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Improved reversible dehydrogenation properties of LiBH4–MgH2 composite by tailoring nanophase structure using activated carbon
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 3 8 ( 2 0 1 3 ) 3 7 1 0 e3 7 1 6
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Improved reversible dehydrogenation properties of LiBH4eMgH2 composite by tailoring nanophase structure using activated carbon Kuikui Wang a,b, Xiangdong Kang a, Junhong Luo a, Chaohao Hu c, Ping Wang a,* a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China University of Science and Technology of China, Hefei 230026, China c Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China b
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abstract
Article history:
In this study, activated carbon (AC) was added to the 2LiBH4eMgH2 composite and
Received 17 September 2012
examined with respect to its effect on the hydrogen storage properties of the system. Our
Received in revised form
study found that AC is an effective additive for promoting the reversible dehydrogenation
12 December 2012
of the 2LiBH4eMgH2 composite. A series of control experiments were carried out to opti-
Accepted 7 January 2013
mize the sample preparation method, milling time and addition amount of AC. In com-
Available online 5 February 2013
parison with the neat LiBH4eMgH2 system, the LiBH4eMgH2eAC composite prepared under optimized conditions exhibits enhanced dehydrogenation kinetics, improved cyclic sta-
Keywords:
bility and particularly, eliminated incubation period between the two dehydrogenation
Hydrogen storage
stages. A combination of phase/microstructure/chemical state analyses has been con-
Lithium borohydride
ducted to gain insight into the promoting effect of AC on the reversible dehydrogenation of
Reactive hydride composite
the 2LiBH4eMgH2 system. Our study found that AC exerts its promoting effect via tailoring
Activated carbon
nanophase structure of the 2LiBH4eMgH2 composite.
Nanoparticle
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The widespread implementation of hydrogen fuel cell technology requires advanced materials that can store and deliver large amounts of hydrogen at moderate temperatures with fast kinetics [1]. Recently, lithium borohydride (LiBH4) and other light metal borohydrides received considerable attention as potential hydrogen storage media [2e4]. Several strategies have been developed to address their thermodynamic and/or kinetic problems that are essentially imposed by the strong and directional chemical bonds [5,6]. These strategies include reactant destabilization [7], nano-confinement using scaffolds [8e10], catalyst doping [11e14], and cation/
anion substitution [15e20]. In a general view, these technological advances have enabled significant improvements in the reversible dehydrogenation properties of LiBH4. Particularly, the employment of reactant destabilization strategy has given rise to a wealth of new reactive hydride composites, which offers new possibilities for the development of highperformance hydrogen storage materials [21e24]. 2LiBH4 þ MgH2 4 2LiH þ MgB2 þ 4H2
(1)
Combination of LiBH4 and MgH2 in a 2:1 M ratio constitutes a representative reactive hydride composite, which undergoes reversible dehydrogenation/rehydrogenation reactions
* Corresponding author. Tel.: þ86 24 2397 1622; fax: þ86 24 2389 1320. E-mail address: [email protected] (P. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.034
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 3 8 ( 2 0 1 3 ) 3 7 1 0 e3 7 1 6
following Eq. (1). In comparison with pristine LiBH4, the 2LiBH4eMgH2 composite exhibits significantly lowered reaction enthalpy (by around 25 kJ/mol H2), remarkably improved reversibility and comparable deliverable hydrogen capacity [7]. But owing to the sluggish kinetics, the 2LiBH4eMgH2 composite typically requires a temperature of at least 400 C for fulfilling dehydrogenation at a reasonable rate [25,26]. This is much higher than the predicted value (around 225 C) using thermodynamic data [7]. In the past years, impressive progresses have been made in improving the dehydrogenation kinetics of the LieMgeBeH system. A number of transition metal (TM) compounds have been identified to be effective for promoting the reversible dehydrogenation of the 2LiBH4eMgH2 composite [27e33]. According to the studies by Bo¨senberg and coworkers [27e29], the various TM additives may share a common promoting mechanism, that is, formation of TM borides which act as heterogeneous nucleation sites for MgB2 product. Alternatively, several nanoconfined systems were prepared by melt infiltration method using carbon aerogel scaffold materials [34,35]. Indeed, the nanoconfined 2LiBH4eMgH2 samples showed property advantages over their bulk counterpart. But the obtained kinetics improvement is much less pronounced than that observed in the nanoconfined LiBH4 systems [8e10]. This might be understood from the technical difficulties in simultaneously and efficiently incorporating LiBH4 and MgH2 phases into the nanopores of scaffold materials. In this paper, we report our trial of using activated carbon (AC) to improve the reversible dehydrogenation properties of LieMgeBeH system. The selection of AC as structure-tailoring agent is based on its chemical inertness, nanoporous texture, commercial availability and low cost [36,37]. Our study found that melt infiltration of LiBH4 in AC followed by mechanically milling with MgH2 provides an optimized procedure for preparation of 2LiBH4eMgH2eAC nanocomposite. In comparison with the neat 2LiBH4eMgH2 system, the nanocomposite with AC additive exhibits notable property advantages in terms of dehydrogenation kinetics and cycling stability. According to a combination of phase/microstructure/chemical state analyses, the observed property improvements should be attributed to the AC-tailored nanophase structure of LiBH4 and MgH2 reactants.
comparison, the LiBH4/MgH2/AC, LiBH4/MgH2/graphite and LiBH4/MgH2 powder mixtures were mechanically milled under Ar atmosphere at 400 rpm for 1 h. In all cases, the LiBH4:MgH2 molar ratio was adjusted to be 2:1 in accordance with Eq. (1). All sample operations were performed in an Ar (99.999%)-filled glovebox, wherein the H2O/O2 levels were typically <0.1 ppm. Hydriding/dehydriding properties of the samples were examined using a carefully calibrated Sievert’s type apparatus. A typical cyclic experiment entailed dehydrogenation at 400 C under 0.3 MPa H2 and hydrogenation at 350 C under an initial H2 pressure of 10 MPa for 6 h. To minimize H2O/O2 contamination during the volumetric measurements, the hydrogen feed (with an initial purity of 99.999%) was further purified using a hydrogen storage alloy system. For comparison with the neat sample, the weight of AC additive was not taken into account in determination of the H-capacity of the 2LiBH4eMgH2eAC nanocomposite sample. The 2LiBH4eMgH2eAC and relevant samples at varied states were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu Ka radiation), Fourier transform infrared spectroscopy (FTIR, Bruker TENSOR 27, 4 cm1 resolution) and scanning electron microscope (SEM, LEO Supra 35) equipped with an energy dispersive X-ray analysis unit (EDS, Oxford). The XRD powder sample was covered by an amorphous polymer tape to avoid oxidation during the measurement. FTIR spectra of the samples were collected using the KBr-pellet method and the obtained spectra were normalized using OPUS 6.5 software. The SEM samples were prepared by spreading the dry powder onto conductive carbon tape supported on a copper pole. All the sample preparations were performed in the glovebox and special measures were taken to minimize H2O/O2 contamination during the sample transfer processes. Solid-state 11B MAS NMR experiments were conducted in a Bruker AVANCE III 400 WB spectrometer operating at a magnetic field of 9.4 T on 128.3 MHz 11B frequency. The samples were packed in the ZrO2 rotor closed with a Kel-F cap and spun at 12 kHz rate. A total of 16 scans were recorded with a 2 s recycle delay for each sample. All 11B chemical shifts are referenced to the resonance of solid LiBH4 (41 ppm).
3. 2.
Experimental section
LiBH4 (95% purity) and graphite (99.99þ% purity, <45 mm particle size) from SigmaeAldrich, and MgH2 (98% purity) from Alfa Aesar were used as received. AC (with a maximum ash content <4 wt.%) from Alfa Aesar was calcined in a quartz reactor at 1000 C for 5 h under a hydrogen gas flow to remove impurities such as sulfur, nitrogen, oxygen and chlorine [38]. The 2LiBH4eMgH2eAC nanocomposite sample was prepared following a three-step procedure. The LiBH4/AC mixture was first milled under Ar atmosphere for 5 min using a Fritsch 7 Planetary mill at 200 rpm in a stainless steel vial together with eight steel balls (10 mm in diameter). The post-milled mixture was then calcined at 320 C under 3.0 MPa H2 pressure for 1 h. Finally, MgH2 powder was added and mechanically milled together with LiBH4/AC under Ar atmosphere at 400 rpm. For
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Results and discussion
3.1. Dehydrogenation properties of the 2LiBH4eMgH2eAC composite
MgH2 4 Mg þ H2
(2)
2LiBH4 þ Mg 4 2LiH þ MgB2 þ 3H2
(3)
It has been well established that the 2LiBH4eMgH2 composite fulfills two-step dehydrogenation reactions following Eqs. (2) and (3), respectively, under appropriate H2 back pressure. One of the major problems of this LieMgeBeH system is the interruption of the two dehydrogenation steps by a long incubation period. Out study showed that this problem can be alleviated by addition of AC. As seen in Fig. 1, the long
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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 3 8 ( 2 0 1 3 ) 3 7 1 0 e3 7 1 6
Fig. 1 e Dehydriding curves of the sample (a) neat 2LiBH4eMgH2; (b) directly milled 2LiBH4eMgH2eAC; (c) the 2LiBH4eMgH2eAC sample prepared following the threestep procedure and (d) 2LiBH4eMgH2egraphite at 400 C under 0.3 MPa H2. In the samples (b), (c) and (d), the LiBH4:C weight ratio is 1:1.
incubation period of around 14 h as observed in the neat 2LiBH4eMgH2 sample was markedly shortened to around 3 h after milling with AC additive (the LiBH4:AC weight ratio is 1:1). Furthermore, our study found that the incubation period can be further reduced to less than 2 h via changing the sample preparation method from routine one-step milling to a three-step procedure, which involves ordinal steps of premilling LiBH4/AC mixture, melt infiltration of LiBH4 into AC and milling together with MgH2. This three-step procedure was applied in the subsequent sample preparation. For comparison, the nonporous graphite was also used as additive and examined with respect to its effect on the dehydrogenation property of the 2LiBH4eMgH2 system. It was observed that the addition of graphite also caused reduction of incubation period, but with a much less pronounced effect than AC. This finding clearly indicates that the promoting effect of AC should be predominantly attributed to its nanoporous texture. Notably, the 2LiBH4eMgH2eAC samples rapidly released around 4.5 wt.% hydrogen in the first dehydrogenation stage, which is much higher than that delivered from MgH2 (2.85 wt.% hydrogen). As no evidence has been obtained supporting the occurrence of the reaction between Mg and LiBH4 at the first dehydrogenation stage (as discussed at Section 3.2), the extra amount of hydrogen should come from the decomposition of the LiBH4 nanophase that was structurally tailored by AC. This deduction was well confirmed by the control experiment using LiBH4/AC nanocomposite prepared by a melt infiltration method (result not shown here). Clearly, tailoring nanophase structure of LiBH4 causes a modification of thermodynamics [8,9]. But currently, whether or not the thermodynamics modification is a just consequence of nanosize effect of LiBH4 is still unclear. In this regard, detailed study is still required for better mechanistic understanding. Next, we conducted a series of control experiments to optimize the addition amount of AC and the milling condition.
Fig. 2 presents a comparison of the dehydrogenation profiles of three samples with different LiBH4:AC weight ratios. It was observed that increasing the AC amount resulted in an increasingly favorable dehydrogenation property owing to the promoted formation of LiBH4 nanoparticles. But meanwhile, addition of AC additive causes a serious capacity penalty of the system. To make a compromise between hydrogen capacity and kinetic property, we selected the sample with a LiBH4:AC weight ratio of 1:1 for further study. Fig. 3 shows the dehydrogenation profiles of the 2LiBH4eMgH2eAC samples milled for a duration ranging from 1 to 10 h. Increasing the milling time from 1 to 5 h resulted in enhanced dehydrogenation kinetics, slightly increased hydrogen capacity and particularly, the elimination of the incubation period. But further prolongation of the milling time caused no appreciable improvement of the dehydrogenation property. We therefore fixed the milling time at 5 h in the subsequent sample preparation. Fig. 4 presents the cyclic dehydrogenation curves of the 2LiBH4eMgH2eAC sample prepared under optimized condition. It was observed that the sample released nearly identical H2 amount in the first two cycles, but with a subtle difference on the dehydrogenation behavior. In the second cycle, the sample released less hydrogen at the first dehydrogenation step, but more hydrogen at the second step in comparison with the corresponding steps in the first cycle. Presumably, these are two correlated phenomena associated with particle coalescence. In the sample preparation process, the intensive milling may partially destroy the nanoporous structure of AC. As a consequence, some of the LiBH4 nanoparticles may not be well confined within the nanopores of AC. This offers chance for coalescence of some LiBH4 nanoparticles in the rehydrogenation process. The coalesced particles larger than the critical size will behave like bulk LiBH4, that is, reacting with Mg instead of undergoing self-decomposition under the applied condition. This will cause lowered H2 desorption at the first dehydrogenation stage and increased H2 contribution from the second step. The 2LiBH4eMgH2eAC sample can
Fig. 2 e Dehydriding curves of the composite samples with different LiBH4:AC weight ratios. The measurements were conducted at 400 C under 0.3 MPa H2.
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 3 8 ( 2 0 1 3 ) 3 7 1 0 e3 7 1 6
Fig. 3 e Dehydriding curves of the 2LiBH4eMgH2eAC composite milled for different periods.
reversibly store about 9 wt.% hydrogen at a cyclic condition of dehydrogenation at 400 C for 7 h and rehydrogenation at 350 C under 10 MPa H2 for 6 h. This cyclic performance compares favorably with the reported results of the nanocomposite systems using carbon aerogel scaffolds [34,35].
3.2.
Structural characterization and mechanism study
A combination of XRD and FTIR analyses has been conducted to characterize the 2LiBH4eMgH2eAC sample at different states. As seen in Fig. 5, the as-prepared sample showed weak peaks of MgH2 and very weak peaks of LiBH4 and AC additive, indicative of its nanocomposite feature. After the two-step dehydrogenation reactions, only MgB2 and Mg crystalline phases were clearly identified in the XRD pattern. Here, the invisibility of LiH product should be attributed to its nanophase structure and the low X-ray scattering cross sections of the light component elements. The identification of Mg in the
Fig. 4 e Cyclic dehydriding (DH) curves of the 2LiBH4eMgH2eAC composite sample at 400 C under 0.3 MPa H2.
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Fig. 5 e XRD patterns of the 2LiBH4eMgH2eAC composite sample at different states: (a) as-prepared sample, (b) the sample collected at the end of the first dehydrogenation stage, (c) dehydrogenated sample and (d) rehydrogenated sample.
final dehydrogenation product is an indication of the local deficiency of LiBH4 reactant, which has twofold reasons. One is the formation of LiBH4 nanoparticles, which undergoes selfdecomposition. The other is segregation of LiBH4 from contacting with Mg by AC. In accordance with the latter speculation, the FTIR analysis detected residual BH4 signal in the dehydrogenated sample, as seen in Fig. 6 pattern (b). The XRD and FTIR results well confirmed the reversible dehydrogenation and rehydrogenation reactions of the 2LiBH4eMgH2eAC sample. But owing to the presence of unreacted LiBH4, the deliverable hydrogen capacity is lower than the theoretical value. As demonstrated above, AC is an effective additive for promoting the reversible dehydrogenation of 2LiBH4eMgH2 system. A central issue in understanding the promoting effect of AC is to clarify the mechanistic reason for the elimination of
Fig. 6 e FTIR spectra of the 2LiBH4eMgH2eAC composite sample at different states: (a) as-prepared sample, (b) dehydrogenated sample and (c) rehydrogenated sample.
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Fig. 7 e Solid-state 11B MAS NMR spectrum of the 2LiBH4eMgH2eAC sample collected at the end of the first dehydrogenation stage. The asterisks denote spinning sidebands. The arrow shows the expected peak position of MgB2.
the incubation period between the two dehydrogenation steps. According to the literature reports [27,28], direct addition of MgB2 or the formation of TM borides can effectively shorten or even eliminate the incubation period. Motivated by these findings, the first possibility that came to our mind is the formation of MgB2 at the first dehydrogenation stage. To check
this point, we collected sample at the end of the first dehydrogenation stage and carefully examined it using a combination of XRD and solid-state 11B MAS NMR techniques (Here, FTIR is an invalid tool because the characteristic IR bands of MgB2 completely overlap with those of LiBH4). But all the measurement results clearly showed the absence of MgB2 at the end of first dehydrogenation stage, as seen in Figs. 5b and 7. Now that the possibility of in situ formation of MgB2 nuclei was precluded, we believed that the tailored nanostructure of the 2LiBH4eMgH2 composite by AC should be responsible for the elimination of incubation period. In the sample preparation process, the applied melt infiltration step produced LiBH4eAC nanocomposite, in which the LiBH4 nanoparticles either reside in the nanopores or adhere to the surface of AC. In the subsequent milling process together with MgH2, AC may effectively preventing agglomeration and/or coldwelding of MgH2 powder, thus promoting uniform and homogeneous mixing of MgH2 with supported LiBH4. Evidence supporting this speculation has been obtained from SEM morphology observation. As seen in Fig. 8, the EDS maps of Mg, B and C elements showed quite similar distribution, indicating a homogenous mixing of the MgH2, LiBH4 and AC component phases in a nanometer scale. In view of the small pore size (around 2 nm) of AC and the applied preparation procedure, the MgH2 nanoparticles should predominantly dispersed on the surface of AC. If the LiBH4 and MgH2 nanoparticles are in intimate contact, the generated Mg nanoparticles from the first dehydrogenation step may then react
Fig. 8 e SEM morphology and EDS maps of the as-prepared 2LiBH4eMgH2eAC sample.
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with the molten LiBH4. Possibly, the lack of incubation period is associated with the size-dependent free energy and the unique surface atomic structure of nanoparticles. In this regard, detail theoretical studies are still required for better understanding the stability and reaction kinetics of nanoparticles. An important advantage of using scaffold materials like AC is the stability of the tailored nanostructure of the reactant composites in the H-cycles at elevated temperature. This is supported by the observed stability of cyclic property of the 2LiBH4eMgH2eAC nanocomposite. But meanwhile, the usage of currently available scaffolds causes serious capacity loss of the nanocomposites. Development of advanced technologies for preparation of light-weight scaffolds with suitable pore structure is therefore a prerequisite for the practical hydrogen storage application of the nano-confinement strategy.
4.
Conclusions
Addition of AC to the 2LiBH4eMgH2 system results in significant improvements in dehydrogenation kinetics and cyclic stability. In particular, the problematic long incubation period prior to the second dehydrogenation step of the 2LiBH4eMgH2 system can be eliminated by addition of AC. For example, the 2LiBH4eMgH2eAC composite sample prepared by a three-step method can reversibly deliver about 9 wt.% hydrogen at 400 C within 7 h, which is comparable to the performance of the systems with TM additives. A combination of phase/microstructure/chemical state analyses showed that AC exerts its promoting effect via tailoring nanophase structure of the 2LiBH4eMgH2 composite. This study, together with the previous works, illustrates the feasibility of using foreign scaffold materials to improve the hydrogen storage property of reactive hydride composite.
Acknowledgments The financial supports from the National Program on Key Basic Research Project (973 Program, Grant No. 2010CB631305), the National Outstanding Youth Science Foundation of China (Grant No. 51125003), the Natural Science Foundation of Liaoning Province (Grant No. 201202219) and Guangxi Key Laboratory of Information Materials (Grant No. 1110908-01-K) are gratefully acknowledged.
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journal homepage: www.elsevier.com/locate/he
Improved reversible dehydrogenation properties of LiBH4eMgH2 composite by tailoring nanophase structure using activated carbon Kuikui Wang a,b, Xiangdong Kang a, Junhong Luo a, Chaohao Hu c, Ping Wang a,* a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China University of Science and Technology of China, Hefei 230026, China c Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China b
article info
abstract
Article history:
In this study, activated carbon (AC) was added to the 2LiBH4eMgH2 composite and
Received 17 September 2012
examined with respect to its effect on the hydrogen storage properties of the system. Our
Received in revised form
study found that AC is an effective additive for promoting the reversible dehydrogenation
12 December 2012
of the 2LiBH4eMgH2 composite. A series of control experiments were carried out to opti-
Accepted 7 January 2013
mize the sample preparation method, milling time and addition amount of AC. In com-
Available online 5 February 2013
parison with the neat LiBH4eMgH2 system, the LiBH4eMgH2eAC composite prepared under optimized conditions exhibits enhanced dehydrogenation kinetics, improved cyclic sta-
Keywords:
bility and particularly, eliminated incubation period between the two dehydrogenation
Hydrogen storage
stages. A combination of phase/microstructure/chemical state analyses has been con-
Lithium borohydride
ducted to gain insight into the promoting effect of AC on the reversible dehydrogenation of
Reactive hydride composite
the 2LiBH4eMgH2 system. Our study found that AC exerts its promoting effect via tailoring
Activated carbon
nanophase structure of the 2LiBH4eMgH2 composite.
Nanoparticle
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The widespread implementation of hydrogen fuel cell technology requires advanced materials that can store and deliver large amounts of hydrogen at moderate temperatures with fast kinetics [1]. Recently, lithium borohydride (LiBH4) and other light metal borohydrides received considerable attention as potential hydrogen storage media [2e4]. Several strategies have been developed to address their thermodynamic and/or kinetic problems that are essentially imposed by the strong and directional chemical bonds [5,6]. These strategies include reactant destabilization [7], nano-confinement using scaffolds [8e10], catalyst doping [11e14], and cation/
anion substitution [15e20]. In a general view, these technological advances have enabled significant improvements in the reversible dehydrogenation properties of LiBH4. Particularly, the employment of reactant destabilization strategy has given rise to a wealth of new reactive hydride composites, which offers new possibilities for the development of highperformance hydrogen storage materials [21e24]. 2LiBH4 þ MgH2 4 2LiH þ MgB2 þ 4H2
(1)
Combination of LiBH4 and MgH2 in a 2:1 M ratio constitutes a representative reactive hydride composite, which undergoes reversible dehydrogenation/rehydrogenation reactions
* Corresponding author. Tel.: þ86 24 2397 1622; fax: þ86 24 2389 1320. E-mail address: [email protected] (P. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.034
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following Eq. (1). In comparison with pristine LiBH4, the 2LiBH4eMgH2 composite exhibits significantly lowered reaction enthalpy (by around 25 kJ/mol H2), remarkably improved reversibility and comparable deliverable hydrogen capacity [7]. But owing to the sluggish kinetics, the 2LiBH4eMgH2 composite typically requires a temperature of at least 400 C for fulfilling dehydrogenation at a reasonable rate [25,26]. This is much higher than the predicted value (around 225 C) using thermodynamic data [7]. In the past years, impressive progresses have been made in improving the dehydrogenation kinetics of the LieMgeBeH system. A number of transition metal (TM) compounds have been identified to be effective for promoting the reversible dehydrogenation of the 2LiBH4eMgH2 composite [27e33]. According to the studies by Bo¨senberg and coworkers [27e29], the various TM additives may share a common promoting mechanism, that is, formation of TM borides which act as heterogeneous nucleation sites for MgB2 product. Alternatively, several nanoconfined systems were prepared by melt infiltration method using carbon aerogel scaffold materials [34,35]. Indeed, the nanoconfined 2LiBH4eMgH2 samples showed property advantages over their bulk counterpart. But the obtained kinetics improvement is much less pronounced than that observed in the nanoconfined LiBH4 systems [8e10]. This might be understood from the technical difficulties in simultaneously and efficiently incorporating LiBH4 and MgH2 phases into the nanopores of scaffold materials. In this paper, we report our trial of using activated carbon (AC) to improve the reversible dehydrogenation properties of LieMgeBeH system. The selection of AC as structure-tailoring agent is based on its chemical inertness, nanoporous texture, commercial availability and low cost [36,37]. Our study found that melt infiltration of LiBH4 in AC followed by mechanically milling with MgH2 provides an optimized procedure for preparation of 2LiBH4eMgH2eAC nanocomposite. In comparison with the neat 2LiBH4eMgH2 system, the nanocomposite with AC additive exhibits notable property advantages in terms of dehydrogenation kinetics and cycling stability. According to a combination of phase/microstructure/chemical state analyses, the observed property improvements should be attributed to the AC-tailored nanophase structure of LiBH4 and MgH2 reactants.
comparison, the LiBH4/MgH2/AC, LiBH4/MgH2/graphite and LiBH4/MgH2 powder mixtures were mechanically milled under Ar atmosphere at 400 rpm for 1 h. In all cases, the LiBH4:MgH2 molar ratio was adjusted to be 2:1 in accordance with Eq. (1). All sample operations were performed in an Ar (99.999%)-filled glovebox, wherein the H2O/O2 levels were typically <0.1 ppm. Hydriding/dehydriding properties of the samples were examined using a carefully calibrated Sievert’s type apparatus. A typical cyclic experiment entailed dehydrogenation at 400 C under 0.3 MPa H2 and hydrogenation at 350 C under an initial H2 pressure of 10 MPa for 6 h. To minimize H2O/O2 contamination during the volumetric measurements, the hydrogen feed (with an initial purity of 99.999%) was further purified using a hydrogen storage alloy system. For comparison with the neat sample, the weight of AC additive was not taken into account in determination of the H-capacity of the 2LiBH4eMgH2eAC nanocomposite sample. The 2LiBH4eMgH2eAC and relevant samples at varied states were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu Ka radiation), Fourier transform infrared spectroscopy (FTIR, Bruker TENSOR 27, 4 cm1 resolution) and scanning electron microscope (SEM, LEO Supra 35) equipped with an energy dispersive X-ray analysis unit (EDS, Oxford). The XRD powder sample was covered by an amorphous polymer tape to avoid oxidation during the measurement. FTIR spectra of the samples were collected using the KBr-pellet method and the obtained spectra were normalized using OPUS 6.5 software. The SEM samples were prepared by spreading the dry powder onto conductive carbon tape supported on a copper pole. All the sample preparations were performed in the glovebox and special measures were taken to minimize H2O/O2 contamination during the sample transfer processes. Solid-state 11B MAS NMR experiments were conducted in a Bruker AVANCE III 400 WB spectrometer operating at a magnetic field of 9.4 T on 128.3 MHz 11B frequency. The samples were packed in the ZrO2 rotor closed with a Kel-F cap and spun at 12 kHz rate. A total of 16 scans were recorded with a 2 s recycle delay for each sample. All 11B chemical shifts are referenced to the resonance of solid LiBH4 (41 ppm).
3. 2.
Experimental section
LiBH4 (95% purity) and graphite (99.99þ% purity, <45 mm particle size) from SigmaeAldrich, and MgH2 (98% purity) from Alfa Aesar were used as received. AC (with a maximum ash content <4 wt.%) from Alfa Aesar was calcined in a quartz reactor at 1000 C for 5 h under a hydrogen gas flow to remove impurities such as sulfur, nitrogen, oxygen and chlorine [38]. The 2LiBH4eMgH2eAC nanocomposite sample was prepared following a three-step procedure. The LiBH4/AC mixture was first milled under Ar atmosphere for 5 min using a Fritsch 7 Planetary mill at 200 rpm in a stainless steel vial together with eight steel balls (10 mm in diameter). The post-milled mixture was then calcined at 320 C under 3.0 MPa H2 pressure for 1 h. Finally, MgH2 powder was added and mechanically milled together with LiBH4/AC under Ar atmosphere at 400 rpm. For
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Results and discussion
3.1. Dehydrogenation properties of the 2LiBH4eMgH2eAC composite
MgH2 4 Mg þ H2
(2)
2LiBH4 þ Mg 4 2LiH þ MgB2 þ 3H2
(3)
It has been well established that the 2LiBH4eMgH2 composite fulfills two-step dehydrogenation reactions following Eqs. (2) and (3), respectively, under appropriate H2 back pressure. One of the major problems of this LieMgeBeH system is the interruption of the two dehydrogenation steps by a long incubation period. Out study showed that this problem can be alleviated by addition of AC. As seen in Fig. 1, the long
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Fig. 1 e Dehydriding curves of the sample (a) neat 2LiBH4eMgH2; (b) directly milled 2LiBH4eMgH2eAC; (c) the 2LiBH4eMgH2eAC sample prepared following the threestep procedure and (d) 2LiBH4eMgH2egraphite at 400 C under 0.3 MPa H2. In the samples (b), (c) and (d), the LiBH4:C weight ratio is 1:1.
incubation period of around 14 h as observed in the neat 2LiBH4eMgH2 sample was markedly shortened to around 3 h after milling with AC additive (the LiBH4:AC weight ratio is 1:1). Furthermore, our study found that the incubation period can be further reduced to less than 2 h via changing the sample preparation method from routine one-step milling to a three-step procedure, which involves ordinal steps of premilling LiBH4/AC mixture, melt infiltration of LiBH4 into AC and milling together with MgH2. This three-step procedure was applied in the subsequent sample preparation. For comparison, the nonporous graphite was also used as additive and examined with respect to its effect on the dehydrogenation property of the 2LiBH4eMgH2 system. It was observed that the addition of graphite also caused reduction of incubation period, but with a much less pronounced effect than AC. This finding clearly indicates that the promoting effect of AC should be predominantly attributed to its nanoporous texture. Notably, the 2LiBH4eMgH2eAC samples rapidly released around 4.5 wt.% hydrogen in the first dehydrogenation stage, which is much higher than that delivered from MgH2 (2.85 wt.% hydrogen). As no evidence has been obtained supporting the occurrence of the reaction between Mg and LiBH4 at the first dehydrogenation stage (as discussed at Section 3.2), the extra amount of hydrogen should come from the decomposition of the LiBH4 nanophase that was structurally tailored by AC. This deduction was well confirmed by the control experiment using LiBH4/AC nanocomposite prepared by a melt infiltration method (result not shown here). Clearly, tailoring nanophase structure of LiBH4 causes a modification of thermodynamics [8,9]. But currently, whether or not the thermodynamics modification is a just consequence of nanosize effect of LiBH4 is still unclear. In this regard, detailed study is still required for better mechanistic understanding. Next, we conducted a series of control experiments to optimize the addition amount of AC and the milling condition.
Fig. 2 presents a comparison of the dehydrogenation profiles of three samples with different LiBH4:AC weight ratios. It was observed that increasing the AC amount resulted in an increasingly favorable dehydrogenation property owing to the promoted formation of LiBH4 nanoparticles. But meanwhile, addition of AC additive causes a serious capacity penalty of the system. To make a compromise between hydrogen capacity and kinetic property, we selected the sample with a LiBH4:AC weight ratio of 1:1 for further study. Fig. 3 shows the dehydrogenation profiles of the 2LiBH4eMgH2eAC samples milled for a duration ranging from 1 to 10 h. Increasing the milling time from 1 to 5 h resulted in enhanced dehydrogenation kinetics, slightly increased hydrogen capacity and particularly, the elimination of the incubation period. But further prolongation of the milling time caused no appreciable improvement of the dehydrogenation property. We therefore fixed the milling time at 5 h in the subsequent sample preparation. Fig. 4 presents the cyclic dehydrogenation curves of the 2LiBH4eMgH2eAC sample prepared under optimized condition. It was observed that the sample released nearly identical H2 amount in the first two cycles, but with a subtle difference on the dehydrogenation behavior. In the second cycle, the sample released less hydrogen at the first dehydrogenation step, but more hydrogen at the second step in comparison with the corresponding steps in the first cycle. Presumably, these are two correlated phenomena associated with particle coalescence. In the sample preparation process, the intensive milling may partially destroy the nanoporous structure of AC. As a consequence, some of the LiBH4 nanoparticles may not be well confined within the nanopores of AC. This offers chance for coalescence of some LiBH4 nanoparticles in the rehydrogenation process. The coalesced particles larger than the critical size will behave like bulk LiBH4, that is, reacting with Mg instead of undergoing self-decomposition under the applied condition. This will cause lowered H2 desorption at the first dehydrogenation stage and increased H2 contribution from the second step. The 2LiBH4eMgH2eAC sample can
Fig. 2 e Dehydriding curves of the composite samples with different LiBH4:AC weight ratios. The measurements were conducted at 400 C under 0.3 MPa H2.
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Fig. 3 e Dehydriding curves of the 2LiBH4eMgH2eAC composite milled for different periods.
reversibly store about 9 wt.% hydrogen at a cyclic condition of dehydrogenation at 400 C for 7 h and rehydrogenation at 350 C under 10 MPa H2 for 6 h. This cyclic performance compares favorably with the reported results of the nanocomposite systems using carbon aerogel scaffolds [34,35].
3.2.
Structural characterization and mechanism study
A combination of XRD and FTIR analyses has been conducted to characterize the 2LiBH4eMgH2eAC sample at different states. As seen in Fig. 5, the as-prepared sample showed weak peaks of MgH2 and very weak peaks of LiBH4 and AC additive, indicative of its nanocomposite feature. After the two-step dehydrogenation reactions, only MgB2 and Mg crystalline phases were clearly identified in the XRD pattern. Here, the invisibility of LiH product should be attributed to its nanophase structure and the low X-ray scattering cross sections of the light component elements. The identification of Mg in the
Fig. 4 e Cyclic dehydriding (DH) curves of the 2LiBH4eMgH2eAC composite sample at 400 C under 0.3 MPa H2.
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Fig. 5 e XRD patterns of the 2LiBH4eMgH2eAC composite sample at different states: (a) as-prepared sample, (b) the sample collected at the end of the first dehydrogenation stage, (c) dehydrogenated sample and (d) rehydrogenated sample.
final dehydrogenation product is an indication of the local deficiency of LiBH4 reactant, which has twofold reasons. One is the formation of LiBH4 nanoparticles, which undergoes selfdecomposition. The other is segregation of LiBH4 from contacting with Mg by AC. In accordance with the latter speculation, the FTIR analysis detected residual BH4 signal in the dehydrogenated sample, as seen in Fig. 6 pattern (b). The XRD and FTIR results well confirmed the reversible dehydrogenation and rehydrogenation reactions of the 2LiBH4eMgH2eAC sample. But owing to the presence of unreacted LiBH4, the deliverable hydrogen capacity is lower than the theoretical value. As demonstrated above, AC is an effective additive for promoting the reversible dehydrogenation of 2LiBH4eMgH2 system. A central issue in understanding the promoting effect of AC is to clarify the mechanistic reason for the elimination of
Fig. 6 e FTIR spectra of the 2LiBH4eMgH2eAC composite sample at different states: (a) as-prepared sample, (b) dehydrogenated sample and (c) rehydrogenated sample.
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Fig. 7 e Solid-state 11B MAS NMR spectrum of the 2LiBH4eMgH2eAC sample collected at the end of the first dehydrogenation stage. The asterisks denote spinning sidebands. The arrow shows the expected peak position of MgB2.
the incubation period between the two dehydrogenation steps. According to the literature reports [27,28], direct addition of MgB2 or the formation of TM borides can effectively shorten or even eliminate the incubation period. Motivated by these findings, the first possibility that came to our mind is the formation of MgB2 at the first dehydrogenation stage. To check
this point, we collected sample at the end of the first dehydrogenation stage and carefully examined it using a combination of XRD and solid-state 11B MAS NMR techniques (Here, FTIR is an invalid tool because the characteristic IR bands of MgB2 completely overlap with those of LiBH4). But all the measurement results clearly showed the absence of MgB2 at the end of first dehydrogenation stage, as seen in Figs. 5b and 7. Now that the possibility of in situ formation of MgB2 nuclei was precluded, we believed that the tailored nanostructure of the 2LiBH4eMgH2 composite by AC should be responsible for the elimination of incubation period. In the sample preparation process, the applied melt infiltration step produced LiBH4eAC nanocomposite, in which the LiBH4 nanoparticles either reside in the nanopores or adhere to the surface of AC. In the subsequent milling process together with MgH2, AC may effectively preventing agglomeration and/or coldwelding of MgH2 powder, thus promoting uniform and homogeneous mixing of MgH2 with supported LiBH4. Evidence supporting this speculation has been obtained from SEM morphology observation. As seen in Fig. 8, the EDS maps of Mg, B and C elements showed quite similar distribution, indicating a homogenous mixing of the MgH2, LiBH4 and AC component phases in a nanometer scale. In view of the small pore size (around 2 nm) of AC and the applied preparation procedure, the MgH2 nanoparticles should predominantly dispersed on the surface of AC. If the LiBH4 and MgH2 nanoparticles are in intimate contact, the generated Mg nanoparticles from the first dehydrogenation step may then react
Fig. 8 e SEM morphology and EDS maps of the as-prepared 2LiBH4eMgH2eAC sample.
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with the molten LiBH4. Possibly, the lack of incubation period is associated with the size-dependent free energy and the unique surface atomic structure of nanoparticles. In this regard, detail theoretical studies are still required for better understanding the stability and reaction kinetics of nanoparticles. An important advantage of using scaffold materials like AC is the stability of the tailored nanostructure of the reactant composites in the H-cycles at elevated temperature. This is supported by the observed stability of cyclic property of the 2LiBH4eMgH2eAC nanocomposite. But meanwhile, the usage of currently available scaffolds causes serious capacity loss of the nanocomposites. Development of advanced technologies for preparation of light-weight scaffolds with suitable pore structure is therefore a prerequisite for the practical hydrogen storage application of the nano-confinement strategy.
4.
Conclusions
Addition of AC to the 2LiBH4eMgH2 system results in significant improvements in dehydrogenation kinetics and cyclic stability. In particular, the problematic long incubation period prior to the second dehydrogenation step of the 2LiBH4eMgH2 system can be eliminated by addition of AC. For example, the 2LiBH4eMgH2eAC composite sample prepared by a three-step method can reversibly deliver about 9 wt.% hydrogen at 400 C within 7 h, which is comparable to the performance of the systems with TM additives. A combination of phase/microstructure/chemical state analyses showed that AC exerts its promoting effect via tailoring nanophase structure of the 2LiBH4eMgH2 composite. This study, together with the previous works, illustrates the feasibility of using foreign scaffold materials to improve the hydrogen storage property of reactive hydride composite.
Acknowledgments The financial supports from the National Program on Key Basic Research Project (973 Program, Grant No. 2010CB631305), the National Outstanding Youth Science Foundation of China (Grant No. 51125003), the Natural Science Foundation of Liaoning Province (Grant No. 201202219) and Guangxi Key Laboratory of Information Materials (Grant No. 1110908-01-K) are gratefully acknowledged.
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