Nano-confined magnesium for hydrogen storage from reactive milling with anthracite carbon as milling aid

The structure and properties of magnesium nanoparticles for hydrogen storage from reactive milling under hydrogen atmosphere with the carbon from anthracite coal carbonization as milling aid were investigated. Experiment showed that after 3 h of milling under 1 MPa of hydrogen with 30 wt.% of carbon additive, the magnesium particles were milled to 20–60 nm and hydrided into β-MgH₂ with a crystallite size of 29.7 nm. For the hydrogen desorption of the material, the onset temperature was determined to be 270 °C. In 270–390 °C, the enthalpy and entropy changes were calculated to be 44.5 kJ/mol and 83.8 J/(mol K), respectively, and the activation energy as pseudo first-order reaction was 127.1 kJ/mol. The carbon still played a role of nano-confinement for magnesium to prevent particles from coalescing in the process of repeatedly heating for hydrogen storage.     

Oxygen vacancy in magnesium/cerium composite from ball milling for hydrogen storage improvement

The hydrogen sorption performance of Mg is constrained by the difficulties of hydrogen dissociation on particle surface and mass transfer in particle bulk. This work focuses on oxygen vacancy and its effect on the performance of Mg-xCeO₂ (x = 0.7, 1.5, 3, and 6 mol.%) from ball milling for hydrogen storage. The HRTEM observation shows that the crystal domains of Mg from ball milling are reduced to nanoscale by the addition of hard CeO₂ nanoparticles. The XRD and XPS characterization shows that during heating for hydrogenation, some O atoms in CeO₂ transfer to Mg and form MgO, and CeO₂ converts to Ce₆O₁₁ with oxygen vacancies. The isothermal absorption (p-c-T) analysis shows that the hydrogen capacity of the materials increases with the increase of CeO₂ additive, and the optimum addition is 3.0 mol.%. The DSC analysis shows that with the addition of 3.0 mol.% of CeO₂, the hydrogen desorption peak temperature is 35 °C lower than that of pure MgH₂, and the calculated activation energy deceases by 31.3 kJ/mol. The improvement of hydrogen sorption performance is mainly attributed to the formation of oxygen vacancies.     

Iron and niobium based additives in magnesium hydride: Microstructure and hydrogen storage properties

In this study, powder mixtures of MgH₂ + 2 mol.% X, with X = Nb, Nb₂O₅, NbF₅, Fe, Fe₂O₃, FeF₃, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF₃) in the MgH₂ matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH₂ sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH₂, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH₂ particle size. DSC analysis revealed that fluorides (NbF₅, FeF₃) are much more effective than oxides (Nb₂O₅, Fe₂O₃) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb₂O₅ is more efficient than Fe₂O₃. Finally, the best results for desorption kinetics were observed for the fluorides: NbF₅ and FeF₃ (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb₂O₅, Fe₂O₃ and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample.     

Characterization of Mg–20 wt% Ni–Y hydrogen storage composite prepared by reactive mechanical alloying

Mg–20 wt% Ni–Y composite was successfully prepared by reactive mechanical alloying (RMA). X-ray diffraction (XRD) measurement showed that both MgH₂ and Mg₂NiH₄ co-exist in the milled composite. The composite exhibits excellent hydrogen sorption kinetics and does not need activation on the first hydrogen storage process. It can absorb 3.92 and 5.59 wt% hydrogen under 3.0 MPa hydrogen pressure at 293 and 473 K in 10 min, respectively, and desorb 4.67wt% hydrogen at 523 K in 30 min under 0.02 MPa hydrogen pressure. The equilibrium desorption pressure of the composite are 0.142, 0.051 and 0.025 MPa at 573, 543 and 523 K, respectively. The differential scanning calorimetry (DSC) measurement showed that dehydrogenation of Mg–20 wt% Ni–Y composite was depressed about 100 K comparing to that of milled pure MgH₂. It is deduced that both the catalysis effect of Mg₂Ni and YH₃ distributed in Mg substrate and the crystal defects formed by RMA are the main reason for improving hydrogen sorption kinetics of the Mg–20 wt% Ni–Y composite.     

Alternative method for generating hydrogen through high-energy mechanical milling using magnesium and methanol

This paper presents a new way to generate hydrogen through mechanical milling. Hydrogen was generated “in-situ” inside a stainless steel container where methanol and magnesium were utilized. The methanol atoms due to the high-impact process were broken to obtain hydrogen. In this same process, magnesium was selectively reacted with oxygen to form the corresponding oxide and hydrogen remain in gas form. Relatively short milling times were programmed. After the programmed times, hydrogen from the stainless steel container through liquid displacement was measured. Likewise, solids were analyzed before and after hydrogen production using XRD and SEM techniques. The volume of hydrogen was a function of the programmed milling time; the average values between 160 and 310 mL were obtained when the most appropriate amounts of magnesium and methanol were used. Only magnesium oxide was formed as by-product. XRD analyses demonstrated the transformation of magnesium and methanol to generate hydrogen.     

MgH2-based nanocomposites prepared by short-time high energy ball milling followed by cold rolling: A new processing route

A new processing route consisting of a short-time high energy ball milling (HEBM) step followed by cold rolling (CR) to produce MgH₂-based nanocomposites was investigated. Samples of pure MgH₂ and MgH₂-mixtures containing 2 mol% of FeF₃ were processed under air atmosphere and their effects on the microstructure and the hydrogen storage properties were studied in detail. X-ray Diffraction (XRD) analysis on all samples revealed crystallite sizes in the nanometer range for the beta-MgH₂ and FeF₃ phases (beta (beta): ∼8–13 nm and FeF₃: ∼16–21 nm). No extra crystallite size reduction was observed for the HEBM + CR samples in comparison with those only cold rolled. Scanning electron microscopy (SEM) evaluation together with elemental composition analysis indicated a finer size distribution of additive particles and also a more intimate level of mixing for the HEBM + CR mixture than for the mixture obtained only by CR. These features were associated with the HEBM step applied before the CR. These characteristics led to a lower hydrogen desorption temperatures and enhanced desorption kinetics behavior for the HEBM + CR samples in comparison with the CR samples.     

Effect of Ni/tubular g-C3N4 on hydrogen storage properties of MgH2

Additive doping is one of the effective methods to overcome the shortcomings of MgH₂ on the aspect of relatively high operating temperatures and slow desorption kinetics. In this paper, hollow g-C₃N₄ (TCN) tubes with a diameter of 2 μm are synthesized through the hydrothermal and high-temperature pyrolysis methods, and then nickel is chemically reduced onto TCN to form Ni/TCN composite at 278 K. Ni/TCN is then introduced into the MgH₂/Mg system by means of hydriding combustion and ball milling. The MgH₂–Ni/TCN composite starts to release hydrogen at 535 K, which is 116 K lower than the as-milled MgH₂ (651 K). The MgH₂–Ni/TCN composite absorbs 5.24 wt% H₂ within 3500 s at 423 K, and takes up 3.56 wt% H₂ within 3500 s, even at a temperature as low as 373 K. The apparent activation energy (E_a) of the MgH₂ decreases from 161.1 to 82.6 kJ/mol by the addition of Ni/TCN. Moreover, the MgH₂–Ni/TCN sample shows excellent cycle stability, with a dehydrogenation capacity retention rate of 98.0% after 10 cycles. The carbon material enhances sorption kinetics by dispersing and stabilizating MgH₂. Otherwise, the phase transformation between Mg₂NiH₄ and Mg₂NiH_0.3 accelerates the re/dehydrogenation reaction of the composite.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

Effective synthesis of Mg2CoH5 by reactive mechanical milling and its hydrogen sorption behavior after cycling

Mg₂CoH₅ was synthesized by reactive mechanical milling (RMM) under hydrogen atmosphere (0.5 MPa) from 2MgH₂–Co and 3MgH₂–Co mixtures, with a yield >80%. The microstructure, structure and thermal behavior of the phases formed during the processing were investigated by transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Kinetic properties of the reaction with hydrogen of the 2MgH₂–Co and 3MgH₂–Co mixtures after RMM were evaluated using modified Sieverts-type equipment. The 3MgH₂–Co mixture showed better properties for storage applications, with its highest rate of hydrogen absorption and desorption at 300 °C, its storage capacity of about 3.7 wt% in less than 100 s, and good stability after cycling. Although the starting material presents Mg₂CoH₅ as majority phase, the cycling leads to disproportion between Mg and Co. We obtained a mixture of Mg₂CoH₅, Mg₆Co₂H₁₁ and MgH₂ hydrides, as well as other phases such as Co and/or Mg, depending on experimental conditions.     

Enhanced hydrogen desorption kinetics and cycle durability of amorphous TiMgVNi3-doped MgH2

MgH₂ is considered as a promising hydrogen storage material for on-board applications. In order to improve hydrogen storage properties of MgH₂, the amorphous TiMgVNi₃-doped MgH₂ is prepared by ball milling under hydrogen atmosphere. It is found that the catalytic (Ti,V)H₂ and Mg₂NiH₄ nanoparticles are in situ formed after activation. As a result, the amorphous TiMgVNi₃-doped MgH₂ exhibits enhanced dehydrogenation kinetics (the activation energy for hydrogen desorption is 94.4 kJ mol^?1 H₂) and superior cycle durability (the capacity retention rate is up to 92% after 50 cycles). These results demonstrate that the in situ formation of highly dispersed catalytic nanoparticles from an amorphous phase is an effective pathway to enhance hydrogen storage properties of MgH₂.     

Enhanced hydrogen storage kinetics in Mg@FLG composite synthesized by plasma assisted milling

Mg-based materials as potential hydrogen storage candidates, however, are suffering from sluggish kinetics during absorption and desorption processes. Here in this work, embedding Mg particles on few-layer graphene nanosheets (FLG) via dielectric barrier discharge plasma (DBDP) assisted milling was synthesized to improve hydrogen storage properties of Mg particles. The SEM observation demonstrates that Mg particles are distributed uniformly on the surface of the graphite layer in the Mg@FLG composite. The obtained Mg-based composite (Mg@FLG) shows a hydrogen storage capacity of ～5 wt%. From the isothermal dehydrogenation kinetic curves, the composite could desorb ～4.5 wt% hydrogen within 25 min at 300 °C. Compared with pure Mg, the dehydriding kinetics of the hydrogenated Mg@FLG composite is significantly elevated, showing an activation energy of 155 J/(mol·K). In addition, the dehydrogenation peak temperature of the Mg@FLG decreases dramatically from 431 to 329 °C for MgH₂. This work implies a promising composite formation technique in Mg-based materials to enhance hydrogen storage kinetics.     

Thermal performance of stearic acid/carbon nanotube composite phase change materials for energy storage prepared by ball milling

In this work, stearic acid/carbon nanotubes composite phase change materials (SA/CNTs composite PCMs) were fabricated by ball milling for the first time to enhance the heat conduction of SA and prevent the delamination of SA and CNTs components. The results of suspension stability study conducted using a gravity sedimentation method showed that polyvinylpyrrolidone (PVP) used as dispersant has the best effect on the stability of composite PCMs. Then, the thermal cycling test further proved the stability of prepared composite. The SEM and FT‐IR results revealed that ball milling led to the formation of highly homogeneous composites. The thermal properties of the fabricated SA/CNTs composites with CNTs contents of 2, 6, and 10 wt.% characterized by differential scanning calorimetry (DSC) demonstrated that their phase change temperatures varied slightly while the latent heat decreased with the increased CNTs content. Furthermore, the thermal conductivity of the SA/CNTs composites were greater than that of pure SA by 61.5%, 92.3%, and 119.2%, respectively. The addition of CNTs also increased the thermal release rates of the prepared PCMs and decreased their storage rates. Therefore, the produced materials can be potentially used in thermal management.     

Metal hydride synthesis through reactive milling of metals with solid acids in a planetary ball mill

In this paper we report on a new type of synthesis of calcium hydride through reactive milling under an argon atmosphere. Therefore we have used two different protic acids (phosphoric acid and phenylphosphonic acid) which are solid at room temperature and have milled them together with calcium. For the synthesis calcium had two functions: It shall work as reduction agent and as reaction partner for the hydride formation. The received product has been characterised by XRD measurements. Using phenylphosphonic acid and calcium as reactants, calcium hydride could be synthesised as the only crystalline reaction product with a crystallite size of 20 nm. A similar reaction using magnesium could not be realised.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling

This work deals with the assessment of the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. Samples are prepared from Mg_xFe (x: 2, 3 and 15) elemental powder mixtures via low energy ball milling under hydrogen atmosphere at room temperature. The highest yield is obtained with Mg₁₅Fe after 150 h of milling (90 wt% of MgH₂). The thermodynamic characterization carried out between 523 and 673 K shows that the obtained Mg–Fe–H hydride systems have similar thermodynamic parameters, i.e. enthalpy and entropy. However, in equilibrium conditions, Mg₁₅Fe has higher hydrogen capacity and small hysteresis. In dynamic conditions, Mg₁₅Fe also shows better hydrogen capacity (4.85 wt% at 623 K absorbed in less than 10 min and after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability in comparison to the other studied compositions. From hydrogen uptake rate measurements performed at 573 and 623 K, the rate-limiting step of the hydrogen uptake reaction is determined by fitting particle kinetic models. According to our results, the hydrogen uptake is diffusion controlled, and this mechanism does not change with the Mg–Fe proportion and temperature.     

New horizon in mechanochemistry—high-temperature,high-pressure mechanical synthesis in a planetary ball mill—with magnesium hydride synthesis as an example

A new route of materials synthesis, namely, high-temperature, high-pressure reactive planetary ball milling (HTPRM), is presented. HTPRM allows for the mechanosynthesis of materials at fully controlled temperatures of up to 450 °C and pressures of up to 100 bar of hydrogen. As an example of this application, a successful synthesis of magnesium hydride is presented. The synthesis was performed at controlled temperatures (room temperature (RT), 100, 150, 200, 250, 300, and 325 °C) while milling in a planetary ball mill under hydrogen pressure ($>$50 bar). Very mild milling conditions (250 rpm) were applied for a total milling time of 2 h, and a milling vial with a relatively small diameter (φ = 53 mm, V = ～0.06 dm³) was used. The effect of different temperatures on the synthesis kinetics and outcome were examined. The particle morphology, phase composition, reaction yield, and particle size were measured and analysed by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) techniques. The obtained results showed that increasing the temperature of the process significantly improved the reaction rate, which suggested the great potential of this technique for the mechanochemical synthesis of materials.     

High-value utilization of intermetallic Cu9Al4 as an additive to improve the hydrogen storage performance of magnesium

As a product of in the preparation of Cu–Al clad composites, intermetallic Cu₉Al₄ greatly affects the mechanical properties of the composites due to its high hardness and poor plasticity, which limits its application. Here, Cu₉Al₄ is used as an additive to improve the hydrogen storage performance of Mg. Thereby, the Cu₉Al₄ was introduced into Mg to prepare the Mg‒x wt.% Cu₉Al₄ composites (x = 0, 5, 10, 15, 20 and 25) by high-energy ball milling. The microstructure and phase composition of the composites under different states were analyzed. The hydrogen absorption and desorption kinetics and thermodynamics of the composites at different temperatures are investigated. Compared with pure Mg, the addition of Cu₉Al₄ can significantly improve the dehydrogenation kinetics of Mg. With the increase of Cu₉Al₄ content, the hydrogen desorption rate gradually increased. When the Cu₉Al₄ content was 20 wt.%, the dehydrogenation activation energy calculated according to JMAK kinetic model and Arrhenius equation was the lowest (96.84 kJ mol⁻¹), and it had the best hydrogen desorption kinetics. Especially, the phase transition of the first hydrogen absorption process for the Mg‒20 wt.% Cu₉Al₄ was studied in detail. The results show that the Cu₉Al₄ phase first transforms into (Cu_1.3Al_0.7)Mg during the first hydrogenation, and then Mg reacts with H₂ to generate MgH₂. In the subsequent dehydrogenation and re-hydrogenation cycles, the (Cu_1.3Al_0.7)Mg is stable and does not change.     

Isothermal activation,thermodynamic and hysteresis of MgH2 hydrides catalytically modified by high-energy ball milling with MWCNTs and TiF3

MgH₂-M (M = MWCNTs or TiF₃ or both of them) composites prepared by high-energy ball milling (HEBM) are used in this work to illustrate the effect of catalysts on isothermal activation, thermodynamic and hysteresis of MgH₂ hydrides. The phase compositions, microstructures, particle morphologies and distributions of MgH₂ with catalysts have been evaluated. The isothermal synergetic catalytic-activation and dehydrogenation effect of MWCNTs and TiF₃ evaluated by P-C-T give the evidences that the addition of catalysts is an effective strategy to destabilize MgH₂ and reduce the dehydrogenation temperatures. The isothermal activation process can be remarkly accelerated by adding MWCNTs or TiF₃. It's worthnoting that fast initial absorption rate and high hydrogenation capacity are obtained for modified MgH₂ when adding MWCNTs coupling with TiF₃ and no special activation treatment is needed. The activation effect is mainly attributed to the large contraction/expansion stresses caused by accelerated catalytic desorption/absorption cycles. The catalytic effect on thermodynamic is mainly attributed to electronic exchange reactions with hydrogen molecules during the dissociation-absorption or recombination-desorption process. The thermodynamic hysteresis of catalyzed MgH₂ is also investigated to evaluate the energy consumption and estimate the efficiency of the isothermal hydrogen absorption/desorption process. A probable synergetic catalytic-activation mechanism is probed.     

Hydrogen storage behaviors of magnesium hydride catalyzed by transition metal carbides

In this study, some transitional metal carbides (Ti₃C₂, Ni₃C, Mo₂C, Cr₃C₂ and NbC) were prepared to enhance the hydrogen storage behaviors of magnesium-based materials. The carbides with a weight ratio of 5 wt% were introduced into magnesium hydride (MgH₂) by mechanical ball milling, and the microstructure, phase composition and hydrogen storage properties of the composites were studied in detail. The phase compositions of Ni₃C, Mo₂C, Cr₃C₂ and NbC in the ball-milled composites have not changed during hydrogen absorption and desorption cycles. However, Ti₃C₂ decompose into multivalent Ti during hydrogenation process. All of these metal carbides can enhance the hydrogen absorption and desorption kinetics of MgH₂. Among them, Ti₃C₂ shows the best catalytic effect on dehydrogenation kinetic properties of MgH₂, followed by the Ni₃C, NbC, Mo₂C and Cr₃C₂.     

Phase-structural and morphological features,dehydrogenation/re-hydrogenation performance and hydrolysis of nanocomposites prepared by ball milling of MgH2 with germanium

Ball milling of magnesium hydride with germanium additives in argon for up to 10 h was shown to result in the formation of nanocomposites of α- and γ-modifications of MgH₂ together with individual Ge phase. When the as milled samples are heated-up, Ge interacts with MgH₂ to form Mg₂Ge at T = 350–400 °C; the phase transformation overlaps with MgH₂ decomposition and Mg₂Ge phase remains in the material after its re-hydrogenation at T = 300 °C and P(H₂)∼15 bar.The ball milled Ge-MgH₂ nanocomposites were found to be characterised by an improved hydrogen release via hydrolysis in organic acid solutions as compared to pure MgH₂ prepared and tested at the same conditions. It was found that the composite containing 5 wt% of Ge and ball milled for 5 h showed the best hydrogen generation performance, with total hydrogen release exceeding 1.55 NL/g (yield close to 100%) in one minute.