Onset Temperature Of Desorption Research Articles

Uncovering the Effect of Al Addition on the Hydrogen Storage Properties of the Ternary TiVNb Alloy.

The effect of Al addition on the structure, microstructure and hydrogen storage properties of the ternary TiVNb alloy was investigated from small amounts to equimolar composition. Alx(TiVNb)1-x (x = 0.05, 0.175 and 0.25) alloys are bcc single-phase materials with decreasing lattice parameters with increasing Al content. Al addition progressively decreases the hydrogen storage capacity but also destabilizes fcc dihydride formation for alloys with x ≤ 0.10. Among the different compositions, the most promising alloy was found to be that with x = 0.05 Al content that exhibited high initial storage capacity (2.96 wt.%), a less stable hydride (ΔH = -52 kJ/mol H2 and ΔS = -141 J/K∙mol H2), better desorption properties (desorption onset temperature around 100 °C) and enhanced reversible capacity during cycling (2.83 wt.%) compared to the ternary TiVNb. In situ and ex situ synchrotron X-ray powder diffraction, together with thermal desorption experiments, showed improved desorption properties with Al addition, together with a two-step reaction with hydrogen. These findings highlight the use of small quantities of lightweight Al in refractory multi-principal element alloys as a promising approach for enhancing the solid-state hydrogen storage performance of bcc-type alloys.

Compositional effects on the hydrogen storage properties in a series of refractory high entropy alloys

The possible combinations in the multidimensional space of high entropy alloys are extremely broad, which makes the incremental experimental research limited. As a result, establishing trends with well-known empirical parameters (lattice distortion, valence electron concentration etc.) and predicting effects of the chemical composition change are vital to guide future research in the field of materials science. In this context, we propose a strategy to rationalize the effect of chemical composition change on the hydrogen sorption properties in a series of high entropy alloys: Ti0.30V0.25Zr0.10Nb0.25M0.10 with M = Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ta and ∅ (corresponding quaternary alloy). All materials are bcc alloys and absorb hydrogen at room temperature forming fcc or pseudo-fcc dihydride phases. The maximum hydrogen storage capacity at room temperature strongly depends on the valence electron concentration (VEC) of the alloys: the capacity is high (1.5–2.0 H/M) for low values of VEC (<4.9) whereas, a drastic fading is observed for VEC ≥4.9 which is the case for alloys with M being a late 3d transition metal. The structural analysis suggests that steric effects might not be responsible for this trend and electronic reasons may be invoked. Increasing the VEC by alloying with late 3d transition metals will fill the unoccupied valence states and the electrons from interstitial hydrogens can no longer be accommodated, which is unfavorable for hydrogen storage. Moreover, the onset temperature of desorption increases almost linearly with VEC for this composition series. These findings suggest that alloys with low VEC are more likely to become promising candidates for hydrogen storage.

Design of TiVNb-(Cr, Ni or Co) multicomponent alloys with the same valence electron concentration for hydrogen storage

Recently, it has been hypothesized that the hydrogen sorption properties of body-centered cubic (BCC) multicomponent alloys might be related to the alloy’s valence electron concentration (VEC). In this work, we employed CALPHAD method to design three BCC alloys with the same VEC to evaluate this hypothesis. The (TiVNb)85Cr15, (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 multicomponent alloys with VEC = 4.87 were produced by arc melting. Although some amount of segregation could not be avoided, the as-cast microstructures were composed of a BCC phase with highly homogeneous chemical composition. The alloys quickly absorb hydrogen at room temperature and formed FCC hydrides with high capacities of 2 H/M (3.1–3.2 wt%). For the three alloys, it was observed a two-step hydrogenation sequence indicating the formation of an intermediate BCC monohydride prior to the formation of the FCC dihydride. Moreover, the three alloys presented very similar values of enthalpy and entropy of hydrogenation, showing that the initial hypothesis seems to hold for the thermodynamic properties. The alloys hydrogenation/dehydrogenation cycling behavior were also evaluated. (TiVNb)85Cr15 had a small and continuous drop in capacity over cycling that was addressed to incomplete transformation of the BCC monohydride to the FCC dihydride, whereas (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 showed a more stable behavior achieving a reversible capacity of 1.76 H/M (2.77 wt%) after 20 cycles. The desorption is strongly affected by absorption/desorption cycling for all alloys, as revealed by thermo desorption spectroscopy (TDS). The onset temperature of desorption depends on the alloys’ composition as well as the number of hydrogenation/dehydrogenation cycles. The results suggest that cycling and desorption properties of the alloys are less related to their VEC.

Improved kinetic behaviour of Mg(NH2)2-2LiH doped with nanostructured K-modified-LixTiyOz for hydrogen storage

The system Mg(NH2)2 + 2LiH is considered as an interesting solid-state hydrogen storage material owing to its low thermodynamic stability of ca. 40 kJ/mol H2 and high gravimetric hydrogen capacity of 5.6 wt.%. However, high kinetic barriers lead to slow absorption/desorption rates even at relatively high temperatures (>180 °C). In this work, we investigate the effects of the addition of K-modified LixTiyOz on the absorption/desorption behaviour of the Mg(NH2)2 + 2LiH system. In comparison with the pristine Mg(NH2)2 + 2LiH, the system containing a tiny amount of nanostructured K-modified LixTiyOz shows enhanced absorption/desorption behaviour. The doped material presents a sensibly reduced (∼30 °C) desorption onset temperature, notably shorter hydrogen absorption/desorption times and reversible hydrogen capacity of about 3 wt.% H2 upon cycling. Studies on the absorption/desorption processes and micro/nanostructural characterizations of the Mg(NH2)2 + 2LiH + K-modified LixTiyOz system hint to the fact that the presence of in situ formed nanostructure K2TiO3 is the main responsible for the observed improved kinetic behaviour.

Mg-based Nanocomposites for hydrogen storage containing La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloys as additives

Abstract Mg based nanocomposites containing different concentrations of La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloy has been undertaken at present work. Doped alloy was prepared by arc melting furnace and ball milled to get its nano structured. Hydrogen sorption properties of MgH2+ x wt% (x=10, 25 and 50) La23Nd8.5Ti1.1Ni33.9Co32.9Al0.65 alloy was characterized for their thermal, morphological and structural properties. Result shows that the alloy significantly reduced onset temperature of desorption. The TGA analysis showed higher desorption temperature for milled MgH2 compared to x wt% alloy added MgH2. Furthermore, the estimated activation energy for hydrogen desorption was found to be -177.90 , -204.65, -79.15 and -185.29 kJ/mol for milled ,10 wt%, 25 wt% and 50 wt % MgH2 respectively and also shows the reduction of activation energy by 98 kJ/mol due to the addition of 25 wt% alloy additives resulted the improved hydrogen storage capacity. The phase and purity of MgH2 during milling was studied by XRD and it is revealed that maximum stable and improved phase alteration for Hydrogen sorption was obtained during 25 wt % alloy milling. Morphological studies by SEM were undertaken to investigate the effect of hydrogenation of nanostructured alloy. DSC investigations were carried out to investigate the effect of alloy on hydrogen absorption behaviour of MgH2.

Electrochemical Synthesis of Highly Ordered Porous Al Scaffolds Melt-Infiltrated with LiBH4 for Hydrogen Storage

Two highly ordered porous Al scaffolds were synthesized by applying a soft template assisted electrodeposition method, using an ionic liquid as the electrolyte. Polystyrene (PS) spheres with an average diameter of 399 ± 2 nm or 89 ± 20 nm were deposited on a polished Cu electrode using a dip-coater. An imidazolium-based ionic liquid mixed with aluminium chloride [EMIm]/AlCl3 (40/60 mol%) was used as the electrolyte for the Al electrodeposition. The PS spheres that were used as a soft template were removed after the Al electrodeposition method by chemically dissolving them in tetrahydrofuran (THF). Lithium borohydride (LiBH4) was then melt-infiltrated into the porous Al scaffold. Morphological observations of the dip-coated Cu electrodes with the PS spheres, the as-synthesized porous Al scaffolds, and the LiBH4 melt-infiltrated samples were carried out using Scanning Electron Microscopy (SEM). The scaffolds exhibited a highly ordered porous Al structure with an open network of pores and an average pore size of 355 ± 25 and 56 ± 20 nm respectively. The porous Al acts as a reactive scaffold which interacts with LiBH4 at elevated temperature. Temperature Programmed Desorption (TPD) experiments revealed that the melt-infiltrated LiBH4 samples exhibited faster H2 desorption kinetics in comparison to the bulk material. In particular, the 56 ± 20 nm Al scaffold showed a H2 desorption onset temperature (Tdes) at 100°C which is 250°C lower than for bulk LiBH4. This temperature drop can be attributed to the size reduction of LiBH4 down to the nanoscale, together with the high contact surface area with the Al scaffold.

Superior effect of RbF on decreasing the dehydrogenation operating temperature of the LiNH2[sbnd]LiH system

In this study, the superior effect of RbF on reducing the hydrogen desorption operating temperature of the LiNH system was demonstrated. By doping 5 mol% RbF in the LiNH2LiH system, we obtain the lowest hydrogen desorption peak temperature (211 °C) at a heating rate of 5 °C/min, which is approximately 69 °C lower than that of the pristine LiNH2LiH system. Moreover, the system starts to desorb H2 at 120 °C, which is the lowest hydrogen desorption onset temperature. The hydrogen desorption peak and onset temperatures can be further decreased by doping more RbF. It is proposed that the RbH produced during heat treatment improves the dehydrogenation properties of the RbF-doped LiNH2LiH sample.

Effect of cycling on hydrogen storage properties of Ti2CrV alloy

In the present work, we have studied the hydrogen absorption–desorption properties of the Ti2CrV alloy, and effect of cycling on the hydrogen storage capacity. The material has been characterized for the structure, morphology, pressure composition isotherms, hydrogen storage capacity, hydrogen absorption kinetics and the desorption profile at different temperatures in detail. The Ti2CrV crystallizes in body centered cubic (bcc) structure like TiCrV. The pressure composition isotherm of the alloy has been measured at room temperature and at 373K. The Ti2CrV alloy shows maximum hydrogen storage capacity of 4.37 wt.% at room temperature. The cyclic hydrogen absorption capacity of Ti2CrV alloy has been investigated at room temperature upto 10th cycle. The hydrogen storage capacity decreased progressively with cycling initially, but the alloy can maintain steady cyclic hydrogen absorption capacity 3.5 wt.% after 5th cycle. To get insight about the desorption behavior of the hydride in-situ desorption has been done at different temperatures and the amount of hydrogen desorbed has been calculated. The TG (Thermo gravimetric) and DTA analysis has been done on uncycled hydride shows that the surface poisoned sample gives a desorption onset temperature of 675K. The DSC measurement of uncycle and multi-cycled saturated hydrides shows that the hydrogen desorption temperature decreasing with cycling.

Hydrogen sorption performance of MgH 2 doped with mesoporous nickel- and cobalt-based oxides

The effect of mesoporous Co 3O 4, NiCo 2O 4 and NiO on the hydrogen sorption performance of MgH 2 was investigated. These oxides were synthesized by multi-step nanocasting and introduced during the high-energy ball milling of MgH 2 powder to act as catalysts. Hydrogen desorption on the as-milled powders was assessed upon heating the samples from room temperature to 400 °C. In all cases, the onset temperature for desorption was lowered by taking advantage of the introduced additives. The NiO-doped sample displayed the best response, the desorption rate being 7 times faster than in pure MgH 2. Complementary kinetic studies on this particular sample revealed that the sorption activation energies were much lower (50 kJ/mol for absorption and 335 kJ/mol for desorption) than the corresponding ones for undoped MgH 2 (57 kJ/mol for absorption and 345 kJ/mol for desorption), thus proving the catalytic activity of the mesoporous NiO oxide. Significantly, the X-ray powder diffraction (XRPD) patterns taken on the NiO-doped sample after discharging/charging cycles revealed that Mg could fully hydrogenate at the end of the charging process, while Mg metal was still detected in the undoped (pure) sample. Favored conditions for dissociative chemisorption of hydrogen could be ascribed to the formation of metallic Ni arising from complete or partial reduction of NiO, as observed in the XRPD patterns.

2Mg–Fe alloys processed by hot-extrusion: Influence of processing temperature and the presence of MgO and MgH 2 on hydrogenation sorption properties

2Mg–Fe alloy powder produced by high-energy ball milling was processed by hot extrusion at temperatures of 200 °C and 300 °C to produce bulk samples. The alloys were hydrogenated for 24 h under hydrogen pressures of 24 bar (to produce the Mg 2FeH 6 phase) and 15 bar (to produce a mixture of MgH 2 + Mg 2FeH 6 phases), respectively. After the hydrogenation treatments, the complex hydride Mg 2FeH 6 was identified in both conditions, while the MgH 2 and MgO phases were observed only after extrusion at 200 °C. Desorption temperatures varied with the extrusion conditions; extrusion at 300 °C resulted in a desorption onset temperature about 68 °C lower than that of samples extruded at 200 °C, and about 200 °C lower than that of commercial MgH 2. Extrusion at the lower temperature did not change the number of stored defects (point defects, dislocations, voids, stacking faults, vacancies and others) produced in the milling process and increased the preferential sites for hydride nucleation, increasing the hydrogen storage capacity. The presence of MgO produced the beneficial effect of grain boundary pinning, but delayed the onset temperature of desorption. The combined presence of MgH 2 and Fe after hydrogenation at 15 bar seems to play a catalytic role that considerably hastened the Mg–H reactions and increased the desorption kinetics. However, the desorption kinetics in both conditions was still low.

Hydriding/dehydriding properties of MgH 2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH 2/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg 2Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.

Mechanism of the reaction NO + H2 on the Pt(100)-hex surface under conditions of the spatially nonuniform distribution of reacting species

The interaction of hydrogen with NO ads /1 × 1 islands produced by NO adsorption on the recon- structed surface Pt(100)-hex was studied by high-resolution electron energy loss spectroscopy (HREELS) and the temperature-programmed reaction (TPR) method. The islands are areas of the unreconstructed surface Pt(100)- 1 × 1 saturated with NO ads molecules. The hexagonal phase around these islands adsorbs much more hydrogen near room temperature than does the clean Pt(100)-hex surface. It is assumed that hydrogen is adsorbed on the hexagonal surface areas that are adjacent to, and are modified by, the NO ads /1 × 1 islands. The reaction of adsorbed hydrogen atoms with NO ads takes place upon heating and has the character of so-called surface explo- sion. The TPR peaks of the products of this reaction—nitrogen and water—occur at T des ~ 365-370 K, their full width at half-maximum being ~5-10 K. In the case of the NO ads /1 × 1 islands preactivated by heating in vacuo above the NO desorption onset temperature (375-425 K), after the admission of hydrogen at 300 K, the reaction proceeds in an autocatalytic regime and the product formation rate increases monotonically at its initial stage. In the case of activation at 375 K, during the initial, slow stage of the reaction (induction period), hydrogen reacts with nitric oxide molecules bound to structure defects (NO def ). After activation at 425 K, the induction period is characterized by the formation and consumption of imido species (NH ads ). It is assumed that NH ads formation involves N ads atoms that have resulted from NO ads dissociation on defects upon thermal activation. The induction period is followed by a rapid stage of the reaction, during which hydrogen reacts with NO 1 × 1 molecules adsorbed on 1 × 1 areas, irrespective of the activation temperature. After the completion of the reac- tion, the areas of the unreconstructed phase 1 × 1 are saturated with adsorbed hydrogen. The formation of H ads is accompanied by the formation of a small amount of amino species (NH 2ads ).

Cyclohexene as a chemical probe of the low-temperature hydrogenation activity of Pt/Ni( [formula omitted]) bimetallic surfaces

The reactions of H 2 and cyclohexene on Pt/Ni(1 1 1) surfaces were investigated using temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Hydrogen desorbs from Pt/Ni(1 1 1) bimetallic surfaces at temperatures approximately 40–50 K lower than the clean Ni(1 1 1) surface. The reaction pathways of cyclohexene are also significantly modified on the Pt/Ni(1 1 1) surfaces. On Ni(1 1 1), cyclohexene undergoes dehydrogenation and decomposition to produce benzene (c-C 6H 6), atomic C (a) and H 2(g). HREELS results also reveal that cyclohexene (c-C 6H 10) is strongly bonded to the Ni(1 1 1) surface. However, when Pt is added to the Ni(1 1 1) surface at coverages from 0.5 to 3 ML, a fraction of the cyclohexene undergoes self-hydrogenation to produce cyclohexane (c-C 6H 12) at 237 K. HREELS results indicate that cyclohexene is more weakly bonded to the Pt/Ni(1 1 1) surface, with the ν(CC) stretching frequency (∼1638 cm −1) and skeletal distortions (649 cm −1) still present at 200 K. When a submonolayer coverage (40–50% saturation) of hydrogen is pre-adsorbed on the Pt/Ni(1 1 1) surfaces, the cyclohexene hydrogenation activity is enhanced by 10-fold, with a lower onset temperature of desorption of the cyclohexane product at ∼180 K.

Crystal structures and thermal properties of titanium carbo-deuterides as prepared by combustion synthesis

Titanium carbo-deuterides of various compositions have been prepared by thermal radiation and cold syntheses, and by self-propagating high-temperature synthesis. While the former methods favored the formation of a cubic structure of composition TiC0.48(1)D0.60(2) (Fm3̄m, a=4.30963(3) Å) the latter yielded a trigonal structure of composition TiC0.50(−)D0.707(4) (P3̄m1,a=3.08208(2), c=5.04052(6) Å). Differential thermal analysis and thermogravimetry data suggest the cubic structure to be thermally less stable (desorption onset temperature 823 K) than the trigonal structure (1093 K). Joint Rietveld structure refinements based on neutron and X-ray powder diffraction data show that carbon occupies octahedral interstices in the f.c.c. and h.c.p. metal atom networks of the cubic and trigonal phase, respectively, while deuterium occupies both octahedral and tetrahedral interstices in the cubic but tetrahedral interstices only in the trigonal phase. The different thermal stability of the compounds is attributed to these structural differences.

Hydrogen adsorption site on the Ni{111}-(2×2)–H: a TOF-LERS study

The adsorption site of hydrogen on the Ni{111}-(2×2)–H surface resulting from the H 2 exposure at low temperature was investigated by time-of-flight low energy recoil spectrometry (TOF-LERS). Recoiling spectra induced by pulsed 1 keV Ne + primary-ion beams were monitored as a function of polar and azimuth angles. From classical trajectory calculations and shadowing and blocking analyses, it was found that the H atoms occupy both the fcc- and hcp-adsorption sites on the Ni(111) terraces at a height of approximately 1.1–1.5 Å above the plane of the first layer of Ni atoms. Furthermore, adsorption and desorption measurements show that the initial sticking coefficient of a H atom is s( θ)≈0.45 and the activation energy of H atoms adsorbed on the surface is evaluated to be 0.42 eV. The onset temperature for desorption was found to be around 250 K.

Stable Ti-based quasicrystal offers prospect for improved hydrogen storage

The desorption of hydrogen from a novel material, a Ti45Zr38Ni17-H quasicrystal, was observed using high-temperature powder x-ray diffraction, demonstrating the potential utility of Ti-based quasicrystals in place of crystalline or amorphous hydrides for hydrogen storage applications. The maximum observed change in hydrogen concentration was from 61 at. %, corresponding to a hydrogen-to-metal ratio (H/M) of 1.54, at 91 °C to less than 2.5 at. % (H/M=0.025) at 620 °C. The onset temperature of desorption is below 350 °C. Surface oxidation was found to promote the formation of crystalline hydride phases. Highly oxidized samples transformed to a mixture of the C14 Laves and C15 Laves crystalline hydrides, and the Ti2Ni phase. When the oxidation was less severe, a reversible transformation between the quasicrystal and crystalline hydride phases was clearly observed, demonstrating the stability of the Ti45Zr38Ni17 quasicrystal at very low hydrogen concentrations, and temperatures as high as 661 °C. This is the first evidence for a stable Ti-based quasicrystal and for reversible hydrogen storage in a quasicrystalline phase.

Helium reemission, desorption and microstructure evolution of graphites under helium ion implantation

Helium reemission, trapping, and thermal desorption from highly ordered pyrolytic graphite (HOPG and HPG) of different orientation, polycrystalline graphite (EK98), and titanium doped graphite (RG-Ti-91) have been measured at irradiation temperatures of 300 K and 800 K. The implantation was performed with a 40 keV 4He ion beam. Detailed transmission electron microscopy (TEM) investigation of the microstructure evolution was made on the implanted specimens. He reemission from basal oriented (BO) highly oriented pyrolytic graphite is accompanied by blistering and flaking leading to repetitive gas bursts. On edge oriented (EO) pyrolytic graphite three reemission peaks are observed during room temperature implantation, the first and smallest one being assigned to He release from intrinsic lenticular cavities, the second one occurs during early bubble formation when a small amount of implanted gas still escapes accumulation, and the third and largest peak being due to He release by bubble coalescence. The He reemission rate grows very slowly at room temperature and does not reach 100% up to the highest implanted fluence of 3.5⋅1018 He/cm2. At 800 K the He reemission rate from EO pyrolytic graphite reaches 100% immediately after starting implantation due to the high diffusive mobility of He. EK98 and RG-Ti-91 show similar reemission behaviour. No gas bursts due to blistering are observed. The initial reemission rate at 300 K is higher than in EO pyrolytic graphite due to release of He via a network of intergranular channels. At 800 K reemission is rather similar to that from EO pyrolytic graphite. No thermal desorption of He from BO HOPG up to 1200 K is observed for implanted fluences ≤1016 He/cm2. At higher fluences the onset temperature of desorption decreases from 750 K at 2⋅1016 He/cm2 to 380 K at 1017 He/cm2 caused by thermal flaking due to pressure increase of He in submicroscopic cracks. In the other materials two desorption peaks are observed, the first one being related to He release from ‘‘solid solution,’’ while the second is attributed to gas escape from He filled bubbles. In contrast to the conclusions of Niwase et al. we find from the reemission kinetics and from selected area electron diffraction patterns (SADP) that graphite implanted at 300 K with He up to a damage of 10 dpa and more cannot be regarded as amorphous and shows a distorted turbostratic structure. The c parameter increases to (3.6..3.7)Å. Radiation effects in graphite implanted at 800 K are less pronounced up to damage levels of 200 dpa but may decrease the He diffusion coefficient.