Synthesis of La2MgNi9 hydrogen storage alloy in molten salt

La₂MgNi₉ alloy is synthesized directly from the sintered mixture of La₂O₃ + NiO + MgO in the molten CaCl₂ electrolyte by the electro-deoxidation method at 740 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy are observed. Sintering (at 1200 °C for 2 h) converts the hygroscopic La₂O₃ (by the reaction with NiO) into the non-hygroscopic La₂NiO₄ and La₃Ni₂O₇ phases. The X-ray diffraction peaks indicate that the electro-deoxidation causes LaOCl, Ni, LaNi₅ and even target phase La₂MgNi₉ to form within 2 h process time. The molten salt synthesis process ends up with the final alloy structure of 79% La₂MgNi₉ and 21% retained LaNi₅. The porous alloy structure (with approximately 31.66 m²g⁻¹ specific surface area) is beneficial for higher hydrogen storage capacity and it is observed that La₂MgNi₉ alloy has promising discharge capacity which is approximately 280 mAhg⁻¹. This work clearly indicates that the electro-deoxidation is a very effective method in the synthesis of the hydrogen storage materials.     

A study on the hydrogen-storage properties of La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys

The La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys were prepared by magnetic levitation melting under Ar atmosphere. The effects of partial substitution Ti for La on the phase structures, hydrogen-storage properties and electrochemical characteristics of the alloys were investigated systematically. For La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys, LaNi₅, LaNi₃ and LaMg₂Ni₉ are the main phases, the maximum hydrogen-storage capacity is 1.51, 1.36, 1.35 and 1.22 wt%, respectively. The absorption–desorption plateau pressure of the alloys first decreases and then increases with increase of Ti content, and the La_1.8MgTi_0.2Ni₉ alloy has the lowest absorption–desorption plateau pressure. The discharge voltage of the alloy electrodes rises with increasing the amount of Ti content. The La_1.8Ti_0.2MgNi₉ alloy electrode presents good electrochemical performance.     

Effect of iron content on the hydrogen production kinetics of electroless-deposited CoNiFeP alloy catalysts from the hydrolysis of sodium borohydride,and a study of its feasibility in a new hydrolysis using magnesium and calcium borohydrides

The effect of Fe content in electroless-deposited CoNi-Fe_x-P alloy catalysts (x = 5.5–11.8 at.%) from the hydrolysis of NaBH₄ is investigated in alkaline sodium borohydride solution. The electroless-deposited CoNiFe_5.5-P and CoNiFe_7.6-P alloy catalysts are composed of flake-like micron particles; however, with an increase in Fe content to 11.8 at.%, the flake-like morphology is changed to a spherical shape and the crystal structure of the electroless-deposited CoNiFeP catalyst is transformed from FCC to BCC. Among all the CoNi-Fe_x-P alloy catalysts, the CoNi-Fe_x-P (x = 7.6 at.%) catalyst has the highest hydrogen production rate of 1128 ml min⁻¹ g⁻¹_catalyst in alkaline solution containing 1 wt% NaOH + 10 wt% NaBH₄ at 303 K. For the optimized catalyst, the activation energy of the hydrolysis of NaBH₄ is calculated to be 54.26 kJ mol⁻¹. Additionally, in this work, we report a new hydrolysis using Mg(BH₄)₂ and Ca(BH₄)₂. As a result, the Mg(BH₄)₂ is stored unstably in an alkaline solution, whereas the Ca(BH₄)₂ is stored stably. When optimizing the hydrogen production kinetics from the hydrolysis of Ca(BH₄)₂, the rate is 784 ml min⁻¹ g⁻¹_catalyst in 10 wt% NaOH + 3 wt% Ca(BH₄)₂ solution.     

Influence of the synthesis route on hydrogen sorption properties of La2MgNi7Co2 alloy

Solid-gas and electrochemical hydrogenation properties of La₂MgNi₇Co₂ alloy are presented. Hydrogen concentration of 1.90 wt% at hydrogen pressure of 10 bar has been reached. The influence of the fabrication technology of La₂MgNi₇Co₂ alloy on electrochemical performance of the hydride electrode were studied and discussed. To evaluate electrochemical characteristics of La₂MgNi₇Co₂ electrodes including discharge capacity, self-discharge and kinetic parameters the galvanostatic charge/discharge technique was used. The studied samples were a multiphase. The presence of Mg-enriched phases (La₂MgO_x, (La, Mg)Ni₃ and LaMgNi₄) raises hydrogen capacity and makes an electrode less susceptible for the self-discharge effect. On the other hand Mg-presence in MH electrodes lowers the hydrogen desorption rate. It was found that, the dominant abundance of the LaNi₅ phase in the tested materials has a positive effect on the kinetic parameters of the hydride electrode.     

Microstructures and hydrogen absorption–desorption behavior of an A2B7-based La-Mg-Ni alloy

The microstructural changes during hydrogen absorption–desorption cycles of an A₂B₇-based La-Mg-Ni alloy with a nominal composition of La_1.5Mg_0.5Ni_7.0 were systematically investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The ternary La-Mg-Ni alloy was mostly composed of 2H-A₂B₇ phase with minor inclusions of 3R-A₅B₁₉, 2H-A₅B₁₉ and 3R-AB₃ phases existing as parts of intergrowth structures with the major A₂B₇ phase. Most parts of the major 2H-A₂B₇ phase containing Mg exhibited an excellent crystal structure retention after the hydrogen absorption–desorption cycles at 80 °C. Two types of defected bands were found to develop after the first hydrogen absorption–desorption cycle. The first ones are amorphous bands developed inside the minor 3R-AB₃ phase, while the second ones develop as heterogeneously strained regions inside the major 2H-A₂B₇ phase. Both the defected bands are considered to be responsible for the irreversible hydrogen capacity of the A₂B₇-based La_1.5Mg_0.5Ni_7.0 alloy during the hydrogen absorption–desorption cycles at 80 °C.     

Structures and electrochemical hydrogen storage behaviours of La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.4) alloys prepared by melt spinning

In order to improve the electrochemical performance of the La–Mg–Ni system A₂B₇-type electrode alloys, La in the alloy was partially substituted by Pr and melt spinning technology was used for preparing La_0.75−xPr_xMg_0.25Ni_3.2Co_0.2Al_0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys. The microstructures and electrochemical performance of the as-cast and spun alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the as-cast and spun alloys have a multiphase structure which consists of two main phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Pr for La leads to an obvious increase of the (La, Mg)Ni₃ phase and a decrease of the LaNi₅ phase in the alloys. The results of the electrochemical measurement indicate that the discharge capacity of the alloys first increases and then decreases with variation of the Pr content. The cycle stability of the alloy monotonically rises with increasing Pr content. When the Pr content rises from 0 to 0.4, the discharge capacity increases from 389.4 (x = 0) to 392.4 (x = 0.1) and then drops to 383.7 mAh/g (x = 0.4) for the as-cast alloy. Discharge capacity increases from 393.5 (x = 0) to 397.9 (x = 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The capacity remaining after 100 cycles increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy.     

Increased interface effects of PtFe alloy/CeO2/C with PtFe selective loading on CeO2 for superior performance in direct methanol fuel cell

Here, a simple two-step solvothermal approach has been employed to synthesize PtFe alloy (or Pt)/CeO₂/C with PtFe (or Pt) selective loading on CeO₂ nanoparticles. In addition, the selective loading of PtFe alloy or Pt nanoparticles on the surface of CeO₂ is achieved under weak alkaline environment, which is mainly attributed to the opposite electrostatic force between H⁺ enriched on the surface of CeO₂ particles and OH⁻ covered with carbon supporters. As-prepared PtFe alloy (or Pt)/CeO₂/C catalysts with two-stage loading structures show more excellent electro-catalytic efficiency for methanol oxidation as well as duration compared with commercial Pt/C and PtCeO₂/C with random loading structure. Further, single-cell assembly based on Pt₃Fe/CeO₂/C as the anode catalyst exhibits a maximum power density of 31.1 mW cm⁻², which is 1.95 times that of an analogous cell based on the commercial Pt/C. These improved performances with considerable low Pt content (<0.3 mg cm⁻²) are mainly ascribed to the abundant three phase interfaces (PtCeO₂ carbon) induced by the selective and efficient dispersion of Pt nanoparticles on ceria.     

Combined steam and CO2 reforming of methane over one-pot prepared Ni/La-Si catalysts

The highly dispersed mNi/xLa−Si catalysts with varied weight percentages of Ni and La were synthesized via one-pot sol-gel process and subsequently applied to combined carbon dioxide and steam reforming of methane (CSDRM) for syngas production. The addition of La improved the catalytic activity and stability as well as the coke resistance of the mNi/xLa−Si catalysts. The effects of preparation routes, Ni contents and CO₂/steam (C/S) ratios on the performances of the Ni/LaSi catalysts were studied in detail for the CSDRM. The 17.5Ni/3.0LaSi catalyst synthesized with the assistance of poly (ethylene glycol) and ethylene glycol exhibited the most excellent catalytic activity, stability and coke resistance. In addition, the H₂/CO ratios in the product gas could be tuned by changing the C/S ratios in the feed. When the C/S ratio was 0.5, the H₂/CO ratio of about 2 was achieved for the 17.5Ni/3.0LaSi catalyst.     

Exploration of Ti substitution in AB2-type YZrFe based hydrogen storage alloys

To improve the hydrogen storage properties of YZrFe alloys, the alloying with Ti was carried out to obtain Y_0.7Zr_(0.3-x)Ti_xFe₂ (x = 0.03, 0.09, 0.1, 0.2) alloys by different processes. It was expected that Ti would substitute Zr and decrease the lattice constant of YFe₂-based C15 Laves phase. All YZrTiFe quaternary alloys consist of the main Y(Zr)Fe₂ phase and the minor YFe₃ phase. Despite the large solubility of Ti in Zr or Zr in Y, the Ti incorporation into YZrFe alloys results in the inhomogeneity of Y and the segregation of Ti, and thus decreases the hydrogen storage capacity. Only the alloy Y_0.7Zr_0.27Ti_0.03Fe₂ containing very few Ti shows the substitution of Ti to Zr and the resultant improvement in the dehydriding equilibrium pressure.     

Mechanochemical reactions and hydrogen storage capacities in MBH4–SiS2 systems (MLi or Na)

The hydrogen storage properties, and phase compositions of mechanochemically prepared mixtures of xMBH₄-SiS₂ (x = 2–8), where M = Li or Na, were investigated using gas sorption analysis, powder X-ray diffraction, and infrared and solid-state NMR spectroscopic methods. The 2LiBH₄:1SiS₂ system forms an amorphous product that releases ca. 4.3 wt % of H₂ below 385 °C with a T_onset of 88 °C without detectable diborane emission. The dehydrogenated sample reversibly absorbs 1.5 wt % of H₂ at 385 °C under 160 bar pressure. The H₂ release from materials with varying LiBH₄:SiS₂ ratios peaks at 8.2 wt % for the 6LiBH₄:1SiS₂ composition, with a reversible hydrogen storage capacity of 2.4 wt %. The H₂ desorption capacities of the Li-containing systems surpass those of Na-containing systems. Solid-state NMR studies indicate that products of mechanochemical reactions in the LiBH₄SiS₂ system consist of one-dimensional chains of edge-sharing SiS_4/2 tetrahedra in which the non-bridging S-ends are terminated with Li⁺, which are further coordinated to the [BH₄]⁻ anions. A variety of possible polymorphs in the LiSiS-(BH₄) composition space have been identified using first principles and thermodynamic modeling that supports the likelihood of formation of such novel complexes.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.     

CoMoB nanoparticles supported on foam Ni as efficient catalysts for hydrogen generation from hydrolysis of ammonia borane solution

We report on CoMoB nanoparticles supported on foam Ni as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃) solution. The CoMoB/foam Ni catalysts with different molar ratios of Co²⁺and MoO₄²⁻ were synthesized via the electroless-deposition technique at ambient temperature. In order to analyze the phase composition, chemical composition, microstructure, and electron bonding structure of the as-prepared samples, powder X–ray diffraction (XRD), inductively coupled plasma-mass spectroscopy (ICP-MS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used. The results showed that CoMoB nanoparticles were variously dispersed on the surface of the foam Ni and the catalytic activity correlated with the molar ratio of Co²⁺ and MoO₄²⁻. The highest hydrogen generation rate was 5331.0 mL min⁻¹ g_cat⁻¹ at 298 K, and the activation energy was calculated to be 45.5 kJ mol⁻¹ toward the hydrolysis of NH₃BH₃ solution. The better catalytic activity was largely attributed to the smaller particle size, higher surface roughness and the novel three-dimensional cone-like architectures of the obtained samples. The kinetic results show that the hydrolysis of NH₃BH₃ is a first-order reaction in catalyst concentration. In addition, the reusability experiment exhibited that the catalytic activity was reduced after 5 cycles and the reason of the decay was also investigated.     

Electrochemical hydrogen storage properties of MgAlMnNi quaternary alloys

Al was partially substituted by Mn in Mg₃AlNi₂ to improve the discharge capacity and electrochemical kinetic properties of Mg₃AlNi₂ alloy electrode. By means of pretreatment of ultrasonic dispersion, followed by mechanical milling and combustion synthesis, a series of quaternary alloys, namely Mg₃Al_1-xMn_xNi₂ (x = 0, 0.2, 0.4, 0.6, 0.8) were synthesized. X-ray diffraction analysis shows that partial substitution of Mn for Al can cause lattice expansion of Mg₃AlNi₂ and the samples all appear similar multiphase structures. The introduction of Mn enhances obviously the maximum discharge capacity of Mg₃AlNi₂ alloy electrode. The high rate dischargeability of the alloys can also be remarkably enhanced by substitution of Mn for Al. The exchange current density (I₀) and charge transfer resistance (R_ct) of the alloy electrode increase and decrease continuously with increasing the Mn substitution content, respectively, indicating the improvement of electrochemical kinetics properties. Combining with the potentiostatic discharge test, it is concluded that in the MgAlMnNi quaternary alloys, the kinetic properties are mainly controlled by charge transfer reaction on the electrode surface.     

Phase relationships,crystal growth and stoichiometry defects in AICIIID2VI/BIIDVI heterojunction-forming systems

The phase equilibria relationships in some A^IC^IIID₂^VI/B^IID^VI (A^I = Cu, Ag; B^II = Zn, Cd; C^III = Ga, In; D^VI = Se, Te) heterojunction-forming systems have been experimentally determined with particular reference to (CuInSe₂)_1−x−(2CdSe)_x and (CuGaTe₂)_1−x−(2ZnTe)_x. As-grown samples are n-type in CuInSe₂CdSe and p-type in CuGaTe₂ZnTe systems. Within the range of the homostructural regions (chalcopyrite, zinc blende and wurtzite) of these systems, single-phase crystals of composition A^I_1−xB^II_2xC^III_1−xD^VI₂ have been grown. Results are also reported for stoichiometry induced p → n and n → p conversions by intrinsic doping in the cation sub-lattice of AgCd₂InTe₄ and CuCd₂InTe₄ quaternary phases. The effect on the electrical properties of A^IB₂^IIC^IIID₄^VI diamond-like phases induced by different types of cation disorder is briefly discussed.     

Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3.5 − x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys

The crystal structure, hydrogen storage property and electrochemical characteristics of the La_0.7Mg_0.3Ni_3.5 − x(Al_0.5Mo_0.5)_x (x = 0–0.8) alloys have been investigated systematically. It can be found that with X-ray powder diffraction and Rietveld analysis the alloys are of multiphase alloy and consisted of impurity LaNi phase and two main crystallographic phases, namely the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase, and the lattice parameter and the cell volume of both the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase increases with increasing Al and Mo content in the alloys. The P–C isotherms curves indicate that the hydrogen storage capacity of the alloy first increases and then decreases with increasing x, and the equilibrium pressure decreases with increasing x. The electrochemical measurements show that the maximum discharge capacity first increases from 354.2 (x = 0) to 397.6 mAh g⁻¹ (x = 0.6) and then decreases to 370.4 mAh g⁻¹ (x = 0.8). The high-rate dischargeability of the alloy electrode increases lineally from 55.7% (x = 0) to 73.8% (x = 0.8) at the discharge current density of 1200 mA g⁻¹. Moreover, the exchange current density of the alloy electrodes also increases monotonously with increasing x. The hydrogen diffusion coefficient in the alloy bulk increases with increasing Al and Mo content and thus enhances the low-temperature dischargeability of the alloy electrode.     

Hydrogen production by oxidative reforming of ethanol in a fluidized bed reactor using a PtNi/CeO2SiO2 catalyst

Oxidative steam reforming of ethanol (OESR) was investigated over PtNi/CeO₂SiO₂ catalysts prepared from different cerium salt precursors. During stability tests performed at 500 °C, steam/ethanol ratio (S/E) of 4 and oxygen/ethanol ratio of 0.5 (O/E), the highest performance was recorded over the catalyst prepared form organic precursors. The most promising formulation was tested at different temperatures, S/E and O/E. Complete conversion was recorded during 100 h of the test at 600 °C, with a very low carbon formation rate (6.9·10⁻⁷ g_{coke, oxidized}·g_catalyst⁻¹·g_carbon,fed⁻¹·h⁻¹). The increase of oxygen content in the reacting mixture from 5 to 7.5% had a beneficial effect on H₂ yield, which rose of more than 20% after 100 h of test, and on the conversion of by-products. When steam/ethanol ratio grew from 4 to 6, a slightly lower performance improvement was observed. Therefore, the highest activity and stability during OESR over the PtNi/CeO₂SiO₂ catalyst prepared from organic salts was achieved at 600 °C, O/E = 0.75 and S/E = 6.     

A novel AlBiOCl composite for hydrogen generation from water

In this paper, a novel AlBiOCl material has been prepared by milling Al powder and BiOCl firstly. Experimental results show that BiOCl-doped can prevent an inert alumina film forming on the surface of Al particles and induce the rapid hydrogen generation as well as high conversion rate. SEM, XRD, EDS, TEM, XPS and calorimetric techniques are used for the mechanism analysis of the samples. The results demonstrate the fresh surface of Al, AlCl₃, Bi and Bi₂O₃ are produced in situ under ball milling Al and BiOCl, which play an important role in hydrolysis reaction of Al. The hydrogen yield of Al-15 wt% BiOCl rises to 1058.1 mL g⁻¹ in about 5 min, corresponding to the high conversion yield of 91.6% at room temperature. After doping additives (such as LiH, Bi or AlCl₃), hydrogen generation performances of AlBiOCl-additive are further improved. For example, the conversion yield and maximum hydrogen generation rate (MHGR) of AlBiOClLiH can increase to 94.9% and 3178.5 mL g⁻¹ min⁻¹, respectively. Therefore, the proposed materials in this paper are expected to serve as a hydrogen generation material for the fuel cells.     

Microstructures and electrochemical hydrogen storage performances of La0.75Ce0.25Ni3.80Mn0.90Cu0.30(V0.81Fe0.19)x (x = 0–0.20) alloys

Electrochemical hydrogen storage performances of a La_0.75Ce_0.25Ni_3.80Mn_0.90Cu_0.30 alloy are improved by adding V_0.81Fe_0.19 combined with hyper-stoichiometry, and microstructures and electrochemical characteristics of La_0.75Ce_0.25Ni_3.80Mn_0.90Cu_0.30(V_0.81Fe_0.19)_x (x = 0–0.20) hydrogen storage alloys are investigated. X-ray diffraction and backscattered electron results indicate that all alloys are a LaNi₅ phase with a hexagonal CaCu₅-type structure and the lattice parameters a, c and cell volume V of the LaNi₅ phase decrease with increasing x value. The alloy electrodes keep excellent activation performance with increasing V_0.81Fe_0.19 content. Maximum discharge capacity of alloy electrodes first increases from 330.2 (x = 0) to 335.4 (x = 0.10) mAh/g, and then decreases to 328.8 mAh/g (x = 0.20) with further increasing x value. The high-rate dischargeability at the discharge current density of 1200 mA/g first increases from 67.2% (x = 0) to 76.7% (x = 0.10), and then decreases to 65.3% (x = 0.20). The cycling capacity retention rate at the 100th cycle increases from 52.3% (x = 0) to 77.9% (x = 0.20), which is mainly ascribed to the improvement of anti-pulverization.     

The structure and 233 K electrochemical properties of La0.8−xNdxMg0.2Ni3.1Co0.25Al0.15 (x = 0.0–0.4) hydrogen storage alloys

The effect of neodymium content on the structure and low-temperature (233 K) electrochemical properties of the La_0.8−xNd_xMg_0.2Ni_3.1Co_0.25Al_0.15 (x = 0.0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys was investigated systematically. The result of the Rietveld analyses suggested that all these alloys mainly consist of two phases: the (La, Mg)₂Ni₇ phase and the LaNi₅ phase. The electrochemical studies revealed that, at temperature 233 K, the maximum discharge capacity first increased from 188.5 mAh/g (x = 0.0) to 201.7 mAh/g (x = 0.1) and then decreased to 153.9 mAh/g (x = 0.4). The low-temperature dischargeability (LTD) first increased and then decreased with increasing x, also reaching an extreme when x was 0.10. The LTD was in agreement with the I₀, but was irrespective of the diffusion of hydrogen. From our work, the optimum composition of the La_0.8−xNd_xMg_0.2Ni_3.1Co_0.25Al_0.15 (x = 0.0–0.4) alloy electrodes was found to be x = 0.10.