Improved Dehydrogenation Properties of Ca(BH4)

Article pubs.acs.org/JPCC

Improved Dehydrogenation Properties of Ca(BH4)2·nNH3 (n = 1, 2, and 4) Combined with Mg(BH4)2 Xiaowei Chen, Feng Yuan, Yingbin Tan, Ziwei Tang, and Xuebin Yu* Department of Materials Science, Fudan University, Shanghai, China 200433 S Supporting Information *

ABSTRACT: As Ca(BH4)2·nNH3 (n = 1, 2, and 4) tends to release ammonia rather than hydrogen when heated in argon, an aided-cation strategy via combining these compounds with Mg(BH4)2 is employed to advance their dehydrogenation. It shows that the interaction between the two hydrogen storage systems, based on a promoted recombination reaction of BH and NH groups, enables a significant mutual dehydrogenation improvement beyond them alone. Dehydrogenation results show that the Ca(BH4)2·4NH3/Mg(BH4)2 composite starts to release hydrogen at around 62 °C and presents a hydrogen desorption capacity of >9 wt % below 300 °C with a H-purity of 92.3 wt %. Furthermore, in the cases of Ca(BH4)2·4NH3/2Mg(BH4)2, Ca(BH4)2·2NH3/Mg(BH4)2, and Ca(BH4)2·NH3/Mg(BH4)2, fairly pure hydrogen (>99 wt %) is released upon heating from RT to 500 °C. Further investigation via introduction of isotope deuterium in the combined system reveals that the dehydrogenation reactions are mainly mediated by the combination of Hδ+···Hδ‑ interactions, whereas Hδ‑···Hδ‑ interactions also contribute in a complementary way.

1. INTRODUCTION Hydrogen, as an ideal clear energy carrier, has attracted a lot of attention due to its highly abundant, lightweight, and environmentally friendly oxidation product (water). However, it is still a great challenge to develop an ideal hydrogen storage material which would be safe, dense, lightweight, inexpensive, and highly reversible.1,2 Up to now, tremendous efforts have been devoted to developing high hydrogen density materials with low dehydrogenation temperature.3−10 Metal borohydrides have recently received great attention owing to their high gravimetric and volumetric hydrogen densities.11−16 However, due to the thermodynamic and kinetic limitations, most of these compounds release hydrogen at high temperatures, which make them unfavorable in practical application. For example, Mg(BH4)2 and Ca(BH4)2 start to release H2 at about 300 °C, and a complete dehydrogenation from them requires temperatures higher than 500 °C.14,15 Although the thermodynamics of hydrogen storage in metal borohydrides have been improved using catalysts,17,18 dopants,9,19,20 or nanostructural templates,21,22 almost all of these materials still exhibit poorer hydrogen release kinetics/thermodynamics than would be desirable. For instance, dehydrogenation of LiBH4 can be significantly improved by doping small amounts of carbon© 2012 American Chemical Society

supported Pt nanoparticles or incorporating into an activated carbon scaffold.17,18 However, the dehydrogenation temperatures of these composites are still relative high (>220 °C). Following the prior work of Mg(BH4)2·2NH3, many reports have suggested that the introduction of dihydrogen bonds in M(BH4)n by coordinating with NH3 to form ammine metal borohydrides (AMBs) is an effective strategy to explore advanced solid-state hydrogen storage substrates, resulting in tunable thermo-dynamical stability and increased hydrogen densities.23−34 However, the main problem suffered for this system is that most of the AMBs give rise to the undesirable release of ammonia during the dehydrogenation process. For instances, LiBH4·NH3 and Ca(BH4)2·nNH3 (n = 1, 2, and 4) mainly release ammonia rather than hydrogen below 300 °C under dynamic situation;25,31,34 Al(BH4)3·6NH3 mainly releases hydrogen upon decomposition, but a high ammonia content of >32 wt % is still evolved from RT to 300 °C along with the hydrogen release;24 In the case of Mg(BH4)2·2NH3, only a trace of ammonia released during decomposition.23 More Received: March 26, 2012 Revised: September 11, 2012 Published: September 17, 2012 21162

dx.doi.org/10.1021/jp302866w | J. Phys. Chem. C 2012, 116, 21162−21168

The Journal of Physical Chemistry C

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recent studies suggest that the dehydrogenation properties of AMBs could be tuned extensively by an aided-cation strategy, e.g., the Al(BH4)3·6NH3/2LiBH4, LiBH4·NH3/Mg(BH4)2, and Ca(BH4)2·NH3/LiBH4 mixtures show significant improvement in dehydrogenation kinetic and purity of gas release compared to that of the sole ammine metal borohydrides.27,30,31 This provides a feasible solution to advance the hydrogen storage properties of AMBs. Guided by the previous aided-cation strategy studies, in this paper, we report a new combined system of Ca(BH4)2·nNH3/ Mg(BH4)2 complexes (n = 1, 2, and 4). Interestingly, our results show that, after combination, these composites exhibit a significant mutual dehydrogenation improvement compared with the Mg(BH4)2 and Ca(BH4)2·nNH3 alone.

C H2 + C NH3 = 1

(1)

(C H2 2.02 + C NH317.03)M p = Wp

(2)

The powder X-ray diffraction (XRD; Rigaku D/max 2400) measurements were also conducted to confirm the phase structure. Powders were spread and measured on a Si single crystal. Amorphous polymer tape was used to cover the surface of the powder to avoid oxidation during the XRD measurement. Fourier transform infrared (FT-IR; Magna-IR 550 II, Nicolet) analyses were conducted to confirm the chemical bonds in the sample. Products were pressed with KBr and then loaded in a sealed chamber for the measurement. The solid-state nuclear magnetic resonance (NMR) spectra were measured using a Bruker Avance 300 MHz spectrometer, using a Doty CP-MAS probe with no probe background. The powder samples collected after the decomposition reaction were spun at 5 kHz using 4 mm ZrO2 rotors filled up in purified argon atmosphere glove boxes. A 0.55 ms single-pulse excitation was employed, with repetition times of 1.5 s.

2. EXPERIMENTAL SECTION 2.1. Reagents and Synthesis. Mg(BH4)2 {Mg(BD4)2} was synthesized from MgCl2 (99.9% purity, Sigma) and NaBH4 (NaBD4) (95% purity, Sigma) in dried diethyl ether as described in ref 36. Ca(BH4)2·4NH3 was prepared by solid− gas reaction between calcium borohydride (95% purity, Sigma) and anhydrous ammonia as described in ref 34. Ca(BH4)2·2NH3 and Ca(BH4)2·NH3 were obtained by heating Ca(BH 4 ) 2 ·4NH 3 at 100 and 150 °C under vacuum, respectively.31,34 Approximately 0.3 g of mixtures of Ca(BH4)2·4NH3/ Mg(BH4)2 with mole ratios of 1:1 and 1:2, Ca(BH4)2·4NH3/ Mg(BD4)2 with a mole ratio of 1:1, Ca(BH4)2·2NH3/ Mg(BH4)2 with a mole ratio of 1:1, and Ca(BH4)2·NH3/ Mg(BH4)2 with a mole ratio of 1:1 were mixed and ball milled using a QM-3SP2 planetary ball mill at 260 rpm for 4 h in a 100 mL hardened steel bowl. The mass ratio of the sample to steel balls is 1:30. All sample handling was done in an argon-filled glovebox equipped with a recirculation system to keep the H2O and O2 levels below 1 ppm. 2.2. Instrumentation and Analyses. Hydrogen release property measurements were performed by thermogravimetry/ differential thermal analysis (TG/DSC, STA 449 C) connected to a mass spectrometer (MS, QMS 403) using a heating rate of 5 °C/min under a 1 atm argon atmosphere. Typical sample quantities were 5−10 mg, which is sufficient for getting accurate results due to the high sensitivity of the employed equipment. Before the MS measurement, flowing argon was introduced for more than 30 min to blow away the remainder gas such as O2, N2, and H2O from the instrument. The m/z = 27 and 28 were employed for the detection of B2H6, and the m/ z = 16 and 17 were employed for the detection of NH3. Temperature-programmed desorption (TPD) was also performed to determine the decomposition behavior of the sample on a semiautomatic Sievert’s apparatus, connected with a reactor filled with sample (∼0.1 g) under an argon atmosphere (1 bar) at a heating rate of 5 °C/min. The H 2 and NH 3 contents were determined using gravimetric and volumetric results based on the fact that the emission gas mainly consisted of H2 and NH3. First, the mole per gram (Mp) data of gas released from the sample was obtained by the Sievert’s type apparatus as a volumetric result, while the mass percent (Wp) of gas released from the sample was calculated from the weight change of the sample, and then the mole proportion of H2 (CH2) and NH3 (CNH3) from gas released could be calculated from the following two equations:

3. RESULTS AND DISCUSSION 3.1. Dehydrogenation Properties of the Ca(BH4)2·4NH3/Mg(BH4)2 Composite. As Ca(BH4)2·4NH3 owns the highest theoretical hydrogen capacity among the Ca(BH4)2·nNH3 (n = 1, 2, and 4), the initial study is focused on the dehydrogenation improvement of Ca(BH4)2·4NH3 combined with Mg(BH4)2. The comparative decomposition properties of Ca(BH4)2·4NH3 and Ca(BH4)2·4NH3/Mg(BH4)2 mixture (with a mole ratio of 1:1) via utilizing MS/TG are exhibited in Figure 1. The gas release of Ca(BH4)2·4NH3

Figure 1. MS/TG results for (a) Ca(BH4)2·4NH3 and (b) Ca(BH4)2·4NH3/Mg(BH4)2 (with a mole ratio of 1:1) with a heating rate of 5 °C/min.

occurring between 20 and 300 °C is mainly ascribed to great amounts of NH3 and a trace of H2 emission (with a small peak at ∼220 °C). This is attributed to the weak coordination bonding of Ca−NH3 in this compound,31,39 which breaks easily at low temperatures to generate NH3 and finally remains as Ca(BH4)2 alone to decompose at high temperature. Moreover, the boranes are hardly detectable as other ammine metal borohydride systems reported,27 suggesting that the weight loss of Ca(BH4)2·4NH3 is ascribed to the evolution of NH3 and a small amount of H2. These results are consistent with the previous report of Ca(BH4)2·4NH3 decomposed under the dynamic flow mode (i.e., MS/TG).34 However, the MS/TG 21163



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results of the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite display significant depression of NH 3 production and tremendously improved H2 liberation compared with the Ca(BH4)2·4NH3. The onset dehydrogenation temperature is reduced to 62 °C, which is about 160 °C lower than that of Ca(BH 4)2 ·4NH 3. Gas release in the first step of the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) mixture proceeding from RT to 280 °C is predominantly attributed to H2, with a small amount of NH3, whereas the following dehydrogenation step, from 280 to 500 °C, is dominated by pure H2. For the detection of diborane, m/z = 23, 24, 25, 26, and 27 were used; however, no B2H6 is observed in any point of the measured temperature range (see in Figure S1). To further clarify the different decomposition properties of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) mixture with the Ca(BH4)2·4NH3, Mg(BH4)2·2NH3 and Mg(BH4)2, TPD measurements were conducted and the results are shown in Figure 2. It shows that Mg(BH4)2 mainly releases hydrogen at above

the composite at 5 bar hydrogen pressure was conducted. As shown in Figure S3, the dehydrogenation temperature under hydrogen atmosphere is slightly higher than that under Ar atmosphere. By combination of the TPD and gravimetric results, the hydrogen purity was calculated to be 97.8 mol % (84.2 wt %). These results indicate that the presence of hydrogen pressure was unfavorable for the hydrogen release reaction, resulting in the increased decomposition temperature, as well as the decreased hydrogen purity. 3.2. Decomposition Process of the Ca(BH4)2·4NH3/ Mg(BH4)2 Composite. To gain insights into the dehydrogenation process of the Ca(BH4) 2·4NH3/Mg(BH4) 2 (1:1) composite, the XRD, 11B NMR, and FTIR measurements were conducted. The XRD patterns (Figure 3) reveal the

Figure 3. XRD results for Mg(BH4)2, Ca(BH4)2·4NH3, as-prepared Ca(BH4)2·4NH3/Mg(BH4)2 (with a mole ratio of 1:1), and the mixture after heating to 310 and 500 °C, respectively. Figure 2. TPD results for (I) Ca(BH4)2·4NH3, (II) Ca(BH4)2·4NH3/ Mg(BH4)2 (with a mole ratio of 1:1), (III) Mg(BH4)2·2NH3, and (IV) Mg(BH4)2 with a heating rate of 5 °C/min.

synthesized Mg(BH4)2 and Ca(BH4)2·4NH3 with good crystallinity, in accordance with the refs 34 and 36. Interestingly, the XRD patterns of the as-prepared Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) sample show some different features compared with that of the previous reported LiBH 4 ·NH 3 /Ca(BH 4 ) 2 and LiBH 4 ·NH 3 /Mg(BH 4 ) 2 systems.30,31 In the case of LiBH4·NH3/Ca(BH4)2 composite, the ammonia can be transferred from Li+ to Ca2+ during ball milling, leading to the same product of Ca(BH4)2·NH3/ LiBH4,31 while ball milling the LiBH4·NH3 and Mg(BH4)2 samples results in the formation of a dual-metal ammine borohydride, LiMg(BH4)3(NH3)2.30 However, as shown in Figure 3, the diffraction peaks of Ca(BH4)2·4NH3 and Mg(BH4)2 almost disappear after ball milling the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) mixture. With further annealing the as-prepared sample under 50 and 150 °C for 4 h, no apparent diffraction peaks corresponding to crystalline phases were observed from the XRD pattern (see Figure S4) as well, suggesting the formation of amorphous state derived from the interaction between Mg(BH4)2 and Ca(BH4)2·4NH3. A previous work on the ammine magnesium borohydride complex has suggested that the Mg cation is able to coordinate with ammonia to form Mg(BH4)2·2NH3, resulting in relatively low hydrogen desorption temperature and high H-purity (only a trace of ammonia released during the decomposition of Mg(BH4)2·2NH3).23 A further study about the electronic structure of M(BH4)2·2NH3 indicated that the interaction between the metal cations and NH3 groups in the Mg(BH4)2·2NH3 is relatively stronger than that in the Ca-

300 °C and Mg(BH4)2·2NH3 starts to release hydrogen at around 120 °C. However, in the case of Ca(BH4)2·4NH3, there are four steps for the decomposition in the temperature range of RT to 300 °C, which are mainly attributed to the emission of NH3 as confirmed by the MS results. As for the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) mixture, the decomposition occurs in two distinct stages. The first stage begins at around 70 °C and reaches a maximum rate at about 240 °C with ca. 0.048 mol g−1 gas released. The second one starts at around 280 °C, and an additional gas emission of ca. 0.0115 mol g−1 is achieved by 500 °C. By combination of the TG and TPD results, the capacities of H2 and NH3 in the evolved gas below 300 °C can be calculated to be 0.04906 and 0.00048 mol g−1, respectively, which correspond to a hydrogen purity of 99 mol % (92.3 wt %). It should be noted that the volumetric measurement result of 12.1 wt % is lower than the theoretical value of 14.6 wt % in the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite, suggesting that part of H still remains in the final products, which is confirmed by the NMR results (see the discussion below). Furthermore, the dehydrogenation of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) was investigated using isothermal volumetric hydrogen release measurements at 200 °C. As shown in Figure S2, approximately 8.5 wt % hydrogen can be released within 14 h. To further investigate the influence of hydrogen backpressure on the release of ammonia, the TPD measurement for 21164



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(BH4)2·2NH3.39 So ammonia may transfer from Ca2+ to Mg2+ during ball milling the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) mixture. However, no diffraction peaks of Mg(BH4)2·nNH3 (n = 2 or 6) or Ca(BH4)2 were observed for the ball milled Mg(BH4)2·4NH3/Ca(BH4)2 (1:1) sample (see Figure 3 and S4), indicating that the formation of amorphous state may be due to the combination of the NH3 groups with both Ca2+ and Mg2+, which results in the NH3 to be stabilized within the composite and then suppresses its emission. Besides, no diffraction peak can be observed after the mixture being heated to 310 and 500 °C, suggesting the formation of polymer or amorphous products during dehydrogenation. Figure 4 shows the 11B NMR results of the Ca(BH4)2, Mg(BH4)2, and Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite. Figure 5. FTIR spectra for the as-prepared Ca(BH4)2·4NH3/ Mg(BD4)2 with a mole ratio of 1:1 (a) and Ca(BH4)2·4NH3/ Mg(BH4)2 with a mole ratio of 1:1 acquired at room temperature (b), 310 °C (c), and 500 °C (d), respectively.

bending modes.37,38 However, no N-D absorption peak is observed in Figure 5a. These results indicate the occurrence of the isotopic exchange between (B)D and (B)H during the ball milling. Upon heating the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) sample to 310 °C, a dramatic decay of the B−H bend mode at 1118 cm−1 is observed in the dehydrogenation products, accompanied by the diminishing of the N−H vibrations at 3200−3400 and 1364 cm−1, suggesting that the dehydrogenation of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite is mainly based on the combination mechanism of Hδ+ and Hδ‑. Meanwhile, the appearance of a B−N bending vibration peak at 799 cm−1 indicates the generation of B−N−H compounds, in agreement with the NMR observation. Upon further heating of the sample to 500 °C, no vibrations related to the NH groups can be detected and the intensity of the BH groups is weakened distinctly, which indicates the consumption of the BH groups in the second decomposition step. In addition, the intensity of the B−N bending modes is increased, indicating the formation of B−N based polymer in the final products. The above results indicate that the dehydrogenation mechanism for Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite may be similar to that of other B−N−H systems,24−32,39 i.e., hydrogen evolution is mainly derived from the Hδ+···Hδ‑ combination, leading to the formation of BN-compounds. However, recent studies on LiNH2BH3 and NH3BH3 suggest that the H δ‑ ···H δ‑ interactions may also play a role complementary to that of their Hδ+···Hδ‑ counterparts in the dehydrogenation reaction.41,42 To further understand the dehydrogenation mechanism of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite, MS and DSC measurements for Ca(BH4)2·4NH3/Mg(BD4)2 (1:1) sample were conducted as shown in Figure 6. Although the MS results (Figure 6a) indicate that the majority of the hydrogen desorbed from the Ca(BH4)2·4NH3/Mg(BD4)2 mixture is produced through the combination of Hδ+···Hδ‑ interactions, the appreciable amount of D2 at 247 and 368 °C clearly indicate that Hδ‑···Hδ‑ interactions also contribute in a complementary way to the liberation of hydrogen gas. The DSC curve of Ca(BH4)2·4NH3/Mg(BD4)2 (1:1) in Figure 6b shows a major exothermic peak with its onset temperature at around 200 °C and its maximum at 231 °C, which would be linked with the combination of Hδ+...Hδ‑

Figure 4. Solid-state 11B NMR spectra for the Ca(BH4)2, Mg(BH4)2, Ca(BH4)2·4NH3/Mg(BH4)2 (with a mole ratio of 1:1), and the sample dehydrogenated to 310 and 500 °C, respectively.

It can be seen that the pristine Ca(BH4)2 has a resonance at −31.5 ppm and Mg(BH4)2 presents a dominant peak at −42.1 ppm. In the case of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite, a single boron species resonating at −35.3 ppm between that of Ca(BH4)2 and Mg(BH4)2 is observed, which may be assigned to the BH4− induced by Ca(BH4)2·4NH3 and Mg(BH4)2. Upon heating the sample to 310 °C, peaks belonging to B−N contacts can be seen at 21.2 and −0.6 ppm, which may be owing to the combination of its B−H and N−H groups, resulting in the formation of tetravalent borane nitrogen substances,40 similar to the chemical shift of B in the decomposed LiMg(BH4)3(NH3)2.30 At this stage, it is likely that an amorphous Mg−Ca−B−N−H compound is formed from the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite. Upon further heating to 500 °C, two kinds of tridentate B nucleus peaks at 13.3 and −0.6 ppm, corresponding to the formation of BN3 and/or HBN2,40 together with a signal at −34.6 ppm for the residue BH4−, are observed. Figure 5 shows the FTIR results acquired from the asprepared Ca(BH4)2·4NH3/Mg(BD4)2 (1:1) and the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) sample dehydrogenated to various temperatures. For the as-prepared Ca(BH4)2·4NH3/ Mg(BH4)2 (1:1) sample, typical features of the B−H stretching bands in the region between 2170 and 2450 cm−1, B−H bending mode at 1118 cm−1, and N−H vibrations at 3200− 3400 and 1364 cm−1 are observed in the spectrum. In the case of Ca(BH4)2·4NH3/Mg(BD4)2 sample, several additional absorption peaks around 1600−1768 and 880−1062 cm−1 are presented, which can be assigned to the B-D stretching and 21165



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Figure 6. MS and DSC patterns for the Ca(BH4)2·4NH3/Mg(BD4)2 (with a mole ratio of 1:1) sample with a heating rate of 5 °C/min.

interactions. The exothermic reaction character in this step is similar to previous reports of ammine metal borohydrides.24,25,27 After this exothermic event, the mixture presents two significantly endothermic signals at around 251 and 378 °C, corresponding to the release of H2, HD and D2 as detected in MS patterns, suggesting that these two endothermic events are related to the dehydrogenation reaction mediated by Hδ+···Hδ‑ and Hδ‑···Hδ‑ interactions, respectively. It should be noted that the DSC curve of Ca(BH4)2·4NH3/Mg(BD4)2 (1:1) mixture is different from that of Mg(BH4)2,16 Ca(BH4)2,18 and their amide compounds,23,34 which also indicates that the improved dehydrogenation in the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite is due to a combination reaction between Mg(BH4)2 and Ca(BH4)2·4NH3. According to the above analysis, although the intermediate phase is not very clear currently, it is concluded that the Mg(BH4)2 is able to introduce effective Hδ‑ sources through the coordination between Mg(BH4)2 and Ca(BH4)2·4NH3 to form new complexes, which adjusts the B−H and N−H groups for the combination. In addition, the adding of Mg(BH4)2 to Ca(BH4)2·4NH3 may also lead to form new metal (Mg)− NH3 ligand bonds, thus stabilizing the NH3 groups and reducing the ammonia emission. 3.3. Further Improvement on the Dehydrogenation of Ca(BH4)2·nNH3 (n = 1, 2, and 4). The above results clearly suggest that the dehydrogenation kinetic and the purity of gas release of Ca(BH4)2·4NH3 can be tuned by combining with Mg(BH4)2. However, the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite still released about 7.7 wt % ammonia below 300 °C. To further improve the dehydrogenation properties of Ca(BH4)2·nNH3 (n = 1, 2, and 4), the dehydrogenation of Ca(BH4)2·4NH3/Mg(BH4)2 (with a mole ratio of 1:2), Ca(BH4)2·2NH3/Mg(BH4)2 (with a mole ratio of 1:1), and Ca(BH4) 2·NH3 /Mg(BH 4) 2 (with a mole ratio of 1:1) composites were investigated, and the XRD and MS/TG results are shown in Figure S5 and Figure 7, respectively. The XRD patterns of the as-prepared Ca(BH4)2·4NH3/Mg(BH4)2 (1:2), Ca(BH4)2·2NH3/Mg(BH4)2 (1:1), and Ca(BH4)2·NH3/ Mg(BH4)2 (1:1) samples show very poor crystallinity, similar to that of Ca(BH4)2·4NH3/Mg(BH4)2 (1:1), suggesting the formation of the new phases derived from the chemical reaction between Mg(BH4)2 and Ca(BH4)2·nNH3. Moreover, the MS/TG results of the three composites show complete depression of NH3 production and tremendous improvement of H 2 liberation compared with the Ca(BH 4 ) 2 ·nNH 3 .

Figure 7. MS/TG results for (a) Ca(BH4)2·4NH3/Mg(BH4)2 (with a mole ratio of 1:2), (b) Ca(BH4)2·2NH3/Mg(BH4)2 (with a mole ratio of 1:1), and (c) Ca(BH4)2·NH3/Mg(BH4)2 (with a mole ratio of 1:1). The heating rate is 5 °C/min.

Interestingly, the maximum dehydrogenation rates of all of the Ca(BH4)2·nNH3/Mg(BH4)2 composites are located at around 220 °C, suggesting that the aided-metal cations may play an important role in determining the dehydrogenation properties of these composites. In order to compare the dehydrogenation performance of Ca(BH4)2·nNH3/Mg(BH4)2 (n = 1, 2 and 4) with that of ammonia complexes of Ca(BH4)2, the gravimetric and volumetric measurements (Figure S6 and S7) for all these samples were conducted until 300 °C. A summary of the calculated results based on eqs 1 and 2 is listed in Table S1 and illustrated in Figure 8. It is observed that Ca(BH4)2·nNH3 have

Figure 8. Histogram of capacity and purity of hydrogen evolved recorded from the volumetric and gravimetric measurements on the composites of ammonia complexes of Ca(BH4)2 and the various composites of Ca(BH4)2·nNH3/Mg(BH4)2 by 300 °C. The heating rate is 5 °C/min.

poor dehydrogenation properties in view of their low capacity and purity of hydrogen released, in agreement with the reported works.31,34 After Mg(BH4)2 was added to these ammonia complexes, all samples show much higher capacity and purity of hydrogen release than those of the initial ammine monometallic borohydrides. Particularly, for the Ca(BH4)2·4NH3/Mg(BH4)2 (1:2), Ca(BH4)2·2NH3/Mg(BH4)2 21166



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(1:1), and Ca(BH4) 2·NH3/Mg(BH4)2 (1:1) samples, a complete suppress of ammonia emission is achieved. Since the coordination bond of Mg−NH3 is stronger than that of Ca−NH3,39 the involvement of Mg2+ cation into Ca(BH4)2·nNH3 may play a crucial role in assisting Ca2+ to stabilize the NH3 within the composite and then prevents ammonia emission. Previous works about ammine metal borohydrides also suggest that the release of ammonia during decomposition can be prevented by changing the NH3/BH4 ratio to promote the consumption of the NH groups during the heating process.23,27 The introduction of Mg(BH4)2 to Ca(BH4)2·nNH3 clearly increases the BH-containing materials, resulting in a change of the NH3/BH4 ratio. For instances, the NH3/BH4 ratios of Ca(BH4)2·4NH3/Mg(BH4)2 (1:2), Ca(BH4)2·2NH3/Mg(BH4)2 (1:1), and Ca(BH4)2·NH3/Mg(BH4)2 (1:1) samples are 2:3, 1:2, and 1:4, respectively. The excessive BH4 groups would facilitate the consumption of NH3 groups, and then minimize the ammonia emission. So the suppression of ammonia evolution during the decomposition of Ca(BH4)2·nNH3/Mg(BH4)2 combination system can be attributed to the synergetic effects of the aided-cation and the introduction of excessive BH4 groups. We further compared the TPD results of Ca(BH4)2·2NH3/ Mg(BH4)2 (1:1) with that of Mg(BH4)2·2NH3. As shown in Figure S8, these two composites have the similar dehydrogenation peak temperature. However, the onset dehydrogenation temperature of Ca(BH4)2·2NH3/Mg(BH4)2 (1:1) is lower than that of Mg(BH4)2·2NH3. In addition, it was reported that there is a trace of ammonia released during the decomposition of Mg(BH4)2·2NH3, whereas the ammonia release was completely suppressed in the case of Ca(BH4)2·2NH3/Mg(BH4)2 (1:1) composite. The depressed ammonia release could be attributed to the increase of the BH4 groups and the combination of Mg2+ and Ca2+ cations in the composite, which promotes the NH/ HB recombination, and then prevents the formation of ammonia.

(BH4)2·4NH3/Mg(BH4)2 (1:1 and 1:2), Ca(BH4)2·2NH3/ Mg(BH4)2 (1:1), Ca(BH4)2·NH3/Mg(BH4)2 (1:1), and Mg(BH4)2·2NH3 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*Phone and Fax: +86-21-5566 4581. E-mail: yuxuebin@fudan. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology of China (2010CB631302), the National Natural Science Foundation of China (Grant No. 51071047), the PhD Programs Foundation of Ministry of Education of China (20110071110009) ,and Science and Technology Commission of Shanghai Municipality (11JC1400700 and 11520701100).

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REFERENCES

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4. CONCLUSIONS In summary, we have demonstrated a combined hydrogen storage system, Ca(BH4)2·nNH3/Mg(BH4)2 (n = 1, 2, and 4), which shows significant suppression of ammonia formation and tremendous improvement of dehydrogenation kinetic compared to the pristine Ca(BH4)2·nNH3 and Mg(BH4)2. It was observed that the Ca(BH4)2·4NH3/Mg(BH4)2 (1:1) composite exhibits an onset dehydrogenation temperature of ∼62 °C and presents a hydrogen desorption capacity of >9 wt % below 300 °C with a H-purity of 92.3 wt %. With further increase of the Mg(BH4)2 or decrease of the coordinated NH3 molecules, pure hydrogen release can be achieved in this combined system upon heating from RT to 500 °C. It was confirmed that the synergetic effects of the aided-cation and the introduction of excessive BH groups guarantees the depression of ammonia evolution and the promotion of H2 generation. Further investigation via introduction of isotope deuterium in the combined system indicates that the dehydrogenation reactions of these composites are mainly attributed the combination of Hδ+···Hδ‑ interaction, whereas Hδ‑···Hδ‑ interactions also contribute in a complementary way.

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S Supporting Information *

Figures showing the XRD patterns and TPD results for the synthesized Ca(BH 4 ) 2 ·nNH 3 (n = 1, 2, and 4), Ca21167



Article

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Improved Dehydrogenation Properties of Ca(BH4)

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