J. Phys. Chem. C 2008, 112, 4005-4010
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Enhanced Hydrogen Storage Performance of LiBH4-SiO2-TiF3 Composite Yao Zhang,* Wan-Sheng Zhang, Mei-Qiang Fan, Shu-Sheng Liu, Hai-Liang Chu, Yan-Hua Zhang, Xiu-Ying Gao, and Li-Xian Sun Department of Aerospace Catalysis and New Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed: October 8, 2007; In Final Form: December 3, 2007
The destabilization approach for SiO2-doped LiBH4 hydrogen storage composite is identified as 4LiBH4 + 2SiO2 f Li4SiO4 + 4B + Si + 8H2, and Li4SiO4 is the thermodynamic obstacle for its reverse action. TiF3 was doped in the composite for avoiding the formation of Li4SiO4 and thus enhancing the reversible hydrogen storage properties. Experimental analysis on LiBH4-SiO2-TiF3 composite was performed via thermogravimetry (TG), temperature programmed desorption (TPD), mass spectral analysis (MS), differential scanning calorimetry (DSC), isothermal sorption, and powder X-ray diffraction (XRD). For LiBH4-20 wt % SiO2-30 wt % TiF3 composite, the dehydrogenation temperature starts from 70 °C and decreases by an average of 100 °C from that of LiBH4-20 wt % SiO2. Its maximum amount attains 8.3 wt % below 500 °C. The whole dehydrogenation can be regarded as a two-step process: (i) preferential reaction (3LiBH4 + TiF3 f 3LiF + TiB2 + B + 6H2) occurring at around 70 °C, and (ii) principal reactions occurring simultaneously both at interface (LiBH4 + TiF3 + SiO2) and inside the bulk (self-decomposition of LiBH4). Doped TiF3 noticeably reduces the energy activation of the reaction at interface. However, the reaction inside the bulk is the rate-controlled process. This composite also demonstrates the ability of rehydrogenation under the pressure of 4.5 MPa. The hydrogen absorption is temperature-dependent and reaches 4 wt % H2 within 14 000 s at 500 °C.
1. Introduction Hydrogen is one of the environmentally clean energy carriers, which could attain nearly zero emission of pollutants from such power generators as fuel cells. Nevertheless, its safe storage and transportation is a major problem, which hinders its further applications. Some solid-state materials were developed for hydrogen storage under ambient conditions during the past decades.1,2 However, only those lightweight compounds such as MgH2,3 sodium alanates,4,5 borohydrides,6 and amides,7,8 etc., can adequately meet the gravimetric target designated by the U.S. Department of Energy.9 They are regarded as favored hydrogen storage materials for mobile applications based on hydrogen fuel cell techniques. LiBH4 is competitive among these materials, due to its large theoretical hydrogen capacity (18.3 wt %) and efficient amount (13.8 wt %).10 Unfortunately, it suffers from unfavorable thermal stability, inferior reversibility, and sluggish kinetics.11 Catalysts have been extensively utilized for the purpose of overcoming its thermodynamic and kinetic disadvantages. Au et al.12,13 found that some transition metal oxides or chlorides are able to destabilize LiBH4. Their onset temperature for dehydrogenation was reduced to minimum 100 °C. The reverse hydrogenation is even feasible for some modified composites, although the temperature condition is very harsh (above 600 °C). Furthermore, their rehydrogenation pressure (7 MPa) is greatly reduced from 15 MPa obtained by Muller6 and 30 MPa by Orimo,14 respectively. The LiBH4-LiNH2 system developed independently by Aoki15 and Pinkerton16,17 is able to liberate more than 10 wt % H2 above 300 °C. However, the dehydrogenation is exothermic, which makes it unfavorable for reverse action in * Corresponding author. Tel.: 86-411-84379215. Fax: 86-411-84379213. E-mail:
[email protected].
thermodynamics. Vajo et al.18 successfully destabilized LiBH4 with MgH2 and achieved more advantageous dehydrogenation enthalpy (42 kJ/mol H2-1) as well as a large amount of reversible capacity. Its van’t Hoff plot gives an extrapolated temperature of 225 °C for an equilibrium hydrogen pressure of 0.1 MPa. This system was also investigated by other groups, which gave separate explanations on reaction mechanisms.19,20 Zu¨ttel et al.11 found that SiO2 plays a significant role in destabilizing LiBH4 and helps to liberate large amount (9 wt % for LiBH4) of hydrogen from a temperature of 200 °C. However, it is still uncertain whether SiO2 has served as reactant in LiBH4 decomposition. Moreover, the cause of adverse rehydrogenation for this composite is unclear. Some suggestion ascribed it to the formation of lithium silicates,20 but no detailed evidence has yet been presented to the best of our knowledge. As we know, lithium silicates compose such species as Li4SiO4, Li2SiO3, Li2Si2O5, Li6Si2O7, Li8SiO6, etc. Each phase corresponds to a specified approach and thermodynamic behaviors. Driven by these questions, XRD measurements were conducted on asmilled and as-dehydrogenated samples. The cause of inferior reversibility for the composite of LiBH4-SiO2 would be explained initially in this article. LiBH4-SiO2-TiF3 composite was synthesized for avoiding the formation of lithium silicates and enhancing the hydrogen sorption properties further. The de-/hydrogenation performances were observed by thermogravimetry (TG), temperature programmed desorption (TPD), and mass spectral analysis (MS). Differential scanning calorimetric (DSC) and isothermal sorption measurements were conducted to reflect its thermodynamic and kinetic behaviors, respectively. Correlated with X-ray diffraction (XRD) measurements, the reactions within the system during de-/hydrogenation could be identified. The above studies would
10.1021/jp709814b CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008
4006 J. Phys. Chem. C, Vol. 112, No. 10, 2008
Figure 1. (a) TG profile for LiBH4-20 wt % SiO2 composite with a heating rate of 10 °C min-1. (b) Profiles of temperature programmed desorption (TPD) and its mass spectral analysis results (MS).
Zhang et al.
Figure 2. XRD patterns for LiBH4-20 wt % SiO2 composite at different states: (a) before dehydrogenation; (b) after dehydrogenation at 265 °C; and (c) after dehydrogenation at 380 °C.
give a primary insight into the performances of the LiBH4SiO2-TiF3 hydrogen storage composite. 2. Experimental Section The commercial LiBH4 (95%, Alfa-Aesar), silicon dioxide (99.8%, CAB-O-SIL, Cabot Co.), and TiF3 (99%, Alfa-Aesar) powders were directly used without further purification. The LiBH4-20 wt % SiO2 and LiBH4-SiO2-TiF3 mixtures were ground upon QM-1SP planetary ball mill for 1 h at the rate of 450 rpm. In each stainless milling pot (100 mL), the ball-topowder weight ratio was 50:1 and the protection atmosphere was 0.3 MPa Ar. All handlings of the samples were conducted in an MBraun unilab glove box filled with high-purity Ar (99.9999%) and low-density H2O and O2 (both
