Ammonia borane–metal alanate composites: hydrogen desorption properties and decomposition processes

Abstract

Hydrogen desorption properties and decomposition processes of NH₃BH₃–MAlH₄ (M = Na, Li) composites were investigated by using thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses. We prepared the composites by ball-milling and the mixtures by hand-milling. The ball-milled composites desorbed 4–5 wt% hydrogen at three exothermic steps below 260 °C. The emissions of by-product gases, NH₃, B₂H₆ and B₃H₆N₃, were effectively suppressed. From XRD analysis, the formation of a mixed-metal (Na(Li), Al) amidoborane phase was suggested. Very different results were obtained using hand-milling. They showed only one exothermic reaction at 80–90 °C. The emission of by-product gases was not suppressed. By comparing the differences between ball-milled composites and hand-milled mixtures, the importance of mixed-metal amidoborane in this system was proposed.

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1. Introduction

Hydrogen storage is a big challenge for a future hydrogen energy society. The U.S. Department of Energy (DOE) introduced a set of technical targets for on-board hydrogen storage systems in 2003.¹ Then, these were revised to the new targets in 2009.² Ultimate targets for system gravimetric and volumetric capacities of hydrogen were set to 7.5 wt% and 70 g L⁻¹, respectively.² These values are based on the tank-system, which takes into account the weight and volume of all of tank components. Therefore, capacities on a materials-basis should be much larger than those on a system-basis.³ Hereafter, hydrogen capacities on a materials-basis are described. Many kinds of candidate hydrogen storage materials have been investigated, such as interstitial metal hydrides, complex hydrides, chemical hydrides and adsorbents.⁴ For example, sodium alanate (NaAlH₄) is one of complex hydrides and has a reversible hydrogen capacity of 5.6 wt%.⁴

Ammonia borane (NH₃BH₃, AB) is one of chemical hydrides and attracts much attention as hydrogen storage materials. AB has high hydrogen capacities (19.6 wt%, 145 g L⁻¹, respectively) and desorbs hydrogen in a relatively low temperature range.⁵ Nevertheless, sluggish kinetics below 100 °C, poor recyclability, and emission of by-product gases during heating (e.g., ammonia (NH₃), diborane (B₂H₆) and borazine (B₃H₆N₃)) are disadvantages for practical applications.^6–8 For instance, release of ammonia causes damage to the fuel cell performance even at trace levels.⁹ Also, NH₃ and B₂H₆ are toxic materials for living things.^10,11

To overcome these disadvantages, several approaches have been developed, such as infusion of AB in nanoscaffolds,¹² doping with transition metals as catalysts,¹³ size and catalytic effects from graphitic carbon nitride,¹⁴ and chemical modification of AB by replacing one of H atoms with an alkali or alkaline earth metal to form metal amidoboranes.¹⁵ In previous reports, many kinds of AB–MH (Metal Hydride) composites, such as AB–LiH,^15–20 AB–NaH,^{15–17,21,22} AB–KH,^16,17,23 AB–MgH₂,^24–26 AB–CaH₂,²⁴ AB–LiNH₂,²⁷ AB–LiBH₄,²⁸ and AB–Li₃AlH₆,²⁹ were synthesized and their dehydrogenation properties were investigated. Recently, AB–amine metal borohydride composites, which showed superior dehydrogenation properties, were also reported.^30,31 In our previous report, we experimentally verified that AB–MAlH₄ (M = Na, Li) composites, which were prepared based on the indicator we proposed, can suppress the emission of NH₃, B₂H₆ and B₃H₆N₃.³² However, their decomposition processes have not been clarified yet.

In this study, we investigated the decomposition processes of AB–MAlH₄ (M = Na, Li) composites. We prepared the composites by ball-milling and the mixtures by hand-milling. We analysed the hydrogen desorption properties by thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS) and performed phase identification by powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. By comparing the ball-milled composites and hand-milled mixtures, the decomposition processes were proposed.

2. Experimental

The starting materials NH₃BH₃, NaAlH₄, LiAlH₄ (purity 97%, 90%, 95%, respectively) were purchased from Sigma Aldrich Co. Ltd. These materials were used as-received without any purification. All samples were handled in an argon-filled glovebox to prevent sample oxidation. AB–MAlH₄ (M = Na, Li) composites were prepared by ball-milling of AB and MAlH₄ (M = Na, Li) with a molar ratio of 1 : 1 under a 1.0 MPa H₂ atmosphere with 300 rpm for 5 min. Ball-milling was performed by using a planetary ball-mill apparatus (Fritsch Pulverisette 7) with 20 stainless steel balls (7 mm in diameter) and 300 mg samples (ball: powder ratio = 70 : 1, by mass). We also prepared the mixtures by hand-milling. Hand-milled mixtures were prepared by mixing AB and MAlH₄ (M = Na, Li) in an agate mortar in the glove box for 90 seconds. Hand-milling over 120 seconds is dangerous because it often causes gas eruptions. The hydrogen desorption properties were examined by thermal desorption mass spectrometry measurements (TDMS, ULVAC, BGM-102) combined with thermogravimetry and differential thermal analysis (TG-DTA, Bruker, 2000SA). The heating rate was 5 °C min⁻¹ and the helium gas flow rate was 300 mL min⁻¹. Powder X-ray diffraction (XRD, PANalytical, X'Pert Pro with Cu Kα radiation) measurements were performed to observe the phases of composites. The samples used for XRD measurements were placed on a greased glass plate in an argon-filled glovebox and then sealed with a polyimide sheet (Kapton, The Nilaco Co. Ltd.) to avoid oxidation during measurement. Fourier transform infrared spectrometry (FT-IR, Spectrum One, Perkin-Elmer) measurements were performed using a diffuse reflection cell to investigate chemical bonds in the composites. All the samples were diluted with KBr to a mass ratio of 5 : 95 (sample: KBr).

3. Results and discussion

3.1 Hydrogen desorption properties of ball-milled composites

TG-DTA-MS results of ball-milled AB–MAlH₄ (M = Na, Li) composites are shown in Fig. 1. As shown in Fig. 1(a), exothermic peaks were observed at 66, 127, and 164 °C in DTA profile of AB–NaAlH₄ composite. These peaks correspond to H₂ desorption peaks in mass spectra. The composite did not desorb NH₃, B₂H₆, and B₃H₆N₃ at all within the accuracy of our apparatus. From TG profile, the amount of desorbed H₂ was estimated at 5 wt%. AB–LiAlH₄ composite showed similar H₂ desorption properties as those of AB–NaAlH₄ composite. Three exothermic peaks (53, 117 and 131 °C) were observed in DTA profile. The composite did not desorb NH₃, B₂H₆ and B₃H₆N₃. The suppression of by-product gas emission was also found in AB–Li₃AlH₆ composite.²⁹ The amount of desorbed H₂ was about 4 wt% for AB–LiAlH₄ composite. These results were quite different from the TG-DTA-MS results of AB⁶ or MAlH₄ ³³ (M = Na, Li) itself, suggesting the reactions between AB and MAlH₄ during milling and heating. Theoretical hydrogen capacities of AB–NaAlH₄ and AB–LiAlH₄ composites are 11.9 wt% and 14.6 wt%, respectively. H₂ desorption during ball-milling and M(BH₄) formation during heating resulted in a reduction in the amount of desorbed H₂ by TG experiments (see Section 3.2). Each exothermic peak of AB–LiAlH₄ composite was lower than the corresponding peak of AB–NaAlH₄ composite. This would be correlated with the lower thermal stability of LiAlH₄ than that of NaAlH₄.³⁴

Fig. 1 TG-DTA-MS profiles of ball-milled AB–MAlH₄ composites; (a) AB–NaAlH₄ composite, (b) AB–LiAlH₄ composite. The heating rate was 5 °C min⁻¹.

3.2 Structure and phase analyses of ball-milled composites

The pressure increase due to H₂ desorption was observed during ball-milling, which suggested the mixed-metal amidoborane formed by the reaction between NaAlH₄ and AB. NH₃, B₂H₆, and B₃H₆N₃ desorption was not observed during ball-milling. One of the driving forces for the reaction would be the affinity of H^δ− in NaAlH₄ and H^δ+ in NH₃ of AB. Fig. 2 shows the XRD profiles of ball-milled AB–MAlH₄ (M = Na, Li) composites after heating to each temperature. Broad diffraction peaks at around 20° and 27° in all profiles originate from the polyimide film and grease to prevent sample oxidation. In AB–NaAlH₄ composite, both AB and NaAlH₄ phases were observed at room temperature (RT). Besides, small unknown peaks appeared in the range of 15–30°. These peaks don't match with any diffraction pattern of decomposition products of starting materials or mono-metal amidoborane, suggesting the formation of mixed-metal (Na, Al) amidoborane phase during ball-milling. After heating to 80 °C, the peak intensities of mixed-metal amidoborane became stronger compared to RT. The reaction between AB and NaAlH₄ proceeded further to form the mixed-metal amidoborane, resulting in the H₂ desorption at 66 °C as shown in Fig. 1(a). After heating to 140 °C, the mixed-metal amidoborane phase disappeared, indicating its decomposition. It is interesting that NaBH₄ phase appeared at 140 °C. After heating to 170 °C, strong peak intensities of NaBH₄ were observed, while most of NaAlH₄ phase disappeared. The formation process of NaBH₄ will be described in Section 3.3. Furthermore, a new set of peaks were observed in the range of 10–25°. This could be another mixed-metal amidoborane formed by the reaction between Na₃AlH₆ and AB. After heating to 260 °C, this unknown phase decomposed and only NaBH₄ and Al phases were observed. In case of AB–LiAlH₄ composite, similar results were obtained as AB–NaAlH₄ composite. At RT, unknown peaks, which were considered as mixed-metal (Li, Al) amidoborane, were observed in the range of 10–25°. After heating to 170 °C, further new peaks were observed in the range of 10–40°. The peak positions of AB–Li₃AlH₆ composite reported by Xia et al. were also shown as reference in Fig. 2(b).²⁹ The positions of observed peaks were similar to the reference, suggesting the formation of mixed-metal (Li, Al) amidoborane. Though borohydride phase was not observed in the XRD profiles of AB–LiAlH₄ composite, the FTIR spectra (Fig. 3(b)) showed the strong B–H stretching. This indicated that the amorphous LiBH₄ formed during heating.

Fig. 2 XRD profiles of ball-milled AB–MAlH₄ composites after heating to each temperature; (a) AB–NaAlH₄ composite, (b) AB–LiAlH₄ composite.

Fig. 3 In situ FTIR spectra of ball-milled AB–MAlH₄ composites at each temperature; (a) AB–NaAlH₄ composite, (b) AB–LiAlH₄ composite. AB and MAlH₄ (M = Na, Li) spectra was presented for comparison. The heating rate was 5 °C min⁻¹.

Fig. 3 shows the in situ FTIR spectra of ball-milled AB–MAlH₄ (M = Na, Li) composites during heating. The spectra of AB and MAlH₄ (M = Na, Li) at RT were also shown as references. In AB–MAlH₄ (M = Na, Li) composites, peak intensities corresponding to N–H stretching between 3150 and 3500 cm⁻¹ decreased as temperature increased, whereas peaks corresponding to B–H stretching between 2200 and 2400 cm⁻¹ were remained after heating to 260 °C in both composites. This phenomenon was also observed in other metal amidoboranes.^{21,24,25,28,29} A new compound containing nitrogen would be formed during H₂ desorption.

3.3 Comparison with hand-milled mixtures

To clarify the reaction process in detail, we prepared the mixtures by hand-milling and investigated their H₂ desorption properties and phases. Interestingly, results were quite different from the ball-milled composites. Fig. 4 shows TG-DTA-MS results of hand-milled AB–MAlH₄ (M = Na, Li) mixtures. Sharp exothermic peaks were observed at 90 °C (AB–NaAlH₄) and 84 °C (AB–LiAlH₄) in DTA profiles. The weight losses of about 30 wt% (AB–NaAlH₄) and 50 wt% (AB–LiAlH₄) were also observed. From the mass spectra, H₂, NH₃, B₂H₆ and B₃H₆N₃ peaks were observed in both mixtures. Except this exothermic reaction, any reactions were not observed up to 260 °C.

Fig. 4 TG-DTA-MS profiles of hand-milled AB–MAlH₄ mixtures; (a) AB–NaAlH₄ mixture, (b) AB–LiAlH₄ mixture. The heating rate was 5 °C min⁻¹.

Fig. 5 shows the XRD profiles of hand-milled AB–MAlH₄ (M = Na, Li) mixtures before and after heating to 260 °C. Before heating, AB and MAlH₄ (M = Na, Li) were observed. Unknown peaks were not observed in the range of 10–30°, which was different from the results of ball-milled composites. After heat treatment, NaBH₄ was observed in the AB–NaAlH₄ mixture, which was similar to the results of ball-milled composites.

Fig. 5 XRD profiles of hand-milled AB–MAlH₄ mixtures at RT and after heating to 260 °C; (a) AB–NaAlH₄ mixture, (b) AB–LiAlH₄ mixture.

The reaction observed in the hand-milled mixture was quite similar to the solid state reaction of MAlH₄ (M = Na, Li) with NH₄Cl. In this reaction, MCl and [H₄Al·NH₄] is formed and soon [H₄Al·NH₄] decomposes to [HAlNH] and H₂, accompanied by a large exothermic heat.³⁵ The previous study showed diammoniate of diborane (DADB), [(NH₃)₂BH₂]⁺[BH₄]⁻, an ionic isomer of AB, is formed during the induction period before H₂ desorption occurs.³⁶ MAlH₄ was also confirmed to be an ionic compound, consisting of M⁺ cation and AlH₄⁻ anion.³⁷ Considering the reaction between DADB and NaAlH₄, the reaction between BH₄⁻ anion and Na⁺ cation would cause the formation of NaBH₄. On the one hand, the reaction between [(NH₃)₂BH₂]⁺ and AlH₄⁻ would cause the H₂ and by-product gas emissions. However, the ball-milled composites showed the different results from the hand-milled mixtures. This would be attributed to the formation of mixed-metal amidoborane. Though this phase was not observed in the hand-milled mixtures, it was observed in the ball-milled composites at not only RT but also other temperatures (e.g., 170 °C). The interaction between metal amidoborane and AB like LiNH₂BH₃·NH₃BH₃ showed the significantly low H₂ desorption temperature.^19,20 Similarly, the interaction between mixed-metal amidoborane and AB could occur in the ball-milled composites. Mixed-metal amidoborane would stabilize the reaction between Al–H bonds and N–H bonds, resulting in the suppression of by-product gases. Thus, it is suggested that mixed-metal amidoborane plays an important role in suppressing the emission of by-product gases.

4. Conclusions

AB–MAlH₄ (M = Na, Li) composites were successfully synthesized by ball-milling and their hydrogen desorption properties and decomposition processes were investigated. The composites desorbed 4–5 wt% hydrogen below 260 °C, accompanied by H₂ desorption. They did not desorb NH₃, B₂H₆, and B₃H₆N₃ at all. They showed three exothermic reactions below 260 °C, accompanied by H₂ desorption. The first reaction is ascribed to the formation of mixed-metal amidoborane phase. The second reaction is ascribed to the decomposition of mixed-metal amidoborane. In the last, the reactions described as below occurred. One is the reaction between AB and MAlH₄ (M = Na, Li), which result in the formation of MBH₄ (M = Na, Li). The other is the reaction between M₃AlH₆ (M = Na, Li) and AB, which result in the formation of another mixed-metal amidoborane. The hand-milled mixtures showed quite different results from the ball-milled composites. They showed only one exothermic reaction at 80–90 °C. The emission of by-product gases was not suppressed. By comparing the results of the ball-milled composites with those of the hand-milled mixtures, the importance of the mixed-metal amidoborane as a barrier against by-product gas emission in this system was proposed. These results would be helpful for clarifying reaction mechanisms of AB–MH composites.

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