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Enhanced dehydrogenation performance of LiBH4 by confinement in porous NiMnO3 microspheres
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 4 2 ( 2 0 1 7 ) 2 5 8 2 4 e2 5 8 3 0
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Enhanced dehydrogenation performance of LiBH4 by confinement in porous NiMnO3 microspheres Xiaohong Xu b,1, Lei Zang b,1, Yaran Zhao b, Yongchang Liu a,b,**, Yijing Wang b, Lifang Jiao b,* a
Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b Key Laboratory of Advanced Energy Materials Chemistry (MOE), College of Chemistry, Nankai University, Tianjin 300071, China
article info
abstract
Article history:
The dehydrogenation behavior of LiBH4 has been investigated when confined into porous
Received 17 June 2017
NiMnO3 microsphere via a wet chemical impregnation method. The confinement of LiBH4
Received in revised form
in the pores of NiMnO3 nanoparticles leads to a significant decrease of the onset and the
13 August 2017
maximum desorption temperatures. The composites begin to release hydrogen at 150 C
Accepted 15 August 2017
and the maximum desorption temperature is 300 C, which are much lower compared to
Available online 19 September 2017
the raw LiBH4. Also, the hydrogen release amount is found to be increased. Moreover, the LiBH4@NiMnO3 composites exhibit excellent dehydrogenation kinetics, with 2.8 wt%
Keywords:
hydrogen released in 1 h at 300 C. X-ray diffraction and Fourier transform infrared
LiBH4
spectroscopy are used to deduce the desorption mechanism of NiMnO3.
Nanoconfinement
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Synergistic Destabilization Porous NiMnO3 nanoparticles Hydrogen release
Introduction Light weight element complex hydrides with high gravimetric and volumetric hydrogen capacities, such as amides (NH1 2 ), 1 ) and borohydrides (BH ), have attracted alanates (AlH1 4 4 numerous attention and been investigated intensively [1e4]. Recently, a wide range of interests have been focused on lithium borohydride (LiBH4) for its high hydrogen content and
potential use in hydrogen storage. However, the harsh thermodynamics, kinetics and reversibility of LiBH4 limit its practical application as a hydrogen storage medium [5e10]. LiBH4 melts at around 280 C and dehydrogenates slowly owing to the strong chemical bonding of BeH and chemical inertness of the element boron [2,3,11e15]. To overcome these problems, several attempts have been explored including destabilization [15,16], catalyst additives [17e19] and partial anion/cation substitution [20,21]. For
* Corresponding author. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Chemistry College, Nankai University, No.94 Weijin Road, Tianjin 300071, PR China. ** Corresponding author. Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (L. Jiao). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ijhydene.2017.08.088 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 4 2 ( 2 0 1 7 ) 2 5 8 2 4 e2 5 8 3 0
example, Zhang et al. found that nano-sized nickel ferrite NiFe2O4 can effectively destabilize LiBH4, which liberates hydrogen at 89 C and the hydrogen capacity is 10.75 wt% [15]. According to Xu et al., LiBH4 doped with the Pd/C catalysts shows superior dehydrogenation performance compared to raw LiBH4. Importantly, it observed a reversible hydrogenation/dehydrogenation with a capacity of approximately 4.3 wt % [22]. Another effective approach that enhances the performance of LiBH4 is nanoconfinement [16,23e27]. Nanoconfinement is a great potential strategy that utilizes nanoporous materials as scaffolds to confine the complex metal hydrides. The scaffolds are including porous carbon, carbon tubes or other porous structure materials [23,28e32]. LiBH4 confined in these scaffolds can limit the particles size to nanoscale and suppress the growth and the agglomeration of particles [25,27,28,33e36]. In addition, it also increases the contacting area between reactants, therefore, significantly enhances the hydrogen desorption properties of LiBH4 [27e29,37e42]. For instance, a pioneering study by Ngene found that nanosizing and confinement of NaBH4 into porous carbon resulted in much faster hydrogen desorption kinetics and the dehydrogenated nanocomposites were partially rehydrogenated to form NaBH4 at relatively mild conditions (60 bar H2 and 325 C) [30]. Zhang revealed that an ammonia borane (AB)@PPY (Polypyrrole) system is able to liberate H2 at temperature as low as 48 C, and up to 15.3 wt% can be released before 150 C without any harmful impurities. The improvement can be ascribed to the synergetic catalysis of nitrogen atoms with nanoconfinement in nanotubes [43]. Apart from the carbon scaffolds, other porous nanostructures are applied as carrier to confine LiBH4 to improve the dehydrogenation performance of LiBH4 [13,16,24,33,44e47]. For example, our previous work prepared LiBH4@2ZZCO (ZnO/ ZnCo2O4) composites through chemical impregnation, finding that the onset temperature was reduced to 169 C and the sample releases 3.2 wt% of hydrogen at 300 C within 1 h. Therefore, nanoconfinement has the great potential to enhance the dehydrogenation performance [48]. Despite the great progress achieved in the research of LiBH4@2ZZCO, the amount of additive ZZCO is larger compared to the bulk LiBH4. In this paper, LiBH4 is successfully confined into the 50 wt% as-synthesized porous NiMnO3 microsphere. Porous NiMnO3 is selected as the scaffold to explore the synergetic effect of destabilization and nanoconfinement. The results show the desorption performance of LiBH4 has a great enhancement compared to pure LiBH4, the onset and the maximal temperature are reduced to 170 C, 300 C, respectively. On the other hand, the kinetic performance improves a lot and the activation energy is 129.8 kJ/ mol, lower than LiBH4. Moreover, the possible mechanism is also proposed and discussed.
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Sigma. All the other reagents were used as received, except THF, which needed to be distilled. Porous NiMnO3 microsphere was prepared in terms of the reported procedures. Typically, 0.594 g NiCl2$4H2O, 0.693 g MnCl2$4H2O were dissolved in 50 mL 1 M NaHCO3 solvent under constant magnetic stirring for 30 min. Then the obtained uniform solvent were centrifuged at 3000 r min1 and washed with water and ethanol for several times and finally dried at 80 C for 24 h. Then the as-fabricated precursor was annealed at 500 C for 1 h with a rate of 5 C min1 to obtain NiMnO3 microsphere.
Confinement into porous NiMnO3 microsphere To prepare LiBH4@NiMnO3 composites (2:1, mass ratio), 0.2 g LiBH4 was dissolved in 4 mL THF (tetrahydrofuran) in glovebox with a ratio of 2 mL THF/0.100 g LiBH4. Next, the solution was injected into the 0.1 g NiMnO3 power in Ar atmosphere in a three necked-flask which was attached to a Schlenk line. Then the mixture was heated at 80 C for 72 h at a continuous stirring to remove the THF residue. At the last, the final product was transferred in the glove-box with an argon purity (99.9999%).
Characterizations The crystal structure and morphology of the materials were characterized by X-ray (XRD, Rigaku D/Max-2500, Cu, Ka, radiation), Scanning electron microscopy (SEM, JEOLJSM7500) and transmission electron microscopy (TEM, JEOL-2100F, 200 kV). During the XRD measurement, the samples were packed with plastic film to avoid the contact with oxygen in Ar-filled glove-box. N2 adsorption-desorption isotherms were measured with Quantachrome Instruments (NoVA 2200e) and the N2 desorption temperature was 300 C. The surface areas were measured by the Brunauer-Emmett-Teller (BET) method and the pore size distributions were derived from the adsorption branches of the Barrett-Joyner-Halenda (BJH) method. FT-IR spectra were collected with a FT-IR-650 spectrometer (Tianjin Gangdong) at a resolution of 4 cm1. Temperature programmed desorption (TPD) was carried out in a reactor, and about 70 mg sample was heated at a rate of 2 C min1 from 30 C to 500 C. The dehydrogenation kinetics was detected via a Sieverts-type isothermal measurement at different temperatures. Sieverts-type isothermal was measured at different temperatures to detect the decomposition properties of the samples. Differential scanning calorimetry (DSC) was conducted via a Q20P, TA Instruments.
Results and discussion Materials characterization
Experiment Materials and methods Preparation of porous NiMnO3 LiBH4 (95%) was purchased from Acros, NaHCO3, NiCl2$6H2O, MnCl2$4H2O, tetrahydrofuran (THF) were purchased from
The NiMnO3 microsphere was successfully synthesized by the chemical impregnation method. Fig. 1 shows the XRD patterns of LiBH4, NiMnO3, LiBH4@NiMnO3 composites, respectively. For NiMnO3 microsphere, all the diffraction peaks can be assigned to the standard card (JCPDS-75-2089). For LiBH4@NiMnO3 composite sample, it contains the characteristic peaks of NiMnO3 and LiBH4 except for the parafilm, which
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Fig. 1 e XRD patterns of as-prepared NiMnO3, LiBH4, LiBH4@NiMnO3 composites.
means that LiBH4 may be successfully confined into the composites. However, the phase of LiBH4 is rather weak, the reason is that the degree of crystallinity of LiBH4 is reduced when it is confined into the carrier [2]. To investigate the structure and morphology of the materials, SEM and TEM experiments were carried out. Fig. 2 shows the SEM and TEM images of LiBH4 and LiBH4@NiMnO3 composites, respectively. As depicted in Fig. 2(a,b), NiMnO3 microspheres are composed of numerous interconnected nanoparticles. Plenty of pores with 12 nm between the interconnected nanoparticles are exited, which is corresponding to the N2 adsorption-desorption curve. It is this porosity structure that makes it possible for NiMnO3 microsphere as a carrier to effectively confine the LiBH4. Fig. 2(c,d) shows the size
of the NiMnO3 microsphere is around 1.5 mm. As shown in Fig. 2e, it can be noticed that the lattice space is 0.36 nm, which is assistant with the (110) lattice plane of NiMnO3. Fig. 2f shows the TEM images of LiBH4@NiMnO3 composites. As presented in Fig. 2f, well defined lattice fringes with 0.37 nm can be seen, which is matched well with the (110) facet of LiBH4. This result reveals that LiBH4 is existed in the LiBH4@NiMnO3 composites. To clarify the influence of NiMnO3 in the confinement process, N2 adsorption-desorption analysis was conducted. Fig. 3 demonstrates that the adsorption-desorption isotherm is typically IV curve. The specific surface area and pore volume of the as-fabricated porous NiMnO3 microsphere is 24 m2/g, 17.5 nm. While, NiMnO3@LiBH4 composite is reduced to 15 m2/ g, 12.5 nm. The reduction of surface area and pore size reveal that LiBH4 was successfully confined into the NiMnO3 porosity structure.
Hydrogen release performance The effects of the as-synthesized NiMnO3 on the dehydrogenation properties of LiBH4 were systematically investigated. Fig. 4 shows the TPD curves of pristine LiBH4 and LiBH4@NiMnO3. It is clearly showed that the composites with LiBH4 confined in the porous NiMnO3 microsphere display a significantly decreased onset temperature and the maximum temperature. As a case of commercial LiBH4, it presented a multiple desorption peaks when heated up to 500 C. It begins to decompose at about 280 C and emerges a maximal decomposition at 420 C, which is reported in the previous literature [48e54]. On the whole, it released 11 wt% of hydrogen at 500 C (Fig. 4a and b). Similarly, the LiBH4@NiMnO3 also presented a multi-step desorption pattern during the dehydrogenation process. However, the onset and the maximum temperature of LiBH4@NiMnO3
Fig. 2 e (a,b) SEM images of NiMnO3; (c,d) TEM and (e) HR-TEM images of as-prepared NiMnO3; (f) TEM image of LiBH4@NiMnO3 composites.
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 4 2 ( 2 0 1 7 ) 2 5 8 2 4 e2 5 8 3 0
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Fig. 3 e (a) N2 adsorption-desorption isotherms and (b) pore size distribution of as-prepared (A) NiMnO3; (B) LiBH4@ NiMnO3.
composites are 170 C, 300 C, lower than the corresponding temperature of bulk LiBH4 110 C, 120 C, respectively and it release 7.3 wt% H2 when heated to 500 C. Obviously, this result gives direct evidence that the dehydrogenation performance of LiBH4 was greatly improved. And these reasons may be accounted for it. Firstly, porous NiMnO3 microsphere composed of numerous interconnected nanoparticles can act as an effective agent to confine LiBH4, lessen the size of LiBH4 to nanoscale and finally enhance desorption properties. Secondly, the tuning of thermodynamics is possible ascribed to the changes of surface chemistry of the porous host which determines the interfacial energy between the scaffold and the LiBH4 [2]. Isothermal dehydrogenation measurements were carried out to explore the dehydrogenation kinetics of the as-prepared LiBH4@NiMnO3 composite. As shown in Fig. 5, pure LiBH4 presented a lower desorption rates and the capacity is 1.5 wt% within 240 min at 300 C, while the LiBH4@NiMnO3 composites produce 2.8 wt% hydrogen when adding the NiMnO3, indicating that NiMnO3 has a positive effect on the desorption performance of LiBH4. For the sample LiBH4@NiMnO3, it still has a faster desorption rates even at 260 C compared to the pristine
LiBH4. When reached up to 300 C, the dehydrogenation rate of the sample LiBH4@NiMnO3 was increased owing to the boosting of reaction activity and the sample reached desorption plateau within 1 h. In addition, the composite sample released 1.8 wt% hydrogen at 260 C within 1 h, whereas, it released 2.8 wt% hydrogen in an hour with the temperature increasing to 300 C. The above results showed the superior catalytic effect of porous NiMnO3 microsphere on the desorption performance of LiBH4. Clearly, desorption properties were greatly improved when porous NiMnO3 was drawn into the LiBH4 system. And the higher the temperature, the larger the released hydrogen capacity and the faster the desorption kinetics. Moreover, to gain sight into the nanoconfinement effect of NiMnO3 on the desorption kinetics, the activation energy of LiBH4@NiMnO3 composite was determined by analyzing the desorption performance at four different desorption rates. The equation is as follows: d ln b T2m Ea ¼ dð1=Tm Þ R where b is the heating rate, Tm is the absolute temperature for the maximum desorption rate and R is the gas constant,
Fig. 4 e (a) Thermally programmed H2 desorption curves and (b) the corresponding thermally programmed H2 desorption capacity curves of pure LiBH4 and LiBH4@NiMnO3.
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separately. Fig. 6 shows the Kissinger plot of the LiBH4@NiMnO3 sample. The fitted apparent activation energy of LiBH4@NiMnO3 sample is 129.8 kJ/mol, which is much lower than that of the raw LiBH4 (146 kJ/mol) [55].The reduction of activation energy demonstrates the improved kinetics. As shown in Fig. 2f, LiBH4 particles were homogeneously dispersed in the interconnected NiMnO3 pores among the confining process, forming a uniform hydrogen diffusion pathway and promoted the hydrogen diffusion when dehydrogenated [1,2]. Given the above findings, it can be understood that the confinement and destabilization of NiMnO3 occurs during the hydrogen desorption, which lead the improved dehydrogenation kinetics.
Deduction of the reaction mechanism Fig. 5 e Sieverts' hydrogen desorption profiles of pure LiBH4, LiBH4@NiMnO3 at 300 C, 260 C.
On the basis of above results, we conclude that the destabilization and nanoconfinement of NiMnO3 are the main factors for the drastically enhanced dehydrogenation behavior of LiBH4. Nanoconfinement can result in shorter diffusion
Fig. 6 e (a) DSC profiles at different heating rates; (b) Kissinger plots of the dehydrogenation of LiBH4@2NiMnO3 composite.
Fig. 7 e (a) FT-IR spectra of LiBH4@NiMnO3 and the dehydrogenated products at different temperatures; (b) XRD patterns of (i) LiBH4@NiMnO3, dehydrogenated of LiBH4@NiMnO3 sample at (ii) 260 C, (iii) 300 C.
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 4 2 ( 2 0 1 7 ) 2 5 8 2 4 e2 5 8 3 0
path, low energy barrier and provide a close contacting between LiBH4 and NiMnO3 nanoparticles. NiMnO3 microsphere not only acts as a destabilization agent but also as a confined scaffold that inhibits the growth and agglomeration of LiBH4 particles. Fig. 7a reveals the FT-IR results of the desorption LiBH4@NiMnO3 at different temperatures. As seen from Fig. 7a, 2388, 2292 cm1 are ascribed to the typical BeH vibrations in the LiBH4@NiMnO3 composites, indicating that LiBH4 is successfully confined into the NiMnO3 particles. In addition, the peak at 1637 cm1 is attributed to the OeH bond, which should be due to the invasion of oxygen and moisture during the storage and measurement process. For the dehydrogenated products at 260 C, 300 C, the BeH vibration peaks still exists, implying the incomplete desorption of LiBH4. This result is compliant with the desorption capacity that LiBH4@NiMnO3 liberated at 260 C and 300 C. Fig. 7b gives the X-ray diffraction patterns of the confined LiBH4@NiMnO3 composites before dehydrogenation, after dehydrogenation (260 C, 300 C). As shown in Fig. 7b, the weak LiBH4 phase is contained for the LiBH4@NiMnO3 composites, revealing that LiBH4 was successfully incorporated in the porous NiMnO3. Also, the weak peak intensity of the composites is attributed to the effect of confinement. However, after heating to 300 C, Ni2B, LiNiO2 are detected as the desorption products, as shown in Fig. 7b. It can be deduced from the XRD measurements that LiBH4 reacts with NiMnO3, resulting the break of BeH bond. In the dehydrogenation products, LiH phase is not appeared, demonstrating that LiH may exist in an amorphous state or may react with NiMnO3 to form LiNiO2. However, in order to make the decomposition mechanism more clear, the limited data based on FT-IR, XRD analysis are far from enough. Further researches on this issue are in urgent need. Generally, the mechanism could be attributed to the following aspects. Firstly, porous NiMnO3 nanoparticles act as the confining agent to LiBH4. It not only reduces the diameter of LiBH4 to the size of nanometer, but also inhibits the particle aggregation of LiBH4. Secondly, this structure gives more active sites between LiBH4 and NiMnO3, promoting the reaction proceeding. Thus, the performance of LiBH4 has a greatly improvement.
Conclusion In general, LiBH4@NiMnO3 composite was successfully synthesized, which uses porous NiMnO3 microsphere as scaffold to confine. And the desorption performance of LiBH4@NiMnO3 composite are significantly improved with LiBH4 incorporated into the NiMnO3 carrier via a chemical impregnation method. The obtained composite presents superior dehydrogenation performance, such as lower desorption temperature and rapid desorption kinetics. The onset and the maximum temperatures of LiBH4@NiMnO3 are 170 C, 300 C, lower about 100 C compared to the raw LiBH4. Moreover, the sample shows superior releasing kinetics, with 2.8 wt% hydrogen liberated from composite within 1 h. These results testify that NiMnO3 has the potential to improve the desorption properties of LiBH4. However, the mechanism
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about confinement and destabilization is needed to be further research.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51622102, 51571124) and the 111 Project (B12015).
references
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Enhanced dehydrogenation performance of LiBH4 by confinement in porous NiMnO3 microspheres Xiaohong Xu b,1, Lei Zang b,1, Yaran Zhao b, Yongchang Liu a,b,**, Yijing Wang b, Lifang Jiao b,* a
Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b Key Laboratory of Advanced Energy Materials Chemistry (MOE), College of Chemistry, Nankai University, Tianjin 300071, China
article info
abstract
Article history:
The dehydrogenation behavior of LiBH4 has been investigated when confined into porous
Received 17 June 2017
NiMnO3 microsphere via a wet chemical impregnation method. The confinement of LiBH4
Received in revised form
in the pores of NiMnO3 nanoparticles leads to a significant decrease of the onset and the
13 August 2017
maximum desorption temperatures. The composites begin to release hydrogen at 150 C
Accepted 15 August 2017
and the maximum desorption temperature is 300 C, which are much lower compared to
Available online 19 September 2017
the raw LiBH4. Also, the hydrogen release amount is found to be increased. Moreover, the LiBH4@NiMnO3 composites exhibit excellent dehydrogenation kinetics, with 2.8 wt%
Keywords:
hydrogen released in 1 h at 300 C. X-ray diffraction and Fourier transform infrared
LiBH4
spectroscopy are used to deduce the desorption mechanism of NiMnO3.
Nanoconfinement
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Synergistic Destabilization Porous NiMnO3 nanoparticles Hydrogen release
Introduction Light weight element complex hydrides with high gravimetric and volumetric hydrogen capacities, such as amides (NH1 2 ), 1 ) and borohydrides (BH ), have attracted alanates (AlH1 4 4 numerous attention and been investigated intensively [1e4]. Recently, a wide range of interests have been focused on lithium borohydride (LiBH4) for its high hydrogen content and
potential use in hydrogen storage. However, the harsh thermodynamics, kinetics and reversibility of LiBH4 limit its practical application as a hydrogen storage medium [5e10]. LiBH4 melts at around 280 C and dehydrogenates slowly owing to the strong chemical bonding of BeH and chemical inertness of the element boron [2,3,11e15]. To overcome these problems, several attempts have been explored including destabilization [15,16], catalyst additives [17e19] and partial anion/cation substitution [20,21]. For
* Corresponding author. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Chemistry College, Nankai University, No.94 Weijin Road, Tianjin 300071, PR China. ** Corresponding author. Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (L. Jiao). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ijhydene.2017.08.088 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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example, Zhang et al. found that nano-sized nickel ferrite NiFe2O4 can effectively destabilize LiBH4, which liberates hydrogen at 89 C and the hydrogen capacity is 10.75 wt% [15]. According to Xu et al., LiBH4 doped with the Pd/C catalysts shows superior dehydrogenation performance compared to raw LiBH4. Importantly, it observed a reversible hydrogenation/dehydrogenation with a capacity of approximately 4.3 wt % [22]. Another effective approach that enhances the performance of LiBH4 is nanoconfinement [16,23e27]. Nanoconfinement is a great potential strategy that utilizes nanoporous materials as scaffolds to confine the complex metal hydrides. The scaffolds are including porous carbon, carbon tubes or other porous structure materials [23,28e32]. LiBH4 confined in these scaffolds can limit the particles size to nanoscale and suppress the growth and the agglomeration of particles [25,27,28,33e36]. In addition, it also increases the contacting area between reactants, therefore, significantly enhances the hydrogen desorption properties of LiBH4 [27e29,37e42]. For instance, a pioneering study by Ngene found that nanosizing and confinement of NaBH4 into porous carbon resulted in much faster hydrogen desorption kinetics and the dehydrogenated nanocomposites were partially rehydrogenated to form NaBH4 at relatively mild conditions (60 bar H2 and 325 C) [30]. Zhang revealed that an ammonia borane (AB)@PPY (Polypyrrole) system is able to liberate H2 at temperature as low as 48 C, and up to 15.3 wt% can be released before 150 C without any harmful impurities. The improvement can be ascribed to the synergetic catalysis of nitrogen atoms with nanoconfinement in nanotubes [43]. Apart from the carbon scaffolds, other porous nanostructures are applied as carrier to confine LiBH4 to improve the dehydrogenation performance of LiBH4 [13,16,24,33,44e47]. For example, our previous work prepared LiBH4@2ZZCO (ZnO/ ZnCo2O4) composites through chemical impregnation, finding that the onset temperature was reduced to 169 C and the sample releases 3.2 wt% of hydrogen at 300 C within 1 h. Therefore, nanoconfinement has the great potential to enhance the dehydrogenation performance [48]. Despite the great progress achieved in the research of LiBH4@2ZZCO, the amount of additive ZZCO is larger compared to the bulk LiBH4. In this paper, LiBH4 is successfully confined into the 50 wt% as-synthesized porous NiMnO3 microsphere. Porous NiMnO3 is selected as the scaffold to explore the synergetic effect of destabilization and nanoconfinement. The results show the desorption performance of LiBH4 has a great enhancement compared to pure LiBH4, the onset and the maximal temperature are reduced to 170 C, 300 C, respectively. On the other hand, the kinetic performance improves a lot and the activation energy is 129.8 kJ/ mol, lower than LiBH4. Moreover, the possible mechanism is also proposed and discussed.
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Sigma. All the other reagents were used as received, except THF, which needed to be distilled. Porous NiMnO3 microsphere was prepared in terms of the reported procedures. Typically, 0.594 g NiCl2$4H2O, 0.693 g MnCl2$4H2O were dissolved in 50 mL 1 M NaHCO3 solvent under constant magnetic stirring for 30 min. Then the obtained uniform solvent were centrifuged at 3000 r min1 and washed with water and ethanol for several times and finally dried at 80 C for 24 h. Then the as-fabricated precursor was annealed at 500 C for 1 h with a rate of 5 C min1 to obtain NiMnO3 microsphere.
Confinement into porous NiMnO3 microsphere To prepare LiBH4@NiMnO3 composites (2:1, mass ratio), 0.2 g LiBH4 was dissolved in 4 mL THF (tetrahydrofuran) in glovebox with a ratio of 2 mL THF/0.100 g LiBH4. Next, the solution was injected into the 0.1 g NiMnO3 power in Ar atmosphere in a three necked-flask which was attached to a Schlenk line. Then the mixture was heated at 80 C for 72 h at a continuous stirring to remove the THF residue. At the last, the final product was transferred in the glove-box with an argon purity (99.9999%).
Characterizations The crystal structure and morphology of the materials were characterized by X-ray (XRD, Rigaku D/Max-2500, Cu, Ka, radiation), Scanning electron microscopy (SEM, JEOLJSM7500) and transmission electron microscopy (TEM, JEOL-2100F, 200 kV). During the XRD measurement, the samples were packed with plastic film to avoid the contact with oxygen in Ar-filled glove-box. N2 adsorption-desorption isotherms were measured with Quantachrome Instruments (NoVA 2200e) and the N2 desorption temperature was 300 C. The surface areas were measured by the Brunauer-Emmett-Teller (BET) method and the pore size distributions were derived from the adsorption branches of the Barrett-Joyner-Halenda (BJH) method. FT-IR spectra were collected with a FT-IR-650 spectrometer (Tianjin Gangdong) at a resolution of 4 cm1. Temperature programmed desorption (TPD) was carried out in a reactor, and about 70 mg sample was heated at a rate of 2 C min1 from 30 C to 500 C. The dehydrogenation kinetics was detected via a Sieverts-type isothermal measurement at different temperatures. Sieverts-type isothermal was measured at different temperatures to detect the decomposition properties of the samples. Differential scanning calorimetry (DSC) was conducted via a Q20P, TA Instruments.
Results and discussion Materials characterization
Experiment Materials and methods Preparation of porous NiMnO3 LiBH4 (95%) was purchased from Acros, NaHCO3, NiCl2$6H2O, MnCl2$4H2O, tetrahydrofuran (THF) were purchased from
The NiMnO3 microsphere was successfully synthesized by the chemical impregnation method. Fig. 1 shows the XRD patterns of LiBH4, NiMnO3, LiBH4@NiMnO3 composites, respectively. For NiMnO3 microsphere, all the diffraction peaks can be assigned to the standard card (JCPDS-75-2089). For LiBH4@NiMnO3 composite sample, it contains the characteristic peaks of NiMnO3 and LiBH4 except for the parafilm, which
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Fig. 1 e XRD patterns of as-prepared NiMnO3, LiBH4, LiBH4@NiMnO3 composites.
means that LiBH4 may be successfully confined into the composites. However, the phase of LiBH4 is rather weak, the reason is that the degree of crystallinity of LiBH4 is reduced when it is confined into the carrier [2]. To investigate the structure and morphology of the materials, SEM and TEM experiments were carried out. Fig. 2 shows the SEM and TEM images of LiBH4 and LiBH4@NiMnO3 composites, respectively. As depicted in Fig. 2(a,b), NiMnO3 microspheres are composed of numerous interconnected nanoparticles. Plenty of pores with 12 nm between the interconnected nanoparticles are exited, which is corresponding to the N2 adsorption-desorption curve. It is this porosity structure that makes it possible for NiMnO3 microsphere as a carrier to effectively confine the LiBH4. Fig. 2(c,d) shows the size
of the NiMnO3 microsphere is around 1.5 mm. As shown in Fig. 2e, it can be noticed that the lattice space is 0.36 nm, which is assistant with the (110) lattice plane of NiMnO3. Fig. 2f shows the TEM images of LiBH4@NiMnO3 composites. As presented in Fig. 2f, well defined lattice fringes with 0.37 nm can be seen, which is matched well with the (110) facet of LiBH4. This result reveals that LiBH4 is existed in the LiBH4@NiMnO3 composites. To clarify the influence of NiMnO3 in the confinement process, N2 adsorption-desorption analysis was conducted. Fig. 3 demonstrates that the adsorption-desorption isotherm is typically IV curve. The specific surface area and pore volume of the as-fabricated porous NiMnO3 microsphere is 24 m2/g, 17.5 nm. While, NiMnO3@LiBH4 composite is reduced to 15 m2/ g, 12.5 nm. The reduction of surface area and pore size reveal that LiBH4 was successfully confined into the NiMnO3 porosity structure.
Hydrogen release performance The effects of the as-synthesized NiMnO3 on the dehydrogenation properties of LiBH4 were systematically investigated. Fig. 4 shows the TPD curves of pristine LiBH4 and LiBH4@NiMnO3. It is clearly showed that the composites with LiBH4 confined in the porous NiMnO3 microsphere display a significantly decreased onset temperature and the maximum temperature. As a case of commercial LiBH4, it presented a multiple desorption peaks when heated up to 500 C. It begins to decompose at about 280 C and emerges a maximal decomposition at 420 C, which is reported in the previous literature [48e54]. On the whole, it released 11 wt% of hydrogen at 500 C (Fig. 4a and b). Similarly, the LiBH4@NiMnO3 also presented a multi-step desorption pattern during the dehydrogenation process. However, the onset and the maximum temperature of LiBH4@NiMnO3
Fig. 2 e (a,b) SEM images of NiMnO3; (c,d) TEM and (e) HR-TEM images of as-prepared NiMnO3; (f) TEM image of LiBH4@NiMnO3 composites.
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Fig. 3 e (a) N2 adsorption-desorption isotherms and (b) pore size distribution of as-prepared (A) NiMnO3; (B) LiBH4@ NiMnO3.
composites are 170 C, 300 C, lower than the corresponding temperature of bulk LiBH4 110 C, 120 C, respectively and it release 7.3 wt% H2 when heated to 500 C. Obviously, this result gives direct evidence that the dehydrogenation performance of LiBH4 was greatly improved. And these reasons may be accounted for it. Firstly, porous NiMnO3 microsphere composed of numerous interconnected nanoparticles can act as an effective agent to confine LiBH4, lessen the size of LiBH4 to nanoscale and finally enhance desorption properties. Secondly, the tuning of thermodynamics is possible ascribed to the changes of surface chemistry of the porous host which determines the interfacial energy between the scaffold and the LiBH4 [2]. Isothermal dehydrogenation measurements were carried out to explore the dehydrogenation kinetics of the as-prepared LiBH4@NiMnO3 composite. As shown in Fig. 5, pure LiBH4 presented a lower desorption rates and the capacity is 1.5 wt% within 240 min at 300 C, while the LiBH4@NiMnO3 composites produce 2.8 wt% hydrogen when adding the NiMnO3, indicating that NiMnO3 has a positive effect on the desorption performance of LiBH4. For the sample LiBH4@NiMnO3, it still has a faster desorption rates even at 260 C compared to the pristine
LiBH4. When reached up to 300 C, the dehydrogenation rate of the sample LiBH4@NiMnO3 was increased owing to the boosting of reaction activity and the sample reached desorption plateau within 1 h. In addition, the composite sample released 1.8 wt% hydrogen at 260 C within 1 h, whereas, it released 2.8 wt% hydrogen in an hour with the temperature increasing to 300 C. The above results showed the superior catalytic effect of porous NiMnO3 microsphere on the desorption performance of LiBH4. Clearly, desorption properties were greatly improved when porous NiMnO3 was drawn into the LiBH4 system. And the higher the temperature, the larger the released hydrogen capacity and the faster the desorption kinetics. Moreover, to gain sight into the nanoconfinement effect of NiMnO3 on the desorption kinetics, the activation energy of LiBH4@NiMnO3 composite was determined by analyzing the desorption performance at four different desorption rates. The equation is as follows: d ln b T2m Ea ¼ dð1=Tm Þ R where b is the heating rate, Tm is the absolute temperature for the maximum desorption rate and R is the gas constant,
Fig. 4 e (a) Thermally programmed H2 desorption curves and (b) the corresponding thermally programmed H2 desorption capacity curves of pure LiBH4 and LiBH4@NiMnO3.
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separately. Fig. 6 shows the Kissinger plot of the LiBH4@NiMnO3 sample. The fitted apparent activation energy of LiBH4@NiMnO3 sample is 129.8 kJ/mol, which is much lower than that of the raw LiBH4 (146 kJ/mol) [55].The reduction of activation energy demonstrates the improved kinetics. As shown in Fig. 2f, LiBH4 particles were homogeneously dispersed in the interconnected NiMnO3 pores among the confining process, forming a uniform hydrogen diffusion pathway and promoted the hydrogen diffusion when dehydrogenated [1,2]. Given the above findings, it can be understood that the confinement and destabilization of NiMnO3 occurs during the hydrogen desorption, which lead the improved dehydrogenation kinetics.
Deduction of the reaction mechanism Fig. 5 e Sieverts' hydrogen desorption profiles of pure LiBH4, LiBH4@NiMnO3 at 300 C, 260 C.
On the basis of above results, we conclude that the destabilization and nanoconfinement of NiMnO3 are the main factors for the drastically enhanced dehydrogenation behavior of LiBH4. Nanoconfinement can result in shorter diffusion
Fig. 6 e (a) DSC profiles at different heating rates; (b) Kissinger plots of the dehydrogenation of LiBH4@2NiMnO3 composite.
Fig. 7 e (a) FT-IR spectra of LiBH4@NiMnO3 and the dehydrogenated products at different temperatures; (b) XRD patterns of (i) LiBH4@NiMnO3, dehydrogenated of LiBH4@NiMnO3 sample at (ii) 260 C, (iii) 300 C.
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path, low energy barrier and provide a close contacting between LiBH4 and NiMnO3 nanoparticles. NiMnO3 microsphere not only acts as a destabilization agent but also as a confined scaffold that inhibits the growth and agglomeration of LiBH4 particles. Fig. 7a reveals the FT-IR results of the desorption LiBH4@NiMnO3 at different temperatures. As seen from Fig. 7a, 2388, 2292 cm1 are ascribed to the typical BeH vibrations in the LiBH4@NiMnO3 composites, indicating that LiBH4 is successfully confined into the NiMnO3 particles. In addition, the peak at 1637 cm1 is attributed to the OeH bond, which should be due to the invasion of oxygen and moisture during the storage and measurement process. For the dehydrogenated products at 260 C, 300 C, the BeH vibration peaks still exists, implying the incomplete desorption of LiBH4. This result is compliant with the desorption capacity that LiBH4@NiMnO3 liberated at 260 C and 300 C. Fig. 7b gives the X-ray diffraction patterns of the confined LiBH4@NiMnO3 composites before dehydrogenation, after dehydrogenation (260 C, 300 C). As shown in Fig. 7b, the weak LiBH4 phase is contained for the LiBH4@NiMnO3 composites, revealing that LiBH4 was successfully incorporated in the porous NiMnO3. Also, the weak peak intensity of the composites is attributed to the effect of confinement. However, after heating to 300 C, Ni2B, LiNiO2 are detected as the desorption products, as shown in Fig. 7b. It can be deduced from the XRD measurements that LiBH4 reacts with NiMnO3, resulting the break of BeH bond. In the dehydrogenation products, LiH phase is not appeared, demonstrating that LiH may exist in an amorphous state or may react with NiMnO3 to form LiNiO2. However, in order to make the decomposition mechanism more clear, the limited data based on FT-IR, XRD analysis are far from enough. Further researches on this issue are in urgent need. Generally, the mechanism could be attributed to the following aspects. Firstly, porous NiMnO3 nanoparticles act as the confining agent to LiBH4. It not only reduces the diameter of LiBH4 to the size of nanometer, but also inhibits the particle aggregation of LiBH4. Secondly, this structure gives more active sites between LiBH4 and NiMnO3, promoting the reaction proceeding. Thus, the performance of LiBH4 has a greatly improvement.
Conclusion In general, LiBH4@NiMnO3 composite was successfully synthesized, which uses porous NiMnO3 microsphere as scaffold to confine. And the desorption performance of LiBH4@NiMnO3 composite are significantly improved with LiBH4 incorporated into the NiMnO3 carrier via a chemical impregnation method. The obtained composite presents superior dehydrogenation performance, such as lower desorption temperature and rapid desorption kinetics. The onset and the maximum temperatures of LiBH4@NiMnO3 are 170 C, 300 C, lower about 100 C compared to the raw LiBH4. Moreover, the sample shows superior releasing kinetics, with 2.8 wt% hydrogen liberated from composite within 1 h. These results testify that NiMnO3 has the potential to improve the desorption properties of LiBH4. However, the mechanism
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about confinement and destabilization is needed to be further research.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51622102, 51571124) and the 111 Project (B12015).
references
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