Electrodeposition of iron(II) on platinum in chloride melts at 700-750 °C

The electrodeposition of iron(II) on platinum electrodes in chloride melts was studied by cyclic voltammetry. The process is irreversible and is complicated by platinum-iron alloy formation. The deposition is accompanied by iron dendrite formation. Thereby, after achieving a steady state for iron deposition on platinum in the melt, the Pt surface was covered by up to 40% atom iron only. The electrochemical parameters of this process were determined. A plausible mechanism scheme was proposed.     

The tensile properties of carbon matrices at temperatures up to 2200 °C

Tensile properties of carbon matrices for carbon/carbon composites made via chemical vapor infiltration (CVI) of a fiber preform, were determined from room temperature to 2200 °C. Microcomposite test specimens were used. For this purpose, a carbon coating was deposited on low modulus carbon fibers via chemical vapor deposition based techniques. The mechanical behavior of the carbon matrix was derived from the stress-strain curves obtained with the single filament reinforced microcomposites. It was related to features of nanostructure characterized using X-ray diffractometry. The trends are discussed with respect to those evidenced on carbon fibers in a previous paper.     

Phase equilibrium data and thermodynamic modeling of the system propane + NMP + methanol at high pressures

This work reports phase equilibrium data at high pressures for the binary and ternary systems formed by propane + n-methyl-2-pyrrolidone (NMP) + methanol. Phase equilibrium measurements were performed in a high-pressure variable-volume view cell, following the static synthetic method for obtaining the experimental bubble and dew points transition data in the temperature range of 363-393 K, pressures up to 16 MPa and overall molar fraction of the lighter component varying from 0.1 to 0.998. For the systems investigated, vapor-liquid (VLE), liquid-liquid (LLE) and vapor-liquid-liquid (VLLE) phase transitions were visually recorded. Results show that the systems investigated present UCST (upper critical solution temperature) phase transition curves with an UCEP (upper critical end point) at a temperature higher than the propane critical temperature. The experimental data were modeled using the Peng-Robinson equation of state with the Wong-Sandler and the classical quadratic mixing rules, affording a satisfactory representation of the experimental data.     

Mechanism of lithium transport through an MCMB heat-treated at 800-1200 °C

Mechanism of lithium transport through a mesocarbon-microbeads (MCMB) heat-treated at 800-1200 °C was elucidated in 1 M LiPF₆-ethylene carbonate-diethyl carbonate (50:50 vol.%) solution by the quantitative analysis of potentiostatic current transient considering the difference in the relative amount of lithium deintercalation sites having different activation energies for lithium deintercalation. From the coincidence between the current transients experimentally measured and theoretically calculated based upon the modified McNabb-Foster equation along with ‘cell-impedance-controlled’ constraint as the governing equation with the boundary condition, respectively, it is suggested that lithium transport through the MCMB electrode is limited by the ‘cell-impedance’, and at the same time the difference in the kinetics of lithium transport between through the four different lithium deintercalation sites is due to the difference in activation energy for lithium deintercalation between from the four different lithium deintercalation sites present within the MCMB. Moreover, it is realised that since the degree of microcrystallinity of the MCMB is increased with rising heat-treatment temperature, the relative charge amount of lithium deintercalated from the lattice-site is increased, but that amount from the extra-sites is decreased. Thus, the inflexion point, i.e. ‘quasi-current plateau’ in the current transient is less clearly observed with rising heat-treatment temperature.     

PEM fuel cells operated at 0% relative humidity in the temperature range of 23-120 °C

Operation of a proton exchange membrane (PEM) fuel cell without external humidification (or 0% relative humidity, abbreviated as 0% RH) of the reactant gases is highly desirable, because it can eliminate the gas humidification system and thus decrease the complexity of the PEM fuel cell system and increase the system volume power density (W/l) and weight power density (W/kg). In this investigation, a PEM fuel cell was operated in the temperature range of 23-120 °C, in particular in a high temperature PEM fuel cell operation range of 80-120 °C, with dry reactant gases, and the cell performance was examined according to varying operation parameters. An ac impedance method was used to compare the performance at 0% RH with that at 100% RH; the results suggested that the limited proton transfer process to the Pt catalysts, mainly in the inonomer within the membrane electrode assembly (MEA) could be responsible for the performance drop. It was demonstrated that operating a fuel cell using a commercially available membrane (Nafion^® 112) is feasible under certain conditions without external humidification. However, the cell performance at 0% RH decreased with increasing operation temperature and reactant gas flow rate and decreasing operation pressure.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Continuous synthesis of Zn2SiO4:Mn fine particles in supercritical water at temperatures of 400-500 °C and pressures of 30-35 MPa

Sub-micron sized Zn₂SiO₄:Mn²⁺ phosphors particles were continuously synthesized in supercritical water with a flow reactor. Colloidal silica or sodium silicate was used as the Si source. Zn and Mn sources were chosen from their nitrates, sulfates, and acetates. The syntheses were carried out at temperatures from 400 to 500 °C, at pressures from 30 to 35 MPa, at NaOH concentrations from 0.014 to 0.025 M, and for residence times from 0.025 to 0.18 s. Sodium silicate formed α- and β-Zn₂SiO₄:Mn²⁺ phases regardless of the Zn and Mn sources, while colloidal silica formed phases dependent on the type of Zn and Mn sources used in addition to the use of alkali. As the reaction temperature increased, the crystallinity of α-Zn₂SiO₄:Mn²⁺ phase increased and the Mn substitution into the Zn sites of the α-Zn₂SiO₄ phase decreased. Of the conditions studied, the most highly crystalline α-Zn₂SiO₄:Mn²⁺ was produced at a temperature of 400 °C, a pressure of 30 MPa, a NaOH concentration of 0.14 M, and a residence time of 0.13 s with Zn and Mn sulfates and colloidal silica as starting materials. The α-Zn₂SiO₄:Mn²⁺ fine particles synthesized were round in shape, had an average diameter of 268 nm, and exhibited a green-emission with a peak wavelength of 524 nm.     

High-pressure gas-liquid equilibrium of the system carbon dioxide-citral at 50 and 70 °C

Measurements of high-pressure gas-liquid equilibria of the binary system carbon dioxide-citral were carried out in the present work. The knowledge of the phase equilibrium behaviour of this system is relevant with regard to the design and optimization of the supercritical deterpenation process. The measurements were carried out at 50 and 70 °C, in the pressure range 7.8-15.6 MPa, by means of a two-chamber gas-phase recirculation apparatus of 340 cm³. Both the liquid and the gas phase composition were measured. The data at 50 °C measured in this work were compared with literature data, whereas no comparison was possible at 70 °C because of their lack. The experimental data measured in this work were successfully correlated by means of a thermodynamic model based on the Peng-Robinson equation of state.     

RAFT cryopolymerizations of acrylamides and acrylates in dioxane at −5 °C

Reversible addition-fragmentation chain transfer (RAFT) cryopolymerizations of acrylamides and acrylates were successfully carried out at −5 °C with cumene hydroperoxide/ascorbic acid as redox initiation couple and 2-dodecylsulfanyl- thiocarbonylsulfanyl-2-methylpropinoic acid as chain transfer agent. The cryopolymerization features of N,N-dimethylacrylamide (DMA) and tert-butyl acrylate (tBA) were investigated in view of kinetics, molecular weight and its distribution by proton nuclear magnetic resonance analysis and gel permeation chromatography. Furthermore, sequential block cryopolymerizations of N-isopropylacrylamide were performed with the obtained trithiocarbonate- functionalized PDMA or PtBA as macro-CTA and the corresponding block polymers were obtained. All the results demonstrated that these cryopolymerizations bear all the characteristics of controlled/living radical polymerizations.     

Effect of catalyst addition on gasification reactivity of HyperCoal and coal with steam at 775-700 °C

HyperCoal is a clean coal with ash content <0.05 wt%. HyperCoal was prepared from a bituminous coal by solvent extraction method with an extraction yield of 66.7%. Effect of K₂CO₃ loading and temperature on steam gasification rate of HyperCoal and parent raw coal was investigated. Experiments were carried out at 775 and 700 °C for both HyperCoal and coal with 0-25% catalyst loading. Gasification rates increased with increased catalyst loading and reached a value above which rates did not change with catalyst loading. The catalyst loadings were 6% for HyperCoal and 20% for coal. The difference is most likely due to the difference in ash content of HyperCoal and coal and not due to the effect of H₂ inhibition on the rate. In the presence of catalyst effect of temperature was only on gasification rate and total gas yield and gas composition were unaffected.     

PEM fuel cell reaction kinetics in the temperature range of 23-120 °C

The performance of a Nafion 112 based proton exchange membrane (PEM) fuel cell was tested at a temperature range from 23 °C to 120 °C. The fuel cell polarization curves were divided into two different ranges based on current density, namely, <0.4 A/cm² and >0.4 A/cm², respectively. These two ranges were treated separately with respect to electrode kinetics and mass transfer. In the high current density range, a linear increase in membrane electrode assembly (MEA) power density with increasing temperature was observed, indicating the advantages of high temperature operation.Simulation based on electrode reaction kinetic theory, experimental polarization curves, and measured cathodic apparent exchange current densities all gave temperature dependent apparent exchange current densities. Both the calculated partial pressures of O₂ and H₂ gas in the feed streams and the measured electrochemical Pt surface areas (EPSAs) decrease with increasing temperature. They were also used to obtain the intrinsic exchange current densities. A monotonic increase of the intrinsic exchange current densities with increasing temperature in the range of 23-120 °C was observed, suggesting that increasing the temperature does promote intrinsic kinetics of fuel cell reactions.There are two sets of cathode apparent exchange current densities obtained, one set is for the low current density range, and the other is for the high current density range. The different values of cathode current densities in the two current density ranges can be attributed to the different states of the cathode Pt catalyst surface. In the low current density range, the cathode catalyst surface is a Pt/PtO, and in the high current density range, the catalyst surface becomes pure Pt.     

Reaction of d-glucose in water at high temperatures (410 °C) and pressures (180 MPa) for the production of dyes and nano-particles

An autoclave (120-mL) and an optical micro-reactor (50-nL) were used to study the hydrothermal decomposition of d-glucose at high temperatures and high pressures. During slow heating (0.18 °C/s) to 350 °C in the autoclave, water-soluble glucose (0.9 M) began to decompose at 220 °C and reacted completely at 280 °C. The initial decomposition products were 5-(hydroxymethyl)furfural and levoglucosan, and these subsequently converted into oil and solid residue, and finally to solid particles at a 65 wt% yield at 350 °C. When the same heating rate and temperature were used on glucose solutions in the micro-reactor, yellow and orange materials decomposed from glucose were produced. Numerous particles precipitated at 251 °C, and at 350 °C, all the glucose changed to an orange film and solid particles, which were nanoparticles as confirmed by SEM. However, when the glucose solution was rapidly heated to 410 °C (9.5-17 °C/s), yellow, brown and orange sugar-like materials were produced. A homogeneous phase with yellow color still remained at temperatures as high as 380 °C, and few particles formed until 410 °C. It can be concluded that micron-sized particles and colored solutions can be produced by slow heating, while rapid heating resulted in the formation of dye-like substances with glucose-like structures. The formation of colored solutions and particles may have technological implications in food or materials formation processes that use high temperature water with biomass feedstocks.     

Electrochemical impedance spectroscopy (EIS) studies of the corrosion of pure Fe and Cr at 600 °C under solid NaCl deposit in water vapor

The corrosion behavior of pure Fe and pure Cr at 600 °C under a deposit of solid NaCl, with and without the presence of water vapor, was studied by using electrochemical impedance spectroscopy (EIS) and mass gain measurements. The mass gain of both metals sharply increased when water vapor was introduced into the system. In EIS measurement, only one capacitive loop obtained on the pure Fe and Cr coated with solid NaCl and gave the information of oxide layer on them. For the oxide in air, there is a good relationship between the R_ox and the reaction rate for both pure Fe and Cr with different oxide time. The lower the R_ox is, the higher the reaction rate is. Although no good relationship can be set up between the R_ox and the reaction rate when water vapor presented, its trend with oxide time for both metals is generally in accordance with that of the corrosion rate measured by the mass gain curves. The electrochemical technique is an effective method for studying corrosion performance at high temperature.     

Study of lepidocrocite γ-FeOOH electrochemical reduction in neutral and slightly alkaline solutions at 25 °C

The electrochemical reduction of lepidocrocite γ-FeOOH was investigated at 25 °C in neutral or slightly alkaline solutions containing chloride, sulphate or bicarbonate anions by means of thin lepidocrocite film electrodeposited on inert gold substrate or graphite/lepidocrocite powder composite electrode. Electrochemical measurements were coupled to in situ electrochemical quartz crystal microbalance (EQCM) and ex situ SEM and FTIR analysis. The reduction of lepidocrocite occurs in all the electrolytes considered here. The initial reduction product is adsorbed ferrous ion, Feads²⁺. The desorption of Feads²⁺ is promoted as pH increases, leading to an increase of reduction depth. This promotion is related to the formation of secondary Fe^II-containing species, as revealed by SEM and FTIR. The comparison of γ-FeOOH reduction potentials and iron corrosion potentials let us state that the galvanic coupling is possible.     

EIS-assisted performance analysis of non-noble metal electrocatalyst (Fe-N/C)-based PEM fuel cells in the temperature range of 23-80 °C

A carbon-supported non-noble metal catalyst, Fe-N/C, was used as the cathode catalyst to construct membrane electrolyte assemblies (MEAs) for a proton exchange membrane (PEM) fuel cell. The performance of such a fuel cell was then tested and diagnosed using electrochemical impedance spectroscopy (EIS) in the temperature range of 23-80 °C. Based on the EIS measurements, individual resistances, such as charger transfer resistance and membrane resistance, were obtained and used to simulate polarization curves (current-voltage (I-V) curves). A close agreement between the simulated I-V curves and the measured curves demonstrates consistency between the polarization and EIS data. The temperature-dependent parameters obtained via EIS, such as apparent exchange current densities and electrolyte membrane conductivities, were also used to acquire activation energies for both the oxygen reduction reaction (ORR) catalyzed by an Fe-N/C catalyst and the proton transport process across the electrolyte membrane. In addition, the maximum power densities for such a fuel cell were also analyzed.     

Experimental Determination and Thermodynamic Calculation of the Zirconia–Calcia–Magnesia System at 1600°, 1700°, and 1750°C

Phase relations in the ternary system ZrO₂–CaO–MgO were experimentally established at 1600°, 1700°, and 1750°C. The investigation was based on powder X-ray diffractometry, scanning electron microscopy–energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO₂–CaZrO₃ phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO₂–CaO–MgO.     

Analysis of proton exchange membrane fuel cell polarization losses at elevated temperature 120 °C and reduced relative humidity

Polarization losses of proton exchange membrane (PEM) fuel cells at 120 °C and reduced relative humidity (RH) were analyzed. Reduced RH affects membrane and electrode ionic resistance, catalytic activity and oxygen transport. For a cell made of Nafion^® 112 membrane and electrodes that have 35 wt.% Nafion^® and 0.3 mg/cm² platinum supported on carbon, membrane resistance at 20%RH was 0.407 Ω cm² and electrode resistance 0.203 Ω cm², significantly higher than 0.092 and 0.041 Ω cm² at 100%RH, respectively. In the kinetically controlled region, 20%RH resulted in 96 mV more cathode activation loss than 100%RH. Compared to 100%, 20%RH also produced significant oxygen transport loss across the ionomer film in the electrode, 105 mV at 600 mA/cm². The significant increase in polarization losses at elevated temperature and reduced RH indicates the extreme importance of designing electrodes for high temperature PEM fuel cells since membrane development has always taken most emphasis.     

The electrochemical performance of a La-Mg-Ni-Co-Mn metal hydride electrode alloy in the temperature range of −20 to 30 °C

The effect of temperature on the overall electrochemical properties of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 hydrogen storage alloy has been studied systematically. The results show that temperature has a striking effect on the overall electrochemical properties, especially the electrochemical kinetic performance. The maximum discharge capacity and the high rate dischargeability (HRD) of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 alloy electrode both decrease with decreasing test temperature, mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. Detailed studies on the temperature effect on the polarization resistance (R_D), the exchange current density (I₀), the limiting current density (I_L) and the hydrogen diffusion coefficient (D), indicate that the diffusion of hydrogen in the bulk for La-Mg-Ni-Co system hydrogen storage alloy electrodes is the rate-determining factor for the discharge process of the alloy electrode for the temperature over 10 °C and the charge-transfer reaction is rate-determining step at lower temperature.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.