Hydrogenation properties of pure magnesium and magnesium–aluminium thin films

We have studied the hydrogenation/dehydrogenation behaviour of multilayered stacks of Pd/Mg/Pd and Pd–Fe(Ti)–Mg–Al–Mg–Fe(Ti)–Pd grown by electron beam physical vapour deposition. The palladium coating was deposited at both sides of the structure to ensure a fast dissociation rate and good transport properties for hydrogen as well as to avoid oxidation of magnesium either from atmosphere as from the substrate surface. Fe and Ti layers were included in the stack composition in order to assess their possible catalyst effect as well as to prevent the formation of Mg_xPd_y intermetallics during the thermal treatments. We have studied the structure evolution after thermal treatments as well as after the hydrogenation and dehydrogenation processes using XRD. We have also followed the reactions kinetics by resistometry and differential scanning calorimetry. The nanostructured Mg films have been hydrogenated at temperature as low as 50 °C in few minutes. Adding aluminium to magnesium has improved its hydrogenation capacity. We have also observed that the formation of an Mg_xAl_y intermetallic before hydrogenation improves the storage capacity. We have confirmed that titanium is a better catalyst for the hydrogenation/dehydrogenation of the Mg films.     

Hydrogen storage studies in Pd/Ti/Mg films

This paper describes investigations (a) on the efficacy of Ti layer as a barrier against the intermixing of Pd and Mg in Pd/Ti/Mg films and (b) the hydrogen storage characteristics of the tri-layered films and the related bulk composites. The Mg film was prepared by resistive evaporation while the Pd and Ti films were deposited by e-beam evaporation. The analysis by Rutherford backscattering spectrometry (RBS) and glancing-incidence X-ray diffraction (GI-XRD) of the Pd/Ti/Mg/Si(substrate) films annealed in vacuum in 348–573 K temperature range revealed that Ti effectively prevents the intermixing of Pd and Mg up to ～523 K. However, mixing across Pd/Ti, Ti/Mg and Mg/Si interfaces commences around 523 K that progresses with the temperature of annealing though PdMg phases are not formed even at 573 K. The as-deposited Pd/Ti/Mg films are hydrogenated to ～7 wt % (62 at%) at 323–423 K at 0.15 MPa hydrogen pressure and dehydrogenated completely at ～ 473 K. The extent of (de)hydrogenation of the films was determined non-destructively by the ¹H(¹⁹F,αγ)¹⁶O nuclear reaction. The powder composites derived from the films, on the other hand, reversibly stored ～2.2 wt% hydrogen up to 18 cycles in 323–473 K temperature range. The superior cyclic stability is attributed to the inhibition of mixing between Pd and Mg and, as a result, the formation of PdMg inter-metallics by titanium.     

Interface mixing and hydrogen absorption in Pd/Mg and Pd/Al/Mg thin films

Pd/Mg bilayers and Pd/Al/Mg trilayers were prepared onto glass substrates at room temperature (RT) by UHV magnetron sputtering. Mixing effects at the Pd–Mg and Al–Mg interfaces were studied in-situ, immediately after deposition, by X-ray Photoelectron Spectroscopy (XPS). Additionally, the interfaces of the Pd/Al/Mg trilayer for the Al thickness equal to 1 nm were examined. Hydrogen absorption was monitored in-situ at RT by simultaneous resistivity and optical transmittance measurements. Formation of MgH₂ phase was confirmed by ex-situ X-ray diffraction measurements. The XPS studies revealed rather sharp interface between Al and Mg layers. On the other hand, a significant interface mixing for the Pd/Mg bilayers and Pd/1 nm – Al/Mg trilayers was observed. Further studies showed that an additional layer of Al, deposited between magnesium and palladium layers, can significantly improve the hydrogen absorption kinetics at RT. The optimal thickness of the Al layer was found to be 0.5 nm.     

Synthesis of Pd/TiO2 nanotubes/Ti for oxygen reduction reaction in acidic solution

Highly ordered and uniformly distributed TiO₂ nanotubes on a pure titanium substrate (TNTs/Ti) are successfully fabricated by a pulse anodic oxidation method as the support for Pd electrocatalyst. Pd is electrochemically deposited onto TNTs/Ti support. The sensitization with SnCl₂ and activation with PdCl₂ are critical for the formation of highly dispersed Pd nanoparticles on the TNTs/Ti support. It has been found that both Pd/TNTs/Ti and Pt electrodes show the similar electrochemical behavior in H₂SO₄, implying the possibility to develop the Pt-free alternative electrocatalyst based on the Pd/TNTs/Ti system in acid medium. The preliminary results in this work show that the Pd/TNTs/Ti catalysts have an acceptable catalytic activity for the oxygen reduction reaction (ORR) in acid medium. The factors influencing the structure of TNTs and the catalytic activity of Pd/TNTs/Ti for the ORR are also studied in detail.     

Intermetallics as cathode materials in the electrolytic hydrogen production

The intermetallics of transition metals have been investigated as cathode materials for the production of hydrogen by electrolysis from water–KOH solutions, in an attempt to increase the electrolytic process efficiency. We found that the best effect among all investigated cathodes (Hf₂Fe, Zr–Pt, Nb–Pd(I), Pd–Ta, Nb–Pd(II), Ti–Pt) exhibits the Hf₂Fe phase. These materials were compared with conventional cathodes (Fe and Ni), often used in the alkaline electrolysis. A significant upgrade of the electrolytic efficiency using intermetallics, either in pure KOH electrolyte or in combination with ionic activators added in situ, was achieved.The effects of these cathode materials on the process efficiency were discussed in the context of transition metal features that issue from their electronic configuration.     

The effect of microstructure on the hydrogenation of Mg/Fe thin film multilayers

Nanoconfined magnesium hydride can be simultaneously protected and thermodynamically destabilized when interfaced with materials such as Ti and Fe. We study the hydrogenation of thin layers of Mg (<14 nm) nanoconfined in one dimension within thin film Fe/Mg/Fe/Pd multilayers by the optical technique Hydrogenography. The hydrogenation of nanosized magnesium layers in Fe/Mg/Fe multilayers surprisingly shows the presence of multiple plateau pressures, whose nature is thickness dependent. In contrast, hydrogen desorption occurs via a single plateau which does not depend on the Mg layer thickness. From structural and morphological analyses with X-ray diffraction/reflectometry and cross-section TEM, we find that the Mg layer roughness is large when deposited on Fe and furthermore contains high-angle grain boundaries (GB's). When grown on Ti, the Mg layer roughness is low and no high-angle GB's are detected. From a Ti/Mg/Fe multilayer, in which the Mg layer is flat and has little or no GB's, we conclude that MgH₂ is indeed destabilized by the interface with Fe. In this case, both the ab- and desorption plateau pressures are increased by a factor two compared to the hydrogenation of Mg within Ti/Mg/Ti multilayers. We hypothesize that the GB's in the Fe/Mg/Fe multilayer act as diffusion pathways for Pd, which is known to greatly alter the hydrogenation behavior of Mg when the two materials share an interface.     

Pd‐Ni/Cd loaded PPy/Ti composite electrode: Synthesis,characterization, and application for hydrogen storage

A wide compositional range of Pd‐Ni/Cd on polypyrrole (PPy)‐modified Ti plates (Pd‐Ni/Cd/PPy/Ti) was fabricated via electrochemical deposition. The hydrogen absorption properties of the prepared Pd‐Ni/Cd/PPy/Ti electrodes were evaluated using cyclic voltammetry and chronoamperometry in acidic media. The optimal Pd₃₆‐Ni₇/Cd₅₇/PPy/Ti electrode achieved a hydrogen storage capacity of 331.3 mC cm^?2 mg^?1 and an H/Pd ratio of 0.77. The enhancement of the hydrogen storage was attributed to a synergistic effect between the Pd‐Ni/Cd catalysts. The surface morphology, crystallinity, and chemical composition of the Pd‐Ni/Cd/PPy/Ti electrode were characterized using scanning electron microscope (SEM), X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS), respectively. Hydrogen spillover occurred on the trimetallic catalysts, and secondary hydrogen spillover occurred on the PPy/Ti support. The enhanced hydrogen sorption capacity was due to both the synergistic effect of the trimetallic catalysts and the assistance of PPy, making Pd‐Ni/Cd/PPy/Ti a promising hydrogen storage material.     

Hydrogen storage property of sandwiched magnesium hydride nanoparticle thin film

Hydrogen sorption property of magnesium (Mg) in the form of sandwiched Pd/Mg/Pd films is investigated. Pulsed laser deposition method was applied to deposit the samples consisting of films of nanoparticles. The enthalpy of formation of MgH₂ was found to be −68 kJ/mol H₂ for films with nanoparticle size on the order of 50 nm, which is smaller than the value for bulk MgH₂ and may be explained by the concept of excess volume.     

A comparison of the performance and stability of Pd/BCC metal composite membranes for hydrogen purification

Composite membranes were fabricated by sputtering 100 nm of Pd on to both sides of dense BCC metal foils (V, Ta, Nb). Under pure H₂ gas testing at 500 °C the maximum permeability of all three metals exceeds previously reported values and closely approach theoretical limits. However, the stability of each membrane varied significantly due to unique failure mechanisms. Pd/V membranes failed quickly (<20 h) due to a combination of Pd–V interdiffusion and high susceptibility to oxidation as shown through microscopy and compositional analysis. The Pd/Ta membranes were the most resilient to oxygen, but their mechanical integrity was relatively poor and they failed within 48 h due to Pd–Ta interdiffusion. In contrast, Pd/Nb membranes exhibited high permeability throughout the 168 h of testing, with no Pd–Nb interdiffusion observed. The decline in permeability observed during testing was attributed to partial Pd delamination as a result of membrane deformation. These results provide pathways for further development of these membranes.     

Hydrogen dissociation and diffusion on transition metal (= Ti,Zr, V,Fe, Ru,Co, Rh,Ni, Pd,Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H₂ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab initio density functional theory investigation of H₂ dissociation and subsequent atomic H diffusion on TM (= Ti, V, Zr, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TMs on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TMs on the right of the periodic table do not bind H, however, they do not reduce the barrier to dissociate H₂ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.     

Electronic structure and stability of new FCC magnesium hydrides Mg7MH16 and Mg6MH16 (M = Ti,V, Nb): An ab initio study

MgH₂ is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH₂ with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg₇TiH_x, Mg_6.5NbH_x and Mg₆VH_x of Ca₇Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH₂. Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self-consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties.     

High dispersion and electrocatalytic activity of Pd/titanium dioxide nanotubes catalysts for hydrazine oxidation

Pd/titanium dioxide nanotubes (Pd/TiO₂-NTs) catalysts were prepared by a simple reduction method using TiO₂-NTs as support. The structure and morphology of the resulting Pd/TiO₂-NTs were characterized by transmission electron microscopy and X-ray diffraction. The results showed that Pd nanoparticles with a size range from 6 to 13 nm were well-dispersed on the surface of TiO₂-NTs. The electrocatalytic properties of Pd/TiO₂-NTs catalysts for hydrazine oxidation were also investigated by cyclic voltammetry. Compared to that of pure Pd particles and Pd/TiO₂ particles, Pd/TiO₂-NTs catalyst showed much higher electrochemical activity. This may be attributed to the uniform dispersion of Pd nanoparticles on TiO₂-NTs, smaller particle size and unique properties of TiO₂-NTs support. In addition, the mechanism of hydrazine electrochemical oxidation catalyzed by Pd/TiO₂-NTs are also investigated. The oxidation of hydrazine was an irreversible process, which might be controlled by diffusion process of hydrazine.     

The effects of Ti-based additives on the kinetics and reactions in LiH/MgB2 hydrogen storage system

In this work we investigated the effect of Ti, TiH₂, TiB₂, TiCl₃, and TiF₃ additives on the hydrogen de/re-sorption kinetics and reaction pathways of LiH/MgB₂ mixture. From high pressure differential scanning calorimeter (HP-DSC) measurements it was found that these additives all effectively decrease the onset temperature of hydrogenation. The isothermal hydrogenation/dehydrogenation measurements suggest that Ti, TiH₂, and TiB₂ can significantly improve the hydrogen sorption kinetics of LiH/MgB₂ mixture. The absorption kinetics of TiF₃ and TiCl₃ doped samples are slower than the baseline (2LiH-MgB₂ without additive), but their desorption kinetics are faster than the baseline and other additives doped systems. X-ray diffraction (XRD) analysis reveals that the additive Ti in LiH/MgB₂ actively participates in both hydrogenation and dehydrogenation process, which can be regarded as an effective additive of this system.     

Influence of transition metals Fe,Ni, and Nb on dehydrogenation characteristics of Mg(BH4)2: Electronic structure mechanisms

First principles calculations on Fe, Ni, and Nb doped Mg(BH₄)₂ were carried out to study the influence of dopants on dehydrogenation properties of Mg(BH₄)₂. It was shown that all dopants considered prefer to substitute for Mg with relatively smaller occupation energies comparing to the B substitution and the interstitial occupation. However, the B substitution shows smaller hydrogen dissociation energy than the Mg substitution. Mechanisms that dopants used to improve dehydrogenation properties of Mg(BH₄)₂ are different. For Mg substitution, Fe strongly interacts with one H atoms of the [BH₄] group, distorts its structural stability and therefore lowers the hydrogen dissociation energy, Ni may attract one particular H atom of the [BH₄] group and weakens the interactions between the B and other H atoms reducing the hydrogen dissociation energy, and the Nb however may drive the formation of NbB₂ and improves the dehydrogenation properties as well. In the B substitution, Fe interacts with the one of H atoms and decreases its structure stability, the Ni will attract its neighbor atoms to form a regular group which is almost identical in structure to that of the NiH₄ group in Mg₂NiH₄, and the NbH₂ and MgH₂ are likely to be generated by Nb doping.     

Pd/Mg/Pd thin films prepared by pulsed laser deposition under different helium pressures: Structure and electrochemical hydriding properties

Three-layered Pd/Mg/Pd thin films were prepared by pulsed laser deposition in the presence of helium gas. For Pd layer deposition, the He pressure was fixed at 200 mTorr whereas different pressures of He were used for Mg layer deposition (50, 200 and 600 mTorr). The degree of crystallinity and of (001) texture in the Mg layer increase with increasing He pressure. In addition, the increase in He pressure upon Mg deposition greatly accentuates the roughness of the Mg layer, which induces an extension of the outer Pd/Mg interface region. In contrast, the inner Pd/Mg interface is sharp for all the Pd/Mg/Pd films. The electrochemical hydrogen sorption properties of the Pd/Mg/Pd films are improved by increasing the He pressure for Mg layer deposition. However, the maximum H-solubility in the Mg layer remains low (H/Mg ∼0.26) and is not significantly increased by the presence of the inner Pd layer, indicating that Mg hydride phase is confined in the outer Pd/Mg interface region.     

Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

The hydrogen absorption and desorption properties of a MgH₂ – 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH₂. Nb ethoxide reacts with MgH₂ during heating, releasing C₂H₆ and H₂, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH₂ without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H₂ to 94 kJ/mol H₂, and from 176 kJ/mol H₂ to 75 kJ/mol H₂, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbH_x achieved by the dispersion of the liquid additive.     

Optimization of TiH2 content for fast and efficient hydrogen cycling of MgH2-TiH2 nanocomposites

Magnesium hydride is extensively examined as a hydrogen store due to its high hydrogen content and low cost. However, high thermodynamic stability and sluggish kinetics hinder its practical application. To overcome this last drawback, different Ti amounts (y = 0, 0.025, 0.05, 0.1, 0.2 and 0.3) were added to magnesium to form (1-y)MgH₂+yTiH₂ nanocomposites (NC) by reactive ball milling under hydrogen gas. Thermodynamic stability of the MgH₂ phase in NCs was determined using a manometric Sieverts rig. Reversible hydrogen capacity and reaction kinetics were determined at 573 K over 20 sorption cycles under a limited reaction time of 15 min. On increasing Ti amount, reaction kinetics are enhanced both in absorption and desorption leading to a higher reversibility for hydrogen storage with the MgH₂ phase. However, titanium increases the molar weight of NCs and forms irreversible titanium hydride. The highest reversible capacity (4.9 wt% H) was obtained for the lowest here studied TiH₂ content (y = 0.025).     

Comparative study on hydrogenation properties of Pd capped Mg and Mg/Al films

Recent emergence of Mg as a promising hydrogen storage material with 7.6 wt% hydrogen encourages study on its thin films to understand physics of storage mechanism. The present study investigates the variations in hydrogen storage properties of Pd sandwiched Mg films upon introduction of Al layer. Multilayered stack of Pd/Mg/Pd and Pd/Al/Mg/Pd were grown on Si substrate using vapor deposition method and further hydrogenated at 150° C under 2 bar H₂ pressure for 2 h. Elastic Recoil Detection Analysis (ERDA) technique with 120 MeV Ag⁹⁺ ions was used to obtain hydrogen concentration versus incident ion fluence. ERDA study reveals that Pd/Mg/Al/Pd films absorb 6.01 × 10¹⁸hydrogen atoms/cm² in comparison to 4 × 10¹⁷ atoms/cm² absorbed by Pd/Mg/Pd system.     

Synthesis by reactive ball milling and cycling properties of MgH2–TiH2 nanocomposites: Kinetics and isotopic effects

MgH₂, MgH₂–TiH₂ nanocomposites and their deuterated analogues have been obtained by reactive ball milling and their kinetic and cycling hydrogenation properties have been analysed by isotope measurements and high-pressure differential scanning calorimetry (HP-DSC). Kinetics of material synthesis depends on both Ti-content and the isotopic nature of the gas. For pure Mg, the synthesis is controlled by isotope diffusion in Mg and therefore MgH₂ forms faster than MgD₂. For the MgH₂–TiH₂ nanocomposites, the synthesis is controlled by the efficiency of milling. Kinetics of reversible hydrogen/deuterium sorption in nanocomposites have been studied at 548 K. The rate limiting step is isotope diffusion for absorption and Mg/MgH₂ interface displacement for desorption. HP-DSC measurements demonstrate that the TiH₂ phase acts as a gateway for hydrogen sorption even in presence of MgO and provides abundant nucleation sites for Mg and MgH₂ phases. The 0.7MgH₂–0.3TiH₂ nanocomposite exhibits steady hydrogen storage capacity after 100 cycles of absorption–desorption.     

Tunable microstructure,de-/hydrogenation kinetics and thermodynamics performance of Mg–Ni–La–Ti–H systems

In the light of positive effects of rare earth and transition metals on the hydrogen absorption/desorption properties of magnesium, the Mg20La–5TiH₂, Mg20Ni–5TiH₂ and Mg10Ni10La–5TiH₂ composites have been prepared in this work to ameliorate the de-/hydrogenation kinetics and thermodynamic performance. The results indicate that the as-prepared composites are mainly composed of Mg, Mg₂Ni/LaH₃ and TiH₂ phases after activation, and LaH₃ and TiH₂ are stable during de-/hydrogenation cycles. The morphology observations give evidences that LaH₃ with size about ~20 nm and Mg₂Ni with size about ~1 μm are uniformly distributed in the composites. It is noted that the de-/hydriding kinetics of the as-prepared composites are significantly improved after internal and surface modification, of which the Mg10Ni10La–5TiH₂ composite can desorb as high as 5.66 wt% hydrogen within 3 min at 623 K. Moreover, the thermodynamic properties of the experimental composites have also been investigated and discussed according to the pressure-composition isothermal curves and corresponding calculation by Van't Hoff equation. The improved hydrogen storage properties of the as-prepared composites are mainly attributed to the uniformly distributed LaH₃, Mg₂Ni and TiH₂ phases, which provide a large amount of phase boundaries, diffusion paths and nucleation sites for de-/hydrogenation reactions.