Article pubs.acs.org/JPCC
Rapidly Releasing over 9 wt % of H2 from NH3BH3−Mg or NH3BH3− MgH2 Composites around 85 °C Junhong Luo,*,†,∥ Xiangdong Kang,§ Changan Chen,† Jiangfeng Song,† Deli Luo,† and Ping Wang*,‡ †
Institute of Materials, China Academy of Engineering Physics, Jiangyou 621908, Sichuan, PR China School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, PR China § Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China ∥ Sichuan Juneng Nuclear Technology Engineering CO., LTD, Jiangyou 621700, Sichuan, PR China ‡
S Supporting Information *
ABSTRACT: It is ideal that the hydrogen storage materials for vehicular applications can desorb substantial amounts of hydrogen below 85 °C, the operating temperature of polymer electrolyte membrane (PEM) fuel cells. Ammonia borane (NH3BH3, AB for short), because of its intriguingly high hydrogen density (i.e., 19.6 wt %) and moderate thermal stability, is widely regarded as a promising on-board hydrogen storage medium. However, at this temperature, both its dehydrogenation kinetics and deliverable H-capacity are far from meeting the requirements for practical applications. Here, we report that the Mg- or MgH2modified AB can deliver over 9 wt % of H2 within 1.5 h at approximately 85 °C. Such pronounced dehydrogenation properties are found to be enabled by the combination of three factors, including partial phase transition of normal AB to its mobile phase AB* in the starting material, adequate sample thermal conductivity, and sufficiently intensive external energy input. A further mechanistic study indicates that the dehydrogenation of the AB−Mg or AB−MgH2 sample should likely involve a three-step mechanism, with the formation of a metastable or even unstable magnesium amidoborane phase being a central event. promoters15−18 all have proven effective at promoting hydrogen release from AB. In particular, chemical modification of AB by substituting one amine hydrogen of AB with alkali and/or alkaline earth metal via mechanochemical or wet-chemistry methods has led to the synthesis of a series of mono- and mixed-metal amidoboranes.19−30 These new derivatives typically exhibited improved dehydrogenation properties with respect to the parent AB in terms of reduced threshold temperatures, accelerated dehydrogenation rates, and suppressed toxic gaseous byproducts. However, these established approaches have proven to be inapplicable to the synthesis of Mg(NH2BH3)2 (MgAB), largely because the occurrence of chemical reactions between AB and Mg or MgH2 requires the conversion of normal AB to its mobile phase (denoted as AB* hereafter) at first.31−34 Depending on the temperature applied, their reactions can either lead to the formation of amorphous Mg−N−B−H species (at temperatures above 70 °C)31−33 or result in the generation of dominantly crystalline MgAB (at temperatures from room temperature to ∼70 °C).34 Although the synthesis of crystalline MgAB is of scientific importance, chemical modification of AB in the former way is clearly more favorable
1. INTRODUCTION The development of safe and energy-efficient hydrogen carriers is widely recognized as a “grand challenge” facing the implementation of a hydrogen-based economy. In particular, the hydrogen storage materials for transportation applications must meet the stringent system volumetric (≥70 g H2 L−1) and gravimetric (7.5 wt % H2) targets set by the U.S. Department of Energy.1 Moreover, in order to be energy-efficient, the dehydrogenation of these materials should be operated at temperatures below 85 °C, the operating temperature of polymer electrolyte membrane (PEM) fuel cells, to make the best of the waste heat.1,2 Ammonia borane (NH3BH3, AB for short), because of its intriguingly high hydrogen density (19.6 wt %) and moderate thermal stability, has garnered considerable attention in recent years as a promising on-board hydrogen storage candidate.3−5 However, at approximately 85 °C, both the dehydrogenation kinetics and the deliverable H-capacity of AB are incommensurate with the requirements for transportation applications;6 whereas at elevated temperatures, the hydrogen released is heavily contaminated by the concomitant release of volatile byproducts, including NH3, B2H6, NH2BH2, and c-(NHBH)3,6,7 which are detrimental to the PEM fuel cells. To address these problems, many approaches have been developed. Incorporation of AB in nanoscaffolds,8,9 catalytic solvolysis of AB by transition metal or acids,10−14 and addition of chemical © XXXX American Chemical Society
Received: April 27, 2016 Revised: August 5, 2016
A
DOI: 10.1021/acs.jpcc.6b04230 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C because the reaction “byproducts”, i.e., hydrogen and chemical energy, produced during the formation of Mg−N−B−H species may be properly utilized. Moreover, it has been demonstrated that AB would experience complicated physical and chemical transformations prior to decomposition,35,36 so it presenting as an individual phase in the postmilled 2AB/MgH2 or 2AB/Mg composites may allow a greater degree of flexibility in controlling the dehydrogenation properties with sophisticated chemical and physical control. In this paper, we report that AB−Mg or AB−MgH2 samples may show substantially different dehydrogenation characteristics depending upon the storing and measuring conditions. Particularly, the AB−Mg and AB−MgH2 samples can release over 9 wt % of H2 at around 85 °C within 1.5 h if the factors like phase transition of normal AB to AB*, sample thermal conductivity, and external energy input can be simultaneously addressed and properly controlled. A mechanistic study indicated that the formation of a metastable or even unstable MgAB phase (denoted as MgAB* hereafter) is likely the dictating factor accounting for the unprecedented dehydrogenation properties.
(WL%) were calculated via the following formula: WL% = (W2 − W3)/W1 × 100%. Here, the weight of the reactor is about 100 g. Solid-state 11B magic-angle spinning (MAS) NMR experiments were conducted at room temperature on a Bruker Avance III 400 WB spectrometer operating at a magnetic field of 9.4 T on 128.3 MHz 11B frequency. The samples were packed in the ZrO2 rotor closed with Kel-F cap and then spun at a 12 kHz rate. A total of 16 scans were recorded with 2 s recycle delay for each sample. All 11B MAS chemical shifts are referenced to the resonances of LiBH4 standard (δ = −41 ppm). The spectrum of the postmilled 2AB/Mg sample was fitted by Gauss bands using the PeakFit commercial analysis software for windows. The AB−Mg and AB−MgH2 samples at varied states as well as MgAB with about 6 wt % or 2.5 equiv of H2 released were also characterized by X-ray diffraction (XRD; Rigaku D/max 2500, Cu Kα radiation) and/or Fourier transform infrared (FTIR) spectroscopy (Bruker TENSOR 27, 4 cm−1 resolution). Special measures were taken to minimize H2O/O2 contamination during the sample transfer process. FTIR spectra of the samples were collected using the KBr-pellet method, and the obtained spectra were normalized using OPUS 6.5 software.
2. EXPERIMENTAL METHODS AB (97% purity) and Mg powder (>99%) from Sigma-Aldrich, Al and MgH2 powders (98%) from Alfa Aesar, and graphene from Deyang Carbonene Technology were all used as received. The AB−Mg and AB−MgH2 with or without additives were mechanically milled under an argon (99.999% purity) atmosphere for 2 h using a SPEX8000 mill. The ball-to-powder ratio was around 100:1. The preheated AB (denoted as p-AB) was prepared by heating AB at 85 °C and then quenching to room temperature when slow H2 release began. MgAB was prepared by aging the postmilled 2AB/MgH2 sample at room temperatures.34 The dehydrogenation product of MgAB with around 6 wt % or 2.5 equiv of H2 released was obtained by heating the as-prepared MgAB sample at 2 °C min−1 to yield the said amount of H2 and then quickly quenching it to around room temperature to prevent further hydrogen desorption. All sample handlings were carried out in an argon (99.999% purity)-filled glovebox, wherein the H2O/O2 levels were typically
