NMR spectroscopic and thermodynamic studies of the etherate and the α, α′, and γ phases of AlH3

Aluminum hydride (alane; AlH₃) has been identified as a leading hydrogen storage material by the US Department of Energy. With a high gravimetric hydrogen capacity of 10.1 wt.%, and a hydrogen density of 1.48 g/cm³, AlH₃ decomposes cleanly to its elements above 60 °C with no side reactions. This study explores in detail the thermodynamic and spectroscopic properties of AlH₃; in particular the α, α′ and γ polymorphs, of which α′-AlH₃ is reported for the first time, free from traces of other polymorphs or side products. Thermal analysis of α-, α′-, and γ-AlH₃ has been conducted, using DSC and TGA methods, and the results obtained compared with each other and with literature data. All three polymorphs were investigated by ¹H MAS-NMR spectroscopy for the first time, and their ²⁷Al MAS-NMR spectra were also measured and compared with literature values. AlH₃·nEt₂O has also been studied by ¹H and ²⁷Al MAS-NMR spectroscopy and DSC and TGA methods, and an accurate decomposition pathway has been established for this adduct.     

Combustion,performance and emissions of a diesel engine with H2, CH4 and H2–CH4 addition

Experiments were conducted to investigate the combustion and emission characteristics of a diesel engine with addition of hydrogen or methane for dual-fuel operation, and mixtures of hydrogen–methane for tri-fuel operation. The in-cylinder pressure and heat release rate change slightly at low to medium loads but increase dramatically at high load owing to the high combustion temperature and high quantity of pilot diesel fuel which contribute to better combustion of the gaseous fuels. The performance of the engine with tri-fuel operation at 30% load improves with the increase of hydrogen fraction in methane and is always higher than that with dual-fuel operations. Compared with ULSD–CH₄ operation, hydrogen addition in methane contributes to a reduction of CO/CO₂/HC emissions without penalty on NO_x emission. Dual-fuel and tri-fuel operations suppress particle emission to the similar extent. All the gaseous fuels reduce the geometry mean diameter and total number concentration of diesel particulate. Tri-fuel operation with 30% hydrogen addition in methane is observed to be the best fuel in reducing particulate and NO_x emissions at 70 and 90% loads.     

Phase formation of Ce5Co19-type super-stacking structure and its effect on electrochemical and hydrogen storage properties of La0.60M0.20Mg0.20Ni3.80 (M = La,Pr, Nd,Gd) compounds

In order to investigate the formation mechanism of Ce₅Co₁₉-type super-stacking structure phase, La_0.60M_0.20Mg_0.20Ni_3.80 (M = La, Pr, Nd, Gd) compounds are synthesized by powder sintering method. Rietveld refinements of X-ray diffraction patterns find that La_0.80Mg_0.20Ni_3.80 compound has a single Pr₅Co₁₉-type structure. The Ce₅Co₁₉-type phase appears and increases with the decrease of atomic radius of M, until the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound shows a Ce₅Co₁₉-type single phase structure. The cycling stability and high rate dischargeability (HRD) of the alloy electrodes both improve with the increase of Ce₅Co₁₉-type phase. The capacity retention of La_0.60Gd_0.20Mg_0.20Ni_3.80 compound at the 100th cycle is high to 93.6% and the HRD reaches 66.9% at a discharge current density of 1500 mA g^?1. Moreover after 50 charge/discharge cycles, the Ce₅Co₁₉-type particle retains an intact crystal structure while severe amorphization occurs to Pr₅Co₁₉-type particle as shown in graphical abstract. The cohesive energy obtained from the First-principle calculations is analyzed combined with the experimental results. It is found that the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound with Ce₅Co₁₉-type single phase structure has the highest cohesive energy indicating a more stable structure. This work provides new insights into the superior composition-structure design of LaMgNi system hydrogen storage alloys that may improve the cycling stability.     

Characterization and activity test of commercial Ni/Al2O3, Cu/ZnO/Al2O3 and prepared Ni–Cu/Al2O3 catalysts for hydrogen production from methane and methanol fuels

In this study, methane and methanol steam reforming reactions over commercial Ni/Al₂O₃, commercial Cu/ZnO/Al₂O₃ and prepared Ni–Cu/Al₂O₃ catalysts were investigated. Methane and methanol steam reforming reactions catalysts were characterized using various techniques. The results of characterization showed that Cu particles increase the active particle size of Ni (19.3 nm) in Ni–Cu/Al₂O₃ catalyst with respect to the commercial Ni/Al₂O₃ (17.9). On the other hand, Ni improves Cu dispersion in the same catalyst (1.74%) in comparison with commercial Cu/ZnO/Al₂O₃ (0.21%). A comprehensive comparison between these two fuels is established in terms of reaction conditions, fuel conversion, H₂ selectivity, CO₂ and CO selectivity. The prepared catalyst showed low selectivity for CO in both fuels and it was more selective to H₂, with H₂ selectivities of 99% in methane and 89% in methanol reforming reactions. A significant objective is to develop catalysts which can operate at lower temperatures and resist deactivation. Methanol steam reforming is carried out at a much lower temperature than methane steam reforming in prepared and commercial catalyst (275–325 °C). However, methane steam reforming can be carried out at a relatively low temperature on Ni–Cu catalyst (600–650 °C) and at higher temperature in commercial methane reforming catalyst (700–800 °C). Commercial Ni/Al₂O₃ catalyst resulted in high coke formation (28.3% loss in mass) compared to prepared Ni–Cu/Al₂O₃ (8.9%) and commercial Cu/ZnO/Al₂O₃ catalysts (3.5%).     

Performance and stability of nano-structured Pd and Pd0.95M0.05 (M = Mn,Co, Ce,and Gd) infiltrated Y2O3–ZrO2 oxygen electrodes of solid oxide electrolysis cells

Nano-structured Pd infiltrated and Pd_0.95M_0.05 (M = Mn, Co, Ce, and Gd) co-infiltrated Y₂O₃–ZrO₂ (YSZ) electrodes are studied as the oxygen electrodes of solid oxide electrolysis cells (SOECs). The infiltrated Pd-YSZ electrodes show good electrocatalytic activity for the oxygen evolution reaction. For example, the electrode polarization resistance (R_E) for 2.0 mg cm⁻² Pd infiltrated YSZ is 0.36 Ω cm⁻² at 800 °C. R_E is not significantly affected by co-infiltrating Pd with Mn and Co, but is enhanced by co-infiltration of Ce and Gd. The co-infiltration of low concentrations of metals in particular Co, Ce and Gd significantly enhances the microstructure and performance stability of the Pd-YSZ electrodes. The results demonstrate that the addition of dopants to the Pd in the form of either an alloy (Co) or a separate phase (Ce and Gd) is beneficial to enhance the performance and stability of Pd based oxygen electrodes of SOECs.     

Synthesis,characterization, and electrochemical performance of the ternary layered composite (1-x-y) LiNi1/3Co1/3Mn1/3O2·xLi2MnO3·yLiCoO2

Composite ceramic cathodes represented by the formula (1-x-y) LiNi_1/3Co_1/3Mn_1/3O₂·xLi₂MnO₃·yLiCoO₂ were studied. A ternary compositional diagram was built with these ceramic materials as end-members, and selected points were chosen to represent the compositional space. Synthesized ceramic composite materials were investigated as to whether integration of structurally compatible units leads to improved electrochemical performance. Detailed structural (X-ray diffraction – XRD), elemental (X-ray photoelectron spectroscopy-XPS), microstructure (Scanning electron microscopy – SEM), and electrochemical (galvanostatic testing of half-cells) studies were performed and are presented. Within eight samples studied three compositions are found to exhibit first discharge capacity of around 230 mAh/g.     

A cobalt-free oxygen transport membrane,BaFe0.9Zr0.1O3−δ, and its application for producing hydrogen

Mixed ionic and electronic conductors are being explored for use as oxygen transport membrane (OTM) materials. An OTM material, BaFe_0.9Zr_0.1O_3−δ (BFZ), was fabricated by conventional solid-state synthesis, and its oxygen permeation flux was measured from 600 to 900 °C. The BFZ is attractive for producing hydrogen because it is a cobalt-free material (resulting in low cost for fabrication) and has high oxygen permeation flux. The oxygen flux through a ≈0.45-mm-thick BFZ membrane exposed to flowing air and helium is ≈2.1 mL min⁻¹ cm⁻² at 900 °C, and the activation energy for oxygen transport is 0.43 eV. With the results of the oxygen flux and the electrical conductivity for BFZ, its high oxygen permeability was explained. To show its potential application, the BFZ was tested in coal-gas-assisted water-splitting and ethanol (EtOH) reforming experiments. The hydrogen production rate of a 1.05-mm-thick BFZ tube was comparable to that of a much thinner (≈30 μm) La_0.7Sr_0.3Cu_0.2Fe_0.8O_3−δ thin-film tube. The EtOH reforming results also indicated significantly better performance of a BFZ disk compared with that of a Ba_0.5Sr_0.5Cu_0.2Fe_0.8O_3−δ/40 vol.% Ag disk. In addition, the crystal structure and the microstructural behavior of BFZ fabricated in different conditions are discussed.     

Rh promoted and ZrO2/Al2O3 supported Ni/Co based catalysts: High activity for CO2 reforming,steam–CO2 reforming and oxy–CO2 reforming of CH4

Ni (2.5 wt%) and Co (2.5 wt%) supported over ZrO₂/Al₂O₃ were prepared by following a hydrolytic co-precipitation method. The synthesized catalysts were further promoted by Rh incorporation (0.01–1.00 wt%) and tested for their catalytic performance for dry CO₂ reforming, combined steam–CO₂ reforming and oxy–CO₂ reforming of methane for production of syngas. The catalysts were characterized by using N₂ physical adsorption, XRD, H₂–TPR, SEM, CO₂–TPD, NH₃–TPD, TEM and TGA. The results revealed that ZrO₂ phase was in crystalline form in the catalysts along with amorphous Al oxides. Ni and Co were confirmed to be in their respective spinel phases that were reducible to metallic form at 800 °C under H₂. Ni and Co were well dispersed with their nano-crystalline nature. The catalyst with 0.2% loading of Rh showed superior performance in the studied reactions for reforming of methane. This catalyst also showed good coke resistance ability for dry CO₂ reforming reaction with 3.8 wt% of carbon formation during the reaction as compared to 11.6 wt% carbon formation over the catalyst without Rh. The catalyst performance was stable throughout the reaction time for CH₄ conversions, irrespective of carbon formation with slight decline (～1%) in CO₂ conversion. For dry CO₂ reforming reaction, this catalyst showed good conversion for both CH₄ and CO₂ (67.6% and 71.8% respectively) with a H₂/CO ratio of 0.84, while for the Oxy-CO₂ reforming reaction, the activity was superior with CH₄ and CO₂ conversions (73.7% and 83.8% respectively) and H₂/CO ratio of 1.05.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Microstructure, oxygen stoichiometry and electrical conductivity of flame-sprayed Sm0.7Sr0.3CoO3−δ

A SSC deposit has been prepared by flame spraying using Sm_0.7Sr_0.3CoO_3−δ (SSC) powder synthesized by a solid-state reaction. A post-spray annealing treatment of the SSC deposit has been performed. The coating characterization includes: the electrical conductivity of the SSC deposit along the lamellar direction measured by a four-electrode D.C. approach, the microstructures of SSC powders and deposits characterized using X-ray diffraction and scanning electron microscopy, the oxygen stoichiometry in both the as-sprayed and annealed deposits and starting powder determined by redox titration. The results show that a significant oxygen deficiency (12%) occurs in the sprayed powder particles during high temperature flame spraying, leading to reduction of the electrical conductivity of the as-sprayed SSC deposit. It is found that oxygen can be recovered through post-spray annealing treatment. After annealing at 900 °C for 5 h or at 1100 °C for 10 h, the electrical conductivity of annealed SSC reaches 433 S cm⁻¹ or 510 S cm⁻¹ at 600 °C due to a sharp recovery of deficient oxygen and microstructural change.     

CO2 dry-reforming of methane over La0.8Sr0.2Ni0.8M0.2O3 perovskite (M = Bi,Co, Cr,Cu, Fe): Roles of lattice oxygen on C–H activation and carbon suppression

La_0.8Sr_0.2Ni_0.8M_0.2O₃ (LSNMO) (where M = Bi, Co, Cr, Cu and Fe) perovskite catalyst precursors have been successfully developed for CO₂ dry-reforming of methane (DRM). Among all the catalysts, Cu-substituted Ni catalyst precursor showed the highest initial catalytic activity due to the highest amount of accessible Ni and the presence of mobile lattice oxygen species which can activate C–H bond, resulting in a significant improvement of catalytic activity even at the initial stage of reaction. However, these Ni particles can agglomerate to form bigger Ni particle size, thereby causing lower catalytic stability. As compared to Cu-substituted Ni catalyst, Fe-substituted Ni catalyst has low initial activity due to the lower reducibility of Ni–Fe and the less mobility of lattice oxygen species. However, Fe-substituted Ni catalyst showed the highest catalytic stability due to: (1) strong metal–support interaction which hinders thermal agglomeration of the Ni particles; and (2) the presence of the abundant lattice oxygen species which are not very active for C–H bond activation but active to react with CO₂ to form La₂O₂CO₃, hence minimizing carbon formation by reacting with surface carbon to form CO.     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.     

Methane dry reforming on Ni/Ce0.75Zr0.25O2–MgAl2O4 and Ni/Ce0.75Zr0.25O2–γ-alumina: Effects of support composition and water addition

Nickel catalysts (10wt.%) supported on MgAl₂O₄ and γ-Al₂O₃ were prepared by the wet impregnation method and promoted with various contents of Ce_0.75Zr_0.25O₂. X-ray diffraction (XRD), BET surface area, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), H₂-temperature programmed reduction (TPR) and CO₂-temperature programmed desorption (TPD) were employed to observe the characteristics of the prepared catalysts. Ni/γ-Al₂O₃ and Ni/Ce_0.75Zr_0.25O₂ (5wt.%)–MgAl₂O₄ showed better activity in CO₂ methane reforming with 75.7(0.93) and 75.4(0.82) CH₄ conversions (and H₂/CO ratio). H₂O was added to feed in the range of H₂O/(CH₄ + CO₂): 0.1–0.5 to suppress reverse water gas shift (RWGS) effect and adjusting H₂/CO ratio. The CH₄ conversions (and H₂/CO) increased to 81(1.1) with 0.5 water/carbon mole ratio in Ni/γ-Al₂O₃ and 85(1.2) with 0.2 water/carbon mole ratio in Ni/Ce_0.75Zr_0.25O₂ (5wt.%)–MgAl₂O₄. The stability of Ni/Ce_0.75Zr_0.25O₂ (5wt.%)–MgAl₂O₄ in the presence and absence of water was investigated. Coke formation and amount in used catalysts were examined by SEM and TGA, respectively. The results showed that the amount of carbon was suppressed and negligible coke formation (less than 3%) was observed in the presence of 0.2 water/carbon mole ratio over Ni/Ce_0.75Zr_0.25O₂ (5wt.%)–MgAl₂O₄ catalyst.     

H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1−x)Zr(x)O2 catalysts

To develop a single stage water–gas shift reaction (WGS) catalyst for compact reformers, Pt/CeO₂, Pt/ZrO₂, and Pt/Ce_(1−x)Zr_(x)O₂ catalysts have been applied for the target reaction. The CeO₂/ZrO₂ ratio was systematically varied to optimize Pt/Ce_(1−x)Zr_(x)O₂ catalysts. Pt/CeO₂ showed the highest turnover frequency (TOF) and the lowest activation energy (E_a) among the catalysts tested in this study. It has been found that the reduction property of the catalyst is more important than the dispersion for a single stage WGS. Pt/CeO₂ catalyst also showed stable catalytic performance. Thus, Pt/CeO₂ can be a promising catalyst for a single stage WGS for compact reformers.     

Microstructure and hydrogen storage properties of Zr0.8Ti0.2Co1−xFex (x=0, 0.1, 0.2, 0.3) alloys

In the present study, Zr_0.8Ti_0.2Co_1?xFe_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared by arc melting method. The effect of Fe substitution on microstructure and hydrogen storage properties was studied systematically. The phase structure and hydrogen storage properties were characterized by X-ray diffraction (XRD), Electron Probe Micro-analysis (EMPA) and Sievert's type volumetric apparatus. XRD and EPMA analysis show that Zr_0.8Ti_0.2Co alloy forms cubic phase ZrCo and traces of ZrCo₂, while the alloys of composition with x = 0.1, 0.2 and 0.3 form cubic phase ZrCo with the secondary Laves phases Zr(Co,Fe)₂ and Zr₂Co. The cell volumes and content of the secondary phase increase gradually as the content of Fe substitution increases. The hydrogen storage experiment shows that Fe substitution for Co ameliorates initial hydriding kinetic property and shortens the incubation duration of the Zr_0.8Ti_0.2Co_1?xFe_x (x = 0.1, 0.2 and 0.3) alloys, compared with Zr_0.8Ti_0.2Co alloy. The improved kinetic property is due to the catalyst effect of the secondary phase, which makes it favorable for the application in International Thermonuclear Experimental Reactor (ITER).     

A simple and effective process for Sr0.995Ce0.95Y0.05O3−δ and BaCe0.9Nd0.1O3−δ solid electrolyte ceramics

A simple and effective reaction-sintering process for Sr_0.995Ce_0.95Y_0.05O_3−δ and BaCe_0.9Nd_0.1O_3−δ solid electrolyte ceramics was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Sr_0.995Ce_0.95Y_0.05O_3−δ ceramics with 98.4% of the theoretical density were obtained after being sintered at 1350 °C for 2 h. A total conductivity 1.42 mS cm⁻¹ at 900 °C could be obtained in Sr_0.995Ce_0.95Y_0.05O_3−δ sintered at 1500 °C for 4 h. BaCe_0.9Nd_0.1O_3−δ ceramics with 91.7% of the theoretical density were obtained after being sintered at 1500 °C for 2 h. A total conductivity 11.54 mS cm⁻¹ at 900 °C could be obtained in BaCe_0.9Nd_0.1O_3−δ sintered at 1350 °C for 6 h. The reaction-sintering process has proven a simple and effective method to obtain useful Sr_0.995Ce_0.95Y_0.05O_3−δ and BaCe_0.9Nd_0.1O_3−δ ceramics for solid electrolyte applications in solid oxide fuel cells.     

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.     

Microstructural characterization and electrochemical properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and its application for anode of SOEC

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was synthesized successfully by a novel citric acid–nitrate combustion method and employed as the anode of solid oxide electrolysis cells (SOEC) for hydrogen production for the first time in this paper. The crystal structure, chemical composition and electrochemical properties of BSCF were investigated in detail. The results showed that BSCF is in good stoichiometry of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−σ formation. ASR of BSCF/YSZ is only 0.077 Ω cm² at 850 °C, remarkably lower than the commonly used oxygen materials LSM as well as the current focus materials LSC and LSCF. Also, BSCF electrode exhibited much better performance than LSM under both SOEC and SOFC operating modes. The hydrogen production rate of BSCF/YSZ/Ni-YSZ can be up to 147.2 mL cm⁻² h⁻¹, about three times higher than that of LSM/YSZ/Ni-YSZ, which indicates that BSCF could be a very promising candidate for the practical application of SOEC technology.     

Bismuth doping effects on the structure,electrical conductivity and oxygen permeability of Ba0.6Sr0.4Co0.7Fe0.3O3−δ ceramic membranes

A series of Ba_0.6Sr_0.4Co_0.7Fe_0.3−xBi_xO_3−δ (BSCFB, x = 0–0.2) ceramic membranes were prepared by solid state reaction method. The doping effects on the phase structure, structural stability, electrical conductivity and oxygen permeability were investigated. Little amount of Bi (x = 0–0.08) can maintain the cubic perovskite structure of BSCFB materials while more Bi (x > 0.08) will result in the generation of other impurities. Even in the Bi solid solution range, Bi doping is unfavorable for the enhancement of structural stability of BSCFB membranes. The electrical conductivity decreases with Bi doping level, while the oxygen permeability of BSCF membrane can be increased remarkably with little amount of Bi doping (x = 0.05). More Bi leads to the structure deterioration of membrane surface under oxygen permeation condition, resulting in a severe decrease in oxygen permeability. Considering the overall performance, a low Bi doping amount such as x = 0.05 is favored for the membrane applications.