NbN nanoparticles as additive for the high dehydrogenation properties of LiAlH4

Abstract

The effects of NbN nanoparticles synthesized via a simple "urea glass" route on the dehydrogenation properties of LiAlH4 have been systematically investigated. The particle size of the as-synthesized NbN nanoparticles is determined to be about 10 nm. The surface configuration and dehydrogenation behaviors of the 2 mol% NbN-doped LiAlH4 (2% NbN-LiAlH4) system are also discussed. It is found that the 2% NbN-LiAlH4 sample starts to decompose at about 95 °C and releases a total of 7.10 wt% hydrogen, which is 55 °C lower than that of as-milled LiAlH4. The isothermal dehydrogenation kinetics shows that the 2% NbN-LiAlH4 sample could release approximately 6.10 wt% hydrogen in 150 min at 130 °C, whereas as-received LiAlH4 only releases about 0.63 wt% hydrogen under the same conditions, revealing that the enhancements arising upon adding NbN nanoparticles are almost 8-9 times that of as-milled LiAlH4. The activation energy (Ea) is calculated to be 71.91 and 90.87 kJ mol(-1) for the first and second hydrogen desorption of the NbN-LiAlH4 sample, a 38% and 32% reduction relative to as-received LiAlH4, respectively. A detailed modeling study shows that the first dehydrogenation step can be sufficiently interpreted with the nucleation and growth in a one-dimensional model based on the first-order reaction. More interestingly, the dehydrogenated LiAlH4 sample can recharge H2 under a 5.5 MPa hydrogen pressure. An SEM image of the dehydrogenated 8% NbN-LiAlH4 sample after HP-DSC under 5.5 MPa H2 shows that some nanorods appear.