Hydrogen production from ethanol steam reforming over core–shell structured NixOy–, FexOy–, and CoxOy–Pd catalysts

The present work focused on the investigation of the hydrogen generation through the ethanol steam reforming over the core–shell structured Ni_xO_y–, Fe_xO_y–, and Co_xO_y–Pd loaded Zeolite Y catalysts. The transmission electron microscopy (TEM) image of Ni_xO_y–Pd represented a very clear core–shell structure, but the other two catalysts, Co_xO_y– and Fe_xO_y–Pd, were irregular and non-uniform. The catalytic performances differed according to the added core metal and the support. The core–shell structured Co_xO_y–Pd/Zeolite Y provided a significantly higher reforming reactivity compared to the other catalysts. The H₂ production was maximized to 98% over Co_xO_y–Pd(50.0 wt%)/Zeolite Y at the conditions of reaction temperature 600 °C, CH₃CH₂OH:H₂O = 1:3, and GHSV (gas hourly space velocity) 8400 h⁻¹. In the mechanism that was suggested in this work, the cobalt component played an important role in the partial oxidation and the CO activation for acetaldehyde and CO₂ respectively, and eventually, cobalt increased the hydrogen yield and suppressed the CO generation.     

La(1−x)SrxMnO3−δ perovskites as redox materials for the production of high purity hydrogen

Materials of the perovskite structure and of the general formula La_1−xSr_xMnO₃ (x = 0, 0.3, 0.7) are investigated as redox catalysts for the two-step steam reforming of methane towards the production of high purity hydrogen. During the activation step, methane is oxidized with lattice oxygen to carbon dioxide and carbon monoxide, while oxygen is withdrawn from the material until a maximum deficiency level which depends on the strontium content and the reaction temperature. During the reaction step water is splitted to gaseous hydrogen and lattice oxygen that fills the oxygen vacancies. It appeared that, after the achievement of a characteristic oxygen deficiency level, La_1−xSr_xMnO₃ materials exhibit good activity for the water-splitting reaction. The activity is further found to be proportional to the oxygen vacancy concentration. At high activity levels, initial water conversions per 15 μmol pulse of up to 70% are achieved at 1000 °C. The cumulatively produced hydrogen during the water-splitting step, per injected water, increases with increasing strontium content, reaching a production of 60 μmol H₂ per 500 μmol water passed over 200 mg La_0.3Sr_0.7MnO₃ at 1273 K and no coke formation. The materials exhibit stable behavior after eight successive oxidation–reduction cycles. The relations between the redox behavior and the material defect chemistry are discussed. Finally the energy efficiency of the process, future prospects and ways for its optimization are discussed.     

Microstructure and hydrogen storage properties of Ti10V84−xFe6Zrx (x = 1–8) alloys

Ti₁₀V_84−xFe₆Zr_x (x = 1, 2, 4, 6, 8) hydrogen storage alloys were prepared by induction melting with magnetic levitation, and the effects of Zr content on the microstructures and hydrogen storage properties have been investigated systematically. The results show that the alloy with x = 1 has a single V-based solid solution phase with BCC structure, while other alloys with x = 2–8 consist of a BCC main phase and a C14 type Laves secondary phase, and the abundance ratio of the secondary phase increases with increasing Zr content. As the Zr content in the alloy increases, the activation behavior is improved, but the hydrogen absorption and desorption capacities decrease gradually. For the alloy with the Zr content of x = 1, the best overall hydrogen storage properties are obtained.     

An electrochemical investigation of melt-spun nanocrystalline Mg20Ni10−xCux (x = 0–4) electrode alloys

The nanocrystalline Mg₂Ni-type electrode alloys with nominal compositions of Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) were synthesized by the melt spinning technique. The microstructures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that all the as-spun alloys hold typical nanocrystalline structure instead of an amorphous phase. The melt spinning does not modify the major phase Mg₂Ni, but it leads to the formation of crystal defects such as stacking faults, dislocations, sub-grain boundary and twin-grain boundary. The melt spinning significantly improves the electrochemical hydrogen storage capacity of the alloys, whereas it slightly impairs the electrochemical cycle stability of the alloys. The substitution of Cu for Ni significantly ameliorates the electrochemical hydrogen storage performances of the alloys, involving both the electrochemical hydrogen storage capacity and the electrochemical charging and discharging stability.     

Investigation on electrochemical performances of melt-spun nanocrystalline and amorphous Mg2Ni1−xMnx (x = 0–0.4) electrode alloys

In order to enhance the glass forming ability of the Mn₂Ni-type electrode alloy, Ni in the Mg₂Ni compound is partially substituted by Mn. The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg₂Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with increasing amount of Mn substitution.     

Preparation and characterization of La1−xSrxNiyFe1−yO3−δ cathodes for low-temperature solid oxide fuel cells

Perovskite-type La_1−xSr_xNi_yFe_1−yO_3−δ (x = 0.3, 0.4, 0.5, 0.6, y = 0.2; x = 0.3, y = 0.2, 0.3, 0.4) oxides have been synthesized and employed as cathodes for low-temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The segregation of La₂NiO_4±δ is observed to increase with the increasing Sr²⁺ incorporation content according to X-ray diffraction (XRD) results. The as-prepared powders appear porous foam-like agglomeration with particle size less than 1 μm. Maximum power densities yield as high as 725 mW cm⁻² and 671 mW cm⁻² at 600 °C for fuel cells with the LSNF4628 and LSNF7337 composite cathodes. The maximum power densities continuously increase with the increasing Sr²⁺ content in LSNF cathodes, which can be mainly ascribed to the possible charge compensating mechanism. The maximum power densities first increase with the Ni ion incorporation content up to y = 0.3 due to the increased oxygen vacancy, ionic conductivity and oxygen permeability. Further increase in Ni ion content results in a further lowering of fuel cell performance, which can be explained by the association of oxygen vacancies and divalent B-site cations in the cathode.     

Synthesis and characterization of BaIn0.3−xYxCe0.7O3−δ (x = 0, 0.1, 0.2, 0.3) proton conductors

The morphological and electrical properties of yttrium (Y) and indium (In) doped barium cerate perovskites of the form BaIn_0.3−xY_xCe_0.7O_3−δ (with x = 0–0.3) prepared by a modified Pechini method were investigated as potential high temperature proton conductors with improved chemical stability and conductivity. The sinterability increased with the increase of In-doping, and the perovskite phase was found in the BaIn_0.3−xY_xCe_0.7O_3−δ solid solutions over the range 0 ≤ x ≤ 0.3. The conductivities decreased from x = 0.3 to 0 while the tolerance to wet CO₂ improved for BaIn_0.3−xY_xCe_0.7O_3−δ samples with an increase of In-doping. BaIn_0.1Y_0.2Ce_0.7O_3−δ was found to have relatively high conductivity as well as acceptable wet CO₂ stability.     

Conductivity study of dense BaCexZr(0.9−x)Y0.1O(3−δ) prepared by solid state reactive sintering at 1500 °C

A cost and time effective process was used to prepare the solid solutions BaCe_xZr_(0.9−x)Y_0.1O_(3−δ) (0 ≤ x ≤ 0.4). 98% dense samples were obtained by solid state reactive sintering at 1500 °C for 4 h, with the addition of 1 wt% of NiO to the quantity of synthesized/sintered compound. Scanning electron micrographs reveal polygonal grains of 1–5 microns, whose size increases from the compound with no cerium (BCZY09) to the samples containing cerium (BCZY18–BCZY45). The conductivity, measured in wet reducing atmosphere (9% H₂ in N₂, p(H₂O) = 0.015 atm) by impedance spectroscopy, increases with the cerium content. Some samples have also been prepared using barium sulfate (BaSO₄) as barium precursor (instead of barium carbonate BaCO₃) due to its non toxicity. The corresponding samples (prepared at 1575 °C) showed similar properties as the ones prepared with barium carbonate. Furthermore, different geometries (rods, tubes, pellets) could be made.     

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.     

Synthesis and electrochemical properties of (Pr–Nd)1−ySryMnO3−δ and (Pr1−xNdx)0.7Sr0.3MnO3−δ as cathode materials for IT-SOFC

(Pr–Nd)_1−ySr_yMnO_3−δ (P-NSM, y = 0.2, 0.25, 0.3, 0.35) powders made from commercial Pr–Nd mixed oxide, as well as (Pr_1−xNd_x)_0.7Sr_0.3MnO_3−δ (PN3SM, x = 0, 0.5, 0.7, 1) were synthesized by a glycine-nitrate process and characterized as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). XRD patterns showed the powders had formed pure perovskite phase after being calcined at 800 °C for 2 h. (Pr–Nd)_0.7Sr_0.3MnO_3−δ (P-N3SM) achieved a high conductivity of 194 S cm⁻¹ at 500 °C and showed a good chemical stability against YSZ at 1150 °C. And the thermal expansion coefficient of P-N3SM/YSZ cathode was 11.1 × 10⁻⁶ K⁻¹, which well matched YSZ electrolyte film. The tubular SOFC with P-N3SM/YSZ cathode exhibited the maximum power densities of 415, 367, 327 and 282 mW cm⁻² at 850, 800, 750 and 700 °C, respectively, which indicated P-N3SM was potentially applied in SOFC for low cost.     

Investigation on thermal, electrical, and electrochemical properties of scandium-doped Pr0.6Sr0.4(Co0.2Fe0.8)(1−x)ScxO3−δ as cathode for IT-SOFC

A systematic study and evaluation were performed on the effect of scandium doping at the B site of Pr_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (PSCF) on key material properties as cathode for intermediate temperature solid oxide fuel cells (IT-SOFC). The doped products Pr_0.6Sr_0.4(Co_0.2Fe_0.8)_(1−x)Sc_xO_3−δ (PSCFS_x, x=0.0-0.2) retained perovskite structure confirmed by X-ray diffraction, and their particles were smaller than the non-doped materials as evidenced by TEM. The electrical conductivity (EC) of PSCFS_x decreased with increasing Sc³⁺ content, but EC values were still larger than 100 S cm⁻¹ in temperature range of 300-800 °C as x ≤ 0.1. The thermal expansion coefficients (TEC) of PSCFS_x were observed to generally decrease with increasing x especially at lower temperature range of 50-600 °C. In addition, the AC impedance revealed better electrochemical performance of PSCFS_x cathode as x ≤ 0.1 than that of the undoped sample PSCF. Therefore, PSCFS_x (x ≤ 0.1) shows some potential as cathode electrode for IT-SOFC. The function of Sc³⁺ dopant was tentatively elucidated and discussed.     

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.     

Catalytic activity of perovskite-type doped La0.08Sr0.92Ti1−xMxO3−δ (M = Mn,Fe, and Co) oxides for methane oxidation

Transition metal doped La_0.08Sr_0.92M_0.20Ti_0.80O_3−δ (M = Mn, Fe, and Co) perovskite oxides were synthesized by the Pechini method. The methane oxidation behavior and the polarization resistance of the solid oxide fuel cells (SOFCs) with the perovskite oxides as anode was subsequently measured as a function of operation temperature. Surface atomic concentrations of the perovskite oxides were evaluated using X-ray photoelectron spectroscopy (XPS) and their relationship to the catalytic activity were discussed with respect to the transition metal dopant. The complete oxidation of methane was predominant in the low-temperature region, while the partial oxidation of methane occurred at high temperatures. Fe- and Co-doped perovskites showed better catalytic activity for the methane oxidation reaction than Mn-doped powder. This phenomenon could be explained by the high atomic concentration with low oxidation states and the resulting high oxygen vacancy concentration in the Fe- and Co-doped perovskite powder samples.     

Nanospheres and nanorods structured Fe2O3 and Fe2−xGaxO3 photocatalysts for visible-light mediated (λ ≥ 420 nm) H2S decomposition and H2 generation

This article reports our investigation on H₂ generation from visible light (λ ≥ 420 nm) photodecomposition of H₂S by nanomaterial catalysts, α-Fe₂O₃ and its chemically modified Fe_2−xGa_xO₃ (Ga substitution at x = 0.6, FeGaO₃-I and x = 1.0, FeGaO₃-II). Simple template-free hydrothermal technique was employed to synthesize the three photocatalysts. XRD study reveals rhombohedral nanocrystalline structure and FESEM shows nanospheres morphology for Fe₂O₃ and nanosticks/nanorods for both FeGaO₃-I, and FeGaO₃-II. In H₂ generation, Fe₂O₃ and FeGaO₃-II perform moderate and almost same activities in the fresh and used conditions (quantum yield, QY = 6.0–6.8% at 550 nm). Contrarily, fresh FeGaO₃-I exhibits a greater activity (11.2% QY) and the activity is further enhanced (QY = 15.3%) on regeneration and reuse. The intricacy, as resolved by XRD and FESEM, appears to take place through morphology transformation. The present work, thus, successfully demonstrates H₂ generation from H₂S by nanostructured photocatalysts involving morphology dependent activity enhancement.     

Effects of iron content on the structural evolution,electrical properties and thermochemical stability of BaCo0.9−xFexNb0.1O3−δ ceramic membrane

Cubic perovskite oxygen permeation materials BaCo_0.9−xFe_xNb_0.1O_3−δ (BCFN, x = 0.1–0.7) are prepared by the conventional solid state reaction process. The crystal structure development, structural stability, electrical conductivity and oxygen permeation flux are investigated. The introduction of iron makes the formation of cubic perovskite structure for BCFN materials much easier. BCFN exhibits a p-type semiconductor and obeys the thermally activated small polarons hopping mechanism. The electrical conductivity of BCFN increases with increasing temperature and decreases with the Fe-doping concentration. The incorporation of Fe decreases slightly the oxygen permeability of BCFN membranes, but enhances significantly the structure stability of the oxygen permeation membrane in reducing atmosphere. A high oxygen permeation flux of 1.7 ml cm⁻² min⁻¹ at 900 °C through 1 mm densified membrane under air/helium condition is obtained for the composition of BaCo_0.6Fe_0.3Nb_0.1O_3−δ.     

Structure and electrochemical properties of (La1−xDyx)0.8Mg0.2Ni3.4Al0.1 (x = 0.0–0.20) hydrogen storage alloys

The structure and electrochemical characteristics of (La_1−xDy_x)_0.8Mg_0.2Ni_3.4Al_0.1 (x = 0–0.20) hydrogen storage alloys have been investigated. Dysprosium was adopted as a partial substitution element for lanthanum in order to improve electrochemical properties. The XRD, SEM and EDX results showed that the alloys were composed of (La, Mg)₂Ni₇, LaNi₅ and (La, Mg)Ni₂ phases. The introduction of Dy promoted the formation of (La, Mg)₂Ni₇ phase which possesses high hydrogen storage capacity, and controlling dysprosium content at 0.05 can obtain the maximum (La, Mg)₂Ni₇ phase abundance in the alloys. The maximum discharge capacity was heightened from 382.5 to 390.2 mAh/g, which was ascribed to (La, Mg)₂Ni₇ phase abundance increasing from 54.8% to a maximum (60.5%). Also, the biggest discharge capacity retention remained 82.7% after 100 cycles at discharge current density of 300 mA/g.     

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.     

Photovoltaic solar cell properties of CdxZn1−xO films prepared by sol–gel method

Cd_xZn_1−xO films have been deposited by sol–gel spin-coating method onto glass substrates. The Cd/Zn ratio in solution was changed from 0 to 1. Zinc acetate dehydrates, cadmium acetate dehydrates, 2-methoxyethanol and monoethanolamine were used as a starting material (zinc and cadmium), solvent and stabilizer, respectively. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns show that the films are polycrystalline nature. As x varies from 0 to 1, it was observed that the crystal structure changed from wurtzite (ZnO) to cubic (CdO) structure. The optical properties of these films have been investigated by means of the optical transmittance and reflectance spectra. A significant change in optical absorption edge, optical band gap and optical constant with variation in composition was observed.     

Ba1−xCo0.9−yFeyNb0.1O3−δ (x = 0-0.15, y = 0-0.9) as cathode materials for solid oxide fuel cells

Perovskite oxide Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3−δ has been reported as oxygen transport membrane and cathode material for solid oxide fuel cells (SOFCs). In this study, the effects of A-site cation deficiency and B-site iron doping concentration on the crystal structure, thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance of Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.15, y = 0-0.9) have been systematically evaluated. Ba_1−xCo_0.9−yFe_yNb_0.1O_3−δ (x = 0-0.10, y = 0.2 and x = 0.10, y = 0.2-0.6) can be indexed to a cubic structure. Increased electrical conductivity and decreased cathode polarization resistance have been achieved by A-site deficiency. No obvious variation can be observed in TEC by A-site deficiency. The electrical conductivity and TEC of Ba_0.9Co_0.9−yFe_yNb_0.1O_3−δ decrease while the cathode polarization resistance increases with the increase in iron doping concentration. The highest conductivity of 13.9 S cm⁻¹ and the lowest cathode polarization resistance of 0.07 Ω cm² have been achieved at 700 °C for Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ. The composition Ba_0.9Co_0.3Fe_0.6Nb_0.1O_3−δ shows the lowest TEC value of 13.2 × 10⁻⁶ °C⁻¹ at 600 °C and can be a potential cathode material for SOFCs.     

Electrical performance and structural analysis of La1−xBaxGa0.8Mg0.2O3−δ solid electrolyte

The aim of this study was to develop La_1−xBa_xGa_1−yMg_yO_3−δ (x = 0.03–0.1, y = 0.2–0.25) (LBGM) electrolytes for intermediate-temperature solid-oxide fuel cells (SOFCs); these electrolytes were synthesized via a solid-state reaction. In the study, the La_1−xBa_xGa_1−yMg_yO_3−δ samples crystallized in an orthorhombic (Imma) structure, and a BaLaGa₃O₇ phase was detected for x ≥ 0.08 at a fixed y = 0.2. The solubility limit of the Ba ions increased with an increase in the Mg content in the matrix. Two active Raman bands at ca. 677 and 739 cm⁻¹ were observed, and they were attributed to the oxygen vacancies. The La_0.95Ba_0.05Ga_0.75Mg_0.25O_3−δ sample had a higher conductivity ca. 0.1 S/cm at 800 °C, and an activation energy of ca. 0.83–1.27 eV at 500–800 °C. The thermal expansion coefficient (TEC) of the LBGM samples at 200–800 °C was in the range of 10 × 10⁻⁶ to 14 × 10⁻⁶/°C.