Thermophysical properties and devitrification of SrO–La2O3–Al2O3–B2O3–SiO2-based glass sealant for solid oxide fuel/electrolyzer cells

Thermal behaviors and stability of glass/glass–ceramic-based sealant materials are critical issues for high temperature solid oxide fuel/electrolyzer cells. To understand the thermophysical properties and devitrification behavior of SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system, glasses were synthesized by quenching (25 − X)SrO–20La₂O₃–(7 + X)Al₂O₃–40B₂O₃–8SiO₂ oxides, where X was varied from 0.0 mol% to 10.0 mol% at 2.5 mol% interval. Thermal properties were characterized by dilatometry and differential scanning calorimetry (DSC). Microstructural studies were performed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). All the compositions have a glass transition temperature greater than 620 °C and a crystallization temperature greater than 826 °C. Also, all the glasses have a coefficient of thermal expansion (CTE) between 9.0 × 10⁻⁶ K⁻¹ and 14.5 × 10⁶ K⁻¹ after the first thermal cycle. La₂O₃ and B₂O₃ contribute to glass devitrification by forming crystalline LaBO₃. Al₂O₃ stabilizes the glasses by suppressing devitrification. Significant improvement in devitrification resistance is observed as X increases from 0.0 mol% to 10.0 mol%.     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

Synthesis and electrochemistry of cubic rocksalt Li–Ni–Ti–O compounds in the phase diagram of LiNiO2–LiTiO2–Li[Li1/3Ti2/3]O2

On the basis of extreme similarity between the triangle phase diagrams of LiNiO₂–LiTiO₂–Li[Li_1/3Ti_2/3]O₂ and LiNiO₂–LiMnO₂–Li[Li_1/3Mn_2/3]O₂, new Li–Ni–Ti–O series with a nominal composition of Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ (0 ≤ z ≤ 0.5) was designed and attempted to prepare via a spray-drying method. XRD identified that new Li–Ni–Ti–O compounds had cubic rocksalt structure, in which Li, Ni and Ti were evenly distributed on the octahedral sites in cubic closely packed lattice of oxygen ions. They can be considered as the solid solution between cubic LiNi_1/2Ti_1/2O₂ and Li[Li_1/3Ti_2/3]O₂ (high temperature form). Charge–discharge tests showed that Li–Ni–Ti–O compounds with appropriate compositions could display a considerable capacity (more than 80 mAh g⁻¹ for 0.2 ≤ z ≤ 0.27) at room temperature in the voltage range of 4.5–2.5 V and good electrochemical properties within respect to capacity (more than 150 mAh g⁻¹ for 0 ≤ z ≤ 0.27), cycleability and rate capability at an elevated temperature of 50 °C. These suggest that the disordered cubic structure in some cases may function as a good host structure for intercalation/deintercalation of Li⁺. A preliminary electrochemical comparison between Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ (0 ≤ z ≤ 0.5) and Li_6/5Ni_2/5Ti_2/5O₂ indicated that charge–discharge mechanism based on Ni redox at the voltage of >3.0 V behaved somewhat differently, that is, Ni could be reduced to +2 in Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ while +3 in Li_6/5Ni_2/5Ti_2/5O₂. Reduction of Ti⁴⁺ at a plateau of around 2.3 V could be clearly detected in Li_1+z/3Ni_1/2−z/2Ti_1/2+z/6O₂ with 0.27 ≤ z ≤ 0.5 at 50 °C after a deep charge associated with charge compensation from oxygen ion during initial cycle.     

Superior low-temperature hydrogen release from the ball-milled NH3BH3–LiNH2–LiBH4 composite

In the present study, we employed a multi-component combination strategy to constitute an AB/LiNH₂/LiBH₄ composite system. Our study found that mechanically milling the AB/LiNH₂/LiBH₄ mixture in a 1:1:1 molar ratio resulted in the formation of LiNH₂BH₃ (LiAB) and new crystalline phase(s). A spectral study of the post-milled and the relevant samples suggests that the new phase(s) is likely ammoniate(s) with a formula of Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄) (0 < x < 1). The decomposition behaviors of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite were examined using thermal analysis and volumetric method in a wide temperature range. It was found that the composite exhibited advantageous dehydrogenation properties over LiAB and LiAB·NH₃ at moderate temperatures. For example, it can release ∼7.1 wt% H₂ of purity at temperature as low as 60 °C, with both the dehydrogenation rate and extent far exceeding that of LiAB and LiAB·NH₃. A selectively deuterated composite sample has been prepared and examined to gain insight into the dehydrogenation mechanism of the Li_2−x(NH₃)(NH₂BH₃)_1−x(BH₄)/xLiAB composite. It was found that the LiBH₄ component does not participate in the dehydrogenation reaction at moderate temperatures, but plays a key role in strengthening the coordination of NH₃. This is believed to be a major mechanistic reason for the favorable dehydrogenation property of the composite at moderate temperatures.     

Nanostructured palladium–La0.75Sr0.25Cr0.5Mn0.5O3/Y2O3–ZrO2 composite anodes for direct methane and ethanol solid oxide fuel cells

A palladium-impregnated La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ/yttria-stabilized zirconia (LSCM/YSZ) composite anode is investigated for the direct utilization of methane and ethanol fuels in solid oxide fuel cells (SOFCs). Impregnation of Pd nanoparticles significantly enhances the electrocatalytic activity of LSCM/YSZ composite anodes for the methane and ethanol electrooxidation reaction. At 800 °C, the maximum power density is increased by two and eight times with methane and ethanol fuels, respectively, for a cell with the Pd-impregnated LSCM/YSZ composite anode, as compared with that using a pure LSCM/YSZ anode. No carbon deposition is observed during the reaction of methane and ethanol fuels on the Pd-impregnated LSCM/YSZ composite anode. The results show the promises of nanostructured Pd-impregnated LSCM/YSZ composites as effective anodes for direct methane and ethanol SOFCs.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Interaction in the Ce3Co8Si–H2 system

Interaction of hydrogen with Ce₃Co₈Si intermetallic compound (IMC) has been studied. IMC Ce₃Co₈Si absorbs hydrogen and forms a hydride phase at 11 atm and 50 °C. X-ray analysis of Ce₃Co₈Si H_10.2 saturated hydride phase lattice showed that it has the symmetry of the initial compound and is expanded with strong anisotropy due to increased c parameter. Analysis of hydrogen desorption isotherms in Ce₃Co₈Si–H₂ system has revealed that the decomposition of hydride phase occurred in one stage. The heat of hydride phase formation was calculated on the base of obtained equilibrium pressures data at 50, 60 and 70 °C. The results obtained demonstrate that Ce₃Co₈Si intermetallic compound may be used as reversible accumulator of hydrogen in medium temperatures interval.     

Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

H2 and CO production over a stable Ni–MgO–Ce0.8Zr0.2O2 catalyst from CO2 reforming of CH4

Ni–Ce_0.8Zr_0.2O₂ and Ni–MgO–Ce_0.8Zr_0.2O₂ catalysts were investigated for H₂ production from CO₂ reforming of CH₄ reaction at a very high gas hourly space velocity of 480,000 h⁻¹. Ni–MgO–Ce_0.8Zr_0.2O₂ exhibited higher catalytic activity and stability (CH₄ conversion >95% at 800 °C for 200 h). The outstanding catalytic performance is mainly due to the basic nature of MgO and an intimate interaction between Ni and MgO.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Hydrogen production aided by new (1−x)SrTi0.5Fe0.5O3–δ–xCe0.8(Sm0.8Sr0.2)0.2O2–δ (MIEC) composite membranes

In the present work, composite materials of the type (1–x)SrTi_0.5Fe_0.5O_3–δ–xCe_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ (with х = 0, 0.25, 0.5, 0.75 and 1) are obtained by the two step solid state technique. Their transport properties are investigated in terms of their usage as mixed ionic-electronic conducting (MIEC) membrane materials for hydrogen production. It is found that, in reducing conditions the composites are characterized by mixed conductivity, which level is controlled by the electrical properties of the prevailing phase. Moreover, at 900 °C and pO₂ = 10⁻¹⁸ atm, total conductivity, ambipolar conductivity and oxygen permeability of composites dramatically grow (each of about 500%), when the fluorite component content x increases from 0 to 1. High-conducting and strengthened material 0.5SrTi_0.5Fe_0.5O_3–δ–0.5Ce_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ is chosen for making tube shaped membranes using the tape rolling method, which are successfully applied for hydrogen production in laboratory scale. The hydrogen flux reached 0.176 ml cm⁻² min⁻¹ for x = 1, T = 900 °C and emf = 10 mV.     

Ce–K-promoted Co–Mo/Al2O3 catalysts for the water gas shift reaction

The effect of doping traditional Co–Mo/Al₂O₃ catalysts with Ce and K on their catalytic activity for the water gas shift reaction in coke oven gas was investigated. Doped Co–Mo/Al₂O₃ catalysts were prepared by adding different amounts of Ce and K (CeO₂, K₂O, and CeO₂–K₂O ∼10 wt%) by a wetness impregnation method and characterized by BET specific surface area measurements and scanning electron microscopy (SEM). The characterization results reveal that CeO₂ addition mainly produced an electronic effect and aided to disperse the active ingredient. At the same time, the synergistic effect between Ce and K contributed to the catalytic activity. Activity tests showed that Ce–K-promoted Co–Mo/Al₂O₃ catalysts exhibited greater activity and selectivity than Co–Mo–Ce/Al₂O₃ catalysts and Co–Mo–K/Al₂O₃ catalysts. The maximum promotion of the water gas shift reaction was observed when 3.0 wt% CeO₂ and 6.0 wt% K₂O were added.     

Nanoconfined 2LiBH4–MgH2–TiCl3 in carbon aerogel scaffold for reversible hydrogen storage

Nanoconfinement of 2LiBH₄–MgH₂–TiCl₃ in resorcinol–formaldehyde carbon aerogel scaffold (RF–CAS) for reversible hydrogen storage applications is proposed. RF–CAS is encapsulated with approximately 1.6 wt. % TiCl₃ by solution impregnation technique, and it is further nanoconfined with bulk 2LiBH₄–MgH₂ via melt infiltration. Faster dehydrogenation kinetics is obtained after TiCl₃ impregnation, for example, nanoconfined 2LiBH₄–MgH₂–TiCl₃ requires ∼1 and 4.5 h, respectively, to release 95% of the total hydrogen content during the 1st and 2nd cycles, while nanoconfined 2LiBH₄–MgH₂ (∼2.5 and 7 h, respectively) and bulk material (∼23 and 22 h, respectively) take considerably longer. Moreover, 95–98.6% of the theoretical H₂ storage capacity (3.6–3.75 wt. % H₂) is reproduced after four hydrogen release and uptake cycles of the nanoconfined 2LiBH₄–MgH₂–TiCl₃. The reversibility of this hydrogen storage material is confirmed by the formation of LiBH₄ and MgH₂ after rehydrogenation using FTIR and SR-PXD techniques, respectively.     

Hydrogen storage properties of Li–Ca–N–H system with different molar ratios of LiNH2/CaH2

In this work, dehydrogenation and rehydrogenation of three LiNH₂/CaH₂ samples with LiNH₂/CaH₂ molar ratio of 2/1, 3/1 and 4/1 were systematically investigated. Remarkable differences were observed in the temperature dependence of hydrogen desorption and subsequent absorption. LiNH₂/CaH₂ in a molar ratio of 2/1 transforms to ternary imide Li₂Ca(NH)₂ after desorbing about 4.5 wt.% H₂ at 350 °C. And it has a reversible hydrogen storage capacity of 2.7 wt.% at 200 °C. As for the LiNH₂/CaH₂ mixture in a molar ratio of 4/1, it transforms to a new compound with a composition of Li₄CaN₄H₆ after being dehydrogenated at 350 °C. The rehydrogenation of both LiCa(NH)₂ and Li₄CaN₄H₆ gives LiNH₂, LiH and the solid solution of 2CaNH–Ca(NH₂)₂.     

Structure and electrochemical characteristics of Ti0.10Zr0.15V0.35Cr0.10Ni0.30–LaNi4Al0.4Mn0.3Co0.3 composite hydrogen storage alloy

Structure and electrochemical characteristics of Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 (matrix alloy) — 1 mass% LaNi₄Al_0.4Mn_0.3Co_0.3 composite hydrogen storage alloy have been investigated systematically. The main phase of composite alloy is composed of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure, while secondary phase which has a composition close to Zr (Ti, V, Ni, Cr, Al, Co, La)_1.8 also exists in the composite alloy. The real maximum discharge capacity of the composite alloy electrode is 354.9 mAh g⁻¹, and distinct synergetic effect appears during composite process. Comparing with the matrix alloy, the thermodynamic performances, electrochemical characteristics, dynamic performances for the composite alloy electrode have been improved. The secondary phase is probably responsible for the improvement of electrochemical characteristics of the matrix alloy.     

Steam reforming of ethanol over Co3O4–Fe2O3 mixed oxides

Co₃O₄, Fe₂O₃ and a mixture of the two oxides Co–Fe (molar ratio of Co₃O₄/Fe₂O₃ = 0.67 and atomic ratio of Co/Fe = 1) were prepared by the calcination of cobalt oxalate and/or iron oxalate salts at 500 °C for 2 h in static air using water as a solvent/dispersing agent. The catalysts were studied in the steam reforming of ethanol to investigate the effect of the partial substitution of Co₃O₄ with Fe₂O₃ on the catalytic behaviour. The reforming activity over Fe₂O₃, while initially high, underwent fast deactivation. In comparison, over the Co–Fe catalyst both the H₂ yield and stability were higher than that found over the pure Co₃O₄ or Fe₂O₃ catalysts. DRIFTS-MS studies under the reaction feed highlighted that the Co–Fe catalyst had increased amounts of adsorbed OH/water; similar to Fe₂O₃. Increasing the amount of reactive species (water/OH species) adsorbed on the Co–Fe catalyst surface is proposed to facilitate the steam reforming reaction rather than decomposition reactions reducing by-product formation and providing a higher H₂ yield.