NaAlH4 system

JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011, P. 599

Hydrogen storage properties of the CeH2 doped Li-Mg-N-H/NaAlH4 system ZHANG Xugang (ᓴᯁ߮), LI Zhinian (ᴢᖫᗉ), WANG Shumao (⥟ᷥ㣖), MI Jing (㉇ 㦕), JIANG Lijun (㩟߽‫)ݯ‬, LÜ Fang (৩ 㢇), LIU Xiaopeng (߬ᰧ吣) (Department of Energy Materials and Technology, General Research Institute for Non-Ferrous Metals, Beijing 100088, China) Received 9 November 2010; revised 17 January 2011

Abstract: The mutual destabilization between complex hydrides and lithium amide has been comprehensively reported. In this paper, CeH2 doped Li-Mg-N-H/NaAlH4 composite was successfully synthesized by ball milling Li-Mg-N-H mixture and NaAlH4 in a molar ratio of 1:2. It was found that a total of 5 wt.% of hydrogen could be desorbed from the newly synthesized composite with a three-step reaction. Temperature-programmed-desorption (TPD) measurements showed that the composite ball milled for 10 min began to desorb hydrogen below 100 °C, which was about 75 °C lower than the pristine materials. XRD analysis revealed that NaAlH4 firstly reacted with LiH to yield Na2LiAlH6 and Al below 150 °C, then the newly developed Na2LiAlH6 reacted with Mg(NH2)2 to form NaH, Al, and Li2MgN2H2 in the temperature range of 180–250 °C. From 200 to 300 °C, the newly formed Al and Li2MgN2H2 reacted further to form Li2NH and some stable phase (AlN and Mg3N2). The H-cycling properties of the composite were further investigated by a standard Sievert’s type apparatus at 150, 200 and 250 °C, respectively. Finally, the reversibility of the newly synthesized composite was discussed. Keywords: Li-Mg-N-H; NaAlH4; ball milling; reversibility; rare earths

Exploiting a high efficient hydrogen storage system for the hydrogen fuel cell vehicles is still a huge challenge for human[1]. Although various kinds of candidates for hydrogen storage have been developed, none of them can meet all of the standards for on-board applications[2–4]. Among them, complex hydrides (such as LiAlH4, NaAlH4 and LiBH4), light metal hydrides (LiH and MgH2) and amides (LiNH2 and Mg(NH2)2) have been considered as promising materials due to their light weight and high capacity[3,5–8]. However, both of the high decomposition temperature and ugly kinetics make them all unpractical when used alone. It has been comprehensively reported that there exists a mutual destabilization among the following systems: metal hydrides-amides, complex hydrides-amides, complex hydrides-metal hydrides and even complex hydrides-metal hydrides-amides. Such a destabilization has been confirmed to be effective to decrease the decomposition temperature and accelerate the desorption kinetics[9–15]. Luo et al.[9] first reported a promising binary system 2LiNH2-MgH2, which was calculated to possess a practical desorption plateau pressure of 0.1 MPa H2 at 90 °C. Xiong et al.[10] reported that the mixture of LiAlH4-LiNH2 can desorb a total of 8 wt.% H2 with a low enthalpy of ~27 kJ/H2. A mixture of LiAlH4NaNH2 was also found to quickly release more than 5 wt.% H2 near ambient temperature[11]. Lu et al.[12] investigated the mixture of 2LiAlH4-LiNH2 and found that LiNH2 effectively destabilizes the LiAlH4 by reacting with LiH during the de-

hydrogenation process of LiAlH4. Liu et al.[15] discovered a total of 5.08 wt.% H2 release from the Na2LiAlH6-Mg(NH2)2 system and systematically investigated the detailed mechanism for dehydrogenation. Based upon the actuality that an individual Li-Mg-N-H system or NaAlH4 still could not satisfy the intended use and the understanding of the mutual destabilizing effect between amides and alanates, we tried to compound Li-Mg-N-H mixture with CeH2 doped NaAlH4 (which has a superior kinetic) to synthesize a more destabilized composite in this work, and investigated the hydrogen storage performances of the composite.

1 Experimental The raw materials NaAlH4 and LiNH2 were purchased from Sigma-Aldrich with claimed purities of 95% and 97%, respectively. The high purity MgH2 and CeH2 were prepared domestically by high-energy reactive ball milling. The Li-MgN-H mixture was prepared by performing a ball-milling for 10 h of 2:1.1 LiNH2/MgH2 mixture followed by three dehydrogenation/rehydrogenation cycles at 200 °C. Then the 2 mol.% CeH2 doped Li-Mg-N-H/NaAlH4 composite was synthesized by ball-milling the Li-Mg-N-H mixture and CeH2 doped NaAlH4 at a molar ratio of 1:2. Different ballmilling and time were applied to investigate the performance of Li-Mg- N-H/2NaAlH4 mixture. In this paper, all of the

Foundation item: Project supported by the Hi-Tech Research and Development Program of China (2009AA034400) and the National Basic Research Program of China (2010CB631305) under the Ministry of Science and Technology of China Corresponding author: LI Zhinian (E-mail: [email protected]; Tel.: +86-10-82241238) DOI: 10.1016/S1002-0721(10)60505-4

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milling process was performed on a Spex-8000 type miller, and all handling of the samples were performed in an argonfilled glove-box to keep the air contamination from the samples. The dehydrogenation behaviors were measured by a home-made temperature-programmed-desorption apparatus. Around 0.3 g of sample was loaded into the stainless steel tube reactor and gradually heated from room temperature to 300 °C at a ramp of 1 °C/min. Quantitative measurements on hydrogen release and uptake at specific temperature were determined by a standard Sievert’s type apparatus. Approximately 0.5 g of sample was used each time. The dehydrogenation pressure was fixed to be 0.1 MPa H2. Differential scanning calorimeter (DSC) was applied to measure the heat of reaction. 10 mg of samples were tested in a Netzsch STA 409 unit. Ar was applied as carrier gas. Heating rate was 5 °C/min. The phase structures were measured by X-ray diffraction (XRD) using X’Pert Pro MPD diffractometerwith Cu KĮ radiation. The XRD specimen holder was covered by a fine layer of parafilm (Pechiney Plastic Packing) to protect the sample from air or moisture induced contamination. X-ray photoelectron spectra were collected in an ESCALAB 250, using Al KĮ 1ph (120 W) as the X-ray source. The pass energy was 200 eV for survey and 30 eV for high resolution scans. Peak decomposition was obtained using mixed Gaussian/Lorentzian curves after applying Shirley background subtraction.

2 Results and discussion 2.1 Dehydrogenation properties It has been comprehensively reported that ball-milling is an effective means to prepare a homogeneous composite with low dehydrogenation/re-hydrogenation activation energy and prominent kinetics. We adopted the high-energy ball-milling hereby to synthesize the CeH2 doped Li-Mg-NH/NaAlH4 composite. According to previous studies, the hydrogen storage performances of NaAlH4 and Li-Mg-N-H mixture are both extremely dependent on the ball-milling time, so, it is necessary to focus on the influence of ball-milling time on the new milled composite. Fig. 1 shows the TPD curves of the milled composite with different ball-milling times. One can see that the ball-milling time significantly affects the initial decomposition temperature and the hydrogen release capacity. The composite without ball-milling represents a maximum of 5 wt.% hydrogen and an initial desorption temperature of 160 °C. Extending the ball-milling time from 10 to 30 and 60 min, the composite demonstrates a gradually reduced initial decomposition temperature from 160 to 110 °C and 70 °C, respectively. But the hydrogen release capacity correspondingly decreases from 5 to 1.9 wt.%. Furthermore, a clear multiple-step dehydrogenation which seriously depends on the ball-milling time can be observed in the milled composite. It can be conclusively seen that the 10 min milling

JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011

Fig. 1 TPD curves of the newly synthesized composite for different ball-milling time with a heating rate of 1 °C/min

composite exhibits an optimal performance, which starts to desorb hydrogen below 100 °C and releases a maximum of 4.5 wt.% hydrogen with a three-step reaction. 2.2 Dehydrogenation mechanism Considering the interesting conclusions resulted from the variable ball-milling time, we were stimulated to make out what had happened during the milling course. Fig. 2 shows the phase characteristics of the composite milled for 10 and 60 min, respectively. It was found that the sample milled for 10 min had already generated some new phases while NaAlH4, Mg(NH2)2 and LiH were remained for the main phases. The three new diffraction peaks located at 34.3°, 49.1° and 61.1°, corresponding to Na2LiAlH6 (PDF number: 42-848), and some other new weak peaks located at 38.2°, 44.9° and 65.3°, indicated the formation of Al, respectively. One can see that the main phases changed abruptly as the milling time was extended to 60 min. The newly formed Na2LiAlH6, in place of the raw material NaAlH4, has dominated the phase composition. And the peaks of LiH has correspondingly disappeared. Furthermore, the peaks of in-situ formed Al and the original reactant Mg(NH2)2 both demonstrated a weakened and broadened profile with the increasing of milling time, which may be due to the amorphization and particles refinement caused by high energy ball milling. We can therefore conclude that the original reactant NaAlH4

Fig. 2 XRD spectra of the composite milled for 10 and 60 min, respectively

ZHANG Xugang et al., Hydrogen storage properties of the CeH2 doped Li-Mg-N-H/NaAlH4 system

should react with LiH to yield Na2LiAlH6 and Al during milling course. Furthermore, the gradually soared pressure occurring in the mill jar reveals a hydrogen release. All of the results from phase changes are well evident of the following reaction (Eq. (1)): 3 2 NaAlH 4  LiH l Na 2 LiAlH 6  Al  H 2 (1) 2 A further specific reaction scheme looks more valuable for us to understand the hydrogen storage properties. In present work, we have made some efforts to reveal the chemical process of hydrogen desorption from the newly milled composite. As shown in Fig. 3, the TPD curve of the newly milled composite for 10 min is given, from which we can see about 4.35 wt.% of hydrogen is released in an apparent three-step reaction from 70 to 300 °C. Based upon the characteristic of TPD curves, the samples dehydrogenated at different stages were collected and subjected to XRD measurements. Fig. 4 shows the XRD spectra of the newly milled composite dehydrogenated at 150, 200, 250 and 300 °C, respectively. It can be seen that the composite milled for 10 min is composed of the original chemicals of NaAlH4, CeH2, Mg(NH 2) 2 , LiH and a little newly formed phase of Na2LiAlH6 (marked with “Ƒ”). The XRD pattern of the composite dehydrogenated at 150 °C indicates that Na2LiAlH6 phase has become more prominent and the original

Fig. 3 TPD curve of the new milled composite for 10 min (the heating ramp is 1 °C/min)

Fig. 4 XRD spectra of the milled composite for 10 min at different dehydrogenation stages (each spectrum was measured after quick cooling to room temperature)

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phase NaAlH4 has completely disappeared. Another new phase is confirmed to be Al, which locate at 38.53°, 44.71°, 65.15° and 78.18°, respectively. These phase transformations indicate the occurrence of Eq. (1) from milling course to 150 °C. After dehydrogenation at 200 °C, one can witness a significant change in the phase composition. Na2LiAlH6 phase has completely disappeared, instead, the newly formed NaH and Al (LiH) dominate the phase composition. The main peaks of LiH are overlapped with those of Al in the present XRD spectra. At the same time, a new set of diffraction peaks emerges from 30.8°, 50.0° and 51.8°, which may be assigned to ternary imides of Li2MgN2H2. These facts make us believe that the decomposition of Na2LiAlH6 (as Eq. (2)) dominates the dehydrogenation process from 150 to 200 °C. 3 Na 2 LiAlH 6 l 2 NaH+ LiH+ Al+ H 2 (2) 2 As the dehydrogenation temperature increased to 250 °C, Li2MgN2H2 phase was increasingly developing along with the complete disappearance of Mg(NH2)2. NaH and Al (LiH) still dominate the phase composition, although a little weakness occurs in their peak intensity. These evidences adequately indicate that the following Eq. (3) dominates the hydrogen desorption from 200 to 250 °C. Mg(NH 2 ) 2 + LiH l Li 2 MgN 2 H 2 + 2 H 2 (3) For the sample dehydrogenated at 300 °C, some new peaks belonging to Mg3N2 (marked with “ȕ”) arise inclusively in the XRD spectrum. Consequently, the intensity of Li2MgN2H2 peaks is identified to be weakened. Furthermore, a Li2NH phase can be fixed at peaks of 30.6°, 35.7° and 51.2°. It has been reported that the AlN phase as a final dehydrogenated product is often included in the AlH4–/NH2– system[15]. It is also speculated hereby that the AlN phase may be formed due to the strong affinity between Al and N. However, the main diffraction peaks of the cubic AlN with a space group Fm3m and Al are very close and can not be distinguished in present XRD spectra. As for that, an XPS apparatus is applied to determine the chemical status of Al and N element in the dehydrogenated composite at 300 °C. Fig. 5 shows the XPS spectra of Al2p and N1s in the dehydrogenated composite, respectively. As shown in Fig. 5(a), Al exhibits one asymmetric peak positioned at 73 eV with FWHM of 1.85 eV. As the typical XPS peak width of contaminated C and O is ~1.3 eV (not shown in this paper), it may be reasonable to deconvolve Al into some separated peaks. As an optimism fitting result, Al may be composed of two chemical states. The reduced chemical state Al (72.15 eV) may coincide with metal Al, and the Al with a higher binding energy (73.05 eV) should be combined with some elements with larger electronegativity (as N ~3.04 in the present system). The chemical state of N, on the other hand, is relatively complicated (Fig. 5(b)). The broad and asymmetric peak can be optimized to three separated peaks positioned at 395.45, 397.05 and 399.32 eV, respectively. It has been reported[11] that N with the lowest 1S binding energy (395.45 eV) should be surrounded largely by the electron donating elements,

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Fig. 5 XPS spectra of Al2p (a) and N1s (b) of the dehydrogenated composite at 300 °C

Fig. 6 H-cycling tests of the milled composite for 10 min under 0.1 MPa H2 and different temperatures (the dehydrogenated samples are recharged under 8 MPa H2 and 120 °C (a), 150 °C (b) and 200 °C (c), respectively)

such as Li in Li2NH. The combination of N and H may result in a higher 1S binding energy (397.05 eV) for N. The highest chemical state of N (399.32 eV) may be rooted in some covalent bonds between N and Al or Mg which accords with the above XRD results. Combining the XRD with XPS results, the following transformation can be confirmed from 250 to 300 °C: 3 3 Li 2 MgN 2 H 2 + Al l Mg 3 N 2 + AlN+ 3 Li 2 NH+ H 2 (4) 2 2.3 Reversibility

The reversibility of dehydrogenation/re-hydrogenation properties is an important indicator for practical application. In present work, we also paid some attention to the reversibility of the newly synthesized composite by investigating the H-cycling properties at different temperatures. Fig. 6 shows the H-cycling curves of the composite milled for 10 min at 120, 150 and 200 °C, respectively. As indicated in Fig. 6(a), the composite releases about 1.5 wt.% of hydrogen in the first desorption and preserves only 0.2 wt.% in the next two cycles under 0.1 MPa H2 and 150 °C. However, a total of 2.1 wt.% of hydrogen can be deduced in the first desorption and around 1.5 wt.% is retained in the next two cycles under 0.1 MPa H2 and 200 °C as shown in Fig. 6(b). When the cycling temperature is increased to 250 °C, over 2.6 wt.% of hydrogen desorbs in the first cycle and then remains the same cycling capacity as that of 200 °C in the next two cycles. After each H-cycling test, the recharged sample was sub-

jected to the XRD measurement (not figured in this paper) for illustrating the hydrogen attenuation occurred during the H-cycling. We discovered that the Eqs. (1) and (3) can not well be re-hydrogenated under present temperature and pressure, but the Eq. (2) is easily re-hydrogenated below 150 °C. That is why the composite shows no reversible capacity at 150 °C, but the same reversible capacity at 200 and 250 °C. The H-cycling tests demonstrated that only a partial reversibility adheres to the milled composite, which casts a deep shadow over its applying on on-board fuel cell. It seems that an in-depth investigation on the potential reversibility of this system shows its necessity. Accordingly, the DSC measurement was applied to characterize the thermodynamics of the dehydrogenation reactions. The result is shown in Fig. 7, which clearly displays four endothermic

Fig. 7 DSC profile of Li-Mg-N-H/NaAlH4 composite milled for 10 min

ZHANG Xugang et al., Hydrogen storage properties of the CeH2 doped Li-Mg-N-H/NaAlH4 system

events corresponding to hydrogen desorption when heated from 50 to 500 °C. The endothermic nature of dehydrogenation indicates that the desorption and absorption of the CeH2 doped Li-Mg-N-H/NaAlH4 system should be thermodynamically reversible, which makes it a promising candidate for hydrogen storage. However, the stable phases of Mg3N2 and AlN deadly stunts its re-hydrogenation ability kinetically. It follows that in-depth work will be extremely urgent to make the CeH2 doped Li-Mg-N-H/NaAlH4 system an applicable one.

3 Conclusions The 2 mol.% CeH2 doped Li-Mg-N-H/NaAlH4 composite was successfully synthesized and its hydrogen storage performances was systematically investigated in this paper. It was found that a total of 5 wt.% hydrogen could be desorbed from the newly synthesized composite with a three-step reaction and the sample for 10 min milling had an optimal integrated performance in initial decomposition temperature and desorption kinetics. Taking combining use of the XRD and XPS apparatus, the dehydrogenation scheme proceeding in the dehydrogenating course from RT to 300 ć were clearly determined as Eqs. (1)–(4). A potential thermodynamic reversibility which enabled it a promising candidate for hydrogen storage was finally confirmed to be adhered to the new composite, although the stable production AlN and Mg3N2 inhibited the reversibility to a certain extent.

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