Improved hydrogen desorption properties of Co-doped Li2BNH6

The hydrogen desorption properties of Li2BNH6 were improved by doping with cobalt. With the addition of CoCl2 (7 wt%), more than 8 wt% of hydrogen was released from Li2BNH6 at temperatures below 210°C, which is approximately 90°C lower than that of pristine Li2BNH6. X-ray diffraction, Fourier transform-infrared and Raman characterizations revealed that the dehydrogenation was a stepwise process with the formation of intermediates Li4BN3H10 and LiBH4 and final products of Li3BN2 and LiH. The introduction of Co greatly accelerated the dehydrogenation of Li4BN3H10. X-ray absorption near-edge structure measurements revealed that Co and CoB species formed during ball milling of CoCl2 with LiBH4 and LiNH2, which may function as catalyst in the subsequent dehydrogenation.

To meet the requirements for hydrogen fuel cell technology, tremendous efforts have been devoted to the development of hydrogen storage materials with high hydrogen contents [1]. Alanates [2], borohydrides [3], amide-hydride systems [4][5][6][7], ammonia borane [8,9], and metal amidoboranes [10,11] have been explored as potential hydrogen storage materials. LiBH 4 with a hydrogen capacity of 18.4 wt% [12], has attracted considerable research attention [3]. However, its thermal decomposition to LiH + B + 3/2H 2 is a highly endothermic process (approximately 67 kJ mol -1 H 2 ) [13], and has a relatively high kinetic barrier [3], and thus requires high temperatures (320-600°C) [3]. Several attempts to improve the thermodynamic properties of hydrogen release from LiBH 4 have been made by reacting LiBH 4 with chemicals such as SiO 2 [3], MgH 2 [13] and CaH 2 [14]. Recent investigations revealed that complex hydrides (LiAlH 4 , LiBH 4 and NaAlH 4 ) could be destabilized by reaction with alkali and alkali earth amides. It was reported that approximately 8.1 wt% of H 2 can be released from 2LiAlH 4 -LiNH 2 *Corresponding author (email: wgt@dicp.ac.cn) at 85-320°C [15]. Four equivalent of H 2 were also found to evolve from 1LiAlH 4 -2LiNH 2 at 50-500°C [16]. Similar improvements were achieved by mixing LiBH 4 with amides. Ball milling LiBH 4 with 2 equivalent of LiNH 2 produces Li 3 BN 2 H 8 , which can release >10 wt% of H 2 at 250-350°C [6,7]. In these hydride-amide systems where protic H and hydridic H coexist, the large potential for the combination of oppositely charged H atoms to form H 2 and the establishment of a strong B-N (or Al-N) bond should be the driving force for the favorable dehydrogenation. A series of quaternary complex hydrides, such as Li 2 BNH 6 , Li 3 BN 2 H 8 and Li 4 BN 3 H 10 , can be prepared by mechanically milling LiBH 4 and LiNH 2 with various molar ratios [17]. However, those complex hydrides encounter very large kinetic barriers in dehydrogenation [6,18]. The LiBH 4 -LiNH 2 system can be catalytically modified with transition metal additives, such as PdCl 2 , Pt/Vulcan carbon, NiCl 2 , CoCl 2 , can greatly reduce the dehydrogenation temperature [19][20][21]. In particular, >9 wt% of H 2 can be released from Li 3 BN 2 H 8 at ca. 182°C in the presence of 5 wt% of Co-based catalyst [20]. Approximately 7.5 wt% of H 2 was released from Li 4 BN 3 H 10 at 160-260°C with the addition of 11 wt% of NiCl 2 [21]. Co-based additives were also found to have a catalytic effect on the hydrogen release from boron-containing hydrogen storage materials [22,23]. The hydrogen capacity of Li 2 BNH 6 (1LiBH 4 -1LiNH 2 ) is 13.3 wt%, which is higher than that in Li 3 BN 2 H 8 (11.8 wt%) or Li 4 BN 3 H 10 (11.0 wt%). However, there are few reports on the catalytic modification of this complex hydride. Herein, the dehydrogenations of pristine and the Co-doped Li 2 BNH 6 are systematically investigated. It was found that the Co-additive promoted the dehydrogenation kinetics of Li 2 BNH 6 . The pathway of dehydrogenation was identified by X-ray diffraction (XRD), and Fourier transform-infrared (FT-IR) and Raman spectroscopy characterizations.

Materials and methods
LiBH 4 (95%), LiNH 2 (95%) and anhydrous CoCl 2 (97%) were purchased from Acros, Fluka and Sigma-Aldrich, respectively. To prevent oxygen and moisture contamination, all the sample loading and handling operations were conducted in an MBRAUN glovebox filled with purified argon. A mixture of LiBH 4 -LiNH 2 (molar ratio 1 : 1) was mechanically milled with 7 wt% of CoCl 2 using a planetary Retch PM 400 mill. The ball-to-sample mass ratio was about 30 : 1. The rotation speed was regulated at 200 r/min and the milling time was 8 h. At the end of ball milling the vessels were connected to a pressure gauge to measure the pressure inside. Then, the gaseous products were passed through a mass spectrometer (MS) for analysis. In contrast to the undoped LiBH 4 -LiNH 2 sample, a small amount of H 2 (0.07 equivalent) was generated during ball milling.
A temperature-programmed desorption (TPD) system connected with a MS was constructed in our laboratory and used to investigate gas release. Approximately 50 mg of sample was loaded and tested each time. The temperature was increased to 500°C at 2°C/min. The carrier gas was purified argon and the flow rate was set at 40 mL/min. Quantitative measurements on hydrogen release were performed on a commercial automatic Sieverts-type apparatus (Advanced Materials Co.). A sample of approximately 150 mg was tested with a heating rate of 2°C/min. The ammonia concentration in the gaseous product was measured by a Thermo conductivity meter with an accuracy of 0.1 μs/cm. The NH 3 concentration was proportional to the decrease in the ion conductivity measured after the outlet gas was passed through a dilute H 2 SO 4 solution. A PANalytical X'pert diffractometer equipped with Cu Kα radiation (40 kV, 40 mA) and an in situ cell was employed for structural identification. FT-IR measurements were conducted on a Varian 3100 FT-IR spectrometer. Raman spectra were recorded on a commercial micro-Raman spectrometer (Renishaw, UK) using a He/Ne laser with a wavelength of λ = 514 nm. X-ray absorption near-edge structure (XANES) spectra were collected in fluorescence mode at the X-ray absorption fine structure (XAFS) station of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai). Samples for XAFS measurements were pressed into pellets in glovebox to avoid air contamination. Metallic cobalt foil and cobalt chloride were also measured as references and for energy calibration.

Hydrogen desorption from Li 2 BNH 6
Li 2 BNH 6 can be prepared by mechanically milling LiBH 4 and LiNH 2 in a molar ratio of 1 : 1 [17]. In this study, formation of Li 2 BNH 6 was observed by XRD in the pristine and Co-doped samples after 8 h of ball milling. The results of TPD measurements on the pristine and Co-doped Li 2 BNH 6 samples are shown in Figure 1. For the pristine sample, hydrogen desorption began at approximately 170°C and peaked at 295°C. The second step of desorption started at 370°C and reached the maximum desorption rate at around 410°C. This dehydrogenation behavior is different from that of Li 3 BN 2 H 8 and Li 4 BN 3 H 10 [6,7,21]. By contrast, the Co-doped sample began to release hydrogen at approximately 110°C, and reached the maximum desorption rate at approximately 230°C, which is about 65°C lower than that of the pristine Li 2 BNH 6 . However, little improvement was observed in the second step of dehydrogenation. Quantitative measurements of hydrogen desorption from the pristine and Co-doped samples were conducted on a Sieverts-type apparatus. As shown in Figure 2, approximately 3.9 equivalent of H (8.6 wt%) was evolved from the pristine Li 2 BNH 6 sample at 170-300°C. In total, 5.3 equivalent of H (11.7 wt%) was desorbed when the temperature approached 450°C. An acceleration of hydrogen release at about 290°C was observed, which was consistent with the TPD result. Nevertheless, it is clear that a large kinetic  barrier resulted in hydrogen evolution occurring at higher temperatures. Introduction of CoCl 2 into Li 2 BNH 6 resulted in hydrogen evolution at approximately 110°C and acceleration of dehydrogenation was observed at around 200°C. This is a major improvement compared with the pristine Li 2 BNH 6 . Approximately 3.9 equivalent of H (8.1 wt%) was desorbed in the first step at <210°C. The concentration of NH 3 in the gaseous phase was below 890 ppm.

Dehydrogenation mechanism
As shown in Figure 1, dehydrogenation of Li 2 BNH 6 is a two-step process. In order to gain insight into the reaction path, the Co-doped samples collected after ball milling and dehydrogenation at different stages were analyzed by XRD, FTIR, and Raman analyses. After 8 h of ball milling, the LiBH 4 , LiNH 2 and CoCl 2 reagents had disappeared and a new set of diffraction peaks attributed to Li 2 BNH 6 was observed ( Figure 3(b)) [17]. In the FTIR spectrum ( Figure  4(b)) obtained after ball milling, peaks at 3293 and 3246 cm -1 were assigned to the symmetric and asymmetric N-H stretching vibrations of Li 2 BNH 6 [24]. The corresponding  Raman spectrum of Li 2 BNH 6 ( Figure 5(b)) also displayed peaks for the N-H vibrations at 3296 cm -1 and 3247 cm -1 , and the B-H vibrations at 2159 cm -1 (w), 2236 cm -1 (sh), 2296 cm -1 (s), and 2390 cm -1 (w). Upon heating the Co-doped Li 2 BNH 6 sample at 90°C, Li 4 BN 3 H 10 and LiBH 4 rather than Li 2 BNH 6 were observed as the dominant phases by XRD (Figure 3(c)). FTIR spectrometry revealed that the peaks for the N-H vibrations had shifted to 3301 cm -1 and 3243 cm -1 (Figure 4(c)). The intensities of the N-H stretches decreased after approximately 2.2 equivalent of H was desorbed and a broad band centered at about 1680 cm -1 developed, which can be assigned to the vibration of N=B=N in Li 3 BN 2 [25]. After releasing about 3.9 equivalent of H atoms at <210°C, the N-H stretches disappeared and the broad band at 1680 cm -1 intensified.
After release of about 5.3 equiv. of H the B-H stretches disappeared (Figure 4(f)). XRD characterization (Figure 3) indicated that the Li 4 BN 3 H 10 phase was gradually weakened during dehydrogenation, and this was accompanied by a gradual increase of the monoclinic phase of Li 3 BN 2 [26]. Finally, after the evolution of 5.3 equivalent of H at <450°C, LiBH 4 disappeared and LiH and tetragonal Li 3 BN 2 appeared. In accordance with the XRD and FTIR characterizations, Raman spectra show the gradual decrease of the N-H and B-H vibrations, and the development of the N=B=N vibration in Li 3 BN 2 (1060 cm -1 ) ( Figure 5) [27].
The pristine Li 2 BNH 6 samples at different stages of dehydrogenation were also collected for structural characterization. The results showed an almost identical process with that of the Co-doped sample although the dehydrogenation occurred at much higher temperatures. This indicates that the main reaction pathway is unchanged by the introduction of CoCl 2 . Upon heating to approximately 90°C, the conversion of Li 2 BNH 6 to Li 4 BN 3 H 10 and LiBH 4 takes place (reaction (1) [28]. Hence, the decomposition of Li 2 BNH 6 is likely to occur through dehydrogenation of Li 4 BN 3 H 10 + 1/2LiBH 4 (reaction (2)) and then that of LiBH 4 (reaction (3)). Therefore, the hydrogen release from the Co-doped and pristine Li 2 BNH 6 can be expressed as follows: Li 4 BN 3 H 10 releases hydrogen exothermically and its rehydrogenation is thermodynamically unfavorable [29]. Storage and release of hydrogen by LiBH 4 is partially reversible [30]; therefore, the Li 2 BNH 6 storage system may be partially reversible.

Chemical state of Co catalyst
After ball milling LiBH 4 and LiNH 2 with 7 wt% of CoCl 2 for 8 h, 0.07 equiv alent of H 2 was generated and the sample changed color from white to black. This indicates that the reduction of CoCl 2 occurred during the milling process. However, no Co-related phases were observed in the XRD pattern, which is probably due to the low loading and/or poor crystallinity. More information regarding the chemical state and local environment of Co were obtained from the XAFS analyses. The XANES of the freshly ball-milled sample resembled that of metallic Co (Figure 6), which indicates that reduction of CoCl 2 by LiBH 4 occurred during ball-milling. Fourier transformation of the XAFS spectra showed that the ball milled sample had a dominant peak at around 2.53 Å with a shoulder at 1.94 Å (Figure 7), which are close to the second shell Co-Co distance (2.67 Å) and the first shell Co-B distance (2.08 Å) of CoB, respectively [31]. As Tang et al. [20] revealed, both Co and CoB species have catalytic effects on the dehydrogenation of Li 3 BN 2 H 8 . Similarly, in the present case, Co and CoB species might also co-function as the catalyst. Co particles might have poor crystallinity and are finely dispersed in the sample, which leads to a high catalytic efficiency. It is likely that the Co particles could facilitate the formation of Li 3 BN 2 , and consequently decrease the dehydrogenation temperature and  improve the dehydrogenation kinetics, as well as in the Co-doped dehydrogenation of NH 3 BH 3 and Li 3 BN 2 H 8 [32,33].

Conclusion
Li 2 BNH 6 undergoes a stepwise dehydrogenation with initial transformation into Li 4 BN 3 H 10 and LiBH 4 at about 90°C. Subsequent decomposition to Li 3 BN 2 and H 2 occurs at around 300°C, and final full dehydrogenation to LiH, B and Li 3 BN 2 occurs at 450°C. The addition of CoCl 2 (7 wt%) to Li 2 BNH 6 greatly reduced the hydrogen desorption temperature. More than 8 wt% of hydrogen can be released at <210°C. XANES measurements revealed that Co and CoB were formed after ball milling of CoCl 2 with LiNH 2 and LiBH 4 .