Improved hydrogen cycling kinetics of nano-structured magnesium/transition metal multilayer thin films

A magnesium-based nano-structured multilayer material was developed by magnetron-assisted physical vapour deposition with promising hydrogen storage properties. The material has a reversible capacity of 4.6 wt% at temperatures between 250 and 350 °C and hydrogenates in <10 min at 250 °C. An activation energy of the dehydrogenation reaction of E_a = 71.6 kJ mol⁻¹ was measured by differential scanning calorimetry. Structural analysis by TEM and SEM showed that the thin magnesium layers of 16.5 nm thickness interspersed with 2.5 nm of an amorphous, nickel-rich transition metal mix resulted in a favourable nanostructure after hydrogen cycling at up to 350 °C. The material also retained its fast kinetics and capacity for the 50 cycles that the material underwent. Comparison of XRD data with TEM shows that the layer thickness of such nano-structured, directional Mg layers in PVD multilayers can be reliably estimated by XRD. In addition the XRD texture relates to the microstructural evolution of the multilayered structure pre- and post-cycling.     

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.     

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 absorption in magnesium based crystalline thin films

A study is carried out for hydrogenation of Mg based thin films produced via thermal evaporation. Films produced were pure Mg, Mg capped with Au–Pd, Mg–Cu co-deposited and Mg–Cu multilayered. The films under the experimental conditions employed were crystalline with columnar grains with some degree of preferred orientation. In multi-component systems, the films in the as-deposited state were made of individual elements, but upon hydrogenation at temperatures greater than 473 K, the elements react with each other yielding the intermetallic phases. The study showed that, of the systems studied in this work, Mg–Cu multilayer yielded the most favorable result as useful storage system, for Mg portion of the film can be converted totally into _MgH2${\mathrm{MgH}}_2$, this occurring at temperatures not greater than 473 K. The study implies that if the as-deposited structure were to be used and preserved as hydrogen storage medium, there is a narrow temperature window for hydrogenation.     

Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

Structural evolution of Pd-capped Mg thin films under H2 absorption and desorption cycles

In this paper we present a study on the relation between the evolution, upon successive H₂ cycling, of the crystalline order and the H₂ sorption properties of Pd-capped textured Mg thin films grown on Si and glass substrates.     

Enhanced oxidation resistance of magnesium nanorods grown by glancing angle deposition

Oxidation behavior of magnesium thin films and nanorods were investigated in the temperature range of 25-550 °C by using thermal gravimetric analysis. Arrays of vertical magnesium nanorods were deposited by the DC magnetron sputtering glancing angle deposition technique, while the magnesium thin films were deposited using the same system but at normal incidence. The morphologies and corresponding crystal structure of the samples were analyzed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction methods, respectively. We report that the Mg thin films showed oxidation induced weight gain starting from room temperature. On the other hand, Mg nanorods did not show any indication of significant oxidation at temperatures below 350 °C. Enhanced oxidation resistance of Mg nanorods was also confirmed by quartz crystal microbalance measurements. At temperatures higher than 350 °C, Mg nanorods started to get oxidized and their weight increased at a similar rate to that of Mg thin films. We argue that reduced oxidation of Mg nanorods is mainly attributed to their single crystal nature. Magnesium nanorods’ reduced oxidation can potentially play a key role in hydrogen storage and gas sensing applications.     

Superior hydrogen absorption and desorption behavior of Mg thin films

Pd-capped Mg films prepared by magnetron sputtering achieved complete dehydrogenation in air at room temperature and behaved as favorable gasochromic switchable mirrors. Their cyclic hydrogen absorption and desorption kinetics in air were investigated by using the Bruggeman effective medium approximation. The overall activation energy was 80 kJ mol⁻¹, while the reaction orders controlling desorption were deduced to be n = 2 at 328 K and n = 1 at lower temperatures by analyzing the transmittance data. The hydrogen diffusion coefficient and the corresponding activation energy were calculated by electrochemical measurements. Mg thin films exhibited the smaller activation energy and remarkable diffusion kinetics at room temperature which implied potential applications in smart windows.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

Hydrogen absorption–desorption,optical transmission properties and annealing effect of Mg thin films prepared by magnetron sputtering

We prepared Pd-capped Mg thin films by direct current (DC) magnetron sputtering. We investigated their structural, optical and electrical properties, as well as the effect of annealing on these films during hydrogenation and dehydrogenation. The hydrogen absorption behaviors at both room temperature and 353 K, and dehydrogenation at 353 K, were studied. The Mg film annealed at 473 K for 2 h exhibited the best absorption kinetics and superior switchable mirror properties among all the samples annealed at different temperatures. We concluded that annealing temperature is a crucial parameter in forming high-quality Mg films. The improved hydriding kinetics can be attributed to the favorable morphology and structure induced by the annealing process.     

Color change of V2O5 thin films upon exposure to organic vapors

The reaction of amorphous V₂O₅ thin films with various organic vapors is investigated using in-situ optical transmission and in-situ Raman spectroscopic measurements. When V₂O₅ thin films are exposed to vapors of methanol, ethanol, acetone, and isopropanol, changes in the Raman spectrum are observed. These changes are similar to those due to alkali ion intercalation and most pronounced for methanol and ethanol. The optical transmission also increases when the thin films are exposed to methanol and ethanol vapors. Depositing a thin catalyst layer of palladium does not promote the reaction. This result has implications for using this material in hydrogen sensor applications, as extended exposure to organic vapors may not be differentiated from the presence of hydrogen.     

Hydrogen desorption properties of Mg thin films at room temperature

Pd–Mg–Pd thin films prepared by magnetron sputtering could absorb hydrogen entirely at room temperature and dehydrogenate completely and rapidly in ambient air. Our investigations of the structural, optical and electrical properties gave a detailed insight into the desorption mechanism. The overall activation energy and the hydrogen diffusion coefficient were deduced to be 48 kJ mol⁻¹ and 8.0 × 10⁻¹⁵ cm² s⁻¹ based on optical and electrochemical measurements, respectively. The desorption process followed the nucleation and growth mechanism by modeling and simulating the resistance data. The small activation energy and remarkable diffusion kinetics highlighted the applicability as on-board hydrogen storage systems.     

Nano-scale bi-layer Pd/Ta, Pd/Nb, Pd/Ti and Pd/Fe catalysts for hydrogen sorption in magnesium thin films

We analyzed the elevated temperature volumetric hydrogen sorption behavior of magnesium thin films catalyzed by nano-scale bi-layers of Pd/Ta, Pd/Nb, Pd/Ti and Pd/Fe. Sorption of magnesium catalyzed by pure Pd was determined as a baseline. Sorption cycling demonstrated that when utilizing pure Pd and the Pd/Fe bi-layer catalysts the sorption kinetics of the Mg films rapidly degraded. However with the Pd/Nb, Pd/Ti and Pd/Ta bi-layer catalysts the composite remained cycleable. After multiple sorption cycles the Pd/Nb and Pd/Ti catalyst combinations possessed the fastest kinetics. X-ray diffraction analysis showed that NbH_0.5 and TiH₂ are formed during testing. Basic thermodynamic analysis indicates that NbH_0.5 and TiH₂ should be stable both during absorption and during desorption. We believe that this is why Nb and Ti are the most effective intermediate layers: The elements form stable hydrides at the Mg surfaces preventing complete Pd-Mg interdiffusion and/or acting as hydrogen catalysts and pumps.     

Crystal structures of MgyTi100−y thin film alloys in the as-deposited and hydrogenated state

In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated Mg_yTi_100-y thin film alloys containing 70, 80 and 90 at.% Mg. The preferred crystallographic orientation of the films in both the as-prepared and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 10⁵ Pa H₂ transforms the unit cell into face-centered cubic for the Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ compounds, whereas the unit cell of hydrogenated Mg₉₀Ti₁₀ has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc-structured hydrides to sluggish for hydrides with a bct symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.     

Thermodynamics, kinetics and microstructural evolution during hydrogenation of iron-doped magnesium thin films

New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg-4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH₂ is the mechanism controlling the hydride layer growth.Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg-Fe powders of 1-2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.     

Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance

Thermochromic films of VO₂, as well as multilayer films of VO₂ and TiO₂, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τ_c≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO₂/VO₂/TiO₂ films could display a luminous transmittance significantly higher than that of bare VO₂ films, and that TiO₂/VO₂/TiO₂/VO₂/TiO₂ films could yield a large change of solar transmittance for temperatures above and below τ_c. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.     

The role of differently distributed vanadium nanocatalyst in the hydrogen storage of magnesium nanostructures

Using a glancing angle (co)deposition technique, ∼4.6 at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E_a^a = 35.3 ± 0.9 kJ/mol H₂) and hydrogen desorption (E_a^d = 38.9 ± 0.3 kJ/mol H₂) than the V-decorated Mg sample when deposited at the same deposition angle of θ = 70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples.     

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.     

Catalysis derived from flower-like Ni MOF towards the hydrogen storage performance of magnesium hydride

Herein, a novel flower-like Ni MOF with good thermostability is introduced into MgH₂ for the first time, and which demonstrates excellent catalytic activity on improving hydrogen storage performance of MgH₂. The peak dehydrogenation temperature of MgH₂-5 wt.% Ni MOF is 78 °C lower than that of pure MgH₂. Besides, MgH₂-5 wt.% Ni MOF shows faster de/hydrogenation kinetics, releasing 6.4 wt% hydrogen at 300 °C within 600 s and restoring about 5.7 wt% hydrogen at 150 °C after dehydrogenation. The apparent activation energy for de/hydrogenation reactions are calculated to be 107.8 and 42.8 kJ/mol H₂ respectively, which are much lower than that of MgH₂ doped with other MOFs. In addition, the catalytic mechanism of flower-like Ni MOF is investigated in depth, through XRD, XPS and TEM methods. The high catalytic activity of flower-like Ni MOF can be attributed to the combining effect of in-situ generated Mg₂Ni/Mg₂NiH₄, MgO nanoparticles, amorphous C and remaining layered Ni MOF. This research extends the knowledge of elaborating efficient catalysts via MOFs in hydrogen storage materials.