J. Phys. Chem. C 2008, 112, 17023–17029
17023
Improved Reversible Dehydrogenation of Lithium Borohydride by Milling with As-Prepared Single-Walled Carbon Nanotubes Zhan-Zhao Fang, Xiang-Dong Kang, Ping Wang,* and Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ReceiVed: May 3, 2008; ReVised Manuscript ReceiVed: July 3, 2008
Single-walled carbon nanotubes (SWNTs) were mechanically milled with lithium borohydride (LiBH4) and examined with respect to its effect on the reversible dehydrogenation reactions of LiBH4. It was found that as-prepared SWNTs possess pronounced promoting effects on the dehydrogenation/rehydrogenation reactions of LiBH4, thus rendering the composite material reversible hydrogen storage at markedly reduced operation temperature and pressure conditions. For the LiBH4-30 wt % SWNTs composite milled for 1 h, 11.4 wt % hydrogen can be discharged within 50 min at 450 °C; over 6.0 wt % hydrogen can be recharged at 400 °C under an initial hydrogen pressure of 10 MPa. The systematic property/phase/structure studies of the LiBH4-SWNTs composite and related materials suggest that the observed property enhancement should be understood as combined effects of the metal nanoparticles or their borides, and carbon nanotubes. Additionally, the mechanism underlying the characteristic capacity loss in the dehydrogenation/rehydrogenation cycles of LiBH4 was also discussed. 1. Introduction Hydrogen storage is a key enabling technology in commercialization of hydrogen-powered vehicles.1,2 Compared to pressurized tank and cryogenic liquid hydrogen, solid-state hydrogen storage materials hold greater promise to provide safe and efficient on-board hydrogen sources. Nevertheless, decades of extensive efforts on traditional metal/alloy hydrides and nanostructured carbon materials have led to no viable system that can reversibly store over 6 wt % hydrogen at a moderate temperature condition relevant to the practical operation of photon exchange membrane fuel cell. Many recent efforts have been directed to develop light-metal complex hydrides as potential hydrogen storage media, among which lithium borohydride (LiBH4) is a leading candidate. LiBH4 possesses extremely high gravimetric hydrogen density, with a theoretical value of 18.3 wt %.3-20 However, the reversible dehydrogenation/rehydrogenation of LiBH4 are essentially restricted by the strong covalent/ionic bonds. Pure LiBH4 requires a temperature of over 400 °C for rapid hydrogen release and a rigorous condition of 600 °C and 35 MPa hydrogen for partial restoration of the hydride.3,4 Recent efforts have resulted in considerable progress in improving the reversible dehydrogenation properties of LiBH4 and related material systems. Vajo et al.5,6 and Barkhordarian et al.,7,8 independently, used MgH2 to stabilize the dehydrogenated state of LiBH4, which resulted in a favorable thermodynamics modification. The H-exchange enthalpy change was reduced from 67 kJ/mol H2 for pure LiBH4 to around 40 kJ/ mol H2 for the LiBH4/MgH2 system.5 The coupled experimental/ theoretical studies of Orimo et al.4 and Miwa et al.9 suggested that partial Li+ substitution by other elements (e.g., Mg) with higher eletronegativity might suppress Li+ f [BH4]- electron transferring and, accordingly, destabilize the hydride. Additionally, Vajo et al. demonstrated a novel nanoengineering approach * To whom correspondence should be addressed. Fax: +86 24 2389 1320. E-mail:
[email protected].
for improving the H-exchange properties of LiBH4.10 As a result of the nanoconfinement effect arising upon incorporating LiBH4 into nanoporous scaffolds, both dehydrogenation kinetics and cycling stability of the hydride were markedly improved. In contrast to the significant achievements in tuning thermodynamics, catalytic enhancement of the H-exchange kinetics of LiBH4 and the related systems is lack of progress. While the recharging process of LiBH4 is thermodynamically favorable, no effective catalyst has been identified that can accelerate the rehydrogenation reaction under moderate conditions. Zu¨ttel et al. reported that dehydrogenation performance of LiBH4 could be improved by adding three times weightier SiO2 additive, but leaving the recharging problem unsolved.3 Subsequent reports of Au et al. claimed that transition metal oxides and/or chlorides possessed catalytic effect on the dehydrogenation process of LiBH4.11,12 But the constituted systems can only be partially recharged even at 600 °C. In our recent efforts to improve the H-exchange kinetics of LiBH4, we found that the as-prepared SWNTs possess pronounced promoting effect on both the dehydrogenation and rehydrogenation reactions of LiBH4. Compared to pure LiBH4, the LiBH4 samples with SWNTs additive can fulfill reversible dehydrogenation under markedly reduced temperature/pressure conditions. We herein report this experimental finding as well as our efforts to further mechanistic understanding toward the effect of SWNTs additive. 2. Experimental Section LiBH4 with a purity of 95% was purchased from SigmaAldrich Corp. and used as received. SWNTs were prepared by hydrogen arc discharge method21 and purified using a procedure as detailed in ref 22. The as-prepared SWNTs consist of ca. 80 wt % C (composed mainly of SWNTs and small amounts of amorphous carbon and carbon nanoparticles) and 20 wt % metal nanoparticles (composed mainly of Ni (ca. 16 wt %) and small amounts of Fe and Co). But for simplicity, the as-prepared
10.1021/jp803916k CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
17024 J. Phys. Chem. C, Vol. 112, No. 43, 2008 SWNTs are referred to as “SWNTs” hereinafter, other than when specified. The metal catalyst particles and carbon impurities can be largely removed by using the purification process, which was typically repeated three times.22 As determined by temperatureprogrammed oxidation (TPO) method, the amount of residual metal particles in the post-purified SWNTs is less than 1 wt %. LiBH4 was mechanically milled with varied amounts of SWNTs additive for different periods under Ar (99.9999% purity) atmosphere by using a Fritsch 7 planetary mill at 400 rpm in a stainless steel vial together with eight steel balls (10 mm in diameter). The ball-to-powder ratio was around 40:1. Thus-prepared samples are referred to as LiBH4-xSWNTs, where x represents the weight percentage of SWNTs additive relative to the weight of LiBH4. All sample operations were performed in an Ar (99.9999% purity)-filled glovebox equipped with a recirculation system to keep the H2O and O2 levels below 0.1 ppm. For comparison purpose, graphite (99.99+% purity,
