Mechanistic investigations on significantly

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH 4)2-added 2LiNH 2/MgH 2system
Bo Li,Yongfeng Liu *,Jian Gu,Yingjie Gu,Mingxia Gao,Hongge Pan *
State Key Laboratory of Silicon Materials,Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province &Department of Materials Science and Engineering,Zhejiang University,Hangzhou 310027,People’s Republic of China
a r t i c l e i n f o
Article history:
Received 12December 2012Received in revised form 31January 2013
Accepted 2February 2013Available online 6March 2013Keywords:
Hydrogen storage Amide Borohydrides Kinetics Mechanisms
a b s t r a c t
The hydrogen storage properties and mechanisms of the Ca(BH 4)2-added 2LiNH 2e MgH 2system were systematically investigated.The results showed that the addition of Ca(BH 4)2pronouncedly improved hydrogen storage properties of the 2LiNH 2e MgH 2system.The onset temperature for dehydrogenation of the 2LiNH 2e MgH 2e 0.3Ca(BH 4)2sample is only 80 C,a ca.40 C decline with respect to the pristine sample.Further hydrogenation ex-amination indicated that the dehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample could absorb ca.4.7wt%of hydrogen at 160 C and 100atm while only 0.8wt%of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions.Structural analyses revealed that during ball milling,a metathesis reaction between Ca(BH 4)2and LiNH 2firstly occurred to convert to Ca(NH 2)2and LiBH 4,and then,the newly developed LiBH 4reacted with LiNH 2to form Li 4(BH 4)(NH 2)3.Upon heating,the in situ formed Ca(NH 2)2and Li 4(BH 4)(NH 2)3work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH 4)2-added 2LiNH 2e MgH 2system.
Copyright ª2013,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights
reserved.
1.Introduction
One of the key requirements for widespread commercializa-tion of hydrogen-fueled vehicles is the availability of safe,compact and efficient methods for on-board hydrogen storage [1].In the past few years,considerable attention has been paid to the solid-state hydrogen storage systems consisting of light metal complex hydrides such as alanates,borohydrides and amides [2e 6].These materials are more attractive due to their high gravimetric and volumetric densities compared to the conventional high-pressure and cryogenic hydrogen storage technologies.Among them,metal-N e H systems have been widely acknowledged as one of the most promising families for on-board hydrogen storage because of their relatively high
reversible hydrogen capacity.In 2002,Chen et al.firstly re-ported the hydrogen storage properties of lithium nitride (Li 3N),which could reversibly store 11.5wt%of H 2by the following two-step reactions [7]:Li 3N þH 24Li 2NH þLiH (1)Li 2NH þH 24LiNH 2þLiH
(2)
In spite of the quite high hydrogen capacity,the equilibrium pressure was determined to be ca.0.01atm at 255 C for re-action (1)and ca.1atm at 280 C for reaction (2),which is too low for practical on-board applications.A large number of studies have been conducted to increase the equilibrium pressure and lower the operating temperature of the Li e N e H
*Corresponding authors .Tel./fax:þ8657187952615.
E-mail addresses:mselyf@ (Y.Liu),hgpan@ (H.
Pan).
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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 38(2013)5030e 5038
0360-3199/$e see front matter Copyright ª2013,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights reserved./10.1016/j.ijhydene.2013.02.011
system [8e 12].Orimo et al.predicted by first-principle calcu-lations that partially substituting Li by other elements with larger electronegativity (such as Mg)could effectively desta-bilize LiNH 2and thus expedited its dehydriding reaction [13].At almost the same time,Luo et al.and Xiong et al.demon-strated experimentally that partial substitution of lithium with magnesium delivered an equilibrium pressure of 30atm at 200 C by forming the Li e Mg e N e H system [8,10],which is a dramatic improvement in the plateau property compared to the LiNH 2e LiH system.However,the operating temperature and the de-/hydrogenation rate of the 2LiNH 2e MgH 2system are not satisfactory for practical applications yet.
To fulfill the technical targets for on-board hydrogen storage systems set by the US Department of Energy (DOE)[14],tremendous efforts have been devoted to modifying the 2LiNH 2e MgH 2system by doping various additives,such as Si,Al,KH,and so on [15,16].Nayebossadri et pared the effect of elemental Si and Al on the dehydrogenation behav-iors of the LiNH 2e LiH and 2LiNH 2e MgH 2systems,and iden-tified the kinetic rate-limiting step to be the diffusion of H Àanion for the LiNH 2e LiH system,but it is the diffusion of Li þcation for the 2LiNH 2e MgH 2system [15].Recently,Luo et al.reported that the addition of less than 4mol%KH significantly enhanced the hydrogen absorption rate of the desorbed 2LiNH 2e MgH 2sample,but had a less dramatic effect on the kinetics of hydrogen desorption [16].Interestingly,it was found that metal borohydrides,such as LiBH 4,NaBH 4and Mg(BH 4)2,were a sort of effective dopants for improving the reaction kinetics of both dehydrogenation and hydrogenation of the metal-N e H systems due to the “self-catalyzing”effect and/or the enhanced mass transport [17e 21].Moreover,it was also revealed that the dehydrogenation thermodynamics of the metal-N e H system could be improved by the introduction of Ca 2þcation [22e 24].The desorption enthalpy of the Mg(NH 2)2e CaH 2combination was determined to be about 28.2kJ/mol-H 2by DSC,which is distinctly lower than those of the LiNH 2e LiH system (66kJ/mol-H 2)and the Mg(NH 2)2e 2LiH system (39kJ/mol-H 2)[23,25,26].As an alkaline earth metal borohydride,Ca(BH 4)2consists of not only the [BH 4]Àanion but also the Ca 2þcation.It is therefore expected to possess
synergetic effects on improving the hydrogen storage prop-erties of the metal-N e H system.
In this work,to further improve the hydrogen storage properties of the Li e Mg e N e H system,Ca(BH 4)2was intro-duced into the 2LiNH 2e MgH 2system to create a 2LiNH 2e MgH 2e x Ca(BH 4)2system.The hydrogen storage properties and mechanism of the 2LiNH 2e MgH 2e x Ca(BH 4)2(x ¼0,0.1,0.2,0.3)composites were systematically investi-gated and discussed by a series of structural analyses and property evaluation.It was found that the metathesis reaction between Ca(BH 4)2and LiNH 2firstly took place to convert to Ca(NH 2)2and LiBH 4during ball milling,and then,the newly formed LiBH 4reacted with LiNH 2to generate Li 4(BH 4)(NH 2)3.The in situ formed Ca(NH 2)2and Li 4(BH 4)(NH 2)3exhibited synergetic positive effects on lowering the operating temper-atures for hydrogen storage in the 2LiNH 2e MgH 2system.
2.Experimental section
The commercial chemicals,LiNH 2(Alfa Aesar,95%),MgH 2(Alfa Aesar,98%),NaBH 4(Alfa Asear,98%),LiBH 4(Aldrich,95%)and anhydrous CaCl 2(Sinopharm,96%)were used as received without any pretreatment.Tetrahydrofuran (THF)with a pu-rity of 99.9%was purchased from Sinopharm and used after a purification process with CaH 2.Ca(BH 4)2was synthesized in our own laboratory by a metathesis reaction between CaCl 2and NaBH 4according to the following reaction:2NaBH 4þCaCl 2/Ca ðBH 4Þ2þ2NaCl
(3)
The detailed synthetic procedure was described elsewhere [27].The 2LiNH 2e MgH 2e x Ca(BH 4)2(x ¼0,0.1,0.2,0.3)com-posites were prepared by ball milling the corresponding chemicals under 50bar of hydrogen pressure on a planetary ball mill at 500rpm for 24h.The ball to sample ratio was about 60:1.To ensure even mixing and milling,the mill was set to rotate for 0.2h for one direction,pause 0.1h,and then rotate in the reverse direction.To prevent air contamination,all handing of samples was performed in an MBRAUN glovebox filled with high purity Ar (H 2O,<1ppm;O 2,<1ppm).
The temperature-dependence of hydrogen desorption be-haviors of the as-milled samples was measured by using a home-built temperature-programmed-desorption (TPD)sys-tem,which is coupled with a QIC-20mass spectrometer (Hiden,England)for the simultaneous monitoring of NH 3generation during dehydrogenation.Approximately 30mg of sample was loaded into a stainless-steel tube reactor which then was connected to TPD-MS system for testing.The tem-perature ramping rate was 2 C/min,and a constant flow of pure argon was maintained during heating.The quantitative evaluations of hydrogen desorption/absorption properties were performed on a homemade Sieverts’-type apparatus,where the temperatures and pressures of the sample and gas reservoirs were monitored and recorded automatically.About 150mg of sample was used for each testing.The sample temperature was gradually raised from the ambient temper-ature at a rate of 2 C/min for dehydrogenation (starting from static vacuum)and 1 C/min for hydrogenation (initial hydrogen pressure being 100bar).The
thermogravimetric
Fig.1e XRD patterns (a)and FTIR spectra (b)of the as-milled 2LiNH 2e MgH 2e x Ca(BH 4)2composites.
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5031
analysis (TGA)was conducted on a Netzsch STA449F3thermal analyzer.About 5mg of sample was used each time and heated in an aluminum crucible under pure argon with a ramping rate of 10 C/min.
X-ray diffraction analysis was conducted on an X’Pert Pro X-ray diffractometer (PANalytical,The Netherlands)with Cu K a radiation at 40kV and 40mA.The data were collected in the 2q range of 10e 90 with step increments of 0.05 .A homemade container was applied to prevent powdery sample from moisture and oxygen contaminations during the sample transfer and scanning.Fourier transform infrared (FTIR)spectra were obtained with a Bruker Tensor 27unit.The testing samples were mixed evenly with potassium bromide (KBr)at the weight ratio of 1:60and then cold pressed to form a pellet.Prior to the collection of each spectrum,the back-ground of the air was measured and subtracted from the sample.The transmission mode was adopted with a resolu-tion of 4cm À1,and each spectrum was created with 16scans on the average.Solid-state 11B NMR spectra were recorded at room temperature by using a Bruker Avance II 300MHz spectrometer operating at 96.3MHz.Samples were packed into 7mm ZrO 2rotors and were sealed with tightly fitting Kel-F caps inside an Ar-filled glovebox.All spectra were obtained after 1024scans with an acquisition time of 38ms per scan and a repetition delay time of 1s.Chemical shifts (d )were reported in parts per million (ppm)externally referenced to solid NaBH 4at 41ppm for 11B nuclei.
3.
Results and discussion
3.1.
Characterization of as-milled samples
Fig.1(a)shows the XRD patterns of the 2LiNH 2e MgH 2e x-Ca(BH 4)2composites milled for 24h.After 24h of ball milling,the characteristic diffraction peaks of the starting chemicals of LiNH 2and MgH 2can be clearly identified in the XRD profile of the pristine sample.For the sample with the addition of 0.1mol Ca(BH 4)2,a slight decline in the relative intensity of the reflections is observed for LiNH 2with respect to MgH 2,and
two weak peaks at 35.6and 50.5 (2q )change to be asymmetric (Fig.S1).As the addition amount of Ca(BH 4)2is increased to 0.3mol,three new diffraction peaks appear at 28.8,34.7and 49.4 along with the disappearance of the typical reflections of LiNH 2.Further analyses reveal that the diffraction peaks at 34.7and 49.4 match quite well with the reflections of the as-milled Ca(NH 2)2as shown in Fig.S2,and the reflection at 28.8 is assigned to Li 4(BH 4)(NH 2)3according to the previous report [28].Moreover,it is noteworthy that the MgH 2phase still dominates in all the post-milled samples with and without Ca(BH 4)2.
FTIR measurements show that only the typical doublet N e H vibrations of LiNH 2at 3312and 3258cm À1are discernable for the as-milled pristine sample as shown in Fig.1(b).However,a new broad absorbance centered at 3280cm À1was detected for the sample with 0.1mol Ca(BH 4)2.As the addition of Ca(BH 4)2is increased to 0.2mol,the relative intensity of the absorbance at 3280cm À1is increased with the weakening of the characteristic absorbance of LiNH 2.At the same time,three new absorption peaks are visible at 3301,3243and 3228cm À1.Further increasing the addition of Ca(BH 4)2to 0.3mol induces the domination of the absorbances at 3301,3280,3243and 3228cm À1in the FTIR spectrum associated with the full disappearance of LiNH 2.According to the previous reports [29,30],the absorbances at 3280and 3228cm À1originate from the N e H vibration of Ca(NH 2)2,and those at 3301and 3243cm À1belong to Li 4(BH 4)(NH 2)3.In particular,there is a broad ab-sorption peak centered at 2293cm À1with two faint shoulders at 2389and 2225cm À1for the as-milled 0.1mol Ca(BH 4)2-doped sample as shown in Fig.2(a),which can be attributed to the B e H vibration in the [BH 4]Àanion of borohydrides [31].Careful ex-aminations on the B e H vibration in the wavenumber range of 1000e 1300cm À1reveal only a broad absorbance at 1125cm À1for the as-milled 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample,which is similar to the single peak of the as-milled LiBH 4(1125cm À1)and differs from the doublet peak of the as-milled Ca(BH 4)2(1190and 1125cm À1)as illustrated in Fig.2(a).Therefore,we believe that for the as-milled 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample,Ca(BH 4)2converts to LiBH 4after ball milling according to the following reaction.
Ca ðBH 4Þ2þ2LiNH 2/Ca ðNH 2Þ2þ2LiBH 4
(4)
Solid-state NMR experiments provide an additional evi-dence for the transformation from Ca(BH 4)2to LiBH 4during ball milling as the 11B MAS NMR spectrum of the as-milled 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample exhibits a single central transition at around À39ppm (Fig.2(b)),which is apparently different from that of the as-milled Ca(BH 4)2(À31ppm)and very close to that of the as-milled LiBH 4(À40ppm).
However,for the samples with the higher content of Ca(BH 4)2(x ¼0.2and 0.3),the B e H absorption peak appears at 2302cm À1in the FTIR spectra,a blue-shift of 9cm À1in com-parison with that of the x ¼0.1sample (2293cm À1)(Fig.2(a)).This fact indicates the formation of Li 4(BH 4)(NH 2)3according to Chater’s report [30],in which the B e H stretching modes of Li 4(BH 4)(NH 2)3are shifted to the higher wavenumbers by around 8cm À1with respect to LiBH 4.Moreover,the typical BH 2deformation bands of Li 4(BH 4)(NH 2)3at 1126and 1082cm À1[30]were also detected for the 2LiNH 2e MgH 2e 0.2Ca(BH 4)2sample,which is further intensified for the x ¼0.3sample.It seems that
Wavenumber (cm -1
)
Chemical shift (ppm)
Fig.2e FTIR spectra (a)and solid-state 11
B NMR spectra (b)
of the as-milled samples.
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the newly developed LiBH4was immediately captured by LiNH2 to form the quarternary compound of Li4(BH4)(NH2)3as shown below.
LiBH4þ3LiNH2/Li4BH4ðNH2Þ3(5)
Apparently,the higher Ca(BH4)2content facilitates the for-mation of Li4(BH4)(NH2)3during ball milling.This notion is further validated by ball milling the newly-designed mixtures of2LiNH2e MgH2e x LiBH4.As shown in Fig.S3,The signature B e H vibration of Li4(BH4)(NH2)3at3301and3243cmÀ1is gradually intensified with increasing the content of LiBH4. 3.2.Dehydrogenation behaviors of Ca(BH4)2-added samples
Fig.3(a)and(b)present the TPD-MS curves of the as-milled 2LiNH2e MgH2e x Ca(BH4)2composites.It is seen that the major-ity of hydrogen release from the pristine sample starts at120 C and peaks at180 C followed by a shoulder at230 C.The whole hydrogen desorption was completed at320 C.After adding Ca(BH4)2,the dehydrogenation process presents two distinct stages in the testing temperature range.With increasing the addition amount of Ca(BH4)2,thefirst dehydrogenation peak shifts to lower temperatures while the operating temperatures for the second one remain almost unchanged.There is a40 C decline in the on-set dehydrogenation temperature for the sample with x¼0.3(ca.80 C)relative to the pristine 2LiNH2e MgH2sample(ca.120 C).Interestingly,the dehydroge-nation peak temperature of thefirst stage is monotonously decreased from180 C for x¼0to150 C for x¼0.3.At the same time,the area of thefirst dehydrogenation peak gradually de-clines with increasing the addition of Ca(BH4)2,suggesting a slight decrease in the hydrogen desorption amount in this dehydrogenation stage.On the contrary,the area of the second dehydrogenation peak increases as the x value increases from 0.1to0.3,delivering an additional dehydrogenation process.
The simultaneous MS analyses on ammonia generation (Fig.3(b))shows that the dehydrogenation process of the pris-tine2LiNH2e MgH2sample is accompanied by the emission of a small amount of ammonia.After adding Ca(BH4)2,the ammonia release is retarded in the low temperature range while it is obviously increased at the high operating temperature.The decreased ammonia emission at low temperatures is possibly attributed to the formation of Li e B e N e H compound by the combination of[BH4]Àwith[NH2]Àin which the strong affinity between the H dÀin[BH4]Àand the H dþin[NH2]Àfavors the hydrogen release and alleviates the ammonia emission.Similar phenomenon was also reported in the borohydride e amide combinations[32e34].At high temperatures,the newly devel-oped borohydride(LiBH4)during ball milling possibly took part in the dehydrogenation reaction as a reactant,which destroys the chemical combination of[BH4]Àwith[NH2]À,consequently resulting in the increased ammonia release.It is evidenced as the typical FTIR absorption peaks of B e H vibration disappear after full dehydrogenation(Fig.S4).
Fig.3(c)demonstrates the quantitative hydrogen desorp-tion behaviors of the2LiNH2e MgH2e x Ca(BH4)2composites.It is seen that totally ca.5.4wt%of H2is released from the pristine sample as the temperature was elevated from room tempera-ture to320 C,which consists well with the previous in-vestigations on the Li e Mg e N e H system[8,35e38].For the samples with the addition of Ca(BH4)2,the hydrogen desorp-tion process exhibits two distinct stages at80e220 C and 220e400 C,respectively.The total amount of hydrogen release is increased from6.7wt%for x¼0.1to8.2wt%for x¼0.3, however,it is decreased from5.0wt%to4.0wt%for thefirst-stage dehydrogenation.Taking comprehensive consideration of the operating temperature and capacity of thefirst-stage dehydrogenation,the0.1Ca(BH4)2-added sample exhibits the optimal dehydrogenation property as hydrogen release amounts to5.0wt%with a starting temperature of100 C.
To understand the chemical events occurring in the dehy-drogenation process of the Ca(BH4)2-added samples,the 2LiNH2e MgH2e0.1Ca(BH4)2sample was selected as an example to examine the compositional and structural changes upon heating.Fig.4shows the XRD patterns and FTIR spectra of the dehydrogenated2LiNH2e MgH2e0.1Ca(BH4)2samples at different stages.It can be seen that as the sample was heated from room temperature to100 C,a typical diffraction peak of Li4(BH4)(NH2)3appears at28.8 in the XRD profile(Fig.4(a)),and correspondingly,the characteristic N e H vibrations of Li4BH4(NH2)3at3301/3243cmÀ1are visible in the FTIR spectrum (Fig.4(b)).This indicates that a new quarternary compound of Li4BH4(NH2)3wasfirst formed during the initial heating process by reaction(5)due to the favorable thermodynamics.Further raising the temperature to150 C,XRD result shows that the weak reflection of Ca(NH2)2at34.7 is invisible.FTIR exami-nation confirms the disappearance of Ca(NH2)2as its signature absorbance at3280cmÀ1is undetectable.However,a broad absorption peak at3151cmÀ1is observed in the FTIR spectrum, which originates from a ternary imide of MgCa(NH)2as re-ported previously[23].Therefore,the following reaction occurs as the sample was heated to150 C:
CaðNH2Þ2þMgH2/MgCaðNHÞ2þ2H2(6) As the dehydrogenation temperature was increased to 180e220 C,it is found that the reflections of MgH2and Li4(BH4)(NH2)3are gradually weakened and even disappear in the XRD profiles while the characteristic diffraction peaks of the cubic Li2Mg(NH)2phase were detected.The FTIR absorption peaks of LiNH2(3312and3258cmÀ1)and Li4(BH4)(NH2)3(3301and
Temperature (o C)
Fig.3e TPD-MS(a,b)and volumetric dehydrogenation
curves(c)of the2LiNH2e MgH2e x Ca(BH4)2composites.
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3243cm À1)exhibit the apparently decreased intensities at 180 C and are undetectable at 220 C.At the same time,a broad absorbance assignable to the cubic Li 2Mg(NH)2appears at 3170cm À1.In addition,it is interesting that for the dehydro-genated sample at 220 C,a distinct red shift was found for the absorption peaks of the B e H vibration in the BH 4group as the central peak shift from 2302to 2293cm À1,indicative of the re-generation of LiBH 4from the intermediate phase Li 4(BH 4)(NH 2)3.The absence of LiBH 4in the XRD profiles (Fig.4(a))is mainly attributed to its quite low relative content (5.6%)in the dehydrogenation product.XRD and FTIR results indicate that as the sample was heated to 220 C,LiNH 2,MgH 2and the newly developed Li 4BH 4(NH 2)3were gradually consumed,and Li 2Mg(NH)2and LiBH 4were yielded with hydrogen release.Thus,the following reactions proceed at 150e 220 C.2LiNH 2þMgH 2/Li 2Mg ðNH Þ2þ2H 2
(7)2Li 4BH 4ðNH 2Þ3þ3MgH 2/3Li 2Mg ðNH Þ2þ2LiBH 4þ6H 2
(8)
The presence of LiBH 4indicates the persistence of [BH 4]Àin this dehydrogenation process,which is reasonably responsible for the decrease in the dehydrogenation amount for the first-stage dehydrogenation (Fig.3(c))due to the “dead weight”of borohydride.
After full dehydrogenation at 400 C,the diffraction pat-terns of LiCa 4(BN 2)3,Li 3BN 2,Mg 3N 2and LiH were identified in the XRD profile (Fig.4(a)).In the FTIR spectrum (Fig.4(b)),the typical absorption peak of Li 2Mg(NH)2dominates while the signature absorbance of the B e H vibration of the BH 4group is invisible.Taking into account the emission of a small amount of ammonia in the temperature range of 220e 400 C as shown in Fig.3(b),the second-stage reaction of the Ca(BH 4)2-added samples can be expressed as follows:
11Li 2Mg ðNH Þ2þ4MgCa ðNH Þ2þ9LiBH 4/LiCa 4ðBN 2Þ3þ5Mg 3N 2
þ12LiH þ6Li 3BN 2þ2NH 3þ28H 2
(9)
Since there is only 0.2mol LiBH 4in the post-milled 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample,it can react with 0.24mol
Li 2Mg(NH)2and 0.09mol MgCa(NH)2to release 0.62mol H 2according to reaction (9).This induces that ca.0.65mol Li 2Mg(NH)2remains in the fully dehydrogenated sample at 400 C,which is responsible for the persistence of the ab-sorption peak of Li 2Mg(NH)2at 3170cm À1as shown in Fig.4(b).
According to the above discussion,the overall reaction process of the 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample during ball milling and subsequent heating is expressed as follows:2LiNH 2þMgH 2þ
1
10
Ca ðBH 4Þ2!BM 95LiNH 2þMgH 2þ110Ca ðNH 2Þ2þ1
5LiBH 4!RT e 100
C
65LiNH 2þMgH 2þ110Ca ðNH 2Þ2þ1
5
Li 4BH 4ðNH 2Þ3/100
C e 150
C
65LiNH 2þ910MgH 2þ110MgCa ðNH Þ2þ1
5
Li 4BH 4ðNH 2Þ3þ1
5H 2/150
C e 220
C
910Li 2Mg ðNH Þ2þ110MgCa ðNH Þ2þ1
5
LiBH 4þ2H 2
!
220 C e 400
C
5990Li 2Mg ðNH Þ2þ190MgCa ðNH Þ2þ1
45
LiCa 4ðBN 2Þ3þ1Mg 3N 2þ4LiH þ2Li 3BN 2þ2NH 3þ118H 2(10)
The theoretical weight loss was calculated to be 7.6wt%,which is in good agreement with the TGA result (7.5wt%)as shown in Fig.S5.The theoretical dehydrogenation capacity amounts to 6.6wt%,which is slightly lower than that of the experimental value of 6.7wt%because it actually contains a small amount of ammonia emission as mentioned above.Moreover,it is noteworthy that there are two dehydrogenation reactions at 100e 220 C as shown in reaction (10),but only one corresponding dehydrogenation peak was detected in the TPD curve (Fig.3(a)).This is mainly due to the similarity of their dehydrogenation thermodynamics and kinetics.Identical phe-nomenon was also observed in the Mg(NH 2)2e 2LiH system [39].
3.3.Hydrogen storage reversibility of Ca(BH 4)2-added samples
Fig.5shows the temperature-depended hydrogenation curve of the 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample after the first-stage dehydrogenation,in which the pristine sample is also included for comparison.It is seen that the operating tem-perature for hydrogenation of the dehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample is dramatically reduced.The onset and ending temperatures for hydrogenation of the dehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2sample are only 80and 160 C,respectively,which are lowered down by 40 C and 80 C in comparison with the pristine sample (120 C and 240 C).The total hydrogenation amount is decreased from 5.4wt%for the pristine sample to 4.7wt%for the 0.1mol Ca(BH 4)2-added sample due to the dilute effect of the additive because the [BH 4]Àspecies does not take part in the first-stage dehydrogenation as demonstrated above.However,the low temperature hydrogenation properties of the 2LiNH 2e MgH 2e 0.1Ca(BH 4)2system are significantly superior to the pristine sample as it can be completely hydrogenated
at
Fig.4e XRD patterns (a)and FTIR spectra (b)of the dehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2samples at different stages.
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5034
160 C and 100atm while only 15%of hydrogen capacity was achieved for the dehydrogenated pristine sample.
For elucidating the influence of Ca(BH 4)2on the hydroge-nation process,the 0.1Ca(BH 4)2-added sample was first dehydrogenated upon heating to 200 C,and then hydroge-nated under 100bar of H 2.The hydrogenated samples at different stages were collected for XRD and FTIR analyses.The results are shown in Fig.6.It is seen that the dehydrogenated sample at 200 C mainly consists of Li 2Mg(NH)2and MgCa(NH)2.The diffraction peaks of Mg(NH 2)2start to appear at 100 C in the XRD profile,and those of LiH can also be detected at 120 C.At 160 C,Mg(NH 2)2and LiH are the only phases detected by XRD,suggesting that the hydrogenation reaction of the dehydrogenated sample completed.FTIR ex-periments (Fig.6(b))show a small and weak absorbance at 3243cm À1was observed for the hydrogenated sample at 80 C,
which originates from the N e H vibration of Li 4(BH 4)(NH 2)3.When the sample was hydrogenated at 100 C,the charac-teristic absorbance of Mg(NH 2)2at 3272cm À1is discernable.At 140 C,the absorbance of Li 2Mg(NH)2are almost undetectable.After full hydrogenation at 160 C,only three absorption peaks were detected at 3327,3272and 3151cm À1,which are assigned to Mg(NH 2)2and MgCa(NH)2,respectively.It is well known that LiNH 2is one of important intermediates in the hydrogenation process of Li 2Mg(NH)2[39].The newly formed LiNH 2during the initial hydrogenation can be immediately captured by LiBH 4to give rise to the in-situ formation of Li 4(BH 4)(NH 2)3according to reaction (5)as reported by Hu et al.[40],which is responsible for the undetectability of LiNH 2in this case.The amide group in Li 4(BH 4)(NH 2)3was further hydrogenated to form the resultant products,Mg(NH 2)2and LiH,and re-generate LiBH 4as evidenced by the appearance of the characteristic absor-bances of LiBH 4at 2389/2293/2225cm À1with the disappear-ance of the typical N e H vibration of Li 4(BH 4)(NH 2)3at 3243cm À1after full hydrogenation (Fig.S6).Moreover,it should be mentioned that the typical absorption peak of MgCa(NH)2at 3151cm À1is still discernable for the sample hydrogenated at 220 C,indicating that the hydrogenation of MgCa(NH)2is unsuccessful at 220 C and 100bar H 2due to the sluggish kinetics [23],which is an important reason for the reduction of the reversible hydrogen capacity (Fig.5).
3.4.Hydrogen storage kinetics of Ca(BH 4)2-added samples
As shown in Figs.3and 5,the addition of Ca(BH 4)2decreases the operating temperatures of de-/hydrogenation of the 2LiNH 2e MgH 2system.To understand the dehydrogenation kinetics,the activation energy barrier for dehydrogenation from the 2LiNH 2e MgH 2e x Ca(BH 4)2composites was estimated by the determination of its apparent activation energy (E a )using the Kissinger’s approach as described below [41]:dln
b 2
m !d 1T m
¼ÀE a
R (11)
in which b is the heating rate,T m is the peak temperature,and R is the gas constant.In the present study,T m was obtained from the TPD data taken at 2,3,5and 7 C/min.Based on Eqn (11),a plot of ln[b /T m 2]versus 1/T should result in a straight line,the slope of which can be used to determine the activa-tion energy.Fig.7shows the Kissinger’s plots of the as-milled 2LiNH 2e MgH 2e x Ca(BH 4)2composites.It is obvious that there is a good linear relationship between ln[b /T m 2]and 1/T for all the samples.The apparent activation energy for the dehy-drogenation of the pristine sample is calculated to be 135.1kJ/mol,which is in good agreement with the result reported by Markmaitree et al.[42].The introduction of Ca(BH 4)2decreases dramatically the apparent activation energy of the dehydro-genation reaction of the 2LiNH 2e MgH 2sample and the apparent activation energies were calculated to be 128.4,104.7and 101.0kJ/mol for x ¼0.1,0.2and 0.3,respectively,which is intimately responsible for the decreased de-/hydrogenation temperatures as shown in Figs.3and 5
.
Fig.5e Rehydrogenation curves of the dehydrogenated 2LiNH 2e MgH 2sample and the first-step dehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2
sample.
Fig.6e XRD patterns (a)and FTIR spectra (b)of the rehydrogenated 2LiNH 2e MgH 2e 0.1Ca(BH 4)2samples at different stages.
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5035。