) ions and central metal(II) ion. The coordinatively bonded DMF molecules, in the complexes, were observed to abandon the structure between 160–180°C. Further heating resulted primarily in the thermal decomposition of the complexes above 310°C, whereas metal oxide residue mixtures were observed above 650°C.     

Photoelectrochemical characteristics of ferric tungstate

Active ferric tungstate was prepared by fusing an equimolar mixture of tungsten oxide and ferric oxide at 1100 °C and annealing at 800 °C for 20 h. Analysis of the electrode material by X-ray diffraction showed that its composition was Fe₂WO₆. When this material was illuminated by visible light in 0.1 M NaOH solution, an anodic photocurrent at a positive potential of 0.5 V (SCE) was obtained. Therefore, this material is considered as an n-type semiconductor. The d.c. conductivity of this material at 25 °C was 4 × 10⁻⁶ Ω⁻¹ cm⁻¹. In the dark, unexpectedly high anodic currents were observed at positive potentials of 0.8 V (SCE) in 0.1 M NaOH. These currents are attributed to the existence of a high density of electron-hole recombination centers within the band-gap of ferric tungstate. When dimethyl viologen (DMV) was used as an electroactive compound in the electrolyte, the anodic photocurrents increased significantly. The oxidation of DMV is thus expected to compete with the electron-hole recombination process. Furthermore, the process of electron-hole recombination was also predicted from the shape of the photocurrent transients under interrupted illumination. These transients exhibited first-order relaxation effects in the region of the onset time of the photocurrents. The band-gap energy of Fe₂WO₆ was found to be about 1.5 eV and its flat-band potential in 0.1 M NaOH was about −0.3 V (SCE). The photoelectrochemical properties of ferric tungstate are explained according to the formalism of the band model of the semiconductor/electrolyte interface. Received: 16 July 1997 / Accepted: 26 September 1997     

Oxidation Behaviour of PACVD-(Ti,Al)N Wear Resistance Layers

 Sputtered (Ti,Al)N hard coatings are successfully used for dry high speed cutting. These films show a lower oxidation rate than TiN or TiC coatings. In our work (Ti,Al)N films were deposited on WC-6%Co substrates at a temperature of 490°C by plasma-assisted chemical vapour deposition (PACVD) using a gas mixture of TiCl₄/AlCl₃/N₂/Ar/H₂. Investigation of microstructure, crystalline structure and chemical composition was carried out using SEM, WDXS, TEM, AES and XRD techniques. The chemical composition of the deposited films showed a Al to Ti ratio of 1.33. The film thickness was 5.5 μm. Films showed a fine crystalline size, the metastable fcc crystal structure and a columnar growth. The film surface was under low compressive stress up to several 100 MPa. For (Ti,Al)N/WC-Co compounds the oxidation behaviour up to 1100°C (high temperature range) was studied. Therefore, samples were annealed or rapidly heated in air and under high vacuum condition using the laser shock method. The results show decomposition of the (Ti,Al)N structure to the TiN and the AlN phases at temperature values above 900°C. Heating in air causes growing of a thin aluminum oxide layer at the film surface, which is a barrier for further oxygen diffusion to the alumina-film boundary. Additionally, at temperatures above 900°C oxidation of the WC-6%Co substrate surface was obtained in regions of opened cracks and film delamination.     

Experimental and Theoretical Constraints on pH Measurements with an Iridium Oxide Electrode in Aqueous Fluids from 25 to 175 °C and 25 MPa

In-situ measurement of pH at elevated temperatures and pressures is of major importance for investigating chemical and biochemical systems in extreme environments. Based on the performance of the newly developed IrO _x electrode at 25 °C, we initiated a series of experiments to test the electrode at elevated temperatures (100 to 175 °C) and high pressure (25 MPa). The experiment was carried out in a titanium flow-through reactor. Our results revealed good pH response at 100, 150 and 175 °C, with good Nernstian slopes at 100 and 150 °C. Although a greater-than-Nernstian response was observed at 175 °C, the factors that cause this difference are attributed to the accuracy of calculations of the distribution of aqueous species rather than alteration of the IrO _x surface. A key problem that may limit applications of the IrO _x electrode at elevated temperatures and pressures is the noticeable shift in E° during the 175 °C (25 MPa) experiments and between experiments with similar conditions at 150 °C. The results of tests from 25 °C to elevated temperatures provide highly useful information on the reversibility and functionality of the IrO _x -pH sensor with implications for the suitability of its use under challenging chemical and physical conditions.     

Nanocrystalline nickel cobalt hydroxides/ultrastable Y zeolite composite for electrochemical capacitors

A novel nanocomposite of Co(OH)₂−Ni(OH)₂ and ultrastable Y molecular sieves was synthesized by an improved chemical precipitation method for electrochemical capacitors. The Co(OH)₂−Ni(OH)₂/ultrastable Y zeolite (USY) composite and its microstructure were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Electrochemical characterization was performed by cyclic voltammetry and galvanostatic charge–discharge measurements. The results show that Co(OH)₂−Ni(OH)₂/USY microstructure applied for the electrochemical energy storage has displayed superior capacitive performance. The effect of heat treatment conditions on specific capacitance properties was also systemically explored. Upon annealing at 250 °C, the maximum specific capacitance was up to 479 F/g (or 1,710 F/g after correcting for the weight percent of Co(OH)₂−Ni(OH)₂ phase). Annealing temperatures higher than 250 °C may cause the hydroxide to form oxide phase and decrease the surface activity of the oxide, thereby leading to a decline of the specific capacitance.     

Spectral and Thermal Studies of Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) Complexes with 3-methylglutaric Acid

Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) 3-methylglutarates were prepared as solids with general formula MC₆ H₈ O₄ ×n H₂ O, where n =0–8. Their solubilities in water at 293 K were determined (7.0×10⁻² −4.2×10⁻³ mol dm⁻³ ). The IR spectra were recorded and thermal decomposition in air was investigated. The IR spectra suggest that the carboxylate groups are mono- or bidentate. During heating the hydrated complexes lose some water molecules in one (Mn, Co, Ni, Cu) or two steps (Cd) and then mono- (Cu) or dihydrates (Mn, Co, Ni) decompose to oxides directly (Mn, Cu, Co) or with intermediate formation of free metals (Co, Ni). Anhydrous Zn(II) complex decomposes directly to the oxide ZnO. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photoelectrochemical characteristics of oxide layers on copper-nickel alloys

The electrochemical behavior of a series of Cu-(7, 30, 50, 70 wt%)Ni alloys in borate buffer and sodium sulfate solutions with and without argon purging was studied by cyclic voltammetry and the photocurrent response method. It is shown that nickel has an effect on both the dark and photocurrents and stimulates oxide film formation. The measurements show that oxygen leads to the formation of passivating p-type Cu₂O layers. In sulfate solution this type changes at the potential of the onset of intense alloy dissolution. Received: 15 June 1998 / Accepted: 16 November 1998     

MnO x /ZrO2 gel-derived materials for hydrogen peroxide decomposition

Manganese-yttrium-zirconium mixed oxide nanocomposites with three different Mn loadings (5, 15 and 30 wt%) were prepared by sol–gel synthesis. Amorphous xerogels were obtained for each composition. Their structural evolution with the temperature and textural properties were examined by thermogravimetry/differential thermal analysis, X-ray diffraction, diffuse reflectance UV–vis spectroscopy and N₂ adsorption isotherms. Mesoporous materials with high surface area values (70–100 m² g⁻¹) were obtained by annealing in air at 550 °C. They are amorphous or contain nanocrystals of the tetragonal ZrO₂ phase (T-ZrO₂) depending on the Mn amount and exhibit Mn species with oxidation state higher than 2 as confirmed by temperature programmed reduction experiments. T-ZrO₂ is the only crystallizing phase at 700 °C while the monoclinic polymorph and Mn₃O₄ start to appear only after a prolonged annealing at 1,000 °C. The samples annealed at 550 °C were studied as catalysts for H₂O₂ decomposition in liquid phase. Their catalytic activity was higher than that of previously studied Mn/Zr oxide systems prepared by impregnation. Catalytic data were described by a rate equation of Langmuir type. The decrease of catalytic activity with time was related to dissolution of a limited fraction (up to 15%) of Mn into the H₂O₂/H₂O solution.     

Corrosion crack enhancement during the cathodic activation of zirconium AB<Subscript>2</Subscript> alloys

The activation of zirconium nickel alloys with and without the addition of chromium and titanium is investigated through electrochemical and optical techniques. Recent investigations in aqueous 1 M KOH indicate oxide layer growth and occlusion of hydrogen species in the alloys during the application of different cathodic scan potential programmes currently used for the activation process. In this research, several techniques, such as voltammetry, ellipsometry, energy dispersive analysis of X-rays and scanning electron microscopy, are applied to three polished massive alloys, Zr_1−x Ni _x , x = 0.36 and 0.43, and Zr_0.9Ti_0.1NiCr. Data analysis shows that the stability, compactness and structure of the passive layers are strongly dependent on the applied potential programme. The alloy activation depends on the formation of deep crevices that remain after further polishing. The microscopic observation shows an increase in crevice thickness after cathodic sweep potential cycling, which produces fragmentation of the grains and oxide growth during the activation process. This indicates metal breaking and intergranular alloy dissolution that take place together with oxide and hydride formation. In some cases, the resultant crevice thickness is one or two orders higher than the growth of surface oxide indicating localised intergranular corrosion. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Electrochemical behaviour of a lead electrode in phosphoric acid

 A lead electrode was studied in 6 and 12 M H₃PO₄. Oxidation of a freshly polished electrode occurred in the −0.5 to −0.3 V vs. SCE range, and led to PbHPO₄ growth on the electrode surface. The dissolution of this layer by electrochemical reduction occurred between −0.5 and −0.7 V. The influence of temperature (20 °C and 65 °C) was investigated and showed that the anodic and the cathodic peaks were increasing, and more markedly for the 12 M H₃PO₄. The ratio Q _cathodic/Q _anodic (Q=electrical charge flowing through the electrode) was equal or close to the unity at 20 °C and decreased as the temperature was increased. The influence of Cl⁻, Br⁻ and I⁻ ions was also evaluated. The addition of Cl⁻ and Br⁻ predominantly led to Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃Br, respectively, while I⁻ led to a mixture of PbI₂ and PbHPO₄. Received: 18 July 1999 / Accepted: 2 November 1999