Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state ( ²Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: k_NCO+O(T) = (4.3_−2.2^+3.2) × 10⁻⁸ × T^{−1.14_−0.12+0.08} cm³ molecule⁻¹ s⁻¹. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant k_NCO+H = (2.2 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O₂), carbon dioxide (CO₂), molecular hydrogen (H₂), and carbon monoxide (CO), which is a dissociation product of CO₂ in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O₂ and NCO + H₂ become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.     

Electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 hydrogen storage alloy

The electrochemical properties of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 hydrogen storage alloy have been studied through chronopotentiometric, chronoamperometric and cyclic voltammogram measurements. The maximum capacity value obtained was 265 mAh g⁻¹ at rate C/6 and the capacity decrease was recorded by 1.5% after 30 cycles. The values of the hydrogen diffusion coefficient D_H obtained through cyclic volammogram and chronoamperometric techniques were, respectively, 7.01 × 10⁻⁸ cm² s⁻¹ and 4.23 × 10⁻¹¹ cm² s⁻¹.     

Synthesis and electrochemical studies of the 4 V cathode, Li(Ni2/3Mn1/3)O2

Layered Li(Ni_2/3Mn_1/3)O₂ compounds are prepared by freeze-drying, mixed carbonate and molten salt methods at high temperature. The phases are characterized by X-ray diffraction, Rietveld refinement, and other methods. Electrochemical properties are studied versus Li-metal by charge–discharge cycling and cyclic voltammetry (CV). The compound prepared by the carbonate route shows a stable capacity of 145 (±3) mAh g⁻¹ up to 100 cycles in the range 2.5–4.3 V at 22 mA g⁻¹. In the range 2.5–4.4 V at 22 mA g⁻¹, the compound prepared by molten salt method has a stable capacity of 135 (±3) mAh g⁻¹ up to 50 cycles and retains 96% of this value after 100 cycles. Capacity-fading is observed in all the compounds when cycled in the range 2.5–4.5 V. All the compounds display a clear redox process at 3.65–4.0 V that corresponds to the Ni^2+/3+–Ni^3+/4+ couple.     

Structure-conductivity relationsihp in polymer electrolytes formed by network polymers from poly[dimethylsiloxane-g- poly(ethylene oxide)] and litegum perchlorate

The combination of LiClO₄ and network polymers from poly[dimethyl-siloxane-g-poly(ethylene oxide)] has been applied to polymer electrolytes as an Li⁺ ion conductor, and the structure/conductivity relationship has been investigated. The ionic conductivity is about 10⁻⁶ S cm⁻¹ at room temperature. The polymer electrolytes form a micro-heterogeneous structure from the constituent segments, and the incorporated LiClO₄ preferentially interacts with the poly(ethylene oxide) segments. The segmental motion of poly(ethylene oxide) appears to contribute to the ionic migration, while that of poly(dimethylsiloxane) does not. Not all of the incorporated LiClO₄ functions as carrier ions.     

Electrochemical and structural characteristics of niobium(V) oxide in a rechargeable lithium battery

The discharge behaviour of T-Nb₂O₅ in various electrolytes is unaffected by the choice of solvent, but is strongly dependent on the crystal radius of the solute cation species. Thermodynamic and structural studies show that this is due to the insertion of unsolvated Li⁺ ions into the crystal lattice. The graphite content of the Nb₂O₅ electrode has a marked influence on the cycling behaviour on account of the decrease in the oxide conductivity with discharge. Furthermore, the chemical diffusion coefficient of Li⁺ ions in Nb₂O₅ is about 10⁻¹⁰ cm² s⁻¹, which is one order of magnitude smaller than that in V₂O₅ with a layered structure.     

Preparation and properties of transparent conducting zinc oxide and aluminium-doped zinc oxide films prepared by evaporating method

Undoped and aluminium-doped zinc oxide films have been prepared by thermal evaporation of zinc acetate [Zn(CH₃COO)₂ 2H₂O] and aluminium chloride [AlCl₃] onto a heated glass substrate. The structural and optoelectrical properties of the films have been studied. The effects of heat treatment for the as-deposited films in air and vaccum are investigated. Highly transparent films with conductivity as low as 2×10⁻³ Ω cm can be produced by controlling the deposition parameters. The electron carrier densities are in the range 0.2–7×10¹⁹ cm⁻³ with mobilities of 22–58 cm² V⁻¹ s⁻¹.     

Charge–discharge and high temperature reaction of LiCoO2 in ionic liquid electrolytes based on cyano-substituted quaternary ammonium cation

The charge–discharge performance of LiCoO₂ positive electrode was observed in a mixed electrolyte system consisting of two ionic liquids: cyano-substituted quaternary ammonium bis(trifluoromethane)sulfoneimide (TFSI) and a same-anion salt of 1-ethyl-3-methyl imidazolium (EMI). The positive electrode exhibited a discharge capacity rather close to the theoretical one when N,N,N,N-cyanoethyl trimethyl ammonium salt was applied. Differential scanning calorimetry (DSC) studies revealed that these electrolytes exhibited exotherm only around 260 °C, 50 °C higher than conventional carbonate-based electrolytes. This is the first attempt to reveal the thermal stability of ionic liquid electrolyte under a practical situation.     

Effect of rare earth oxides for improvement of MCFC

The solubility of rare earth metal oxides and their effect on the NiO solubility have been discussed to stabilize the cathode of molten carbonate fuel cells. The solubility of Ho, Yb, and Nd oxides were 4.4 × 10⁻⁴, 3.4 × 10⁻⁴, and 1.3 × 10⁻³ (mole fraction) at 923 K, respectively. The solubilities of NiO in (Li_0.52/Na_0.48)₂CO₃ with the saturated Ho, Yb, and Nd were 1.57 × 10⁻⁵, 1.41 × 10⁻⁵, and 9.5 × 10⁻⁶, respectively. Among these three, Nd, which has the highest solubility in the carbonates, reduced the NiO solubility most; although, the La reduced the NiO solubility more than Nd.

The logarithm of the solubility of the rare earth metal oxides has a linear relation to the Coulomb force ratio between the rare earth metal and the alkaline metal. Following this relation, the La should have the highest solubility among all the lanthanides. The basicity which NiO solubility closely relates has a linear relationship to the Coulomb force parameter of the melts. Based on these two models, the La would be the best additive to reduce the NiO solubility in Li/Na eutectic carbonate melt, among all the lanthanides.     

The effect of annealing on the electrochemical properties of Zr0.5Ti0.5Mn0.5V0.3Co0.2Ni1.1 alloy electrodes

The annealing treatment was found to result in the improvement in the cyclic stability but the degradation of discharge capacity, activation and high-rate dischargeability for Zr_0.5Ti_0.5Mn_0.5V_0.3Co_0.2Ni_1.1 alloy electrode. A lower discharge potential in the annealed alloy electrode was found owing to a more homogeneous alloy, which is consistent with the pressure–composition isotherms (P–C–T) measurement. We found that the annealed alloy also had lower and flatter pressure plateaus, and larger pressure hysteresis. At high discharge rates, the hydrogen diffusion in the bulk of the alloy was the rate-determining step. The diffusion coefficients for hydrogen in the annealed and as-cast alloys were calculated to be 1.4×10⁻¹² cm² s⁻¹ and 4.3×10⁻¹² cm² s⁻¹, respectively. The lowering of high-rate discharge capacity can be ascribed to the reason that the hydrogen diffusion coefficient is lower due to homogeneous microstructure in the annealed alloy.     

Influence of chemical modification of activated carbon surface on characteristics of supercapacitors

The influence of the change of Fermi level electrons position of activated carbon μ_C caused by chemical modification of its porous surface by Mn²⁺ ions on its capacitive characteristics in 7.6 m KOH, 4 m KI, 2 m ZnI₂ aqueous solutions is investigated in this work. The detection of adsorbed Mn²⁺ ions on the surface of activated carbon was carried out according to methods of secondary ionic mass spectrometry (SIMS). An increase in electronic density on the Fermi level of modified with Mn²⁺ activated carbon was determined with a help of X-ray photoelectron spectroscopy (XPS) data. Capacitive characteristics of the electrodes have been investigated by means of electrochemical impedance spectroscopy, computer modeling, and galvanic discharge. The correlation between electronic structure of modified activated carbon (MAC) and thermodynamic characteristics of ions of the used electrolytes is established. On the basic of the obtained experimental data, electrochemical system of the hybrid capacitor with a specific capacitance of 1740 F g⁻¹ and with a specific energy of 30 mWh g⁻¹ is developed.     

Effect of salt concentration on poly (vinyl chloride)/poly (acrylonitrile) based hybrid polymer electrolytes

Hybrid, solid polymer electrolyte films consisting of poly (vinyl chloride) (PVC), poly (acrylonitrile) (PAN) and, propylene carbonate (PC) with different concentrations of LiClO₄ are prepared by means of a using solvent-casting technique. The structure and complex formation are studied by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The temperature dependence of the ionic conductivities of the polymer films is explained in terms of a free volume model. The conductivities of PVC–PAN–LiClO₄–PC complexes are determined at different salt concentrations. The highest ionic conductivity (8.35 × 10⁻⁵ S cm⁻¹) is obtained for 8 wt.% LiClO₄ in the polymer complex at 304 K. The thermal stability of the electrolyte is examined by thermogravimetric/differential thermal analysis (TG/DTA).     

A rechargeable lithium battery employing a porous thin film of Cu3+δMo6S7.9

Porous, thin films of copper molybdenum sulfides (Cu_3+δMo₆S_7.9), that have been prepared by the technique of painting and subsequent reaction with mixed H₂/H₂S gases at 500 °C, have been used as a cathode material for lithium secondary batteries. The test cell comprised: Li/2 M LiClO₄ in PC-THF (4:6)/Cu_3+δMo₆S_7.9 (porous, thin film). The discharge reaction proceeded via the intercalation of lithium ions into the structural interstices of the cathode material.

The first discharge curve of the cell showed that the porous film could incorporate up to 18 lithium ions per formula unit. The capacity of the thin film was four times higher than that previously reported for powder or pressed-pellet electrodes. The theoretical energy density was 675 W h kg⁻¹, i.e., higher than that of TiS₂ (455 W h kg⁻¹) which is one of the best materials for high-energy lithium batteries. From X-ray diffraction studies of the lithium incorporated in the thin film at each discharge step, it is suggested that there are four incorporation reactions of lithium ions into the cathode. Finally, cycling tests have been conducted at room temperature.     

The ignition of oxetane in shock waves and oxidation in a jet-stirred reactor: An experimental and kinetic modeling study

The ignition and oxidation of oxetane have been studied in a single-pulse shock tube under reflected shock wave conditions and also in a jet-stirred reactor (JSR). These experiments cover a wide range of conditions: 1–10 atm, 0.5 ≤ φ ≤ 2.0, 800–1780 K. The ignition delays of oxetane measured in a shock tube have been used to propose an overall dependence of ignition delay time on the concentrations of each component in the gas as: τ = 10^−13.5 exp(13389/T₅)[C₃H₆O]^−0.36[O₂]^−0.59[Ar]^0.088 (units: seconds, moles per cubic decimeters, and Kelvin). Concentration profiles of the reactants, intermediates, and products of the oxidation of oxetane were measured in a JSR. A numerical model, consisting of a detailed kinetic reaction mechanism with 423 reactions (most of them reversible) of 63 species describes the ignition of oxetane in reflected shock waves and its oxidation in a jet-stirred reactor. Fairly good agreement between the observations and the model was obtained. The major reaction paths have been identified through detailed kinetic modeling.     

Long-term thermal cycling of Phlogopite mica-based compressive seals for solid oxide fuel cells

Planar solid oxide fuel cells (SOFCs) require sealants to function properly in harsh environments at elevated temperatures. The SOFC stacks are expected to experience multiple thermal cycles (perhaps thousands of cycles for some applications) during their lifetime service in stationary or transportation applications. As a result, thermal cycle stability is considered a top priority for SOFC sealant development. In previous work, we have developed a hybrid mica-based compressive seal with very low leak rates of 2–4 × 10⁻² to 10⁻³ sccm cm⁻¹ at 800 °C, and showed stable leak rates over limited thermal cycles. In this paper we present results of long-term thermal cycle testing (>1000 thermal cycles) of Phlogopite mica-based compressive seals. Open-circuit voltage (OCV) was measured on a 2 in. × 2 in. 8-YSZ plate with the hybrid Phlogopite mica seals during thermal cycling in a dual environment (2.75% H₂/Ar versus air). During two long-term cycling tests, the measured OCVs were found to be consistent with the calculated Nernst voltages. The hybrid mica seal showed excellent thermal cycle stability over 1000 thermal cycles and can be considered a strong candidate for SOFC applications.     

Electrochemistry of poly(vinylferrocene) modified electrodes in aqueous acidic media

A cyclic voltammetric study of the electrochemistry and chemical stability of the poly(vinylferrocene) (PVFc) redox couple, coated on a gold substrate, in aqueous solutions of H₂SO₄, HClO₄ and HCl was carried out. It was found that the anodic peak potential (E_pa) did not depend on the acid concentration in the range (1.0 × 10⁻² to 1.0 × 10⁻⁷ mol L⁻¹). However, the E_pa values shifted linearly to less positive potentials when investigated in more concentrated acid solutions in the range 1–5 mol L⁻¹. The slope of the E_pa versus acid concentration graph was found to be in the order H₂SO₄ > HCl > HClO₄. In this regard PVFc behaved very similar to 1,1′-bis(11-mercaptoundecyl)ferrocene (Fc(C₁₁SH)₂) except for its chemical stability. In H₂SO₄ media the PVFc was found to be much less stable than 1,1′-Fc(C₁₁SH)₂. The dependence of E_pa on acid concentration could be used to monitor state of charge of lead-acid batteries. However, for this application Fc(C₁₁SH)₂ would be a better choice because of its superior chemical stability.     

Outward phase change in a cylindrical annulus with axial fins on the inner tube

A theoretical analysis is presented for the phase change process occurring in a cylindrical annulus in which rectangular, uniformly spaced axial fins, spanning the annulus, are attached to the inner isothermal tube, while the outer tube is kept adiabatic. The model assumes conduction to be the only mode of heat transfer. The governing equations are solved by finite-difference methods. The time-wise evolution of the interface profile, phase-change fraction and energy stored/discharged and the effect of all the nine prescribable parameters are presented here. Based on the analysis a working formula VF = 1.1275 (Fo Ste T_f)^0.624 (N)^0.028 (L)^−1.385(W)^−0.049 is suggested for engineering design purposes.     

The ZEBRA electric vehicle battery: power and energy improvements

Vehicle trials with the first sodium/nickel chloride ZEBRA batteries indicated that the pulse power capability of the battery needed to be improved towards the end of the discharge. A research programme led to several design changes to improve the cell which, in combination, have improved the power of the battery to greater than 150 W kg⁻¹ at 80% depth of discharge. Bench and vehicle tests have established the stability of the high power battery over several years of cycling. The gravimetric energy density of the first generation of cells was less than 100 Wh kg⁻¹. Optimisation of the design has led to a cell with a specific energy of 120 Wh kg⁻¹ or 86 Wh kg⁻¹ for a 30 kWh battery. Recently, the cell chemistry has been altered to improve the useful capacity. The cell is assembled in the over-discharged state and during the first charge the following reactions occur: at 1.6 V: Al+4NaCl=NaAlCl₄+3Na; at 2.35 V: Fe+2NaCl=FeCl₂+2Na; at 2.58 V: Ni+2NaCl=NiCl₂+2 Na. The first reaction serves to prime the negative sodium electrode but occurs at too low a voltage to be of use in providing useful capacity. By minimising the aluminium content more NaCl is released for the main reactions to improve the capacity of the cell. This, and further composition optimisation, have resulted in cells with specific energies in excess of 140 Wh kg⁻¹, which equates to battery energies>100 Wh kg⁻¹. The present production battery, as installed in a Mercedes Benz A class electric vehicle, gives a driving range of 205 km (128 miles) in city and hill climbing. The cells with improved capacity will extend the practical driving range to beyond 240 km (150 miles).     

Electrical and optical properties of F-doped textured SnO2 films deposited by APCVD

This paper presents the structural, electrical and optical properties of transparent conducting F-doped textured SnO₂ films prepared by atmosphere pressure chemical vapour deposition (APCVD). Polycrystalline SnO₂:F films having a variable preferred orientation have been obtained with resistivity as low as 5 × 10⁻⁴ Ωcm, with carrier concentrations between 3.5 × 10²⁰ and 7 × 10²⁰ cm⁻³, and Hall mobilities from 15.7 to 20.1 cm²/V/s. The average transmittance (including diffusion transmittance) is as high as 94% in the wavelength range of the visible spectrum and the maximum infrared reflectance reaches 92% for a film 655 nm thick. The figure of merit ƒ_TC = T10/_sh, (7.12 × 10⁻² S) of these films is the highest amongst the results reported on doped SnO₂ films.     

Production of hydrogen via methane reforming using atmospheric pressure microwave plasma

In this paper, results of hydrogen production via methane reforming in the atmospheric pressure microwave plasma are presented. A waveguide-based nozzleless cylinder-type microwave plasma source (MPS) was used to convert methane into hydrogen. Important advantages of the presented waveguide-based nozzleless cylinder-type MPS are: stable operation in various gases (including air) at high flow rates, no need for a cooling system, and impedance matching. The plasma generation was stabilized by an additional swirled nitrogen flow (50 or 100 l min⁻¹). The methane flow rate was up to 175 l min⁻¹. The absorbed microwave power could be changed from 3000 to 5000 W. The hydrogen production rate and the corresponding energy efficiency in the presented methane reforming by the waveguide-based nozzleless cylinder-type MPS were up to 255 g[H₂] h⁻¹ and 85 g[H₂] kWh⁻¹, respectively. These parameters are better than those typical of the conventional methods of hydrogen production (steam reforming of methane and water electrolysis).     

Capacity fading with oxygen loss for manganese spinels upon cycling at elevated temperatures

The nominal LiMn₂O₄ and Li-doped spinels with different oxygen stoichiometry were prepared and investigated for capacity fading upon cycling at elevated temperatures. The discharge plateau at 3.2 V originating from oxygen defects in manganese spinels is observed to grow very quickly to nearly a maximum scale in initial 15 cycles at 60 °C. Meanwhile, the majority of capacity fading is lost. Therefore, the quick capacity fading in the initial stage is associated with the increase of oxygen deficiencies or oxygen loss upon cycling. It is proposed that the oxygen loss is originated from the decomposition of instable spinel phases that containing little Li cations on the 8a sites ([□₁]_8a[Mn_2−x]_16d[O_4−δ□_δ]_32e, etc.), which are formed upon charging to the upper voltage limit. This phenomenon is much severe for nominal LiMn₂O₄ spinels with oxygen deficiencies. After partial substitution of Mn with Li, part of the Li cations on the 8a sites will be retained upon charging to the upper voltage limit. Thereafter, the cycling performance can be improved for the stabilized spinel phases formed upon charging.