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Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
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Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles Hao Yu, Simona Bennici*, Aline Auroux Universite Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l'environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France
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abstract
Article history:
Commercial metal nanoparticles of Fe, Co, Ni, Cu, Zn were added to MgH2 by ball-milling
Received 14 March 2014
to improve the kinetics of hydrogen release and the reversibility during successive
Received in revised form
absorption/desorption cycles. metal nanoparticles were well dispersed into the MgH2
28 April 2014
matrix without formation of any ternary metal hydrides, nor binary compounds. Acti-
Accepted 13 May 2014
vation energy values were determined for the various samples by temperature
Available online xxx
programmed desorption experiments while the hydride formation enthalpy was deduced from Van't Hoff equation starting from high pressure volumetric isotherms
Keywords:
acquired at different temperatures. The presence of transient effect during the absorp-
Hydrogen
tion process was excluded by comparing successive hydrogenation/dehydrogenation
Magnesium hydride
cycles recorded at 350 C on Ni and Fe-containing samples. Information about hydrogen
Metal nanoparticles
absorption kinetics was also obtained. Promisingly, the Ni, Fe, and Co containing sam-
Energy
ples have shown a good stability, enhanced catalytic performance, and high rate of hydrogen absorption while Zn and Cu nanoparticles worked more like inhibitors than activators. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The use of alternative energies to produce clean fuels like hydrogen could extensively participate to the decrease of CO2 emission, as well as to diminish the production of polluting species like NOx, especially if hydrogen is consumed through the use of fuel cells [1,2]. However, safe storage of hydrogen is still today one of the main problems for its use as an energy carrier. Moreover, the choice of the most suitable storage methodology becomes crucial when hydrogen is produced
starting from renewable and intermittent energies, as wind or solar energy. In the field of hydrogen storage in solid materials, mainly three types of solids are currently investigated: the porous materials with high specific surface area, the intermetallics and the complex hydrides. Many intermetallics (AB5, AB2, solid solutions like TieV …) are able to reversibly absorb hydrogen, even at room temperature for some of them. However, if their hydrogen volumetric capacities are very high (up to 140 g H2/L), their gravimetric capacities remain limited, since these
* Corresponding author. Tel.: þ33 472445379; fax: þ33 472445399. E-mail address: [email protected] (S. Bennici). http://dx.doi.org/10.1016/j.ijhydene.2014.05.069 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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phases are mainly formed of transition metals or lanthanides [3]. A low gravimetric capacity is also a restriction to the storage of intermittent energies where the amount of energy involved can be extremely high and consequently a stable and efficient storage system becomes a must [4]. To reach the demanded targets for the application in these fields, research efforts have been made to develop interstitial, binary or even more complex hydrides capable to store and release hydrogen at temperature and pressure compatible with the different applications [5,6]. Mg-based materials are good candidates for these kinds of applications [7e11]; Mg (non toxic and inexpensive) shows a hydrogen storage capacity of 7.6 wt%, but the major obstacle is its high H2 desorption temperature (>300 C). In practice, the sorption reactions are quite slow and require temperatures of at least 350 C for fast hydrogenation/dehydrogenation processes [12]. In order to decrease the desorption temperature of MgH2 many research groups added other hydrides to MgH2 in order to form complex hydrides with lower hydrogen desorption temperature or to use catalysts and activators to improve the hydrogen release kinetics (to provide hydrogen ondemand), and the reversibility of the reaction for a better recycling of the material [13e16]. Original methodologies as the chemical fluid deposition process in supercritical fluids for the preparation of nanosized plot of metals (Ni and Pd) on the surface of Mg have been employed in order to improve the hydrogen sorption cyclability [17]. Films of Nb-doped Mg deposited by magnetron sputtering with thickness 10e20 mm on graphite wafers strongly act on the dehydrogenation velocity as shown by Bazzanella et al. [18]. Transition metals have shown to be highly effective catalysts due to their high affinity towards hydrogen [14]. Tests performed on MgH2 enriched with nanosized-Ni particles were reported by Hanada et al. [15] in a temperature range of 150e250 C. MgH2eV samples showed a desorption temperature of 235 C, as reported by Liang et al. [15], and also transition metals like Ti, and Nb, mixed to magnesium by ballmilling, act as catalyst to improve the hydrogen sorption kinetics [15,19]. Moreover, MgeLaNi5 revealed a relatively low working temperature (100e150 C) [20], and activated ZreNi alloys [21] present a non negligible effect in enhancing the desorption kinetics (at 250 C) when intimately mixed to MgH2. NieCo, FeeNi, AleNi, YeCe bimetallic catalysts are also considered to improve the desorption kinetics of MgH2 [22e25]. The stability of the commercial MgH2 powder, the possibility to obtain premanufactured commercial metal nanoparticles, and the fact that such a system can be prepared under inert atmosphere make relatively easy to reproducibility of the preparation procedure at industrial scale. For these reasons, in the present work commercial transition metal nanoparticles of Fe, Co, Ni, Cu, and Zn were added to MgH2 by ball-milling to improve the kinetics of hydrogen release and the reversibility during successive absorption/ desorption cycles.
Experimental Materials preparation Magnesium hydride powder (98%) was purchased from Alfa Aesar, while the Fe, Co, Ni, Cu, and Zn nanoparticles were purchased from Nanostructured & Amorphous Materials Inc. Ball-milling of 10 wt% metal nanoparticles with MgH2 was performed using a Retsch P100 planetary ball miller operating under inert atmosphere of argon and equipped with grinds of 12 and 50 mL in volume. MgH2 powder was previously reduced in size and later-on intimately mixed with the metal nanoparticles by ball-milling at a rotation speed of 300 rpm for 4 h, and a ball/powder weight ratio close to 20:1. The rotation of the cell in the planetary ball miller was paused every 30 min to limit the temperature increase in the milling cell. The powders were strictly handled in a glove box under argon atmosphere in order to prevent any oxidation. The prepared composites were labeled as MgH2-10-Menano with Me ¼ Fe, Co, Ni, Cu, Zn.
Materials characterisation X-ray diffraction (XRD) The phase composition and crystallites state of the samples were controlled by X-ray diffraction using a Bru¨ker D5005 powder diffractometer where the sample is fixed while the Xray tube (Cu Ka1 þ a2; l ¼ 0.154184 nm) and the detector rotate. X-ray diffraction patterns were recorded between 20 and 90 (2q) with a step size of 0.02 and acquisition time of 8 s/ step. In order to increase the signal/background ratio, a zero background holder was used. The sample was deposited on a resin support and protected by kapton-tape in the glove box under inert atmosphere.
Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was performed using a Philips 5800 SEM electron microscope. The samples were deposited on carbon tape and metallized by sputtering. A gold film ensured a good conductivity for the observation.
Transmission electron microscopy (TEM) The recording of transmission electron micrographs was carried out using a JEOL 2010 LaB6 equipment operating at 200 kV. The samples finely grinded and handled under inert atmosphere were incorporated in a resin matrix to avoid any contact with air.
Hydrogen storage properties Temperature programmed desorption (TPD) Hydrogen desorption temperatures and amounts were measured for all the samples by means of a temperature programmed desorption device. The experiments were performed using a TPD/O/R-1100 instrument (Thermofisher) equipped with a quartz reactor with a porous septum (ca. 8 mm i.d.) and a filter filled with soda lime. The samples (10e30 mg) were heated at different temperature increasing rates (1, 2, 3, 5, 10 C/min) from room temperature to 500 C under an argon flow of 20 mL/min. The H2 released was
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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detected by a thermal conductivity detector (TCD), and the corresponding volume calculated after calibration of the hydrogen desorption peak areas by injections of known amounts of pure hydrogen.
Hydrogen absorption/desorption isotherms, absorption kinetics and absorption/desorption cycles The hydrogen absorption/desorption isotherms were measured in the 250e400 C temperature range by solidegas reaction in a volumetric apparatus, High Pressure Volumetric Analyzer (HPVA), from Micromeritics, USA. The samples (100e200 mg) were weighted and transferred in the measurement cell in a dry-box under Ar atmosphere to avoid any contamination and oxidation. High purity hydrogen (purity > 99.9999%) was supplied by a cylinder at high pressure. Hydrogen isotherms were performed at 250, 300, 350, and 400 C. The absorption kinetics were measured at 350 C exposing the sample to an initial pressure of 20 bar and a fixed absorption time of 200 min. Hydrogenation/dehydrogenation cycles at 350 C were also performed to verify the samples stability.
Results and discussion MgH2 based samples To obtain nanocrystalline Mg-based materials is crucial to prepare composites with improved hydrogenation kinetics by facilitating hydrogen diffusion into the solid matrix [26,27]. In order to obtain a particle size reduction a permanent deformation of the material must occur. The energy provided for particle interactions must overcome the elastic properties of the material thus allowing the fracture to occur and leading ultimately to size reduction and permanent deformation. Metallic magnesium reveals to be a too ductile material to be easily ball-milled without the formation of large agglomerates by contact welding. To avoid this problem the preparation of the samples used in the present research has been performed starting from commercial MgH2, which is a brittle material that consequently absorbs relatively little energy prior to ballmilling and that can be easily reduced in size. To optimize the ball-milling conditions, the commercial MgH2 powder was ball-milled for different time of 30 min, 4 h, and 10 h, and analyzed by XRD to determine the crystallite size obtained by the Scherrer formula [28]. The crystallite size was strongly reduced from 29 to 14 nm after 30 min milling, while for longer treatment time (4 and 10 h ball-milling) no real size decreasing was detected. 4 h ball-milling time was chosen to guaranty a sufficient low size of the crystallites. In the aim of improving the hydrogen sorption kinetics by activation of magnesium hydride, various metal nanoparticles of iron, copper, nickel, zinc, and cobalt were intimately mixed to the previous ball-milled MgH2 (see procedure in Section 2.1). No new phases' formation was observed by XRD analysis after the ball-milling preparation step and only the diffraction peaks of MgH2 (27.1, 35.4, 39.5, 54.1 2q [29]), metal particles (Fe ¼ 44.5, 65.2, 82.9 2q [30,31]; Co ¼ 44.2 2q [32]; Ni ¼ 44.1, 50.3, 75.3 2q [30.31]; Cu ¼ 43.1, 49.9, 73.6 2q [30]; and Zn ¼ 43.0 2q [33]) and traces of metallic Mg were
Fig. 1 e XRD patterns of the MgH2 based composites.
identified (Fig. 1). As already known the synthesis of magnesium complex hydrides can be problematic due to the difference in vapour pressure and melting point between magnesium and transition metals. The high energy ballmilling carried on during 4 h was effective to well disperse the metal nanoparticles into the MgH2 matrix (see SEM/TEM images for the iron and nickel containing samples in Fig. 2, and the SEM images for the Co, Zn, and Cu-containing samples in the “Supplementary data” published online alongside the electronic version of this article) without formation of ternary metal hydrides [27], nor bimetallic compounds that require much higher temperatures, hydrogen pressure, and long reaction time up to several days [22,34e37]. MgH2 was mainly present in the beta form and only very small X-ray diffraction peaks of the g-phase were detected, showing the distortion of the b-MgH2 phase during milling. The formation of orthorhombic g-phase during milling has been already described in previous articles [38]. Some small peaks corresponding to Mg (36.8 2q [29,31]) and MgO (62.4 2q [30,31]) phases were also detected, due to the oxidation of the impurities of metallic Mg contained in the commercial MgH2 powder during handling of the samples, milling, etc [39]. In addition, we can observe that the MgH2 crystallite size (calculated by the Scherrer formula) was maintained in the 10e12 nm range for all the samples containing the transition metal nanoparticles.
Performance of the catalysts Influence of transition metal nanoparticles addition on the hydrogen desorption temperature and kinetics TPD experiments permitted to make a first comparison among the different metal nano particles additives by comparing the hydrogen desorption temperature peaks of the MgH2-10Menano samples to that of pure 4 h ball-milled MgH2 [12] (Fig. 3). All the samples containing metal nanoparticles present hydrogen desorption peaks at lower temperature than that of pure MgH2 (maximum of the peak centered at 386 C). The best results were observed for the nickel containing sample that shows a unique sharp TPD peak centered at 203 C and a hydrogen release starting at 130 C. Relatively low initial desorption temperatures were observed also for the Co, Fe and
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Fig. 2 e Scanning electron micrographs of the MgH2 based composites containing 10 wt% of iron and nickel nanoparticles.
Cu containing samples (around 170 C). For these three samples (MgH2-10-Conano, MgH2-10-Cunano, and MgH2-10-Fenano) we can observe more complex TPD profiles composed of two main peaks, with the highest desorption peak temperature returning to the baseline (end of the desorption process) only around 400 C. This desorption profile is characteristic of two types of MgH2, one intimately mixed with the metal nanoparticles and presenting improved dehydrogenation kinetics [26], and a second with MgH2 keeping its original behavior characterized by a high desorption temperature. Finally, the addition of Zn nanoparticles did not improve the hydrogen desorption kinetics of MgH2 and for the MgH2-10-Znnano sample a well defined single desorption peak was observed at 368 C. This slight decrease in temperature, compared to the desorption peak of pure MgH2, can be related to the longer ball-milling time required by the mixing of the metal
nanoparticles to the already milled pure MgH2 in 4 supplementary hours, and not to a catalytic effect. The quantity of hydrogen released from the various samples is reported in Table 1 (TPD section). For each sample the amount of hydrogen evolved is lower than the theoretical amount of 862 mL/gMgH2, probably due to a partial decomposition of MgH2 during the ball-milling, in which the temperature can reach values high enough to activate the desorption process. TPD analysis were performed for each sample at different heating rates (from 1 to 10 C/min) and the activation energies reported in Table 1 were calculated applying the Kissinger equation [40]. This equation allows the determination of the activation energy for a decomposition reaction regardless to the reaction order. The linearization of the Kissinger equation in logarithmic form is reported as follow: ln
Fig. 3 e TPD curves obtained for all the samples at an increasing temperature rate of 2 C/min.
a T2max
!
Ea RC0 þ ln 2 ¼ Tmax Tmax Ea
! (1)
with R being the gas constant, a the heating rate, C0 the reaction order, and Tmax the temperature at the maximum of the TPD peak. The activation energy (Ea) values are obtained from the slope of the straight lines interpolating the data obtained by plotting lnða=T2max Þ versus 1=T2max for each sample. All the added metal nanoparticles were able to decrease the activation energies for the hydrogen desorption reaction to a much higher extent than similar systems reported in the literature for which the lower Ea was estimated at 91 kJ/mol [14]. The lowest value of 72 kJ/mol was observed for MgH2-10-Ninano, according to the values given by Shahi et al. [31]. The activation energy values decrease in the range 35e55 kJ/mol by addition of metal nanoparticles if compared to the 121 kJ/mol value, calculated for the pure ball-milled MgH2.
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Table 1 e Data obtained by temperature programmed desorption and high pressure volumetric isotherms for all samples. Sample
TPD Hydrogen released (mL)
MgH2-10-Ninano MgH2-10-Conano MgH2-10-Fenano MgH2-10-Cunano MgH2-10-Znnano MgH2
10.2 17.4 15.6 9.9 12.2 16.0
Hydrogen released (mL/gMgH2 )b 653 739 852 649 690 800
HPVA Activation energy (kJ/mol)c
72 75 86a (66) (67)a 73 84 121
Hydrogen adsorbed (mL/ gMgH2 )d 250 C
300 C
350 C
760 578 484 680 518 831
764 608 674 725 635 831
772 630 718 783 643 827
DHf (kJ/mol)e
75 71 75 62 67 73
The standard formation enthalpy of MgH2 is 75 kJ/mol [42]. a TPD presenting 2 peaks of desorption. b Vmax =gMgH2 ¼ 852 mL corresponding to 7.6 wt%. c Calculated from Kissinger equation after TPD data acquisition at 1, 2, 3, 5 and 10 C/min. d Measured at 25 bars. e Calculated from Van't Hoff equation after performing isotherms at 250, 300, and 350 C for all MgH2-10-Menano samples and at 300, 350, 400 C for pure MgH2.
Fig. 4 e High pressure volumetric isotherms obtained at different temperatures. Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Hydrogen absorption/desorption isotherms, absorption kinetics and absorption/desorption cycles Fig. 4 shows the high pressure isotherms acquired for all samples at 250, 300, and 350 C up to 30 bar of hydrogen pressure and after a pretreatment step of 4 h, at 350 C under vacuum. For pure MgH2 the isotherms were performed at higher temperatures of 300, 350, and 400 C after a pretreatment performed at 400 C. Pure MgH2 adsorbed the highest amount of hydrogen, 830 mL per g of material (see Table 1, HPVA section), independently of the temperature, and with well defined isotherms displaying a flat plateau. Among the MgH2-10-Menano samples, MgH2-10-Ninano is the only one presenting an adsorbed hydrogen volume that remains constant for the isotherms acquired at different temperatures (around 765 mL/gMgH2 ). For all samples except MgH210-Cunano hydrogen absorption/desorption isotherms present a flat plateau and larger hysteresis than pure MgH2 isotherm. The plateau of the 350 C isotherm of the MgH2-10-Cunano sample is not completely flat and presents a change of slope centered around 23 and 18 bar on the absorption and desorption branches, respectively. This peculiar behavior has been confirmed by performing the same experiment three times and can be related to the presence of two different MgH2 populations, more and less activated by the presence of copper, as confirmed by the presence of two desorption peaks in the TPD analysis discussed in Section 3.2.1. For the Co, Fe, Cu, and Zn nanoparticles containing samples the quantity of adsorbed hydrogen increases with absorption performed at higher temperature (see Fig. 4 and the corresponding values reported in Table 1). For all the nanoparticles containing samples (with exception of MgH2-10-Znnano) the beginning of the absorption plateau is located in the 6e6.4 bar range for the 350 C isotherms; this value is close to that measured for pure MgH2 (5.8 bar) and confirms the presence of MgH2 as only hydrogen adsorbing species, excluding the in-situ formation of other hydride alloy phases [41]. Surprisingly, the addition of Zn shows an inhibitor effect on the absorption/desorption performances of MgH2. The MgH2-10-Znnano sample is characterized by isotherms with a very large hysteresis and an absorption plateau (for the 350 C isotherm) starting only at 7.8 bar. The formation enthalpies were calculated by means of Van't Hoff equation: Peq DH DS ¼ ln RT R P0
Fig. 5 e Absorption/desorption cycles for MgH2-10-Ninano and MgH2-10-Fenano samples.
enthalpy between the activated samples and pure MgH2 can be attributed to defects introduced during ball-milling [7,41]. The low values of formation enthalpy, 62 and 67 kJ/mol, measured for MgH2-10-Cunano and MgH2-10-Znnano samples, respectively, suggest the presence of a side reaction involving the Mg, H2 and Zn, Cu nanoparticle species (i.e. alloying, sintering) [7]. Moreover, the small difference between these values and the 75 kJ/mol formation energy of pure MgH2 indicates that only a small amount of the metal nanoparticles is involved in this side reaction process. Generally, when MgH2 is ball-milled with metal catalyst to form alloys [41] an activation procedure, consisting in successive absorption/desorption cycles, is necessary to obtain repeatable high pressure volumetric isotherm experiments. In the present work no alloys were formed, but to exclude the presence of any transient effect during the absorption cycles and to validate the calculated formation
(2)
After plotting lnðPeq =P0 Þ as a function of 1/T, the DH values are calculated from the slope of the obtained straight lines. The samples containing Ni, Fe and Co nanoparticles present enthalpy values in the 71 to 75 kJ/mol range that correspond to the experimental (73 kJ/mol, Table 1) and theoretical formation enthalpy value [42] reported for MgH2. Because the addition of metal nanoparticles can influence only the activation energy and not the enthalpy of the process, the obtained data for Ni, Fe and Co nanoparticles confirm the absence of intermetallic compounds formation [7] as already verified during the preparation step by XRD analysis, and as discussed in Section 3.1. The small differences in the values of formation
Table 2 e Hydrogen absorption times obtained by kinetics experiments performed at 350 C. Sample
MgH2-10-Ninano MgH2-10-Conano MgH2-10-Fenano MgH2-10-Cunano MgH2-10-Znnano MgH2
(H2%wt 80)a/(H2%wt total) at 20 bar 5.0/6.2 4.3/5.3 5.1/6.3 4.6/5.8 4.1/5.1 5.3/6.7
Time (min)a 20 bar 15 bar 10 bar 2.0 2.7 4.2 6.6 11.7 4.6
4.2 e e e e 7.5
6.1 7.7 9.1 15.3 e 18
a
Hydrogen absorption calculated at the time for which the pressure is P80 ¼ Pi e (PiPf) * 0,8.
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Fig. 6 e Hydrogen absorption kinetic curves for the MgH2-10-Menano and MgH2 systems performed at 350 C under 20 bar of hydrogen pressure (the hydrogen quantity is normalized per mass unit of MgH2 in each sample).
enthalpies deduced from the isotherms acquired at different temperatures on fresh samples, four successive absorption/ desorption cycles at 350 C were acquired on MgH2-10-Ninano and MgH2-10-Fenano samples, selected as example. The obtained isotherms are reported in Fig. 5; for both samples the four cycles are nearly superimposable thus confirming that no formation of new intermetallic compounds and hydrides occurs even by exposing the mixtures to hydrogen under high pressure and high temperature. The hydrogen absorption kinetics at 350 C was studied for all samples under 10, 15, and 20 bar hydrogen pressure. The samples were previously dehydrogenated under vacuum at 350 C for 4 h following the same pretreatment procedure as for the determination of high pressure volumetric isotherms. To render possible the comparison among the samples that present different extents of hydrogenation, the hydrogen absorption time necessary to attain 80% of the maximum capacity proper to each sample at the three pressures has been reported in Table 2. The absorption times are inversely proportional to the initial hydrogen pressure and the catalytic effect of Ni, Co, and Fe nanoparticles is evidenced by lower absorption times if compared to pure MgH2 sample. Accordingly to the previous observation, Zn and Cu nanoparticles exhibit an inhibitor effect and, at the highest pressure of 20 bar, the related absorption times are higher than those of pure MgH2, and respectively of 6.6, 11.7 and 4.6 min for MgH2-10-Cunano, MgH210-Znnano and pure MgH2. The hydrogen absorption kinetics curves acquired under 20 bar hydrogen pressure are reported in Fig. 6. For the most active catalysts, the main part of hydrogen is adsorbed in the first minute of the process and gives rise to a fast intake characterized by the steep section of the curve. After this first step, absorption keeps on with a slower rate up to the definitive hydrogen intake (described by the plateau of the curve). The maximum hydrogen absorption amounts (normalized by mass unit of MgH2) vary for the different samples, and never reach the maximum theoretical value of 7.6%.
Conclusion MgH2-10-Menano samples were prepared by ball-milling in Ar atmosphere. Thanks to the structural, morphological, dehydrogenation/hydrogenation properties measured by means of complementary techniques, we can confirm that there is no new phase formation between MgH2 and Ni, Fe, and Co nanoparticles even after several hydrogenation/dehydrogenation cycles. The Ni, Fe, and Co containing samples show good stabilities coupled with enhanced catalytic performances, as confirmed by lower activation energies than for pure ballmilled MgH2, no effect on the formation enthalpy values, and high rate of hydrogen absorption. Among them, the most performing sample in hydrogen desorption is MgH2-10-Ninano that presented a unique TPD desorption peak, the lowest hydrogen desorption activation energy (72 kJ/mol), and the lowest hydrogen desorption temperature (starting at 130 C). Zn and Cu nanoparticles act more like inhibitors than catalysts in the hydrogenation sorption process. Looking at the formation enthalpies the constitution of intermetallic compounds or the presence of other simultaneous reactive processes cannot be excluded at high hydrogen pressure. Finally, analyzing the obtained results and in particular the non-formation of intermetallic compounds for the most active catalysts, we can conclude that the contact surface between MgH2 and Ni, Fe, and Co nanoparticles is crucial to improve hydrogen desorption/absorption by increasing the hydrogen diffusion into the MgH2 matrix.
Acknowledgements AREVA and CNRS for H. Yu PhD thesis granting. The authors gratefully acknowledge the scientific services of IRCELYON, in particular Dr. G. Bergeret and F. Bosselet for the XRD analyses, and L. Burel for the SEM/EDX analyses.
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Appendix A. Supplementary data [18]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.05.069.
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Mg2CoH5 and Mg2NiH4 with lithium ion: far from equilibrium electrochemically driven conversion reactions. Int J Hydrogen Energy 2013;38:4798e808. [38] Varin RA, Czujko T, Chiu Ch, Wronski Z. Particle size effects on the desorption properties nanostructures magnesium dihydride (MgH2) synthesized by controlled reactive mechanical milling (CRMM). J Alloys Compd 2006;424:356e64. [39] Agarwal S, Aurora A, Jain A, Montone A. Structural and H2 sorption properties of MgH2-10 wt% ZrCrM (M ¼ Cu, Ni) nano-composites. J Nanopart Res 2011;13:5719e26.
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[40] Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem 1957;29:1702e6. [41] Jain A, Agarwal S, Jain P, Gislon P, Prosini PP, Jain IP. Hydriding behavior of Mg-50 wt% ZrCrFe composite prepared by high energy ball milling. Int J Hydrogen Energy 2012;37:3665e70. [42] Zeng K, Klassen T, Oerlerich W, Bormann R. Critical assessment and thermodynamic modeling of the MgeH system. Int J Hydrogen Energy 1999;24:989e1004.
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Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles Hao Yu, Simona Bennici*, Aline Auroux Universite Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l'environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France
article info
abstract
Article history:
Commercial metal nanoparticles of Fe, Co, Ni, Cu, Zn were added to MgH2 by ball-milling
Received 14 March 2014
to improve the kinetics of hydrogen release and the reversibility during successive
Received in revised form
absorption/desorption cycles. metal nanoparticles were well dispersed into the MgH2
28 April 2014
matrix without formation of any ternary metal hydrides, nor binary compounds. Acti-
Accepted 13 May 2014
vation energy values were determined for the various samples by temperature
Available online xxx
programmed desorption experiments while the hydride formation enthalpy was deduced from Van't Hoff equation starting from high pressure volumetric isotherms
Keywords:
acquired at different temperatures. The presence of transient effect during the absorp-
Hydrogen
tion process was excluded by comparing successive hydrogenation/dehydrogenation
Magnesium hydride
cycles recorded at 350 C on Ni and Fe-containing samples. Information about hydrogen
Metal nanoparticles
absorption kinetics was also obtained. Promisingly, the Ni, Fe, and Co containing sam-
Energy
ples have shown a good stability, enhanced catalytic performance, and high rate of hydrogen absorption while Zn and Cu nanoparticles worked more like inhibitors than activators. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The use of alternative energies to produce clean fuels like hydrogen could extensively participate to the decrease of CO2 emission, as well as to diminish the production of polluting species like NOx, especially if hydrogen is consumed through the use of fuel cells [1,2]. However, safe storage of hydrogen is still today one of the main problems for its use as an energy carrier. Moreover, the choice of the most suitable storage methodology becomes crucial when hydrogen is produced
starting from renewable and intermittent energies, as wind or solar energy. In the field of hydrogen storage in solid materials, mainly three types of solids are currently investigated: the porous materials with high specific surface area, the intermetallics and the complex hydrides. Many intermetallics (AB5, AB2, solid solutions like TieV …) are able to reversibly absorb hydrogen, even at room temperature for some of them. However, if their hydrogen volumetric capacities are very high (up to 140 g H2/L), their gravimetric capacities remain limited, since these
* Corresponding author. Tel.: þ33 472445379; fax: þ33 472445399. E-mail address: [email protected] (S. Bennici). http://dx.doi.org/10.1016/j.ijhydene.2014.05.069 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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phases are mainly formed of transition metals or lanthanides [3]. A low gravimetric capacity is also a restriction to the storage of intermittent energies where the amount of energy involved can be extremely high and consequently a stable and efficient storage system becomes a must [4]. To reach the demanded targets for the application in these fields, research efforts have been made to develop interstitial, binary or even more complex hydrides capable to store and release hydrogen at temperature and pressure compatible with the different applications [5,6]. Mg-based materials are good candidates for these kinds of applications [7e11]; Mg (non toxic and inexpensive) shows a hydrogen storage capacity of 7.6 wt%, but the major obstacle is its high H2 desorption temperature (>300 C). In practice, the sorption reactions are quite slow and require temperatures of at least 350 C for fast hydrogenation/dehydrogenation processes [12]. In order to decrease the desorption temperature of MgH2 many research groups added other hydrides to MgH2 in order to form complex hydrides with lower hydrogen desorption temperature or to use catalysts and activators to improve the hydrogen release kinetics (to provide hydrogen ondemand), and the reversibility of the reaction for a better recycling of the material [13e16]. Original methodologies as the chemical fluid deposition process in supercritical fluids for the preparation of nanosized plot of metals (Ni and Pd) on the surface of Mg have been employed in order to improve the hydrogen sorption cyclability [17]. Films of Nb-doped Mg deposited by magnetron sputtering with thickness 10e20 mm on graphite wafers strongly act on the dehydrogenation velocity as shown by Bazzanella et al. [18]. Transition metals have shown to be highly effective catalysts due to their high affinity towards hydrogen [14]. Tests performed on MgH2 enriched with nanosized-Ni particles were reported by Hanada et al. [15] in a temperature range of 150e250 C. MgH2eV samples showed a desorption temperature of 235 C, as reported by Liang et al. [15], and also transition metals like Ti, and Nb, mixed to magnesium by ballmilling, act as catalyst to improve the hydrogen sorption kinetics [15,19]. Moreover, MgeLaNi5 revealed a relatively low working temperature (100e150 C) [20], and activated ZreNi alloys [21] present a non negligible effect in enhancing the desorption kinetics (at 250 C) when intimately mixed to MgH2. NieCo, FeeNi, AleNi, YeCe bimetallic catalysts are also considered to improve the desorption kinetics of MgH2 [22e25]. The stability of the commercial MgH2 powder, the possibility to obtain premanufactured commercial metal nanoparticles, and the fact that such a system can be prepared under inert atmosphere make relatively easy to reproducibility of the preparation procedure at industrial scale. For these reasons, in the present work commercial transition metal nanoparticles of Fe, Co, Ni, Cu, and Zn were added to MgH2 by ball-milling to improve the kinetics of hydrogen release and the reversibility during successive absorption/ desorption cycles.
Experimental Materials preparation Magnesium hydride powder (98%) was purchased from Alfa Aesar, while the Fe, Co, Ni, Cu, and Zn nanoparticles were purchased from Nanostructured & Amorphous Materials Inc. Ball-milling of 10 wt% metal nanoparticles with MgH2 was performed using a Retsch P100 planetary ball miller operating under inert atmosphere of argon and equipped with grinds of 12 and 50 mL in volume. MgH2 powder was previously reduced in size and later-on intimately mixed with the metal nanoparticles by ball-milling at a rotation speed of 300 rpm for 4 h, and a ball/powder weight ratio close to 20:1. The rotation of the cell in the planetary ball miller was paused every 30 min to limit the temperature increase in the milling cell. The powders were strictly handled in a glove box under argon atmosphere in order to prevent any oxidation. The prepared composites were labeled as MgH2-10-Menano with Me ¼ Fe, Co, Ni, Cu, Zn.
Materials characterisation X-ray diffraction (XRD) The phase composition and crystallites state of the samples were controlled by X-ray diffraction using a Bru¨ker D5005 powder diffractometer where the sample is fixed while the Xray tube (Cu Ka1 þ a2; l ¼ 0.154184 nm) and the detector rotate. X-ray diffraction patterns were recorded between 20 and 90 (2q) with a step size of 0.02 and acquisition time of 8 s/ step. In order to increase the signal/background ratio, a zero background holder was used. The sample was deposited on a resin support and protected by kapton-tape in the glove box under inert atmosphere.
Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was performed using a Philips 5800 SEM electron microscope. The samples were deposited on carbon tape and metallized by sputtering. A gold film ensured a good conductivity for the observation.
Transmission electron microscopy (TEM) The recording of transmission electron micrographs was carried out using a JEOL 2010 LaB6 equipment operating at 200 kV. The samples finely grinded and handled under inert atmosphere were incorporated in a resin matrix to avoid any contact with air.
Hydrogen storage properties Temperature programmed desorption (TPD) Hydrogen desorption temperatures and amounts were measured for all the samples by means of a temperature programmed desorption device. The experiments were performed using a TPD/O/R-1100 instrument (Thermofisher) equipped with a quartz reactor with a porous septum (ca. 8 mm i.d.) and a filter filled with soda lime. The samples (10e30 mg) were heated at different temperature increasing rates (1, 2, 3, 5, 10 C/min) from room temperature to 500 C under an argon flow of 20 mL/min. The H2 released was
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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detected by a thermal conductivity detector (TCD), and the corresponding volume calculated after calibration of the hydrogen desorption peak areas by injections of known amounts of pure hydrogen.
Hydrogen absorption/desorption isotherms, absorption kinetics and absorption/desorption cycles The hydrogen absorption/desorption isotherms were measured in the 250e400 C temperature range by solidegas reaction in a volumetric apparatus, High Pressure Volumetric Analyzer (HPVA), from Micromeritics, USA. The samples (100e200 mg) were weighted and transferred in the measurement cell in a dry-box under Ar atmosphere to avoid any contamination and oxidation. High purity hydrogen (purity > 99.9999%) was supplied by a cylinder at high pressure. Hydrogen isotherms were performed at 250, 300, 350, and 400 C. The absorption kinetics were measured at 350 C exposing the sample to an initial pressure of 20 bar and a fixed absorption time of 200 min. Hydrogenation/dehydrogenation cycles at 350 C were also performed to verify the samples stability.
Results and discussion MgH2 based samples To obtain nanocrystalline Mg-based materials is crucial to prepare composites with improved hydrogenation kinetics by facilitating hydrogen diffusion into the solid matrix [26,27]. In order to obtain a particle size reduction a permanent deformation of the material must occur. The energy provided for particle interactions must overcome the elastic properties of the material thus allowing the fracture to occur and leading ultimately to size reduction and permanent deformation. Metallic magnesium reveals to be a too ductile material to be easily ball-milled without the formation of large agglomerates by contact welding. To avoid this problem the preparation of the samples used in the present research has been performed starting from commercial MgH2, which is a brittle material that consequently absorbs relatively little energy prior to ballmilling and that can be easily reduced in size. To optimize the ball-milling conditions, the commercial MgH2 powder was ball-milled for different time of 30 min, 4 h, and 10 h, and analyzed by XRD to determine the crystallite size obtained by the Scherrer formula [28]. The crystallite size was strongly reduced from 29 to 14 nm after 30 min milling, while for longer treatment time (4 and 10 h ball-milling) no real size decreasing was detected. 4 h ball-milling time was chosen to guaranty a sufficient low size of the crystallites. In the aim of improving the hydrogen sorption kinetics by activation of magnesium hydride, various metal nanoparticles of iron, copper, nickel, zinc, and cobalt were intimately mixed to the previous ball-milled MgH2 (see procedure in Section 2.1). No new phases' formation was observed by XRD analysis after the ball-milling preparation step and only the diffraction peaks of MgH2 (27.1, 35.4, 39.5, 54.1 2q [29]), metal particles (Fe ¼ 44.5, 65.2, 82.9 2q [30,31]; Co ¼ 44.2 2q [32]; Ni ¼ 44.1, 50.3, 75.3 2q [30.31]; Cu ¼ 43.1, 49.9, 73.6 2q [30]; and Zn ¼ 43.0 2q [33]) and traces of metallic Mg were
Fig. 1 e XRD patterns of the MgH2 based composites.
identified (Fig. 1). As already known the synthesis of magnesium complex hydrides can be problematic due to the difference in vapour pressure and melting point between magnesium and transition metals. The high energy ballmilling carried on during 4 h was effective to well disperse the metal nanoparticles into the MgH2 matrix (see SEM/TEM images for the iron and nickel containing samples in Fig. 2, and the SEM images for the Co, Zn, and Cu-containing samples in the “Supplementary data” published online alongside the electronic version of this article) without formation of ternary metal hydrides [27], nor bimetallic compounds that require much higher temperatures, hydrogen pressure, and long reaction time up to several days [22,34e37]. MgH2 was mainly present in the beta form and only very small X-ray diffraction peaks of the g-phase were detected, showing the distortion of the b-MgH2 phase during milling. The formation of orthorhombic g-phase during milling has been already described in previous articles [38]. Some small peaks corresponding to Mg (36.8 2q [29,31]) and MgO (62.4 2q [30,31]) phases were also detected, due to the oxidation of the impurities of metallic Mg contained in the commercial MgH2 powder during handling of the samples, milling, etc [39]. In addition, we can observe that the MgH2 crystallite size (calculated by the Scherrer formula) was maintained in the 10e12 nm range for all the samples containing the transition metal nanoparticles.
Performance of the catalysts Influence of transition metal nanoparticles addition on the hydrogen desorption temperature and kinetics TPD experiments permitted to make a first comparison among the different metal nano particles additives by comparing the hydrogen desorption temperature peaks of the MgH2-10Menano samples to that of pure 4 h ball-milled MgH2 [12] (Fig. 3). All the samples containing metal nanoparticles present hydrogen desorption peaks at lower temperature than that of pure MgH2 (maximum of the peak centered at 386 C). The best results were observed for the nickel containing sample that shows a unique sharp TPD peak centered at 203 C and a hydrogen release starting at 130 C. Relatively low initial desorption temperatures were observed also for the Co, Fe and
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Fig. 2 e Scanning electron micrographs of the MgH2 based composites containing 10 wt% of iron and nickel nanoparticles.
Cu containing samples (around 170 C). For these three samples (MgH2-10-Conano, MgH2-10-Cunano, and MgH2-10-Fenano) we can observe more complex TPD profiles composed of two main peaks, with the highest desorption peak temperature returning to the baseline (end of the desorption process) only around 400 C. This desorption profile is characteristic of two types of MgH2, one intimately mixed with the metal nanoparticles and presenting improved dehydrogenation kinetics [26], and a second with MgH2 keeping its original behavior characterized by a high desorption temperature. Finally, the addition of Zn nanoparticles did not improve the hydrogen desorption kinetics of MgH2 and for the MgH2-10-Znnano sample a well defined single desorption peak was observed at 368 C. This slight decrease in temperature, compared to the desorption peak of pure MgH2, can be related to the longer ball-milling time required by the mixing of the metal
nanoparticles to the already milled pure MgH2 in 4 supplementary hours, and not to a catalytic effect. The quantity of hydrogen released from the various samples is reported in Table 1 (TPD section). For each sample the amount of hydrogen evolved is lower than the theoretical amount of 862 mL/gMgH2, probably due to a partial decomposition of MgH2 during the ball-milling, in which the temperature can reach values high enough to activate the desorption process. TPD analysis were performed for each sample at different heating rates (from 1 to 10 C/min) and the activation energies reported in Table 1 were calculated applying the Kissinger equation [40]. This equation allows the determination of the activation energy for a decomposition reaction regardless to the reaction order. The linearization of the Kissinger equation in logarithmic form is reported as follow: ln
Fig. 3 e TPD curves obtained for all the samples at an increasing temperature rate of 2 C/min.
a T2max
!
Ea RC0 þ ln 2 ¼ Tmax Tmax Ea
! (1)
with R being the gas constant, a the heating rate, C0 the reaction order, and Tmax the temperature at the maximum of the TPD peak. The activation energy (Ea) values are obtained from the slope of the straight lines interpolating the data obtained by plotting lnða=T2max Þ versus 1=T2max for each sample. All the added metal nanoparticles were able to decrease the activation energies for the hydrogen desorption reaction to a much higher extent than similar systems reported in the literature for which the lower Ea was estimated at 91 kJ/mol [14]. The lowest value of 72 kJ/mol was observed for MgH2-10-Ninano, according to the values given by Shahi et al. [31]. The activation energy values decrease in the range 35e55 kJ/mol by addition of metal nanoparticles if compared to the 121 kJ/mol value, calculated for the pure ball-milled MgH2.
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Table 1 e Data obtained by temperature programmed desorption and high pressure volumetric isotherms for all samples. Sample
TPD Hydrogen released (mL)
MgH2-10-Ninano MgH2-10-Conano MgH2-10-Fenano MgH2-10-Cunano MgH2-10-Znnano MgH2
10.2 17.4 15.6 9.9 12.2 16.0
Hydrogen released (mL/gMgH2 )b 653 739 852 649 690 800
HPVA Activation energy (kJ/mol)c
72 75 86a (66) (67)a 73 84 121
Hydrogen adsorbed (mL/ gMgH2 )d 250 C
300 C
350 C
760 578 484 680 518 831
764 608 674 725 635 831
772 630 718 783 643 827
DHf (kJ/mol)e
75 71 75 62 67 73
The standard formation enthalpy of MgH2 is 75 kJ/mol [42]. a TPD presenting 2 peaks of desorption. b Vmax =gMgH2 ¼ 852 mL corresponding to 7.6 wt%. c Calculated from Kissinger equation after TPD data acquisition at 1, 2, 3, 5 and 10 C/min. d Measured at 25 bars. e Calculated from Van't Hoff equation after performing isotherms at 250, 300, and 350 C for all MgH2-10-Menano samples and at 300, 350, 400 C for pure MgH2.
Fig. 4 e High pressure volumetric isotherms obtained at different temperatures. Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Hydrogen absorption/desorption isotherms, absorption kinetics and absorption/desorption cycles Fig. 4 shows the high pressure isotherms acquired for all samples at 250, 300, and 350 C up to 30 bar of hydrogen pressure and after a pretreatment step of 4 h, at 350 C under vacuum. For pure MgH2 the isotherms were performed at higher temperatures of 300, 350, and 400 C after a pretreatment performed at 400 C. Pure MgH2 adsorbed the highest amount of hydrogen, 830 mL per g of material (see Table 1, HPVA section), independently of the temperature, and with well defined isotherms displaying a flat plateau. Among the MgH2-10-Menano samples, MgH2-10-Ninano is the only one presenting an adsorbed hydrogen volume that remains constant for the isotherms acquired at different temperatures (around 765 mL/gMgH2 ). For all samples except MgH210-Cunano hydrogen absorption/desorption isotherms present a flat plateau and larger hysteresis than pure MgH2 isotherm. The plateau of the 350 C isotherm of the MgH2-10-Cunano sample is not completely flat and presents a change of slope centered around 23 and 18 bar on the absorption and desorption branches, respectively. This peculiar behavior has been confirmed by performing the same experiment three times and can be related to the presence of two different MgH2 populations, more and less activated by the presence of copper, as confirmed by the presence of two desorption peaks in the TPD analysis discussed in Section 3.2.1. For the Co, Fe, Cu, and Zn nanoparticles containing samples the quantity of adsorbed hydrogen increases with absorption performed at higher temperature (see Fig. 4 and the corresponding values reported in Table 1). For all the nanoparticles containing samples (with exception of MgH2-10-Znnano) the beginning of the absorption plateau is located in the 6e6.4 bar range for the 350 C isotherms; this value is close to that measured for pure MgH2 (5.8 bar) and confirms the presence of MgH2 as only hydrogen adsorbing species, excluding the in-situ formation of other hydride alloy phases [41]. Surprisingly, the addition of Zn shows an inhibitor effect on the absorption/desorption performances of MgH2. The MgH2-10-Znnano sample is characterized by isotherms with a very large hysteresis and an absorption plateau (for the 350 C isotherm) starting only at 7.8 bar. The formation enthalpies were calculated by means of Van't Hoff equation: Peq DH DS ¼ ln RT R P0
Fig. 5 e Absorption/desorption cycles for MgH2-10-Ninano and MgH2-10-Fenano samples.
enthalpy between the activated samples and pure MgH2 can be attributed to defects introduced during ball-milling [7,41]. The low values of formation enthalpy, 62 and 67 kJ/mol, measured for MgH2-10-Cunano and MgH2-10-Znnano samples, respectively, suggest the presence of a side reaction involving the Mg, H2 and Zn, Cu nanoparticle species (i.e. alloying, sintering) [7]. Moreover, the small difference between these values and the 75 kJ/mol formation energy of pure MgH2 indicates that only a small amount of the metal nanoparticles is involved in this side reaction process. Generally, when MgH2 is ball-milled with metal catalyst to form alloys [41] an activation procedure, consisting in successive absorption/desorption cycles, is necessary to obtain repeatable high pressure volumetric isotherm experiments. In the present work no alloys were formed, but to exclude the presence of any transient effect during the absorption cycles and to validate the calculated formation
(2)
After plotting lnðPeq =P0 Þ as a function of 1/T, the DH values are calculated from the slope of the obtained straight lines. The samples containing Ni, Fe and Co nanoparticles present enthalpy values in the 71 to 75 kJ/mol range that correspond to the experimental (73 kJ/mol, Table 1) and theoretical formation enthalpy value [42] reported for MgH2. Because the addition of metal nanoparticles can influence only the activation energy and not the enthalpy of the process, the obtained data for Ni, Fe and Co nanoparticles confirm the absence of intermetallic compounds formation [7] as already verified during the preparation step by XRD analysis, and as discussed in Section 3.1. The small differences in the values of formation
Table 2 e Hydrogen absorption times obtained by kinetics experiments performed at 350 C. Sample
MgH2-10-Ninano MgH2-10-Conano MgH2-10-Fenano MgH2-10-Cunano MgH2-10-Znnano MgH2
(H2%wt 80)a/(H2%wt total) at 20 bar 5.0/6.2 4.3/5.3 5.1/6.3 4.6/5.8 4.1/5.1 5.3/6.7
Time (min)a 20 bar 15 bar 10 bar 2.0 2.7 4.2 6.6 11.7 4.6
4.2 e e e e 7.5
6.1 7.7 9.1 15.3 e 18
a
Hydrogen absorption calculated at the time for which the pressure is P80 ¼ Pi e (PiPf) * 0,8.
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Fig. 6 e Hydrogen absorption kinetic curves for the MgH2-10-Menano and MgH2 systems performed at 350 C under 20 bar of hydrogen pressure (the hydrogen quantity is normalized per mass unit of MgH2 in each sample).
enthalpies deduced from the isotherms acquired at different temperatures on fresh samples, four successive absorption/ desorption cycles at 350 C were acquired on MgH2-10-Ninano and MgH2-10-Fenano samples, selected as example. The obtained isotherms are reported in Fig. 5; for both samples the four cycles are nearly superimposable thus confirming that no formation of new intermetallic compounds and hydrides occurs even by exposing the mixtures to hydrogen under high pressure and high temperature. The hydrogen absorption kinetics at 350 C was studied for all samples under 10, 15, and 20 bar hydrogen pressure. The samples were previously dehydrogenated under vacuum at 350 C for 4 h following the same pretreatment procedure as for the determination of high pressure volumetric isotherms. To render possible the comparison among the samples that present different extents of hydrogenation, the hydrogen absorption time necessary to attain 80% of the maximum capacity proper to each sample at the three pressures has been reported in Table 2. The absorption times are inversely proportional to the initial hydrogen pressure and the catalytic effect of Ni, Co, and Fe nanoparticles is evidenced by lower absorption times if compared to pure MgH2 sample. Accordingly to the previous observation, Zn and Cu nanoparticles exhibit an inhibitor effect and, at the highest pressure of 20 bar, the related absorption times are higher than those of pure MgH2, and respectively of 6.6, 11.7 and 4.6 min for MgH2-10-Cunano, MgH210-Znnano and pure MgH2. The hydrogen absorption kinetics curves acquired under 20 bar hydrogen pressure are reported in Fig. 6. For the most active catalysts, the main part of hydrogen is adsorbed in the first minute of the process and gives rise to a fast intake characterized by the steep section of the curve. After this first step, absorption keeps on with a slower rate up to the definitive hydrogen intake (described by the plateau of the curve). The maximum hydrogen absorption amounts (normalized by mass unit of MgH2) vary for the different samples, and never reach the maximum theoretical value of 7.6%.
Conclusion MgH2-10-Menano samples were prepared by ball-milling in Ar atmosphere. Thanks to the structural, morphological, dehydrogenation/hydrogenation properties measured by means of complementary techniques, we can confirm that there is no new phase formation between MgH2 and Ni, Fe, and Co nanoparticles even after several hydrogenation/dehydrogenation cycles. The Ni, Fe, and Co containing samples show good stabilities coupled with enhanced catalytic performances, as confirmed by lower activation energies than for pure ballmilled MgH2, no effect on the formation enthalpy values, and high rate of hydrogen absorption. Among them, the most performing sample in hydrogen desorption is MgH2-10-Ninano that presented a unique TPD desorption peak, the lowest hydrogen desorption activation energy (72 kJ/mol), and the lowest hydrogen desorption temperature (starting at 130 C). Zn and Cu nanoparticles act more like inhibitors than catalysts in the hydrogenation sorption process. Looking at the formation enthalpies the constitution of intermetallic compounds or the presence of other simultaneous reactive processes cannot be excluded at high hydrogen pressure. Finally, analyzing the obtained results and in particular the non-formation of intermetallic compounds for the most active catalysts, we can conclude that the contact surface between MgH2 and Ni, Fe, and Co nanoparticles is crucial to improve hydrogen desorption/absorption by increasing the hydrogen diffusion into the MgH2 matrix.
Acknowledgements AREVA and CNRS for H. Yu PhD thesis granting. The authors gratefully acknowledge the scientific services of IRCELYON, in particular Dr. G. Bergeret and F. Bosselet for the XRD analyses, and L. Burel for the SEM/EDX analyses.
Please cite this article in press as: Yu H, et al., Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.069
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Appendix A. Supplementary data [18]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.05.069.
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