Low-Temperature Hydrogen Generation and Ammonia Suppression

Feb 3, 2011 - decomposition behavior of the sample on a semiautomatic. Sievert's apparatus ... High-resolution X-ray powder diffraction with transmiss...
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Low-Temperature Hydrogen Generation and Ammonia Suppression from Calcium Borohydride Combined with Guanidinium Borohydride Ziwei Tang, Yanhui Guo, Shaofeng Li, and Xuebin Yu* Department of Materials Science, Fudan University, Shanghai 200433, China

bS Supporting Information ABSTRACT: A combined hydrogen storage system of Ca (BH4)2/[C(NH2)3]þ[BH4]- (GBH) complexes is reported. By a short time ball milling of Ca(BH4)2 and GBH, a new composite of Ca(BH4)2/GBH complex was produced. It was found that the newly formed composite exhibits a significant improvement of dehydrogenation, which not only decreases the onset dehydrogenation to around 60 °C, but also depresses the emission of ammonia from GBH thoroughly, leading to more than 10 wt % fairly pure H2 released below 300 °C. Further investigations revealed that the complexation between Ca(BH4)2 and GBH as well as balancing the protic and hydridic hydrogen are two crucial factors in improvement of the dehydrogenation of this system, which may serve as a new strategy for developing the new boronnitrogen-hydrogen system as hydrogen storage materials.

’ INTRODUCTION Storage of hydrogen at high volumetric and gravimetric density and release it on demand provides enabling technology vital to the future energy applications.1-5 The synthesis of functional materials with high hydrogen uptake and delivery under safe and ambient conditions remains a key challenge for the widespread implementation of fuel cells as high power density portable systems.6-10 As potential solid-state hydrogen storage materials, alkali and alkali-earth metal borohydrides11-16 have attracted wide attention due to their combination of low molecular weight and high H2 storage capacity. Among them, Ca(BH4)2, with an attractive hydrogen capacity of 11.6 wt %, has been acknowledged as one of the potential candidates for hydrogen storage materials recently.14,17 Nevertheless, the poor thermodynamic and kinetic properties of this borohydride are severely limiting its practical applications.18,19 Recently, B-N-H compounds have also been of general interest for chemical hydrogen storage due to their high theoretical hydrogen capacity.20-22 In the B-N-H system, the corresponding protic and hydridic character of the hydrogen on the nitrogen and boron, respectively, allows a facile H2 elimination pathway, resulting in hydrogen release at low temperature.23,24 To date, a number of B-N-H composites, such as metal amidoboranes (LiAB, NaAB),25,26 hydrazine borane and hydrazine bisborane (HB, HBB),10 guanidinium borohydride (GBH, [C(NH2)3]þ[BH4]-),27 ammine magnesium borohydride complex,28 and so forth.29-32 have been developed. Among them, GBH with a theoretical H2 capacity of 13.5 wt % was found to be a promising, low cost, reliable, and safe high-density chemical hydrogen storage source.27 However, along with the hydrogen evolution, a large amount of ammonia release also occurred during heating, which is fatal for fuel cell operation. Herein, we report a new complex system of Ca(BH4)2/GBH, in which GBH can coordinate with Ca(BH4)2 to form some new r 2011 American Chemical Society

hydrogen storage sources that make a significant improvement in dehydrogenation over the two substances alone.

’ EXPERIMENTAL SECTION The source materials were obtained commercially, namely, LiBH4 95% Ca(BH4)2 95% NaBH4 95% (Sigma-Aldrich, USA), guanidinium chloride 98% (CH5N3 3 HCl, Sigma-Aldrich, USA), and CaCl2 99.9% NiCl2 99.9% MgCl2 99.999% ZnCl2 99.99% (ultra dry, Alfa Aesar, China), were used without further purification with all handling procedures conducted under argon atmosphere. Approximately 0.5 g of a mixture of LiBH4CH5N3 3 HCl with a mole ratio of 1:1 was mechanically milled for 60 min (planetary QM-1SP2) under argon using stainless steel spheres with a ball-to-powder weight ratio (BPR) of 30:1 to produce GBH, and then ∼0.5 g mixtures of GBH-Ca(BH4)2 with mole ratios of 1:0.5 and 1:1 were prepared in the same manner. In addition, the mixtures of GBH-metal chlorides with a mole ratio of 1:0.5 and two mixtures of NaBH4-CH5N3 3 HCl with mole ratios of 1:1 and 2:1 were also prepared by the same process. The milling process was carried out alternating 6 min of milling and 6 min of stop in order to avoid an increasing temperature of the powders in the vial. All the procedures of the powder handling before and after milling were carried out in a glovebox with argon atmosphere. Hydrogen release property measurements were performed by thermogravimetry/differential thermal analysis (TG, STA 449 C) connected to a mass spectrometer (MS, QMS 403) using a heating rate of 10 °C min-1 under 1 atm argon atmosphere. Typical sample quantities were 5-10 mg, which is sufficient for getting accurate results due to the high sensitivity Received: November 24, 2010 Revised: January 6, 2011 Published: February 3, 2011 3188

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Figure 1. MS of m/e = 2 (H2) and m/e = 16 (NH3), and TG profiles for GBH and GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5. The heating rate is 10 °C/min.

of the employed equipment. Temperature-programmed desorption (TPD) and eliminating-ammonia temperature-programmed desorption (EATPD) were 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 argon atmosphere (1 bar) at a heating rate of 5 °C/min. An EATPD apparatus with molecular sieves was employed to absorb the ammonia during the thermal decomposition, whose performance of ammonia stabilization is shown in Figure S1, Supporting Information. For all calculations of hydrogen capacity, the contents of metal chlorides were not considered. The contents of H2 and NH3 in the emission gas were determined using gravimetric and volumetric results. First, the mass percent (Wp) and mole per gram (Mp) of gas released from the sample were calculated from the weights of the samples and volumetric results, then the mole proportion of H2 (CH2) and NH3 (CNH3) can be calculated from the following two equations: CH2 þCNH3 ¼ 1

ð1Þ

ðCH2  2:02þCNH3  17:03Þ  M p ¼ W p

ð2Þ

High-resolution X-ray powder diffraction with transmission optics (X’Pert PRO MPD, PANalytical, Netherlands) and powder X-ray diffraction (XRD, Rigaku D/max 2400) measurements were conducted to confirm the phase structure. During the highresolution X-ray powder diffraction measurement, samples were sealed by Mylar foil (6 μm) then loaded on transmission sample holder in argon glovebox to avoid oxidation and hydrolysis during the measurement. Decomposition behavior of GBH/ Ca(BH4)2 composite was also studied by in situ XRD by heating the sample with a Cybostar hot gas blower. For in situ measurements, the sample was loaded into a 0.7 mm quartz capillary and kept in argon atmosphere. Then, the sample was heated from 40 to 90 °C at temperature intervals of 10 °C. The heating rate was 6 °C/min, and data were collected for 8 min at each point. The wavelength for all these measurements was 0.688702 Å. Additionally, during the XRD measurement, powders were spread and measured on a Si single crystal and amorphous polymer tape was used to cover the surface of the powder to avoid oxidation. The Raman spectra were collected on a LABRAM-1B spectrograph (Dilor, Franch). The wavelength of the incident radiation

Figure 2. TPD results for GBH and GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5. The heating rate is 5 °C/min.

Table 1. Summary of H2 Evolution from the GBH/Ca(BH4)2 Mixtures wt % a

samples

Ca(BH4)2

GBH, in this work

Mole H2

wt % H2 capacity

mol %

H2b

Mole GBH

0

7.4

88.0

2.8

GBHþ0.5Ca(BH4)2

22.9

11.3

99.4

6.2

GBHþ1Ca(BH4)2 GBH, in ref 26

37.3 0

10.1 10.6

99.8 95.9

7.1 3.9

a

The GBH/Ca(BH4)2 mixtures were heated in 1 bar argon from 30 to 300 °C with a heating rate of 5 °C/min for calculations of H2 evolution. b H2 content in the released gas.

Figure 3. Isothermal EATPD results for GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5.

was 632.8 nm. Data acquisitions were made in the frequency range 3600-180 cm-1 by accumulating 10 spectra of 50 s each. Fourier transform infrared (FTIR, 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.

’ RESULTS AND DISCUSSION Figure 1 shows the MS and TG analysis results for the GBH and GBH/Ca(BH4)2 composite. For the GBH sample, a twostep decomposition, with a total weight loss of 26.7 wt % by 400 °C, was observed, in which the first step is a mixed evolution 3189

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Figure 4. High-resolution XRD pattern for the GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5. (patterns of GBH and Ca(BH4)2 simulated from database).

Figure 5. High-resolution in situ XRD pattern for the GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5.

of H2 and NH3, proceeding from about 90 to 150 °C, while the second step (150-400 °C) is dominated by NH3 release. As no borane evolution was observed during the whole decomposition (Figure S2), the weight loss can be ascribed to the release of H2 and NH3. This decomposition performance is similar to that in a previous report.27 Apparent ammonia evolution without hydrogen above 200 °C indicates only partial consumption of NH in the GBH, which may result from its insufficient BH sources. The

MS result for the GBH/Ca(BH4)2 with a mole ratio of 1:0.5 (named G1) shows that the evolution of NH3 was depressed and the H2 generation was improved significantly (Figure 1). Following the first strong peak centered at around 90 °C, another main hydrogen release peak centered at around 220 °C appeared without ammonia evolution. Moreover, the onset temperature of the first dehydrogenation step was also reduced from 90 °C for GBH to 62 °C for samples G1. Therefore, it is supposed that supplement of more BH sources may be an effective route to consume the excess NH groups in GBH, resulting in promoted H2 generation and depressed ammonia evolution. The TPD results of GBH and G1 below 300 °C are shown in Figure 2. Clearly, introduction of Ca(BH4)2 can lower the onset temperature of H2 release in GBH and make a significant improvement in its dehydrogenation performance, which is agreeable with Figure 1. By combination of the gravimetric and volumetric results, the portion of hydrogen and ammonia release can be calculated, which is listed in Table 1. It can be seen that the ammonia concentration was reduced from 12.0% for GBH to 0.6% for G1 and further decreased to 0.2% for the GBH/ Ca(BH4)2 with a mole ratio of 1:1 (denoted as G2), suggesting that the increased Ca(BH4)2 content may further contribute to the ammonia suppression. In addition, an extra hydrogen desorption peak centered at 316 °C appeared for the G2 sample (Figure S3), which may be due to the decomposition of the excess BH groups. However, this temperature is still 60 °C lower than that of the pure Ca(BH4)2. The results above clearly indicate that a mutual dehydrogenation improvement between GBH and Ca(BH4)2 was achieved. The isothermal dehydrogenation results of sample G1 (Figure 3) showed that favorable dehydrogenation kinetics can be obtained at temperatures above 80 °C, from which hydrogen capacities of 4.4, 5.0, and 5.7 wt% 3190

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Figure 6. Raman spectra of GBH (a) and GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5 (b) before and after heating to 150 and 300 °C.

were released within 60 min at 80, 100, and 120 °C, respectively. The fact that the hydrogen capacity is temperature dependent could be ascribed to its stepwise decomposition, as shown in Figure S4, in which 2.2, 1.5, and 2.5 equiv of H2 were released from the three stages. The DSC results showed a exothermal reaction in dehydrogenation of sample G1 (Figure S5), indicating that indirect chemical routes will have to be adapted to regenerate the composite from its decomposed products, as in the cases of other boron-nitrogen systems.33,34 In order to understand the decomposition process of the GBH/Ca(BH4)2 mixtures, XRD, Raman, and FTIR techniques were employed. The high-resolution XRD results revealed that no phase related to Ca(BH4)2 was present in sample G1, and significant variations in the GBH peaks were observed (Figure 4), suggesting that an interaction of GBH with Ca(BH4)2 has occurred. The high-resolution in situ XRD result (Figure 5) provides further information to understand this procedure. It was found that no obvious variations in the peaks of sample G1 appeared when the scale of treatment temperature was from 40 to 60 °C. After heating to 70 °C, the peaks related to GBH were altered significantly, and some of them even disappeared, revealing that the initiation of dehydrogenation in G1 occurred at 60-70 °C, which was consistent with the MS and TPD results. It is supposed that this interaction was a complexation between GBH and Ca(BH4)2, based on the N:fCa2þ coordination bond, similar to the complexation between alkali and alkali-earth metal borohydrides and ammonia.10,29-32,35,36 After heating GBH and G1 to 150 and 300 °C, only LiCl remained (Figure S6), and peaks assigned to GBH and the supposed GBH/ Ca(BH4)2 complex disappeared, indicating the amorphous structure of the decomposition products and the involvement of Ca(BH4)2 in the dehydrogenation reaction.

Figure 7. FTIR spectra of GBH (a) and GBH/Ca(BH4)2 composite with a mole ratio of 1:0.5 (b) before and after heating to 150 and 300 °C.

Raman results in Figure 6 revealed that frequencies similar to those assigned to B-H and [C(NH2)3]þ were present in GBH and G1, i.e., the B-H stretching band in the region between 2540-2050 cm-1 and at 1340 cm-1, the NH2 stretching vibration at around 3380 cm-1, the NH2 scissors vibrations at around 1655 and 1565 cm-1, and the CN3 symmetric stretching vibration and angle deformation vibration at around 1010 cm-1 and 539 cm-1.37,38 Upon heating, vibrations assigned to NH groups still remained in GBH, even after heating to 300 °C, but the intensity of the BH groups reduced gradually and almost disappeared after heat treatment to 300 °C, implying insufficient BH supplement to consume the NH in GBH. However, in the case of the G1, vibrations assigned to BH groups remained during heat treatment to 300 °C, indicating an excessive supplement of BH sources, but vibrations assigned to NH groups reduced gradually and finally disappeared at 300 °C, consistent with the stepwise dehydrogenation shown in Figure S4, suggesting different or varied reactivity of the protonic H of NH2 in combination with BH groups during the decomposition. Moreover, noticeable intensity reduction for the CN3 vibrations was observed for both GBH and G1 upon heating, indicating variation of the C-N skeletal structure during the dehydrogenation. The same trend for the variations of BH, NH groups, and the C-N skeletal structure indicated above were also observed by FTIR characterization (Figure 7). Characteristic frequencies assigned to [BH4]- and [C(NH2)3]þ can be observed in the two samples of GBH and G1, i.e., the B-H stretching band in the region between 2400 cm-1and 2200 cm-1, the BH2 deformation at 1123 cm-1, and the “scissors” motions of the C, N, and H atoms at around 1600 cm-1. It is noteworthy that a CdN 3191

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Figure 10. A potential dehydrogenation pathway for the GBH/Ca(BH4)2 system.

Figure 8. XRD patterns for GBH, CaCl2, and GBH/CaCl2 composite with a mole ratio of 1:0.5 (a), and GBH/CaCl2 composite with a mole ratio of 1:0.5 (b) heated to 150 and 300 °C in argon.

Figure 9. EATPD results for GBH/metal chloride composites with a mole ratio of 1:0.5. The heating rate is 5 °C/min.

vibration appears in the region of 2200-2000 cm-1 in G1 after heating to 300 °C, implying a C-N bonding change in this sample. Furthermore, B-N vibration emerged at 150 °C in G1 as that at 300 °C in GBH, indicating that the interaction of BH groups with the NH groups in G1 proceeds at lower temperature, which may result from the sufficient BH supplement and the complexation between the two compounds in the G1 sample. On the basis of the analysis above, it is revealed that the dehydrogenation of GBH/Ca(BH4)2 is due to the combination of BH and NH, resulting in the formation of a B-N-containing compound, and that balancing the hydrogen sources represented

by the NH and BH groups will facilitate the dehydrogenation, which suggests that a similar mutual dehydrogenation improvement seems likely to be achieved for GBH combined with other borohydrides. However, further investigation for using excessive NaBH4 did not exhibit expected results. The TG/MS results for the NaBH4/GBH mixture (mole ratio of 1:1) showed distinct ammonia evolution (Figure S7). Additionally, the NaBH4 phase was still present in the as-prepared GBH/NaBH4 (mole ratio of 1:1), indicating that the NaBH4 did not combine with the GBH as Ca(BH4)2 did (Figure S8). Hence, it is proposed that the complexation between borohydride and GBH was a crucial factor in promoting the dehydrogenation of the borohydride/GBH system, igniting the interaction between the positively charged hydrogen of NH and the electronegative hydrogen of BH to forming H 3 3 3 H bonds, which have been demonstrated to have a role in the heterolytic splitting of H2.2,39,40 Therefore, in the GBH/Ca(BH4)2 system, Ca2þ may dominate the key role in the formation of complexation between the two compounds. To confirm the interaction between GBH and Ca2þ, the XRD measurements of GBH/CaCl2 composite with a mole ratio of 1:0.5 were conducted. The results showed that few peaks related to CaCl2 were observed and GBH peaks were altered in the GBH/CaCl2 composite (peaks of notable variation were marked by arrows) (Figure 8a), implying that GBH can interact with Ca2þ. After heating the GBH/CaCl2 composite to 150 °C, LiCl remained and some peaks of CaCl2 appeared, and more CaCl2 peaks emerged when the composite was heated to 300 °C (Figure 8b). This indicates that, with the decomposition of GBH, Ca2þ moves off the GBH gradually. To further confirm the complexation effect of metal ions on the dehydrogenation of GBH, various metal chlorides were mixed with GBH, and the EATPD results are shown in Figure 9. It was observed that the introduction of these metal chlorides enabled lowering the onset dehydrogenation temperature of GBH to around 60 °C and significant improvement in H2 release and suppression of ammonia. Among them, NiCl2 combined with GBH exhibited the best dehydrogenation performance, resulting from the strong complexation effect of Ni2þ, revealing that the complexation effect of metal ions plays a considerable role in facilitating H2 generation and depressing NH3 evolution in GBH. According to the results above, a reaction pathway for GBH/ Ca(BH4)2 is proposed as Figure 10. When mixing GBH with Ca(BH4)2, a complexation reaction between them occurs, resulting in B-H and N-H bonds that are almost in contact on a molecular scale, probably allowing close H 3 3 3 H contact at a distance shorter than 2.4 Å, which tends to promote hydrogen 3192

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The Journal of Physical Chemistry C evolution in this system. During the decomposition of the complex, the combination between B-H and N-H proceeds more easily, and the bonds between Ca(BH4)2 and GBH may enhance the stability of the NH2 groups, thus reducing the ammonolysis of GBH to release ammonia. However, regarding NaBH4, no complexation occurred between NaBH4 and GBH, resulting in the independent decomposition of GBH and excessive NaBH4.

’ CONCLUSION In this paper, we have revealed that GBH can combine with Ca(BH4)2 to produce Ca(BH4)2/GBH complexes. The novel complexes displayed improved dehydrogenation, which not only depressed the emission of ammonia from GBH significantly, but also led to improved dehydrogenation from the Ca(BH4)2 below 300 °C, thus enabling the release of >10 wt % pure hydrogen during the thermal decomposition reaction. Further investigation demonstrated that balancing the hydrogen sources represented by the N-H bonds in GBH by more B-H supplement from Ca(BH4)2 and the complexation between the two hydrides are two crucial factors for improving the dehydrogenation in this system, which may open up a new strategy for designing various boronnitrogen-hydrogen systems with favorable dehydrogenation. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 to S8 as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: Department of Materials Science, Fudan University, Shanghai, China. Fax: þ86-21-65643685. Tel: þ86-21-55664581. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by the National Natural Science Foundation of China (Grant No. 51071047) and the Ph. D. Programs Foundation of Ministry of Education of China (20090071110053). We also wish to thank Qinfen Gu for assistance in using the Australian high-resolution X-ray powder diffraction. ’ REFERENCES

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