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Enhanced catalytic effects of [email protected] additive on dehydrogenation properties of LiAlH4
Journal of Alloys and Compounds xxx (2015) xxx–xxx
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced catalytic effects of Co@C additive on dehydrogenation properties of LiAlH4 Li Li b, Yijing Wang a,⇑, Lifang Jiao a, Huatang Yuan a a Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, China b Key Laboratory of Inorganic Functional Materials in Universities of Shandong, School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Complex hydride LiAlH4 Co@C Dehydrogenation
a b s t r a c t The catalytic effects of Co@C additive on hydrogen storage properties of LiAlH4 prepared by ball milling have been systematically investigated. The onset desorption temperature of Co@C–LiAlH4 sample is 100.0 °C, which is 51 °C lower than that of as-received LiAlH4. Isothermal desorption results show that for Co@C–LiAlH4 dehydrogenated at 130 °C, 4.58 wt% of hydrogen can be released with 180 min, which is 3.56 wt% higher than that of as-received LiAlH4 under the same conditions. Approximately 7.05 wt% of hydrogen is released at 150 °C. Through the Kissinger desorption kinetics analyses, the apparent activation energy, Ea, of Co@C–LiAlH4 is calculated as 95.36 kJ mol1 H2 and 115.6 kJ mol1 H2 for the first two decomposition processes, respectively. It indicates that dehydrogenation kinetics of LiAlH4 matrix considerably improves by doping the Co@C catalyst. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The demand for safe and efficient storage of renewable energy has created significant interest. Hydrogen, as the lightest element on the earth, is expected to play a crucial role in the future energy system [1,2]. Solid-state materials have the potential to provide significantly hydrogen storage densities. Thus, a long list of novel complex metal hydrides [3–18] (eg, LiAlH4, NaAlH4, LiBH4, Mg(BH4)2, LiNH2, NH3BH3) receive a lot of attention among current hydrogen storage technologies. Among these complex hydrides, lithium alanate (LiAlH4) is of particular interest for lightweight hydrogen storage due to its high hydrogen storage capacity (7.9 wt% accessible) and relatively low decomposition temperature in the presence of suitable catalysts [19]. In general, LiAlH4 releases hydrogen in the following stages ((R1) and (R2)) upon heating.
LiAlH4 ðsÞ ! LiAlH4 ðlÞ
ðR1aÞ
3LiAlH4 ðlÞ ! Li3 AlH6 ðsÞ þ 2AlðsÞ þ 3H2 ðgÞ ð150—175 CÞ ð5:3 wt% H2 Þ
ðR1bÞ
Li3 AlH6 ðsÞ ! 3LiHðsÞ þ AlðsÞ þ 1:5H2 ðgÞ ð180—220 CÞ ð2:6 wt% H2 Þ
⇑ Corresponding author. Tel./fax: +86 22 23503639. E-mail address: [email protected] (Y. Wang).
ðR2Þ
However, the utilization of (R1b) and (R2) is still inhibited by a lot of obstacles, especially high dynamic dehydrogenation temperatures, relatively slow dehydrogenation rate and poor reversibility of LiAlH4. In practice, an effective strategy to modifying the hydrogen storage properties of LiAlH4 is to alter the chemical bonding of LiAlH4 by adding transition metals (TMs). In recent years, hydrogen storage properties of LiAlH4 were significantly enhanced by ballmilling with a number of additives, such as pure metals, alloys, metal halides, metal carbide, metal oxides, metallic hydride and multi-hydride systems [18,20–29]. Therefore, it is crucial to find an advanced catalyst which could significantly improve the hydrogen storage of LiAlH4. Recently, in order to further improve the dehydrogenation properties of LiAlH4, cobalt base compounds are becoming a kind of new researching field [18,30,31]. For example, Li et al. [18] reported that the onset desorption temperature of the LiAlH4 + 2 mol% CoFe2O4 sample is 65 °C, which is 90 °C lower that of the as-received LiAlH4, with approximately 7.2 wt% hydrogen released at 250 °C. It is well known that carbon materials [32] have also beneficial effects on the hydrogen storage properties of LiAlH4. Taking above discussions into consideration, it is expected that Co@C can exhibit enhanced catalytic effects on the dehydrogenation properties of LiAlH4. In this work, we studied the catalytic effects of Co@C on the dehydrogenation properties of LiAlH4 matrix. The dehydrogenation properties of as-prepared Co@C–LiAlH4 composites have been systematically investigated. Compared with pure LiAlH4,
http://dx.doi.org/10.1016/j.jallcom.2014.12.080 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Li et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2014.12.080
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L. Li et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
the addition of Co@C remarkably enhances the desorption properties of LiAlH4. 2. Experimental 2.1. Preparation of Co@C–LiAlH4 samples LiAlH4 were purchased from Sigma–Aldrich Co. The mixture of LiAlH4 and Co@C was prepared by high energy ball milling. About 0.01 mol of LiAlH4 was mixed with 10 wt% of Co@C catalyst, and then the mixture was charged into a stainless steel vessel under an Ar atmosphere. Ball milling was carried out for 1 h at 450 rpm with a ball to powder weight ratio of 40:1 using planetary ball mill. After every 12 min of milling there was a 6 min pause and the rotation was automatically reversed. For reference purposes, an undoped LiAlH4 sample was also prepared under the same conditions (as milled LiAlH4). In order to avoid oxidation and moisture, all experimental operations were performed in a glovebox filled with high-purity Ar (H2O: <10 ppm; O2: <10 ppm). 2.2. Characterization Structural characteristics of the samples were identified by powder X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu Ka radiation). Temperature programmed desorption (TPD) of H2 was performed using in a home-made apparatus. About 70 mg amount of sample was loaded in the reactor and heated in a 35 mL/min Ar flow at a heating ramp of 2 °C min1 while heating from 50 to 275 °C. Hydrogen desorption was measured by isothermal dehydrogenation apparatus using a volumetric method. In the dehydrogenation experiment, the sample was quickly heated to and kept at a given temperature. Differential scanning calorimetry measurements (DSC) were performed using a TA apparatus (DSC Q20P).
3. Results and discussion Fig. 1 shows TPD curves of as-milled LiAlH4 and Co@C–LiAlH4. It is obvious that hydrogen desorption behavior exhibits two distinct peaks for as-milled LiAlH4 and Co@C–LiAlH4 samples in the temperature range 50–275 °C, implying an identical hydrogen desorption process as described in (R1b) and (R2) reactions. It is obvious that Co@C nanoparticles significantly enhance the dehydrogenation properties of LiAlH4 and reduce of the dehydrogenation temperatures ((R1b) and (R2)) in comparison with as-milled LiAlH4. As shown in Fig. 1A, as-milled LiAlH4 starts to decompose at 151.0 °C. Approximately 3.50 and 2.81 wt% hydrogen is released in the first and second dehydrogenation reactions ((R1b) and (R2)).
Fig. 2. Thermal dehydrogenation characteristics of Co@C–LiAlH4 sample at different temperatures and as-milled LiAlH4 sample at 130 °C.
When doping Co@C additive into LiAlH4, the onset dehydrogenation temperature of Co@C–LiAlH4 significantly decreases. The onset dehydrogenation temperature of Co@C–LiAlH4 lowers to about 100.0 °C with a peak temperature at 156.9 °C, which is 51 °C lower than that of the as-received LiAlH4. The dehydrogenation temperature of the second step starts at about 179.9 °C, and the dehydrogenation tends to reach equilibrium at about 244.5 °C. The first dehydrogenation step (R1b) releases about 4.48 wt% hydrogen, with the second dehydrogenation reaction (R2) exhibiting a 2.56 wt% hydrogen (Fig. 1B). Thus, we conclude that Co@C additive exhibits the superior catalytic performance and significantly improves the dehydrogenation properties of LiAlH4, which may make it quite attractive for the PEM fuel cell applications. Dehydrogenation kinetic behaviors of as-milled LiAlH4 and Co@C–LiAlH4 were further investigated by isothermal desorption kinetics measurement at different temperatures (Fig. 2). Obviously, the addition of Co@C dramatically decreases the dehydrogenation temperature of LiAlH4. The improvement may also attribute to
Fig. 1. (A) TPD hydrogen signal and (B) Hydrogen desorption curves of as-milled LiAlH4 and Co@C–LiAlH4 samples after ball milling. (Heating ramp of 2 °C min1.)
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L. Li et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
Fig. 3. FTIR spectra of as-milled LiAlH4, Co@C–LiAlH4 samples and after hydrogen desorption of Co@C–LiAlH4 at different temperatures.
the reduction of particle size and increase of defect during ball milling. It can be seen from Fig. 2, the dehydrogenation rate improves with the increase of dehydrogenation temperature, and dehydrogenation weight increases with the increase of dehydrogenation temperature. Isothermal desorption results exhibit that Co@C–LiAlH4 composite can release 2.25, 4.76 and 6.73 wt% hydrogen within 240 min at 100, 130 and 150 °C, respectively. Besides, Co@C–LiAlH4 sample releases 4.58 wt% of hydrogen in 180 min at 130 °C, while the undoped LiAlH4 sample just releases about 1 wt% H2 under the same conditions. The dehydrogenation capacity of Co@C–LiAlH4 is 3.56 wt% higher than that of as-milled LiAlH4 sample. Hydrogen release capacity of Co@C–LiAlH4 is approximately 7.05 wt%. Thus, it is reasonable to conclude that Co@C catalyst demonstrates excellent catalytic performance and enhances the dehydrogenation kinetics of commercial LiAlH4.
3
Fig. 3 shows FTIR spectra of as-milled LiAlH4, Co@C–LiAlH4 samples and after hydrogen desorption at different temperatures of Co@C–LiAlH4 in the wave-number range of 480–2200 cm1. For as-milled LiAlH4, the distinct Al–H stretching modes m3 of [AlH4] observed at 1783 and 1640 cm1 and Li–Al–H bending modes m4 of [AlH4] are observed at 886 and 710 cm1, respectively. It confirms that the sample after ball-milling is LiAlH4 and LiAlH4 does not decompose after high energy ball milling. However, there is a weak IR absorption peak at 1398 cm1 after adding Co@C additive, indicating that LiAlH4 decomposes partially. After dehydrogenation of Co@C–LiAlH4 at 130 °C, it shows a very broad band centered at 1399 cm1, which corresponds to the characteristic bending mode m4 of the Al–H vibration in [AlH6]3 group, indicating that LiAlH4 decomposes into Li3AlH6. However, no band of LiAlH4 and Li3AlH6 is found after hydrogen desorption at 150 °C, suggesting that LiAlH4 and Li3AlH6 have fully decomposed. From the above results, it can be inferred that Co@C additive is highly reactive to catalyze the decomposition of LiAlH4. The apparent activation energy (Ea) is determined with Kissinger’s approach, as described below, to quantitatively elucidate the kinetic improvement of the Co@C–LiAlH4 sample [33].
d lnðb=T 2m Þ Ea ¼ dð1=T m Þ R
ðR3Þ
Here, b, Tm and R are the heating rate, the absolute temperature for the maximum desorption rate and the gas constant, respectively. The non-isothermal DSC curves were used to determine the Ea of Co@C–LiAlH4 sample. The typical DSC curves of Co@C–LiAlH4 were shown in Fig. 4A. In this work, Tm was obtained at several heating ramps (1, 3 and 5 °C min1). Fig. 4B plots the dependence of ln(b/ Tm2) on 1/Tm. Clearly, a good linear relationship exists between ln(b/Tm2) and 1/Tm. By fitting, the values of Ea are calculated to be 95.36 and 115.6 kJ mol1 for the first and second hydrogen desorption of Co@C–LiAlH4 sample, which are lower than those of as-received LiAlH4 (116.2 kJ mol1 and 133.0 kJ mol1). The decreased Ea implied that the enhanced kinetics of Co@C–LiAlH4 sample is caused by the decreased energy barrier during the dehydrogenation reaction.
Fig. 4. (A) DSC profiles of Co@C–LiAlH4 (1, 3 and 5 °C min1). (B) Kissinger plots for the first and second dehydrogenation steps of Co@C–LiAlH4 (the black Kissinger plots: the first-step dehydrogenation; the red Kissinger plots: the second-step dehydrogenation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions In conclusion, dehydrogenation properties of LiAlH4 doped Co@C additive have been systematically investigated. It demonstrates that the Co@C additive can significantly improve the dehydrogenation kinetics and decrease the desorption temperature of Co@C–LiAlH4 system. The onset dehydrogenation temperature of Co@C–LiAlH4 is about 100.0 °C with a peak temperature at 156.9 °C. Approximately 7.05 wt% of H2 is released at 150 °C. Meanwhile, the Co@C–LiAlH4 sample releases 4.58 wt% of hydrogen in 180 min at 130 °C, while the undoped LiAlH4 sample just releases about 1 wt% H2 under the same conditions. The Ea values of LiAlH4 sample for the first and second dehydrogenation decrease to 95.36 kJ mol1 and 115.6 kJ mol1 after doping Co@C additive, respectively. From the above analyses, it is reasonable to conclude that Co@C is an effective additive for significantly improving dehydrogenation properties of as-received LiAlH4. Acknowledgments This work was financially supported by MOST Project (2012AA051901), NSFC (51471089 and 51171083), 111 Project (B12015), Excellent Young and Middle-aged Scientists of Shandong Province (BS2014CL026) and MOE (IRT-13R30). References [1] [2] [3] [4] [5] [6] [7] [8]
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Please cite this article in press as: L. Li et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2014.12.080
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced catalytic effects of Co@C additive on dehydrogenation properties of LiAlH4 Li Li b, Yijing Wang a,⇑, Lifang Jiao a, Huatang Yuan a a Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, China b Key Laboratory of Inorganic Functional Materials in Universities of Shandong, School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Complex hydride LiAlH4 Co@C Dehydrogenation
a b s t r a c t The catalytic effects of Co@C additive on hydrogen storage properties of LiAlH4 prepared by ball milling have been systematically investigated. The onset desorption temperature of Co@C–LiAlH4 sample is 100.0 °C, which is 51 °C lower than that of as-received LiAlH4. Isothermal desorption results show that for Co@C–LiAlH4 dehydrogenated at 130 °C, 4.58 wt% of hydrogen can be released with 180 min, which is 3.56 wt% higher than that of as-received LiAlH4 under the same conditions. Approximately 7.05 wt% of hydrogen is released at 150 °C. Through the Kissinger desorption kinetics analyses, the apparent activation energy, Ea, of Co@C–LiAlH4 is calculated as 95.36 kJ mol1 H2 and 115.6 kJ mol1 H2 for the first two decomposition processes, respectively. It indicates that dehydrogenation kinetics of LiAlH4 matrix considerably improves by doping the Co@C catalyst. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The demand for safe and efficient storage of renewable energy has created significant interest. Hydrogen, as the lightest element on the earth, is expected to play a crucial role in the future energy system [1,2]. Solid-state materials have the potential to provide significantly hydrogen storage densities. Thus, a long list of novel complex metal hydrides [3–18] (eg, LiAlH4, NaAlH4, LiBH4, Mg(BH4)2, LiNH2, NH3BH3) receive a lot of attention among current hydrogen storage technologies. Among these complex hydrides, lithium alanate (LiAlH4) is of particular interest for lightweight hydrogen storage due to its high hydrogen storage capacity (7.9 wt% accessible) and relatively low decomposition temperature in the presence of suitable catalysts [19]. In general, LiAlH4 releases hydrogen in the following stages ((R1) and (R2)) upon heating.
LiAlH4 ðsÞ ! LiAlH4 ðlÞ
ðR1aÞ
3LiAlH4 ðlÞ ! Li3 AlH6 ðsÞ þ 2AlðsÞ þ 3H2 ðgÞ ð150—175 CÞ ð5:3 wt% H2 Þ
ðR1bÞ
Li3 AlH6 ðsÞ ! 3LiHðsÞ þ AlðsÞ þ 1:5H2 ðgÞ ð180—220 CÞ ð2:6 wt% H2 Þ
⇑ Corresponding author. Tel./fax: +86 22 23503639. E-mail address: [email protected] (Y. Wang).
ðR2Þ
However, the utilization of (R1b) and (R2) is still inhibited by a lot of obstacles, especially high dynamic dehydrogenation temperatures, relatively slow dehydrogenation rate and poor reversibility of LiAlH4. In practice, an effective strategy to modifying the hydrogen storage properties of LiAlH4 is to alter the chemical bonding of LiAlH4 by adding transition metals (TMs). In recent years, hydrogen storage properties of LiAlH4 were significantly enhanced by ballmilling with a number of additives, such as pure metals, alloys, metal halides, metal carbide, metal oxides, metallic hydride and multi-hydride systems [18,20–29]. Therefore, it is crucial to find an advanced catalyst which could significantly improve the hydrogen storage of LiAlH4. Recently, in order to further improve the dehydrogenation properties of LiAlH4, cobalt base compounds are becoming a kind of new researching field [18,30,31]. For example, Li et al. [18] reported that the onset desorption temperature of the LiAlH4 + 2 mol% CoFe2O4 sample is 65 °C, which is 90 °C lower that of the as-received LiAlH4, with approximately 7.2 wt% hydrogen released at 250 °C. It is well known that carbon materials [32] have also beneficial effects on the hydrogen storage properties of LiAlH4. Taking above discussions into consideration, it is expected that Co@C can exhibit enhanced catalytic effects on the dehydrogenation properties of LiAlH4. In this work, we studied the catalytic effects of Co@C on the dehydrogenation properties of LiAlH4 matrix. The dehydrogenation properties of as-prepared Co@C–LiAlH4 composites have been systematically investigated. Compared with pure LiAlH4,
http://dx.doi.org/10.1016/j.jallcom.2014.12.080 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Li et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2014.12.080
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the addition of Co@C remarkably enhances the desorption properties of LiAlH4. 2. Experimental 2.1. Preparation of Co@C–LiAlH4 samples LiAlH4 were purchased from Sigma–Aldrich Co. The mixture of LiAlH4 and Co@C was prepared by high energy ball milling. About 0.01 mol of LiAlH4 was mixed with 10 wt% of Co@C catalyst, and then the mixture was charged into a stainless steel vessel under an Ar atmosphere. Ball milling was carried out for 1 h at 450 rpm with a ball to powder weight ratio of 40:1 using planetary ball mill. After every 12 min of milling there was a 6 min pause and the rotation was automatically reversed. For reference purposes, an undoped LiAlH4 sample was also prepared under the same conditions (as milled LiAlH4). In order to avoid oxidation and moisture, all experimental operations were performed in a glovebox filled with high-purity Ar (H2O: <10 ppm; O2: <10 ppm). 2.2. Characterization Structural characteristics of the samples were identified by powder X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu Ka radiation). Temperature programmed desorption (TPD) of H2 was performed using in a home-made apparatus. About 70 mg amount of sample was loaded in the reactor and heated in a 35 mL/min Ar flow at a heating ramp of 2 °C min1 while heating from 50 to 275 °C. Hydrogen desorption was measured by isothermal dehydrogenation apparatus using a volumetric method. In the dehydrogenation experiment, the sample was quickly heated to and kept at a given temperature. Differential scanning calorimetry measurements (DSC) were performed using a TA apparatus (DSC Q20P).
3. Results and discussion Fig. 1 shows TPD curves of as-milled LiAlH4 and Co@C–LiAlH4. It is obvious that hydrogen desorption behavior exhibits two distinct peaks for as-milled LiAlH4 and Co@C–LiAlH4 samples in the temperature range 50–275 °C, implying an identical hydrogen desorption process as described in (R1b) and (R2) reactions. It is obvious that Co@C nanoparticles significantly enhance the dehydrogenation properties of LiAlH4 and reduce of the dehydrogenation temperatures ((R1b) and (R2)) in comparison with as-milled LiAlH4. As shown in Fig. 1A, as-milled LiAlH4 starts to decompose at 151.0 °C. Approximately 3.50 and 2.81 wt% hydrogen is released in the first and second dehydrogenation reactions ((R1b) and (R2)).
Fig. 2. Thermal dehydrogenation characteristics of Co@C–LiAlH4 sample at different temperatures and as-milled LiAlH4 sample at 130 °C.
When doping Co@C additive into LiAlH4, the onset dehydrogenation temperature of Co@C–LiAlH4 significantly decreases. The onset dehydrogenation temperature of Co@C–LiAlH4 lowers to about 100.0 °C with a peak temperature at 156.9 °C, which is 51 °C lower than that of the as-received LiAlH4. The dehydrogenation temperature of the second step starts at about 179.9 °C, and the dehydrogenation tends to reach equilibrium at about 244.5 °C. The first dehydrogenation step (R1b) releases about 4.48 wt% hydrogen, with the second dehydrogenation reaction (R2) exhibiting a 2.56 wt% hydrogen (Fig. 1B). Thus, we conclude that Co@C additive exhibits the superior catalytic performance and significantly improves the dehydrogenation properties of LiAlH4, which may make it quite attractive for the PEM fuel cell applications. Dehydrogenation kinetic behaviors of as-milled LiAlH4 and Co@C–LiAlH4 were further investigated by isothermal desorption kinetics measurement at different temperatures (Fig. 2). Obviously, the addition of Co@C dramatically decreases the dehydrogenation temperature of LiAlH4. The improvement may also attribute to
Fig. 1. (A) TPD hydrogen signal and (B) Hydrogen desorption curves of as-milled LiAlH4 and Co@C–LiAlH4 samples after ball milling. (Heating ramp of 2 °C min1.)
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L. Li et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx
Fig. 3. FTIR spectra of as-milled LiAlH4, Co@C–LiAlH4 samples and after hydrogen desorption of Co@C–LiAlH4 at different temperatures.
the reduction of particle size and increase of defect during ball milling. It can be seen from Fig. 2, the dehydrogenation rate improves with the increase of dehydrogenation temperature, and dehydrogenation weight increases with the increase of dehydrogenation temperature. Isothermal desorption results exhibit that Co@C–LiAlH4 composite can release 2.25, 4.76 and 6.73 wt% hydrogen within 240 min at 100, 130 and 150 °C, respectively. Besides, Co@C–LiAlH4 sample releases 4.58 wt% of hydrogen in 180 min at 130 °C, while the undoped LiAlH4 sample just releases about 1 wt% H2 under the same conditions. The dehydrogenation capacity of Co@C–LiAlH4 is 3.56 wt% higher than that of as-milled LiAlH4 sample. Hydrogen release capacity of Co@C–LiAlH4 is approximately 7.05 wt%. Thus, it is reasonable to conclude that Co@C catalyst demonstrates excellent catalytic performance and enhances the dehydrogenation kinetics of commercial LiAlH4.
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Fig. 3 shows FTIR spectra of as-milled LiAlH4, Co@C–LiAlH4 samples and after hydrogen desorption at different temperatures of Co@C–LiAlH4 in the wave-number range of 480–2200 cm1. For as-milled LiAlH4, the distinct Al–H stretching modes m3 of [AlH4] observed at 1783 and 1640 cm1 and Li–Al–H bending modes m4 of [AlH4] are observed at 886 and 710 cm1, respectively. It confirms that the sample after ball-milling is LiAlH4 and LiAlH4 does not decompose after high energy ball milling. However, there is a weak IR absorption peak at 1398 cm1 after adding Co@C additive, indicating that LiAlH4 decomposes partially. After dehydrogenation of Co@C–LiAlH4 at 130 °C, it shows a very broad band centered at 1399 cm1, which corresponds to the characteristic bending mode m4 of the Al–H vibration in [AlH6]3 group, indicating that LiAlH4 decomposes into Li3AlH6. However, no band of LiAlH4 and Li3AlH6 is found after hydrogen desorption at 150 °C, suggesting that LiAlH4 and Li3AlH6 have fully decomposed. From the above results, it can be inferred that Co@C additive is highly reactive to catalyze the decomposition of LiAlH4. The apparent activation energy (Ea) is determined with Kissinger’s approach, as described below, to quantitatively elucidate the kinetic improvement of the Co@C–LiAlH4 sample [33].
d lnðb=T 2m Þ Ea ¼ dð1=T m Þ R
ðR3Þ
Here, b, Tm and R are the heating rate, the absolute temperature for the maximum desorption rate and the gas constant, respectively. The non-isothermal DSC curves were used to determine the Ea of Co@C–LiAlH4 sample. The typical DSC curves of Co@C–LiAlH4 were shown in Fig. 4A. In this work, Tm was obtained at several heating ramps (1, 3 and 5 °C min1). Fig. 4B plots the dependence of ln(b/ Tm2) on 1/Tm. Clearly, a good linear relationship exists between ln(b/Tm2) and 1/Tm. By fitting, the values of Ea are calculated to be 95.36 and 115.6 kJ mol1 for the first and second hydrogen desorption of Co@C–LiAlH4 sample, which are lower than those of as-received LiAlH4 (116.2 kJ mol1 and 133.0 kJ mol1). The decreased Ea implied that the enhanced kinetics of Co@C–LiAlH4 sample is caused by the decreased energy barrier during the dehydrogenation reaction.
Fig. 4. (A) DSC profiles of Co@C–LiAlH4 (1, 3 and 5 °C min1). (B) Kissinger plots for the first and second dehydrogenation steps of Co@C–LiAlH4 (the black Kissinger plots: the first-step dehydrogenation; the red Kissinger plots: the second-step dehydrogenation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions In conclusion, dehydrogenation properties of LiAlH4 doped Co@C additive have been systematically investigated. It demonstrates that the Co@C additive can significantly improve the dehydrogenation kinetics and decrease the desorption temperature of Co@C–LiAlH4 system. The onset dehydrogenation temperature of Co@C–LiAlH4 is about 100.0 °C with a peak temperature at 156.9 °C. Approximately 7.05 wt% of H2 is released at 150 °C. Meanwhile, the Co@C–LiAlH4 sample releases 4.58 wt% of hydrogen in 180 min at 130 °C, while the undoped LiAlH4 sample just releases about 1 wt% H2 under the same conditions. The Ea values of LiAlH4 sample for the first and second dehydrogenation decrease to 95.36 kJ mol1 and 115.6 kJ mol1 after doping Co@C additive, respectively. From the above analyses, it is reasonable to conclude that Co@C is an effective additive for significantly improving dehydrogenation properties of as-received LiAlH4. Acknowledgments This work was financially supported by MOST Project (2012AA051901), NSFC (51471089 and 51171083), 111 Project (B12015), Excellent Young and Middle-aged Scientists of Shandong Province (BS2014CL026) and MOE (IRT-13R30). References [1] [2] [3] [4] [5] [6] [7] [8]
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