Article pubs.acs.org/JPCC
Unexpected Dehydrogenation Behaviors of the 2LiBH4−MgH2 Composite Confined in a Mesoporous Carbon Scaffold Kuikui Wang,†,‡ Xiangdong Kang,*,‡ Yujie Zhong,‡ Chaohao Hu,§ Jianwei Ren,⊥ and Ping Wang*,‡ †
University of Science and Technology of China, Hefei 230026, P. R. China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China § Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, P. R. China ⊥ HySA Infrastructure Centre of Competence, Materials Science and Manufacturing, Council for Scientific and Industrial Research, Pretoria 0001, South Africa ‡
S Supporting Information *
ABSTRACT: Nanoconfinement has been widely employed as a promising strategy to improve dehydrogenation kinetics, reversibility, and equilibrium pressure of complex metal hydrides. In this paper, we report a careful study of the influence of nanoconfinement on the reversible dehydrogenation property and reaction mechanism pathway of the 2LiBH4−MgH2 composite system. Compared to the bulk 2LiBH4−MgH2 composite, the 2LiBH4−MgH2 confined in the mesoporous carbon (CMK-3) scaffold host exhibits the significantly enhanced dehydrogenation kinetics but meanwhile shows the serious loss of hydrogen capacity upon cycling, particularly in the first two cycles. Moreover, the observed dehydrogenation property is independent of the hydrogen back pressure. The combination analyses of XRD, FTIR, and NMR definitely detected the dominant Mg and B phases in the dehydrogenation products, suggesting the mainly individual desorption of MgH2 and LiBH4 in the confined 2LiBH4−MgH2 system. This unfavorable change of the dehydrogenation reaction pathway would result in the poor reversibility, which is not expected for the combined hydride systems. These findings might provide renewed insight into the nanoconfinement effect on the hydrogen storage property for multiple phase combined systems.
1. INTRODUCTION
that possess superior hydrogen storage property relative to the parent LiBH4.9−16
Complex hydrides containing light alkali or alkaline-earth metal cations and AlH4−, NH2−, or BH4− anionic units have been extensively investigated for potential high-capacity hydrogen storage applications.1−3 Among them, lithium borohydride (LiBH4) is of special interest due to its high gravimetric (18.5 wt %) and volumetric hydrogen capacities (121 kg/m3).4 However, owing to the strong ionic/covalent bonding between the constituent elements, the hydrogen release of LiBH4 (eq 1) is restricted by high thermodynamic stability and sluggish reaction kinetics. As a result, the hydrogen desorption from LiBH4 proceeds only at high temperatures over 400 °C. On the other hand, the reverse hydrogenation process is still very limited due to the chemical inertness of the elemental B.5,6 For example, the dehydrogenated products (LiH/B) can only be partially hydrogenated even at a rigorous condition of 600 °C and 35 MPa hydrogen.6,7 To date several strategies have been developed to address these problems. A promising strategy is destabilization of complex hydrides using reactive additives that form mixed compounds upon dehydrogenation.7,8 In past decades, the employment of reactant destabilization strategy has given rise to a wealth of new reactive hydride composites © 2014 American Chemical Society
LiBH4 → LiH + B + 3/2H 2
(1)
LiBH4 + 1/2MgH2 → LiBH4 + 1/2Mg + 1/2H 2 → LiH + 1/2MgB2 + 2H 2
(2)
The combination of LiBH4 and MgH2 in a 2:1 molar ratio constitutes a prototypical reactive hydride composite system with a high theoretical hydrogen capacity of 11.4 wt %. Experimental studies have demonstrated that the hydrogen storage property and dehydrogenation reaction pathway of the combined system strongly depend on the measurement conditions, especially hydrogen back pressure.17,18 Under a low H2 back pressure (
