Destabilization of LiAlH4 by nanocrystalline MgH2

international journal of hydrogen energy 34 (2009) 2333–2339

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Destabilization of LiAlH4 by nanocrystalline MgH2 Andrew W. Vittetoea,b, Michael U. Niemanna,b, Sesha S. Srinivasana,b,*, Kimberly McGrathc, Ashok Kumara,b, D. Yogi Goswamia,b, Elias K. Stefanakosa,b, Sylvia Thomasb a

Clean Energy Research Center, University of South Florida, Tampa, FL 33620, USA College of Engineering, University of South Florida, Tampa, FL 33620, USA c QuantumSphere Inc., 2905 Tech Center Drive, Santa Ana, CA 92705, USA b

article info

abstract

Article history:

In this work, we report the synthesis, characterization and destabilization of lithium

Received 22 December 2008

aluminum

Received in revised form

LiAlH4 þ nanoMgH2). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was

7 January 2009

obtained by solid-state mechano-chemical milling of the parent compounds at

Accepted 10 January 2009

ambient temperature. Nanosized MgH2 is shown to have greater and improved hydrogen

Available online 10 February 2009

performance in terms of storage capacity, kinetics, and initial temperature of decompo-

hydride

by

ad-mixing

nanocrystalline

magnesium

hydride

(e.g.

sition, over the commercial MgH2. The pressure–composition isotherms (PCI) reveal that Keywords:

the destabilized LiAlH4 þ nanoMgH2 possess w5.0 wt.% H2 reversible capacity at T  350  C.

LiAlH4

Van’t Hoff calculations demonstrate that the destabilized (LiAlH4 þ nanoMgH2) complex

Hydrogen storage

materials have comparable enthalpy of hydrogen release (w85 kJ/mole H2) to their pristine

Mechano-chemical synthesis

counterparts, LiAlH4 and MgH2. However, these new destabilized complex hydrides exhibit

Complex hydrides

reversible hydrogen sorption behavior with fast kinetics.

Dehydrogenation

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Nanocrystalline

1.

Introduction

For the wide-spread use of PEM Fuel Cell powered vehicles to become a reality, it is necessary for light weight, inexpensive reversible hydrogen storage systems to be developed and deployed [1–4]. The discovery of using NaAlH4 for reversible onboard hydrogen storage may prove to be an insurmountable obstacle in attaining the DOE 2010 and FreedomCAR technical targets [5]. This is due to the low usable hydrogen storage capacity of NaAlH4 (5.4 wt.%), which is well below the set DOE goals [6–9]. Alternatively, LiAlH4 possesses a high reversible hydrogen capacity of 10.5 wt.% and could prove to be ideal for use in hydrogen storage systems [10–12]. Although it was demonstrated to possess a slight reversibility of 0.8 wt.% by

doping LiAlH4 with LaCl3 [13] or VCl3 [14], to date no conclusive demonstration of reversibility has been reported. However, it is theorized that through destabilization of the compound, reversibility can be realized [15–19]. In general, LiAlH4 has a three-stage decomposition, as represented by the following equations: 3LiAlH4 / Li3AlH6 þ 2Al þ 3H2 (w150  C)

Li3AlH6 / 3LiH þ Al þ 3/2H2 (w180  C)

3LiH þ 3Al / 3LiAl þ 3/2H2 (>400  C)

(i)

(ii)

(iii)

* Corresponding author. College of Engineering, University of South Florida, Tampa, FL 33620, USA. Tel.: þ1 813 974 4787; fax: þ1 813 974 2050. E-mail address: [email protected] (S.S. Srinivasan). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.025

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Reaction (i) approximately releases 5.3 wt.% hydrogen and reactions (ii) and (iii) both decompose approximately 2.6 wt.% each. However, reaction (iii) is generally not considered usable because of its high temperature for decomposition. It is also generally known that pristine MgH2 theoretically can store a content of w7.6 wt.% hydrogen [20]. However, so far, magnesium hydride-based materials have limited practical applications because both hydrogenation and dehydrogenation reactions are very slow and relatively high temperatures are required [3]. Magnesium hydride forms ternary and quaternary hydride structures by reacting with various transition metals (Fe, Co, Ni, etc.) which often possess improved kinetics [21]. Moreover, the nanoscale version of these transition metal particles offers an additional hydrogen sorption mechanism via its active surface sites [20,22]. Research has in recent years shown mechano-chemical milling to decrease the temperature of decomposition of LiAlH4 by increasing the surface area through particle size reduction. In this work we examine the destabilization effects of combining LiAlH4 with nanocrystalline MgH2 through the use of mechano-chemical milling, as it pertains to the problem of reversibility and kinetics.

2.

Experimental details

2.1.

Materials and methods

Starting materials such as LiAlH4 (95þ%, Sigma Aldrich) and MgH2 (98%, Alfa Aesar) are procured and used without further purification. High purity H2 (99.9999%), N2 (99.99%) and He (99.99%) are purchased from Airgas Inc. for the synthesis and analytical measurements. All chemical reactions and operations are performed in a nitrogen filled glove box. LiAlH4 and MgH2 with a 1:1 mole ratio were mixed in a stainless steel bowl (80 ml) and the lid sealed with a viton O-ring in the glove box. The bowl was then evacuated for at least 1 h to remove the residual oxygen and moisture down to ppm levels. A specially designed lid with inlet and outlet valves was used for this purpose. The mechano-chemical process employing high energy milling has been carried out with a Fritsch Pulverisette P6 planetary mono mill in an inert atmosphere. The milling parameters such as ball to powder weight ratio and milling speed are optimized to 20:1 and 300 rpm, respectively. Milling duration has been varied as 1 h, 2 h, and 5 h. The mechano-chemically processed complex hydride mixture, LiAlH4 þ MgH2, was immediately transferred to the glove box for further characterization measurements, ensuring the sample was not exposed to the atmosphere. In the next batch of samples, pre-ball milled (12 h) MgH2 under hydrogen atmosphere was prepared, hereafter called nanoMgH2. An equal amount (1:1 mole ratio) of LiAlH4 was milled with nanoMgH2 under positive hydrogen pressure for 2 h. Similar milling conditions were followed as previously mentioned. The mechano-chemically processed nanoparticulate complex mixture (LiAlH4 þ nanoMgH2) was immediately transferred to the glove box for further characterization measurements.

2.2.

Structural characterization: X-ray diffraction

The powder X-ray diffraction of the mechano-chemically milled complex hydride mixture, LiAlH4 þ MgH2, and the nano particle-size complex hydride mixture, LiAlH4 þ nanoMgH2, has been carried out by the Philips X’pert diffractometer with ˚ . The incident and diffraction Cu Ka radiation of l ¼ 1.54060 A slit width used for the measurements are 1 and 2 , respectively. The incident mask of 10 mm was used for all the samples. The sample holder has been covered with polyethylene tape (foil) with O-ring seal in an N2 filled glove box in order to avoid the O2/moisture pickup during the XRD measurements. The diffraction from the Parafilm tape was calibrated without the actual sample and found to be occurring at the 2q angles of 21 and 23 . The XRD phase identification and particle size calculation have been carried out using PANalytical X’pert Highscore software, version 1.0f with built-in Scherer calculator.

2.3. Simultaneous thermogravimetric and differential scanning calorimetric measurements The simultaneous DSC and TGA (SDT) analysis pertaining to the weight loss and the heat flow for the reaction enthalpy during thermal decomposition of the complex hydride mixture, LiAlH4 þ MgH2, and the nano particle-size complex hydride mixture, LiAlH4 þ nanoMgH2, was performed by using the TA instrument’s SDT-Q600 analytical tool. The calibration of SDT was performed in four steps with empty pan and standard sapphire disc. The four calibration subroutines such as TGA weight, DTA baseline, and temperature and DSC heat flow were carried out before an actual measurement of the sample. A pre-weighed sample was loaded into the ceramic pan and covered with the ceramic lid inside the glove box to prevent the moisture from getting into the sample during transfer. The ramp rate of 5  C/min was used for all the measurements. TA’s Universal Analysis 2000 software program was used to analyze the TGA and DSC profiles.

2.4.

Temperature programmed desorption

Temperature programmed desorption (TPD) was carried out by Quantachrome Instrument’s Autosorb-1 equipment. A pre-weighed sample was loaded in between fiber wool and the tube was sealed with a filler rod. The carrier gas used for the TPD measurement was nitrogen. The thermal desorption profiles are recorded and analyzed using TPRWIN software package.

2.5. Hydrogenation and dehydrogenation: pressure–composition isotherms, life-cycle kinetics The volumetric hydrogen sorption measurements are of paramount importance in understanding the hydrogen storage behavior of LiAlH4 þ MgH2 complex hydrides. High pressure hydrogen sorption was executed at different temperatures (250, 300, 350  C) with a pre-calibrated reservoir. These isothermal volumetric measurements were carried out by Hy-Energy’s PCTPro 2000 sorption equipment. This fully automated Sievert’s type instrument uses an internal PID

international journal of hydrogen energy 34 (2009) 2333–2339

controlled pressure regulator with maximum pressure of 170 bars. It also includes five built-in and calibrated reservoir volumes of 4.66, 11.61, 160.11, 1021.30 and 1169.80 ml. The volume calibration with and without the sample was performed at a constant temperature with an accuracy of 1  C using helium. The software subroutines for hydrogen purging cycles, leak test, kinetics, PCT and cycling were performed by the HyDataV2.1 Lab-View program. The data collected for each run were analyzed using the Igor Pro 5.03 program with a built-in HyAnalysis Macro.

2.6. Microstructural characterization – scanning electron microscopy The microstructural characteristics or surface morphologies of LiAlH4 þ nanoMgH2 in the different stages (before and after hydrogen sorption) were observed by a Hitachi S800 scanning electron microscope (SEM). A fixed working distance of 5 mm and a voltage of 25 kV were used. Sample preparation for the SEM measurement was carried out inside the glove box by covering the sample holder with parafilm for minimal exposure to oxygen while transferring into the secondary emission chamber. EDAX Genesis software was used to analyze the SEM images.

3.

Results and discussion

3.1. X-ray diffractions of LiAlH4 þ MgH2 and LiAlH4 þ nanoMgH2 – structural characterization Fig. 1 represents the powder X-ray diffraction patterns of plain LiAlH4 and MgH2 before and after being subjected to mechanical milling. The presence of both LiAlH4 and MgH2 phases has been indexed and no other impurities were found in these samples. In comparing the XRD profiles of commercial MgH2 and nanoMgH2 (w12 h of ball milling under H2 ambient), significant changes have been observed such as (i) decrease in the relative intensities of XRD peaks (ii) increase

Fig. 1 – X-ray diffraction patterns of (a) LiAlH4, (b) mechanochemically milled LiAlH4, (c) MgH2, (d) mechano-chemically milled MgH2, (e) LiAlH4 D MgH2, and (f) LiAlH4 D nanoMgH2.

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in the full width at half maximum (FWHM), (iii) reduced average crystallite sizes and (iv) a slight shift in the Bragg orientation. The average crystallite size of nanoMgH2 was 20–30 nm whereas the commercial MgH2 was of the order of few micrometers. Similarly, both the complex hydride mixtures LiAlH4 þ MgH2 and LiAlH4 þ nanoMgH2 prepared by 2 h of mechano-chemical milling under hydrogen atmosphere show the presence of the intimate mixed phases of LiAlH4 and MgH2 with no other impurities. The main differentiation between LiAlH4 þ MgH2 and LiAlH4 þ nanoMgH2 was the reduction in the relative intensities of the peaks and thus caused the reduction in overall crystallite sizes. This is also confirmed when comparing the relative intensity of the Parafilm peaks to the intensity of the sample peaks.

3.2. Thermogravimetric and temperature programmed desorption analyses of LiAlH4 þ MgH2 – optimization of ball milling duration The thermogravimetric analysis of the complex hydride mixture (LiAlH4 þ MgH2) mechano-chemically prepared with different milling durations is shown in Fig. 2. An actual hydrogen storage capacity of approximately 3.9–4.2 wt.% at 200  C was obtained for LiAlH4 þ MgH2 milled for 2 h. The capacity was lower when mechano-chemically milled for a lesser duration due to the lack of a homogenous mixture formation. However, the capacity also decreases as the milling duration increases. This is apparently due to the partial release of hydrogen during the extended milling process. The TGA profiles in Fig. 2 clearly show that the on-set of hydrogen decomposition occurs at around 118  C following the stepwise reactions (i) and (ii). Fig. 3 shows the thermal programmed desorption (TPD) curves of the complex hydride mixture, LiAlH4 þ MgH2, mechano-chemically milled for durations of 1 h, 2 h, and 3 h. As TPD signal is proportional to the quantity of molecules desorbed as the temperature increases, it can be seen in the graph that for both the 2 h and 3 h sample, the decomposition starts early (w100  C) and the 1 h sample starts at w120  C, 2  C of difference from the TGA data (118  C). It is interesting to note

Fig. 2 – Thermogravimetric analysis profiles of LiAlH4 D MgH2 mechano-chemically milled for durations of 1 h, 2 h, 3 h, and 5 h.

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international journal of hydrogen energy 34 (2009) 2333–2339

Fig. 3 – Temperature programmed desorption profiles of LiAlH4 D MgH2 mechano-chemically milled for durations of 1 h, 2 h, and 3 h.

the difference in the peaks of the reaction for the different mechano-chemical milling durations. The 1 h sample clearly shows two distinct peaks of decomposition at w148  C and w180  C; however, both the 2 h and the 3 h samples show three noticeable peaks at w132  C, w150  C, and w174  C for the 2 h sample and w142  C, w173  C, and w176  C for the 3 h sample. It is possible to surmise that this is evidence of a new mixture being formed. Hence from both the TGA and TPD analysis, it is confirmed that the optimum ball milling duration of 2 h is necessary to obtain a three-step hydrogen decomposition reaction with early on-set temperature and higher hydrogen release characteristics. Further experiments were carried out for the samples LiAlH4 þ MgH2 and LiAlH4 þ nanoMgH2 with optimized ball milling duration of 2 h.

3.3. Simultaneous DSC, TGA and TPD analyses of LiAlH4 þ MgH2 and LiAlH4 þ nanoMgH2 – comparison of thermal decomposition performance of nano over commercial MgH2 The simultaneous DSC and TGA profiles of the commercial (LiAlH4 þ MgH2) and nanosized complex hydride mixture (LiAlH4 þ nanoMgH2) samples mechanically milled for 2 h are shown in Fig. 4. The LiAlH4 þ nanoMgH2 material exhibits a greater hydrogen weight loss of about 5.2 wt.% (w200  C), at least 1.0 wt.% higher capacity than the commercial grade sample (LiAlH4 þ MgH2). The DSC profiles of these complex hydrides reveal endothermic transitions due to hydrogen decomposition according to the reactions (i)–(iii). To further confirm the superiority of the nanosized complex hydrides (LiAlH4 þ nanoMgH2), they were subjected to TPD measurements as previously mentioned and are shown in Fig. 5. The TPD profile of the nano complex hydride (LiAlH4 þ nanoMgH2), shows a decrease in the temperature required for decomposition, from 100  C to only 90  C. The area between the baseline and the signal corresponds to the amount of hydrogen desorbed; therefore, a higher downward increase of the TPD signal translates into greater amounts of hydrogen desorption. Overall, the simultaneous DSC, TGA and TPD profiles

Fig. 4 – Comparison of TGA/DSC profiles of LiAlH4 D MgH2, and LiAlH4 D nanoMgH2 mechano-chemically milled for duration of 2 h.

undoubtedly confirm that the nanosized complex hydrides (LiAlH4 þ nanoMgH2) are superior over the commercial samples (LiAlH4 þ MgH2) in terms of kinetics, decomposition temperature and hydrogen storage capacity.

3.4. Hydrogenation–dehydrogenation of LiAlH4 þ nanoMgH2 – life-cycle kinetics and pressure–composition isotherms Fig. 6(a) represents the hydrogenation–dehydrogenation cycling kinetic curves of LiAlH4 þ nanoMgH2 mechanically milled for 2 h. This material was subjected to repeated hydrogen absorption (w80 bars) and desorption (against vacuum) for two different temperatures, 300 and 350  C. Reversible hydrogen uptake and release of w3–5 wt.% was observed at these temperatures. However, two noticeable facts have been found (Fig. 6(b)) while increasing the temperature of sorption from 300 to 350  C; (i) increasing the rate of hydrogen desorption by 15-fold and (ii) decreasing

Fig. 5 – Comparison of TPD profiles of LiAlH4 D MgH2 and LiAlH4 D nanoMgH2 mechano-chemically milled for durations of 2 h.

international journal of hydrogen energy 34 (2009) 2333–2339

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Fig. 7 – Pressure–composition isotherms of LiAlH4 D nanoMgH2 at 250, 300 and 350 8C.

pristine compounds such as LiAlH4 and MgH2. Though the thermodynamic value of these materials (enthalpy of reaction) remains unchanged, the new destabilized complex hydride, LiAlH4 þ nanoMgH2 possesses higher reversibility in hydrogen sorption behavior with rapid kinetics. Further improvement is being currently carried out to dope the LiAlH4 þ nanoMgH2 system with nanocatalysts and the results will be forthcoming.

3.5. Microstructural characterization of LiAlH4 þ nanoMgH2 – SEM explorations Fig. 6 – (a) Hydrogenation and dehydrogenation cycling kinetics of LiAlH4 D nanoMgH2 at two different temperatures, 300 and 350 8C. (b). Hydrogenation and dehydrogenation kinetic curves of LiAlH4 D nanoMgH2 at temperatures, 300 and 350 8C; 15-fold increase in kinetics was observed at 350 8C.

the hydrogen absorption capacity by 1.0 wt.%. These features agree well with many of the typical complex hydrides (e.g. Tidoped NaAlH4) where the hydrogen absorption proceeds with maximum hydride conversion at lower temperatures, whereas the dehydrogenation requires higher temperature ranges. The hydrogen desorption pressure–composition–temperature (PCT) profiles of LiAlH4 þ nanoMgH2 at three different temperatures, 250, 300 and 350  C are demonstrated in Fig. 7. It can be clearly seen that by increasing the temperature from 250 to 350  C, an increase in the hydrogen storage capacity from 4.0 to 5.5 wt.% was observed. Additionally, the plateau pressure at 350  C (the flat region of the PCT isotherm) increases 3–4 times when compared to that at 300  C and more than 10 times in comparison to the one observed at 250  C. The three phase regions, a (hydrogen in solid solution with metal), a þ b (metal–hydrogen at equilibrium), b (metal hydride phase) are distinctly observed in the PCT isotherms of the samples treated at 350  C. From this figure and the data obtained from PCT, a Van’t Hoff plot was drawn to estimate the enthalpy, DH, of hydrogen desorption and is depicted in Fig. 8. The enthalpy of hydrogen desorption from LiAlH4 þ nanoMgH2 (DH w85 kJ/mol H2) was comparable to the

The microstructural characterization using SEM was performed for the complex hydride LiAlH4 þ nanoMgH2 after milling for 2 h and is shown in Fig. 9(a). From this micrograph, it can be observed that two different grains, (larger LiAlH4 particles and smaller nanoMgH2 grains) intimately mixed during the mechano-chemical process. After the many hydrogen absorption–desorption experiments at different temperatures, these LiAlH4 þ nanoMgH2 hydrides have been carefully examined under the scanning electron microscope. Fig. 9(b) shows the SEM image of LiAlH4 þ nanoMgH2 after hydrogen cycling and found to be relatively smaller and

Fig. 8 – Van’t Hoff plot of LiAlH4 D nanoMgH2 as evaluated from the PCT isotherms.

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from the TPD concurred with that of the TGA in that the nano particle-size MgH2 combined with the LiAlH4 had a much improved performance in both capacity and initial temperature of decomposition than that of the as-received MgH2 combined with the LiAlH4. The PCT clearly exhibited that the destabilized LiAlH4 combined with the nano particle-size MgH2 to have w5.0 wt.% reversibility with moderate kinetics at temperatures, 300 and 350  C. Van’t Hoff calculations reveal that destabilized LiAlH4 þ nanoMgH2 have a comparable enthalpy of reaction and also possess higher reversible sorption behavior and rapid kinetics. Many possibilities for improvements are currently being carried out. A complete study of the destabilized LiAlH4 þ nanoMgH2 sample with the addition of nano catalytic doping could prove to increase the hydrogen capacity even more and to decrease the initial temperature of decomposition. A further investigation into different mechano-chemical milling times and procedures could also prove to be of profitable use in an endeavor to even further optimize the kinetics, hydrogen capacity, and decomposition temperature.

Acknowledgements Authors wish to acknowledge the US Department of Energy (Hydrogen Fuel Initiative code: DE-FG36-04G014224) for funding and the National Science Foundation (Research for Experience for Undergraduates Grant No. 0552864) for funding. USF Nanomaterials & Nanomanufacturing Research Center for analytical studies is gratefully acknowledged.

references Fig. 9 – SEM micrographs of LiAlH4 D nanoMgH2. (a) Mechano-chemically milled for 2 h and (b) after dehydrogenation–rehydrogenation cycling.

uniform grains with highly porous matrix. This may be due to the effective hydrogen uptake and release from the host structure and its associated pulverization effects.

4.

Conclusions

With a large hydrogen capacity (10.5 wt.%), LiAlH4 could prove to be an ideal medium for hydrogen storage, provided reversibility could be realized. The destabilization of LiAlH4 was achieved through particle size reduction and by lattice substitution through ad-mixing of the matrices of LiAlH4 with MgH2. From the TGA data it is evident that reducing the particle size of MgH2 by mechano-chemical milling for 12 h had the effect of increasing the surface area, creating more area for the hydrogen to interact, thereby increasing the hydrogen capacity and decreasing the temperature for decomposition. The XRD demonstrates the absence of the formation of an alloy between the LiAlH4 and the nanoMgH2. However, the XRD and SEM do indicate the co-existence of LiAlH4 and MgH2 as an intimate complex mixture. The results

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