X-ray photoelectron spectroscopy study of anodically oxidized SIMFUEL surfaces

The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range −500 to +500 mV (versus SCE). Cathodically cleaned UO₂ specimens were anodically oxidized for 1 h and subsequently analyzed by XPS. Using published binding energies, the U (4f_7/2) and O (1s) peaks were resolved into contributions from U^IV, U^V, U^VI, O^II, OH⁻ and H₂O. It was shown that over the potential range −500 to approximately +50 mV a thin surface layer of U^IV/U^V oxide (UO_2+x) formed. At more positive potentials, a U^VI hydrated oxide (UO₃·yH₂O) was deposited on the electrode surface. At very positive potentials (≥400 mV) the rapid anodic formation and hydrolysis of UO₂²⁺ led to local acidification in pores in the deposited UO₃·yH₂O layer and its subsequent re-dissolution.     

Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions

The reaction of H₂O₂ on SIMFUEL electrodes has been studied electrochemically and under open circuit conditions in 0.1 mol l⁻¹ NaCl (pH 9.8). The composition of the oxidized UO₂ surface was determined by X-ray photoelectron spectroscopy (XPS). Peroxide reduction was found to be catalyzed by the formation of a mixed U^IV/U^V (UO_2+x) surface layer, but to be blocked by the formation of U^VI (UO₂²⁺) species on the electrode surface. The formation of this U^VI layer blocks both H₂O₂ reduction and oxidation, thereby inhibiting the potentially rapid H₂O₂ decomposition process to H₂O and O₂. Decomposition is found to proceed at a rate controlled by desorption or reduction of the adsorbed O₂ species. Reduction of O₂ is coupled to the slow oxidative dissolution of UO₂ and formation of a corrosion product deposit of UO₃·yH₂O.     

Electrochemical synthesis and characterization of mixed molybdenum-rhenium oxides

The electrodeposition of Mo_xRe_1−xO_y films (0.6 ≤ x ≤ 1) on indium-tin oxide (ITO) coated glass substrates from acidic peroxo-polymolybdo-perrhenate solutions is described. Trends in film growth were established as a function of potential from +0.4 V to −0.7 V vs Ag/AgCl by analyzing the composition and stoichiometry of the deposit using inductively coupled plasma mass spectrometry (ICPMS) and X-ray photoelectron spectroscopy (XPS). These experiments show that the concentration of rhenium increases linearly with the deposition potential and that the deposits are mixed-valent containing up to five different metal oxidation states (i.e., Mo^IV, Mo^V, Mo^VI, Re⁰, Re^IV). Electroanalytical techniques were used to explore the deposition mechanism, including chronocoulometry, cyclic voltammetry, spectroelectrochemistry, and electrochemical quartz crystal nanogravimetry (EQCN). At potentials positive to −0.26 V, perrhenate (Re^VIIO₄⁻) behaves as a redox mediator to accelerate the deposition of a mixed-valent molybdenum oxide, but at more negative potentials mixed molybdenum-rhenium oxides are produced.     

Electrochemical characteristics of cobalt-substituted lithium nickel oxides synthesized from lithium hydro-oxide and nickel and cobalt oxides

LiNi_1−yCo_yO₂ (y=0.1, 0.3 and 0.5) were synthesized by solid state reaction method at 800 °C and 850 °C from LiOH·H₂O, NiO and Co₃O₄ as starting materials. The electrochemical properties of the synthesized LiNi_1−yCo_yO₂ were investigated. As the content of Co decreases, particle size decreases rapidly and particle size distribution gets more homogeneous. When the particle size is compared at the same composition, the particles synthesized at 850 °C are larger than those synthesized at 800 °C. LiNi_0.7Co_0.3O₂ synthesized at 850 °C has the largest intercalated and deintercalated Li quantity Δx among LiNi_1−yCo_yO₂ (y=0.1, 0.3 and 0.5). LiNi_0.7Co_0.3O₂ synthesized at 850 °C has the largest first discharge capacity (178 mAh/g), followed by LiNi_0.7Co_0.3O₂ (162 mAh/g) synthesized at 800 °C. LiNi_0.7Co_0.3O₂ synthesized at 800 °C has discharge capacities of 162 and 125 mAh/g at n=1 and n=5, respectively.     

Solubility studies and thermophysical properties of uranium–neodymium mixed oxides system

Uranium–neodymium mixed oxides (U_1−yNd_y)O_2±x (y=0.2–0.85) were prepared by citrate gel-combustion and characterized by XRD. Single phase fluorite structure was observed up to y=0.80. For solid solutions with y>0.80 additional lines pertaining to hexagonal neodymium oxide were observed. Lattice thermal expansion of these samples was investigated by using high temperature X-ray diffraction (HTXRD). The coefficients of thermal expansion for (U_1−yNd_y)O_2±x for y=0.2, 0.4, 0.6, and 0.8 in the temperature range 298–1973 K were found to be 16.46, 16.64, 16.79, and 16.89×10⁻⁶ K⁻¹, respectively. Heat capacity and enthalpy increment measurements were carried out by using DSC and drop calorimetry in the temperature range 298–800 K and 800–1800 K respectively. The C_p,m values at 298 K for (U_1−yLa_y)O_2±x (y=0.2, 0.4, 0.6, and 0.8) are 63.4, 64.3, 61.8, and 58.9 J K⁻¹ mol⁻¹ respectively.     

Oxygen evolution anodes composed of anodically deposited Mn-Mo-Fe oxides for seawater electrolysis

Preparation of anodes for oxygen evolution in seawater electrolysis was carried out. Manganese-molybdenum double oxides, Mn_1−xMo_xO_2+x, prepared by anodic deposition from MnSO₄-Na₂MoO₄ solutions showed the 100% oxygen evolution efficiency at a current density of 1000 A m⁻² in 0.5 M NaCl at 30 °C and pH 12, but an increase in solution temperature resulted in dissolution of the oxides as molybdate and permanganate ions. In order to increase the stability of the electrodes at higher temperatures the addition of iron to the manganese-molybdenum oxides was performed by anodic deposition in MnSO₄-Na₂MoO₄-FeNH₄(SO₄)₂ solutions. The electrodes thus prepared showed the 100% oxygen evolution efficiency at 1000 A m⁻² in 0.5 M NaCl at 30-90 °C, when proper amounts of molybdenum and iron were contained. The iron addition also enhanced the oxygen evolution efficiency. The electrodes were not composed of oxide mixtures but triple oxides, Mn_1−x−yMo_xFe_yO_2+x−0.5y, consisting of Mn⁴⁺, Mo⁶⁺ and Fe³⁺. The formation of the triple oxides seemed responsible for enhancement of both oxygen evolution efficiency and stability.     

The anodic dissolution of SIMFUEL (UO2) in slightly alkaline sodium carbonate/bicarbonate solutions

The corrosion of nuclear fuel under waste disposal conditions is likely to be influenced by the bicarbonate/carbonate content of the groundwater since it increases the solubility of the U^VI corrosion product, [UO₂]²⁺. As one of the half reactions involved in the corrosion process, the anodic dissolution of SIMFUEL (UO₂) has been studied in bicarbonate/carbonate solutions (pH 9.8) using voltammetric and potentiostatic techniques and electrochemical impedance spectroscopy. The reaction proceeds by two consecutive one electron transfer reactions (U^IV → U^V → U^VI). At low potentials (≤250 mV (vs. SCE) the rate of the first electron transfer reaction is rate determining irrespective of the total carbonate concentration. At potentials >250 mV (vs. SCE) the formation of a U^VIO₂CO₃ surface layer begins to inhibit the dissolution rate and the current becomes independent of potential indicating rate control by the chemical dissolution of this layer.     

Single crystal nanobelts of V3O7·H2O: A lithium intercalation host with a large capacity

In this study, single crystal V₃O₇·H₂O nanobelts were successfully synthesized using a simple hydrothermal route, in which templates or catalysts were absent. The synthesized V₃O₇·H₂O nanobelts are highly crystalline and have lengths up to several tens of micrometers. The width and thickness of the nanobelts are found to be about 30-50 and 30 nm, respectively. A lithium battery using V₃O₇·H₂O nanobelts as the positive electrode exhibits a high initial discharge capacity of 409 mAh g⁻¹, corresponding to the formation of Li_xV₃O₇·H₂O (x = 4.32). Such a high degree of electrochemical performance is attributed to the intrinsic properties of the single-crystalline V₃O₇·H₂O nanobelts.     

Investigation of the surface properties and the hydrogen evolution reaction, HER, at thermal rhodium oxide electrodes

A systematic investigation was conducted of the surface properties and the HER at electrodes of nominal composition Ti/Rh_xTi_(1−x)O_y prepared by thermal decomposition (T_cal: 500 °C; t_cal: 2 h; O₂ flux: 5 dm³ min⁻¹) from salt precursor solutions dissolved in 6.0 mol dm⁻³ HNO₃. Films were characterized ex situ by SEM, EDX, XPS and XRD and in situ by open circuit potential measurements and CV. The electrochemical behaviour was investigated by CV as function of the anodic, E_λ,a, and cathodic, E_λ,c, switching potentials showing the Rh surface oxidation states strongly depend on these experimental variables. Surface Rh-sites are reduced to metallic rhodium in the cathodic potential region while higher oxidation states (I-III) are formed at more positive potentials (E ≥ 0.5 V/RHE). Hydrogen adsorption and desorption peaks as well as a short double layer charging region are observed at intermediate potential values. The HER was investigated by Tafel coefficients and reaction order with respect to H⁺ as function of nominal Rh-content.     

Li-intercalation behaviour of vanadium oxide thin film prepared by thermal oxidation of vanadium metal

In order to produce thin films of crystalline V₂O₅, vanadium metal was thermally oxidised at 500 °C under oxygen pressures between 250 and 1000 mbar for 1-5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V₂O₅ films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O₂ at 500 °C resulted in a surface oxide film composed of V₂O_5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V₂O₅ oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 μA/cm². The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10⁻¹³ cm² s⁻¹, respectively. Beneath the V₂O₅ layer, lower oxides (mainly VO₂) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V⁴⁺ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V₂O₅ and V₆O₁₃ was found in the oxide film.     

Assessment of Ba0.5Sr0.5Co1−yFeyO3−δ (y = 0.0-1.0) for prospective application as cathode for IT-SOFCs or oxygen permeating membrane

The influence of iron doping level in Ba_0.5Sr_0.5Co_1−yFe_yO_3−δ (y = 0.0-1.0) (BSCF) oxides on their phase structure, oxygen nonstoichiometry, electrical conductivity, performance as symmetrical cell electrode and oxygen permeating membranes was systematically investigated. A cubic perovskite structure was observed for all the compositions with the presence of iron. The increase of iron doping level resulted in the decrease of the lattice constant, room-temperature oxygen nonstoichiometry, total electrical conductivity, and the increase of area specific resistance (ASR) as cathode with samaria doped ceria electrolyte. However, promising cathode performance with an ASR as low as 0.613 Ω cm² was still obtained at 600 °C for Ba_0.5Sr_0.5FeO_3−δ (BSF). The ceramic membranes composing of BSCF with various iron doping level are all oxygen semi-permeable at elevated temperatures. The increase of iron doping level resulted in the decrease of oxygen permeation flux from JO₂ = 2.28 μmol cm⁻² s⁻¹ (STP) for Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) to ∼0.45 μmol cm⁻² s⁻¹ (STP) at 900 °C for BSF (y = 1.0) with the same membrane thickness of 1.1 mm, alongside with the change of the rate-determination step from the oxygen surface exchange to the slow oxygen bulk diffusion. The formation of composite oxide with a proper electronic conducting phase and the thin film technology are important for their prospective application as cathode in IT-SOFCs and oxygen permeating membrane, respectively.     

Effects of Zn or Ti substitution for Ni on the electrochemical properties of LiNiO2

LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05, and 0.1) were synthesized with a solid-state reaction method by calcination at 750 °C for 30 h under oxygen stream after preheating at 450 °C for 5 h in air. LiNi_0.995Zn_0.005O₂ among the Zn-substituted samples and LiNi_0.995Ti_0.005O₂ among the Ti-substituted samples showed the best electrochemical properties. For similar values of y, LiNi_1−yTi_yO₂ had in general better electrochemical properties than LiNi_1−yZn_yO₂. Electrochemical properties seem to be closely related to R-factor but less related to I_0 0 3/I_1 0 4 value. In the FT-IR absorption spectra of LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05 and 0.1), Li₂CO₃ was detected even if it is not observed from XRD pattern, with the samples LiNi_1−yZn_yO₂ (y = 0.05 and 0.1) showing Li₂ZnO₂ additionally. The smaller cation mixing of the Ti-substituted samples is considered to lead to their better electrochemical properties than the Zn-substituted samples.     

The key factor determining the anodic deposition of vanadium oxides

This work demonstrates that anodic deposition of vanadium oxide (denoted as VO_x·nH₂O) can be considered as the chemical co-precipitation of V⁵⁺ and V⁴⁺ oxy-/hydroxyl species and the accumulation of V⁵⁺ species at the vicinity of electrode surface is the key factor for the successful anodic deposition of VO_x·nH₂O at a potential much more negative than the equilibrium potential of the oxygen evolution reaction (OER). The results of in situ UV-vis spectra show that the V⁴⁺/V⁵⁺ ratio near the electrode surface can be controlled by varying the applied potential, leading to different, three-dimensional (3D) nanostructures of VO_x·nH₂O. The accumulation of V⁵⁺ species due to V⁴⁺ oxidation at potentials ≥0.4 V (vs. Ag/AgCl) has been found to be very similar to the phenomenon by adding H₂O₂ in the deposition solution. The X-ray photoelectron spectroscopic (XPS) results show that all VO_x·nH₂O deposits can be considered as aggregates consisting of mixed V⁵⁺ and V⁴⁺ oxy-/hydroxyl species with the mean oxidation state significantly increasing with the applied electrode potential.     

Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

TPR,XRD and XPS characterisation of ceria-based materials synthesized by freeze-drying precursor method

A freeze-drying precursor method was used to obtain submicrometric powders of ceria-based materials such as Ce_1−xGd_xO_2−δ (x=0, 0.01, 0.05, 0.10 and 0.20), 80%CeO₂–20%ZrO₂, 80%CeO₂–20%Al₂O₃ and (1−y)Ce_0.99Gd_0.01O_2−δ– (y)Al₂O₃ (y=0.01, 0.02, 0.05, 0.10 and 0.30) at temperatures as low as 400 °C. The phase formation and evolution with the temperature was studied by X-ray diffraction (XRD). Also, temperature programmed reduction (TPR) was performed to investigate the reducibility of the ceramic powders. It was observed that after reduction of the ceria-based materials the fluorite structure of the samples was retained. The TPR profiles showed two peaks which are associated to the surface and bulk ceria reduction processes. Likewise, after the TPR measurements the resulting powders have showed high phase stability and reproducibility. XPS results confirmed the reduction of Ce⁴⁺ to (Ce³⁺+Ce⁴⁺) ratio with alumina doping.     

Enhanced luminescence of novel Ca3B2O6:Dy phosphors by Li-codoping for LED applications

The Ca_3−xB₂O₆:xDy³⁺ (0.0 ≤ x ≤ 0.105) and Ca_2.95−yDy_0.05B₂O₆:yLi⁺ (0 ≤ y ≤ 0.34) phosphors were synthesized at 1100 °C in air by solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation (PLE) and photoluminescence (PL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm is due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions upon 350 nm excitation is observed at 480 nm (blue) due to the ⁴F_9/2 → ⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2 → ⁶H_13/2 transitions and a weak 660 nm (red) due to ⁴F_9/2 → ⁶H_11/2 emissions, respectively. The optimal PL intensity of the Ca_3−xB₂O₆:xDy³⁺ phosphors is found to be x = 0.05. Moreover, the PL results from Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors show that Dy³⁺ emissions can be enhanced with the increasing codopant Li⁺ content till y = 0.22. By simulation of white light, the CIE of the investigated phosphors can be tuned by varying the content of Li⁺ ions, and the optimal CIE value (0.300, 0.298) is realized when the content of Li⁺ ions is y = 0.22. All the results imply that the Ca_2.95−yDy_0.05B₂O₆:yLi⁺ phosphors could be potentially used as white LEDs.     

Ba1−xPrxCo1−yFeyO3−δ as cathode materials for low temperature solid oxide fuel cells

Ba_1−xPr_xCo_1−yFe_yO_3−δ (BPCF) perovskite oxides have been synthesized and investigated as cathode materials for low temperature solid oxide fuel cells (LT-SOFCs). Compared with those of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and Sm_0.5Sr_0.5CoO₃ (SSCo) cathode materials, BPCF has a lower polarization resistance at decreased temperatures. In particular, Ba_0.5Pr_0.5Co_0.8Fe_0.2O_3−δ showed the lowest polarization loss among the different compositions as a cathode material for LT-SOFCs. The area specific resistance (ASR) of Ba_0.5Pr_0.5Co_0.8Fe_0.2O_3−δ as a cathode material is 0.70 and 0.185 Ω cm² at 500 °C and 550 °C, respectively. The maximum power density of the cell BPCF/SDC/Ni-SDC with humidified hydrogen as fuel and air as oxidant reaches 860 mW cm⁻² at 650 °C.     

Synthesis and electrical conductivity of La-Sr-X-Mg-O (X = Ti, Zr, Al) perovskite solid solution

Perovskite solid solutions of (La_0.6Sr_0.4)(X_1−yMg_y)O_3−δ (X = Ti, Zr, Al) were prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders were sintered in air at 1000-1500 °C for 1-36 h. Single phase solid solutions were produced in the compositions of (La_0.6Sr_0.4)(Zr_0.6Mg_0.4)O_3−δ and (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ where (3 − δ) < 3. For the compositions of X = Ti and Zr for y = 0.1 where (3 − δ) > 3, two phases including perovskite solid solution were produced at 1400-1500 °C. The stability of perovskite solid solution was closely related to the fraction of lattice oxygen atom (3 − δ). A relatively high conductivity was measured for (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ (σ = 4.15 × 10⁻⁴ S/cm at 600 °C, activation energy 113.4 kJ/mol). The influence of fraction of oxide ion vacancy on the activation energy was small for δ = 0.1-0.3 of perovskite solid solution.     

Electrocatalyst materials for fuel cells based on the polyoxometalates [PMo(12 − n)VnO40] (n = 0-3)

We report on the use of the polyoxometalate acids of the series [PMo_(12 − n)V_nO₄₀]^(3 + n)− (n = 0-3) as electrocatalysts in both the anode and the cathode of polymer-electrolyte membrane (PEM) fuel cells. The heteropolyacids were incorporated as catalysts in a commercial gas diffusion electrode based on Vulcan XC-72 carbon which strongly adsorbed a low loading of the catalyst, ca. 0.1 mg/cm². The moderate activity observed was independent of the number of vanadium atoms in the polyoxometalate. In the anode the electrochemistry is dominated by the V^3+/4+ couple. With a platinum reference wire in contact with the anode, polarization curves are obtained withV_OC of 650 mV and current densities of 10 mA cm⁻² at 100 mV at 80 °C. These catalysts showed an order of magnitude more activity on the cathode after moderate heat treatment than on the anode,V_OC = 750 mV, current densities of 140 mA cm⁻² at 100 mV. The temperature dependence of the catalysts was also investigated and showed increasing current densities could be achieved on the anode up to 139 °C and the cathode to 100 °C showing the potential for these materials to work at elevated temperatures.