Study the effects of mechanical activation on Li–N–H systems with H and Li solid-state NMR

To gain insight into the effects of mechanical activation (MA) on the hydrogen desorption of the lithium amide (LiNH₂) and lithium hydride (LiH) mixture, LiNH₂ and LiH + LiNH₂ were mechanically activated by high-energy ball milling. The formed products were studied with in situ ¹H and ⁶Li nuclear magic angle spinning (MAS) magnetic resonance (NMR) spectroscopy from ambient temperature to 180 °C. Up-field chemical shift was observed in ⁶Li MAS NMR spectra with increased milling time, indicating that average local electronic structure around Li nuclei was modified during MA. ¹H MAS NMR was used to dynamically probe ammonia release from the activated LiNH₂ at temperature as low as 50 °C. In the case of activated LiH + LiNH₂ mixtures, the ¹H MAS NMR results implied that MA enhanced the dehydrogenation reaction of LiNH₂ + LiH = Li₂NH + H₂.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Sm0.5Sr0.5CoO3 + Sm0.2Ce0.8O1.9 composite cathode for cermet supported thin Sm0.2Ce0.8O1.9 electrolyte SOFC operating below 600 °C

The cathode is a key component in low temperature solid oxide fuel cells. In this study, composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSC) + 25 wt.% Sm_0.2Ce_0.8O_1.9 (SDC), was applied on the cermet supported thin SDC electrolyte cell which was fabricated by tape casting, screen-printing, and co-firing. Single cells with the composite cathodes sintered at different temperatures were tested from 400 to 650 °C. The best cell performance, 0.75 W cm⁻² peak power operating at 600 °C, was obtained from the 1050 °C sintered cathode. The measured thin SDC electrolyte resistance R_s was 0.128 Ω cm² and total electrode polarization R_p(a + c) was only 0.102 Ω cm² at 600 °C.     

Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures

Iron offers the possibility of transformation of a syngas or gaseous hydrocarbons into hydrogen by a cycling process of iron oxide reduction (e.g. by hydrocarbons) and release of hydrogen by steam oxidation. From the thermodynamic and chemical equilibrium point of view, the reduction of magnetite by hydrogen, CO, CH₄ and a model syngas (mixtures CO + H₂ or H₂ + CO + CO₂) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe₃O₄ to iron at temperatures 400–800 K but also on the possible formation of undesired soot, Fe₃C and iron carbonate as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of magnetite at low temperatures requires a relatively high H₂/H₂O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe₃C formation. At lower temperatures and higher CO₂ concentrations in the reducing gas, the possibility of FeCO₃ formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe₃C and FeCO₃ in the iron after the reduction step. Magnetite reduction is the more difficult stage in the looping process. Pressurized conditions during the reduction step will enhance formation of soot and carbon containing iron compounds.     

Electrical properties of CuInSe2 films prepared by evaporation of Cu2Se and In2Se3 compounds

We prepared CuInSe₂ films by evaporating In₂Se₃ and Cu₂Se compounds instead of elemental sources. The resulting CuInSe₂ film grown at 680°C had a smooth and dense microstructure with the grain size of 2 3 μm. But the CuInSe₂ films were Cu-rich, with a low resistivity of about 0.1 Ω cm. So we conducted H₂ post annealing to control the electrical resistivity and composition of CuInSe₂ films. In a H₂ atmosphere, the resistivity increased to about 100 Ω cm by annealing at 350°C for 1 h. The resistivity decreased again when the annealing temperature was above 350°C. This resistivity change might be related to the contents of Cu, In, Se atoms and the valency states of Cu and In ions in the films. We discussed the reason of resistivity change caused by H₂ post annealing in this paper.     

Proton conductivity and chemical stability of Li2SO4 based electrolyte in a H2S–air fuel cell

Proton conductivity of Li₂SO₄-Al₂O₃ (LA) based electrolyte was determined under non-reducing dynamic conditions using current interruption technique. The performance of LA as electrolyte has been examined at 600 °C in a H₂S fuel cell with MoS₂-NiS as anode catalyst and NiO as cathode catalyst. XRD and XPS results show that Li₂SO₄ is not stable when heated in pure H₂S as it is reduced to Li₂S by hydrogen produced in equilibrium amounts from the thermal decomposition of H₂S. In contrast, under dynamic operation in a H₂S fuel cell the concentration of H₂ is much lower, the reduction reaction does not occur and, surprisingly, Li₂SO₄ is a chemically stable electrolyte.     

Investigation of a eutectic mixture of sodium acetate trihydrate and urea as latent heat storage

In this paper, the pseudobinary system CH₃COONa·3H₂O---CO(NH₂)₂ is studied by means of differential scanning calorimetry (DSC). Its eutectic mixture is found to melt congruently at 30°C, its heat of fusion is 200.5 J/g, considering the temperature of phase change and its heat storage capacity. This eutectic mixture is an excellent material for latent heat storage of solar energy.     

Method of gasification in IGCC system

A coal gasifier is designed to operate at the temperature range of 1200–1300 °C. The 1200 °C sets the lower limit to the carbon reforming efficiency of the high temperature reformer, and the 1300 °C is the lower limit of the fluid temperature of coal slags, below which they may be collected as non-fluid slag. The gasifier is connected to two syngas burners where a portion of product syngas is combusted with O₂ gas and produce ultra hot H₂O and CO₂ gases, these two gases enter into the gasifier and maintain the gasifier temperature at above 1200 °C and reform carbon into syngas. The temperature of the gasifier is controlled by the flow of O₂ gas into the syngas burner, where O₂ gas is completely consumed and none left to enter into the gasifier. This removes any possibility of forming oxidated products, and compressed CO₂ gas spray coal powder into the gasifier column and non-fluid slag is collected at the bottom. A higher level integration of oxidation–reduction cycle is shown for a IGCC system, wherein the exhaust gas of syngas turbine drives the reduction reaction of coal gasification.

A smooth and uniform temperature control within the gasifier assures high efficiency of carbon reforming and quality of product syngas. Conventional Lurgi gasifier relies on its large heat capacity and accumulating coal slag along the inner walls of the gasifier has made the gasifier bigger, lately as large as a three story building. The gasifier of the present design is constructed much smaller in its size, but with greater reforming efficiency.     

A comprehensive study of hydrogen production from ammonia borane via PdCoAg/AC nanoparticles and anodic current in alkaline medium: experimental design with response surface methodology

In this paper, the optimization of hydrogen (H₂) production by ammonia borane (NH₃BH₃) over PdCoAg/AC was investigated using the response surface methodology. Besides, the electro-oxidation of NH₃BH₃ was determined and optimized using the same method to measure its potential use in the direct ammonium boran fuel cells. Moreover, the ternary alloyed catalyst was synthesized using the chemical reduction method. The synergistic effect between Pd, Co and Ag plays an important role in enhancement of NH₃BH₃ hydrolysis. In addition, the support effect could also efficiently improve the catalytic performance. Furthermore, the effects of NH₃BH₃ concentration (0.1–50 mmol/5 mL), catalyst amount (1–30 mg) and temperature (20°C–50°C) on the rate of H₂ production and the effects of temperature (20°C–50°C), NH₃BH₃ concentration (0.05–1 mol/L) and catalyst amount (0.5–5 µL) on the electro-oxidation reaction of NH₃BH₃ were investigated using the central composite design experimental design. The implementation of the response surface methodology resulted in the formulation of four models out of which the quadratic model was adjudged to efficiently appropriate the experimental data. A further statistical analysis of the quadratic model demonstrated the significance of the model with a p-value far less than 0.05 for each model and coefficient of determination (R²) of 0.85 and 0.95 for H₂ production rate and NH₃BH₃ electrroxidation peak current, respectively.     

Structural, magnetic and electrochemical properties of lithium iron orthosilicate

We report the structural and electronic characterization of Li₂FeSiO₄ synthesized by solid-state reaction. X-ray diffraction, Raman scattering, Fourier transform infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and magnetization measurements are analyzed. Magnetic susceptibility experiments give evidence that Li₂FeSiO₄ powders possess an antiferromagnetic ordering below T_N = 25 K due to long range Fe–O–Li–O–Fe interactions. Analysis of the paramagnetic region giving the Curie–Weiss parameters θ_p = −93.5 K and C_p = 4.13 emu K mol⁻¹ shows the divalent state of Fe cations. Electron paramagnetic resonance experiments confirm this electronic configuration. Electrochemical measurements were carried out in lithium cells with LiTFSI in a poly(ethylene oxide) (PEO) polymer electrolyte at 80 °C. The resulting cyclic voltammogram indicates a stable structure for the first cycle with redox peaks at 2.80 and 2.74 V versus Li⁰/Li⁺.     

A screen-printed Ce0.8Sm0.2O1.9 film solid oxide fuel cell with a Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode

Screen-printing technology was developed to fabricate Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte films onto porous NiO–SDC green anode substrates. After sintering at 1400 °C for 4 h, a gas-tight SDC film with a thickness of 12 μm was obtained. A novel cathode material of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was subsequently applied onto the sintered SDC electrolyte film also by screen-printing and sintered at 970 °C for 3 h to get a single cell. A fuel cell of Ni–SDC/SDC (12 μm)/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ provides the maximum power densities of 1280, 1080, 670, 370, 180 and 73 mW cm⁻² at 650, 600, 555, 505, 455 and 405 °C, respectively, using hydrogen as fuel and stationary air as oxidant. When dry methane was used as fuel, the maximum power densities are 876, 568, 346 and 114 mW cm⁻² at 650, 600, 555 and 505 °C, respectively. The present fuel cell shows excellent performance at lowered temperatures.     

Effect of Co content on performance of LiAl1/3−xCoxNi1/3Mn1/3O2 compounds for lithium-ion batteries

Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were studied via the combination of computational and experimental approach. The calculated voltage curve of LiNi_1/3Al_1/3Mn_1/3O₂ compound is presented, indicating it is of great potential for a cathode material of lithium-ion batteries. Unfortunately, it was found that the LiNi_1/3Al_1/3Mn_1/3O₂ compound without impurity phase could not be synthesized via a sol–gel process. To obtain a layered compound without impurity phase, partial of Al is replaced by Co in LiNi_1/3Al_1/3Mn_1/3O₂ compound in this study. Layered LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ (0  x  1/3) compounds were synthesized via sol–gel reaction at 900 °C under a oxygen stream. Single phase of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ in 1/6  x  1/3 region could be prepared successfully. The discharge capacity and conductivity increased with an increase in the Co-substitution content. The enhancement of the conductivity and phase purity by the introduction of Co content shows profound influence on the performance of the LiAl_1/3−xCo_xNi_1/3Mn_1/3O₂ compounds.     

Importance of the band gap energy and flat band potential for application of modified TiO2 photoanodes in water photolysis

The influence on water photolysis of two important parameters of the electronic structure of photocatalytic semiconductors: the forbidden band gap, E_g, that decides about the absorption spectrum and the flat band potential, V_Fb, that affects the recombination probability, was studied. The photoelectrochemical experiments were performed in a three-electrode cell PEC with a TiO₂ thin film photoanode immersed in liquid electrolyte of variable pH. Titanium dioxide photoanodes doped with chromium (up to 16 at.%) and tin (up to 50 at.%) were prepared by rf reactive sputtering. Different methods of flat band potential determination: Mott–Schottky plots and photocurrent versus voltage characteristics were used. The energy band gap was derived from the spectrophotometric measurements of optical transmittance and reflectance coefficients of thin films. For TiO₂ + 7.6 at.% Cr high and negative flat band potential V_Fb = −0.72 eV (at pH 4) has been found but the recombination time τ = 8 s was the shortest of all TiO₂ modifications. Despite additional absorption feature at about 2.8 eV, i.e., at wavelength corresponding to visible range of the light spectrum, the photoconversion efficiency of TiO₂ + 7.6 at.% Cr was found to be much smaller (η_c = 0.1%) than that of undoped TiO₂ (η_c = 1.8%) and TiO₂ doped with 8 at.% of Sn (η_c = 1.0%).     

Reactive and selective redox system of Ni(II)-ferrite for a two-step CO and H2 production cycle from carbon and water

A number of redox systems composed of carbon and bivalent metal-ferrites, MFe₂O₄(M = bivalent metal ions), were studied to find the most reactive and selective working materials for a thermochemical water-decomposition cycle combined with CO production from a carbon compound. Magnetite and Mg(II)-, Mn(II)-, Co(II)-, Ni(II)-, and Zn(II)-ferrites mixed with carbon powder have been screened for reactivity and selectivity in the CO-production step (first step) and subsequent water-decomposition step (second step). The Ni(II)-ferrite showed the most reactivity and selectivity for CO formation from carbon in the first step at temperatures above 700°C. The Ni(II)-ferrite could be completely reduced with carbon to the metallic phase of the Ni---Fe alloy and Ni in the first step although only a small portion of magnetite was reduced to wustite under similar conditions. The ferrites reduced in the first step were oxidized with water vapor to generate H₂ in the second step. The highest conversion of H₂O to H₂ was obtained using the Ni(II)-ferrite. The total amount of evolved H₂ using Ni(II)-ferrite was 10 times larger than for the magnetite at 500°C. The processes could be repeated using the phase transition between Ni(II)-ferrite and Ni---Fe alloy in the temperature range 700–800°C, with the highly efficient net reaction H₂O + C → H₂ + CO.     

The role of temperature on the corrosion and passivation of type 310S stainless steel in eutectic (Li + K) carbonate melt

The corrosion behavior of type 310S stainless steel was studied in the eutectic Li + K carbonate as a function of temperature by several electrochemical methods. Within the range 600–675 °C the steel passivated spontaneously at OCP condition after a few hours of immersion. Active-passive transition was observed in the polarization curves below 600 °C and above 675 °C indicating a failure to reach a stable passive condition even at prolonged exposure times. Impedance analysis indicates that passivity does not lead to the formation of an impervious barrier layer as denoted by the presence of diffusional components at low frequencies indicating oxide growth. Corrosion rates exhibited a minimum at 675 °C at both OCP and at cathode polarization conditions. A mechanism to explain the active–passive transition has been proposed based on the phase transition from LiFe₅O₈ to LiFeO₂.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Fermentative biohydrogen production by a new chemoheterotrophic bacterium Citrobacter sp. Y19

A newly isolated Citrobacter sp. Y19 for CO-dependent H₂ production was studied for its capability of fermentative H₂ production in batch cultivation. When glucose was used as carbon source, the pH of the culture medium significantly decreased as fermentation proceeded and H₂ production was seriously inhibited. The use of fortified phosphate at 60–180 mM alleviated this inhibition. By increasing culture temperatures (25–36°C), faster cell growth and higher initial H₂ production rates were observed but final H₂ production and yield were almost constant irrespective of temperature. Optimal specific H₂ production activity was observed at 36°C and pH 6–7. The increase of glucose concentration (1–20 g/l) in the culture medium resulted in higher H₂ production, but the yield of H₂ production (mol H₂/mol glucose) gradually decreased with increasing glucose concentration. Carbon mass balance showed that, in addition to cell mass, ethanol, acetate and CO₂ were the major fermentation products and comprised more than 70% of the carbon consumed. The maximal H₂ yield and H₂ production rate were estimated to be 2.49 molH₂/mol glucose and 32.3 mmolH₂/gcellh, respectively. The overall performance of Y19 in fermentative H₂ production is quite similar to that of most H₂-producing bacteria previously studied, especially to that of Rhodopseudomonas palustris P4, and this indicates that the attempt to find an outstanding bacterial strain for fermentative H₂ production might be very difficult if not impossible.     

A proton-conducting solid state H2S-O2 fuel cell. 1. anode catalysts, and operation at atmospheric pressure and 20–90°C

A stable solid state H₂S--O₂ fuel cell has been developed and operated at 1 atm and 20-90°C. A series of anode catalysts has been examined using Nafion^® as a common proton conducting membrane; those containing Pd and Pt were found to be effective using H₂ or H₂S as the anode feed gas, but MoS₂--C catalysts were effective for use of H₂S but not for H₂. The highest potential attained using H₂S and Pd/C catalyst was 722 mV (theory: 1140 mV). When H₂S was used as anode feed the potential decreased up to 35% over 24 h as sulfur was deposited on the anode. The efficiency of the cell increased with temperature up to 90°C.     

Performance analysis and impedance spectral signatures of high temperature PBI–phosphoric acid gel membrane fuel cells

Polarization curves, i.e., dc performance, and impedance spectral signatures of polybenzimidazole (PBI)–phosphoric acid (H₃PO₄) membrane fuel cells are obtained in the temperature range of 160–180 °C, in an effort to investigate the effect of temperature, anode humidification, various cathode stoichs, and use of oxygen versus air. Thus, in situ electrochemical impedance spectroscopy (EIS) was used to obtain various resistances, ohmic as well as charge-transfer resistances, under these conditions. The results obtained show that PBI–H₃PO₄ gel membrane fuel cells exhibit very good performance in the temperature range of 160–180 °C with an ohmic resistance similar to Nafion. Mass transfer limitations were determined by comparing performance polarization curves with air and oxygen along with EIS. Further EIS was also used to obtain signatures during fuel starvation, and electrical shorting across the cell.