Effect of Li 3 N additive on the hydrogen storage properties of Li-Mg-N-H system

  • PDF / 595,226 Bytes
  • 7 Pages / 584.957 x 782.986 pts Page_size
  • 117 Downloads / 231 Views

DOWNLOAD

REPORT


The effect of Li3N additive on the Li-Mg-N-H system was examined with respect to the reversible dehydrogenation performance. Screening study with varying Li3N additions (5, 10, 20, and 30 mol%) demonstrates that all are effective for improving the hydrogen desorption capacity. Optimally, incorporation of 10 mol% Li3N improves the practical capacity from 3.9 wt% to approximately 4.7 wt% hydrogen at 200  C, which drives the dehydrogenation reaction toward completion. Moreover, the capacity enhancement persists well over 10 de-/rehydrogenation cycles. Systematic x-ray diffraction examinations indicate that Li3N additive transforms into LiNH2 and LiH phases and remains during hydrogen cycling. Combined structure/property investigations suggest that the LiNH2 “seeding” should be responsible for the capacity enhancement, which reduces the kinetic barrier associated with the nucleation of intermediate LiNH2. In addition, the concurrent incorporation of LiH is effective for mitigating the ammonia release.

I. INTRODUCTION

A major technical obstacle to the widespread use of hydrogen-powered vehicles is the lack of a safe and efficient system for onboard hydrogen storage. Hydrogen storage in solid hydrides is a promising option because it offers a volumetric density greater than that of either compressed or liquefied hydrogen.1 However, conventional metal hydrides are greatly limited by their low gravimetric density or high operating temperature.2,3 Considerable recent efforts have been directed at developing light-metal complex hydrides (alanate, amide, or borohydride) and their hybrid systems as potential H-storage media.4–6 In 2002, Chen et al.7 reported the reversible storage of over 10 wt% hydrogen by Li3N. The reversible hydrogenation/dehydrogenation of Li3N proceeds via a twostep process involving the reactions as follows: Li3 N þ H2 ⇆ Li2 NH þ LiH 5:7 wt% Li2 NH þ H2 ⇆ LiNH2 þ LiH 6:5 wt%

;

ð1Þ :

ð2Þ

This revolutionary discovery immediately attracted worldwide interest in Li-N-H and related metal-N-H systems.6 Among them, Li-Mg-N-H system receives the most extensive attention due to its combined advantages a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0248

1936

http://journals.cambridge.org

J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 15 Mar 2015

of favorable thermodynamics, good reversibility, and moderate hydrogen capacity.8–15 Starting from a 2LiNH2 + MgH2 mixture, the reversible de-/rehydrogenation reactions can be described by following reactions8,9,12,13: 2LiNH2 þ MgH2 ! Li2 MgðNHÞ2 þ 2H2 ⇆ MgðNH2 Þ2 þ 2LiH 5:5 wt% : ð3Þ Compared with the Li-N-H system (e.g., DH = 66 kJ/ mol H2 for reaction 2), Li-Mg-N-H possesses a much lower dehydrogenation reaction enthalpy, just around 44 kJ/mol H2.16–18 Thermodynamically, it predicts a dissociation pressure of 0.1 MPa at 90  C.17 However, two barriers still remain for the potential application of this system.19 On the one hand, the dehydrogenation reaction is kinetically limited. For example, even at an o

Data Loading...