Article pubs.acs.org/JPCC
Comparative Study of Structural Changes in NH3BH3, LiNH2BH3, and KNH2BH3 During Dehydrogenation Process Keiji Shimoda,† Koichi Doi,‡ Tessui Nakagawa,§ Yu Zhang,† Hiroki Miyaoka,† Takayuki Ichikawa,*,† Masataka Tansho,∥ Tadashi Shimizu,∥ Anthony K. Burrell,⊥ and Yoshitsugu Kojima† †
Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan § Materials Physics and Applications Division, Los Alamos National Laboratory, Mail Stop J514, Los Alamos, New Mexico 87545, United States ∥ National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan ⊥ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States ‡
ABSTRACT: The thermal decomposition pathways of ammonia borane (AB, NH3BH3), lithium amidoborane (LiAB, LiNH2BH3), and potassium amidoborane (KAB, KNH2BH3) have been investigated in detail by using solid state 11B MAS NMR spectroscopy. During the first decomposition process of AB, the complex structural entities were observed, which have been attributed to polyaminoborane (PAB, (NH2BH2)n). On the other hand, polyiminoborane (PIB, (NHBH)n) formed in the second process showed a single 3-fold coordination IIIB signal above 125 °C, suggesting a [BN]n network structure. LiAB and KAB did not show the PAB-like structure, and the PIB-like structure was directly formed by the hydrogen desorption. Quantitative analyses of the 11B NMR spectra suggested that the general route of the thermal decomposition of alkali metal amidoboranes (LiAB, NaAB, and KAB) is similar. Also, 39K MAS NMR spectra of KAB and its decomposition products indicated the formation of KH and an amorphous K−N−B−H phase during the hydrogen release. ammonia proton H+ in AB, leading to ionic molecular crystal M+[NH2BH3]−.14 The thermal hydrogen releases from lithium amidoborane (LiAB, LiNH2BH3) and sodium amidoborane (NaAB, NaNH2BH3) have been extensively studied by several researchers.12,13,15−17 The crystal structure and hydrogen release from potassium amidoborane (KAB, KNH2BH3) were also reported very recently.18 These alkali metal amidoboranes release 10.9, 7.5, and 6.5 mass % H2, respectively, at temperatures of 80−90 °C, and all show improved desorption kinetics. Furthermore, there is no apparent release of borazine or diborane from these materials.12,13,18 There has been some attempt to characterize the solid residue based on the amount of hydrogen release from both AB and MAB during the dehydrogenation process; however, because the decomposition products are generally X-ray amorphous, solid state nuclear magnetic resonance (NMR) spectroscopy is the only technique which has proven to be successful.12,13,17,19 The previous studies reported in situ 11B MAS NMR spectra of AB and LiAB
I. INTRODUCTION The development of a safe and economical on-board hydrogen storage material as an energy carrier is a crucial element if fuelcell vehicles are to become an efficient and effective form of transportation in any forthcoming low-carbon society. In the past decade major efforts have focused on the development of accessible hydrogen storage media. The most well-known are the metal−organic frameworks (MOFs), metal alloys (LaNi5, etc.), and simple or complex metal hydrides (metal hydrides such as LiH and MgH2, alanates, amides, borohydrides, and composites). 1−6 Among these, ammonia borane (AB, NH3BH3) and its derivatives have many of the desirable properties that were sought for a potential hydrogen carrier. AB stores 19.6 mass % H2, which meets the 2015 U.S. Department of Energy (DOE) target.7 Hydrogen can be released to ∼13 mass % below 200 °C.8 However, its practical application is limited by its slow dehydrogenation kinetics below 100 °C and concurrent release of the fuel cell poisons, borazine (NHBH)3 and diborane B2H6, at temperatures above 145 °C.9−12 Recently, AB derivatives called metal amidoboranes (MABs) have been developed as novel hydrogen storage materials13 where a cation M+ (M = Li, Na, K, etc.) replaces one of the © 2012 American Chemical Society
Received: December 22, 2011 Revised: February 16, 2012 Published: February 16, 2012 5957
dx.doi.org/10.1021/jp212351f | J. Phys. Chem. C 2012, 116, 5957−5964
The Journal of Physical Chemistry C
Article
at ∼90 °C.17,19 These studies provided an important implication for the formation of the diammoniate of diborane (DADB, [NH3BH2NH3]+[BH4]−) during the initial induction period of AB dehydrogenation but have not enabled sufficient experimental evidence to fully elucidate the decomposition pathways of AB and MAB. The thermogravimetric and volumetric measurements suggested that solid phase AB decomposes to polyaminoborane (PAB, (NH2BH2)n) in the first dehydrogenation step, which is further changed to socalled polyiminoborane (PIB, (NHBH)n) or polyborazilene (PB, (BNHx)n, x < 2) during the second step.9−11,13,20 Above 500 °C, it is considered that hexagonal BN is generated as a final product.9,21 In the case of the MABs, much less information has been published as to the thermal decomposition process.12,13,15,17 Xiong et al. assumed MNBH as a decomposition product after the desorption of 2 mol equiv of H2, whereas they concluded the product as a mixture of MH and BN in their subsequent paper.13,15 Our previous study revealed that the hydrogen desorption from NaAB leads to the formation of NaH and amorphous Na−N−B−H phase above ∼90 °C.22 In the present study, we have examined and compared the thermal decomposition of AB, LiAB, and KAB, on the basis of the structural characterization of their decomposition products using quantitative 11B MAS NMR technique.
Figure 1. (a) Thermogravimetric curve and mass profile for desorbed H2 gas and (b) mass profiles for NH3, B2H6, NH2BH2, and (NHBH)3 gases from AB up to 500 °C.
shows that the sample weight drastically decreases between 90 and 120 °C, and further weight loss appears between 140 and 160 °C. The H2 desorption profile also represented two desorption peaks with a relatively sharp one at 105 °C and a broader one at 148 °C. Above 200 °C, there were no clear peaks in the profile. The thermal decomposition of AB proceeds in a two-step reaction with H2 release, as has been already reported.9−12 Figure 1b shows the gas desorption profiles for ammonia (NH3), diborane (B2H6), monomeric aminoborane (NH2BH2), and borazine ((NHBH)3). NH3, B2H6, and NH2BH2 gases desorbed in the two steps similar to the H2 profile, and the small amount of (NHBH)3 was also detected only in the second gas desorption step. We should mention that the weight loss obtained in this study was much larger than those reported by the previous studies. There is a broad range of weight losses reported in the literature. For example, Baitalow et al. reported the weight loss of ∼14 mass % (heating ramp; 1 °C/min), while Kang et al. showed 46 mass % loss (2 °C/min) up to 200 °C.11,12 As we know that compounds other than hydrogen were released from AB as it was heated, we expect the weight losses to be much greater than the theoretical. This implies that the purity, particle size, and chemical history of the AB sample, and measurement condition of TG-MS (heating rate, gas flow, etc.), all impact the weight loss, or percentage of non-hydrogen volatiles produced. The TG curve and H2 desorption profile for LiAB are shown in Figure 2a. The sample weight started to decrease above 50 °C, and a weight loss of 8.1 mass % was achieved by 200 °C. The H2 desorption profile represented a relatively sharp peak at 87 °C and a partially overlapping broad peak centered at 116 °C. Therefore, the dehydrogenation of LiAB proceeds in twostep reaction similar to that of AB. Furthermore, there seems to be a small peak between 250 and 300 °C, and another one above 450 °C in the profile. Although B2H6, NH2BH2, and (NHBH)3 were not detected, a small amount of NH3 was observed concomitantly with the first H2 release (Figure 2b). The release of NH3 was not referred to in the previous
II. EXPERIMENTAL DETAILS The starting materials NH3BH3 (97%, Sigma-Aldrich) and LiNH2BH3 (>90%, Sigma-Aldrich) were used without further purification. KNH2BH3 was synthesized according to the procedure reported by Diyabalanage et al.18 All the samples were handled in an Ar-purified glovebox with oxygen and water contents of
