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Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst
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Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst He Zhou a, Liuting Zhang b, Shichao Gao a, Haizhen Liu c, Li Xu c, Xinhua Wang a,*, Mi Yan a,** a
State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China b School of Energy and Power, Jiangsu University of Science and Technology, 2 Meng Xi Road, Zhenjiang 212003, China c State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, State Grid Corporation of China, Beijing 102209, China
article info
abstract
Article history:
CeF3 as a catalyst is first added to activated carbon (AC) by ball milling under low rotation
Received 24 April 2017
speed. Then the treated AC was used as the scaffold to confine LiBH4 by melt infiltration
Received in revised form
process. The combined effects of confinement and CeF3 doping on the hydrogen storage
22 June 2017
properties of LiBH4 are studied. The experimental results show that LiBH4 and CeF3 are well
Accepted 23 June 2017
dispersed in the AC scaffold and occupy up to 90% of the pores of AC. Compared with pristine
Available online xxx
LiBH4, the onset dehydrogenation temperature for LiBH4-AC and LiBH4-AC-CeF3 decreases by 150 and 190 C, respectively. And the corresponding dehydrogenation capacity increases
Keywords:
from 8.2 wt% to 13.1 wt% for LiBH4-AC and 12.8 wt% for LiBH4-AC-CeF3, respectively. The
Hydrogen storage materials
maximum dehydrogenation speed of LiBH4-AC and LiBH4-AC-CeF3 is 80 and 288 times higher
Lithium borohydride
than that of pristine LiBH4 at 350 C. And LiBH4-AC andLiBH4-AC-CeF3 show good reversible
Activated carbon
hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release
Nanoconfinement
capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively.
Cerium fluoride
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to develop a new energy carrier system, different kinds of hydrogen storage materials have been extensively studied to optimize their properties [1e4]. Among these materials, complex hydrides (e.g. LiBH4, Mg(BH4)2, and NaBH4) comprised of light metal cations and borohydride have received much attentions due to their high gravimetric hydrogen density (e.g. 18.3 wt% H2 for LiBH4) [5e8]. These
hydrides are considered to be potential candidates for solid state hydrogen storage system [2,9]. However, the BeH covalent bonds in these borohydrides are too thermodynamically stable that they hamper the de/rehydrogenation process [10,11]. For example, the decomposition of LiBH4 requires over 400 C with sluggish kinetics and inferior reversibility [12e14]. In order to achieve a better dehydrogenation condition, several attempts have been made to increase the dehydrogenation properties of hydrides. One of the methods is catalysts
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Yan). http://dx.doi.org/10.1016/j.ijhydene.2017.06.193 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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additions, which is mixing the hydrides with metal oxides, chlorides or fluoride [15e17]. Such catalysts could lower the onset dehydrogenation temperature. Guo et al. [18] reported that when doped with TiF3, the decomposition temperature of LiBH4 could be reduced to 100 C while the system reaches a hydrogen capacity of 5.0 wt% at 250 C. Nielsen et al. [19] reported that nanoconfined NaAlH4 in carbon aerogel catalyzed with 3.0 wt% TiCl3 showed superior dehydrogenation kinetics over both nanoconfined NaAlH4 and bulk catalyzed NaAlH4. Kim et al. [7] reported that when doped with NbF5, 6LiBH4 þ CaH2 composite shows a reversible hydrogen capacity of 6 wt% at 450 C. Sun et al. [20] reported that nano Ni particles could significantly decrease the activation energy of LiBH4 in the LiBH4-AC system to 88 kJ/mol, which is much lower than that of pristine LiBH4 (156 kJ/mol) [10]. Zhou et al. [21] proved that addition of REF3 (RE ¼ Y, La, Ce) not only reduces the peak temperatures of both dehydrogenation steps of LiAlH4, but also reduces the activation energy for the two desorption steps. Another effective approach to improve the dehydrogenation properties is the adoption of nanoconfinement [22e26]. After space confinement, the metal hydrides are confined in the nano-structure of the scaffold materials, hence decrease the particle size of the hydrides while prevent agglomeration and phase segregation during decomposition [23]. Many reports have confirmed great enhancement in the thermodynamic and kinetic properties of metal hydrides using this method. Fang et al. [27] reported incorporation of LiBH4 into activated carbon using chemical impregnation. The sample starts to release hydrogen at over 220 C. Liu et al. [28] proved that when LiBH4 was confined with nanoporous hard carbon with hexagonally pores of 2 nm diameter, the onset dehydrogenation temperature reduced drastically while emission of diborane (B2H6) gas was suppressed. Xu et al. [29] confirmed that the onset dehydrogenation temperature of LiBH4-graphene ball-milling sample could be reduced from 420 C to 195 C. Ward et al. [30] showed that LiBH4:C60 nanocomposite had great reversible hydrogen storage properties as well as the ability to absorb hydrogen under mild conditions. Shao et al. [31] used a mesoporous carbon scaffold containing dispersed NbF5 to load LiBH4 and reported a combined effect of nanoconfinement and nanocatalysis on the dehydrogenation properties of LiBH4. Although their hydrogen storage properties have been greatly enhanced using these methods, complex hydrides still could not meet the requirement for onboard applications. Some works have been focused on combining the effect of catalyst doping and nano-confinement [8,16,17,19,20,32]. In this paper, the scaffold we used was activated carbon (AC) doped with CeF3 as catalyst. Activated carbon is a very commonly used porous structure and is very easy to obtain. CeF3 was added to activated carbon through ball milling and melt infiltration was used to prepare LiBH4-AC-CeF3 confined sample. The hydrogen storage properties of LiBH4 were greatly enhanced.
Experimental LiBH4 (95% purity) was purchased from Acros Organics and used without further purification. Activated carbon (AC, 99%
purity) was also purchased from Acros Organics and was purified under 500 C and vacuum in a reactor for 5 h to remove moisture and other possible impurities. CeF3 (99.99% purity) was purchased from Alfa Aesar. First, different amount of CeF3 catalyst was added to activated carbon in a milling vessel and they were ball milled for 1 h using a QM-3SP4 planetary ball mill with a rotation speed of 100 rpm. Then the mixed samples were removed from the vessel and used as scaffold in further preparation. The LiBH4-AC-CeF3 melt infiltration (MI) nanocomposite was prepared following a two-step procedure. First, the scaffold AC-CeF3 and LiBH4 (mass ratio 3:1) were mixed using the same planetary mill at 100 rpm for 5 min. The mixture was then delivered into a reactor and calcined at 290 C under 5 MPa H2 pressure for 1 h. All sample operations were performed in an Ar atmosphere. Dehydriding/rehydriding measurements were carried out on a homemade Sieverts-type apparatus. About 200 mg of 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 non-isothermal desorption measurements, i.e. the temperature programmed desorption (TPD) measurements, the samples were heated gradually from room temperature to 500 C at a heating rate of 2 C/min. For the isothermal desorption measurements, the samples were heated quickly to 350 C under a back pressure of 8 MPa H2 to prevent the decomposition of the samples. Then the reactor was evacuating quickly to start the isothermal measurements. Hydrogen capacity was estimated by the ideal gas equation using the temperature and pressure previously obtained. The rehydriding process was carried out under the conditions of 350 C and 6 MPa H2. The reactor was filled with H2 and then heated to 350 C rapidly and kept at that temperature for 5 h. The AC scaffold was characterized by gas absorption and desorption using an Autosorb-1-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, CuKa, 40 kV, 40 mA) where the samples were sealed with an amorphous membrane during the measurements to avoid oxidation. Field emission scanning electronic microscopes (SEM, Hitachi S-4800) were applied to analyze the morphology of the samples, while the energy dispersive spectroscopy (EDS) detector was used to study the elemental distributions.
Results and discussion Sample preparation In order to characterize the effect of melt infiltration, the surface area and pore volume were determined by BET measurements. Fig. 1 shows the results of N2 adsorption measurements for the as received activated carbon (AC), the activated carbon ball milled with CeF3 to be used as scaffold (AC-CeF3). Table 1 lists the surface area and pore volumeof the
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 1 e Nitrogen adsorption curves (a) and calculated pore size distribution (b) of AC scaffolds (black), AC-CeF3 (red) and prepared LiBH4-AC-CeF3 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 e Pore volume and specific surface area of AC scaffolds, AC-CeF3 and LiBH4-AC-CeF3.
AC (as received) AC-CeF3 LiBH4-AC-CeF3
Pore volume (cm3/g)
Specific surface area (m2/g)
0.5836 0.5828 0.0595
292.1 291.8 25.2
above samples calculated from Fig. 1. It can be seen from Fig. 1 and Table 1 that the nitrogen adsorption volume of AC scaffold doesn't change much after ball milled with catalyst CeF3, and the pore volume and the specific surface remains almost the same. After the melt infiltration process, the nitrogen adsorption volume decrease remarkably, and the pore volume and the specific surface also drastically decrease. One of the reasons for these phenomena is that the content of catalyst added is very low, so that limited effect is caused to the structure of AC scaffold. This also indicates that the ball milling process we adopted doesn't affect the porous structure of AC scaffold. After melt infiltration process, the porosity of the scaffold drops drastically, which is probably because the pores of the AC scaffold are filled with LiBH4. Fig. 2 shows the morphology of pure AC, AC-CeF3 and LiBH4-AC-CeF3 observed by SEM. Corresponding EDS images of C, Ce and B of LiBH4-AC-CeF3 are also showed in Fig. 2. It can be seen from Fig. 2 that the main particle size of AC is around 100 mm, and the size changes are not eminent during the whole preparation process. This indicates that the preparation method adopted here would not affect the structure of the scaffold. The EDS images indicates that Ce and B elements are also evenly distributed on the AC particles, which suggest that LiBH4 and CeF3 are well dispersed in the scaffold after the preparation process.
Dehydrogenation properties The dehydrogenation curves and DSC curves of the prepared sample with different catalyst loading are showed in Fig. 3.
From the dehydrogenation curves we can see thatThe pristine LiBH4 starts to decompose at around 350 C, and its decomposition is still not finished upon heating to 500 C, and it releases 8.2 wt% hydrogen after heated to 500 C. The onset dehydrogenation temperature of the confined samples LiBH4AC, LiBH4-AC-CeF3 decrease from 350 C to 200 C and 160e180 C, respectively, which is 150 and 190 C lower than that of pristine LiBH4. And the hydrogen desorption capacities increase to 13.1 and 12.8 wt% for LiBH4-AC, LiBH4-AC-CeF3 respectively. Compared to the results (220 C) observed by Fang et al. [27] in their LiBH4/AC system, the onset dehydrogenation temperature of LiBH4-AC-CeF3 sample is lower, while that of LiBH4-AC sample is very close. This result is caused by the addition of CeF3, which has a catalytically effect on the system. From the DSC curves in Fig. 3(b) we can see that an endothermic peak occurs at ~110 C in each sample, which represents the phase transformation of LiBH4, and a peak occurs at ~280 C, which is caused by the melting of LiBH4. For the pure LiBH4 sample, the decomposition peak occurs at 463.1 C. The peak reaction temperature of the LiBH4-AC is 356.0 C, while those of the nano-confined LiBH4 doped with different amount of catalyst LiBH4-AC-CeF3 are 320.1 C, 323.3 C and 326.2 C. Compared with pure LiBH4 sample, the decomposition temperature of LiBH4-AC and LiBH4-AC-CeF3 lowered drastically. It is clear that when doped with catalyst, the dehydrogenation temperatures of LiBH4-AC-CeF3 are 30e36 C lower then undoped LiBH4-AC. This phenomenon is in consistence with the result from TPD curves in Fig. 1(a). The peak reaction temperature also decreases slightly when more catalyst is added. We can find a small endothermic peak at over 450 C in all the samples. This might be caused by the partially decomposition of the LiH in the dehydrogenation reaction of LiBH4. The reason that this reaction is not showed in the TPD results is that during TPD tests, the H2 pressure in the system is too high for LiH to decompose. However in DSC tests, a constant Ar flow is going through in the system so the H2 pressure is proximity zero. Thus the decomposition of LiH could take place here. The TG/DSC/MS curves of LiBH4-AC-5 wt% CeF3 and LiBH4AC are showed in Fig. 4. It is clear that the dehydrogenation
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 2 e SEM images of AC (a), AC-CeF3(b) and LiBH4-AC-5 wt% CeF3 (c) and elemental mapping of Carbon (d), Cerium (e) and Boron (f) in the as-prepared LiBH4-AC-5 wt% CeF3.
Fig. 3 e TPD curves (a) and DSC curves (b) of as-prepared LiBH4-AC-x wt% CeF3 and LiBH4-AC.
characteristics are consistent with the TPD measurements. From the TG and H2-MS curves, we can see that the weight loss of both samples and the emission of H2 follow the same pattern. We also use MS to detect other possible product or impurity such as diborane or water, and no other gas is detected during the process. It suggests that the weight losses of both samples are caused solely by the emission of H2. To further understand the dehydrogenation mechanism of LiBH4-AC-5 wt% CeF3, it was heated to different temperatures and then cool down to determine its phase composition by XRD analysis. Fig. 5 shows the XRD patterns of LiBH4-AC-5 wt% CeF3 at different stages. The as prepared sample composes LiBH4 and carbon scaffold, as was discussed in sample preparation part. The amount of CeF3 is too small to be detected by XRD measurements during the whole decomposition process.
After the sample is heated to 100 C and 200 C, the XRD patterns remain unchanged. When the sample is heated to 300 C, the diffraction peaks of LiBH4 still exist but become weaker, while those of LiH start to appear. When the sample is heated to 400 C, the diffraction peaks of LiBH4 disappear completely whilst the peaks of LiH intensify. The XRD pattern of the sample doesn't change much when heated to 500 C. The diffraction peaks of LiH still exist while some weak peaks of Li start to appear in the sample. These results suggest that LiBH4 starts to decompose between the temperature of 200 and 300 C. However the reaction is completely down when the temperature reaches 400 C. The main reaction temperature of the sample is between 300 C and 400 C. There is an additional decomposition of LiH at over 400 C. This conclusion is in consistent with the results from the DSC/TG/MS analysis.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 4 e DSC/TG/MS curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC.
Kinetic properties The dehydriding kinetics of LiBH4-AC and LiBH4-AC-5 wt% CeF3 is further studied by estimation of the kinetic barrier using the Kissinger method. The apparent activation energy (Ea) for the dehydriding of sample is determined following ln
Fig. 5 e XRD patterns of LiBH4-AC-x wt% CeF3 sample after heating to various temperatures.
c Ea ¼ þA T2p RTp
In the equation, c is the heating rate in thermal analysis, and Tp is the absolute temperature for the maximum reaction rate. In addition, R is the universal gas constant and A is also constant, which is irrelevant in the linear fitting. In our study, theTp data for both LiBH4-AC and LiBH4-AC-5 wt% CeF3 are extracted from the DSC measurements at various heating rates (c ¼ 5, 8, 12 and 16ºC/min), as demonstrated in Fig. 6(a) and (b). The linear fitting of the data is demonstrated in Fig. 6(c). Theapparent activation energy of LiBH4-AC is estimated to be121 kJ/mol, while that of LiBH4-AC-5 wt% CeF3 is 108 kJ/mol. The activation energy of pure LiBH4 was estimated to be ~154 kJ/mol [10]. The activation energy of both samples is much lower than that of pure LiBH4. While doped with CeF3 as catalyst, the apparent activation energy of the sample is reduced and the dehydriding reaction kinetics is improved. Isothermal dehydriding experiments are also conducted and the results are showed in Fig. 6(d). As is showed, the amount of hydrogen desorbed in the pristine LiBH4 is very low, the maximum hydrogen release speed is around 0.4 wt% H2/h.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 6 e DSC curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC with various heating rates (c ¼ 5, 8, 12, 16 ºC/min), (c) estimation of apparent activation energies for both sample and (d) isothermal dehydriding curves of LiBH4-AC-x wt% CeF3, LiBH4-AC and pristine LiBH4.
As for the LiBH4-AC, it reaches a hydrogen capacity of 8.7 wt% within 1000 s, and has a full dehydrogenation capacity of 12.5 wt%. The maximum dehydrogenation speed reaches 32 wt% H2/h, which is 80 times higher than that of pristine LiBH4. The 5 wt% CeF3 doped sample LiBH4-AC-5 wt% CeF3 achieves a hydrogen capacity of 11.8 wt% within 500 s, and reaches a full dehydrogenation capacity of 13.5 wt%. The maximum dehydrogenation speed reaches 115 wt% H2/h which is around 288 and 3.6 times higher than that of pristine LiBH4 and LiBH4-AC.
Rehydrogenation properties In order to study the cyclic property, rehydriding tests are carried out under 350 C and 5 MPa H2 for 5 h. The first three dehydriding curves of the samples after rehydriding are showed in Fig. 7. As we can see that both LiBH4-AC and LiBH4-AC-5 wt% CeF3 show good reversible hydrogen storage
properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively. The components of the rehydrided LiBH4-AC-5 wt% CeF3 sample are determined by XRD and FTIR analysis. The results are showed in Fig. 7. We can seefrom the XRD patterns that the diffraction peaks of LiBH4 appear again after rehydriding. Although the diffraction peaks of LiH and AC still exist, which indicates that the sample is not fully rehydrided. The FTIR analysis results show that the vibrational peaks of BeH stretching (2395, 2298 and 2234 cm1) and bending (1125 cm1) are all detected in both rehydrided samples. The peak at around 1640 cm1 is the OeH bond from the contamination of moisture and air during the experiments. This suggests the presence of LiBH4 in the rehydrided sample. Clearly the AC confinement and addition of CeF3 catalyst could effectively improve the reversible hydrogen storage properties of LiBH4.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 7 e Dehydriding curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC. (c) XRD patterns and (d) FTIR patterns of LiBH4-AC5 wt% CeF3 before and after rehydrogenation.
Conclusions
Acknowledgements
In this paper, the AC nanoconfined LiBH4 (LiBH4-AC) and the confined sample doped with CeF3 are prepared. CeF3 was mixed with AC by ball milling and LiBH4 was confined into pores of AC by melt infiltration. BET and SEM results exhibit that CeF3 and LiBH4 are well dispersed in the AC scaffold. AC nanoconfinement and CeF3 doping remarkably improve the hydrogen storage properties of LiBH4. The onset dehydrogenation temperature for LiBH4-AC and LiBH4-AC-CeF3 decreases 150 and 190 C compared with pristine LiBH4, respectively. The maximum dehydrogenation speed of LiBH4AC and LiBH4-AC-CeF3 is 80 and 288 times higher than that of pristine LiBH4. And LiBH4-AC and LiBH4-AC-CeF3 show good reversible hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively.
This work was supported by the National Natural Science Foundation of China (No.51471149 and 51171168), Program for Innovative Research Team in University of Ministry of Education of China (No. IRT13037), and Public Project of Zhejiang Province (No.2015C31029).
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Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
Available online at www.sciencedirect.com
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Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst He Zhou a, Liuting Zhang b, Shichao Gao a, Haizhen Liu c, Li Xu c, Xinhua Wang a,*, Mi Yan a,** a
State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China b School of Energy and Power, Jiangsu University of Science and Technology, 2 Meng Xi Road, Zhenjiang 212003, China c State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, State Grid Corporation of China, Beijing 102209, China
article info
abstract
Article history:
CeF3 as a catalyst is first added to activated carbon (AC) by ball milling under low rotation
Received 24 April 2017
speed. Then the treated AC was used as the scaffold to confine LiBH4 by melt infiltration
Received in revised form
process. The combined effects of confinement and CeF3 doping on the hydrogen storage
22 June 2017
properties of LiBH4 are studied. The experimental results show that LiBH4 and CeF3 are well
Accepted 23 June 2017
dispersed in the AC scaffold and occupy up to 90% of the pores of AC. Compared with pristine
Available online xxx
LiBH4, the onset dehydrogenation temperature for LiBH4-AC and LiBH4-AC-CeF3 decreases by 150 and 190 C, respectively. And the corresponding dehydrogenation capacity increases
Keywords:
from 8.2 wt% to 13.1 wt% for LiBH4-AC and 12.8 wt% for LiBH4-AC-CeF3, respectively. The
Hydrogen storage materials
maximum dehydrogenation speed of LiBH4-AC and LiBH4-AC-CeF3 is 80 and 288 times higher
Lithium borohydride
than that of pristine LiBH4 at 350 C. And LiBH4-AC andLiBH4-AC-CeF3 show good reversible
Activated carbon
hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release
Nanoconfinement
capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively.
Cerium fluoride
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to develop a new energy carrier system, different kinds of hydrogen storage materials have been extensively studied to optimize their properties [1e4]. Among these materials, complex hydrides (e.g. LiBH4, Mg(BH4)2, and NaBH4) comprised of light metal cations and borohydride have received much attentions due to their high gravimetric hydrogen density (e.g. 18.3 wt% H2 for LiBH4) [5e8]. These
hydrides are considered to be potential candidates for solid state hydrogen storage system [2,9]. However, the BeH covalent bonds in these borohydrides are too thermodynamically stable that they hamper the de/rehydrogenation process [10,11]. For example, the decomposition of LiBH4 requires over 400 C with sluggish kinetics and inferior reversibility [12e14]. In order to achieve a better dehydrogenation condition, several attempts have been made to increase the dehydrogenation properties of hydrides. One of the methods is catalysts
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Yan). http://dx.doi.org/10.1016/j.ijhydene.2017.06.193 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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additions, which is mixing the hydrides with metal oxides, chlorides or fluoride [15e17]. Such catalysts could lower the onset dehydrogenation temperature. Guo et al. [18] reported that when doped with TiF3, the decomposition temperature of LiBH4 could be reduced to 100 C while the system reaches a hydrogen capacity of 5.0 wt% at 250 C. Nielsen et al. [19] reported that nanoconfined NaAlH4 in carbon aerogel catalyzed with 3.0 wt% TiCl3 showed superior dehydrogenation kinetics over both nanoconfined NaAlH4 and bulk catalyzed NaAlH4. Kim et al. [7] reported that when doped with NbF5, 6LiBH4 þ CaH2 composite shows a reversible hydrogen capacity of 6 wt% at 450 C. Sun et al. [20] reported that nano Ni particles could significantly decrease the activation energy of LiBH4 in the LiBH4-AC system to 88 kJ/mol, which is much lower than that of pristine LiBH4 (156 kJ/mol) [10]. Zhou et al. [21] proved that addition of REF3 (RE ¼ Y, La, Ce) not only reduces the peak temperatures of both dehydrogenation steps of LiAlH4, but also reduces the activation energy for the two desorption steps. Another effective approach to improve the dehydrogenation properties is the adoption of nanoconfinement [22e26]. After space confinement, the metal hydrides are confined in the nano-structure of the scaffold materials, hence decrease the particle size of the hydrides while prevent agglomeration and phase segregation during decomposition [23]. Many reports have confirmed great enhancement in the thermodynamic and kinetic properties of metal hydrides using this method. Fang et al. [27] reported incorporation of LiBH4 into activated carbon using chemical impregnation. The sample starts to release hydrogen at over 220 C. Liu et al. [28] proved that when LiBH4 was confined with nanoporous hard carbon with hexagonally pores of 2 nm diameter, the onset dehydrogenation temperature reduced drastically while emission of diborane (B2H6) gas was suppressed. Xu et al. [29] confirmed that the onset dehydrogenation temperature of LiBH4-graphene ball-milling sample could be reduced from 420 C to 195 C. Ward et al. [30] showed that LiBH4:C60 nanocomposite had great reversible hydrogen storage properties as well as the ability to absorb hydrogen under mild conditions. Shao et al. [31] used a mesoporous carbon scaffold containing dispersed NbF5 to load LiBH4 and reported a combined effect of nanoconfinement and nanocatalysis on the dehydrogenation properties of LiBH4. Although their hydrogen storage properties have been greatly enhanced using these methods, complex hydrides still could not meet the requirement for onboard applications. Some works have been focused on combining the effect of catalyst doping and nano-confinement [8,16,17,19,20,32]. In this paper, the scaffold we used was activated carbon (AC) doped with CeF3 as catalyst. Activated carbon is a very commonly used porous structure and is very easy to obtain. CeF3 was added to activated carbon through ball milling and melt infiltration was used to prepare LiBH4-AC-CeF3 confined sample. The hydrogen storage properties of LiBH4 were greatly enhanced.
Experimental LiBH4 (95% purity) was purchased from Acros Organics and used without further purification. Activated carbon (AC, 99%
purity) was also purchased from Acros Organics and was purified under 500 C and vacuum in a reactor for 5 h to remove moisture and other possible impurities. CeF3 (99.99% purity) was purchased from Alfa Aesar. First, different amount of CeF3 catalyst was added to activated carbon in a milling vessel and they were ball milled for 1 h using a QM-3SP4 planetary ball mill with a rotation speed of 100 rpm. Then the mixed samples were removed from the vessel and used as scaffold in further preparation. The LiBH4-AC-CeF3 melt infiltration (MI) nanocomposite was prepared following a two-step procedure. First, the scaffold AC-CeF3 and LiBH4 (mass ratio 3:1) were mixed using the same planetary mill at 100 rpm for 5 min. The mixture was then delivered into a reactor and calcined at 290 C under 5 MPa H2 pressure for 1 h. All sample operations were performed in an Ar atmosphere. Dehydriding/rehydriding measurements were carried out on a homemade Sieverts-type apparatus. About 200 mg of 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 non-isothermal desorption measurements, i.e. the temperature programmed desorption (TPD) measurements, the samples were heated gradually from room temperature to 500 C at a heating rate of 2 C/min. For the isothermal desorption measurements, the samples were heated quickly to 350 C under a back pressure of 8 MPa H2 to prevent the decomposition of the samples. Then the reactor was evacuating quickly to start the isothermal measurements. Hydrogen capacity was estimated by the ideal gas equation using the temperature and pressure previously obtained. The rehydriding process was carried out under the conditions of 350 C and 6 MPa H2. The reactor was filled with H2 and then heated to 350 C rapidly and kept at that temperature for 5 h. The AC scaffold was characterized by gas absorption and desorption using an Autosorb-1-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, CuKa, 40 kV, 40 mA) where the samples were sealed with an amorphous membrane during the measurements to avoid oxidation. Field emission scanning electronic microscopes (SEM, Hitachi S-4800) were applied to analyze the morphology of the samples, while the energy dispersive spectroscopy (EDS) detector was used to study the elemental distributions.
Results and discussion Sample preparation In order to characterize the effect of melt infiltration, the surface area and pore volume were determined by BET measurements. Fig. 1 shows the results of N2 adsorption measurements for the as received activated carbon (AC), the activated carbon ball milled with CeF3 to be used as scaffold (AC-CeF3). Table 1 lists the surface area and pore volumeof the
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 1 e Nitrogen adsorption curves (a) and calculated pore size distribution (b) of AC scaffolds (black), AC-CeF3 (red) and prepared LiBH4-AC-CeF3 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 e Pore volume and specific surface area of AC scaffolds, AC-CeF3 and LiBH4-AC-CeF3.
AC (as received) AC-CeF3 LiBH4-AC-CeF3
Pore volume (cm3/g)
Specific surface area (m2/g)
0.5836 0.5828 0.0595
292.1 291.8 25.2
above samples calculated from Fig. 1. It can be seen from Fig. 1 and Table 1 that the nitrogen adsorption volume of AC scaffold doesn't change much after ball milled with catalyst CeF3, and the pore volume and the specific surface remains almost the same. After the melt infiltration process, the nitrogen adsorption volume decrease remarkably, and the pore volume and the specific surface also drastically decrease. One of the reasons for these phenomena is that the content of catalyst added is very low, so that limited effect is caused to the structure of AC scaffold. This also indicates that the ball milling process we adopted doesn't affect the porous structure of AC scaffold. After melt infiltration process, the porosity of the scaffold drops drastically, which is probably because the pores of the AC scaffold are filled with LiBH4. Fig. 2 shows the morphology of pure AC, AC-CeF3 and LiBH4-AC-CeF3 observed by SEM. Corresponding EDS images of C, Ce and B of LiBH4-AC-CeF3 are also showed in Fig. 2. It can be seen from Fig. 2 that the main particle size of AC is around 100 mm, and the size changes are not eminent during the whole preparation process. This indicates that the preparation method adopted here would not affect the structure of the scaffold. The EDS images indicates that Ce and B elements are also evenly distributed on the AC particles, which suggest that LiBH4 and CeF3 are well dispersed in the scaffold after the preparation process.
Dehydrogenation properties The dehydrogenation curves and DSC curves of the prepared sample with different catalyst loading are showed in Fig. 3.
From the dehydrogenation curves we can see thatThe pristine LiBH4 starts to decompose at around 350 C, and its decomposition is still not finished upon heating to 500 C, and it releases 8.2 wt% hydrogen after heated to 500 C. The onset dehydrogenation temperature of the confined samples LiBH4AC, LiBH4-AC-CeF3 decrease from 350 C to 200 C and 160e180 C, respectively, which is 150 and 190 C lower than that of pristine LiBH4. And the hydrogen desorption capacities increase to 13.1 and 12.8 wt% for LiBH4-AC, LiBH4-AC-CeF3 respectively. Compared to the results (220 C) observed by Fang et al. [27] in their LiBH4/AC system, the onset dehydrogenation temperature of LiBH4-AC-CeF3 sample is lower, while that of LiBH4-AC sample is very close. This result is caused by the addition of CeF3, which has a catalytically effect on the system. From the DSC curves in Fig. 3(b) we can see that an endothermic peak occurs at ~110 C in each sample, which represents the phase transformation of LiBH4, and a peak occurs at ~280 C, which is caused by the melting of LiBH4. For the pure LiBH4 sample, the decomposition peak occurs at 463.1 C. The peak reaction temperature of the LiBH4-AC is 356.0 C, while those of the nano-confined LiBH4 doped with different amount of catalyst LiBH4-AC-CeF3 are 320.1 C, 323.3 C and 326.2 C. Compared with pure LiBH4 sample, the decomposition temperature of LiBH4-AC and LiBH4-AC-CeF3 lowered drastically. It is clear that when doped with catalyst, the dehydrogenation temperatures of LiBH4-AC-CeF3 are 30e36 C lower then undoped LiBH4-AC. This phenomenon is in consistence with the result from TPD curves in Fig. 1(a). The peak reaction temperature also decreases slightly when more catalyst is added. We can find a small endothermic peak at over 450 C in all the samples. This might be caused by the partially decomposition of the LiH in the dehydrogenation reaction of LiBH4. The reason that this reaction is not showed in the TPD results is that during TPD tests, the H2 pressure in the system is too high for LiH to decompose. However in DSC tests, a constant Ar flow is going through in the system so the H2 pressure is proximity zero. Thus the decomposition of LiH could take place here. The TG/DSC/MS curves of LiBH4-AC-5 wt% CeF3 and LiBH4AC are showed in Fig. 4. It is clear that the dehydrogenation
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 2 e SEM images of AC (a), AC-CeF3(b) and LiBH4-AC-5 wt% CeF3 (c) and elemental mapping of Carbon (d), Cerium (e) and Boron (f) in the as-prepared LiBH4-AC-5 wt% CeF3.
Fig. 3 e TPD curves (a) and DSC curves (b) of as-prepared LiBH4-AC-x wt% CeF3 and LiBH4-AC.
characteristics are consistent with the TPD measurements. From the TG and H2-MS curves, we can see that the weight loss of both samples and the emission of H2 follow the same pattern. We also use MS to detect other possible product or impurity such as diborane or water, and no other gas is detected during the process. It suggests that the weight losses of both samples are caused solely by the emission of H2. To further understand the dehydrogenation mechanism of LiBH4-AC-5 wt% CeF3, it was heated to different temperatures and then cool down to determine its phase composition by XRD analysis. Fig. 5 shows the XRD patterns of LiBH4-AC-5 wt% CeF3 at different stages. The as prepared sample composes LiBH4 and carbon scaffold, as was discussed in sample preparation part. The amount of CeF3 is too small to be detected by XRD measurements during the whole decomposition process.
After the sample is heated to 100 C and 200 C, the XRD patterns remain unchanged. When the sample is heated to 300 C, the diffraction peaks of LiBH4 still exist but become weaker, while those of LiH start to appear. When the sample is heated to 400 C, the diffraction peaks of LiBH4 disappear completely whilst the peaks of LiH intensify. The XRD pattern of the sample doesn't change much when heated to 500 C. The diffraction peaks of LiH still exist while some weak peaks of Li start to appear in the sample. These results suggest that LiBH4 starts to decompose between the temperature of 200 and 300 C. However the reaction is completely down when the temperature reaches 400 C. The main reaction temperature of the sample is between 300 C and 400 C. There is an additional decomposition of LiH at over 400 C. This conclusion is in consistent with the results from the DSC/TG/MS analysis.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 4 e DSC/TG/MS curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC.
Kinetic properties The dehydriding kinetics of LiBH4-AC and LiBH4-AC-5 wt% CeF3 is further studied by estimation of the kinetic barrier using the Kissinger method. The apparent activation energy (Ea) for the dehydriding of sample is determined following ln
Fig. 5 e XRD patterns of LiBH4-AC-x wt% CeF3 sample after heating to various temperatures.
c Ea ¼ þA T2p RTp
In the equation, c is the heating rate in thermal analysis, and Tp is the absolute temperature for the maximum reaction rate. In addition, R is the universal gas constant and A is also constant, which is irrelevant in the linear fitting. In our study, theTp data for both LiBH4-AC and LiBH4-AC-5 wt% CeF3 are extracted from the DSC measurements at various heating rates (c ¼ 5, 8, 12 and 16ºC/min), as demonstrated in Fig. 6(a) and (b). The linear fitting of the data is demonstrated in Fig. 6(c). Theapparent activation energy of LiBH4-AC is estimated to be121 kJ/mol, while that of LiBH4-AC-5 wt% CeF3 is 108 kJ/mol. The activation energy of pure LiBH4 was estimated to be ~154 kJ/mol [10]. The activation energy of both samples is much lower than that of pure LiBH4. While doped with CeF3 as catalyst, the apparent activation energy of the sample is reduced and the dehydriding reaction kinetics is improved. Isothermal dehydriding experiments are also conducted and the results are showed in Fig. 6(d). As is showed, the amount of hydrogen desorbed in the pristine LiBH4 is very low, the maximum hydrogen release speed is around 0.4 wt% H2/h.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 6 e DSC curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC with various heating rates (c ¼ 5, 8, 12, 16 ºC/min), (c) estimation of apparent activation energies for both sample and (d) isothermal dehydriding curves of LiBH4-AC-x wt% CeF3, LiBH4-AC and pristine LiBH4.
As for the LiBH4-AC, it reaches a hydrogen capacity of 8.7 wt% within 1000 s, and has a full dehydrogenation capacity of 12.5 wt%. The maximum dehydrogenation speed reaches 32 wt% H2/h, which is 80 times higher than that of pristine LiBH4. The 5 wt% CeF3 doped sample LiBH4-AC-5 wt% CeF3 achieves a hydrogen capacity of 11.8 wt% within 500 s, and reaches a full dehydrogenation capacity of 13.5 wt%. The maximum dehydrogenation speed reaches 115 wt% H2/h which is around 288 and 3.6 times higher than that of pristine LiBH4 and LiBH4-AC.
Rehydrogenation properties In order to study the cyclic property, rehydriding tests are carried out under 350 C and 5 MPa H2 for 5 h. The first three dehydriding curves of the samples after rehydriding are showed in Fig. 7. As we can see that both LiBH4-AC and LiBH4-AC-5 wt% CeF3 show good reversible hydrogen storage
properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively. The components of the rehydrided LiBH4-AC-5 wt% CeF3 sample are determined by XRD and FTIR analysis. The results are showed in Fig. 7. We can seefrom the XRD patterns that the diffraction peaks of LiBH4 appear again after rehydriding. Although the diffraction peaks of LiH and AC still exist, which indicates that the sample is not fully rehydrided. The FTIR analysis results show that the vibrational peaks of BeH stretching (2395, 2298 and 2234 cm1) and bending (1125 cm1) are all detected in both rehydrided samples. The peak at around 1640 cm1 is the OeH bond from the contamination of moisture and air during the experiments. This suggests the presence of LiBH4 in the rehydrided sample. Clearly the AC confinement and addition of CeF3 catalyst could effectively improve the reversible hydrogen storage properties of LiBH4.
Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193
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Fig. 7 e Dehydriding curves of (a) LiBH4-AC-x wt% CeF3 and (b) LiBH4-AC. (c) XRD patterns and (d) FTIR patterns of LiBH4-AC5 wt% CeF3 before and after rehydrogenation.
Conclusions
Acknowledgements
In this paper, the AC nanoconfined LiBH4 (LiBH4-AC) and the confined sample doped with CeF3 are prepared. CeF3 was mixed with AC by ball milling and LiBH4 was confined into pores of AC by melt infiltration. BET and SEM results exhibit that CeF3 and LiBH4 are well dispersed in the AC scaffold. AC nanoconfinement and CeF3 doping remarkably improve the hydrogen storage properties of LiBH4. The onset dehydrogenation temperature for LiBH4-AC and LiBH4-AC-CeF3 decreases 150 and 190 C compared with pristine LiBH4, respectively. The maximum dehydrogenation speed of LiBH4AC and LiBH4-AC-CeF3 is 80 and 288 times higher than that of pristine LiBH4. And LiBH4-AC and LiBH4-AC-CeF3 show good reversible hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH4-AC and LiBH4-AC-5 wt% CeF3 reaches 8.1 and 9.3 wt%, respectively.
This work was supported by the National Natural Science Foundation of China (No.51471149 and 51171168), Program for Innovative Research Team in University of Ministry of Education of China (No. IRT13037), and Public Project of Zhejiang Province (No.2015C31029).
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Please cite this article in press as: Zhou H, et al., Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.193