Hydrogen Storage Properties of New Hydrogen-Rich BH3NH3-Metal

Article pubs.acs.org/JPCC

Hydrogen Storage Properties of New Hydrogen-Rich BH3NH3-Metal Hydride (TiH2, ZrH2, MgH2, and/or CaH2) Composite Systems Young Joon Choi, Yimin Xu, Wendy J. Shaw, and Ewa C. E. Rönnebro* Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States ABSTRACT: Ammonia borane (AB = NH3BH3) is one of the most attractive materials for chemical hydrogen storage due to its high hydrogen contents of 19.6 wt. %; however, impurity levels of borazine, ammonia, and diborane in conjunction with foaming and exothermic hydrogen release calls for finding ways to mitigate the decomposition reactions. In this paper we present a solution by mixing AB with metal hydrides (TiH2, ZrH2, MgH2, and CaH2) which can control impurity levels impurity levels from AB upon decomposition. The composite materials were prepared by mechanical ball milling, and their H2 release properties were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The formation of volatile products from decomposition side reactions, such as borazine (N3B3H6) was determined by mass spectrometry (MS). Sieverts type pressure−composition−temperature (PCT) gas−solid reaction instrument was adopted to observe the kinetics of the H2 release reactions of the combined systems and neat AB. In situ 11B MAS NMR of AB/MgH2/TiH2 revealed for the first time a competing decomposition pathway via cyclic −BH2NH2− species, previously only observed in solution. We found that by adding specific metal hydrides to AB we can eliminate the impurities and mitigate the heat release. Ammonia borane (NH3BH3, referred to as “AB” hereafter) has recently received considerable attention because of its combined low molecular weight (30.86 g/mol) and high gravimetric storage capacity (19.6 wt.%). The dehydrogenation of AB occurs in three steps that are generally represented by the transformation of AB to polyaminoborane (PAB, (H2NBH2)x) and H2, PAB to polyiminoborane (PIB, (HNBH)x) and H2, and PIB to BN and H2.7−9 These steps can be described by the following eqs 1−3, respectively. The two first steps release ∼16 wt % at PEM fuel cell relevant temperatures.7−9

1. INTRODUCTION Hydrogen is widely regarded as the most promising alternative energy source to replace fossil fuels as a clean energy carrier. It can be produced from a variety of renewable sources and yields a nonpolluting product as waste, i.e., water. However, a major challenge for using hydrogen as a fuel today, especially for vehicles, is to develop efficient and effective methods for hydrogen storage that can not only store hydrogen safely but also supply it where and when it is needed. Because hydrogen is a gas under practical conditions, it is difficult to store it compactly and safely. Hydrogen storage systems developed so far are liquid hydrogen, compressed gas cylinders, and solid-state storage materials. Compared to the physical approaches such as liquefaction and compression, hydrogen storage in the solid state has merits in terms of high volumetric and gravimetric hydrogen contents and, most importantly, safety.1 Thus, the development of solid-state hydrogen storage materials that will work at sufficiently low temperatures has been recognized as a key requirement for hydrogen PEM fuel cell applications.1 Substantial progress has been made in the past few years in the discovery of new materials.2 Chemical hydrides are hydrogen storage materials that typically release large amounts of hydrogen at low temperatures (500 °C)

(3)

The potential for practical use of AB as a hydrogen storage source is limited by the poor rate of hydrogen release, especially at temperatures below 100 °C,10−15 coproduction of volatile byproduct, such as diborane (B2H6) and borazine (c-(NHBH)3), and the lack of energy- and cost-effective methods for spent fuel regeneration.16−18 Therefore, many researchers have focused on additives such as nanoscaffolds,11 acid catalysts,14 transition metal,19 hexagonal boron nitride,20 carbon cryogels,21 and hydrolysis22,23 in order to improve the dehydrogenation performance of AB. However, to the best knowledge of the authors, there is no single approach that can overcome all of these challenges. In more recent studies, efforts have been addressing chemical modifications of AB through replacing one of its hydrogens Received: October 31, 2011 Revised: March 14, 2012 Published: March 21, 2012 8349

dx.doi.org/10.1021/jp210460w | J. Phys. Chem. C 2012, 116, 8349−8358

The Journal of Physical Chemistry C

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properties of the mixtures were examined by the use of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) on a TGA/DSC STA 449 Jupiter Netzsch instrument equipped with an Aelos QMS 403C MS. The MS uses a standard electron impact ionization detector. Typically, the samples were loaded in the glovebox into alumina crucibles equipped with a lid with a 60 μm hole, transferred to the TGA/DSC/MS instrument in which ∼4 mg samples were heated under flowing argon (∼25 mL/min) up to 180 °C at a predetermined heating rate. The hydrogen release/uptake properties of the milled mixtures were further evaluated by using a commercial Sieverts’ type apparatus (PCT Pro-2000, Setaram) upon heating to 100−350 °C at a heating rate of 10−13 °C/min. About 40 mg of the milled sample is loaded into a stainless steel container as loose powder, which is then sealed inside the PCT autoclave in the glovebox. Hydrogen pressures were measured by a Teledyne Taber model 206 piezoelectric transducer, 0−20 MPa, with a resolution of 10−4 MPa. During dehydrogenation, the sample temperature and applied pressure were monitored and recorded by a Lab View-based software program. The amount of hydrogen release was calculated by the pressure changes in calibrated volumes, of which the details are described elsewhere.33 2.3. X-ray Diffraction. The identification of the products after milling and dehydrogenation was carried out using an X’Pert Philips multipurpose diffractometer (XRD) with a fixed Cu anode (λ = 1.5406 Å) operating at 45 kV and 40 mA. Each sample was mounted in the glovebox in an in-house made sample holder consisting of a stainless steel plate covered with a dome made of Kapton tape. The X-ray intensity was measured over a 2θ range from 10° to 70° with a scanning rate of 0.2 °/s. 2.4. Nuclear Magnetic Resonance Spectroscopy. A milled sample of 2AB+MgH2+0.1TiH2 was investigated by in situ 11B MAS NMR measurements. NMR samples were packed into a boron-free zirconia rotor with a pinhole in the top of the rotor cap to release hydrogen, preventing pressure from building during decomposition. Samples were packed inside of the glovebox. The temperature was calibrated by following the chemical shift of 207Pb in Pb(NO3)2 as a function of temperature similar to the experiment described by Stowe et al.8 11 B spectra were referenced to NaBH4 (−41 ppm). Experiments were performed at 7.1 T, 300 MHz 1H frequency, using a 5 mm Chemagnetics 2-channel variable temperature probe and a Chemagnetics Infinity console.

with alkali or alkali earth element to form metal amidoborane (MAB).24−29 Xiong et al.25 reported that alkalimetal amidoboranes, such as LiNH2BH3 (LiAB) and NaNH2BH3 (NaAB) crystallizing in orthorhombic space group Pbca, release large amount of hydrogen (10.9 wt. % for LiNH2BH3 and 7.5 wt. % for NaNH2BH3) without borazine formation at 91 °C. On the other hand, Kang et al.30 reported that mechanical milling with magnesium hydride (MgH2) can significantly improve the dehydrogenation properties of AB in terms of the kinetic aspects. More specifically, the dehydrogenation reaction of the AB/0.5MgH2 becomes considerably less exothermic compared to neat AB without formation of undesired volatile byproduct. Recently, we have been exploring a variety of hydrides mixed with AB to find ways to improve sorption properties and to reduce impurity levels. In particular, introducing the combination of binary hydrides to AB is not only aiming at the rate enhancement of H2 release from AB, but also to improve the purity of gaseous dehydrogenated products. Herein, we describe the preparation and characterization of the hydrogen storage properties of the combined systems of AB-MH (MH = TiH2, ZrH2, MgH2, and CaH2) utilizing thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mass spectrometry (MS) and pressure−composition−temperature (PCT) Sieverts apparatus. Materials phase content was verified by X-ray diffraction (XRD). To better understand the dehydrogenation mechanism for AB under influence of a metal hydride, we performed in situ 11B MAS NMR measurements. As described in the sections below, we found the most favorable AB+MH system reported so far. The dehydrogenation temperature and enthalpy of AB were significantly reduced without measurable detection of byproduct when MgH2/0.1TiH2 was added and we propose a changed reaction mechanism in section 3.2.

2. EXPERIMENTAL APPARATUS AND PROCEDURE 2.1. Mechanical Alloying. The starting materials, ammonia borane (NH3BH3, 97%) from JSC Aviabor (Nizhny Novgorod region, Russia), magnesium hydride (MgH2, hydrogen storage grade) and calcium hydride (CaH2, 99.9%) from Sigma-Aldrich (Milwaukee, WI), zirconium hydride (ZrH2, 97%, grade G) from Chemetall (Frankfurt), and titanium hydride (TiH2, 99%) from Alfa-Aesar, were used as received without any further purification. All of the material handling was carried out in a glovebox filled with purified argon, which can keep a low oxygen concentration (less than 1 ppm) by a recycling purification system to protect samples and starting materials from oxidation and/or hydroxide formation. Approximately 1.0 g mixtures of AB and MH (MH = TiH2, ZrH2, MgH2, and CaH2) in various molar ratios and combinations were prepared using SPEX vibratory ball mill (SPEX SamplePrep 8000 M Mixer/Mill, 1725 rpm) under argon atmosphere. It is known that the temperature during the continuous high energy ball-milling can rise above 200 °C,31,32 and this condition would cause the decomposition of AB. Thus, we performed 8 cycles of 15 min milling followed by 45 min rest per each cycle in order to avoid temperature build-up, minimize the loss of H2, and achieve a uniform mixture. The ratio of ball to powder in the mill was 21: 1 by weight, and the total milling time per each sample was 2 h. The milling jar, with an inner volume of 35 mL, was sealed by a Viton-type O-ring, which kept the inside atmosphere inert during milling. Six stainless steel balls with 1/8 in. diameter were used. 2.2. Thermal and Kinetic Hydrogen Release Properties and Mass Spectrometry. The hydrogen release

3. RESULTS AND DISCUSSION 3.1.1. Dehydrogenation Properties of the AB-MH (MH = TiH2, ZrH2, MgH2, and CaH2) Systems. AB-TiH2 Systems. The first AB-MH system to be discussed is the mixing of TiH2 with AB. Ti is a well- known catalyst for complex metal hydrides.34 It has been reported that a small amount of doping with a 3d-transition metal catalyst, especially TiH2 nanoparticles, induced by the mechanical milling, affects not only the thermodynamics of dehydrogenation but also the kinetics of H2 release/uptake of MgH2.35 Mechanical milling has been found to be an effective way to improve the kinetic rates of metal hydrides. For instance, the formation of nanocrystalline or amorphous structures produced by milling results in considerable changes in hydrogenation properties, especially by eliminating the need for activation and improving hydrogenation and dehydrogenation kinetics.36 Therefore, it is anticipated that the mechanical activation by TiH2 enhances hydrogen release of solid state AB. To demonstrate the effects 8350



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investigate the effects of TiH2 on the hydrogen release properties of AB. However, there is no indication under the current experimental conditions that the AB/0.1TiH2 and 3AB/TiH2 mixtures exhibit notable improvement compared to neat AB in terms of changing the onset or peak temperature. More specifically, the mixtures start to release hydrogen at about 114 °C and reach the maximum reaction rate at 122 °C, which are close to neat AB’s decomposition temperature. 3.1.2. AB-MgH2−TiH2 Systems. In the authors’ previous paper,38 it was demonstrated that nanosized Mg−Ti−H system synthesized by dual-planetary high-energy milling under 15 MPa hydrogen pressure enhanced H2 release properties of MgH2. More specifically, the decomposition temperature of MgH2−TiH2 mixtures varies as a function of the TiH2 content, and the lowest onset temperature (∼110 °C) based on TG analysis was achieved when MgH2/TiH2 molar ratio was 10:1. We also discovered that the 10MgH2/TiH2 system can be rehydrogenated under high-hydrogen pressures at room temperature which would be practical for regeneration of the mixture with AB.39 Inspired by these results, we aimed at preparing a new combined system of AB/MgH2/TiH2 in 2:1:0.1 molar ratio (referred to as 2AB/ MgH2/0.1TiH2) as an optimal composition for the dehydrogenation properties of AB. In other words, our hypothesis was that the exothermicity of AB dehydrogenation could be compensated with the endothermic effect in the MgH2/0.1TiH2, indicating that the overall enthalpy of AB dehydrogenation may become less exothermic by the addition of MgH2/0.1TiH2. Our results below, however, did not confirm this hypothesis, as will be discussed. In addition, the total amount of hydrogen release from the mixtures could potentially be increased due to the TiH2-doped MgH2 dehydrogenation within a temperature range of ∼200−250 °C. For reference, we investigated the AB/MgH2 system in different molar ratios (1:1, 2:1, and 1:2). We found that AB/ MgH2 mixtures have favorable dehydrogenation properties, especially if the molar ratio is 2:1, however, this was already discussed in a different study by Kang et al.,30 who reported that the rate of H2 release from AB can be increased by mechanical milling with MgH2 as an additive. Our results were similar to Kang et al.;30 however, our in situ NMR data shows different mechanism features compared to Kang et al.30 which will be further discussed in section 3.2. We will hereafter focus on the results of the 2AB/MgH2/0.1TiH2 system which compared to 2AB/MgH2 system has more favorable hydrogen release properties. To examine the effect of MgH2/0.1TiH2 on the AB dehydrogenation properties, Figure 2, panels A and B, shows the weight and enthalpy changes vs time. The TG/DSC/MS analyses were run under the same conditions as described above. First of all, the TG profiles in Figure 2A indicate that there are substantial differences in the onset dehydrogenation temperature and time for complete hydrogen release for the two samples (2AB/MgH2/0.1TiH2 and neat AB). The result shows that the first equivalent hydrogen started to release at ∼85 °C from the 2AB/MgH2/0.1TiH2 mixture, which is approximately 29 °C lower than that of neat AB. The weight loss from the 2AB/MgH2/0.1TiH2 accelerated at ∼95 °C and completed the first dehydrogenation step below 100 °C, which is much lower than the onset dehydrogenation temperature of neat AB. It shows that the temperature is considerably lower and the time is notably shorter to complete hydrogen release from the as-milled 2AB/MgH2/0.1TiH2 mixture than from the unmilled, neat AB, implying a significant improvement of the

of TiH2 on the dehydrogenation properties of AB, two mixtures of AB and TiH2 in molar ratios of 10:1 and 3:1 (referred to as AB/0.1TiH2 and 3AB/TiH2, respectively, hereafter) were milled for 2 h. It is noted that the degree of mixing and particle size reduction did not change much by increasing milling time beyond 2 h. The milled samples were subsequently analyzed using TG/DSC in combination with MS. Figure 1 presents a comparison of the thermal decomposition behavior for AB/0.1TiH2, 3AB/TiH2 and neat AB.

Figure 1. TG/DSC profiles for as-milled AB/0.1TiH2, as-milled 3AB/ TiH2, and neat AB with a heating rate of 5 °C/min up to 180 °C.

Weight and enthalpy changes as a function of time were measured while the samples were heated from room temperature to 180 °C under an argon atmosphere at a ramping rate of 5 °C/min. DSC profile for neat AB shows a sharp endothermic melting at 108 °C, followed by two exothermic effects with the peaks centering at ∼113 and ∼153 °C. They correspond to the two decomposition steps, which were described in section 1, associated with the formation of solid residues PAB (NH2BH2)x and PIB (NHBH)x, respectively.7−9 According to Wolf et al.,2 considerable amounts of borazine and diborane were coproduced during the hydrogen release reaction of neat AB, particularly at the second decomposition step. This qualitatively accounts for our present TG result of neat AB, which shows a significantly larger weight loss (46 wt. %) than the expected value (13 wt. %) upon releasing two equivalents pure hydrogen. The reason for the excessive weight loss is not only because of the volatile species from the side reactions, but also because of the foaming and buoyancy effect. Note in Figure 1 that a visible mass increase at ∼120 °C was repeatedly observed in the TG measurements of neat AB, which is attributed to the foaming of molten AB. On the other hand, the TG/DSC results show that the area of exothermic effects for the AB/0.1TiH2 and 3AB/TiH2 mixtures become gradually smaller relative to neat AB, which results in lower hydrogen storage capacity. This is further confirmed by the fact that integration of each exothermic peak in the AB/0.1TiH2 and 3AB/ TiH2 mixtures gave the values of heat of reaction of −22.0 ± 0.4 and −17.8 ± 0.5 kJ/mol H2, respectively. Note that the sample weight was quite similar and the heat of reaction was obtained from the integration of the peak based on only the AB dehydrogenation in these two cases. TiH2 is not known to release hydrogen until above 400 °C.37 Thus, it can be concluded that the reaction enthalpy of the first step hydrogen release becomes less exothermic as more TiH2 is added. The beneficial influence of TiH2 on AB prompted further work to 8351



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the dehydrogenation process to damage the membrane. Although their direct quantitative measurements are yet to be performed, any borazine, diborane and ammonia possibly generated are likely to be suppressed by MgH2/0.1TiH2 through a different decomposition mechanism as discussed in section 3.2 on in situ NMR analysis. 3.1.3. AB-MgH2−ZrH2 and AB-MgH2−TiH2−CaH2 Systems. The above results demonstrated that the hydrogen release reaction of AB can occur without the formation of volatile species from side reactions and become significantly less exothermic than that of neat AB by the addition of MgH2/ 0.1TiH2. In order to further understand the effect of TiH2 on the dehydrogenation properties and impurity levels, various combinations including ZrH2 and CaH2 based on the 2AB/ MgH2 system were investigated. These mixtures were milled under the same condition as 2AB/MgH2/0.1TiH2, and their dehydrogenation properties were investigated by TG/DSC/MS analyses. First of all, the results of 2AB/MgH2/0.1ZrH2 are shown in Figure 3, which includes those of 2AB/MgH2/0.1TiH2 for

Figure 2. (A) TG/DSC curves for as-milled 2AB/MgH2/0.1TiH2 and neat AB, and (B) synchronous MS profiles of m/e = 2 (H2), m/e = 17 (ammonia, NH3), m/e = 27 (diborane, B2H6), and m/e = 80 (borazine, c-(NHBH)3) under argon at a heating rate of 5 °C/min up to 180 °C. The solid and dashed lines correspond to as-milled 2AB/ MgH2/0.1TiH2 and neat AB, respectively.

kinetics compared to neat AB. The total weight loss amounted to 7.5 wt. % of the initial weight, which takes place within the temperature range from 85 to 100 °C. The experimental weight loss is close to the theoretical hydrogen release of 8.6 wt. % from reaction 1 and 2. The amount of released hydrogen using the gravimetric analysis (TGA) is further confirmed by the volumetric measurements (PCT), which will be introduced in section 3.3. Second, the dehydrogenation reaction of the 2AB/MgH2/ 0.1TiH2 mixture becomes significantly less exothermic than that of neat AB, hence, it appears that a stabilization mechanism takes place. By comparing our AB/MH mixtures with neat AB, we can get a relative understanding of the impact of metal hydride addition to ammonia borane. According to the DSC results of the 2AB/MgH2/0.1TiH2, shown in Figure 2A, the reaction enthalpy of the first dehydrogenation step obtained by integrating the exothermic peak is −8.7 ± 0.2 kJ/mol H2, which is ∼14 kJ/mol H2 less exothermic than that of neat AB. Therefore, the heat released from the dehydrogenation of AB becomes substantially smaller when MgH2/0.1TiH2 was applied in comparison with the sample with only TiH2 added. Again, it is noted that the heat of reaction was obtained from the integration of the peak based on only the AB dehydrogenation in this case. Finally, the MS spectra in Figure 2B demonstrate that no measurable coproduction of the volatile byproduct was detected throughout the thermal decomposition process of the 2AB/ MgH2/0.1TiH2 mixture. One of the concerns for considering AB as a chemical hydrogen storage material for fuel-cell-powered applications is the possibility of undesirable byproduct during

Figure 3. (A) TG/DSC curves for as-milled 2AB/MgH2/0.1ZrH2 and as-milled 2AB/MgH2/0.1TiH2 and (B) synchronous MS profiles of m/e = 2 (H2), m/e = 17 (ammonia, NH3), m/e = 27 (diborane, B2H6), and m/e = 80 (borazine, c-(NHBH)3) under argon at a heating rate of 5 °C/min up to 180 °C. The solid and dashed lines correspond to asmilled 2AB/MgH2/0.1ZrH2 and as-milled 2AB/MgH2/0.1TiH2, respectively.

comparison. As seen in Figure 3A, the onset dehydrogenation temperature of 2AB/MgH2/0.1ZrH2 is similar to 2AB/MgH2/0.1TiH2. In addition, the reaction enthalpy of the first dehydrogenation step, −8.5 ± 0.2 kJ/mol, is similar to that of 2AB/MgH2/0.1TiH2 when TiH2 is replaced by ZrH2. However, according to the combined 8352



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the presence of MgH2/0.1TiH2 is an important factor for lowering the desorption enthalpy, but also eliminating, or at least suppressing, the volatile byproduct in the thermal decomposition process. The reaction products before and after dehydrogenation will be further investigated in the following section to better elucidates the effect of MgH2/0.1TiH2 on the AB thermal decomposition. 3.2. Analysis of Chemical Composition and Reaction Mechanism by ex Situ XRD and in Situ NMR. X-ray powder diffraction was used to analyze phase composition of the samples after mechanical alloying and after dehydrogenation. Crystalline phases were identified by comparing the experimental data with JCPDS files. The XRD patterns in Figure 5A

MS analysis results shown in Figure 3B, considerable coproduction of borazine and diborane were measured during the thermal decomposition process up to 180 °C for the 2AB/MgH2/ 0.1ZrH2 sample. Thus, TiH2 is a better choice than ZrH2 to improve hydrogen release properties of AB. Second, CaH2 was added to the 2AB/MgH2/0.1TiH2 mixtures, and the results were compared with those of 2AB/ MgH2/0.1TiH2. It is noted that the addition of CaH2 has been found to lower dehydrogenation enthalpy of AB considerably along with suppressing the formation of diborane and borazine in our preliminary work (not shown). Zhang et al.40 found that addition of CaH2 destabilizes AB and thus makes it more difficult to control hydrogen release rates. As shown in Figure 4A, TG and

Figure 4. (A) TG/DSC profiles for as-milled 2AB/MgH2/0.1TiH2/ 0.1CaH2 and as-milled 2AB/MgH2/0.1TiH2 and (B) synchronous MS profiles of m/e = 2 (H2), m/e = 17 (ammonia, NH3), m/e = 27 (diborane, B2H6), and m/e = 80 (borazine, c-(NHBH)3) under argon at a heating rate of 5 °C/min up to 180 °C. The solid and dashed lines correspond to as-milled 2AB/MgH2/0.1TiH2/0.1CaH2 and as-milled 2AB/MgH2/0.1TiH2, respectively.

Figure 5. XRD patterns of 2AB/MgH2/0.1TiH2: (A) after milling and after dehydrogenation at 200 °C for 2 h. Circle is AB, M is MgH2 ,and T is TiH2. (B) Fresh sample and after 3 months.

DSC curves are almost identical and there are no substantial differences in the onset temperature and weight loss between the two samples. The reaction enthalpy of the first dehydrogenation step from the 2AB/MgH2/0.1TiH2/0.1CaH2 (−8.1 ± 0.1 kJ/mol) is slightly less exothermic than that of 2AB/MgH2/ 0.1TiH2. However, the MS results shown in Figure 4B indicate formation of undesired volatile byproduct, i.e., diborane and ammonia. Thus, the addition of CaH2 did not further enhance the hydrogen release properties of AB. In summary, the above TG/DSC/MS analyses of the ABMH (MH = TiH2, ZrH2, MgH2, and CaH2) systems show that enthalpy and impurity levels are dependent on the choice of combinations of metal hydrides mixed with AB. We found that

revealed that no chemical reaction occurred during mechanical milling and only starting materials; that is, AB and MH were detected. After dehydrogenation by heating up to 200 °C and holding for 2 h, the XRD pattern only contained the metal hydrides with no traces of AB, indicating decomposition of AB, but not of the metal hydrides. However, if the cooling intervals during milling are too short, AB will decompose as evidenced by the presence of diammoniate of diborane (DADB), [(NH3)2BH2]+[BH4]−, in the NMR data. Since NMR-data indicated decomposition products forming during ball milling procedures that were allowed to reach higher temperatures, we decided to investigate the stability of our composite materials. Therefore, we also performed an 8353



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Table 1. Chemical Shifts from 11B MAS-NMR Observed during in Situ Heating of Mechanically Milled 2AB/MgH2/ 0.1TiH2a

XRD-analysis of a 2AB/MgH2/0.1TiH2 sample (which was not allowed to become heated during ball milling) after it is been kept in the glovebox for three to four months. Interestingly, three minor peaks were observed in the 2θ-range of 16−22° as is shown in Figure 5B. We were not able to identify these diffraction peaks from a JCPDS database search, but, we find it reasonable to conclude that they are due to decomposition, thus, indicating that the addition of MgH2/TiH2 to AB enhances decomposition of AB. To better understand the role of the addition of metal hydride on the hydrogen release properties of ammonia borane, we performed an in situ 11B MAS NMR experiment on the 2AB/MgH2/0.1TiH2 mixture to monitor the transformation of AB into its decomposition products under possible influence of the metal hydride addition. The sample was analyzed shortly after sample preparation/milling and shortly after the XRDanalysis to exclude presence of any decomposition products at room temperature. We chose a heating temperature of 75 °C to obtain a slow enough reaction rate to allow monitoring all events. The spectra are shown in Figure 6 and the identities of the chemical shifts are summarized in Table 1. It is noteworthy

peak

chemical shift (ppm)

BHx species

identity

1 2 3 4 5 6 7 8 9

−25.0 −31.1 −22.7 −39.9 −11.0 −6.2 −25.3 −20.8 −6.5

BH3 BH3 BH3 BH4 BH2 BH BH3 BH3 BH2

AB AB new phase AB (AB*) DADB BCDC and/or linear dimer BCDB BCDB linear dimer DADB

a

The peak numbers are consistent with the labels in Figure 6.

how much faster the decomposition of AB mixed with MgH2/ 0.1TiH2 is compared to previously published in situ NMR experiments on neat AB. As can be seen in Figure 6, DADB started to form already after 6 min at 75 °C. Stowe et al.8 reported the formation of DADB after 14 min at 88 °C. Hydrogen is known to be released after formation of DADB. If we take a closer look at the series of spectra of 2AB/ MgH2/0.1TiH2 and chemical shifts (δ) in Figure 6, the BH3 groups from AB can clearly be identified at room temperature at δ = −25.0 and −31.1 ppm corresponding to peaks 1 and 2. After 4 min at 75 °C, a large second peak is observed at −22.7 ppm, peak 3, which is consistent with “new phase AB” or AB*, a mobile phase of AB discovered by Stowe et al.8 and characterized by Shaw et al.41 AB* is a prerequisite for the formation of DADB. At 4 min of heating, a very small amount of DADB is observed at δ = −39.9 ppm (BH4) represented by peak 4, and accompanied by a broad BH2 peak at −11.0, which increases after 6 min of heating. The formation of DADB is the observed first step for hydrogen release from solid-state AB.8 As with the neat AB material, the metal hydride modified AB shows a measurable DADB resonance throughout the experiment. In contrast to neat AB, however, the DADB resonance continues to grow throughout our experiment with 2AB/ MgH2/0.1TiH2 mixture, possibly due to the lower temperature employed. As the metal hydride/AB starts to lose hydrogen, −BH2− polyaminoborane species appears, peak 5, as was previously observed for neat AB. However, there are also more prominent intermediate species that are somewhat different compared to the neat AB spectra reported by Stowe et al.8 When the original features have disappeared after 8 min, two other species are observed at −25.3 and −20.8 ppm, peaks 7 and 8, respectively. The species at −20.8 ppm is tentatively identified as BH3 from the linear dimer of AB based on comparison with the chemical shifts reported by Stowe et al.8 Luo et al.42 performed an in situ 11 B MAS NMR experiment at 75 °C of 2AB/MgH2 and in comparing our spectra with theirs, we can see that the general trend is similar; that is, AB is decomposing upon formation of DADB and intermediate polymerization products. Luo et al.42 tentatively assign a species at δ = −19.3 ppm to be a Mg−B− N−H intermediate complex due to possible interaction of magnesium hydride with ammonia borane. Based on our previous results from ex situ and in situ NMR experiments of AB both in solid state and in solution, we can explain the shift at ca. −20 ppm to be the linear dimer of AB and not from a Mg-containing intermediate complex.

Figure 6. In situ 11B MAS NMR spectra of 2AB/MgH2/0.1TiH2 at room temperature (RT) and 75 and 95 °C. 8354



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Shaw et al.43 discuss thermal decomposition of ammonia borane in solution and describe three cyclic −BH2NH2− species occurring during the decomposition pathway of AB, proceeding from isomerization of AB to DADB which cyclizes to form cyclodiborazane (CDB) and further forms cyclotriborazane (CTB) and B-(cyclodiborazanyl) aminoborohydride (BCDB). BCDB has three peaks at δ = −5.0, −10.9, and −23.4 ppm. Although our current experiment does not have sufficient resolution to allow us to distinguish between the BH2’s of CDB, CTB and BCDB, the observation of three resonances at δ = −6.2, −11.0, and −25.3 ppm is most consistent with BCDB. With the occurrence of both cyclic species and the linear dimer, it seems that the addition of MgH2/0.1TiH2 to AB promotes two competing decomposition pathways when heated, i.e., (1) AB reacting with DADB to form acyclic polyamineborane, similar to solid AB and (2) DADB decomposing into cyclic species, similar to AB in solution. As decomposition continues, peak 8 continues to grow. After 16 min, peaks 5 and 6 have broadened and peak 9 starts to grow, likely due to −BH2 from DADB. At 172 min, the distribution of peaks 3, 7, and 8 is nearly equal and thereafter decomposes simultaneously. To fully decompose the sample, we raised the temperature to 95 °C and within 20 min, the intermediates were gone, leaving DADB as the sole component in the spectra. We were able to tentatively assign the peaks based on reference to literature revealing a different mechanism of 2AB/MgH2/0.1TiH2 as relative to neat AB. In order to fully characterize the true nature of the intermediate species, carefully designed 11B and 15N NMR experiments are necessary. Shaw et al.43 discuss that the cyclization is much slower in the solid state and DADB reacts by a dehydrogenation pathway with the remaining AB. In an organic solvent, the ion pair [NH3BH2BH3]+[BH4]− is destabilized due to less polarity of the environment, and thus, the barrier to cyclization is lower. Interestingly, our in situ NMR experiment on 2AB/MgH2/ 0.1TiH2 suggests that the addition of MgH2/0.1TiH2 to AB also impacts stability of the ion pair which explains why we unexpectedly observe cyclic compounds, leading to a more rapid decomposition mechanism compared to solid state AB. Thus, based on the findings by Shaw et al and our current observations, we believe it is reasonable to conclude that DADB is destabilized to form CDB and hence forms cyclic species in a competing reaction pathway that enhances decomposition. We have no evidence for a Mg-containing intermediate complex. Instead, we propose that the addition of MgH2/0.1TiH2 to AB provides a chemical environment that significantly reduces the induction period and stimulates the formation of cyclic intermediates upon decomposition of metal hydride modified AB. Moreover, the addition of MgH2/0.1TiH2 is responsible for the elimination of impurities. The reaction mechanism for formation of borazine and ammonia is not known. We suggest that the elimination could be due to solid−gas recombination reactions involving MgH2/0.1TiH2 or the cyclic species being less prone to impurity formations as was indicated by Shaw et al.43 Shaw et al observed that borazine was not formed from CTB decomposition, thus, it may be that borazine forms mainly from DADB decomposition. In other words, if destabilizing DADB to form cyclic species, borazine formation may be suppressed. In the next section, we will discuss the role of MgH2/0.1TiH2 addition on AB dehydrogenation kinetics. 3.3. Kinetics of the Dehydrogenation Reactions. As discussed in section 3.1 and verified by the in situ NMR experiments described in section 3.2, the kinetics of AB dehydrogenation

has been found to be improved significantly by the addition of MgH2 + 0.1TiH2. In order to further verify the effects of additive, nonisothermal runs were performed to determine the activation energy for dehydrogenation reactions by using the Kissinger’s method. The method is based on the following rate equation: ⎡ ⎛ β ⎞⎤ ∂⎢ln⎜ 2 ⎟⎥ ⎢⎣ ⎝ Tm ⎠⎥⎦ E =− a ⎛ 1 ⎞ R ∂⎜ ⎟ ⎝ Tm ⎠

(5)

where Tm is the temperature at which the maximum reaction rate peaks, β the heating rate, R the gas constant, Ea the activation energy. According to the results of temperature desorption-mass spectroscopy (TD-MS) profiles of the neat AB, as-milled 2AB/MgH2 and 2AB/MgH2/0.1TiH2 mixtures, the peak temperatures for the maximum reaction-rate have been determined at various heating rates from 1 to 10 °C/min (not shown). Based on eq 5, the Kissinger plots, i.e., ln(β/Tm2) as a function of the inverse of Tm, can be given for the three samples. The activation energies, Ea, are then evaluated from the slope (Ea/R) of the fitted line. Figure 7 presents the results

Figure 7. Activation energy (Ea) for the dehydrogenation of neat AB, as-milled 2AB/MgH2 and as-milled 2AB/MgH2/0.1TiH2.

for the dehydrogenation of neat AB, as-milled 2AB/MgH2, and as-milled 2AB/MgH2/0.1TiH2. The results show that the activation energy (Ea) for the dehydrogenation of the as-milled 2AB/MgH2/0.1TiH2 is approximately 98.6 kJ/mol, which is much lower than the Ea determined for neat AB (143.6 kJ/mol) and 2AB/MgH2 (131.9 kJ/mol), respectively. Thus, on the basis of this activation energy study, it can be concluded that the as-milled mixtures of 2AB/MgH2/0.1TiH2 shows considerably enhanced properties in terms of the dehydrogenation kinetics compared with the neat AB, as-milled AB and 2AB/ MgH2. In order to further investigate the H2 release kinetics, dehydrogenation of neat AB, as-milled 2AB/MgH2, and asmilled 2AB/MgH2/0.1TiH2 were performed using the Sieverts apparatus, and the detailed results are given below. Temperature-programmed desorption (TPD) is a common technique for surveying the overall hydrogen storage behavior of solid hydride materials. During TPD, temperature is ramped up at a constant heating rate. The pressure change during the temperature ramping is measured in a calibrated volume and converted to corresponding percentages of hydrogen release. 8355



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It should be noted that a cold trap filled with dry ice was adopted between the pressure transducer and the sample holder in order to capture undesirable byproduct such as borazine (N3B3H6) or diborane (B2H6) during the dehydrogenation process. Thus, the pressure change is considered not to be affected by the small pressure error that arises upon release of volatile byproduct from neat AB. Figure 8 shows the TPD profiles of the milled mixtures of 2AB/MgH2/0.1TiH2 at various temperatures. Approximately

Figure 9. TPD profiles of hydrogen desorption from neat AB (orange), as-milled 2AB/MgH2 (red), and as-milled 2AB/MgH2/ 0.1TiH2 (blue) at 100 °C. The TPD and temperature profiles are given as solid and dotted lines, respectively.

Figure 8. TPD profiles of hydrogen desorption from as-milled 2AB/ MgH2/0.1TiH2 at 100 (red), 160 (blue), and 200 °C (orange). The TPD and temperature profiles are given as solid and dotted lines, respectively.

50 mg samples were heated under atmospheric pressure up to 100−200 °C at a heating rate of 10−13 °C/min, thereafter held at those temperatures for a predetermined length of time. The total weight loss up to 100, 160, and 200 °C amounted to ∼4.8, 7.1, and 10.1 wt. % of the initial weight, respectively, within 6 h. More importantly, the mixtures release nearly 90% of hydrogen during the first 30 min by the time the temperature reaches 160 and 200 °C. The observed mass loss of 7.1 wt. % at 160 °C, determined by this volumetric measurement agrees well with the result from TG-gravimetric measurement, i.e., 7.5 wt. %. The results also indicate that the hydrogen release rates of 2AB/MgH2/0.1TiH2 accelerated significantly as the target temperatures increased from 100 to 200 °C. Note that the acute fluctuation of the TPD profiles at 100 and 160 °C is attributed to the rapid release of the confined hydrogen rather than an indication of dehydrogenation steps. Recently, Wang et al.18 reported that some released hydrogen may be confined in the bubbles formed when sample foaming occurs in the dehydrogenation, and when these unstable bubbles break, the rapid release of the captured hydrogen causes the sharp fluctuation in the TG profile. The hydrogen release kinetics of the AB with MgH2 or MgH2/0.1TiH2 destabilizer compared to neat AB are shown in Figures 9 and 10. Figure 9 presents the plot of TPD profiles as a function of time for neat AB, as-milled 2AB/MgH2 and as-milled 2AB/ MgH2/0.1TiH2. When the comparison was made at 100 °C, there are substantial differences in the onset temperature of dehydrogenation and the time required to release the same amount of hydrogen among those samples. The result shows that the hydrogen release started at about 84 °C (at 13 °C/min heating rate) and the weight loss amounted to ∼3.1 wt. % of the initial mass within 1 h for the as-milled 2AB/MgH2/ 0.1TiH2. Although the onset temperature of the neat AB drops

Figure 10. TPD profiles of hydrogen desorption from neat AB (orange), as-milled 2AB/MgH2 (red), and as-milled 2AB/MgH2/ 0.1TiH2 (blue) at 200 °C. The TPD and temperature profiles are given as solid and dotted lines, respectively.

significantly from 117 to 82 °C when MgH2 was added, a total of 3.0 wt. % hydrogen was released from the as-milled 2AB/MgH2 during 6 h. These results indicate the beneficial effect of MgH2/0.1TiH2 on reducing induction period for AB decomposition from around 40 min to less than 16 min at 100 °C. As shown in Figure 10, upon further elevating the operation temperature to 200 °C, both samples of neat AB and as-milled 2AB/MgH2/0.1TiH2 exhibit a distinctive two-step decomposition feature.7−9 On the other hand, the as-milled mixture of 2AB/MgH2 show relatively obscure dehydrogenation steps. The measured hydrogen capacity of neat AB is comparable to the theoretical value for a two-step desorption, 16.2 wt. %, and the capacities of 2AB/MgH2 and 2AB/MgH2/0.1TiH2 are 9.7 and 10.1 wt. %, respectively, within 6 h at 200 °C. It is noteworthy that approximately 90% of hydrogen was released within 30 min based on the mass loss from the neat AB and 2AB/MgH2/0.1TiH2, and their maximum reaction rates are 0.49 mg/s 1 g AB. The key conclusion from the experiments in this section is that it appears that MgH2/0.1TiH2 addition to AB enhances kinetics of decomposition by acting as a catalyst to lower the activation energy barrier. Moreover, reducing exothermicity, i.e., stabilization of AB, is important for practical applications to 8356



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avoid a potential “run-away” reaction of a self-propagated exotherm reaction. To put our results in a context and to compare the accuracy of our results with similar investigations, we would like draw attention to a study by Rongeat et al.47 It has been reported that DSC measurements can provide thermodynamic data such as reaction enthalpy as well as kinetics properties from activation energy determinations.44−47 It is a nonequilibrium measurement, meaning that directly exchanged heat does not correspond to the “true” value of reaction enthalpy. Thus, an interpretation of the DSC results in this work may include potential limitations on the interpretation of the data. Nevertheless, approximate reaction enthalpies can be extracted from DSC measurements to evaluate new compounds such as the AB-MgH2−TiH2 system. By comparing DSC data with TPD (temperature programmed desorption) data, we learned that the enthalpy and activation energy measurements can be considered accurately estimated. Rongeat et al.47 made a similar conclusion on determining enthalpies of reversible metal hydride materials and found that DSC data correlates well with TPD data taken up on a Sieverts apparatus. This is an important finding since a Sieverts analysis is lengthy and not a common equipment, whereas DSC is usually available and a faster method to estimate enthalpies and to extract kinetics. It is worthwhile to point out that the study by Rongeat et al. was performed on reversible metal hydrides under hydrogen pressures, whereas our study was performed on a nonreversible chemical hydride under argon flow. The AB decomposition reaction is an irreversible chemical nucleation and growth reaction that occurs with hydrogen release. It is important to note that our present study shows that the DSC data correlates well with the TPD data. A major difference between reversible metal hydrides and exothermic chemical hydrides is that Sieverts data of a reversible metal hydride can be performed under equilibrium conditions by dosing hydrogen in aliquots to collect isotherms at a set temperature. For a chemical hydride, the reaction is exothermic and to some extent self-propagating; thus, temperature will not be fixed and equilibrium states are difficult to obtain, and to our knowledge, performing isotherms is most difficult. Finally, we find it intriguing that the a MgH2/0.1TiH2 addition appears to have multiple impacts on AB decomposition: (1) destabilize DADB to form CDB in a different reaction pathway previously only observed in solution; (2) eliminate formation of gaseous impurities, i.e., borazine, diborane, and ammonia; (3) reduce exothermicity, i.e., stabilize AB; and (4) act as a catalyst to lower activation energy barrier for AB decomposition.

AB was significantly reduced without measurable detection of byproduct when MgH2/0.1TiH2 was added. In situ 11B MAS NMR measurements suggested a competing reaction mechanism for metal hydride modified AB via cyclic −BH2NH2− species which reduces the induction period and eliminates impurities.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (509) 372-6877. Fax: (509) 375-4448. E-mail: ewa.ronnebro@ pnnl.gov. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. This work was performed as part of the Center of Excellence in Chemical Hydrogen Storage. We appreciate fruitful discussions with T. Autrey, A. Karkamkar and J. Holladay at PNNL. A portion of the research described in this paper was performed in the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for U.S. DOE by Battelle.

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REFERENCES

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4. CONCLUSIONS In the present work, various combinations of Ammonia BoraneMH (MH = TiH2, ZrH2, MgH2, and CaH2) mixtures were prepared by ball milling and their dehydrogenation properties were investigated by the use of TG/DSC/MS, XRD, NMR, and Sieverts apparatus. We found that the choice of combinations of AB and MH is a way to reduce the exothermicity of the dehydrogenation reaction of AB and to possibly eliminate impurities. Moreover, according to the TG/DSC/MS data the 2AB/MgH2/0.1TiH2 mixture has an activation energy of 98.6 kJ/mol and a heat of reaction of −8.7 kJ/mol H2, which is lower by about 45 and 14 kJ/mol H2 respectively than those of neat AB. It has been determined that hydrogen release from AB occurs at a lower temperature and decomposition enthalpy of 8357



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Hydrogen Storage Properties of New Hydrogen-Rich BH3NH3-Metal

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