Hydrogen Sorption Behavior Research Articles

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ∼5 nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si.

Destabilization of LiAlH4 by nanocrystalline MgH2

Abstract In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH4 + nanoMgH2). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH2 is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH2. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH4 + nanoMgH2 possess ∼5.0 wt.% H2 reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH4 + nanoMgH2) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H2) to their pristine counterparts, LiAlH4 and MgH2. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.

Hydrogen Sorption in Magnesium Nanoparticles: Size- and Surface-related Phenomena

AbstractThe aim of this work is the investigation of the metal-hydride transformation in magnesium (Mg) nanoparticles both as a function of particle size and in response to surface functionalization by clusters of transition metals (TM): Pd, Ni, Ti.Mg nanoparticles were synthesized by the inert-gas condensation technique, which yields single crystals with six-fold symmetry whose average size can be controlled by tuning the inert gas pressure. After the synthesis the nanoparticles were passivated by slow exposure to oxygen, obtaining a core-shell morphology where a metallic core is coated by a MgO shell of about 5 nm thickness.The material structure was investigated by Transmission Electron Microscopy (TEM), also in High Resolution (HRTEM) mode, and by X-Ray Diffraction (XRD). The sorption kinetics were analysed by a volumetric Sievert apparatus, which also allowed for a determination of the activation energies.Small nanoparticles (≈35 nm) display interesting kinetics with gravimetric capacity of 4.5 wt.% at saturation, limited by the oxide fraction. Hydride formation proceeds by one-dimensional growth controlled by diffusion through the hydride, while the reverse transformation to metal involves interface-controlled three-dimensional growth of nuclei formed at constant rate.On the contrary, large nanoparticles (≈450 nm) exhibit very low reactivity due to reduced probability of hydrogen dissociation/recombination and nucleation at the particle surface. For this reason, large nanoparticles were surface-decorated by TM through in situ evaporation in the inert-gas condensation chamber. This procedure results in clusters of 3-4 nm located over a portion of the MgO shell, as shown by XRD and HRTEM on Pd-decorated sample. This treatment results in dramatically improved hydrogen sorption behavior. In fact, previously inert nanoparticles now exhibit of up to 5.6 wt.%.Real-time diffraction studies using Synchrotron Radiation were carried out during hydrogen desorption on the Pd-decorated nanoparticles. We clearly show that a Mg-Pd intermetallic phase is formed after the first heating treatment and takes active part in the transformation.

Pd electrodeposited from membrane-separated thin layer cell

Polycarbonate (PC) membranes of different porosity (from 50 nm to 400 nm pore size) were used as separators in a thin layer cell for palladium electrodeposition. Atomic force microscopy (AFM) confirmed the formation of Pd layer between the cathode and porous membrane, with subsequent growth inside the pores induced by space limitations of further growth of initial layer. Our estimates confirm that at this stage Pd deposit feels the pronounced mechanical pressure and consider it as a possible reason of specific hydrogen sorption behaviour in the region of β-phase hydride formation. Up to c.a. 1.0 H/Pd atomic ratio is observed for some samples. We consider possible (nano)structural peculiarities responsible for this behaviour.

A preliminary study of some “pseudo- [formula omitted]” compounds: [formula omitted] with [formula omitted], Ce and Gd. Structural and hydrogen sorption properties

The compounds RENi 4 Mg have been elaborated by mechanical alloying (MA) and a subsequent heat treatment at 600 ∘ C for 1 h. The lattice parameters are in good agreement with those already reported: 7.165 A ˚ for LaNi 4 Mg , 7.030 Å for CeNi 4 Mg and 7.038 A ˚ for GdNi 4 Mg . The hydrogen sorption behaviors are strongly influenced by the rare earth used: for La compound, hydrogen absorption leads to a structural change and no desorption can be observed, for Ce compound, no reaction towards hydrogen can be noticed and for Gd compound, hydrogen can be absorbed and desorbed reversibly at room temperature. Moreover, for GdNi 4 Mg , no structural change is reported after hydrogenation and only a slight volume expansion can be noticed (e.g. 5%). The MA process has also been used successfully for the elaboration of GdNi 4 - x Al x Mg up to x = 1 . In these substituted compounds, hydrogen can also be absorbed and desorbed reversibly at room temperature.

MgH2 by Gas Phase Condensation: Nanostructure Morphology and Hydrogen Sorption Behaviour

AbstractInert gas condensation was employed to prepare nanoparticles of Mg and MgH2 which morphology, clustering degree and structural stability have been investigated by X-ray diffraction and electron microscopy. Thermodynamic functional properties of the Mg and MgH2 nanostructured samples were investigated by high pressure differential scanning calorimetry. Some specific features of the morphology of the samples prepared by inert gas condensation are compared with powders obtained by ball milling through desorption kinetics behavior.

Hydrogen Sorption on Palladium-Doped Sepiolite-Derived Carbon Nanofibers

The hydrogen sorption behavior of novel Pd-doped sepiolite-derived carbon nanofibers (SDCNs) was investigated. Two chemical doping methods of ethanol/toluene reduction and a polyol process were applied to control Pd(0) formation in the SDCNs at different Pd doping levels. Hydrogen storage capacity was observed to vary depending on the Pd particle size and doping amount as the Pd particle surface area and the carbon Brunauer-Emmett-Teller (BET) surface area change with them, suggesting the existence of an optimum Pd doping level at each doping method for the best hydrogen storage capacity. Among the samples prepared using the polyol method at different Pd amounts, the maximum hydrogen uptake of about 0.41 wt % was obtained at 298 K and 90 bar for the 5 wt % Pd-doped SDCN that has a relatively high Pd particle surface area and carbon BET surface area. Samples prepared using the ethanol/toluene reduction method exhibited a higher hydrogen uptake of about 0.59 wt % at lower Pd doping levels (3 wt % Pd) due to a smaller Pd particle size and relatively high carbon BET surface area. The hydrogen desorption behavior investigated by differential scanning calorimeter exhibited that a higher amount of hydrogen desorbed at around 860 K from the Pd-doped SDCNs compared to that from the undoped SDCN. Fourier transform infrared (FTIR) spectroscopic analysis suggested that some of the weak chemisorbed hydrogen changes to a normal covalent bond during the heating and effuses at around 860 K.

Hydrogen Sorption Behavior of Nanodeposits of Palladium and Its Alloys

not Available.

Hydrogen Desorption from MgH2 Based Nano-Micro Composites

ABSTRACTThe microstructure and hydrogen sorption behaviour of MgH2 based nano-hydrides with different additives prepared by ball milling and inert gas condensation has been investigated by XRD, Differential Scanning Calorimetry and Mechanical Spectroscopy. Preliminary results on materials with similar composition and different nanostructures show that suitable mechanical processing and additive additions induce in the nanostructured composites tailored channels for improved ab/de-sorption kinetics at reduced temperatures.

On the Diffusivity, Solubility and Adsorption of Hydrogen in Isotropic, Mechanically Nanostructured Graphite and Graphite Nanofibers

Thermodynamic analysis of experimental data on the hydrogen sorption behavior in graphite and novel carbon-based nanomaterials shows that the mechanism of the hydrogen storage is not well known. The interpretation of the thermodynamic and kinetic results is proposed basing on the “ab initio” molecular orbital study of the adsorption of atomic hydrogen on the graphite and carbon nanostructures. Characteristics and nature of five desorption peaks (processes) have been determined and analyzed in respect to the technological requirements for electric vehicles. Both experimental and theoretical developments are discussed.

Surface Characteristics, Hydrogen Sorption, and Catalytic Properties of Pd−Zr Alloys

Significant differences are observed in the hydrogen sorption behavior of as-received, HF-treated, and oxidized PdZr alloys. Removal by HF treatment of the surface ZrO2 layer, which blocks sorption of hydrogen, and a part of the bulk Zr generates a porous surface layer of increased surface area. The resulting specimen sorbs hydrogen even at low H2 partial pressure. A thick oxide layer composed of a mixture of PdO and ZrO2 is formed by aerobic oxidation (553 K, 3 h). A diffusion controlled slow H2 sorption by Pd in the surface layer followed by a rapid and large H2 sorption by the bulk alloy is observed. This results in the fracturing of the surface oxide layer and the formation of the hydrides of the constituent elements. The pretreated alloy samples studied exhibit high catalytic activities and selectivities in the hydrogenation of compounds with multiple unsaturation (cycloocta-1,3-diene, phenylacetylene, 1-pentyne).

Electrochemical Reactivity of Zirconium-Based Bulk Metallic Glasses

ABSTRACTAmorphous Zr-(Ti)-(Nb)-Al-Cu-Ni alloy samples were prepared by melt-spinning and copper mould casting in an argon atmosphere and characterized regarding their microstructure and thermal behavior. Their anodization behavior in aqueous environments with pH= 0.5 – 13 was studied by electrochemical polarization techniques in combination with surface analytical investigations, i.e. SEM/EDX, AES. In chloride-containing solutions the macroscopic corrosion resistance of bulk amorphous alloys is affected by the presence of heterogeneities, such as concentrated cluster zones of selected components or crystalline defects. Pitting phenomena are studied in neutral and acidic chloride solutions and as a result a local corrosion mechanism is proposed. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in fluoride solutions was investigated. After pre-etching melt-spun amorphous samples exhibit a significant increase in surface reactivity as expressed by a drastic increase in electrochemical capacities and in cathodic current densities as well as by a significant reduction of overpotentials for the hydrogen reduction reaction. The hydrogen sorption behavior was studied on samples galvanostatically charged at various cathodic current densities by means of XRD, DSC, TEM and thermal desorption analysis TDA. At room temperature Zr-based alloys absorb hydrogen up to H/M=1.65 mainly by interstitial solution of hydrogen atoms in the amorphous structure. The effect of absorbed hydrogen on the thermal stability and the crystallization behavior is described.

Thermodynamic and structural aspects of hydrogen absorption in the Zr(Al xFe 1− x) 2 system

Hydrogen desorption isotherms have been measured for Zr(Al x Fe 1− x ) 2 (0.12≤ x≤0.4) compounds at pressures up to 60 atm and temperatures between −30 °C and 140 °C. The enthalpies of hydride formation, derived from the temperature variation of the equilibrium pressures for x=0.12, 0.167, 0.2 and 0.4, show an increase of the hydride stability with increase of x. The shapes of the isotherms in the plateau regions indicate decrease of the critical temperature for the metal-to-hydride transition as a function of x. These two features closely resemble the observations for the recently investigated Zr(Al x Co 1− x ) 2 system and support further a proposed explanation for the anomalous hydrogen sorption behavior in these and other Al-containing intermetallics. No hysteresis between absorption and desorption isotherms was observed for the x=0.12 and 2/12 compounds. The lattice constants of the expanded crystal structures upon hydrogenation have been determined by employing X-ray diffraction.

Thermodynamic and structural aspects of hydrogen absorption in the Zr(AlxCo1−x)2 system

Hydrogen absorption and desorption isotherms have been measured for Zr(AlxCo1−x)2 (0.1≤x≤0.5) compounds at pressures up to 60 atm and temperatures between −30 and 100°C. Enthalpies and entropies of hydride formation have been derived from desorption isotherms for x=0.1, 0.167, 0.25 and 0.35. Below these x values the pressures, necessary for hydrogen absorption, are higher than 60 atm. The critical temperatures for hydride formation of the x=0.35 compounds are lower than the experimental temperatures employed in the present work. In general, the hydride stability increases while the critical temperature decreases as a function of x. Consequently, we suggest that a competition between two opposing factors, namely the metal–hydrogen interaction and the hydrogen–hydrogen interaction (or hydrogen-induced long range elastic strain interaction), accounts for the anomalous hydrogen sorption behavior in this and other Al-containing systems. The hydrogen equilibrium pressure for ZrCo2 is estimated by extrapolation. The estimation compares fairly with available experimental information. The volume occupied by a single hydrogen atom is obtained for all the investigated compounds, and supports the idea of approximately constant value for this quantity.

Host Effect on the Properties of AM-GICs

Carbon materials (A-1, A-2 and A-3 derived from pitch cokes) with different graphitization degree, were allowed to react with potassium. The hydrogen-sorption behavior at 77 K, electrical resistivity. ESR and Raman spectra of the resulting compounds with the composition of KC60 were determined. The sorbed amount at saturation, (n H2/n K)sat, was 0.55 and 1.43 for KC60s prepared from A2 (d 002 = 0.3377 nm) and A3 (d 002 = 0.3361 nm), respectively. No H2 sorption was observed for KC60 from A-1 (d 002=0.342 nm). Temperature dependence of the resistivity of KC60s from A-2 and A-3 showed metallic behavior, contrary to semiconductive one for the host materials. However, KC60 (A-1) showed semiconductive temperature dependence, similarily to the host material. Raman spectra of KC60s from A-2 and A-3 showed doublet structure, similarly to that of K-GICs from HOPG, characteristic for graphite intercalation compounds with stage n > 2. On the contrary, KC60 (A-1) gave single peak at around 1597 cm−1. Those facts suggest that potassium exists in the interlayer spaces for KC60s (A-2, A-3), but not for KC60 (A-1). It was also shown that g-factor of ESR spectra of KC60 can be useful to predicting the H2-sorption behavior.

Characteristics of electrodes using amorphousAB 2 hydrogen storage alloys

The hydrogen sorption behaviour of RENi 2 (RE=La, Ce, Pr, Mm (misch metal))Laves phase alloys was investigated by both the coulometric titration technique and the gas volumetric method. These alloys became amorphous by charging and kept the amorphous structure during charge-discharge cycles. The shapes of the pressure-composition isotherms indicated that the amorphous hydrides were compounds of the solid solution type. The discharge capacities of the amorphous hydride electrodes in the electrochemical cell were less than one-fourth of the capacities evaluated from the gas volumetric measurement, though they increased with increasing temperature. The discharge capacity did not decay during the first 500 cycles.

HYDROGEN SORPTION BY SOME EARLY-LATE TRANSITION METAL GLASSES

The hydrogen sorption behaviour of amorphous Cu50Zr50, Ni50 Ti50, Ni61Ti39, Ni62Ti29Zr9, Ni64Zr36, Ni60Nb40 and Ni50Nb50 obtained by melt spinning is described. The first three alloys have a relatively low crystallization temperature and these alloys either could not be made completely amorphous or crystallized during hydrogen sorption. The highest absorption occurs in Ni64Zr36 : about 0.6 hydrogen atom per metal atom at a temperature of 333 K and a hydrogen pressure of 30 bar. Pressure-composition isotherms of Ni64Zr36 show no pressure plateau in the range of temperatures and pressures investigated. The isotherms of crystalline Ni64Zr36 and the intermetallic compound Ni10Zr7 do show such a plateau. During 100 cycles of hydration and dehydration of amorphous Ni64Zr36 splinters, of 0.1 mm spalled off from the pieces of ribbon, but disintegration into still smaller particles was not observed. The X-ray diffraction and DSC-scan after 100 cycles are similar to those of as-quenched material. Magnetization and Rutherford-back-scattering results indicate surface segregation of Ni in a 100 times cycled sample.

Hydrogen sorption characteristics of Ce-3d and Y-3d intermetallic compounds

A study of the hydrogen sorption behaviour of several intermetallic compounds based on 3d metals (M) and cerium or yttrium (R) is reported. As well as pressure-composition isotherms, an X-ray diffraction study was made of the structural changes that accompany hydrogen absorption. It was found that, in compounds of the types R 2M 7 and RM 3, the structure type is preserved during hydrogen absorption. In yttrium compounds expansion occurs in the c direction as well as in the basal plane of the hexagonal or rhombohedral unit cells. In cerium compounds the expansion affects the c direction exclusively. This expansion is particularly large in CeCo 3H 4 where, with respect to CeCo 3, the c axis increases by more than 30%.

Film thickness effects on hydrogen sorption at palladium electrodes

Hydrogen sorption at thin film evaporated Pd electrodes is quite different from that at bulk Pd. Diffusion of dissolved hydrogen in the thin films is slow. The grain size of the evaporated films is small, and the large number of grain boundaries present a high resistance to hydrogen diffusion. Thin films sorb a greater quantity of hydrogen than bulk Pd. Annealing of the evaporated films, due to an increase in grain size, results in hydrogen sorption behavior resembling that of bulk Pd and a decrease in the charge due to sorbed hydrogen. The dependence of sorption charge on Pd morphology could be explained by segregation of sorbed hydrogen at grain boundaries.