稀有金属(英文版) 2019,38(04),321-326

Improved hydrogen storage properties of LiBH₄ confined with activated charcoal by ball milling

He Zhou Xin-Hua Wang Hai-Zhen Liu Shi-Chao Gao Mi Yan

作者简介：*Xin-Hua Wang,e-mail:xinhwang@zju.edu.cn;

收稿日期：23 February 2017

基金：financially supported by the National Natural Science Foundation of China(Nos. 51471149 and 51171168);the Public Project of Zhejiang Province (No. 2015C31029);

Improved hydrogen storage properties of LiBH₄ confined with activated charcoal by ball milling

He Zhou Xin-Hua Wang Hai-Zhen Liu Shi-Chao Gao Mi Yan

State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Zhejiang University

State Key Laboratory of Advanced Transmission Technology,Global Energy Interconnection Research Institute, State Grid Corporation of China

Abstract：

In order to enhance the hydrogen storage properties of LiBH₄, activated charcoal(AC) was used as the scaffold to confine LiBH₄ in this paper. Ball milling was used to prepare LiBH₄/AC composites. Experimental results show that dehydrogenation properties of ball-milled LiBH₄/AC(LiBH₄/AC-BM) are greatly improved compared with that of pristine LiBH₄, ball-milled LiBH₄(LiBH₄-BM) and hand-milled LiBH₄/AC(LiBH₄/ACHM). The onset dehydrogenation temperature of LiBH₄ for LiBH₄/AC-BM is around 160 ℃, which is 170 ℃ lower than that of pristine LiBH₄. At around 400 ℃, LiBH₄/ACBM finishes the dehydrogenation with a hydrogen capacity of 13.6 wt%, which is approximately the theoretical dehydrogenation capacity of pure LiBH₄(13.8 wt%), while the dehydrogenation processes for LiBH₄-BM and LiBH₄/AC-BM do not finish even when they were heated to 600 ℃. The isothermal dehydriding measurements show that it takes only 15 min for LiBH₄/AC-BM to reach a dehydrogenation capacity of 10.1 wt% at 350 ℃, whereas the pristine LiBH₄ and the LiBH₄/AC-HM release hydrogen less than 1 wt% under the same conditions. The dehydrogenation process and the effect of AC were discussed.

Keyword：

Hydrogen storage properties; Hydrogen storage materials; Lithium borohydride; Activated charcoal;

Received： 23 February 2017

1 Introduction

Hydrogen is considered as one of the cleanest and most sustainable energy carrier which has great potential to operate on stationary and onboard applications [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] .Unfortunately,hydrogen is a gas that is very difficult to store efficiently under ambient conditions.As for the irreversible hydrogen storage methods,the production of hydrogen by hydrolysis reaction of hydrides (NaBH₄,MgH₂,etc.) recently attracts increased attention [ 14, 15, 16, 17, 18, 19, 20, 21] because of its safety and its amenability to mild reaction conditions.As for the reversible hydrogen storage methods,complex hydrides are receiving a worldwide attention due to their high hydrogen capacity [ 22, 23] .Among the complex hydrides,lithium borohydride (LiBH₄) is one of the most promising hydrogen storage materials because it possesses a gravimetric hydrogen density of 18.3 wt%and a volumetric hydrogen density of 121 kg·m^-3 H₂.However,the high desorption temperature (～500℃),inaccessible rehydrogenation conditions (35×10⁶ Pa H₂ and600℃) and emission of toxic diborane (B₂H₆) during decomposition are all the barriers that hinder its practical application [ 24, 25, 26, 27] .

To improve the dehydrogenation property of the complex hydride,several methods have been adopted.Some strategies such as reactions with other hydrides [ 28, 29, 30, 31] ,doping with catalyst [ 32, 33, 34, 35] ,nonporous scaffold confinement [ 36, 37, 38, 39] and cation substitution [ 40, 41, 42] ,are confirmed to be successful in enhancing the dehydrogenation property of LiBH₄.Yu et al. [ 43] used carbon nanotube as a catalyst to improve the hydrogen storage properties of LiBH₄.The sample starts to dehydrogenate at 250°C and the majority of hydrogen being released below 600°C.A quarter of the hydrogen capacity could be reversed under the condition of 10 MPa H₂ and 400℃.Zhang et al. [ 44] also used disordered mesoporous carbon as an additive to enhance the hydrogen storage performances of LiBH₄.The sample released a hydrogen capacity of 14 wt%below600℃.Agresti et al. [ 45] used solvent infiltration method to disperse the LiBH₄ on multi-walled carbon nanotubes.The onset dehydrogenation temperature of this sample is60°C lower than that of pristine LiBH₄.Liu et al. [ 46] used a nonporous hard carbon with an average pore size of 2 nm to nanoconfine LiBH₄,which leaded to a dramatically reduced onset dehydrogenation temperature (from 460 to220℃).

Ball-milling method is also a very effective method to improve the dehydrogenation properties of metal hydrides.Several studies have showed that using ball milling could also provide a hydrides/scaffold nanocomposite [ 47, 48, 49, 50, 51] .Zhang et al. [ 44] found that through ball milling,the LiBH4/C could form a nanocomposite.The mesoporous carbon had two effects on the decomposition of LiBH₄,which is nanodispersion of LiBH₄ and reactive agent of carbon.Xu et al. [ 49] also used ball milling as a doping mechanism to combine graphene with LiBH₄.The graphene could lower the onset decomposition temperature from 420 to 195℃.The reasons of the improved properties could be ascribed to the increase in the contact area between graphene and LiBH₄ and confinement in nanoporous graphene.Despite the many favored enhancements,ball milling is known to destroy fragile carbon scaffold nanostructures [ 52, 53] and could introduce Fe impurity into the sample [ 54] .

In this study,a very common carbon scaffold,the activated charcoal (AC),was used as the porous material to confine LiBH_4.Ball milling was used as the method to combine the two components.It is found that the ballmilled LiBH4/AC (LiBH₄/AC-BM) shows remarkable improvement of the dehydrogenation property.

2 Experimental

LiBH₄ (95%purity) was purchased from Acros Organics and used without further purification.Activated charcoal(AC,99%purity) was also purchased from Acros Organics and was purified under 500℃and vacuum in a reactor for5 h to remove moisture and other possible impurities.The LiBH₄/AC ball-milled nanocomposite (LiBH₄/AC-BM)was prepared by directly mixing LiBH₄ and AC,and ballmilled using a QM-3SP4 planetary ball mill (Nanjing NanDa Instrument Plant).The sample was loaded into a100-ml stainless-steel vial with a ball-to-powder ratio of50:1.The milling was carried out at 500 r·min^-1 for 1 h.The process was paused for 6 min every half hour to cool down the vial and sample,preventing the temperature rising during the long-term ball milling.Three control samples were also prepared:(1) hand-milled LiBH₄/AC(LiBH₄/AC-HM),in which LiBH₄ and AC scaffold were mixed by hand to avoid the high energy during the ball milling process;(2) pristine LiBH₄;and (3) ball-milled LiBH₄ (LiBH₄-BM),in which LiBH₄ was ball-milled under the same procedure as described above.All sample operations were performed in an Ar atmosphere.

Dehydriding measurements were taken on a homemade Sieverts-type apparatus.About 200 mg sample was loaded into a stainless-steel reactor,which was connected to a thermocouple and a pressure sensor to monitor the temperature and pressure inside the reactor.For the nonisothermal desorption measurements,i.e.the temperatureprogrammed desorption (TPD) measurements,the samples were heated gradually from room temperature to 600°C at a heating rate of 2℃·min^-1.For the isothermal desorption measurements,the samples were heated quickly to 350°C under a back pressure of 5 MPa H₂ to prevent the decomposition of the samples.Then,the reactor was evacuating quickly to start the isothermal measurements.

The Brunauer-Emmett-Teller (BET) measurements were carried out by using an Autosorb-l-C surface area and pore size analyzer from Quantachrome.Differential scanning calorimetry (DSC) and thermogravimetric analysis(TG) were conducted using a differential scanning calorimeter (Netzsch STA449F3),which was equipped with a mass spectrometer (MS,Netzsch QMS403C) to detect the hydrogen desorption synchronously.X-ray diffraction (XRD) analysis was performed on a PANalytical X-ray diffractometer (X'Pert Pro,CuKα,40 kV,40 mA);a specially designed sample holder was used to avoid sample oxidation.

3 Results and discussion

3.1 BET measurements

The N₂ adsorption measurements of the activated charcoal used as the supporting scaffold of the borohydrides and the prepared LiBH₄/AC-HM were taken to determine the specific surface area and porosity,as shown in Fig.1.The analysis results of absorption/desorption experiments are summarized in Table 1.As shown,the pore volume and specific surface area (SSA) of LiBH₄/AC-HM composite are lower than that of AC.It can be ascribed to two reasons.The first one is that part of the pores of AC is filled with LiBH₄ particles,and the second one is that themicros truc tures of the scaffolds are partly destroyed during the milling process.

Fig.1 BET measurement results:a N₂ adsorption curves (p,partial pressure of nitrogen;p₀,saturation vapor pressure of nitrogen) and b pore size distribution of AC and LiBH₄/AC-BM

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Table 1 Pore volume and specific surface area (SSA) of AC and LiBH₄/AC-BM

3.2 Dehydrogenation properties

The dehydrogenation curves of the LiBH₄/AC-BM and the control samples are shown in Fig.2.As shown,the LiBH₄/AC-BM shows the best dehydrogenation properties.It starts to dehydrogenate at around 160℃,which is 170℃lower than that of pristine LiBH₄,and reaches full dehydrogenation at 400℃with a dehydrogenation capacity of13.6 wt%.This is approximately the theoretical dehydrogenation capacity of pure LiBH₄ (13.8 wt%).As for the three control samples,the dehydrogenation processes do not finish even when they were heated to 600°C.One can also see that the starting dehydrogenation temperatures of LiBH₄/AC-HM,LiBH₄-BM and pristine LiBH₄ are around 260,270 and 330℃,respectively.And the corresponding dehydrogenation capacity at 600°C is12.2 wt%,11.2 wt%and 11.1 wt%,respectively.At400℃,the dehydrogenation capacities of LiBH₄-BM and pristine LiBH₄ are lower than 2 wt%.The addition of AC scaffold has a remarkable positive effect on the dehydrogenation of LiBH₄,including catalytical effect and nanoconfinement effect.

Fig.2 Dehydrogenation curves of LiBH₄/AC-BM,LiBH₄/AC-HM,LiBH₄-BM and pristine LiBH₄

XRD patterns of LiBH₄/AC-BM and LiBH₄/AC-HM heated to various temperatures are shown in Fig.3.In the XRD pattern of the as-prepared sample,the diffraction peaks at about 21°and 27°are assigned to AC scaffolds,while most of the other peaks could be ascribed to LiBH₄.As shown,while heated to 250℃,the diffraction peaks of LiBH₄ are still visible.It could also be found that the diffraction peaks could be assigned to LiH,meaning that the LiBH₄ starts to decompose at this temperature.When heated to 300℃,the peaks of LiBH₄ become weakened and completely disappear when the temperature reaches400℃.We also find some diffraction peaks ascribed to LiC.

Figure 4 shows the synchronous DSC/TG/MS analysis of LiBH₄/AC-BM.The heating rate is 8℃·min^-1.It clearly shows that it undergoes a phase transformation from orthorhombic to hexagonal structure at 112.3℃and a melting process at 287.5 After that there is a main dehydrogenation reaction going on between 300 and400℃where the peak temperature appears at 367.5℃.This peak also appears in TG and MS results.Two small peaks can be found at around 310 and 455.7℃in DSC curve.The first peak is caused by partly decomposition of LiBH₄ during melting,and the second peak is caused by the partly decomposition of LiH produced in the first step of dehydrogenation.This result is confirmed by TG analysis.TG results suggest that the main mass lost is between300 and 400℃,but the onset temperature of the mass lost is at around 150℃.The dehydrogenation is slow at first,while it accelerates when the temperature reaches around320℃.There is also a slight weight loss at over 450℃,which could be caused by the reaction at 455.7℃in DSC curve.TG analysis also shows that the total weight loss is13.6 wt%.From MS analysis,one can see that a hydrogen release peak appears at 295.0℃,corresponding to the hydrogen release during the melting process of LiBH₄.In addition to that,MS analysis indicates that no other possible gas impurities appear during the dehydrogenation.

Fig.3 XRD patterns of LiBH₄/AC-BM heated to various temperatures

Fig.4 DSC/TG/MS curves of LiBH₄/AC-BM

3.3 Dehydriding kinetics

The dehydriding kinetics of LiBH₄/AC-BM was further studied by estimation of the kinetic barrier using the Kissinger method.The apparent activation energy (E_a) for dehydriding of sample is determined as follows:

where c is the heating rate in thermal analysis,T_p is the absolute temperature for the maximum reaction rate,R is the universal gas constant and A is also constant.In this study,the T_p data are extracted from DSC measurements at various heating rates (c=2,5,8,12 and 16℃·min^-1),as demonstrated in Fig.5.The apparent activation energy of LiBH₄/AC-BM for the reaction of (LiBH₄=LiH+B+3/2H₂) is estimated to be 140.7 kJ·mol^-1,which is lower than that of pure LiBH₄ sample (156 kJ·mol^-1).The reduction in the apparent activation energy causes the improvement in the dehydriding reaction kinetics of LiBH₄/AC-BM.

To further study the dehydriding kinetics,the isothermal dehydriding tests of the LiBH₄/AC-BM and some control samples were carried out under 350℃.The results are displayed in Fig.6 and show that LiBH₄/AC-BM has the best kinetic property.This sample finishes dehydrogenation in 15 min and reaches a dehydrogenation capacity of10.1 wt%during this period.This kinetic is much faster than that of other samples.For instance,the LiBH₄-BM reaches the same hydrogenation capacity in about 1h,whereas the pristine LiBH₄ and the LiBH4/AC-HM barely decompose under the provided condition.

4 Conclusion

With activated charcoal (AC) as scaffolds,LiBH4/AC composite was prepared by ball-milling process.The activated carbon scaffolds can enhance the dehydrogenation properties of LiBH₄.The onset dehydrogenation temperature for ball-milled LiBH₄/AC is lowered to160℃,which is 170℃lower than that of pristine LiBH₄,and the dehydrogenation finishes at around 400℃with a hydrogen release capacity of 13.6 wt%.And it reaches a dehydrogenation capacity of more than 10 wt%at 15 min for isothermal dehydriding under the condition of 350℃.The apparent activation energy of ball-milled LiBH₄/AC is estimated to be 140.7 kJ·mol^-1,which is lower than that of pure LiBH₄.The reduction in the apparent activation energy causes the improvement in the dehydriding reaction kinetics of LiBH₄/AC.

Fig.5 a DSC curves of LiBH₄/AC-BM under various heating rates and b apparent activation energy results determined by Kissinger method

Fig.6 Isothermal dehydriding curves of LiBH₄/AC-BM,LiBH₄/AC-HM,LiBIH₄-BM and pristine LiBH₄ at 350℃

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Nos.51471149 and51171168) and the Public Project of Zhejiang Province (No.2015C31029).

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