Article pubs.acs.org/JPCC
Metathesis Reaction-Induced Significant Improvement in Hydrogen Storage Properties of the KF-Added Mg(NH2)2−2LiH System Yongfeng Liu, Chao Li, Bo Li, Mingxia Gao, and Hongge Pan* State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: The hydrogen storage properties and mechanisms of the Mg(NH2)2−2LiH system with potassium halides (KF, KCl, KBr, and KI) were investigated and discussed. The results show that the KF-added sample exhibits superior hydrogen storage properties as ∼5.0 wt % of hydrogen can be reversibly stored in the 0.08KF-added sample via a two-stage reaction with an onset dehydrogenation temperature of 80 °C. However, hydrogen storage behaviors of the samples with KCl, KBr, and KI remain almost unchanged. The fact that KF can readily react with LiH to convert to KH and LiF due to the favorable thermodynamics during ball milling should be the primary reason for its significant effects, as the presence of KH provides a synergetic thermodynamic and kinetic destabilization in the hydrogen storage reaction of the Mg(NH2)2−2LiH system by declining the activation energy of the first-step dehydrogenation as a catalyst and reducing the desorption enthalpy change of the second step as a reactant. The understanding on the role played by KF sheds light on how to further decrease the operating temperature and enhance the hydrogen storage kinetics of the metal−N−H system.
1. INTRODUCTION Hydrogen is the ideal synthetic fuel for on-board application, because it is lightweight, it is highly abundant, and its oxidation product (water) is environmentally benign.1 However, how to store it safely and efficiently remains a big problem.2−7 In 2002, Chen et al. reported that Li3N could reversibly store 11.5 wt % of hydrogen,8 which adds a new family of solid-state hydrogen storage materials. Since then, the metal−N−H system has received more and more attention due to its high hydrogen content, and a variety of metal amide−hydride combinations have been investigated and developed for their hydrogen storage performances.9−19 Significantly, it was found that introducing Mg into the Li−N−H system decreased the desorption enthalpy change by creating a new Mg(NH2)2− 2LiH combination.12,13 Approximately 5.6 wt % of hydrogen could be reversibly stored in the Mg(NH2)2−2LiH system with a desorption enthalpy change of 39 kJ/mol-H2,20 which is lowered by 35% in comparison with the LiNH2−LiH system (60 kJ/mol-H2).8,12,13 The hydrogen desorption process can be described below:12,13 Mg(NH 2)2 + 2LiH ↔ Li 2MgN2H 2 + 2H 2
kinetics of the Li−Mg−N−H system by adjusting composition, doping additives, reducing particle size, and so on.21−27 Sudik et al. reported that the desorption kinetics was enhanced by seeding the dehydrogenation product, that is, Li2MgN2H2, which expedites the nucleation and growth of the resultant product, and consequently lowers the activation energy of reaction.21 The partial substitution of Na for Mg or Li in the Li−Mg−N−H system reduced the dehydrogenation peak temperature by ∼10 °C.22 A 2-fold increase in the average dehydrogenation rate was achieved by introducing the graphitesupported Ru catalyst or the LiBH4 additive.23,24 Our previous work also revealed that the Li2MgN2H2 sample with a particle size of 100−200 nm started to take up hydrogen at only 80 °C, which is lowered by ∼100 °C with respect to the sample with a particle size of >800 nm.25 More encouragingly, Wang et al. demonstrated a significant improvement in hydrogenation/ dehydrogenation properties by partially replacing LiH with KH.26 The dehydrogenation peak temperature was lowered to 132 °C for the Mg(NH2)2−1.9LiH−0.1KH sample from 186 °C for the pristine Mg(NH2)2−2LiH sample, and the reversible hydrogen storage could proceed even at a temperature as low as 107 °C in the PCT model. Such an improvement was further confirmed recently by introducing KH into the 2LiNH2−MgH2 system.27,28 Unfortunately, the role played by KH is not well understood so far. Moreover, the dehydrogenation/hydro-
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Thermodynamic calculation delivered an operating temperature of 90 °C at 1 bar of equilibrium hydrogen pressure. However, a reasonable kinetics for dehydrogenation and hydrogenation was obtained only at temperatures above 200 °C due to a rather high energy barrier (Ea: ∼102 −120 kJ/mol).20,21 Considerable efforts have been devoted to lowering the operating temperature and enhancing the dehydrogenation © 2013 American Chemical Society
Received: October 30, 2012 Revised: December 17, 2012 Published: January 4, 2013 866
dx.doi.org/10.1021/jp3107414 | J. Phys. Chem. C 2013, 117, 866−875
The Journal of Physical Chemistry C
Article
Figure 1. TPD-MS (a, b) and volumetric release (c) curves of the Mg(NH2)2−2LiH−0.08KX samples.
99%), were purchased from Alfa-Aesar, Aladdin, and Sinopharm, respectively. Mg(NH2)2 and Li2MgN2H2 were synthesized in our own laboratory, and the detailed procedure can be found in our previous work.25 Samples with the compositions of Mg(NH2)2−2LiH−0.08KX (X = F, Cl, Br, I) and Mg(NH2)2−2LiH−xKF (x = 0, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15) were prepared by ball milling the corresponding chemicals at 500 rpm on a planetary ball mill (QM-3SP4, Nanjing) for 36 h. The ball milling vessels were filled with 50 bar of hydrogen to prevent hydrogen release during ball milling. The ball-to-sample weight ratio was about 60:1. The handling of samples was carried out in a MBRAUN glovebox filled with pure argon to prevent the air and moisture contaminations (O2,
