Article pubs.acs.org/JPCC
Improvement on Hydrogen Desorption Performance of Calcium Borohydride Diammoniate Doped with Transition Metal Chlorides Hailiang Chu,*,†,‡,§ Shujun Qiu,†,‡ Yongjin Zou,†,‡ Cuili Xiang,†,‡ Huanzhi Zhang,†,‡ Fen Xu,†,‡ Lixian Sun,*,†,‡ and Huaiying Zhou†,‡ †
School of Materials Science and Engineering, Guilin University of Electronic Technology, No. 1, Jinji Road, Guilin 541004, People’s Republic of China ‡ Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, No. 1, Jinji Road, Guilin 541004, People’s Republic of China § Dalian Institute of Chemical Physics, Chinese Academy of Sciences, No. 457, Zhongshan Road, Dalian 116023, People’s Republic of China S Supporting Information *
ABSTRACT: Calcium borohydride diammoniate with a molecular formula of Ca(BH4)2·2NH3 is a novel complex hydride for hydrogen storage. However, it suffers from high temperature and sluggish kinetics for hydrogen desorption. In this work, the temperature and kinetics for hydrogen desorption of calcium borohydride diammoniate were effectively improved through doping transition metal chlorides (such as CoCl2, NiCl2, and FeCl3) in a closed vessel. Three additives could effectively decrease the temperature for hydrogen desorption of Ca(BH4)2·2NH3. Among them, CoCl2-doped Ca(BH4)2·2NH3 could desorb hydrogen even at a low temperature of 170 °C, and at 200 °C, 7.6 wt % hydrogen with high purity is released, which shows superior performance for hydrogen desorption than that of pristine Ca(BH4)2·2NH3. For CoCl2-doped Ca(BH4)2·2NH3, X-ray absorption fine structure (XAFS) revealed that the improvements could be ascribed to the catalytic function of well-dispersed cobalt particles, which results in a much lower activation energy (86.1 kJ/mol) for hydrogen desorption of calcium borohydride diammoniate.
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INTRODUCTION One of the key challenges for the extensive application of hydrogen as a practical substitution for fossil fuels is to develop hydrogen storage materials with high safety and performance.1 Extensive interest has been focused on metal borohydrides for storing hydrogen in solid state because they exhibit much higher weight content of H2 and, therefore, are one of the most promising candidates to meet the criteria for hydrogen storage.2−5 Among all metal borohydrides, Ca(BH4)2 has been acknowledged as a potential candidate for hydrogen storage6,7 owing to its attractive hydrogen content (e.g., 11.4 wt % H2) and intermediate enthalpy of decomposition (40.6 kJ/mol H2, estimated from theoretical calculation8), which is close to the ideal value for reversible hydrogen storage at ambient conditions.9 However, although the enthalpy change determined by calculation is much lower among all metal borohydrides for hydrogen desorption, the higher kinetic barrier prevents Ca(BH4)2 from releasing hydrogen at low temperatures. It has been reported that 9.0 wt % hydrogen could be desorbed from Ca(BH4)2 only at a high temperature of 500 °C.10 Some Nb- or Ti-contianing compounds were introduced to assist hydrogen desorption of Ca(BH4)2, but no obvious catalytic effect was observed.11 © 2014 American Chemical Society
Our strategy for facilitating hydrogen desorption of Ca(BH4)2 is the interaction between NH2 (Hδ+) and BH4 (Hδ−) groups and has been proven to be feasible for improving the properties. For example, the temperature for hydrogen desorption of Ca(BH4)2 can be decreased from 360 °C to 320, 313, 305, and 125 °C after combination with [NH2]-containing materials, such as lithium amide,12−14 calcium amide, magnesium amide,15 and urea,16 respectively. Moreover, in our previous work,17 a new compound of Ca(BH4)2·2NH3 was successfully prepared through the reaction between solid Ca(BH4)2 and gaseous ammonia. There are two steps for Ca(BH4)2·2NH3 decomposition into NH3 and Ca(BH4)2 in an open system (i.e., TG with a dynamic carrier gas). However, hydrogen formation and desorption occurred, rather than NH3 desorption, when we conducted the volumetric release tests in a small and closed vessel. Clearly, the pathway for thermal decomposition of Ca(BH4)2·2NH3 is diverse under the different conditions applied (closed system vs open system). In an open system such as TG-DSC, NH3 desorbed from Ca(BH4)2·2NH3 was immediately removed along with the Received: October 10, 2014 Revised: December 18, 2014 Published: December 18, 2014 913
DOI: 10.1021/jp510248m J. Phys. Chem. C 2015, 119, 913−918
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The Journal of Physical Chemistry C carrier gas. Thus, there is no existence of NH3 around Ca(BH4)2· 2NH3, under which circumstance the probability of interaction between BH4 and NH3 groups at higher temperature more than 200 °C is close to zero. However, due to the existence of the equilibrium pressure in a closed vessel, NH3 is kept in the proximity of the solid raw, either in the gaseous phase closely around Ca(BH4)2·2NH3 or within the lattice of Ca(BH4)2· 2NH3, which results in an opportunity to react with each other and leads to the interaction between BH4 and NH3 groups and the following formation of H2. However, it took considerably more time than 100 h to release ∼5.9 equiv of H2 (11.3 wt %) at 250 °C, which implies the existence of high kinetic barriers to overcome for hydrogen desorption. Therefore, there is an urgent need for catalysts to effectively modify the kinetics for hydrogen desorption. Herein, we report that the hydrogen desorption properties of Ca(BH4)2·2NH3 could be improved by ball milling with transition metal chlorides (CoCl2, NiCl2, and FeCl3) as additives. And the catalytic species is clarified for CoCl2-doped Ca(BH4)2·2NH3 through the characterization of X-ray absorption fine structure (XAFS) spectroscopy.
after performing the tests. The constant weight in all cases indicated no leakage during the experiments. XAFS spectra were collected at the beamline of BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF) at room temperature. Powdery samples were pressed into pellets for XAFS tests under a pressure of 8 MPa in a glovebox. Then the pellets were packed by a kapton film to avoid contamination from air during measurements. The Co K-edge XANES data were recorded in a fluorescence mode. For XANES and EXAFS analysis, the data were analyzed using Athena software.18
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RESULTS AND DISCUSSION The powder XRD patterns of pristine Ca(BH4)2·2NH3 and additive-doped Ca(BH4)2·2NH3 are presented in Figure 1.
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EXPERIMENTAL SECTION Ca(BH4)2·2NH3 was synthesized through the gas−solid reaction between ammonia and calcium borohydride according to our previous work.17 Cobalt chloride (97%), nickel chloride (98%), and iron(III) chloride (98%) from Sigma-Aldrich Corporation were used without any special treatment. Mixtures of Ca(BH4)2· 2NH3 and additives (5% in weight ratio) were mechanical milled using a planetary Retch PM 400 miller. To clarify the roles of adding sequence of dopants, [Ca(BH4)2CoCl2]·2NH3 sample was prepared according to the following procedure: first, Ca(BH4)2 was ball milled with an addition of 5 wt % CoCl2; then it was reacted with ammonia for the preparation of [Ca(BH4)2CoCl2]·2NH3 under the identical conditions for synthesizing Ca(BH4)2·2NH3. The rotational speed was set at 200 rpm and the milling time was set for 5 h. The samples were weighed and loaded into a milling vessel according to a ball-topowder-ratio of 50:1. All manipulations were handled in a glovebox (MBraun Labstar) filled with argon (H2O < 1 ppm, O2 < 1 ppm), unless otherwise stated. For deammoniation of the samples in an open system, the thermogravimetry (TG, SETSYS Evolution) combined with mass spectroscopy (MS, GAM 200) was employed for determining the decomposition behaviors. Using TG-MS with argon as the carrier gas at 40 mL/min, the samples of ∼5 mg were loaded into Al2O3 crucible and then heated to 300 °C at 2 °C/ min. The hydrogen desorption experiments in a closed vessel were conducted on an apparatus made in our laboratory. A sample of approximately 200 mg was loaded into a smaller reactor with a volume of 13.4 mL and then heated to the target temperature. The concentration of ammonia in the gaseous products was determined using homemade equipment mainly consisting of a conductivity meter. The detailed operational procedures have been illustrated in our previous paper.12 XRD patterns were measured on a PANalytical X’pert diffractometer (X’Pert MPD PRO, Cu Kα radiation). DSC measurements were performed using a Calvet calorimeter (C80). In a glovebox, the sample was weighed and loaded into a reactor of 8.5 mL made of stainless steel for high pressure. Then the reactor was sealed and transferred to the C80 calorimeter. To verify that there was no leakage of the reactor during the tests, the weight of the reactor (vessel + sample) was measured before and
Figure 1. XRD patterns of (a) pristine Ca(BH4)2·2NH3, (b) CoCl2doped Ca(BH4)2·2NH3, (c) [Ca(BH4)2CoCl2]·2NH3, (d) NiCl2doped Ca(BH4)2·2NH3, and (e) FeCl3-doped Ca(BH4)2·2NH3.
Ca(BH4)2·2NH3 crystallizes in an orthorhombic structure with space group of Pbcn (no.: 60), which has the lattice parameters of a = 6.4160 Å, b = 8.3900 Å, and c = 12.7020 Å.17 After ball milling Ca(BH4)2·2NH3 with 5 wt % additives of transition metal chlorides for 5 h, Figure 1 indicates that the samples doped with additives have almost identical XRD patterns as the pristine sample. And the XRD pattern of [Ca(BH4)2CoCl2]·2NH3 is very much like that of Ca(BH4)2·2NH3, including the intensity of diffraction peaks. It should be noted that, after ball milling for 5 h with the additives, the peak intensity in XRD patterns is weakened compared to the pristine sample, which can be related to a decrease of the crystalline size and/or partial amorphization after the ball milling treatment. It can be seen from Figure 2 that the mass loss of Ca(BH4)2· 2NH3 begins at a temperature of 80 °C in TG under a dynamic flow. MS results show that there is only NH3 in the gaseous products, without detectable signals of H2 and B2H6. The TG curve of Ca(BH4)2·2NH3 shows two steps for its decomposition. The first one has a weight loss of 16.0 wt % from room temperature to 150 °C, and the second one has a 15.9 wt % from 150 to 210 °C. The theoretical value for mass loss of Ca(BH4)2· 2NH3 is 16.3 wt % for each equiv of NH3, which is very close to our results. As for the samples doped with transition metal chlorides, the dopants have an obvious effect on the deammoniation for Ca(BH4)2·2NH3. For pristine Ca(BH4)2· 2NH3, two peaks of NH3 signal are observed at 131 and 196 °C. After doping with CoCl2, NiCl2, and FeCl3, the first peak of NH3 914
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closed vessel. After ball milling with additives, the temperature for hydrogen desorption dramatically decreased, which means that transition metal chlorides play remarkable roles for improving the properties for hydrogen desorption. It is worth noting that all curves for hydrogen desorption versus temperature have the same shape, with the exception of the temperature difference, which suggests that the reaction pathways may be identical before and after doping transition metal chlorides. As indicated in Figure 3(b), the temperature of rapid rate for hydrogen desorption is obviously decreased from 283 °C for pristine Ca(BH4)2·2NH3 to 217, 227, and 232 °C for CoCl2-, NiCl2-, and FeCl3-doped samples. However, the content of ammonia in the evolved gas from the doped samples is about 3000, 12 000, and 12 000 ppm for CoCl2-, NiCl2-, and FeCl3doped samples, respectively. Also, [Ca(BH4)2-CoCl2]·2NH3 has almost the same behavior for hydrogen desorption as CoCl2doped Ca(BH4)2·2NH3, which implies that the adding sequence of CoCl2 has undetectable effects on the properties for hydrogen desorption. To quantitatively determine the improved kinetics of transition metal chloride-doped Ca(BH4)2·2NH3, we used the model proposed by Kissinger to determine the activation energy for hydrogen desorption,19 i.e., collecting peak temperatures for hydrogen desorption at three different ramping rates and applying the following equation to determine apparent activation energy:19
Figure 2. (a) MS for NH3 signal (m/z = 17) for additive-doped Ca(BH4)2·2NH3. (b) TG profile of pristine Ca(BH4)2·2NH3.
signal is reduced to 115, 116, and 125 °C, and the second one is at 189, 196, and 197 °C. It is clear that the reaction mechanism of thermal decomposition of Ca(BH4)2·2NH3 in a dynamic flow is completely different from that under a closed mode. Figure 3(a) shows that all samples including pristine and transition metal chloride-doped Ca(BH4)2·2NH3 could release about 11.3 wt % or 5.9 equiv of hydrogen upon heating to 500 °C in an
⎛ β⎞ ⎛ AR ⎞ E ln⎜⎜ 2 ⎟⎟ = ln⎜ ⎟− a RTp ⎝ Ea ⎠ ⎝ Tp ⎠
(1)
In eq 1, β is the temperature ramping rate, Tp is the peak temperature at which there is a maximum reaction rate, A is the pre-exponential factor, Ea is the activation energy for hydrogen desorption, and R is the gas constant. The peak temperatures Tp for maximum reaction rate at different heating rates were obtained with a C80 microcalorimeter (Supporting Information Figure S1). The dependence of ln(β/Tp2) to 1/Tm is shown in Figure 4. The values of ln(AR/Ea) and −Ea/R are determined
Figure 4. Kissinger’s plots of the hydrogen desorption for pristine and transition metal chloride-doped samples.
from the intercept and slope of the fitted lines. Then we can calculate A and Ea. Once A and Ea are determined, the rate constant k at a known temperature can be calculated according to Arrhenius equation:
⎛ E ⎞ k = A exp⎜ − a ⎟ ⎝ RT ⎠
Figure 3. (a) Hydrogen release curves of pristine and additive-doped Ca(BH4)2·2NH3 samples in a closed vessel. (b) The derivative of curves in (a). The samples were heated to 500 °C at 0.5 °C/min. 915
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The Journal of Physical Chemistry C The values of A, Ea, and k (at 200 °C) for pristine and transition metal chloride-doped Ca(BH4)2·2NH3 are summarized in Table 1. Compared to pristine Ca(BH4)2·2NH3, the Ea for hydrogen
The change of sample color from white to black was observed before and after ball milling Ca(BH4)2·2NH3 with 5 wt % CoCl2 for 5 h in our experiments, which suggests that the reaction between CoCl2 and metal borohydirde happened during ball milling. However, there are no cobalt-related species detectable from XRD patterns, probably due to the lower amount and/or poorer crystallinity. Therefore, we employed XAFS technique to investigate the local structures and chemical state of cobalt in CoCl2-doped Ca(BH4)2·2NH3 at different states including fresh and postdehydrogenated samples. As references for energy calibration, cobalt chloride and cobalt foil were included. As shown in Figure 6, the Co K-edge XANES of the fresh sample
Table 1. Pre-exponential Factor A, the Activation Energy Ea and the Rate Constant k (at 200 °C) of Pristine and Transition Metal Chloride-Doped Ca(BH4)2·2NH3 Determined from Kissinger’s Model and the Arrhenius Equation sample
Ea (kJ/mol)
A (min−1)
k (min−1)
pristine CoCl2-doped NiCl2-doped FeCl3-doped
129.7 86.1 114.2 117.7
5.47 × 1011 2.88 × 107 2.92 × 1010 6.25 × 1010
2.89 × 10−4 8.99 × 10−3 7.08 × 10−3 6.28 × 10−3
desorption of transition metal chloride-doped samples is decreased to some extent, indicating a substantial improvement in the kinetics for hydrogen desorption of pristine Ca(BH4)2· 2NH3 after being doped with transition metal chlorides. This result is in good agreement with the results for hydrogen releasing rate of the volumetric release tests shown in Figure 3. The Ea for hydrogen desorption of CoCl2-doped Ca(BH4)2· 2NH3 is reduced to 86.1 kJ/mol, and the rate constant k at 200 °C of CoCl2-doped Ca(BH4)2·2NH3 is 31 times as that of pristine Ca(BH4)2·2NH3. Because CoCl2 has better performance in enhancing the properties for hydrogen desorption of Ca(BH4)2·2NH3, to elucidate the kinetic and thermodynamic improvements for hydrogen desorption of this composite, the isothermal volumetric release of CoCl2-doped Ca(BH4)2·2NH3 in the closed system at 200, 225, and 250 °C was further performed and about 7.6, 9.9, and 10.3 wt % of hydrogen can be desorbed, which are shown in Figure 5. At the beginning within 5 h, more than
Figure 6. XANFS spectra for Co K-edge of Co foil, CoCl2, and CoCl2doped Ca(BH4)2·2NH3 samples after ball milling and hydrogen desorption.
after ball milling and the samples after hydrogen desorption are much more similar to each other. And all of them resemble that of metallic cobalt, indicating the existence of metallic cobalt particles in milled and postdehydrogenated samples. Figure 7 shows the Fourier transform of the XAFS functions in Figure 6. It can be seen that the postdehydrogenated sample at 250 °C has a main peak at 2.42 Å with a shoulder at around 1.90 Å, which is almost same as those of the pristine sample. The position of main peak is close to the second shell CoCo distance (2.51 Å) and the position of should peak is very close to the first shell CoB
Figure 5. Isothermal volumetric release measurements of CoCl2-doped Ca(BH4)2·2NH3 at different temperatures.
80% H2 can be detached. It should be noted that, at a lower temperature of 170 °C, CoCl2-doped Ca(BH4)2·2NH3 can desorb more than 2 wt % hydrogen. When the temperature increases to 180 °C, it has a desorption capacity of 6.8 wt % H2. The concentration of NH3 in the gaseous products at 200 °C is determined to be below 200 ppm, which is a solid evidence for the stoichiometric conversion of NH3. For comparison, pristine Ca(BH4)2·2NH3 releases only 1 wt % H2 after keeping it at 200 °C for a duration of approximately 100 h. The amount of ammonia in the evolved hydrogen at 200 °C is much lower than that in the volumetric release measurement from room temperature to 500 °C, probably because of long-term reaction.
Figure 7. Fourier transform (FT) of EXAFS spectra for Co K-edge of (a) Co foil, (b) CoCl2, and (c) CoCl2-doped Ca(BH4)2·2NH3, postdehydrogenated CoCl2-doped Ca(BH4)2·2NH3 at (d) 250, and (e) 500 °C. 916
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distance (1.89 Å) in CoB catalyst supported by mesoporous silica.20 The CoB amorphous alloy is probably formed due to the redox reaction between CoCl2 and BH4− during milling process. When the temperature increases to 500 °C, the reaction for hydrogen desorption proceeds, which results in the shift from 2.42 to 2.49 Å of the main peak and the disappearance of the shoulder peak. The peak at 2.49 Å is close to the CoCo distance (2.50 Å) of metallic cobalt. 21 Previous study demonstrated that CoB might begin to decompose into B and Co at the temperature of more than 300 °C.22 Furthermore, the resultant B may have a probability to react with N-containing species and form the BN bond under the reaction conditions. Although it is difficult for us to rule out the catalytic effect of CoB-like alloy in the CoCl2-doped Ca(BH4)2·2NH3 system, the likeliest catalyst should be cobalt particles formed during the milling process. The cobalt nanoparticles or clusters have enhanced catalytic activity in the CoCl2-catalyzed hydrolysis of ammonia borane23 and NaBH424 and thermolysis of ammonia borane,25 Li3BN2H8,26 Ca(BH4)2LiNH213 and lithium borohydride ammoniate.27
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CONCLUSIONS In summary, transition metal chlorides (CoCl2, NiCl2, and FeCl3) could effectively improve the properties for hydrogen desorption of calcium borohydride diammoniate in a closed vessel, with lower activation energy than that of pristine sample. In the case of the CoCl2-doped sample, it could release 7.6 wt % or 4 equiv of hydrogen at 200 °C. While the pristine sample released only 1.0 wt % hydrogen at this temperature. The likeliest species as a catalyst could be metallic cobalt, as determined by XAFS in this system.
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ASSOCIATED CONTENT
S Supporting Information *
DSC profiles through C80 microcalorimeter of pristine and transition metal chloride-doped Ca(BH4)2·2NH3 at different heating rates. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86-773-2216607; fax: +86-773-2290129; e-mail:
[email protected]. *Tel.: +86-773-2216607; fax: +86-773-2290129; e-mail: sunlx@ guet.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (51101144, 51461010, 51401059, 51361005, 51201041, 51371060, 21173111, 51361006, 51201042, and 51102230), Guangxi Natural Science Foundation (2013GXNSFBA019034, 2013GXNSFBA019239, 2014GXNSFAA118043, and 2014GXNSFAA118333) and Guangxi University Research Project (2013ZD023 and YB2014132). H.C. wishes to thank BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF) for XAFS tests.
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REFERENCES
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