ВОДОРОДНАЯ ЭНЕРГЕТИКА И ТРАНСПОРТ
Хранение водорода
HYDROGEN ENERGY AND TRANSPORT
Hydrogen storage
I BALL MILLING SYNTHESIS AND PROPERTIES OF HYDROGEN 1 SORBENTS IN MAGNESIUM HYDRIDE-GRAPHITE SYSTEM
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I S. N. Klyamkin, B. P. Tarasov*, E. L. Straz, R. V. Lukashev*,
J I. E. Gabis**, E. A. Evard**, A. P. Voyt**
с &
& Chemistry Department, Moscow State University, Moscow, Russia
rn с о
q * Institute of Problems of Chemical Physics, Chernogolovka, Russia
** V. A. Fock Institute of Physics, St. Petersburg State University, Russia
Hydrogen sorbing composites in the MgH2-Graphite system have been synthesized using high pressure and ball milling technique and a comprehensive study of their properties has been carried out. X-Ray diffraction study indicated the presence in the material produced of only the starting phases of magnesium hydride and graphite but a profile analysis of the XRD patterns evidenced a drastic reduction of the coherently diffracting domains and increase of crystal lattice defects concentration. The hydrogen desorption process was studied under dynamic conditions at heating of the sample in vacuum in calibrated volume with heating rate of 2 and 3K/min. The carbon containing samples subjected to mechanochemical treatment as compared with non-treated MgH2 demonstrated a lower temperature of the onset of hydrogen release (150 °C instead of 200 °C). Moreover, at temperatures up to 270 °C the pressure of desorbed hydrogen was higher that the corresponding equilibrium value calculated on the basis of thermodynamic parameters of the magnesium hydride. Some explanation of the phenomenon observed have been suggested.
Introduction
Magnesium hydride MgH2 due to high hydrogen content (more than 7.5 wt. %) is considered as one of the most promising material for hydrogen storage. However, dissatisfied kinetics of hydrogen absorption and desorption and excessive thermal stability put obstacles on the way of its practical use. Intermetallic compounds of magnesium, first with nickel and rare earth metals, produced by conventional metallurgical methods are characterized by improved kinetics of interaction with hydrogen but reduced sorption capacity because of high concentration of heavy elements. The problem thus is to choose such components and such preparation procedure which can ensure a qualitative modification of kinetic and thermo-dynamic parameters of hydride formation and decomposition reactions in the presence of minor amount of dopants.
Mechanochemical treatment in high-energy ball mills is an alternative method for production of novel hydrogen sorbing intermetallic materials. That allows to synthesize metastable phases with various composition and microstructure possessing unusual physico-chemical properties and enhanced reactivity. Effectiveness of such an approach has been experimentally proved for a series of hydride
forming polymetallic composition [1, 2]. In the case of magnesium hydride based systems mecha-nochemical activation particularly with addition of catalytically active components (Ni, RE, V et al.) results in a drastic improvement of the reaction kinetics and complete interaction with hydrogen under milder conditions [3].
However, thermal stability of such modified materials practically does not differ from the customary magnesium hydride. The hopeful results have been obtained in [4-8], where mixtures of magnesium with graphite were ball milled. A special attention was paid to organic additives (benzene, cyclohexene or cyclohexane) during the milling process which modified the characteristics of the resulting nanocomposites [4]. Hydrogenation of the composites produced proceeded at temperature of less than 200 °C and following analysis of thermodesorption spectra allowed the authors to register a shift of the onset of hydrogen release to lower temperatures and to conclude that thermal stability of the material studied reduced as compare with individual MgH2 [7].
The present work is devoted to synthesis and investigation of properties of the composites forming in the magnesium-hydrogen-carbon system. For higher effectiveness of mechanochemical treatment magnesium hydride has been used as a starting
Водородная энергетика и транспорт Хранение водорода
mixture component instead of metallic magnesium. Having lesser plasticity and smaller particle size the hydride shall promote interaction between components during ball milling.
Experimental
A synthesis of MgH2 has been carried at temperature of 400-470 °C and hydrogen pressures up to 1500 atm using high gaseous pressure technique described elsewhere [9]. Mechanochemical treatment was performed in a high energy planetary ball mill (acceleration of 70 g) with stainless steel reactor and balls under an argon atmosphere during 30-90 minutes.
The hydrogen desorption process was studied under dynamic conditions at heating of the sample in vacuum into a calibrated volume with heating rate of 2 K/min and simultaneous registration of temperature and pressure in the system. Thermode-sorption spectra with simultaneous mass-spectro-scopic registration of the gaseous reaction products was also measured at heating rate of 3 K/min.
Electron scanning microscopy and X-ray diffraction analysis (CuKa-radiation and crystalline silicon as an internal standard) were applied to characterize the starting materials and the composites produced.
Results and discussion
A comprehensive study of the influence of hydrogen pressure and temperature on the behaviour of magnesium during the first hydrogenation shown that the reaction had a long incubation period (few hours) regardless of the pressure applied and started at temperature higher than 400 °C. A temperature increase from 410 to 460 °C improved noticeably the reaction kinetics however the use of high pressure resulted in an opposite effect (Fig. 1). We can suggest that in the latter
400 t, min
Fig. 1. Kinetics curves of virgin magnesium hydrogenation: ■ — 410 °C, 290 atm; • — 460 °C, 800 atm; p — 460 °C, 90 atm
case higher value of the reaction driving force (Pexp - Peq) provokes a prompt formation of numerous small nuclei of the hydride phase which cover the surface of magnesium particles in an early stage and thus slows down the further transformation.
According to ESM analysis the MgH2-Graphite composites constitute of very fine particles with average size of 1 to 10 mm and an homogenous distribution of the components. The XRD patterns evidences the presence in the material produced of only the starting phases of magnesium hydride and graphite but a profile analysis indicates a drastic reduction of the coherently diffracting domains and increase of crystal lattice defects concentration.
A comparative study of the samples behaviour during thermodesorption proved a considerable effect of the mechanochemical treatment. The TPD spectrum of the as-prepared MgH2-Graphite composite shows a splitting of the desorption peak and a shift of its maximum to lower temperatures as compared with non-treated hydride (Fig. 2). This phenomenon, however, cannot be reproduced and at the second desorption the peak location practically does not differ from that of individual MgH2.
1st desorption; -tion
0 50 100 150 200 250 300 350 400 450 500 550 600 650
T, °C
Fig. 2. Thermodesorption spectra of individual magnesium hydride and its composites with graphite. Heating rate 3 K/min.--MgH2;--MgH2, Graphite,
--MgH2, Graphite, 2nd desorp-
A registration of pressure of hydrogen released by the samples in vacuum into a closed volume allowed us to precise the desorption regularities during a slow heating. In the case of non-treated MgH2 the onset of hydrogen release was fixed at temperature of about 200 °C (Fig. 3). In the whole temperature range (up to 400 °C) the pressure created in the system by hydrogen from decomposing hydride was noticeably lower than corresponding equilibrium values calculated on the basis of literature thermodynamic parameters of the magnesium hydride [10]. That result can be considered as a direct confirmation of kinetically
С. Н. Клямкин, Б. П. Тарасов, Е. Л. Страз, Р. В. Лукашев, И. Е. Габис, Е. А. Евард, А. П. Войт Механохимический синтез и свойства сорбентов водорода в системе гидрид магния—графит
10.0q
200 250 300 350 400
T, °C
Fig. 3. Hydrogen pressure vs temperature for MgH2 decomposition reaction. Calculated equilibrium and experimental values for various samples. - — calculated (InP = 6.9-3857/T); p — MgH2 + C, 1st des.; • — MgH2 + C, 2nd des.; * — MgH2, untreated
limited phase transformations in the individual magnesium-hydrogen system.
For the carbon containing samples subjected to mechanochemical treatment the temperature of hydride decomposition onset decreased to 150 °C. The most intriguing fact was that up to 270 °C (Peq = 0.6 atm) the pressure in the systems was higher that the corresponding equilibrium value (Fig. 3). Such a behaviour evidenced that there was in the sample some amount of hydrogen for which the bonding energy with matrix was lower than in the case of conventional magnesium hydride.
The experimental data obtained do not allow us to define the nature of the phenomenon observed. Two explanations or their combination can be suggested:
■ the mechanochemical treatment leads to a generation of defects and strain in the crystal lattice and also to a disintegration of the sample and, consequently, a considerable increase of the specific surface of the material. The Mg-H bonding energy in the surface layer and within the lattice defects is apparently different from that of the bulk of a perfect crystal. Increasing contribution in the total amount of hydrogen absorb of such a non-equilibrium constituent can cause above mentioned desorption behavior at low temperatures.
■ the high-energy effects in the ball mill can result in formation of new metastable hydrogen containing phases with lower thermal stability. For example, high-pressure modification
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