Superporous P(2-hydroxyethyl methacrylate) cryogel-M (M:Co,Ni, Cu) composites as highly effective catalysts in H2 generation from hydrolysis of NaBH4 and NH3BH3

Highly porous p(2-hydroxyethyl methacrylate) p(HEMA) cryogels were synthesized via cryopolymerization technique and used as template for Co, Ni, and Cu nanoparticle preparation, then as composite catalyst systems in H₂ generation from hydrolysis of both NaBH₄ and NH₃BH₃. Due to their highly porous and open microstructures, p(HEMA)-Co cryogel composites showed very effective performances in H₂ production from hydrolysis of both chemical hydrides. The characterization of p(HEMA) cryogels, and their metal composites was determined via various techniques including swelling experiments, digital camera images, SEM and TEM images, AAS and TGA measurements. The effect of various parameters on the hydrolysis reaction of NaBH₄ such as metal types, concentration of chemical hydrides, amounts of catalyst, alkalinity of reaction medium and temperature were investigated in detail. It was found that Co nanoparticles are highly active catalysts in H₂ generation reactions from both hydrides. The hydrogen generation rate (HGR) of p(HEMA)-Co was 1596 (mL H₂) (min)⁻¹ (g of Co)⁻¹ which is quite good in comparison to reported values in the literature. Furthermore, kinetic parameters of p(HEMA)-Co metal composites such as energy, enthalpy and entropy were determined as Ea = 37.01 kJmol⁻¹, ΔH# = 34.26 kJmol⁻¹, ΔS# = −176,43 Jmol⁻¹ K⁻¹, respectively.     

Fibrous mixed conducting cathode with embedded ionic conducting particles for solid oxide fuel cells

The Sm_0.5Sr_0.5CoO_3−δ (SSC) fibers with embedded nano-Sm_0.2Ce_0.8O_1.9 (SDC) particles are fabricated by electrospinning process using commercial SDC nanopowders and an SSC precursor gel containing polyvinyl alcohol (PVA) and aqueous metal nitrate. After calcination at 800 °C, fibers with diameters ranged between 300 and 500 nm and well-developed SSC cubic-perovskite structure and SDC fluorite are successfully obtained. The calculated crystallite sizes of SSC and SDC are 20.78 and 45.35 nm, respectively. Over whole measured temperature ranges during the symmetrical cell test, the fiber composite cathode exhibits much lower polarization resistance than conventional powder composite cathodes. The polarization resistances are estimated to 0.06 and 1.23 Ω cm² for the fiber composites and 0.15 and 2.10 Ω cm² for the powder composites at 700 and 550 °C, respectively. The single cell with the fiber composite cathode shows much higher performances; its maximum power density is 380.5 mW cm⁻² at 550 °C and higher than 1278 mW cm⁻² at 700 °C.     

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti_1.4V_0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti_1.4V_0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti_1.4V_0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti₂Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti_1.4V_0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g⁻¹ as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g⁻¹ compared with that of 30 mA g⁻¹, and the cycling life achieved above 200 mAh g⁻¹ after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti_1.4V_0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti_1.4V_0.6Ni. Ti_1.4V_0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.     

Hydrogen storage properties of Mg–TM–La (TM = Ti,Fe, Ni) ternary composite powders prepared through arc plasma method

For the first time, Mg based Mg–Transition metal (TM) –La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg–TM–La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La₂O₃ introduced during passivation. Based on the Pressure–Composition–Temperature measurements, the hydrogenation enthalpies of Mg are determined to be −68.7 kJ/mol H₂ for Mg–Ti–La powder, −72.9 kJ/mol H₂ for Mg–Fe–La powder and −82.1 kJ/mol H₂ for Mg–Ni–La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg–TM–La composites when compared to those in the binary Mg–TM or Mg–RE composites. This is especially true for the Mg–Ni–La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg₂Ni and La₂O₃. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

Magnetically recyclable Fe@Co core-shell catalysts for dehydrogenation of sodium borohydride in fuel cells

Co-based catalysts of the reaction by which hydrogen was obtained from NaBH₄ solution were prepared by chemical reduction in a liquid phase. X-ray diffraction and scanning electron microscopy analyses showed that the as-prepared Fe@Co catalyst was ultrafine and amorphous. The calculated Arrhenius activation energy of the Fe@Co catalyst was 35.62(1) kJ mol⁻¹ while that of the Co catalyst was 38.81(2) kJ mol⁻¹, demonstrating that Fe@Co nanoparticles reduce the activation energy of the reaction more than does a Co nanocatalyst. X-ray absorption spectroscopy (XAS) clearly reveals the valences of Fe and Co. The Fe valence of Fe@Co is smallest among three catalysts because of the Co shell. The molar ration of Fe to Co is 1: 2 as determined by using XPS analysis, indicating that the novel catalyst reduces costs. The generation of hydrogen is schematically elucidated.     

Electrochemical reactivity of magnesium hydride toward lithium: New synthesis route of nano-particles suitable for hydrogen storage

Electrochemical conversion reaction of MgH₂ with Li ion enables the production of in-situ nanometric Mg and MgH₂ particles so-called (nano-Mg) _INSITU and [(nano-MgH₂)_INSITU] showing interesting hydrogen sorption properties with hydrogen absorption at 100 °C under 10 bars of hydrogen pressure (PH₂) and desorption at 200 °C under primary vacuum, respectively. Differential Scanning Calorimetry (DSC) measurements of MgH₂ electrochemically prepared confirmed a decrease in the desorption temperature from 416 °C to 295 °C and in the heat of formation from −74 kJ mole mol⁻¹ (H₂)⁻¹ to −56 kJ mol⁻¹ (H₂)⁻¹ for commercial (particle size diameter: 10 μm–100 μm) and as prepared MgH₂ hydride (particle size: 10 nm–40 nm), respectively.     

A facile two-step synthesis and dehydrogenation properties of NaAlH4 catalyzed with Co–B

Co–B doped NaAlH₄ is successfully synthesized by two-step synthesis process. The first activation step is milling NaH/Al powder and Co–B mixtures under Ar atmosphere. The second step is milling in a lower hydrogen pressure atmosphere. XRD patterns and FTIR spectrum demonstrate that NaAlH₄ is completely formed after 15 h milling in Ar atmosphere following by 40 h milling in 2 MPa H₂ atmosphere. PCT curves of as-prepared NaAlH₄ show that it can release hydrogen at a low temperature of 90 °C. The activation energy value calculated by Arrhenius equation is only 67.95 kJ mol⁻¹. Moreover, the formation mechanism of NaAlH₄ is also discussed.     

Complex quaternary hydrides Mg2(Fe,Co)Hy for hydrogen storage

Complex ternary hydrides based in Mg and transition metals are very attractive materials for hydrogen and energy storage due to their large volumetric capacity, up to 150 kgH₂/m³ in Mg₂FeH₆ and their high dissociation enthalpies. These compounds may be produced at room temperature by mechanical milling of the constituents in H₂ atmosphere. This technique has also served to explore the synthesis of quaternary hydrides Mg₂T_1−zT’_zH_y, combining two transition metals to optimize the properties of the resulting hydride. In the present work we analyze the mechanical synthesis of the compounds Mg₂Fe_1−zCo_zH_y (z = 0, ¼, ½, ¾, 1) by mechanical alloying at room temperature Mg-Fe-Co powder mixtures in adequate proportion, at 0.3 MPa H₂. We follow the mechanosynthesis process trough the analysis of the hydrogen absorption kinetic curves. Samples obtained after a steady state was reached were characterized by X ray diffraction and Mössbauer spectroscopy. The different stages in the mechanosynthesis of these complex hydrides are discussed in terms of the composition and initial state of the powder mixture.     

Study on hydrogen storage properties of Mg nanoparticles confined in carbon aerogels

Magnesium nanoparticles confined in carbon aerogels were successfully synthesized through hydrogenation of infiltrated dibutyl-magnesium followed by hydrogen desorption at 623 K. The average crystallite size of Mg nanoparticle is calculated to be 19.3 nm based on X-ray diffraction analyses. TEM observations showed that the size of MgH₂ particle is mainly distributed in the range from 5.0 to 20.0 nm, with a majority portion smaller than 10.0 nm. The hydrogenation and dehydrogenation enthalpies of the confined Mg are determined to be −65.1 ± 1.56 kJ/mol H₂ and 68.8 ± 1.03 kJ/mol H₂ by Pressure–composition–temperature tests, respectively, slightly lower than the corresponding enthalpies for pure Mg. In addition, the apparent activation energy for hydrogen absorption is determined to be 29.4 kJ/mol H₂, much lower than that of the micro-size Mg particles. These results indicate that the thermodynamic and absorption kinetic properties of confined Mg nanoparticles can be significantly improved due to the ‘nanosize effect’.     

Synthesis and crystal structure analysis of complex hydride Mg(BH4)(NH2)

Complex hydride Mg(BH₄)(NH₂), which consists of double anion BH₄⁻ and NH₂⁻, was synthesized and the crystal structure was analyzed by synchrotron X-ray diffraction. The mixture sample of Mg(BH₄)₂ + Mg(NH₂)₂ prepared by ball milling was reacted and crystallized to Mg(BH₄)(NH₂) by heating at about 453 K. This crystal phase transforms into amorphous phase above 473 K and subsequently the dehydrogenation begins. The crystal structure of Mg(BH₄)(NH₂) was determined from measurement data at 453 K (chemical formula: Mg_0.94(BH₄)₁(NH₂)_0.88, crystal system: tetragonal, space group: I4₁ (No.80), Z = 8, lattice constants: a = 5.814(1), c = 20.450(4) Å at 453 K). Mg(BH₄)(NH₂) is ionic crystal which the cation (Mg²⁺) and the anions (BH₄⁻ and NH₂⁻) are stacking alternately along the c-axis direction. Two BH₄⁻ and two NH₂⁻ tetrahedrally coordinate around Mg²⁺ ion.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

BaCe0.85Tb0.05Co0.1O3−δ perovskite hollow fibre membranes for hydrogen/oxygen permeation

Co-doped BaCe_0.85Tb_0.05Co_0.1O_3−δ (BCTCo) nanopowder was synthesized via a sol–gel method using ethylenediaminetetraacetic acid (EDTA) and citric acid as the chelating agents. Using the resultant powder, BCTCo perovskite hollow fibre membranes were then fabricated by the combined phase inversion and sintering technique. Properties of the BCTCo powder and the hollow fibre membranes in terms of crystalline phase, morphology, electrical conductivity, porosity, mechanical strength and hydrogen/oxygen permeation were investigated by a variety of characterization methods. The results indicated that doping of cobalt in the BCTb oxide led to a higher electrical conductivity and lower calcination temperature for the powder precursor to a perovskite structure as well as sintering temperature for the hollow fibre precursors to gastight membranes. In order to obtain gastight and robust hollow fibre membranes, the sintering temperature should be controlled between 1300 and 1450 °C. The maximum hydrogen flux through the BCTCo hollow fibre membranes reached up to 0.385 mL cm⁻² min⁻¹ at 1000 °C under 50% H₂–He/N₂ gradient, which is higher than that of the un-doped BCTb hollow fibre membranes with the same effective thickness, and especially much higher than that obtained from other proton conductors due to the asymmetric structure of the membrane designed. Moreover, the BCTCo hollow fibre membrane also exhibited noticeable oxygen permeation fluxes, i.e. 0.122 mL cm⁻² min⁻¹ at 1000 °C under the air/He gradient. However, doping of cobalt might damage the mechanical stability of the perovskite membranes in the hydrogen-containing atmosphere.     

Hydrogen generation by the hydrolysis of magnesium–aluminum–iron material in aqueous solutions

Highly activated Mg–Al–Fe materials are prepared from powder by mechanical ball milling method for hydrogen generation. The hydrolysis characteristics of Mg–Al–Fe materials in aqueous solutions under different experimental conditions are carefully investigated. The results show that the hydrolysis reactivity of Mg–Al–Fe material can be significantly improved by increasing the ball milling time and Fe content. The increase of NaCl solution concentration and initial temperature is also found to promote the hydrogen generation reaction. At 25 °C, the Mg60–Al30–Fe10 (wt%) material ball-milled for 4 h shows the best performance in 0.6 mol L⁻¹ NaCl solution, and the reaction can produce 1013.33 ml g⁻¹ hydrogen with a maximum hydrogen generation rate of 499.50 ml min⁻¹ g⁻¹. In comparison to NaCl solution, natural seawater is found to have an inhibiting effect on the hydrolysis of Mg–Al–Fe material. Especially, the presence of Mg²⁺ and Ca²⁺ in seawater can greatly reduce the hydrogen conversion yield, and the SO₄²⁻ can decrease the hydrogen generation rate.     

Mesophilic fermentative hydrogen production from sago starch-processing wastewater using enriched mixed cultures

Biohydrogen (bioH₂) production from starch-containing wastewater is an energy intensive process as it involves thermophilic temperatures for hydrolysis prior to dark fermentation. Here we report a low energy consumption bioH₂ production process with sago starch powder and wastewater at 30 °C using enriched anaerobic mixed cultures. The effect of various inoculum pretreatment methods like heat (80 °C, 2 h), acid (pH 4, 2.5 N HCl, 24 h) and chemical (0.2 g L⁻¹ bromoethanesulphonic acid, 24 h) on bioH₂ production from starch powder (1% w/v) showed highest yield (323.4 mL g⁻¹ starch) in heat-treatment and peak production rate (144.5 mL L⁻¹ h⁻¹) in acid-treatment. Acetate (1.07 g L⁻¹) and butyrate (1.21 g L⁻¹) were major soluble metabolites of heat-treatment. Heat-treated inoculum was used to develop mixed cultures on sago starch (1% w/v) in minimal medium with 0.1% peptone-yeast extract (PY) at initial pH 7 and 30 °C. The effect of sago starch concentration, pH, inoculum size and nutrients (PY and Fe ions) on batch bioH₂ production showed 0.5% substrate, pH 7, 10% inoculum size and 0.1% PY as the best H₂ yielding conditions. Peak H₂ yield and production rate were 412.6 mL g⁻¹ starch and 78.6 mL L⁻¹ h⁻¹, respectively at the optimal conditions. Batch experiment results using sago-processing wastewater under similar conditions showed bioH₂ yield of 126.5 mL g⁻¹ COD and 456 mL g⁻¹ starch. The net energy was calculated to be +2.97 kJ g⁻¹ COD and +0.57 kJ g⁻¹ COD for sago starch powder and wastewater, respectively. Finally, the estimated net energy value of +2.85 × 10¹³ kJ from worldwide sago-processing wastewater production indicates that this wastewater can serve as a promising feedstock for bioH₂ production with low energy input.     

Achieving highly efficient hydrogen generation and uniform Ag nanoparticle preparation via hydrolysis of Mg9Ag alloy milled under hydrogen gas

A Mg₉Ag alloy is employed as a medium for both production of hydrogen and preparation of Ag nanoparticles through hydrolysis. Mg₉Ag milled under H₂ exhibits very favorable structural characteristics, i.e., yielding a fine nanocrystal powder and partial hydrogen-induced phase decomposition. As a result, a hydrogen yield of 730 mL g⁻¹ is obtained in 25 min at 298 K, a much higher rate than produced by samples not milled or milled under argon. Moreover, the hydrolysis by-product can be recycled to obtain Ag nanoparticles by removing insoluble Mg(OH)₂ using an added HCl solution. These results show that this process provides a highly efficient method for economically produce hydrogen and Ag nanoparticles.     

Hydrogen absorption of catalyzed magnesium below room temperature

Hydrogen absorption of magnesium (Mg) catalyzed by 1 mol% niobium oxide (Nb₂O₅) was demonstrated under the low temperature condition even at −50 °C. The kinetic and thermodynamic properties were examined for MgH₂ with and without Nb₂O₅. By considering the remarkable absorption features at such low temperature, the essential hydrogen absorption properties were investigated under accurate isothermal conditions. As the results, the activation energy of hydrogen absorption for the catalyzed Mg was evaluated to be 38 kJ/mol, which was significantly smaller than that of MgH₂ without the catalyst. The kinetic improvement was also found on the hydrogen desorption process. On the other hand, thermodynamic properties were not changed by the catalyst as a matter of course. Therefore, the Nb₂O₅ addition mainly affects the reaction rates between Mg and hydrogen and shows the excellent catalytic effects.     

Kinetics of NaBH4 hydrolysis on carbon-supported ruthenium catalysts

In this work, different shapes (powder and spherical) of ruthenium-active carbon catalysts (Ru/C) were prepared by impregnation reduction method for hydrogen generation (HG) from the hydrolysis reaction of the alkaline NaBH₄ solution. The effects of temperature, amount of catalysts, and concentration of NaOH and NaBH₄ on the hydrolysis of NaBH₄ solution were investigated with different shapes of Ru/C catalysts. The results show that the HG kinetics of NaBH₄ solution with the powder Ru/C catalysts is completely different from that with the spherical Ru/C catalysts. The main reason is that both mass and heat transfer play important roles during the reaction with Ru/C catalysts. The HG overall kinetic rate equations for NaBH₄ hydrolysis using the powder Ru/C catalysts and the spherical catalysts are described as r = A exp (−50740/RT) [catalyst]^1.05 [NaOH]^−0.13 [NaBH₄]^−0.25 and r = A exp (−52,120/RT) [catalyst]^1.00 [NaOH]^−0.21 [NaBH₄]^0.27 respectively.     

Production of an Mg/Mg2Ni lamellar composite for generating H2 and the recycling of the post-H2 generation residue to nickel powder

The magnesium hydrolyzing reaction was catalyzed in situ using a layered Mg₂Ni compound, rapidly producing hydrogen in NaCl solution. The post-H₂ generation residue (mixture of Mg(OH)₂ and Mg₂Ni catalyst) was recycled to recover pure Ni powder from the waste mixture. Pure Mg (153 g) and pure Ni (47 g) in a eutectic composition were easily melted to form a molten alloy by a super-high-frequency (35,000 Hz) induction furnace. The lamellar material had an Mg/Mg₂Ni/Mg/Mg₂Ni… layered structure, in which each layer was ∼0.8 μm thick; Mg was an anodic phase and Mg₂Ni was a cathodic phase (the catalyst). Bulk Mg/Mg₂Ni composite alloy contains many microgalvanic cells. Owing to the lamellar microstructure, no dense hydrated oxide film that might have caused surface passivation was found, allowing continuous H₂ generation until no magnesium remained to participate in the hydrolysis. The activation energy of the hydrolysis reaction in simulated sea water was ∼36.35 kJ mol⁻¹.     

Enhanced hydrogen storage by a variable temperature process

We present a simple method of variable temperature process that can potentially enhance the hydrogen storage properties of a large variety of solid state materials. In this approach, hydrogen gas is first introduced at about room temperature, which is followed by a gradual increase to a preset maximum temperature value, T_max. Using this approach, we investigated hydrogen absorption properties of vertically aligned arrays of magnesium nanotrees and nanoblades fabricated by glancing angle deposition (GLAD) technique, and conventional Mg thin film. Weight percentage (wt%) storage values were measured by quartz crystal microbalance (QCM). After exposing Mg samples to H₂ at 30 bar and 30 °C, dynamic absorption measurements were conducted as the temperature was increased from 30 °C to maximum values of T_max = 100, 200, and 300 ^°C all within 150 min. QCM measurements revealed that variable temperature method results in significant improvements in hydrogen storage values over the ones obtained by conventional constant temperature process. At a low effective temperature T_eff = 165 °C (T_max = 300 °C), we achieved storage values of 6.19, 4.76, and 2.79 wt% for Mg nanotrees, nanoblades, and thin film, respectively.