Kinetics Of Mg Research Articles

Hydrogen storage and hydrolysis properties of core-shell structured Mg-MFx (M=V, Ni, La and Ce) nano-composites prepared by arc plasma method

In this work, core-shell structured Mg-MFx (M = V, Ni, La and Ce) nano-composites are prepared by using arc plasma method. The particle size distribution, phase components, microstructures, hydrogen sorption properties of these composites and hydrolysis properties of their corresponding hydrogenated powders are carefully investigated. It is shown that the addition of MFx through arc plasma method can improve both the hydrogen absorption kinetics of Mg and the hydrolysis properties of corresponding hydrogenated powders. Among them, the Mg-NiF2 composite shows the best hydrogen absorption properties at relatively low temperatures, which can absorb 3.26 wt% of H2 at 373 K in 2 h. Such rapid hydrogen absorption rate is mainly due to the formation of Mg2Ni and MgF2 on Mg particles during arc evaporation and condensation. In contrast, measurements also show that the hydrogenated Mg-VF3 composite has the lowest peak desorption temperature and the fastest hydrolysis rate among all the hydrogenated Mg-MFx composites. The less agglomeration tendency of Mg particles and VO2 covered on MgH2 particles account for the reduced hydrogen desorption temperature and enhanced hydrolysis rate.

Tuning Mg hydriding kinetics with nanocarbons

Mg-nanocarbon composites demonstrate fast reversible hydriding kinetics. The kinetics is strongly correlated with the structure of the added nanocarbon allotrope. We conducted a systematic exploration of the reversible hydriding kinetics of Mg upon addition of nanocarbons with different dimensionalities, namely, 1D carbon nanotubes, 2D graphene nanoplatelets and 3D activated carbon. Our findings showed that the Mg (de)hydriding rate could be tuned by the dimensionality and defect density of the nanocarbon. The resulting kinetic enhancement was explained in terms of the hydrogen spillover mechanism. The use of nanocarbons offers an alternative to the utilization of high concentrations of expensive transition-metal catalysts for hydrogen storage.

Kinetic Control of Hexagonal Mg(OH)2 Nanoflakes for Catalytic Application of Preferential CO Oxidation

Controlling the growth of nanocrystals is one of the most challenged issues in current catalytic field, which helps to further understand the size and morphology related behaviors for catalytic applications. In this work, we investigated the plane growth kinetics of Mg(OH)2 for catalytic application in preferential CO oxidation. Nanoflakes were synthesized through hydrothermal method. The morphology and structure of nanoflakes were characterized by TEM, SEM, and XRD. By varying the reaction temperature and time, Mg(OH)2 nanoflakes underwent an anisotropic growth. Benefited from the Ostwald ripening process, the thickness of nanoflake corresponding to the (110) plane of Mg(OH)2 was tuned from 7.6 nm to 24.0 nm, while the diameter of (001) plane increased from 18.2 nm to 30.2 nm. The grain growth kinetics for the thickness was well described in terms of an equation, D5 = 7.65 + 6.9 × 108exp(−28.14/RT). After depositing Pt nanoparticles onto these Mg(OH)2 nanoflakes, an excellent catalytic performance was achieved for preferential CO oxidation in H2‐rich streams with a wide temperature window from 140 °C to 240 °C for complete CO conversion due to the interaction between Pt and hydroxyl groups. The findings reported here would be helpful in discovering novel catalysts for application of proton exchange membrane fuel cells.

Tailoring the Thermodynamics and Kinetics of Mg–Li Alloy for a MgH2‐Based Anode for Lithium‐Ion Batteries

AbstractUnderstanding the lithiation–delithiation of the Mg–Li alloy during the absorption–desorption of hydrogen is essential for the development of Li‐ion batteries with MgH2 as a negative electrode. Tuning the hydrogenation–dehydrogenation kinetics and thermodynamics of the Mg–Li alloy could also be helpful to develop a lightweight material for on‐board hydrogen‐storage applications. Single‐phase Li3Mg7 (the highest % of lithium compound) was prepared by ball milling of LiH and MgH2 as precursors of Li and Mg followed by dehydrogenation at 400 °C under dynamic vacuum conditions. The cyclic hydrogenation–dehydrogenation behavior of the intermetallic was studied in detail. During hydrogenation, Li3Mg7 was delithiated to LiH and MgH2, whereas during dehydrogenation, LiH and MgH2 were lithiated to form the Li3Mg7 phase along with a Mg–Li solid solution. The hydrogenation–dehydrogenation kinetics of pristine Li3Mg7 were found to be slow. The hydrogenation–dehydrogenation kinetics were remarkably improved by doping with ZrCl4 as a catalyst. The activation energy and the thermodynamic parameters of the uncatalyzed and catalyzed alloy were evaluated, and the results were compared.

Enhanced hydriding kinetics of Mg-10 at% Al composite by forming Al 12 Mg 17 during hydriding combustion synthesis

Abstract Mg-based alloys are widely considered as promising materials for hydrogen storage. However, owing to the high chemical activity of Mg, the surface of Mg powder is always covered with a layer of close packed MgO, which can retard the hydrogenation of Mg. In this paper, 10 at% Al was added to accelerate the hydriding kinetics of Mg through tabletting and grinding treatments. After heating to 420 °C and subsequent holding at 340 °C for 120 min under 1.5 MPa hydrogen pressure, the Mg-10 at% Al composite could absorb 6.34 wt% H2 in the process of hydriding combustion synthesis (HCS) while the same treated pure Mg powder was barely hydrided. Upon heating from room temperature to 420 °C, Al alloyed with Mg to form an Al12Mg17 phase on the surface of Mg particles and the newly formed alloy was oxide-free, which would react with H2 when the temperature decreased to 340 °C. Then hydrogen was able to get into the inner part of the Mg particle from the former Al12Mg17 sites where the MgO shell had been broken in the alloying period to further react with unhydrided Mg.

A new method to produce macroporous Mg-phosphate bone growth substitutes

This paper is a sequel to our previous effort in developing Mg-phosphate orthopedic cements using amorphous Mg-phosphate (AMP) as the precursor. In this paper, we report a new real-time in situ technique to create macroporous bone growth substitute (BGS). The method uses biodegradable Mg-particles as the porogen. As opposed to the conventional wisdom of providing corrosion protection layers to biodegradable Mg-alloys, the present method uses the fast corrosion kinetics of Mg to create macropores in real time during the setting of the cement. An aqueous solution of PVA was used as the setting solution. Using this technique, a macroporous cement containing up to 91% porosity is obtained, as determined by pycnometry. Due to formation of H2 gas bubbles from corrosion of Mg, the cement becomes macroporous. The pore sizes as big as 760μm were observed. The results of SBF soaking indicated change in crystallinity as confirmed via scanning electron microscopy (SEM) and X-ray diffraction (XRD). Our in vitro cytocompatibility evaluation also revealed that the macroporous bone growth substitute composed of bobierrite is cytocompatible and can improve gene expression.

Synergic catalytic effect of Ti hydride and Nb nanoparticles for improving hydrogenation and dehydrogenation kinetics of Mg-based nanocomposite

The Mg-9.3wt% (TiH1.971-TiH)−0.7wt%Nb nanocomposite has been synthesized by hydrogen plasma-metal reaction (HPMR) approach to enhance the hydrogen sorption kinetics of Mg at moderate temperatures by providing nanosizing effect of increasing H “diffusion channels” and adding transition metallic catalysts. The Mg nanoparticles (NPs) were in hexagonal shape range from 50 to 350nm and the average size of the NPs was 177nm. The small spherical TiH1.971, TiH and Nb NPs of about 25nm uniformly decorated on the surface of the big Mg NPs. The Mg-TiH1.971-TiH-Nb nanocomposite could quickly absorb 5.6wt%H2 within 5min at 573K and 4.5wt%H2 within 5min at 523K, whereas the pure Mg prepared by HPMR could only absorb 4 and 1.5wt%H2 at the same temperatures. TiH1.971, TiH and Nb NPs transformed into TiH2 and NbH during hydrogenation and recovered after dehydrogenation process. The apparent activation energies of the nanocomposite for hydrogenation and dehydrogenation were 45.0 and 50.7kJmol−1, which are much smaller than those of pure Mg NPs, 123.8 and 127.7kJmol−1. The improved sorption kinetics of the Mg-based nanocomposite at moderate temperatures and the small activation energy can be interpreted by the nanostructure of Mg and the synergic catalytic effects of Ti hydrides and Nb NPs.

Hydriding kinetics of Mg[sbnd]TiH2 fine dispersions obtained by mechanosynthesis

Mg80Ti20 hydride systems were prepared via reactive ball milling following two different procedures: grinding Mg with Ti or TiH2, in both cases under H2 atmosphere. Such systems were subjected to several cycles of hydrogen uptake and release in volumetric Sieverts apparatus under different pressure and temperature conditions.A structural characterization from SEM images and XRD patterns reveals no essential microstructural differences between the samples obtained by different routes, although a growth in domain size with temperature is observed. Also, the slowing of the kinetics when the temperature goes down is more pronounced in the specimen fabricated by the second route.The influence of the transformed and untransformed phase fractions on the absorption kinetics can be well described using a double Hill function. Such complex function takes into account two distinguishable processes, with and without nucleation, that differentiate themselves as temperature increases. The first one may be related to surface absorption and the second tentatively ascribed to reactions at grain boundaries, dislocations and other extended defects.The fitted values may then be associated to physical constants of the processes occurring during hydrogen absorption in both regions.

Effect of transition metal on the hydrogen storage properties of Mg–Al alloy

Mg–Al alloys were prepared via sintering combined with ball milling, and the effect of a transition metal (TM = Ti, V, Ni) on the hydrogen storage properties of these alloys was investigated; the alloys were characterized via X-ray diffraction, pressure composition isotherms, and differential scanning calorimetry. The results showed that the alloys were mainly composed of Mg and the Mg17Al12 phase, and the cell volume of these phases decreased after the addition of TM (TM = Ti, V, Ni), which is attributed to the improved hydrogenation kinetics of Mg–Al alloy. Moreover, the hydrogenation/dehydrogenation temperature of the Mg–Al alloy decreased with the addition of TM (TM = Ti, V, Ni). Ti, Ni, and V acted as a catalyst, thereby lowering the reaction barrier for dehydrogenation and promoting the reversible hydrogenation reaction of the Mg–Al alloy. The onset temperature of dehydrogenation of the Mg–Al–V alloy was ~244 °C, which was 66 °C lower than that of the Mg–Al alloy (~310 °C). And the apparent activation energy of the Mg–Al–V alloy was 80.1 kJ mol−1, where it was 34.6 kJ mol−1 lower than that of Mg–Al alloy.

Transparent-to-Dark Electrochromic Behavior in Naphthalene-Diimide-Based Mesoporous MOF-74 Analogs

Summary The deliberate insertion of redox-active naphthalene diimide ligands into the versatile family of metal-organic frameworks known as MOF-74 (CPO-27) gives rise to a mesoporous electrochromic MOF that can be switched from transparent to dark, a desirable feature in electrochromic devices such as smart windows. Specifically, we report two new materials that have the MOF-74 topology and display redox activity stemming from a naphthalene diimide salicylic acid (NDISA) ligand. We show that these materials can be deposited as thin films on fluorine-doped tin oxide glass. The surprisingly different morphologies of MOF films obtained from Mg 2+ and Ni 2+ ions is likely controlled by the nucleation kinetics of Mg 2 (NDISA) and Ni 2 (NDISA), respectively. Both materials exhibit well-behaved quasi-reversible redox events associated with the [NDI]/[NDI] ·– and [NDI] ·– /[NDI] 2– redox couples, which are also responsible for the electrochromic switching.

Magnesium/Sulfur Rechargeable Batteries Using Liquid Mg Polysulfides

Rechargeable magnesium (Mg) batteries are a sustainable option for electrochemical energy storage of renewable energy due to the inherent merits associated with Mg such as natural abundance, operational safety and high theoretical volumetric capacity (3,832 mAh cm-3), with a reduction potential of − 2.356 V versus normal hydrogen electrode (NHE).1 Among different types of cathode materials, the combination of Mg with a sulfur cathode and a non-nucleophilic electrolyte is very promising due to its large theoretical energy density of about 3200 Wh l−1, based on a two-electron conversion reaction expressed as, Mg2+ + S + 2e − ↔ MgS, which is beyond that of a Li/S battery (2800 Wh l-1). In addition, these battery systems are reversible and using low cost materials. Compared to the progress made on Li/S batteries, the Mg/S battery is still in a very early stage of research and development due to number of reasons such as low kinetics of Mg, lack of suitable electrolytes, polysulfide dissolution, insulating nature of sulfur, capacity fading etc. Herein, we demonstrate a reversible Mg dissolved polysulfide cell using reduced graphene oxide (rGO) as the cathode host matrix and a non-nucleophilic Mg electrolyte in tetraglyme solution as the electrolyte. The electrolyte was prepared by mixing a non-nucleophilic base of magnesium-bis(hexamethyldisilazide) [(HMDS)2Mg] with the Lewis acid, AlCl3 (1:2 molar ratio) and dissolving the reaction product in a tetraglyme (C10H22O5) ethereal solvent . The liquid Mg polysulfides cell delivers better reversible capacity (~750 mAh g-1, Figure. 1) and cyclability as compared to previous Mg/S cell reports. The high surface area, porous structure and presence of oxygen functional groups over rGO give simultaneous physical and chemical adsorption of Mg polysulfides with the host matrix. Further, the working mechanism of liquid Mg polysulfides within Mg/S cell is investigated by Raman spectroscopy.

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.

Effect of trace Na additions on the hydrogen absorption kinetics of Mg2Ni

Abstract

Ion Exchange Kinetics of Mg(II) from Aqueous Solutions with 732 Cation-exchange Resin

Ion Exchange Kinetics of Mg(II) from Aqueous Solutions with 732 Cation-exchange Resin

Synthesis and hydrogen absorption/desorption properties of Mg–Nb2O5-SWCNT/MWCNT nanocomposite prepared by reactive milling

Mg–Nb2O5-SWCNT and Mg–Nb2O5-MWCNT composites were prepared by ball-milling in a hydrogen reactive atmosphere (P = 3.0 MPa) for different periods of time, and their sorption behaviors and microstructure were investigated systematically. The results showed that the hydriding and dehydriding kinetics of Mg–Nb2O5-SWCNT and Mg–Nb2O5-MWCNT 1 h milled samples were markedly improved. At 573 K, these two composites could absorb 5.81–4.88 wt% hydrogen under initial hydrogen pressure of 3.0 MPa, and desorb 5.14–4.01 wt% hydrogen within 600 s. Furthermore, these 1 h milled composites could also absorb hydrogen in a moderate temperature range between 298 K and 573 K. However, the Mg–Nb2O5-SWCNT 1 h milled sample had better hydriding/dehydriding kinetics than the Mg–Nb2O5-MWCNT 1 h milled sample. The apparent activation energies for the hydrogen absorption and desorption of the Mg–Nb2O5-SWCNT 1 h milled sample were estimated to be 12.84 kJ/mol and 102.69 kJ/mol from the Kissinger's plot. These figures were lower than those of a pure Mg sample with the same milling time, which were 3.14 and 2.0 times higher, respectively. The desorption temperature and the dehydrogenation enthalpy of the Mg–Nb2O5-SWCNT 1 h milled sample were 140 K and 20.72 kJ/mol, respectively, which were lower than those of pure Mg/MgH2.

Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen Storage Properties of Mg-Based Nanocomposite

With the intention of improving the hydrogenation/dehydrogenation kinetics of Mg, Mg-7.5 wt % Nb nanocomposite has been prepared by hydrogen plasma-metal reaction approach. The spherical Nb nanoparticles (NPs) of 12 nm are uniformly decorated on the surface of Mg NPs. The Mg–Nb nanocomposite can quickly uptake 4.0 wt % H2 in 10 min and reach a saturation value of 5.7 wt % H2 in 60 min at 473 K. Furthermore, it can also release 4.0 wt % H2 in 60 min at 573 K. The reversible hydrogen storage capacity is as high as 7.0 wt % at 673 K. Nb NPs transform into NbH during hydrogenation and recover after dehydrogenation process. They restrain the growth of Mg and work as catalysts to accelerate the hydrogen transportation in the Mg-based nanocomposite by decreasing the activation energies of hydrogenation/dehydrogenation to 70.9 and 86.4 kJmol–1, respectively. The catalytic mechanism of Nb NPs is explained in terms of spillover, d-electrons and electronegativity effects. The nanosizing effects of both Mg and Nb and...

Improved hydrogen storage properties of Ti-doped Mg95Ni5 powder produced by hydriding combustion synthesis

Abstract

In-situ synchrotron X-ray diffraction investigation of the hydriding and dehydriding properties of a cast Mg–Ni alloy

In previous research, it has been demonstrated that the hydrogenation kinetics of Mg–Ni alloys is significantly improved by elemental Na doping. To clarify the underlying mechanisms in terms of crystallographic phase changes during the reactions, in-situ hydrogen absorption and desorption as a function of temperature under constant hydrogen atmospheres of 2MPa and 0.2MPa, respectively, was studied. The experiments were performed at the Powder Diffraction beamline of the Australian Synchrotron facility with a gas flow cell and a hot air blower. This study showed that the hydrogenation of Na-doped Mg–Ni alloys can be identified at a temperature as low as 260°C via an interface-controlled nucleation and growth mechanism without any prior activation whilst the dehydrogenation occurred at about 370°C. The sequence of phase transformations associated with these reactions as well as associated expansion properties of individual phases during hydrogen sorption reactions as provided by the high resolution X-ray diffraction data are discussed.

Microstructure characteristics and hydrolysis mechanism of Mg–Ca alloy hydrides for hydrogen generation

The microstructure characteristics and hydrolysis mechanism of Mg–Ca alloy hydrides were investigated in this paper. It was found that x wt.% Ca–Mg alloys (x = 10, 20 and 30) hydrogenated after ball-milling are mainly composed of MgH2 and Ca4Mg3H14 phases, and they show much better hydrolysis properties than pure MgH2 at the temperature range of 25–70 °C. The superior performances of Mg–Ca alloy hydrides may be attributed to an easy hydrolysis nature of Ca4Mg3H14 and a high solubility of the by-product Ca(OH)2, which provides gates to make water penetrate deeply inside the particles. Moreover, increasing the content of Ca enhances the hydrolysis performances of Mg–Ca alloy hydrides further due to the formation of more Ca4Mg3H14 and larger specific surface areas as well. The best comprehensive hydrolysis performances are obtained by 30 wt.% Ca–Mg alloy hydride, which releases 1419.8 mL g−1 hydrogen within 1 h at 70 °C and its conversion yield is about 95%. The hydrogen generation kinetics of Mg–Ca alloy hydrides was also investigated and it showed that the activation energies decrease with the increase of the Ca content, which is in accordance with their hydrolysis properties.

In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc.

The in vitro degradation of magnesium (Mg) alloys containing low levels of strontium (Sr, 0.05, 0.1 and 0.2 wt%), with and without addition of zinc (Zn, 0.5 and 1.0 wt%), was studied for potential use in orthopaedics for fracture treatment. Alloying Mg with Sr was selected as a promising strategy to utilise the biological effect of Sr in inducing accelerated bone tissue growth. The influence of controlled alloying upon degradation rate was studied via electrochemical measurements and immersion tests in minimum essential medium (MEM). Immersion testing revealed a comparable degradation rate of the alloys tested herein, indicating no detrimental effect of Sr on degradation. Cytotoxicity experiments on primary mouse osteoblasts indicated good biocompatibility and enhanced proliferation of osteoblasts for all the tested Mg alloys. Potentiodynamic polarisation testing further confirmed that addition of low-levels of Sr had a minor influence on cathodic kinetics, with a slight inhibition of anodic kinetics. In contrast, the addition of Zn as a ternary element moderated both anodic and cathodic kinetics of Mg-Sr alloys.