J. Phys. Chem. C 2007, 111, 4879-4884
4879
Improving Hydrogen Storage Performance of NaAlH4 by Novel Two-Step Milling Method 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: December 4, 2006; In Final Form: January 26, 2007
A selective two-step doping method was developed to prepare metallic Ti-doped NaAlH4, in which a catalytic amount of metallic Ti was first milled with NaH and then milled again after adding Al powder (NaH:Al)1: 1). As determined by the combined X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) examinations, the utilization of this novel doping method has produced a more homogeneous dispersion of the fine Ti hydride particles in the NaH/Al matrix, compared to the sample prepared by the traditional one-step milling. As a result, both the hydrogen capacity and de-/hydriding kinetics of the doped materials have been markedly improved. For the NaH + Al + 0.04Ti mixture mechanically milled for 20 h, the cycling hydrogen capacity of the two-step milled sample reached to over 4.3 wt %, about 40% higher than the sample prepared by the traditional one-step method. Moreover, it was observed that the cycling performance of the doped material was highly stable over several hydriding/dehydriding cycles. On the basis of these results, a discussion regarding the nature of catalytically active species was given.
1. Introduction Safe and effective storage and delivery of hydrogen has been generally recognized as a key technical challenge in commercialization of fuel cell-power vehicles.1,2 In 1997, Bogdanovic´ and Schwickardi3 reported that, upon mixing the hydride with a few mole percent of metal catalysts, the decomposition/ reconstruction of sodium alanate was markedly accelerated and rendered reversible under moderate temperature. This finding triggered an intensive investigation of the catalytically enhanced NaAlH4 and other related lightweight complex hydrides as a potential hydrogen storage medium. The reversible dehydrogenation of the doped hydrides is governed by catalytic enhancement arising upon doping catalyst. Developing advanced dopant precursors and doping technology, therefore, constitutes a key aspect in the research on the NaAlH4 system. The traditional doping method involves the usage of Ti(III) or Ti(IV) compounds as a dopant precursor.3-10 The utilization of these titanium compounds, however, resulted in the capacity loss associated with the sideproducts generation and/or evolution of gas impurities that are highly detrimental to fuel cell operation. Similar problems may be also encountered in the recently developed systems doped with novel Ti colloid (Ti13‚6THF) (THF)tetrahydrofuran) nanoparticles,11 where the THF ligand was used to stabilize the Ti nanoparticles. Recently, we have found that metallic Ti powder could be directly used as a dopant precursor to prepare catalytically enhanced NaAlH4.12,13 Thus-prepared material is free of inactive byproducts and detrimental gas-impurities, thus promising a high-performance hydrogen storage system. However, as demonstrated, the practical hydrogen cycling capacity stabilized at only about 3 wt %, far below the theoretical value of 5.5 wt % * To whom correspondence should be addressed. Fax: +86 24 2389 1320. E-mail:
[email protected].
for NaAlH4 undergoing the reversible two-step reaction following eq 1:
NaAlH4 a 1/3Na3AlH6 + 2/3Al + H2 a NaH + Al + 3/2H2 (1) Moreover, it was observed that the de-/hydriding kinetics of the metallic Ti doped hydrides was substantially inferior to that of the samples doped with Ti compounds. Further investigations found that the added metallic Ti powder was in situ hydrogenated to Ti hydride during the mechanical milling process and that an unfavorable distribution state of Ti hydride particles might be responsible for the inferior hydrogen storage performance of the metallic Ti doped hydride.14 For example, the Ti hydride particles as large as 1 µm were still detectable after mechanically milling the sample for 10 h. Our further efforts therefore focus on microstructure optimization of the doped system via innovating the doping technology. In this paper, we present a novel two-step doping method, in which a catalytic amount of metallic Ti was first milled with NaH, and then the milled mixture was milled again after adding Al powder (NaH:Al)1:1). It was found that thus-prepared materials possessed a much more favorable microstructure, in comparison with those obtained from the traditional one-step doping process. As a result, a pronounced improvement on the cyclic hydrogen capacity and hydriding/dehydriding kinetics has been achieved. 2. Experimental Section The starting materials, NaH (95%, 200 mesh), Al powder (99.97%, 325 mesh), and Ti powder (99.98%, 325 mesh), were all purchased from Sigma-Aldrich Corp. and used as received. Ball milling of the materials was performed under Ar atmosphere using a Fritsch P7 planetary mill at 400 rpm in a vial together with eight balls (10 mm in diameter) made of
10.1021/jp068309d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
4880 J. Phys. Chem. C, Vol. 111, No. 12, 2007
Figure 1. The XRD patterns of the samples prepared under varied conditions: (a) NaH + 4 mol % Ti milled under H2 atmosphere for 10 h; (b) the milled mixture (a) in addition to Al (NaH:Al)1:1) milled under H2 atmosphere for another 10 h; (c) NaH + 4 mol % Ti milled under Ar atmosphere for 10 h; and (d) the milled mixture (c) in addition to Al (NaH:Al)1:1) milled under Ar atmosphere for another 10 h.
stainless steel. The ball-to-powder weight ratio was around 40: 1. A mixture of NaH and metallic Ti powder at a molar ratio of 1:0.04 was first milled for 10 h, and then after adding Al powder (NaH:Al)1:1) the powder mixture was milled for another 10 h under H2 or Ar atmosphere with an initial pressure of 0.8 or 0.1 MPa, respectively. For comparison, a mixture of NaH + Al + 0.04Ti was directly milled for 20 h under identical milling conditions. All sample operations were performed in an Arfilled glovebox equipped with a recirculation system to keep the H2O and O2 levels < 1 ppm. Hydriding/dehydriding behaviors of the samples were examined by using a carefully calibrated Sievert’s type apparatus. A typical cyclic experiment entailed absorption at 120 °C and desorption at 150 °C with an initial pressure condition about 11 MPa and
