Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders

Several composite aluminum-metal oxide powders were prepared by mechanical milling and considered for hydrogen production in the Al-water split reaction. The powders included compositions capable of independent, highly exothermic thermite reaction between components: Al·MoO₃, Al·Bi₂O₃, and Al·CuO, as well as chemically inert compositions Al·MgO and Al·Al₂O₃. Experiments used a water displacement method to quantify hydrogen production. In most experiments, the flask containing water and composite powder was maintained at 80 °C; additional limited experiments were performed at varied temperatures. Condensed reaction products were collected and examined using electron microscopy and X-ray diffraction. For all compositions, the aluminum-water split reaction was nearly complete. Average reaction rates were comparable to those reported earlier for materials with similar particle sizes prepared by ball milling. Reaction rates were affected by the specific composition; the Al·Bi₂O₃ composite reacted substantially faster than other materials. It was observed that the Al-water split reaction initiated at 80 °C could be completely stopped by cooling the reacting flask to room temperature; the reaction did not restart at room temperature but could be resumed at its previous rate by heating the flask back to 80 °C. For Al·MoO₃ composite, an interruption in the hydrogen production was observed at a constant temperature; it was associated with the formation of MoO_2.4(OH)_0.6, a hydrated MoO₃ phase. Evidence of thermite reactions interfering with the Al-water split reaction and generating metallic Bi and Cu was obtained for experiments with Al·Bi₂O₃, and Al·CuO composites, respectively. A qualitative reaction mechanism is proposed assigning different rate controlling processes to different stages of the Al-water split reaction for the composites prepared by ball milling.     

Preparation and characterization of La0.8Sr0.2Ga0.83Mg0.17O3 electrolyte by polyol method for solid oxide fuel cells

Perovskite type La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ powders were prepared via simple polyol method for the first time in literature. Obtained material was characterized by using XRD, SEM, Impedance Spectroscopy and density measurements. Pure LSGM powders were achieved after heat treatment at 1100 °C for 12 h. 87% relative density was obtained after pressing of these powders under 1 MPa and sintering at 1250 °C for 12 h. Average particle size was calculated as 1.77–2.4 μm from SEM micrographs. Overall conductivity of the LSGM pellet was found to be 0.0014 S/cm at 850 °C with impedance analysis and showed that the preparation process needs improvements.     

Synthesis of Mo/TNU-9 (TNU-9 Taejon National University No. 9) catalyst and its catalytic performance in methane non-oxidative aromatization

Mo-modified catalysts of Mo/TNU-9 were prepared for the non-oxidative aromatization of methane. The effects of different Si/Al ratios, Mo loadings and temperatures on the catalytic performance of Mo/TNU-9 catalysts were also investigated. Furthermore, for comparison, Mo/ZSM-5 was synthesized for the same reaction. The physical properties and acidities of the samples were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), BET (Brunauer-Emmett-Teller method) and IR (infrared) spectroscopy. The best conditions for methane aromatization with Mo/TNU-9 are as follows: Si/Al ratio = 50, 6 wt.% MoO₃ loading, and reaction temperature = 973 K. Compared with Mo/ZSM-5, the Mo/TNU-9 catalyst showed both a higher conversion of methane and higher selectivity to benzene. In addition, the stability of Mo/TNU-9 was also better than that of Mo/ZSM-5. The superior catalytic behavior of Mo/TNU-9 may be attributed to its unique three-dimensional 10-member-ring channel structure with the presence of large 12-member-ring cavities.     

Electrochemical properties of Cu6Sn5 alloy powders directly prepared by spray pyrolysis

Spherical shape Cu–Sn alloy powders with fine size for lithium secondary battery were directly prepared by spray pyrolysis. The mean size and geometric standard deviation of the Cu–Sn alloy powders prepared at a temperature of 1100 °C were 0.8 μm and 1.2, respectively. The powders prepared at a temperature of 1100 °C with low flow rate of carrier gas as 5 l min⁻¹ had main XRD peaks of Cu₆Sn₅ alloy and copper-rich Cu₃Sn alloy phases. Cu and Sn components were well dispersed inside the submicron-sized alloy powders. The discharge capacities of the Cu₆Sn₅ alloy powders prepared at a flow rate of 5 l min⁻¹ dropped from 485 to 313 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C. On the other hand, the discharge capacities of the Cu–Sn alloy powder prepared at a flow rate of 20 l min⁻¹ dropped from 498 to 169 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C.     

Electrochemical performance of Ni-RE (RE = rare earth) as electrode material for hydrogen evolution reaction in alkaline medium

In this work, Ni-RE (RE = rare earth, La, Ce) materials were obtained by the solid state reaction using as Ni source: a) metal acetylacetonates and b) metal powders while rare earth element was added from acetylacetonates. These materials were synthesized for 3 h at different temperatures (800 °C or 900 °C, 1000 °C and 1200 °C) in order to evaluate their electrochemical performance on the Hydrogen Evolution Reaction (HER). The effects of the sintering temperature and the Ni source on the morphology, structure and particle size were evaluated and correlated with the displayed catalytic activity. The results showed that depending of the added rare earth element and Ni source the formed compounds varied from a mixture of oxides (NiO, CeO₂) and intermetallic compounds (LaNiO₃) at low annealing temperatures up to the formation of the NiO-CeO₂, NiO-LaNiO₃ and NiO-La₄Ni₃O₁₀ compounds at 1200 °C. From Scanning Electron Microscopy (SEM) results, it was observed that the agglomerates of Ni-RE electrode materials presented a more uniform shape (semispherical) and lower crystal sizes (0.2-2.0 μm) using acetylacetonate precursors than that obtained with Ni powders (5-50 μm). It was found that the individual organization of the nickel particles and their electrocatalytic activity is affected by diverse factors: a) the type of precursor used in the synthesis, b) the reaction temperature and c) the synergetic effect caused by the addition of the rare earth metal, which seems to be better for lanthanum than for cerium. The Tafel parameters of the stabilized Ni-RE electrodes revealed that the formation of Ni-La intermetallic compounds at low temperature favors the current densities on the HER. Thus a clear dependence of the electrocatalytic activity on the source of these Ni-RE materials was observed.     

New ternary Fe, Co, and Mo mixed oxide electrocatalysts for oxygen evolution

Ternary mixed oxides of Fe, Co, and Mo with composition Fe_xCo_1-xMoO₄ (x = 0.25, 0.50 & 0.75) have been prepared by a co-precipitation method and investigated as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solutions. The study indicates that partial replacement of Co by Fe in the CoMoO₄ matrix increases the specific surface area as well as the apparent electrocatalytic activity of the oxide; the magnitude of increase being the greatest with x = 0.25. The order for the OER with respect to OH⁻ concentration has been observed to be ∼2 on Fe_0.75Co_0.25MoO₄ and ∼1 on Fe_0.25Co_0.75MoO₄ and Fe_0.5Co_0.5MoO₄. The Tafel slopes at low overpotentials were close to 40 mV with each oxide catalyst.     

Hydrogen and multiwall carbon nanotubes production by catalytic decomposition of methane: Thermogravimetric analysis and scaling-up of Fe–Mo catalysts

Fe-based catalysts doped with Mo were prepared and tested in the catalytic decomposition of methane (CDM), which aims for the co-production of CO₂-free hydrogen and carbon filaments (CFs). Catalysts performance were tested in a thermobalance operating either at isothermal or temperature programmed mode by monitoring the weight changes with time or temperature, respectively, as a result of CF growth on the metal particles. Maximum performance of Fe–Mo catalysts was found at the temperature range of 700–900 °C. The addition of Mo as dopant resulted in an increase in the rate and amount of deposited carbon, reaching an optimum in the range 1.7–5.1% (mol) of Mo for Fe–Mo/Al₂O₃ catalysts, whereas for Fe–Mo/MgO catalyst an optimum at 5.1% Mo loading was obtained. XRD study revealed the effect of the Mo addition on the Fe₂O₃/Fe crystal domain size in the fresh and reduced catalysts. Tubular carbon nanostructures with high structural order were obtained using Fe–Mo catalysts, mainly as multiwall carbon nanotubes (MWCNTs) and bamboo carbon nanotubes. Fe–Mo catalysts showing best results in thermobalance were tested in a rotary bed reactor leading to high conversions of methane (70%) and formation of MWCNTs (5.3 g/h).     

Amorphous MoS2 nanosheets on MoO2 films/Mo foil as free-standing electrode for synergetic electrocatalytic hydrogen evolution reaction

A facile oxidation-sulfidation strategy is proposed to fabricate the vertically aligned amorphous MoS₂ nanosheets on MoO₂ films/Mo foil (MF) as free-standing electrode, which features as the integration of three merits (high conductivity, abundant exposures of active sites, and enhanced mass transfer) into one electrode for hydrogen evolution reaction (HER). Density functional theory (DFT) calculations reveal the strong interaction between MoS₂ and MoO₂, which can enhance the intrinsic conductivity with narrow bandgap, and decreases hydrogen adsorption free energy (ΔG_H1 = ~0.06 eV) to facilitate the HER process. Benefiting from the unique hierarchical structure with amorphous MoS₂ nanosheets on conductive MoO₂ films/MF to facilitate the electron/mass transfer by eliminate contact resistance, controllable number of stacking layers and size of MoS₂ slabs to expose more edge sites, the optimal MoS₂/MoO₂/MF exhibits outstanding activity with overpotential of 154 mV at the current density of 10 mA cm⁻², Tafel slope of 52.1 mV dec⁻¹, and robust stability. Furthermore, the intrinsic HER activity (vs. ECSA) on MoS₂/MoO₂/MF is significantly enhanced, which shows 4.5 and 18.6 times higher than those of MoS₂/MF and MoO₂/MF at overpotential of 200 mV, respectively.     

Absorption and desorption of H by NbMo alloys at high temperature

Absorption and desorption of hydrogen have been investigated in Nb₉₅Mo₅ and Nb₈₀Mo₂₀ alloys over wide temperature ranges. On continuous heating H desorption from Nb₉₅Mo₅ was found to take place between 800 and 1000 K and from Nb₈₀Mo₂₀ between 900 and 1100 K. The observed increase in the desorption temperature with increasing Mo content has been attributed to a higher stability of the Mo and NbMo oxides with respect to those of Nb. The solid–gas reaction during absorption was first order and the rate limiting process consisted in the penetration of H atoms through surface oxides. At high temperatures and in the presence of H the oxides are expected to become permeable to H due to the reduction of higher valence to lower valence oxides. The values of the activation energy for H diffusion within the oxide films were 0.82 ± 0.04 eV for Nb₉₅Mo₅ and 1.1 ± 0.1 eV for Nb₈₀Mo₂₀. The thicknesses of the oxide films estimated from the absorption data were of the order of 1 μm.     

In situ construction of oxygen deficient MoO3-x nanosheets/porous graphitic carbon nitride for enhanced photothermal-photocatalytic hydrogen evolution

In this work, the photocatalysts containing oxygen-deficient molybdenum oxide and macroscopic three-dimensional porous graphitic carbon nitride phase composite (MoO_3-x/PCN) were prepared by in situ self-assembly method. The crystal phase and structure were characterized by XRD, XPS, FT-IR, SEM, and TEM measurements. Hydrogen production results showed that introducing of MoO_3-x resulted in a higher hydrogen production rate of MoO_3-x/PCN composite catalyst than that of PCN. Among them, the highest hydrogen production rate of 2336.15 μmol g⁻¹ h⁻¹ was achieved for MoO_3-x-10/PCN, which was 2.23 times higher than PCN (1048.00 μmol g⁻¹ h⁻¹). When the reaction system temperature was 100 °C, the photothermal hydrogen production rate of MoO_3-x-10/PCN was 8902.00 μmol g⁻¹ h⁻¹, which was 3.81 times higher than that at room temperature. PL spectra, UV–vis spectra and photoelectrochemical measurements showed that the localized surface plasmon resonance (LSPR) effect of MoO_3-x effectively enhanced the photo response range and increased the temperature of the reaction system. ESR measurements showed that he composites should follow the Z-scheme charge transfer mechanism, the electrons in the CB of MoO_3-x further migrate to the VB of PCN, which hinders the charge complexation in MoO_3-x and PCN, improving the hydrogen production activity. This study provides a new idea for constructing a plasma-based photothermal synergistic catalytic hydrogen production strategy.     

Morphology control of La(Sr)Fe(Co)O3−a cathodes for IT-SOFCs

A La0.6Sr0.4Co0.2Fe0.8O3−a (LSCF) powder was prepared through a citric synthesis route and subsequent media agitating milling. The milling for 1.5 and 3 h reached the average particle sizes of 0.66 and 0.53 μm, respectively. Then, the LSFC cathodes were formed using the two powders in a conventional manner. It was shown that the cathode performance was strongly influenced by the starting particle size as well as sintering temperature. The smallest cathode polarization for both 700 and 800 °C operations was obtained when using the finer powder (0.53 μm) and sintering at 850 °C, suggesting an excellent cathode morphology. An anode-supported single cell with this cathode structure was fabricated and demonstrated a good generation performance under intermediate temperature operation.     

New ternary mixed oxides of Fe, Ni and Mo for enhanced oxygen evolution

Ternary mixed oxides of Fe, Ni and Mo with molecular formulas Fe_xNi_1−xMoO₄ (x = 0.25, 0.50 and 0.75) have been prepared by a co-precipitation method and investigated for their structural and electrocatalytic properties by XRD, AFM, electrochemical impedance spectroscopy and anodic Tafel polarization. Results indicate that the apparent oxygen evolution activity of the base (NiMoO₄) electrode significantly increases with introduction of Fe from 0.25 to 0.75 mol. The Tafel slope for the oxygen evolution reaction at low overpotentials is found to be only ∼35 mV on Fe-substituted oxides, while it was ∼75 mV on the base oxide. The reaction follows the first order kinetics with respect to OH⁻ concentration, regardless of Fe content in the oxide.     

Low temperature synthesis of Yb doped SrCeO3 powders by gel combustion process

In this work, SrCe_0.9Yb_0.1O_3−δ powders were synthesized by a gel combustion method which combined gel process and combustion process. The effect of ratio of citric acid to metal cations (C/M), oxidizer and calcination temperature on the properties of powders was investigated in detail. It was found that the extra oxidizer NH₄NO₃ increased the flame temperature of combustion and thus promoted the formation of SrCeO₃. The relative amount of SrCeO₃ in powder increased as the C/M ratio increased. The as-ignited powder at 250 °C mainly consisted of the perovskite SrCeO₃, i.e. relative amount of 95.2 wt%. The adiabatic flame temperature of the combustion reaction was calculated to be 1903.1 °C, higher than the required formation temperature of 787.2 °C for SrCeO₃. Furthermore, the pure perovskite phase powder with agglomerated microstructure and average grain size of 2 μm was obtained after calcination at 1200 °C for 5 h. This heat-treatment temperature is 200 °C lower than the conventional solid state reaction method for SrCe_0.9Yb_0.1O_3−δ preparation.     

Electrochemical reactivity of ball-milled MoO3−y as anode materials for lithium-ion batteries

The electrochemical reactivity of ball-milled MoO₃ powders was investigated in Li rechargeable cells. High-energy ball-milling converts highly-crystalline MoO₃ bulk powders into partially reduced low-crystalline MoO_3−y materials with a reduced particle size. Both bulk and ball-milled MoO₃ exhibit a first discharge capacity beyond 1100 mAh g⁻¹ when tested in the 0–3 V (vs. Li/Li⁺) range, which is indicative of a complete conversion reaction. It is found that partial reduction caused by ball-milling results in a reduction in the conversion reaction. Additionally, incomplete re-oxidation during subsequent charge results in the formation of MoO₂ instead of MoO₃, which in turn affects the reactivity in subsequent cycles. As compared to bulk MoO₃, ball-milled MoO_3−y showed significantly enhanced cycle performance (bulk: 27.6% charge capacity retention at the 10th cycle vs. ball-milled for 8 h: 64.4% at the 35th cycle), which can be attributed to the nano-texture wherein nanometer-sized particles aggregate to form secondary ones.     

Mo incorporated Ni nanosheet as high-efficiency co-catalyst for enhancing the photocatalytic hydrogen production of g-C3N4

The design and development of noble metal-free, low-cost and stable co-catalyst are of great significance to the practical application of photocatalysts. In this work, the Mo incorporated Ni nanosheets (MoNi NSs) are successfully prepared and loaded onto g-C₃N₄ via a simple and controllable method. The controlled loading of MoNi NSs with an optimal Mo intake can greatly enhance the photocatalytic H₂-evolution property of g-C₃N₄ (Mo_0.25Ni_0.75/CN5). Specifically, the Mo_0.25Ni_0.75/CN5 exhibits the highest photocatalytic H₂-evolution rate of 273.2 μmol h⁻¹ (5464 μmol h⁻¹ g⁻¹), which is the highest rate under one-solar light in the g-C₃N₄ systems coupled with noble-metal-free co-catalysts. The greatly enhanced photocatalytic H₂-evolution activity of MoNi/g-C₃N₄ is attributed to the role of MoNi as an outstanding co-catalyst to promote the carriers’ separation and transfer, and accelerate the surface H₂ evolution reaction (HER).     

Effects of particle size and type of conductive additive on the electrode performances of gas atomized AB5-type hydrogen storage alloy

The phase composition, morphology, structure, and state of the surface of gas atomized LaNi_4.5Al_0.5 alloy powders constituting a fine (≤50 μm), a medium (160–316 μm), and a coarse (630–1000 μm) fraction have been investigated. The electrochemical and storage characteristics of electrodes made from these powders with addition of electrolytic copper powder or a carbon composite (1 wt.% carbon nanotubes + 7 wt.% nanosized carbon black) as a conductive additive have been studied. In the work, X-ray diffraction, scanning electron microscopy, electron-probe microanalysis, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and several electrochemical methods have been used. It has been established that, in the initial state, the coarse-fraction gas atomized powders show a better kinetics of the hydrogen exchange reactions and higher discharge capacity (∼300 mA h/g). It is shown that electrodes made from the powders of all the fractions have a good high-rate discharge capability. Hydrogen diffusion coefficients during discharge of the electrodes made from the alloy powders of all the fractions and conductive additives have been calculated. It is shown that, for LaNi_4.5Al_0.5 alloy electrodes with the composite carbon additive, the hydrogen diffusion coefficients during discharge computed from data obtained by the electrochemical impedance spectroscopy method agree well with those calculated from cyclic current–voltage curves (2–4 × 10⁻⁹ cm²/s).     

MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite anode for solid oxide fuel cells

MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite has been prepared by a hydrothermal method combined with an in situ diffusion growth process. Single cells based on 300 μm LSGM electrolyte have been fabricated with the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite anode and a composite cathode consisting of Sr_0.9Ce_0.1CoO_3−δ and Sm-doped ceria (SDC). The peak power densities reach 225, 50, 75 mW cm⁻² at 900 °C in H₂, CH₄ and C₃H₈, respectively. The cell shows excellent long-term stability at 850 °C. The preliminary results demonstrate that the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite is a promising alternative anode for solid oxide fuel cells.     

Effect of chemisorbed CO on MoOx–Pt/C electrode on the kinetics of hydrogen oxidation reaction

The influence of poisoning of MoO_x–Pt catalyst by CO on the kinetics of H₂ oxidation reaction (HOR) at MoO_x–Pt electrode in 0.5 mol dm⁻³ HClO₄ saturated with H₂ containing 100 ppm CO, was examined on rotating disc electrode (RDE) at 25 °C. MoO_x–Pt nano-catalyst prepared by the polyole method combined with MoO_x post-deposition was supported on commercial carbon black, Vulcan XC-72. The MoO_x–Pt/C catalyst was characterized by TEM technique. The catalyst composition is very similar to the nominal one and post-deposited MoO_x species block only a small fraction of the active Pt particle surface area. MoO_x deposition on the carbon support can be ruled out from the EDAX results and from the low mobility of these oxides under used conditions. Based on Tafel–Heyrovsky–Volmer mechanism the corresponding kinetic equations from a dual-pathway model were derived to describe oxidation current–potential behavior on RDE over entire potential range, at various CO coverages. The polarization RDE curves were fitted with derived polarization equations according to the proposed model. The fitting showed that the HOR proceeded most likely via the Tafel–Volmer (TV) pathway. A very high electrocatalytic activity observed at MoO_x–Pt catalyst for the hydrogen oxidation reaction in the presence of 100 ppm CO is achieved through chemical surface reaction of adsorbed CO with Mo surface oxides.     

Effect of Mo content on hydrogen evolution reaction activity of Mo2C/C electrocatalysts

Recent research suggests that molybdenum carbide (β-Mo₂C) has the potential to be a cheap and active substitute for Pt-based electrocatalyst for hydrogen evolution reaction. In this article molybdenum carbide (Mo₂C) electrocatalysts immobilized on carbon support were synthesized and evaluated for hydrogen evolution reaction (HER). The quantity of Mo in the samples was varied to understand the effect of Mo content in Mo₂C/C electrocatalyst on the structure, morphology, electrochemical properties and HER. The Mo weight percentages determined by ICP-OES technique in four Mo₂C/C samples prepared were found as ~9.3, 15.8, 20.4 and 28.0. SAXS studies revealed that the pore size of the carbon increased with an increase in Mo content, most probably to accommodate the Mo₂C motifs. X-ray photoelectron spectra showed that the amount of low valent Mo increased as we increased the Mo content up to 20 wt % but decreased in the 28 wt % sample. All the samples were active for electrochemical HER with the sample having ~20 wt % Mo showing the highest activity and exhibited a Tafel slope of 69 mVdec⁻¹. Among all samples the 20 wt% Mo sample exhibited the highest electrochemical surface area (ECSA) of ~2.92 mFcm⁻² and minimum charge transfer resistance for the HER. Thus, it is concluded that 20 wt% Mo in Mo₂C/C electrocatalyst evolves with ideal pore size, highest ECSA, smooth charge transfer and thus exhibits the best electrochemical properties for HER.     

Integrated design of hydrogen production and thermal energy storage functions of Al-Bi-Cu composite powders

In recent years, the hydrolysis of Al-based composite powders to produce hydrogen has become a hot topic in the field of hydrogen energy research. However, the hydrogen generation products of Al-based alloys have not been reasonably utilized. For this purpose, this study proposed a novel research idea to achieve the integrated design of hydrogen production and thermal energy storage functions of Al-based composite powders. Specifically, Al-Bi-Cu composite powders with stable hydrogen production were taken as research objects. The hydrogen was obtained by the reaction of Al-Bi-Cu alloy powders with H₂O for different reaction times, and then the hydrogen generation products were directly sintered at high temperature to obtain Al-Cu alloy based composite phase change thermal energy storage materials. The results indicated that at 50 °C, the hydrogen yield of Al-Bi-Cu alloy powders in 100min, 200min and 400min are 319.9 mL/g, 428.5 mL/g and 665.8 mL/g, respectively. Importantly, the Al-Cu alloy based composite phase change thermal energy storage materials prepared by the hydrogen generation products exhibited an adjustable phase change temperature (577.3 °C ∼ 598.2 °C), high thermal energy storage density (44.1J/g ∼ 153.5J/g), good thermal cycling stability and structural stability.