Mechano-chemical Activation Synthesis Research Articles

High resolution transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction studies of nanocrystalline manganese borohydride (Mn(BH4)2) after mechano-chemical synthesis and thermal dehydrogenation

Abstract In order to synthesize manganese borohydride, Mn(BH4)2, mechano-chemical activation synthesis (MCAS) of the (2LiBH4 + MnCl2) powder mixture was carried out by ball milling in a magneto ball mill. Both X-ray diffraction and TEM selected area electron diffraction patterns (SAEDPs) clearly confirm the presence of the Mn(BH4)2 and LiCl phases in the synthesized nanocomposite. No other phases were detected. Bright field high-resolution TEM imaging of the synthesized composite powder particles reveals the presence of nanograins consistent with LiCl and Mn(BH4)2 within the powder particles. Their respective grain sizes, estimated as the equivalent circle diameters (ECD) from the high-resolution TEM micrographs with the corrected sample standard deviations, are within the range of 14.1 ± 3.7 nm and 10.0 ± 2.9 nm for LiCl and Mn(BH4)2, respectively. The XRD patterns of the thermally dehydrogenated (Mn(BH4)2 + 2LiCl) nanocomposite do not exhibit any Bragg diffraction peaks belonging to either crystalline Mn or B. In contrast, the SAED patterns and EDS elemental maps provide strong evidence that both Mn and B exist in the dehydrogenated powder as crystalline phases α-Mn and β-B, respectively. The results show that the lack of XRD Bragg diffraction peaks is insufficient evidence that the Mn and B elemental products of Mn(BH4)2 thermolysis can be classified as being amorphous.

Mechanical and Thermal Dehydrogenation of the Mechano-Chemically Synthesized Calcium Alanate (Ca(AlH4)2) and Lithium Chloride (LiCl) Composite

LiAlH4 and CaCl2 were employed for mechano-chemical activation synthesis (MCAS) of Ca(AlH4)2 and LiCl hydride composite. After short ball milling time, their X-ray diffraction (XRD) peaks are clearly observed. After ball milling for a longer duration than 0.5 h, the CaAlH5 diffraction peaks are observed which indicates that Ca(AlH4)2 starts decomposing during ball milling into CaAlH5+Al+1.5H2. It is estimated that less than 1 wt % H2 was mechanically dehydrogenated in association with decomposition reaction. After 2.5 h of ball milling, no Ca(AlH4)2 diffraction peaks were observed on XRD patterns which suggests that Ca(AlH4)2 was decomposed. Thermal behavior of ball milled powders, which was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), indicates that a certain fraction of Ca(AlH4)2 could have been disordered/amorphized during ball milling being undetectable by XRD. The apparent activation energy for the decomposition of Ca(AlH4)2 and CaAlH5 equals 135 kJ/mol and 183 kJ/mol, respectively.

Rapid, ambient temperature hydrogen generation from the solid state Li–B–Fe–H system by mechano-chemical activation synthesis

A novel processing method promoting a rapid hydrogen generation from the solid state Li–B–Fe–H system is presented. The Li–B–Fe–H system is quite remarkable since the mechanical dehydrogenation rate at the ambient temperature is much higher than the thermal dehydrogenation rate at the 100–250 °C range. The quantity of 4.02 wt.% H2 is desorbed after barely 0.5 h of ball milling with a very low energy input of 36.4 kJ/g while at 250 °C, after the same desorption time, only 3.2 wt.% H2 is desorbed. Thus this system is a rapid, solid state H2 generator at the ambient temperature. X-ray diffraction after ball milling shows the presence of the crystalline LiCl diffraction peaks. The Fourier transform infrared spectroscopy (FT-IR) measurements show a spectrum characteristic of the disordered Fe(BH4)2 hydride. Samples milled under varying energy inputs are attracted to a permanent magnet. It is shown that the thermal dehydrogenation behavior of the pre-milled samples is initially fast but then quickly slows down. The pre-milled samples after thermal dehydrogenation are also attracted to a permanent magnet. Diffraction peaks of α-Fe are clearly observed after thermal desorption of powders at 100 and 250 °C. For the sample dehydrogenated at 250 °C the crystallite size of the α-Fe phase, estimated from its diffraction peak breadths, is within a nanometric range of 27–47 nm.

Nanostructured, complex hydride systems for hydrogen generation

Complex hydride systems for hydrogen (H2) generation for supplying fuel cells are being reviewed. In the first group, the hydride systems that are capable of generating H2 through a mechanical dehydrogenation phenomenon at the ambient temperature are discussed. There are few quite diverse systems in this group such as lithium alanate (LiAlH4) with the following additives: nanoiron (n-Fe), lithium amide (LiNH2) (a hydride/hydride system) and manganese chloride MnCl2 (a hydride/halide system). Another hydride/hydride system consists of lithium amide (LiNH2) and magnesium hydride (MgH2), and finally, there is a LiBH4-FeCl2 (hydride/halide) system. These hydride systems are capable of releasing from ~4 to 7 wt.% H2 at the ambient temperature during a reasonably short duration of ball milling. The second group encompasses systems that generate H2 at slightly elevated temperature (up to 100 °C). In this group lithium alanate (LiAlH4) ball milled with the nano-Fe and nano-TiN/TiC/ZrC additives is a prominent system that can relatively quickly generate up to 7 wt.% H2 at 100 °C. The other hydride is manganese borohydride (Mn(BH4)2) obtained by mechano-chemical activation synthesis (MCAS). In a ball milled (2LiBH4 + MnCl2) nanocomposite, Mn(BH4)2 co-existing with LiCl can desorb ~4.5 wt.% H2 at 100 °C within a reasonable duration of dehydrogenation. Practical application aspects of hydride systems for H2 generation/storage are also briefly discussed.

The effect of milling energy input during mechano-chemical activation synthesis (MCAS) of the nanocrystalline manganese borohydride (Mn(BH4)2) on its thermal dehydrogenation properties

Abstract A manganese borohydride, Mn(BH4)2, co-existing with a nanocrystalline LiCl salt, which is a reaction “dead-weight” byproduct, was successfully synthesized by the mechano-chemical activation synthesis (MCAS) during ball milling the (nLiBH4 + MnCl2) mixtures having the molar ratios n = 2 and 3, using the total milling energy input, QTR, from 36.4 to 364 kJ/g. The crystallite (grain) size of the synthesized nanocrystalline Mn(BH4)2 hydride attains 21 ± 5.0 nm for the energy input QTR = 36.4 kJ/g and then it is further reduced to 18 ± 1.0 nm for QTR = 145.6 kJ/g and finally to 14 ± 0.5 nm for QTR = 364 kJ/g. The crystallite (grain) size of LiCl is very close to 30 nm regardless of the milling energy input, QTR. During continuous heating in a Differential Scanning Calorimeter (DSC), Mn(BH4)2 decomposes in endothermic reaction releasing H2 and forming amorphous Mn and B in the process. The synthesized nanocrystalline Mn(BH4)2 hydride, co-existing with a nanocrystalline LiCl salt, is capable of desorbing up to ∼ 4.5 wt.% at 100 °C. The values of the apparent activation energy for dehydrogenation obtained in the present work are very low. The apparent activation energy for the n = 3 nanocomposite decreases monotonically from ∼70 to ∼59 kJ/mol with increasing milling energy input whereas the apparent activation energy for the n = 2 nanocomposite decreases from about 65 kJ/mol for QTR = 36.4 kJ/g to about 53 kJ/mol for QTR = 145.6 kJ/g and then again increases to ∼59 kJ/mol for the QTR = 364 kJ/g. These changes closely follow the variations in the average powder particle size obtained with the varying milling energy input. For the milling energy input QTR = 36.4 and 145.6 kJ/g the average powder particle size decreases to 14.9 ± 6.6 and 7.5 ± 2.6 μm, respectively, and subsequently increases reaching the average size of 16.1 ± 6.3 μm for the milling energy input QTR = 364 kJ/g. On the other hand, the apparent activation energy for dehydrogenation doesn't depend on the average crystallite (grain) size. The amorphous Mn and B elements are also formed after isothermal dehydrogenation. The synthesized Mn(BH4)2 hydride is very stable and doesn't excessively release H2 during a long-term storage at room temperature for over 120 days under a slight overpressure of argon.

Mechano-chemical synthesis of manganese borohydride (Mn(BH4)2) and inverse cubic spinel (Li2MnCl4) in the (nLiBH4 + MnCl2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

Manganese borohydride (Mn(BH4)2) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH4) and manganese chloride (MnCl2) by applying high energy ball milling for 30 min. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH4 + MnCl2) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H2 that increases with the molar ratio n but does not exceed ∼0.2 wt.% for n = 23. However, longer milling duration leads to more H2 released. For the equimolar ratio n = 1 the principal phases synthesized are Li2MnCl4, an inverse cubic spinel phase, and the Mn(BH4)2 borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH4)2. With the n increasing from 3 to 23 LiBH4 is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH4)2. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 °C shows the release of hydrogen as a principal gas with a maximum intensity around 130–150 °C accompanied by a miniscule quantity of borane B2H6. The intensity of the B2H6 peak is 200–600 times smaller than the intensity of the corresponding H2 peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH4)2 up to about 270–280 °C. At 100 °C under 1 bar H2 pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ∼4 wt.% H2 which is a very large amount of H2 considering that the mixture also contains 2 mol of LiCl salt. The H2 quantities experimentally desorbed at 100 and 200 °C do not exceed the maximum theoretical quantities of H2 expected to be desorbed from Mn(BH4)2 for various molar ratios n. It clearly confirms that the contribution from B2H6 evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100–200 °C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, EA ∼ 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H2 during storage at room temperature and 40 °C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m2/g substantially enhances the rate of discharge at 40 °C.

Synergistic hydrogen desorption behavior of magnesium aluminum hydride synthesized by mechano-chemical activation method

A mechano-chemical activation synthesis (MCAS) is employed to fabricate Mg(AlH4)2 via milling the precursors, specifically NaAlH4 and MgCl2. The corresponding dehydrogenation behavior of the synthesized powders is investigated. The experimental results showed that incomplete synthesis or premature dehydrogenation may occur if the milling process was not properly controlled. The hydrogen content of each synthesized powder is determined by using a thermal gravimetric analyzer (TGA). The dehydrogenation reactions of the synthesized powders are investigated by employing ex situ X-ray diffraction (XRD), in situ synchrotron XRD and differential thermal analysis (DTA). The results showed that the incompletely synthesized powder consisted of residual NaAlH4 in the synthesized Mg(AlH4)2, which demonstrated an initial dehydrogenation temperature as low as 100°C and accompanied with a maximum amount (3.1wt%) of H2 released below 350°C. The mutual catalytic effect of both NaAlH4 and Mg(AlH4)2 on lowering their initial dehydrogenation temperature is confirmed.

The effects of ball milling and nanometric nickel additive on the hydrogen desorption from lithium borohydride and manganese chloride (3LiBH 4 + MnCl 2) mixture

A mixture of [3LiBH 4 + MnCl 2] was processed by high energy ball milling in ultra-high purity hydrogen gas for 0.5 and 1 h. The XRD patterns of milled powders show the sole diffraction peaks of LiCl. The reaction occurring during milling of [3LiBH 4 + MnCl 2] seems to have all characteristics of the metathesis-type reactions occurring between borohydrides (LiBH 4 and NaBH 4) and metal chlorides (MCl n ) induced in a solid state by a mechano-chemical activation synthesis (MCAS). Under pressure of 0.1 MPa H 2 (atmospheric) the ball milled [3LiBH 4 + MnCl 2] mixture is able to desorb ∼4.0 wt.% H 2 at 100 °C within 21,000 s and ∼4.5 wt.% H 2 at 120 and 200 °C within 8000 s and 4000 s, respectively. The addition of n-Ni with SSA = 60.5 m 2/g allows desorption of ∼3.7wt.%H 2 within 8,700 s at 100 °C. This is one of the highest H 2 desorption capacities obtained for a complex hydride at 100 °C under atmospheric pressure of H 2 taking into account the fact that the microstructure contains some amount of a useless LiCl constituent. The activation energy of hydrogen desorption for a ball milled undoped [3LiBH 4 + MnCl 2] is ∼102 kJ/mol and ∼98 and 92 kJ/mol after doping with 5 wt.% of nanometric Ni having specific surface area (SSA) of 9.5 and 60.5 m 2/g, respectively. After volumetric desorption from 100 to 450 °C the XRD patterns show only LiCl. The n-Ni additive slightly lowers the total quantity of desorbed H 2. Re-absorption tests, under pressure of 10 MPa H 2 at 200 °C, show that the system is, most likely, irreversible. Flammability studies show that the ball milled [3LiBH 4 + MnCl 2] mixture can be ignited by scraping the cylinder walls with a metal tool as well when it is thrown and dispersed in air in a powder form. It also reacts violently in contact with water and a nitric acid.

Mechano-chemical activation synthesis (MCAS) of disordered Mg(BH4)2 using NaBH4

Mechano-chemical activation synthesis (MCAS) of a powder mixture ratio 2NaBH4 and MgCl2 was carried out by ball milling in a magneto-mill from 1 to 100 h duration with the objective of initiating a metathesis reaction 2NaBH4 + MgCl2 → Mg(BH4)2 + 2NaCl. It is found from X-ray diffraction (XRD) that the reaction has been only partially completed resulting in the formation of NaCl and a (Na,Mg)BH4 solid solution. The solutionizing effect of Mg becomes more intense as milling time increases. No XRD peaks of crystalline Mg(BH4)2 are observed. Although the result of XRD analysis in the present work does not indicate the presence of a crystalline Mg(BH4)2 hydride, the similarity of thermal events in the DSC test of the milled powder mixtures to those of crystalline Mg(BH4)2 [D.S. Stasinevich, G.A. Egorenko, Russ. J. Inorg. Chem. 13 (1968) 341–343; V.N. Konoplev, V.M. Bakulina, Izv. Akad. Nauk SSSR 1 (1971) 159–161] indicates the possibility of presence of a “disordered” Mg(BH4)2 in the in the milled powder mixture, as specifically proposed by Nakamori et al. [Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-I. Towata, A. Züttel, S.-I. Orimo, Phys. Rev. B74 (2006) 045126; Y. Nakamori, H.-W. Li, K. Kikuchi, M. Aoki, K. Miwa, S. Towata, S. Orimo, J. Alloys Compd., in press, doi:10.1016/j.jallcom.2007.03.144]. It is hypothesized that the presence of (Na,Mg)BH4 solid solution in the MCAS process inhibits the formation of crystalline Mg(BH4)2. Regardless of the nature of the synthesized hydride phase the milled powders desorb hydrogen within the 275–420 °C temperature range and the total amount of hydrogen desorbed gradually decreases with increasing milling time reaching ∼2.1 wt.% for the powder milled for 100 h. However, regardless of the milling time the total amount of hydrogen desorbed within 275–420 °C is always less than the theoretical hydrogen capacity in the initial mixture of 2NaBH4 and MgCl2 (∼4.7 wt.%).

Mechano-chemical activation synthesis (MCAS) of nanocrystalline magnesium alanate hydride [Mg(AlH 4) 2] and its hydrogen desorption properties

A successful synthesis of the Mg(AlH 4) 2 + 2NaCl mixture by the mechano-chemical activation synthesis (MCAS) for 5 and 10 h has been achieved. The nanocrystalline Mg(AlH 4) 2 hydride in the mixture has the grain size on the order of 18 nm. Even relatively short milling for just 10 h seems to result in a partial decomposition of the initially formed Mg(AlH 4) 2 into the nanocrystalline β-MgH 2, elemental Al (grain size ∼26 nm) and hydrogen gas. A number of DSC tests at the scan rate of 4 °C/min of the Mg(AlH 4) 2 + 2NaCl mixture synthesized for 5 and 10 h show either endothermic or exothermic heat flow changes at the ∼125–180 °C range due to the decomposition of Mg(AlH 4) 2. Endothermic peaks with the maxima at ∼271 and 316 °C due to the decomposition of β-MgH 2 and the formation of Al 3Mg 2 intermetallic as well as Al(Mg) solid solution are present. The peak at ∼452 °C is due to the eutectic melting of the Al 3Mg 2 and Al(Mg) mixture. Prolonged milling for 40 h results in a complete decomposition of the Mg(AlH 4) 2 into two nanocrystalline solid phases such as β-MgH 2 (grain size ∼11 nm) and the elemental Al (grain size ∼20 nm), and hydrogen gas. DSC tests at the scan rate of 4 °C/min up to 500 °C of the 40 h milled powder, which contains β-MgH 2, results in two endothermic peaks with the maxima at ∼292 °C due to the decomposition of β-MgH 2 and ∼452 °C due to the eutectic melting. TGA tests for the 5 and 10 h milled sample give the total weight loss of ∼2.45 and 2.16 wt.%, respectively. The hydrogen desorption kinetics of the as-synthesized powders directly after milling were also evaluated using a Sieverts-type apparatus.