Substitutional V–Pd alloys for the membranes permeable to hydrogen: Hydrogen solubility at 150–400 °C

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V-xPd alloys (x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–10⁶) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10⁻⁴ to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys.     

Cycle life and hydrogen storage properties of mechanical alloyed Ca1−xZrxNi5−yCry; (x = 0, 0.05 and y = 0, 0.1)

CaNi₅–based alloys have been synthesized by mechanical alloying followed by isothermal annealing. The formation of the CaNi₅ structure occurred when the milled powders were heated at 800 °C under vacuum for 3 h. The abundance of CaNi₅ phase in the alloys ranges from 60 to 70 wt.%. Replacement of Zr into the Ca site reduces the unit cell volume of CaNi₅ whilst replacement of Cr into the Ni site slightly increases the unit cell volume. The hydrogen storage capacity of all substituted alloys is decreased and the hydrogen sorption plateau regions are narrowed compared to those of pure CaNi₅. Substitution of Zr into the Ca site extinguishes the flat plateau region unlike replacement of Cr into the Ni site where a flat plateau is maintained. The reaction enthalpy ΔH for both absorption and desorption are directly proportional to the unit cell volume of the alloys. The hydrogen storage capacity of all alloys rapidly decays for the first 50 cycles at 85 °C followed by a more gradual decline after 50 further cycles. The hydrogen storage capacity of the alloys after 200 cycles is in the range of 65–75% of the initial capacity.     

Hydrogen solid solution thermodynamics of V1−xAlx (x: 0, 0.18, 0.37, 0.52) alloys

Thermodynamic parameters such as enthalpy and entropy of the vanadium–hydrogen solid solution are investigated as a function of aluminum content using hydrogen solubility data and the Sievert's constant. The enthalpy decreases with increase in the aluminum content. Entropy shows anomalous behavior as it first increases with the aluminum content for V_1−xAl_x (x: 0, 0.18, 0.37) but then substantially decreases for V_0.48Al_0.52. The lattice parameters and the electrical resistivity of the alloys are calculated to explain the mechanical and electronic effects on the thermodynamic parameters. It is found that the electrical resistivity of vanadium systematically decreases and the lattice constant increases with increase in aluminum content. The hardness of the alloys increases with aluminum which indicates that aluminum hardens the vanadium by simple solid solution effect. The variation of enthalpy and entropy is explained on the basis of change in Fermi energy level of the host matrix vanadium, the strong bonding nature of V–Al in the alloy and increased activity of hydrogen due to aluminum in the alloy.     

Electrochemical hydriding of Mg–Ni–Mm (Mm = mischmetal) alloys as an effective method for hydrogen storage

In this work, the electrochemical hydriding method was used for storing hydrogen in four binary Mg–Ni (Ni content from 15 to 34 wt.%) alloys and one ternary Mg–26Ni–12Mm alloy. Both the as-cast and powdered alloys were hydrided in a 6 M KOH solution at 80 °C for 120–480 min. The structures and phase compositions of the alloys, both before and after hydriding, were studied using optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Differential scanning calorimetry and mass spectrometry were used to study the dehydriding process. In the case of as-cast alloys, the best combination of hydriding parameters (maximum hydrogen concentration on surface; depth of hydrogen penetration) was achieved in the Mg–26Ni alloy. In the case of powdered alloys, the Mg–34Ni alloy absorbed the highest amount of hydrogen, nearly 4.5 wt.%. The only hydride formed during hydriding was the MgH₂ hydride. The results of the mass spectrometry analysis reveal a significant thermodynamic destabilization of magnesium hydride due to Ni and Mm. The decomposition temperature of MgH₂ was reduced by more than 200 °C. The results are discussed in relation to the electronic structure and atomic size of the alloying elements and the structural variations in the alloys.     

Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

Synthesis and hydrogen storage behavior of Mg–V–Al–Cr–Ni high entropy alloys

The ternary MgVAl, MgVCr, MgVNi, quaternary MgVAlCr, MgVAlNi, MgVCrNi and quinary MgVAlCrNi alloys were produced by high energy ball milling (HEBM) under hydrogen pressure (3.0 MPa) as a strategy to find lightweight alloys for hydrogen storage applications. Most of the ternary and quaternary alloys presented multiphase structure, composed mainly of body-centered cubic (BCC) solid solutions and Mg-based hydrides. Only the quinary MgVAlCrNi high entropy alloy (HEA) formed a single-phase structure (BCC solid solution), which is a novel lightweight (ρ = 5.48 g/cm³) single-phase HEA. The hydrogen storage capacity of this alloy was found to be very low (approximately 0.3 wt% of H). Two non-equiatomic alloys with higher fraction of Mg and V (strong hydride former elements), namely Mg₂₈V₂₈Al₁₉Cr₁₉Ni₆ and Mg₂₆V₃₁Al₃₁Cr₆Ni₆, were then designed, aiming at higher storage capacity. Both alloys were produced by HEBM. The results show that the non-stoichiometric alloys also presented low hydrogen storage capacity. The low affinity of these alloys with hydrogen was discussed in terms of enthalpy of hydrogen solution and enthalpy of hydride formation of the single components. This study brought to light the importance of considering both enthalpy of hydrogen solution and enthalpy of hydride formation of the alloying elements for designing Mg-containing HEA for hydrogen storage. Once Mg has a positive enthalpy of hydrogen solution, the alloys composition must be balanced with alloying elements with higher hydrogen affinity, i.e., negative values of enthalpy of solution and hydride formation.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

The cycle life prediction of Mg-based hydrogen storage alloys by artificial neural network

Mg-based hydrogen storage alloys are a type of promising cathode material of Nickel-Metal Hydride (Ni-MH) batteries. But inferior cycle life is their major shortcoming. Many methods, such as element substitution, have been attempted to enhance its life. However, these methods usually require time-consuming charge–discharge cycle experiments to obtain a result. In this work, we suggested a cycle life prediction method of Mg-based hydrogen storage alloys based on artificial neural network, which can be used to predict its cycle life rapidly with high precision. As a result, the network can accurately estimate the normalized discharge capacities vs. cycles (after the fifth cycle) for Mg_0.8Ti_0.1M_0.1Ni (M = Ti, Al, Cr, etc.) and Mg_0.9 − xTi_0.1Pd_xNi (x = 0.04–0.1) alloys in the training and test process, respectively. The applicability of the model was further validated by estimating the cycle life of Mg_0.9Al_0.08Ce_0.02Ni alloys and Nd₅Mg₄₁–Ni composites. The predicted results agreed well with experimental values, which verified the applicability of the network model in the estimation of discharge cycle life of Mg-based hydrogen storage alloys.     

Microstructure and electrochemical hydriding/dehydriding properties of ball-milled TiFe-based alloys

Nanocrystalline/amorphous AB-type hydrogen storage TiFe_1−xCo_x (x = 0.1 and 0.3) and TiFe_0.7Ni_0.2Co_0.1 alloys have been synthesized by mechanical alloying. The high-energy ball milling process is performed in a planetary type mill under an inert atmosphere, and at various milling times (15, 20 and 30 h). The microstructure and the morphology of the as obtained alloy powders are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The SEM analysis showed that the alloy powders are consisted of agglomerations (10–15 μm) of smaller particles cold-welded together. The average particle size, in the range of 5–7 μm, is dependent on both, the time of milling and in a less extent on the amount of Co in the TiFe_1−xCo_x alloys. The addition of Ni (TiFe_0.7Ni_0.2Co_0.1) results in slight increase of the mean particle size of the ball milled powder. The XRD analysis showed that the samples have composite microstructure. Both, the diffraction peaks of the pure metals and the peaks of the fcc TiFe compound are identified in the diffractograms of all samples. This result indicates that the alloying process is not fully completed even after 30 h of ball milling. The electrochemical cycle life tests of the electrodes, prepared from as-obtained alloys, showed that the Co containing AB type alloys possessed lower maximum discharge capacity and better corrosion stability, compared to the TiFe electrode. In contrast to the TiFeCo alloys the Ni containing material does not show a big difference in the discharge capacity and cycle life performance between the samples milled for different time in the range of 15–30 h. Moreover, the TiFe_0.7Ni_0.2Co_0.1 alloy reveals very small discharge capacity decay, about 90% of the capacity is retained after 100 charge/discharge cycles.     

Desorption of hydrogen from Ti–Zr–Ni hydrides using a mass spectrometer

We have performed thermogravimetry (TG) and mass-spectrometry measurements of hydrogen desorbed from fully and partially hydrided ternary Ti–Zr–Ni amorphous, quasicrystalline and crystalline alloys, with four different initial compositions, where the Ti/Zr ratio ranged from 1 to 2.4. The icosahedral, quasicrystalline Ti–Zr–Ni samples were obtained using the melt-spinning technique, and with subsequent annealing of these ribbons at 700 °C for 2 h in vacuum we were able to obtain a mixture of crystalline C14 Laves and α/β solid-solution phases. In addition, using subsequent mechanical alloying we produced amorphous powders of Ti–Zr–Ni from the as-spun ribbons. These various samples were then hydrided and analyzed by TG and mass spectrometry. The TG measurements provided us with the mass% of desorbed hydrogen, whereas the mass-spectrometry revealed information about the hydrogen desorption temperatures in the material. Despite the fact that the amorphous and icosahedral samples undergo some crystallization during the desorption measurements, the resulting mass spectra were different and were closely related to the alloy's structure. In contrast, the shapes of mass spectra were less affected by the composition, the total amount of desorbed hydrogen and the loading pressure.     

Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Hydrogenation properties of LaNi_5 − xIn_x alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi₅ significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi₅. The LaNi_4.9In_0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α-phase (hydrogen solid solution) and β-phase (LaNi₅H₆ hydride), formation of a new σ*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H₂-pressure/composition (p–c) isotherms at several temperatures, applying the Van't Hoff's (lnp − 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi_4.9In_0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.     

Development of Fe–Ni/YSZ–GDC electrocatalysts for application as SOFC anodes: XRD and TPR characterization and evaluation in the ethanol steam reforming reaction

Electrocatalysts based on Fe–Ni alloys were prepared by means of modified Pechini and physical mixture methods and using on a composite of Yttria Stabilized Zirconia (YSZ) and Gadolinia-Doped Ceria (GDC) as support. The former method was based on the formation a polymeric precursor that was subsequently calcined; the later method was based on the mixture of NiO and the support. The resulting composites had 35 wt.% metal load and 65 wt.% support (70 wt.% YSZ and 30 wt.% GDC mixture) (cermets). The samples were then characterized by Temperature-Programmed Reduction (TPR) and X-Ray Diffraction (XRD) and evaluated in the ethanol steam reforming at 650 °C for 6 h in the temperature range of 300–900 °C. The XRD results showed that the bimetallic sample calcined at 800 °C formed a mixed oxide (NiFe₂O₄) with a spinel structure, which, after reduction in hydrogen, formed Ni–Fe alloys. The presence of Ni was observed to decrease the final reduction temperature of the NiFe₂O₄ species. The addition of iron to the nickel anchored to YSZ–GDC increased the hydrogen production and inhibited carbon deposition. The resulting bimetallic 30Fe5Ni sample reached an ethanol conversion of about 95% and a hydrogen yield up to 48% at 750 °C. In general, ethanol conversion and hydrogen production were independent of the metal content in the electrocatalyst. However, the substitution of nickel for iron significantly reduced carbon deposition on the electrocatalyst: 74, 31, and 9 wt.% in the 35Ni, 20Fe15Ni, and 30Fe5Ni samples, respectively.     

Hydrogen absorption characteristics of mechanically alloyed Ti–Zr–Ni and Ti–V–Ni powders

A first investigation into the production of amorphous and nanostructured Ti-based alloys with nominal compositions Ti_41.5Zr_41.5Ni₁₇, Ti₆₁Zr₂₂Ni₁₇, Ti_41.5V_41.5Ni₁₇ and Ti₆₁V₂₂Ni₁₇ by mechanical alloying (MA) technique is presented. This technique was adopted to produce alloys' powders with high fresh surface area that were active for hydrogen storage. Hydrogen absorption characteristics and structure changes in the alloys after hydrogenation were investigated. Gas phase hydrogenation of the Ti–Zr–Ni alloys, at 573 K and an initial hydrogen pressure of 2 MPa, exhibited good hydriding properties and started at a maximal rate without induction period with a hydrogenation capacity up to 1.2 wt%. However, hydriding of Ti–V–Ni alloys at the same conditions exhibited slower rates. The Ti₆₁V₂₂Ni₁₇ composition showed high hydrogen absorption capacity of 1.8 wt% and exceeded 4 wt% at 345 K. In addition, the Ti–V–Ni alloys showed structure stability after hydrogenation and retained the amorphous structure.     

Effects of ultra-high pressure on phase compositions,phase configurations and hydrogen storage properties of LaMg4Ni alloys

The influences of ultrahigh pressure (UHP, under 5 GPa) on phase compositions, phase morphologies and hydrogen storage properties of LaMg₄Ni alloys were studied. The X-ray diffraction patterns show that the as-cast alloy consists of La₂Mg₁₇, LaMg₂Ni and Mg₂Ni phases, whereas a new LaMg₃ phase is observed in the UHP samples in addition to LaMg₂Ni and Mg₂Ni phases. The scanning electron microscopy graphs indicate that the phase distribution is more homogenous in the UHP alloys than in the as-cast one. Additionally, the microstructure of the UHP alloy heat-treated at 973 K is finer than that at 823 K. Both the reversible hydrogen storage capacity and the plateau of hydrogen pressure of the UHP alloys are close to those of the as-cast one. Of particular interest is that both UHP alloys exhibit better activation properties compared with the as-cast alloy. Moreover, the dehydriding onset temperature of the UHP alloys (5 GPa at 973 K) is about 490 K, which is obviously lower than that of the as-cast alloy. The amount of hydrogen desorption in the UHP alloy (5 GPa at 973 K) is 2.69 wt.% at 573 K, which corresponds to 89.6% of the saturated capacity. However, the corresponding values change to 1.75 wt.% and 58.3% in the as-cast alloy, respectively. It is confirmed the UHP treatment is one of effective approaches to tune the hydrogen storage performances of those rare earth–magnesium–nickel alloys.     

Electrochemical hydrogen production from water electrolysis using ionic liquid as electrolytes: Towards the best device

Electrodes constructed with different electroactive materials such as platinum (Pt), nickel (Ni), 304 stainless steel (SS) and low carbon steel (LCS) have been tested in water electrolysis using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF₄). All experiments were performed at room temperature using a classical Hoffman's cell operating at atmospheric pressure and at different cathodic potentials. For the electrodes studied herein, in the presence of a 10 vol.% solution of BMI.BF₄ in water, current densities (j) in the range 10–42 mA cm⁻² were observed, with overall hydrogen production efficiencies (experimental/theoretical hydrogen production ratio) between 82 and 98%. The highest j values obtained with Pt, Ni, SS and LCS electrodes were 30, 12, 10 and 42 mA cm⁻², respectively, and all efficiencies were in the 85–99% range. These comparative results show that the LCS electrocatalyst constitutes an attractive alternative for the technological production of high purity hydrogen by water electrolysis reaction since the LCS electrode gave j and efficiencies as high as those observed with platinum electrodes.     

Effect of rapid solidification on hydrogen solubility in Mg-rich Mg-Ni alloys

The hydrogenation properties of Mg_100−xNi_x alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg₂Ni and metastable Mg₆Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 °C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg_99.5Ni_0.5 and Mg₉₅Ni₅ powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH₂ two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 °C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH₂ precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification.     

Development of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption pressures for hybrid hydrogen storage vessel application

Three series of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption plateau pressures for hybrid hydrogen storage vessel application were prepared by induction levitation melting, as well as their crystallographic characteristics and hydrogen storage properties were investigated. The results show that all of the alloys were determined as a single phase of C14-type Laves structure. As the Fe content in the TiCr_1.9−xMn_0.1Fe_x (x = 0.4–0.6) alloys increases, the hydrogen absorption and desorption plateau pressures increase, and the hydrogen storage capacity and plateau slope factor decrease respectively. The same trends are observed when increasing the Mn content in the TiCr_1.4−yMn_yFe_0.6 (y = 0.1–0.3) alloys, except for the plateau slope factor. Compared with the stoichiometric TiCr_1.1Mn_0.3Fe_0.6 alloy, the titanium super-stoichiometric Ti_1+zCr_1.1Mn_0.3Fe_0.6 (z = 0.02, 0.04) alloys have larger hydrogen storage capacities and lower hydrogen desorption plateau pressures. Among the studied alloys, Ti_1.02Cr_1.1Mn_0.3Fe_0.6 has the best overall properties for hybrid hydrogen storage application. Its hydrogen desorption pressure at 318 K is 41.28 MPa, its hydrogen storage capacity is 1.78 wt.% and its dissociation enthalpy (ΔH_d) is 16.24 kJ/mol H₂.     

Electrodeposition of Ni–Co–Sn alloy from choline chloride-based deep eutectic solvent and characterization as cathode for hydrogen evolution in alkaline solution

The electrodeposition of Ni–Co–Sn alloy were carried out at room temperature from the chlorine chloride (ChCl)–ethylene glycol (EG) deep eutectic solvent (DES). For comparison of properties, Ni–Sn and Co–Sn alloys were also deposited using the same solvent. Deposition mechanism, microstructure, and electrochemical properties of the deposits in 1 M KOH solution were investigated. The deposition of Ni, Co, Sn, Ni–Sn, Co–Sn and Ni–Co–Sn on platinum electrode were also studied using cyclic voltammetry. Interestingly, the electrochemical stability of DES is observed to be increased in the presence of Sn²⁺ ions. The X-ray diffraction (XRD) patterns showed only Ni phases indicating that the other elements get incorporated inside the nickel matrix and the lattice constant have linear relation with Sn content in alloy. The morphologies of Ni–Sn and Ni–Co–Sn alloys were observed to be almost same with fine grains, the XRD studies confirm this. The potentiodynamic polarization measurements showed that the Ni–Co–Sn alloy exhibits the lowest corrosion current density (j_corr), noblest corrosion potential (E_corr) and highest exchange current density (j_c) value than the other two binary alloys, indicating that the ternary alloy is a good candidate for Hydrogen Evolution Reaction (HER).