Hydrogen Absorption and Desorption by Magnesium-Based Nano-Composite Materials
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Hydrogen Absorption and Desorption by Magnesium-Based Nano-Composite Materials Yoshitsugu Kojima, Yasuaki Kawai and Tetsuya Haga Toyota Central R& D Labs., Inc., Nagakute, Aichi, 480-1192 JAPAN ABSTRACT MgH2 was mechanically milled with nano-Ni (nano-Ni/Al2O3/C) and Ni catalysts (Ni), yielding Mg-based nano-composite materials. X-ray and TEM measurements revealed that nano-Ni particles, which are 6-20 nm size, were dispersed in the MgH2 matrix. The nano-composite material with nano-Ni/Al2O3/C showed excellent properties as compared to that with Ni, a ball-milled MgH2 and the mixture of MgH2 and nano-Ni/Al2O3/C in terms of the H2 desorption and absorption. The nano-composite material with nano-Ni/Al2O3/C desorbed H2 of 4.9-5.8 wt% at 423-473 K, while the mixture could not desorb H2 at the temperature. The H2 absorption capacity at 9 MPa and room temperature in 6 hr. increased from below 0.1 wt% for the mixture to 5.0 wt % for the nano-composite material, approaching a maximum of 6.5 wt% in 70 hr. The catalyst activity was improved with decreasing Ni size. The improved kinetics is indicated by the small activation barrier, the short diffusion path length and the high driving force. INTRODUCTION Hydrogen (H2) can be stored in tanks as compressed [1] or liquefied H2 [1] or by adsorption on carbon materials [1-3]. It can also be stored in hydrogen absorbing alloys [4, 5] or as a chemical hydride, such as NaBH4 [6, 7] or NaAlH4 [8, 9], metal nitrides [10-12], MgH2 [13, 14] as well as in an organic hydride, such as methylcyclohexane or decalin [15]. Among these methods, much attention has been given to H2 storage materials with light weight (carbon materials, chemical hydrides). Magnesium (Mg) has a high H2 storage capacity of 7.6 wt%. However, the high work temperature (573 K) and the slow reaction kinetics (high activation energy) limit the practical application of MgH2 system. G. Liang et al. reported that MgH2-Ni composite desorbed hydrogen at 523 K. The composite absorbed H2 at 302 K, but the maximum capacity was only 0.7 wt% [13]. For usable H2 storage, H2 absorption / desorption at ambient temperature is necessary. For the first time, we demonstrated that the H2 absorption capacities of the Mg-based nano-composite material with a nano-Ni catalyst exhibited 5.0-6.5 wt% at high H2 pressure of 9 MPa and room temperature (296 K). EXPERIMENTAL The starting materials for mechanical milling were magnesium hydride (MgH2, Aldrich), Ni (Soekawa Chemicals, 0.8 µm) and a nano-Ni catalyst. The nano-Ni catalyst (nano-Ni/Al2O3/C) was prepared by a co-precipitation method [16]. In distilled water, 5 mol of nickel nitrate hexahydrate [Ni(NO3)2⋅6H2O] and 1 mol of aluminum nitrate nonahydrate [Al(NO3)2⋅9H2O] were dissolved and the resulting solution and a super activated carbon (specific surface area: 3200 m2/g) were mixed. To this solution, an aqueous solution of Na2CO3 was added and Ni6Al2(OH)16CO3⋅4H2O was deposited on the activated carbon. Then, the product was held at
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673 K for 4 h in Ar. The dried catalyst was
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