Synergetic catalyst effect of Ni/Pd dual metal coating accelerating hydrogen storage properties of ZrCo alloy

Aimed at enhancing the hydrogen absorption/desorption performances of ZrCo system, Ni/Pd dual metal coating is employed on ZrCo alloy combined with the electroless plating and displacement plating. The effects of Ni/Pd dual metal coating on the microstructure, hydrogen storage performance of ZrCo alloys were investigated systematically. The results show that Ni/Pd dual metal coating deposits on the surface of ZrCo sample successfully with the thickness of 500 nm. The hydrogen absorption kinetic property is substantially enhanced for ZrCo alloy after Ni/Pd dual metal coating, which is owing to the catalytic effect of Ni/Pd coating. Further, the activation energies (E_a) for hydrogen absorption and desorption are calculated using the Arrhenius Equation and Kissinger method, respectively. Compared with the bare ZrCo, the activation energies of the Ni/Pd coated samples for hydriding/dehydriding process decrease which facilitate the hydrogenation/dehydrogenation reaction. This work introduces a rational approach by building new catalytic coating on the hydrogen storage materials to improve the hydriding/dehydriding kinetic performance.     

Influence of the Fe-doping on hydrogen behavior on the ZrCo surface

Ab initio calculations have been carried out to investigate the adsorption, dissociation, and diffusion of atomic and molecular hydrogen on the Fe-doped ZrCo (110) surface. It is found that the adsorption of H₂ on doped surface seems thermodynamically more stable with more negative adsorption energy than that on the pure surface, and the dissociation energy of H₂ on doped surface is much bigger therefore. However, compared with the pure system, there are fewer adsorption sites for spontaneous dissociation. After dissociation, the higher hydrogen adsorption strength sites would promote the H atom diffusion towards them where they can permeate into the bulk further. Furthermore, the ZrCo (110) surface possesses much higher hydrogen permeability and lower hydrogen diffusivity than its corresponding ZrCo bulk. Moreover, further comparison of the present results to analogous calculations for pure surface reveals that the Fe dopant facilitates the H₂ molecule dissociation. Unfortunately, this does not improve the hydrogen storage performance of ZrCo alloy due to the H atom diffusion on the surface and into bulk are prevented with higher reaction energetic barriers by doping Fe. Consequently, ZrCo (110) surface modified with Fe atoms should not be preferred as a result of its terrible hydrogen permeability. A clear and deep comprehending of the inhibiting effect of Fe dopant on the hydrogen storage of ZrCo materials from the perspective of the surface adsorption of hydrogen are obtained from the present results.     

A numerical investigation of hydrogen desorption phenomena in ZrCo based hydrogen storage beds

In this paper, a three-dimensional (3D) hydrogen desorption model is applied to the thin double layered annulus ZrCo based hydrogen storage bed to precisely study the hydrogen desorption reaction and resultant heat and mass transport phenomena inside the bed. The 3D hydrogen desorption simulations are carried out and calculated results are compared with the experimental data measured by Kang et al. [1]. The present model reasonably captures the bed temperature evolution behavior and the hydrogen discharging time for 90% desorption. In addition, the thin double layered annulus metal hydride bed (MHB) design is numerically evaluated by comparing with a simple cylindrical MHB. More uniform distributions in the bed temperature and H/M atomic ratio and resultant superior hydrogen desorption performance are achieved with the thin double layered annulus bed owing to its high external surface to volume ratio and thus more efficient heating. This numerical study indicates that efficient design of the metal hydride bed is key to achieve rapid hydrogen discharging performance and the present 3D hydrogen desorption model is a useful tool for the optimization of bed design and operating conditions.     

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr_0.8Ti_0.2-xNb_xCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr_0.8Ti_0.2-xNb_xCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr_0.8Ti_0.2-xNb_xCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.     

Effect of Sm content on activation capability and hydrogen storage performances of TiFe alloy

Element substitution is an efficient method to enhance the activation property of TiFe alloys. In this paper, Zr, Mn and Ni were utilized to replace Fe in the alloy partially, and different content rare earth Sm substitute Ti in the alloy. The alloys with nominal compositions of Ti_1.1-xFe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_x (x = 0–0.08) were made through vacuum induction melting. The microstructure, composition and hydrogen storage property of alloys were measured in detail by X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy and automatically Sievert apparatus. The results reveal that the as-cast alloys contain TiFe as major phase and Ti₂Fe as secondary phase. Sm addition refines the grain of alloys obviously. All alloys have good activation properties and can be completely activated without any heat treatment. The activation performance can be further improved by partially replacing Ti with Sm, and the incubation period of activation can be shortened greatly.     

Stable region of ZrCo under hydrogen atmosphere at high temperature

In this work, the unusual reproportionation phenomenon, which occurs under hydrogen atmosphere, has been studied for the first time by combing temperature-programmed dehydrogenation and XRD characterization. The phase transition from ZrCo₂–ZrH₂ to ZrCo for reproportionation was clearly observed without vacuum pumping. It was shown that the reproportionation could take place even under hydrogen pressure of 0.65 bar at 640 °C. In addition, the reproportionation behaviors under different initial hydrogen pressures were investigated and the onset boundary for reproportionation was successfully established. The original insight into the unusual reproportionation phenomenon reveals that there is a stable region for ZrCo under hydrogen atmosphere at high temperature. On the basis of this work, it is hopeful that the application of ZrCo at high temperature will be substantially broadened.     

Novel non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation

This study presents a new non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation. It shows that palladium and ruthenium can be deposited on an aluminum-oxide-modified porous Hastalloy by using our new EDTA-free plating bath at room temperature and 358 K, respectively. A 6.8 μm thick non-alloy Ru/Pd membrane film could be plated and helium leak test confirmed that the membrane was free of defects. Hydrogen permeation test showed that the membrane had a hydrogen permeation flux of 4.5 × 10⁻¹ mol m⁻² s⁻¹ at a temperature of 773 K and a pressure difference of 100 kPa. The hydrogen permeability normalized value with thickness of the membrane was 1.4 times higher than our pure Pd membrane having similar structure. The EDX profiles of the front and back side membrane, cross-sectional EDX line scanning and XRD profile show that there was no alloying progress between the palladium and ruthenium layer after hydrogen permeation test at 773 K.     

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.     

Effects of Zr doping on activation capability and hydrogen storage performances of TiFe-based alloy

Activation difficulty is the key problem limiting the application of TiFe-based hydrogen storage alloys. The addition of transition group elements helps to improve the activation properties of TiFe-based hydrogen storage alloy. In our previous work, the Ti_1.08Y_0.02Fe_0.8Mn_0.2 alloy exhibits extremely high hydrogen storage capacity (1.84 wt%) at room temperature with excellent kinetic properties, but it still needs an incubation period of about 1500s. In this study, the composition of Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_x (x = 0, 0.02, 0.04, 0.06, 0.08) alloys was prepared by electromagnetic induction melting. The quantitative analysis of elements by energy dispersive spectrometer shows that in the second phase region containing Zr, the content of Ti element is significantly higher than that of Fe. Meanwhile, the first-principle calculation on Zr-doped TiFe system indicates that Zr is more attractive to substitute Ti than Fe. Therefore, the doping of Zr partially replaces the Ti. The solubility of Zr in TiFe is limited, when x ≤ 0.04, the alloy consists of pure TiFe phase. When x > 0.4, the excess Zr forms precipitates, which reduces the reversible hydrogen absorption and desorption capacity of the TiFe alloy. The addition of Zr significantly shortens the activation time and reduces the plateau pressure of TiFe alloys. The Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_0.04 alloy can be directly activated without the incubation period and its absolute values of enthalpy change (ΔH) and entropy change (ΔS) are minima (ΔH for 23.2 kJ/mol and ΔS for 83.1 J/mol/K).     

Modifying the hydrogen storage performances of NaBH4 by catalyzing with MgFe2O4 synthesized via hydrothermal method

In this study, the hydrogenation performance of NaBH₄ was modified by the addition of 10 wt% MgFe₂O₄ as the catalyst. The NaBH₄ + 10 wt% of MgFe₂O₄ sample was prepared by a ball milling technique. The onset decomposition temperature of MgFe₂O₄-doped NaBH₄ was decreased to 323 °C and 483 °C for the first and second stage of dehydrogenation as compared to the milled NaBH₄ (497 °C). The desorption kinetics study showed that the addition of MgFe₂O₄ caused the sample to had faster hydrogen desorption with a capacity of 6.2 wt% within 60 min while the milled NaBH₄ had only released 5.3 wt% of hydrogen in the same period of time. For the isothermal absorption kinetics, the total amount of hydrogen absorbed by the milled NaBH₄ was 3.7 wt% while the NaBH₄ + 10 wt% MgFe₂O₄ sample showed better absorption characteristic with a total amount of 4.5 wt% of hydrogen within 60 min. The calculated desorption activation energy from the Kissinger plot of NaBH₄ + 10 wt% MgFe₂O₄ sample was 187 kJ/mol which reduced by 28 kJ/mol than the milled NaBH₄ (215 kJ/mol). The in-situ formation of MgB₆ and Fe₃O₄ after the dehydrogenation process indicates that these new species were responsible for the improved hydrogenation performances of NaBH₄.     

Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors

The effects of CO and H₂S as catalyst inhibitors on the rate of toluene hydrogenation were studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO₂ suffered serious negative effects from catalyst inhibitors; however, Pd/TiO₂–SiO₂ exhibited high CO and H₂S tolerance because the acidic support decreased the electron density of the Pd metal particles, which, in turn, decreased the interaction between the Pd surface and CO (or H₂S). The TiO₂–SiO₂-supported Pd catalyst exhibited activity greater than that of the TiO₂–SiO₂-supported Pt catalyst in the presence of CO; however, it exhibited lower activity in presence of H₂S. Catalyst characterization after sulfidation with H₂S revealed that Pd particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity in the presence of H₂S.     

First-principles investigation on the role of interstitial site preference on the hydrogen-induced disproportionation of ZrCo and its doped alloys

There are two phase structures involved in ZrCo hydrides (ZrCoH_x). When x ≤ 1, the α-phase hydride is generated when hydrogen atoms occupy the 3c and 12i sites. When 1 < x ≤ 3, three interstitial sites of 4c₂, 8f₁, and 8e are occupied by H, and in turn the β-phase hydride is formed. There is a disproportionation reaction in β-phase hydrides during hydrogen discharging process to produce the ZrH₂ phase with higher thermal stability, leading to inferior hydrogen storage performance. In this study, the influence of hydrogen storage capacity on thermodynamic and lattice stabilities of α- and β-phase hydrides for each occupancy position is investigated under the framework of the first-principles study. The results indicate that the binding energy in the 3c site is higher compared with the 12i site under the condition of identical hydrogen storage capacity. Similarly, the binding energy is the largest for the 8e site compared with the other two sites, indicating that there is the least energy released in the reaction process. Thus, the 8e site is proved as the most unfavorable site in β-phase ZrCo hydrides, which is due to its degraded thermodynamic stability. Also, comparisons of mechanical properties and total density of states for each site in two hydride phases are presented to demonstrate that compound lattice stability in the 8e site is the poorest, suggesting that it is more likely to produce disproportionation. Furthermore, the dependence of hydrogen storage performance of β-phase hydrides on Ti/Rh doping is examined as well. It is discovered that there is improved thermodynamic stability and lattice stability in the 8e site for Zr_0.875Ti_0.125Co after Zr is partially substituted by Ti, which significantly enhances the disproportionation resistance. In contrast, when Co is partially replaced by Rh, there is a deterioration in the thermodynamic stability of ZrCo_0.875Rh_0.125 in the 8e site, but its lattice stability is somewhat improved.     

Effects of surface coating with Cu-Pd on electrochemical properties of A2B7- type hydrogen storage alloy

La_0.75Mg_0.25Ni_3.2Co_0.2Al_0.1 hydrogen storage alloy, the nickel-metal hydride (MH/Ni) secondary battery negative electrode, was modified by CuSO₄ solution (3 wt% in Cu in contrast with alloy weight) and PdCl₂ solution varied from 1 wt% to 4 wt% in Pd in contrast with alloy weight with a simplified pollution-free replacement plating method, aiming at improving its comprehensive electrochemical properties. The XRD analysis and SEM images combined with EDS results reveal that Cu and Pd nanoparticles are uniformly plated on the pristine alloy surface. The relative amount of Pd on the Cu-Pd coated alloy surface increases notably as the PdCl₂ concentration increases in the plating solution. Electrochemical tests indicate that alloy electrodes modified by Cu-Pd composite coating show perfect activation performance, which achieve the maximum discharge capacity at the first charge-discharge cycle. Moreover, alloy electrodes coated with Cu-Pd perform dramatically enhanced high rate dischargeability (HRD). The enhancement increases firstly and then decreases as the content of Pd increases in the Cu-Pd coating. Meanwhile, the cycle life of modified alloys is also improved significantly. Among all the samples, the Cu-Pd coated alloy with 3 wt% Pd content in the PdCl₂ solution reinforces the comprehensive electrochemical properties most sufficiently, with dischargeability of 86.4% under 1500 mA/g and remaining capacity of 82.7% after 100 cycles.     

A numerical investigation of hydrogen absorption phenomena in thin double-layered annulus ZrCo beds

In this paper, a three-dimensional (3-D) hydrogen absorption model is applied to a thin double-layered annulus ZrCo hydride bed and validated against the temperature evolution data measured by Kang et al. [1]. In their experiment, the monitored hydrogen tank pressure decreased with time due to continuous hydrogen supply to a ZrCo hydride bed; hence, the effect of decreasing hydrogen feed pressure is considered for simulations. The equilibrium pressure expression for hydrogen absorption on ZrCo is derived as a function of temperature and the H/M atomic ratio based on the pressure–composition isotherm data given by Konishi et al. [2]. In general, the calculated results agreed well with the temperature evolution data, and the hydrogen charging time for 99% absorption was accurately captured by the model. In addition, detailed simulation results, including multidimensional contours, clearly elucidate the hydrogen absorption behavior of the thin double-layered ZrCo MHB.     

Copper deposition on Pd membranes by electroless plating

PdCu membranes were prepared by the electroless plating of Pd membranes prepared on ceramic tubular supports. Different PdCu membranes were prepared with Pd content between 45 and 77 wt% and a total metal layer between 0.5 and 1.9 μm thickness. The alloying step was performed in two ways to compare and establish the required alloying time for obtaining high permeance membranes. The alloying was analysed with EDX composition measurements, and full alloying was not required to obtain a stable hydrogen flux. Finally, permeance tests were performed at different pressures, including temperature cycles in hydrogen and nitrogen, to observe membrane stability. The hydrogen permeance values of the membranes were high, between 1.5 × 10⁻³ and 4.5 × 10⁻³ mol/(s Pa^0.5 m²) at 673 K. The membranes recorded stable permeance values even after thermal cycles in a hydrogen atmosphere. Metal layer thickness was calculated using both the weight difference method and SEM images. SEM images were also used to analyse the surface morphology of the membranes, which was generally fairly uniform and smooth.     

Enhancing antioxidant properties of hydrogen storage alloys using PMMA coating

Oxidation of hydrogen storage alloy leads to the formation of a passive surface oxide layer, which deteriorates its performance. This study introduces a method utilizing polymethyl methacrylate (PMMA) nano-coating to improve the antioxidant properties of hydrogen storage alloys, including LaNi₅, TiMn₂, and Mg₂Ni. PMMA nano-coating was achieved using a solution immersion method. The results show that the PMMA-coating promotes a stable capacity, kinetics, and thermodynamic properties compared to those of uncoated alloys. After 168 h of air exposure, the hydrogen storage capacities of PMMA-coated LaNi₅, TiMn₂, and Mg₂Ni alloys show minor decreases. Moreover, cycling tests demonstrate that PMMA-coated LaNi₅ and PMMA-coated Mg₂Ni have good cyclic stabilities. Our results show that the PMMA coating provides effective protection for variant hydrogen storage alloys from oxygen contamination and oxidation.     

Characterization of microstructure and surface oxide of Ti1.2Fe hydrogen storage alloy

In this study, we investigated the microstructures, hydrogen absorption kinetics, and oxide layers of TiFe and Ti_1.2Fe hydrogen storage alloys. Whereas the TiFe alloy has a single phase, the Ti_1.2Fe alloy is composed of three phases: TiFe, Ti₂Fe, and Ti₄Fe. Under no thermal activation process, the TiFe alloy does not absorb hydrogen, though the Ti_1.2Fe alloy starts to absorb hydrogen after 4 min of incubation time. From the XPS results, it is revealed that the Ti concentration in the oxide layer on the Ti₄Fe phase is higher than that on the TiFe phase, indicating that the Ti concentration in the oxide layer would be important in improving hydrogen absorption kinetics. Based on these results, the hydrogen absorption kinetics could be improved by adjusting composition, enabling the formation of a Ti-rich oxide layer.     

Excellent catalytic effect of LaNi5 on hydrogen storage properties for aluminium hydride at mild temperature

The catalytic effect of rare-earth hydrogen storage alloy is investigated for dehydrogenation of alane, which shows a significantly reduced onset dehydrogenation temperature (86 °C) with a high-purity hydrogen storage capacity of 8.6 wt% and an improved dehydrogenation kinetics property (6.3 wt% of dehydrogenation at 100 °C within 60 min). The related mechanism is that the catalytic sites on the surface of the hydrogen storage alloy and the hydrogen storage sites of the entire bulk phase of the hydrogen storage reduce the dehydrogenation temperature of AlH₃ and improve the dehydrogenation kinetic performance of AlH₃. This facile and effective method significantly improves the dehydrogenation of AlH₃ and provides a promising strategy for metal hydride modification.     

Effect of milling duration on hydrogen storage thermodynamics and kinetics of Mg-based alloy

Mechanical milling is widely recognized as the best method to prepare nano-structured magnesium based hydrogen storage materials. The composites La₇Sm₃Mg₈₀Ni₁₀ + 5 wt% TiO₂ (named La₇Sm₃Mg₈₀Ni₁₀–5TiO₂) whose structures are nano-crystal and amorphous accompanied by great hydrogen absorption and desorption properties were fabricated by mechanical milling. The research focuses on the effect of milling duration on the thermodynamics and dynamics. The instruments of researching the gaseous hydrogen storing performances include Sievert apparatus, DSC and TGA. The calculation of dehydrogenation activation energy was realized by applying Arrhenius and Kissinger formulas. The calculation results show the specimen milled for 10 h exhibits the optimal activation performance and hydrogenation and dehydrogenation kinetics. Extending or shrinking the milling duration will lead to the degradation of hydrogen storage performances. The as-milled (10 h) alloy at the full activated state can absorb 4 wt% hydrogen in 87 s at 473 K and 3 MPa and release 3 wt% H₂ in 288 s at 573 K and 1 × 10⁻⁴ MPa. The changed milling durations have little impact on the thermodynamic properties of experimental samples and the enthalpy change (ΔH) of the alloy milled for 10 h is 74.23 kJ/mol. Moreover, it is found that the as-milled (10 h) alloy displays the minimum apparent activation energy of dehydrogenation (59.1 kJ/mol), suggesting the optimal hydrogen storing property of the as-milled (10 h) alloy.     

Effect of cobalt on the microstructure and hydrogen sorption performances of TiFe0.8Mn0.2 alloy

The microstructures and the hydrogen sorption performances of TiFe_0.8Mn_0.2Co_x (x = 0, 0.05, 0.10, 0.15) and TiFe_0.8Mn_0.2-yCo_y (y = 0.05, 0.10) alloys have been investigated. For TiFe_0.8Mn_0.2Co_x alloys, the lattice parameters of TiFe phase decreased and the Laves phase contents increased with the addition of Co. With the increase of Co content in TiFe_0.8Mn_0.2Co_x alloys, the maximum hydrogen storage capacities of TiFe_0.8Mn_0.2Co_0.05 and TiFe_0.8Mn_0.2Co_0.10 alloys decreased, but the effective hydrogen capacities increased, which is ascribed to the improved flatness of the α-β desorption plateau. Substitution of Co for Mn in TiFe_0.8Mn_0.2-yCo_y alloys can effectively lead to single phase of TiFe alloys. Therefore, TiFe_0.8Mn_0.2-yCo_y alloy showed a deteriorated activation property, but its effective hydrogen capacity increased remarkably due to the obviously improved flatness of the α-β desorption plateau. The addition of Co might adjust the change of the octahedral intersitial environment caused by Mn doping in TiFe phase, which contributes to the improved flatness of the α-β desorption plateau and hence the increased effective hydrogen capacity.