Improved hydrogen desorption properties of ammonia borane by Ni-modified metal-organic frameworks
Introduction
Hydrogen has been suggested as a future carrier of renewable energy because its combustion product is water, a zero pollutant. Though hydrogen production has made remarkable progress [1], [2], safe, efficient, compact and inexpensive hydrogen storage systems remain to be developed [3], [4]. It has been demonstrated that the greatest potential method to store hydrogen is by using solid media, such as hydrides [5], [6] and sorbent materials [7], [8], [9]. Ammonia borane (NH3BH3, denoted as AB) has recently attracted great interest as a promising candidate material for hydrogen storage due to its high gravimetric hydrogen capacity (19.6 wt.%) and low molecular weight (30.7 g mol−1) [10]. However, the practical application of AB is handicapped by its slow thermal kinetics below 100 °C, the emission of poisonous byproducts (borazine, ammonia and diborane), severe material foaming during the dehydrogenation and irreversibility. Proton exchange membrane fuel cells (PEMFC) normally work at 80 °C and in the absence of species poisonous to their catalysts. In the PEMFC, only ppm level ammonia will cause poisoning of the catalysts. The elimination of poisonous byproducts and decrease of decomposition temperature, therefore, play a very important role for AB used in PEMFC. To overcome these barriers, a number of approaches have been developed, including activation by transition metal catalysts [11], [12], [13], [14], [15], ionic liquids [16] and acid catalysts [17]. Despite considerable effort, these have not met the requirement of PEMFC. New approaches that are more practicable remain to be developed.
Recent reports show that materials structured at the nanoscale can decrease their H2 desorption enthalpy and enhance the kinetics relative to the bulk materials [18], [19]. Carbon cryogel and mesoporous silica infiltrated with AB have been demonstrated to significantly improve the kinetics and decrease the temperature for hydrogen release [20], [21]. However, the hydrogen release temperature is still higher than 85 °C and there is no concern on the prevention of ammonia formation. Therefore, a breakthrough in this research area calls upon the synthesis of new nanostructured materials that can provide a stronger nanoscale effect. Metal-organic frameworks (MOFs) are promising candidates as new scaffolds due to their ordered crystalline lattice, adjustable pore sizes and acceptable thermal stabilities, as well as especially excellent catalytic performances [22]. It has been shown that metal clusters and hydrides could be confined in MOFs [23], [24]. Recent study also shows that AB filled in MOFs enhances kinetics and eliminates ammonia [25]. In this system, molar ratio of AB to MOF was 1:1. Because of high molecular weight of MOF, AB mass content is relatively low. In addition, the interaction between AB and MOFs is still unclear. It is important to investigate the interaction between AB and MOFs so that to design new AB/MOFs materials.
Here, we select chromium(III) terephthalate (MIL-101) as a host material for detailed investigation because it has a huge cell volume that is necessary to confine more AB. MIL-101 is thermally stable up to 275 °C, and also stable when treated with various organic solvents at room temperature or under solvothermal conditions [26]. This enables MIL-101 to be used without decomposition during materials preparation and H2 desorption from AB at suitable conditions. The presence of numerous unsaturated CrIII sites in MIL-101 provides an intrinsic chelating property with an electron-rich functional group, making it as a catalyst. Our previous study showed that MOFs can be modified by catalysts to improve their hydrogen storage property [27], and the first-principle calculations play an important role in the study of hydrogen storage materials [28], [29]. Therefore, here we propose a new method to improve hydrogen storage of ammonia borane by doping it with metal-organic frameworks modified by Ni catalyst for the first time. The peak temperature of AB dehydrogenation was shifted to 75 °C (114 °C for neat AB). This is the first report of such a big decrease (40 °C) in the decomposition temperature. Furthermore, poisoning species like NH3 and borazine have been inhibited. In AB/MIL-101 nanocomposites, improved thermal decomposition thermodynamics and kinetics of AB have been observed. The interaction between AB and MOFs is investigated by the experiments and first-principle calculation in the present work.
Section snippets
Sample preparation
MIL-101 was synthesized according to reported procedures [26]. MIL-101 was heated to 200 °C for 3 h under Ar to remove the coordinated water, then stored in a Unilab91200 glove box (MBraun Co., Germany) filled with purified argon. 3% Ni@MIL-101 was prepared by adding 3 ml of 0.18 mol L−1 NiCl2 solution to 1 g activated MIL-101, and dried at 60 °C under vacuum overnight and further dried at 180 °C under Ar for 2 h, finally reduced by 3 MPa H2 at 200 °C for 3 h, then stored in a glove box for
Structure characterization
The metal-organic frameworks used in this paper, MIL-101, was prepared according to a literature procedure [26]. The 3% Ni@MIL-101, 50% AB/MIL-101 and 50% AB/Ni@MIL-101 (where the weight percent is used in all the text unless stated elsewhere) materials were synthesized by an impregnation method. N2 adsorption/desorption analysis (Fig. 2) shows that 50% AB/MIL-101 has much smaller pore volume (0.03 cm3 g−1) and BET surface area (4.3 m2 g−1) than those (1.45 cm3 g−1 and 2540 m2 g−1) of MIL-101.
Conclusions
In conclusion, we have demonstrated that MOFs are effective host materials to improve thermal decomposition of AB and introduction of a catalyst into the nanocomposites is a feasible method for improving both thermodynamics and kinetics. Our research also show that AB confined in MOFs decomposition at a much lower temperature and some undesirable volatile products are suppressed, but some other by-product like aminodiborane is also generated. This is the first study shows that though
Acknowledgments
This work was supported by the “973 Project” (2010CB631303) and NSFC (No. 50671098, 20873148, 20833009, 51071146, 51071081, U0734005 and 20903095), and. This work was carried out in connection with IUPAC Project No. 2008-006-3-100 “Critical evaluation of thermodynamic properties of hydrogen storage materials: metal-organic frameworks and metal or complex hydrides”. We also would like to thank Professor Dr. John H. Dymond of University of Glasgow (UK) and Professor Dr. Jeffrey R. Long of
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