Stability and transport properties of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>

La₂Mo₂O₉ samples were prepared from freeze-dried powder precursors and characterized by XRD, TG/DTA, SEM, electrical and electrochemical measurements. Pellets with different density were obtained by sintering at temperatures between 900 and 1100 °C to obtain nearly dense samples with grain sizes in the range 1–8 m. The electrical conductivity was measured using impedance spectroscopy. The capacitance and relaxation frequencies of the main contributions to the spectra were used to ascribe the contributions of grain interiors and internal interfaces, and their temperature dependence. A coulometric titration technique was used to evaluate the change of oxygen stoichiometry under moderately reducing conditions, and to estimate the stability limits under strongly reducing conditions. An ion-blocking method was used to evaluate the onset of n-type conductivity, and a combination of these results with total conductivity measurements was used to obtain the ionic transport number. A combination of oxygen stoichiometry changes and ion-blocking results was used to obtain estimates of mobility.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

Development of Double‐Perovskite Compounds as Cathode Materials for Low‐Temperature Solid Oxide Fuel Cells

A class of double‐perovskite compounds display fast oxygen ion diffusion and high catalytic activity toward oxygen reduction while maintaining excellent compatibility with the electrolyte. The astoundingly extended stability of NdBa_1−xCa_xCo₂O_5+δ (NBCaCO) under both air and CO₂‐containing atmosphere is reported along with excellent electrochemical performance by only Ca doping into the A site of NdBaCo₂O_5+δ (NBCO). The enhanced stability can be ascribed to both the increased electron affinity of mobile oxygen species with Ca, determined through density functional theory calculations and the increased redox stability from the coulometric titration.     

Effects of Nb substitution on structure and electrochemical properties of LiNi<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>O<Subscript>2</Subscript> cathode materials

Nb-doped cathode materials with the formula Li(Ni_0.7Mn_0.3)_1?xNb_xO₂ (x?=?0, 0.01, 0.02, 0.03, 0.04) have been prepared successfully by calcining the mixtures of LiOH·H₂O, Nb₂O₅, and Ni_0.7Mn_0.3(OH)₂ precursor formed through a simple continuous co-precipitation method. The effects of Nb substitution on the crystal structure and electrochemical properties of LiNi_0.7Mn_0.3O₂ were studied systematically by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and various electrochemical measurements. The results show that the lattice parameters of the Nb substitution LiNi_0.7Mn_0.3O₂ samples are slightly larger than that of pure LiNi_0.7Mn_0.3O₂, and the basic α-NaFeO₂ layered structure does not change with the Nb doping. What’s more, better morphology, lower resistance, and good cycle stability were obtained after Nb substitution. In addition, CV test exhibits that Nb doping results in lower electrode polarization and XPS results indicate that the valence of Mn kept constant but the component of Ni³⁺ decreased after doping. All the results indicate that Nb doping in LiNi_0.7Mn_0.3O₂ is a promising method to improve the properties of Ni-rich lithium-ion batteries positive-electrode materials.     

Sintering behavior and ionic conductivity of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> with a small amount of MnO<Subscript>2</Subscript> doping

A 20% GdO_1.5 doped ceria solid solution with a small amount of MnO₂ doping (≤5% molar ratio) was prepared via the mixed oxide method from high-purity commercial powders with grain size around 0.2–0.5 μm. X-ray diffraction analysis indicated that all the samples exhibited the fluorite structure, and no new phase was found. The data from dilatometeric measurements and scanning electron microscopy observations revealed that 1% Mn doping reduced the sintering temperature by over 150 °C, and enhanced the densification and grain growth. Mn doping has little effect on grain interior conductivity, but a marked deterioration in grain boundary behavior is observed. This leads to a lower total conductivity in comparison with the undoped Ce_0.8Gd_0.2O_2–δ. Therefore, for solid oxide fuel cells (SOFCs) with Mn-containing compounds as electrodes, optimization of electrode fabrication conditions is needed to prevent the formation of a lower conductivity layer at the electrode/electrolyte interface since Mn will diffuse from the electrode side to the electrolyte during fabrication and operation of SOFCs. Electronic Publication     

SO<Subscript>2</Subscript> carry-over and sulphur formation in a SO<Subscript>2</Subscript>-depolarized electrolyser

SO₂-depolarized electrolysis (SDE) is considered as one of the most efficient hydrogen production methods. To maximize H₂ production in SDE, the phenomena occurring in the cell need to be understood and controlled. In this work, electrochemically driven SO₂ carry-over and elemental sulphur formation in cathode space are analysed by various methods: electrochemical, titration and photon correlation spectroscopy (PCS) under different conditions during SDE operation. The results indicate that SO₂ carry-over is a fast process in an operating SDE and is difficult to control with only working parameter optimization. The PCS method provides SO₂ carry-over information at early stages compared to other methods. Moreover, PCS might be further implemented as an in situ method for SDE system control.     

Improving rate performance of cathode material Li<Subscript>1.2</Subscript>Mn<Subscript>0.54</Subscript>Co<Subscript>0.13</Subscript>Ni<Subscript>0.13</Subscript>O<Subscript>2</Subscript> via niobium doping

As a promising Li-ion battery cathode active material, lithium-rich manganese-based layer-structured oxides suffer from inferior cycle performance and poor rate capability. Herein, Nb-doped Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ is prepared by a sol-gel method, and the effects of Nb doping on its electrochemical performance are investigated. It is concluded that the Nb-doped Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, has a good layered structure along c-axis independent on the amount of Nb dopant and little cationic mixing. Nb doping for Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has no obvious influence on its morphology. It is found that Nb doping can enhance the electrochemical activity of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, such as improved rate performance and cycle performance under high rate conditions. Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ doped with 0.015 Nb shows the best cycle performance under the high rate with the capacity maintenance of 95.4% after 100 cycles under 5 C rate, which is higher than that of the undoped one by 10.5%.

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Graphical abstract Rate performance of Li_1.2Mn_0.54-xCo_0.13Ni_0.13Nb _x O₂ materials

     

Synthesis and electrochemical properties of Ca-doped LiNi<Subscript>0.8</Subscript>Co<Subscript>0.2</Subscript>O<Subscript>2</Subscript> cathode material for lithium ion battery

LiNi_0.8Co_0.2O₂ and Ca-doped LiNi_0.8Co_0.2O₂ cathode materials have been synthesized via a rheological phase reaction method. X-ray diffraction studies show that the Ca-doped material, and also the discharged electrode, maintains a hexagonal structure even when cycled in the range of 3.0–4.35 V (vs Li⁺/Li) after 100 cycles. Electrochemical tests show that Ca doping significantly improves the reversible capacity and cyclability. The improvement is attributed to the formation of defects caused by the partial occupancy of Ca²⁺ ions in lithium lattice sites, which reduce the resistance and thus improve the electrochemical properties.     

Effects of Cr doping on the electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material for lithium ion batteries

A series of Cr-doped Li₃V_2???x Cr _x (PO₄)₃ (x?=?0, 0.1, 0.25, and 0.5) samples are prepared by a sol–gel method. The effects of Cr doping on the physical and chemical characteristics of Li₃V₂(PO₄)₃ are investigated. Compared with the XRD pattern of the undoped sample, the XRD patterns of the Cr-doped samples have no extra reflections, which indicates that Cr enters the structure of Li₃V₂(PO₄)₃. As indicated by the charge–discharge measurements, the Cr-doped Li₃V_2???x Cr _x (PO₄)₃ (x?=?0.1, 0.25, and 0.5) samples exhibit lower initial capacities than the undoped sample at the 0.2 C rate. However, both the discharge capacity and cycling performance at high rates (e.g., 1 and 2 C) are enhanced with proper amount of Cr doping (x?=?0.1). The highest discharge capacity and capacity retention at the rates of 1 and 2 C are obtained for Li₃V_1.9Cr_0.1(PO₄)₃. The improvement of the electrochemical performance can be attributed to the higher crystal stability and smaller particle size induced by Cr doping.     

Effects of Mg,Al Co-doping into Mn site on electrochemical performance of LiNi<Subscript>0.5</Subscript>Co<Subscript>0.2</Subscript>Mn<Subscript>0.3</Subscript>O<Subscript>2</Subscript>

In order to study the influence of multiple ions doping into single-site on the structure and electrochemical properties of Ni-rich layered-structure cathode material LiNi_0.5Co_0.2Mn_0.3O₂, the coprecipitation of hydroxides was applied to synthesize Mg, Al co-doped cathode material LiNi_0.5Co_0.2Mn_0.3–x Mg_1/2x Al_1/2x O₂ (x = 0.00, 0.01, 0.02, 0.04) in this paper. Morphology and structure, kinetic parameter, impedance and electrochemical performance of the material were respectively characterized by SEM, XRD, CV, EIS and galvanostatic charge/discharge test. The results of comprehensive analysis showed that the properties of material were improved obviously when the amount of doping was 0.02. At this amount of doping, the corresponding material has smaller cation mixing, higher hexagonal ordering of layered-structure, larger Li⁺ ion diffusion coefficients which are 2.444 × 10^–10 and 4.186 × 10^–10 cm² s^–1 for Li+ intercalation and deintercalation respectively, smaller impedance which is 33.93 Ω, higher specific capacity of first-discharge which is 168.01 mA h g^–1 and higher capacity retention rate which is up to 95.06% after 20 cycles at 0.5 C (100 mA g^–1).     

Preparation and characterization of superconducting YBa<Subscript>2</Subscript>(Cu<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>)<Subscript>4</Subscript>O<Subscript>8</Subscript> oxides by thermal analysis

In this study the formation of chromium substituted YBa₂Cu₄O₈ (Y-124) superconductors has been investigated by TG/DTA measurements. The YBa₂(Cu_1−xCr_x)₄O₈ ceramics with nominal compositions of x=0.01, 0.03, 0.05, 0.10 and 0.20 have been prepared by an aqueous sol-gel method using aqueous mixtures of the corresponding metal acetates and nitrates. Homogeneous precursor gels were obtained by complexing metal ions with tartaric acid. To assist the interpretation of the results obtained the synthesis products were additionally characterized by X-ray powder diffraction (XRD) and resistivity measurements. It was determined that doping the YBa₂Cu₄O₈ phase with chromium has a strong effect on the phase purity and superconducting properties of the synthesis products.     

Ammonia gas sensor based on a spinel semiconductor,Co<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanomaterial

Thick film of nanocrystalline Co_0.8Ni_0.2Fe₂O₄ was obtained by sol–gel citrate method for gas sensing application. The synthesized powder was characterized by X-ray diffraction (XRD) and transmission electron microscopy. The XRD pattern shows spinel type structure of Co_0.8Ni_0.2Fe₂O₄. XRD of Co_0.8Ni_0.2Fe₂O₄ revels formation of solid solution with average grain size of about 30 nm. From gas sensing properties it observed that nickel doping improves the sensor response and selectivity towards ammonia gas and very low response to LPG, CO, and H₂S at 280 °C. Furthermore, incorporation of Pd improves the sensor response and stability of ammonia gas and reduced the operating temperature upto 210 °C. The sensor is a promising candidate for practical detector of ammonia.     

Synthesis and crystal structure of a mixed valence heteropoly green compound, (PPh<Subscript>4</Subscript>)<Subscript>4</Subscript>[PMo<Subscript>12</Subscript>O<Subscript>40</Subscript>], and its use as a catalyst in olefin epoxidation

A green heteropolyblue compound, (PPh₄)₄[PMo₁₂O₄₀] · 3DMF(1), has been synthesized from MoO₃, H₂O₂ and H₃PO₄ in acetylacetone medium and crystallized from N,N-dimethylformamide. Compound 1 was characterized by analytical and spectroscopic methods, and X-ray structure analysis. The compound is a one-electron paramagnet and shows a featureless and cubic EPR spectrum with <g> = 1.95 in DMF glass. The complex shows a Mo(V)–Mo(IV) couple, which has been studied by cyclic voltammetric and coulometric methods. The compound acts as an efficient olefin epoxidation catalyst with H₂O₂ as oxidant and NaHCO₃ as co-catalyst. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Enhancement of visible-light photocatalytic activity of Cu-doped TiO<Subscript>2</Subscript> nanoparticles

Bare TiO₂ and Cu-doped TiO₂ nanoparticles with different nominal doping amounts of Cu ranging from of 0.5 to 5.0 mol% were synthesized using the modified sol–gel method. The samples were physically characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller-specific surface area, UV–Vis diffuse reflectance spectroscopy, zeta potential, X-ray photoelectron spectroscopy, inductively coupled plasma, and photoluminescence techniques. The Cu-doped TiO₂ exhibited good photocatalytic activity in mineralization of oxalic acid and formic acid under visible light irradiation. Photomineralization of oxalic and formic acids under visible light irradiation revealed greatly enhanced photoactivity exhibited by the 2.0 mol% Cu-doped TiO₂ photocatalyst compared to bare TiO₂ . The enhanced photocatalytic performance arises from copper ion doping in the TiO₂ structure, leading to an extended photoresponsive range, enhanced photogenerated charge separation, and transportation efficiency.     

Microstructure of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> hybrid electrospun nanofibers and their application in dye degradation

SiO₂/TiO₂ hybrid nanofibers were prepared by electrospinning and applied for photocatalytic degradation of methylene blue (MB). The phase structure, specific surface area, and surface morphologies of the SiO₂/TiO₂ hybrid nanofibers were characterized through thermogravimetry (TG), X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM), etc. XRD measurements indicated that doping of silica into TiO₂ nanofibers can delay the phase transition from anatase to rutile and decrease the grain size. SEM and BET characterization proved that silica doping can remarkably enhance the porosity of the SiO₂/TiO₂ hybrid nanofibers. The MB adsorption capacity and photocatalytic activity of the SiO₂/TiO₂ hybrid nanofibers were distinguished experimentally. It was found that, although increased silica doping content could enhance the MB adsorption capacity, the intrinsic photocatalytic activity gradually dropped. The SiO₂ (10 %)/TiO₂ composite nanofibers exhibited the highest MB degradation rate, being superior to SiO₂ (20 %)/TiO₂ or pure TiO₂.     

Effect of organic acids on sorption of uranyl ions in solution onto ZrP<Subscript>2</Subscript>O<Subscript>7</Subscript>

Hydration of zirconium diphosphate (ZrP₂O₇) conduced to formation of active sites in solid/liquid interface. In ZrP₂O₇/NaClO₄ 0.5 M suspensions, active sites and their acidity constants are quite determined but the presence of some impurities is now studied. This work was conducted to determine the surface properties changes produced by oxalic and citric acid during the hydration process. Moreover the presence of organic acids with ZrP₂O₇ modified reveals an increase in uranium sorption constants. The zirconium diphosphate has been characterized using X-ray powder diffraction (XRD), Scanning electron microscopy (SEM) and Particle induced X-ray emission and Neutron (PIXE). Furthermore, the specific surface area, measured by the BET method, was 3.5 m²/g. The pH corresponding to the isoelectric point, determined by Zeta Potential measurements and mass titration was 3.6. The sites density calculated using titration curves was around of 5.37 s/nm² for NaClO₄ 0.5 M_, 13.71 s/nm² for NaClO₄ 0.5 M/citric acid 0.1 M and 7.33 s/nm² NaClO₄ 0.5 M/oxalic acid 0.1 M. The surface acidity constants and species distribution in surface were calculated by means of simulation of the titration curves with the FITEQL code (constant capacitance model), for ZrO and PO amphoteric sites of ZrP₂O_7. The uranyl sorption edge was determined for NaClO₄ 0.5 M. It spreads between pH 3 and 4.5 for complete sorption according to the previously published results. In the ZrP₂O₇–citrate modified surface, the uranyl sorption edge begin at pH 2 and was almost complete at pH 3.2 while ZrP₂O₇–oxalate modified surface edge started at 50% of sorption at pH of 1.5 and was complete at pH 3.     

Synthesis and electrochemical properties of yttrium-doped LiMn<Subscript>0.98</Subscript>Y<Subscript>0.02</Subscript>O<Subscript>2</Subscript> for lithium secondary batteries

Yttrium-doped lithium manganese oxide (LiMn_0.98Y_0.02O₂) was prepared by ion exchange of lithium for sodium in NaMn_0.98Y_0.02O₂ precursors obtained by using rheological phase reaction method. This material had small particle size, which was composed of grain size of about 100 nm. Especially, LiMn_0.98Y_0.02O₂ delivered the initial discharge capacity of about 191 mA h g⁻¹ at room temperature when cycled between 2.0 and 4.4 V vs Li/Li⁺. Moreover, it showed an excellent cycling behavior, its specific capacity remained above 173 mA h g⁻¹ after 20 cycles, and the material did not transform into spinel structure during the electrochemical cycling according to the cyclic voltammograms and X-ray powder diffraction. The electrochemical results revealed that the doping of Y³⁺ improved the performance of LiMnO₂ considerably.     

Spezifisches Verfahren zur quantitativen CO<Subscript>2</Subscript>-Bestimmung in natürlichen Wässern

The direct determination of CO₂ is often faulty because of certain contents of the natural water. Due to this reason a specific method was worked out. The water to be analysed and distilled water are placed side by side in a gas-tight vessel with an atmosphere free of CO₂, whereby mixing is avoided but gas-exchange is possible. After adjustment to the gas-equilibration the CO₂ content of the distilled water is determined e.g. by titration, measuring of conductivity or other methods, from where the CO₂ content in the natural water is calculated.     

Narrow span coulometric analysis of neptunium

A narrow span (E _o ±0.1 V) controlled potential coulometric method has been developed for the determination of neptunium in 1M H₂SO₄ with a RSD of 0.2%. The main advantage of this method over the existing coulometric methods is that it can tolerate up to a 5-fold excess of plutonium. The method involves carrying out the electrolysis to about 97% and calculating by an iterative computation the formal electrode potential in situ, which is used to calculate the total amount present in the sample. The method consists in oxidation of all the neptunium to Np (VI) by Ce(IV), destruction of excess Ce(IV) and reduction of Np(VI) to Np(V) by NaNO₂, destruction of excess nitrite by sulfamic acid followed by coulometric titration of Np(V) to Np(VI).     

Eine coulometrische Methode zur kontinuierlichen Bestimmung von Schwefeldioxid in der Luft

Zusammenfassung Ein Gerät zur Registrierung von Schwefeldioxid in Luft, die auf der Basis einer coulometrischen Titration mit Jod beruht, wird beschrieben. Wechselstrom und zwei Platinelektroden dienen zur Kontrolle des Generatorstromes, der ein Maß für die SO₂-Konzentration darstellt.

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A coulometric method for continuous determination of sulfur dioxide in air

Summary An apparatus for the continuous determination of sulfur dioxide in air is described which works on the basis of coulometric titration by iodine. Alternating current and two platinum electrodes are used for controlling the current on the generator electrodes which is a measure of the SO₂ concentration.

     

Influence of ThO<Subscript>2</Subscript> on the photocatalytic activity of TiO<Subscript>2</Subscript>

Microcomposites consisting of TiO₂ (or Ce-doped TiO₂) and ThO₂ (0.5–2% of the TiO₂ mass) are produced by sol-gel synthesis of TiO₂ in presence of ThO₂. X-ray diffraction study reveals the effects of ThO₂ (compared to the ThO₂-free TiO₂, obtained by the same method) on the anatase interplanar distances, crystallites size and phase composition. The photocatalytic tests in presence of the composites under UV irradiation reveal an increase of the Malachite Green degradation rate constant. The effect depends on the Th relative content, temperature of annealing of the catalyst and addition of other doping agent. The highest photocatalytic activity is observed for TiO₂ obtained at 550°C and containing 1% ThO₂. The composite exhibits activity in dark, also. The presence of Ce⁴⁺ ions is not an obligatory requirement for the realization of the ThO₂ effect. The reported results suggest that the radioactivity of the Th and/or its decay products is one of the main factors responsible for the increased photocatalytic activity of TiO₂.