Synergetic catalytic effect of MWCNTs and TiF3 on hydrogenation properties of nanocrystalline Mg-10wt%Ni alloys

The Mg-10wt%Ni master alloy was firstly prepared using an electric resistance furnace under the protection of the cover reagent. Nanocrystalline alloys were obtained by spinning the molten alloy on a rotating copper wheel at a linear velocity of 35 m/s. Subsequently, the nanocrystalline alloy was high-energy ball milled (HEBM) with MWCNTs or TiF₃ or both. The catalytic effect of MWCNTs or TiF₃ and the synergetic catalytic effects on hydrogenation properties of Mg-based alloys were investigated. The alloys were characterized by X-ray diffraction (XRD), transition electron microscope (TEM), scanning electron microscope (SEM) equipped with an electron energy dispersion spectrum (EDS) to evaluate phase compositions, crystal structure, grain size, particle morphology and the distribution of catalyst element. Hydrogen storage capacities and the hydriding kinetics of the materials were measured using a Sievert's apparatus. Hydrogenation properties of the samples with and without catalysts were compared to understand the catalytic effect of MWCNTs and TiF₃ on the hydrogenation performance of Mg-based alloys.     

The influence of Cu substitution on the hydrogen sorption properties of magnesium rich Mg-Ni films

This paper describes the hydrogen storage properties of Magnesium rich ternary Mg-Ni-Cu films of 1.5 μm thickness using binary Mg-Ni and Mg-Cu as baselines, and aims to elucidate the precise influences of alloying element Cu on the hydrogen sorption kinetics, thermodynamics and cycleability. Mg-rich Mg-Ni-(Cu) alloys show two stages during absorption. The first stage due to the absorption of Mg not alloyed in the form of Mg₂Ni and Mg₂Cu, hereafter denoted as free-Mg, is very quick, but the second one due to the absorption of intermetallic Mg₂Ni and/or Mg₂Cu is significantly slower. This sequence is confirmed by XRD characterizations at different absorption stages. The rapid first stage absorption is mainly catalyzed by the intermetallic phase, Mg₂Ni. Cu substitution improves the desorption kinetics, but severely decreases the kinetics of the second absorption stage. Failure to completely absorb Mg₂Cu to MgH₂ and MgCu₂ in consecutive absorption cycles leads to complete loss of desorption-ability in binary Mg-15 at.%Cu. XRD combined with TEM shows that segregation of Mg₂Cu towards the grain boundaries is responsible for this. Pressure-Composition Isotherms are used to examine the thermodynamic properties of the alloys. The thermodynamic properties of the Low-Temperature (LT-) Mg₂NiH₄ are determined for the first time experimentally, and are found to be ΔH = −78.6 kJ/mol H₂ and ΔS = −147.83 J/K-mol H₂. It is found that the Cu substitution has no influence on the plateau pressure of MgH₂ from free-Mg phase, but slightly increases the plateau pressure of LT-Mg₂NiH₄.     

Effect of processing parameter on hydrogen storage characteristics of as quenched Ti45Zr38Ni17 quasicrystalline alloys

The present study deals with the microstructural changes with respect to the processing parameter (quenching rate) and their correlation with hydrogen storage characteristics of Ti₄₅Zr₃₈Ni₁₇ quasicrystalline alloys. The ribbons of the alloy have been synthesized at different quenching rates obtained through different wheel speeds (35, 40, 45 and 50 m/s) and investigated for their hydrogen storage characteristics. The lower cooling rate obtained through low wheel speed (35 m/s) produces, i-phase grains whose size ranges from 300-350 nm, whereas higher cooling rates obtained through high wheel speed (45 and 50 m/s) promote the formation of grains with size ranges from 100-150 nm in Ti₄₅Zr₃₈Ni₁₇ ribbons. It has been found that the ribbons synthesized at 35 m/s absorbed ∼2.0 wt%, whereas ribbons synthesized at 50 m/s absorbed ∼2.84 wt. % of hydrogen. Thus the hydrogen storage capacity of ribbon increases for the ribbons produced at higher quenching rate. One of the salient features of the present study is that the improvement of hydrogen storage capacity obtained through higher quenching rates (∼45 to 50 m/s wheel speed) leading to the formation of lower grain size.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

The effect of rapid solidification on the microstructure and hydrogen storage properties of V35Ti25Cr40 hydrogen storage alloy

The microstructures and hydrogen storage properties of as-cast and rapidly solidified V₃₅Ti₂₅Cr₄₀ alloys have been investigated in this paper. The results showed that the rapid solidification refined the dendritic microstructure and altered the element distribution of the alloy. And through the positron annihilation measurements of the vacancy trapping rate (K_d1) and vacancy-trapped positron annihilation lifetime (τ₂), it was found that the rapid solidification increased the vacancy concentration and at the same time decreased the vacancy size in the alloy. The XRD results showed that the rapid solidification also significantly enlarged the alloy's lattice parameter. As a result of the microstructure change, the hydrogen absorption capacity and hydrogen absorption rate were increased; and the kinetic mechanism of hydrogen absorption was changed from 3-D diffusion control in the as-cast alloy to chemical reaction control in the rapidly solidified alloy; but the activation property was to some extent weakened after the rapid solidification.     

Electrochemical hydriding as method for hydrogen storage?

Electrochemical hydriding of magnesium alloys in alkaline solutions is proposed as a method of storing hydrogen in a solid phase. In this article, we present a new approach to hydrogen storage for mobile applications. Rapidly solidified ribbons of Mg–14Ni alloy were prepared by melt spinning. Subsequently, they were exposed to electrochemical hydriding at 90 °C/120 min in various alkaline electrolytes. It was found that hydrogen reached up to 1.4 wt.%. Higher hydrogen concentrations might be achieved by proper adjustment of hydriding conditions.     

Understanding crystal structure roles towards developing high-performance V-free BCC hydrogen storage alloys

Current bottlenecks in the supply and high cost of V have negatively impacted their application. There is great interest in developing V-based hydrogen storage alloys that use less or free V. Here, we investigate the role of V in deliberately designed V-based alloys. Our results affirm that V plays an undeniable role in enhancing hydrogen storage properties. It is found that V maintains the stable single BCC structure but leads to more residual hydrogen (1.4 wt%) because of the high stability of the dihydride and smaller hydriding rate because of the small lattice parameter, which offers unexpected but encouraging perspectives towards reducing the need of V in such alloys. Mo substitution for V effectively alleviates the higher residual hydrogen to achieve a high dehydriding capacity of 2.5 wt%. Moreover, the suction-cast (Ti_0.46Cr_0.54)_97.5Mo_2.5 alloy, which keeps BCC structure after suction-cast process and contains a low-Mo content, also exhibits dehydriding capacity of 2.3 wt%. The enthalpy change as well as dehydriding capacity of V-Free alloys obtained were similar to those reported V-based alloys. These findings are attractive for developing new V-free BCC hydrogen storage alloys and higher hydrogen capacity.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

Effect of electroless nickel coating on the electrochemical hydrogen storage characteristics of Al and Zr including Mg-based alloys

Mg₂Ni, Mg_1.5Al_0.5Ni, Mg_1.5Zr_0.5Ni and Mg_1.5Al_0.2Zr_0.3Ni alloys were synthesized by the mechanical alloying and the effect of electroless Ni coatings on the electrochemical hydrogen storage characteristics of these alloys were investigated. X-ray diffraction studies showed that the Ni particles formed on the alloy surface during the electroless coating resulted in the formation of two new broad Ni peaks. The application of the Ni coating improved the capacity retention rates of all the alloys. This improvement was attributed to the hydrogen diffusion pathways provided by the external Ni particles. The capacity retention of the coated Mg₂Ni alloy was relatively lower than those of the coated Al and Zr including alloys. This observation was assumed to arise from the contribution of the underlying electrocatalytically active Ni sites formed as a result of the dissolution of the disseminated Al and Zr oxides throughout the Mg(OH)₂ layer to the alloy capacity retention rate. The electrochemical impedance spectroscopy experiments indicated that the Ni crystals nucleated during the electroless coating make the alloy surface more catalytic for the electrochemical charge transfer reactions.     

Effect of nickel addition on the solubility of hydrogen in tantalum

A study of isothermal as well as isobaric P–C–T equilibrium measurements has been investigated for the solubility of hydrogen in tantalum and its alloys with nickel (1.7 and 4.9 atom % Ni) in the temperature range of 673–873 K and hydrogen pressure range of 0.60–1.20 atmospheres. The alloys were prepared by arc melting in an inert atmosphere. The dissolved hydrogen was within the solid solubility range corresponding to the temperature and followed the Sievert's law. The hydrogen solubility in tantalum decreased on the addition of nickel as an alloying element. The change in enthalpy and the change in entropy of solution for hydrogen in the tantalum metal and its alloys were calculated. The heat of reaction for hydrogen solution in all the samples was exothermic. The enthalpy of solution for hydrogen in the tantalum matrix increases on the addition of Ni as an alloying element.     

Improving hydrogen storage properties of magnesium based alloys by equal channel angular pressing

Equal channel angular pressing was applied to a commercial magnesium alloy ZK60 in order to improve its hydrogen storage properties. The microstructure refinement and increase in the density of crystal lattice defects caused by equal channel angular pressing increase hydrogen desorption pressure, change the slope of the pressure plateau in pressure-composition isotherms, decrease the pressure hysteresis, and accelerate the hydrogen desorption kinetics. It is argued that a proper design of the defect structure of materials is a key element in the search for economically viable and environmentally acceptable solutions for mobile hydrogen storage based on metal hydrides.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Effect of cold rolling on the structure and hydrogen properties of AZ91 and AM60D magnesium alloys processed by ECAP

This investigation was conducted to evaluate the effect of cold rolling on the structure and hydrogen properties of two magnesium alloys, AZ91 and AM60D, after processing by equal-channel angular pressing (ECAP). The results show that the use of cold rolling after ECAP significantly increases the preferential texture for hydrogenation and increases the potential for the use of these alloys as hydrogen storage materials. The ECAP was performed through two different numbers of passes in order to give different grain sizes and both materials were subsequently cold-rolled through the same numbers of passes for a comparison of the hydrogenation absorption. It is shown that the hydriding properties are enhanced by an (0001) texture which improves the kinetics primarily in the initial stages of hydrogenation. The results demonstrate that optimum sorption properties may be acquired through a combination of fine grains and appropriate texture.     

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of _{V55Ti22.5Cr16.1Fe6.4}${\mathrm{V}}_{55}{\mathrm{Ti}}_{22.5}{\mathrm{Cr}}_{16.1}{\mathrm{Fe}}_{6.4}$ alloy (RE=La$\mathrm{RE}=\mathrm{La}$, Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.     

Evaluation of hydrogen storage performance of ZrTiVNiCrFe in electrochemical and gas-solid reactions

In the present study, the hydrogen storage performance of multi-principal-component ZrTiVNiCrFe alloy produced through rapid solidification has been examined by electrochemical methods and gas-solid reactions. XRD and EBSD analyses reveal the hexagonal Laves phase structure (type C14) with average grain size of 300 nm and root-mean-square microstrain of 0.19%. Cyclic voltammetry and electrochemical impedance spectroscopy analyses in the hydrogen sorption/desorption region give insight to the sorption/desorption kinetics and the change in the desorption charge in terms of the applied potential. The pressure-composition isotherms measured in course of gas-solid reaction confirm the hydrogen storage capacity reaching 1.6 wt% at the first hydrogenation at room temperature, then reducing to 1.3–1.4% during subsequent cycling. According to the calorimetric titration study, there is a significant hysteresis primarily caused by the non-equilibrium character of the hydrogenation process.     

Effect of microstructure on the phase composition and hydrogen absorption-desorption behaviour of melt-spun Mg-20Ni-8Mm alloys

The effect of microstructure on the phase composition and hydrogen absorption-desorption behaviour of Mg-based Mg-20Ni-8Mm (wt.%) (Mm = La-rich Mischmetal) alloys has been studied. Rapid solidification (RS) processing resulted in the formation of the high-temperature cubic modification of Mg₂NiH₄ and the solid solution hydride Mg₂NiH_0.3, in the disappearance of the monoclinic modification of Mg₂NiH₄, as well as in a decrease in the unit cell volumes of the constituent hydride phases. The above-mentioned tendencies became more pronounced in the order “as-cast < Cu-300 < Cu-1000 ≈ Cu-2000” (where the sample names Cu-#### denote the spinning velocity of the copper wheel in rpm), which is explained by an increase in the mechanical stresses in the materials and/or by an increased interfacial energy of the fine grains of the corresponding hydrides. The hydrogen absorption kinetics was improved in the order “Cu-300 < Cu-1000 < Cu-2000”. The temperature range of hydrogen thermal desorption from the hydrogenated alloys shrank in the order “Cu-300 > Cu-1000 >> Cu-2000”, which is explained by increased uniformity of the hydrides grain size in the hydrides with increasing solidification rate. During PCT (pressure composition temperature) tests, the Cu-1000 and Cu-2000 samples displayed the largest pressure hysteresis and the smallest slope of the higher Mg₂NiH₄ plateau, but also the lowest hydrogen storage capacity.     

Raman spectroscopy study of molecular hydrogen solubility in water at high pressure

We have measured the Raman spectra of gaseous molecular hydrogen dissolved in liquid water at room temperature and as a function of pressure. Vibrational spectra of molecular hydrogen have been clearly detected. Band intensities and profiles have been carefully measured using, for calibration purposes, the water OH stretching band. From the measured intensities of the Raman band, we have obtained the behavior of hydrogen concentration in the liquid water, as a function of the gas partial pressure. The observed behavior is presented and compared to Henry’s law predictions. Additionally, we present a detailed analysis of the spectral band features from which important information on the interaction of hydrogen with water molecules could be derived.     

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.     

Hydrolysis of Mg-based alloys and their hydrides for efficient hydrogen generation

The hydrolysis of Mg-based alloys and their hydrides with high abundance on the earth and low cost could produce hydrogen with high theoretical capacity and the formation of by-products that have no pollution to the environment. Hence, it has been regarded as one of the most promising way for hydrogen generation. Particularly, a gravimetric capacity of 6.4 wt% and 3.4 wt% H₂ could be produced from the hydrolysis of pure Mg and MgH₂, respectively, even when stoichiometric water is included for calculation. The formation of passive magnesium hydroxides with dense structure, however, could immediately interrupt the hydrolysis reaction of Mg/MgH₂, which leads to ultralow yield and sluggish hydrogen generation rate. Recent studies have demonstrated that the hydrolysis reaction of Mg/MgH₂ could be effectively enhanced in terms of both yield and kinetics by the formation of Mg-based alloys and their hydrides. This review aims to summarize the recent progress in the hydrolysis of Mg-based alloys and their hydrides and the involved hydrolysis mechanisms.     

Nanostructured rapidly solidified LaMg11Ni alloy: Microstructure,crystal structure and hydrogenation properties

In our earlier publication (Poletaev et al., J Alloys Compd., 509S (2011) S633) we reported a drastic variation of the structural and hydrogenation properties of LaMg_∼12 intermetallic alloy caused by Rapid Solidification (RS). In present work we have probed the effect of nickel during the chemical modification of LaMg₁₂, in combination with RS, on the structure, microstructure and hydrogen absorption–desorption properties of the alloys.