on the Thermal Decomposition of Light Metal ... - ACS Publications

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“Boron Effect” on the Thermal Decomposition of Light Metal Borohydrides MBH4 (M = Li, Na, Ca) Flavio Pendolino* Departamento de Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain

bS Supporting Information ABSTRACT: The hydrogen release rate of thermal decomposition, after the melting, for the borohydride of lithium (Li), sodium (Na), and calcium (Ca) and their boron mixtures, at selected molar ratios, is investigated under 1 bar hydrogen pressure and nonisothermal conditions. The reaction is studied by means of manometric measurements. The maximum hydrogen release rate for all pure borohydrides is ∼8  10
3 bar/min. By adding boron to the borohydride systems, the hydrogen release rate is affected and, generally, is lowered. For the decomposition process of LiBH4+B, maximum rate is ∼2  10
3 bar/min. On the opposite, hydrogen rate is suppressed in boron mixtures of NaBH4. The addition of boron changed slightly the maximum rate of hydrogen release for Ca(BH4)2+B for first and second decomposition.

’ INTRODUCTION Hydrogen as a future energy carrier requires the development of new technology for efficient storage. An Important issue of a hydrogen economy is a storage material that can release and recharge hydrogen as fast as possible at minimum energy loss. Nowadays, taking advantage of Brown’s research on sodium borohydride,1 alkali, and alkaline earth metals borohydrides (MBH4) are materials that could be used to store hydrogen in the solid state.2
7 These materials decompose thermally starting near the melting point according to the following expressions MBH4 f MH + B + 3/2H2 f M + B + 2H2 for alkali borohydrides and 3Ca(BH4)2 f 2CaH2 + CaB6 + 10H2 for calcium borohydride. Alkali and alkaline earth borohydrides release the most of hydrogen at a temperature range 400
600 °C, which is a problem for onboard utilization. Recently, numerous works3,8
21 have described improved of hydrogen release by destabilizing the borohydrides using various additives; for instance, Au11
13 utilized transition-metal oxides (TiO2, V2O3, SnO3, etc.) and chloride compounds (TiCl3, MgCl2, etc.). In 2007, Kostka22 has shown that Silica-Gel mixtures works well in in decreasing the LiBH4 decomposition temperature. In these last years, several group proposed mixed hydrides composites for obtaining better decomposition properties. Vajo10,16,17 was successful to decrease the decomposition temperature of LiBH4 adding MX (X = H, F, Cl, Se, Si, etc.), whereas Mao18,19 mixed NaBH4 with lithium Alanate or MgH2. More complex alkali borohydrides composites were proposed for combining favorable thermodynamics and kinetics. The behavior of LiBH4/CaH2 ball-milled with NbF5 was studied by Lim20 and the catalytic mechanism of LiBH4
SiO2
TiF3 was investigated by Zhang.15 Extensively, the sodium borohydride system was investigated in a mixture to r 2011 American Chemical Society

decrease the decomposition temperature; that is, Mao18 added LiAlH4 to NaBH4 doping with TiF3. A well-studied mixture of NaBH4 is a ball-milled MgH2 system.8,23
25 Few works investigated the Ca(BH4)2 composite for improving the decomposition process. Rongeat et al.26 described the reversibility of formation/ decomposition of Ca(BH4)2 with TiCl3 or TiF3 and showed that only this latter is a suitable additive. The use of metal hydrides composite is studied by Barkhordarian et al.27 by adding MgH2 and 1% Ti isopropoxide, which lower the decomposition temperature peak to ∼380 °C. Lee et al.9 mixed the borohydrides of calcium and lithium at various molar ratios, observing hydrogen release at lower temperature than individual materials. We approached the problem of adding amorphous boron as an additive. As reported in a previous paper,28 the addition of an excess of amorphous boron to LiBH4 enhanced the decomposition process following an alternative reaction path after partial decomposition of material. The induced changes in the thermal decomposition pathway lower the decomposition temperature by 150 °C at a heating rate of 11 °C/min and activation energy of 55 kJ/mol. Understanding the speed of hydrogen release is the key to use the borohydrides for practical applications. In this present work, the effect of boron addition (boron effect) on hydrogen release rate for a thermal decomposition of the borohydrides of lithium, sodium, and calcium is investigated for various molar ratios. A pressure of 1 bar is applied to systems, and hydrogen gas is chosen because it is part of the reaction of decomposition. All experiments are carried out with a Received: June 27, 2011 Revised: November 27, 2011 Published: November 30, 2011 1390

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temperature over melting up to 600 °C under nonisothermal condition, which allows us to complete the decomposition reaction in a reasonable amount of time compared with the time-consuming for equilibrium measurements.

’ EXPERIMENTAL SECTION Borohydrides of lithium LiBH4 (g95%), sodium NaBH4 (98%), calcium Ca(BH4)2, and amorphous boron powder B (g99% purity) were purchased from Sigma-Aldrich. The hydrogen gas had a purity of 5 N. All samples handling took place under an Ar atmosphere in a glovebox with O2 and H2O content lower than 0.1 ppm. The boron-mixed samples were prepared by mixing, in an Agata mortar, the borohydride and the amorphous boron at various molar ratios. Manometric (Man) experiments under hydrogen pressure were performed in a Sievert-type apparatus PCTPro-2000 (Hy-Energy & Setaram). Pressure measurement accuracy was 1% of reading, and the reservoir volume used for the measurements was 1180 mL for LiBH4 and 167 mL for NaBH4 and Ca(BH4)2 and the corresponding boron mixtures. The amounts of sample were 300 mg (LiBH4) and 50 mg (NaBH4 and Ca(BH4)2). ’ RESULTS AND DISCUSSION In Figure 1, we reported the hydrogen pressure rate (dp/dt) as a function of temperature for LiBH4 and for LiBH4+B mixtures. A small production of hydrogen starts around 285 °C (melting point), and a decomposition between 380 and 500 °C is found. At molar ratio X = 0.8 (poor in boron) (Figure 1b), the decomposition starts at ∼240 to 400 °C. A peak is present at 288 °C, followed by a small shoulder. In the mixture X = 0.08, containing an excess of boron (Figure 1c), decomposition starts at ∼250 °C and increases to 500 °C. We can observe a peak at melting and a peak around 430 °C. Figure 1a, the rate of hydrogen release is maximum at 7.6  10
3 bar/min after almost 75 min, and the total amount of H2 is 6.2  10
4 mole in the temperature range 200
500 °C. Anyhow, when boron (X = 0.8) is added to the system, only one reaction takes place, and maximum rate is lowered to 2.4  10
3 bar/min; the process reaches the maximum in 50 min and producing 3.6  10
4 mole H2. The peak temperature of decomposition is shifted down from 450 (pure material) to 300 °C. In Figure 1c, an excess of boron is added, and two peaks are found: one is near melting point and the other is at decomposition (∼450 °C) with a hydrogen rate that is (1.3 and 2)  10
3 bar/min, respectively (2.0  10
4 mole H2). The peak temperature of each process remains at the same temperature range as pure LiBH4 at a heating rate of 1°/min. In Figure 2 is shown the hydrogen pressure rate as a function of temperature for pure NaBH4 and NaBH4+B under a starting hydrogen pressure of 1 bar at heating rate of 2 °C/min. In Figure 2a, manometric experiments show that the system decomposes with two stages, realizing hydrogen at melting temperature 495 °C and at decomposition peak 572 °C. The maximum hydrogen rate is (1.9 and 7.8)  10
3 bar/min, respectively (1.7  10
4 mole H2 totally). Only a smaller decomposition is observed at melting for X = 0.8 (Figure 2b), and even a smaller broad decomposition is found for boron mixture X = 0.08 (Figure 2c), starting around 470 °C. As occurred for LiBH4, the addition of boron lowers the hydrogen rate toward 1.3  10
3 bar/min at X = 0.8 (1.9  10
5 mole H2). Any maximum rates at peak can be measured for NaBH4+B X = 0.08; however, from ∼470 to 600 °C, the hydrogen rate is almost constant at ∼0.7  10
3 bar/min

Figure 1. Hydrogen pressure rates versus temperature for (a) pure LiBH4, (b) LiBH4+B X = 0.8, and (c) LiBH4+B X = 0.08. The experiments were performed at heating rate of 1 °C/min under a starting pressure of 1 bar H2.

(1.1  10
4 mole H2). Hydrogen pressure rate of Ca(BH4)2 as a function of temperature under 1 bar of hydrogen pressure at 2 °C/ min was studied in Figure 3. The pure Ca(BH4)2 shows a large release with a peak temperature at 357 °C and two small releases at 388 and 453 °C. Maximum hydrogen release at first peak has a rate of 8.1  10
3 bar/min, whereas the following two peaks have a rate ∼2  10
3 bar/min, releasing 2.0  10
4 mole H2 totally. Concerning boron mixtures, little difference is present after the large effect at 357 °C, where only one small peak is shown at peak temperature 430 °C for both molar ratio X = 0.8 and 0.03; this second peak is almost at half of the temperature range (400
500 °C), in which the two peaks of pure material are found. However, hydrogen rates for the boron mixtures of Ca(BH4)2 consist of little changes; at molar ratio X = 0.8, the first decomposition is described by a rate of 7.9  10
3 bar/min, whereas the second one is 1.1  10
3 bar/min (2.1  10
4 mole H2 totally). At molar ration X = 0.03, first and second decomposition have a rate of (6.5 and 0.8)  10
3 bar/min, respectively (1.6  10
4 mole H2). Pure materials reproduce the temperature ranges in agreement with measurements reported in literature. The onset temperature for melting of LiBH4 is between 268 and 285 °C, and the decomposition is a wide peak between 400 and 550 °C, which depends on heating rate and used gas (see comparison in Table 2 of the Supporting Information).29
33 In our 1391

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Figure 2. Hydrogen pressure rates versus temperature for the thermal decomposition of (a) pure NaBH4, (b) NaBH4+B X = 0.8, and (c) NaBH4+B X = 0.08. The experiments were performed at heating rate of 2 °C/min with a starting pressure of 1 bar H2.

experiments, melting temperature is 285 °C, and decomposition occurs in a temperature range 380
500 °C with a maximum peak at 441 °C. The melting of NaBH4 takes place in a temperature range 495
511 °C and a decomposition between 565 and 595 °C.29,34
37 Our experiment in Figure 2a shows that the melting of pure NaBH4 occurs at 495 °C and decomposes with sharp peak at 572 °C. Literature38
41 reports that the decomposition of pure Ca(BH4)2 occurs in two stages, which start from ∼350 and ∼450 °C, respectively. We obtained for the first decomposition a maximum at 356 °C and the second event, which is splitted in two substeps, at 384 and 443 °C, as shown in Figure 3. However, in literature, the second stage of decomposition is a broad peak only; this fact is due to the heating rate effect that splits a broad peak composed by two similar kinetics when heating rate is changing, as we observed for LiBH4.28 In Figures 1
3 (panel a), we observe that borohydrides decompose with the same maximum rate around 8  10
3 bar/min, which is independent from the borohydried chosen. Hydrogen rate is lowered when the amount of boron is increased. The addition of a small amount of amorphous boron to each system (X = 0.8) reduces the hydrogen rate for the Li and Ca borohydride and exhibits an inhibition effect in NaBH4+B. For LiBH4, the decomposition peak at 435 °C is shifted down, overlapping

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Figure 3. Hydrogen pressure rates versus temperature for the thermal decomposition of (a) pure Ca(BH4)2, (b) Ca(BH4)2+B X = 0.8, and (c) Ca(BH4)2+B X = 0.03 under a starting pressure of 1 bar of H2 at 2 °C/min.

the melting peak and reducing the rate to 2.4  10
3 bar/min. Moreover, the two peaks of second decomposition of Ca(BH4)2 join into one at 430 °C with a rate of 1.1  10
3 bar/min. When an excess of boron powder is used in LiBH4+B (X = 0.08), melting and decomposition peak are again separated at that same temperature as in pure material, but hydrogen rate is lowered toward 2  10
3 bar/min, although the hydrogen rate for melting increased slightly from 0.7 to 1.2  10
3 bar/min. Concerning the addition of an excess of boron in NaBH4 (X = 0.08), any peaks are shown in Figure 2, but only a broad decomposition is present, which has a constant low rate (0.7  10
3 bar/min); hence, boron inhibits the reaction. Boron seems does not seem to have any important effects on Ca(BH4)2+B, even adding a large amount of material (X = 0.03) to borohydride. We observed that the rate of first peak is reduced to 6  10
3 bar/min and the second one is reduced to 0.9  10
3 bar/min, and any temperature changes took place under this condition. Taking into account of results in this study, LiBH4+B X = 0.8 (poor in boron) is the only mixture that could be a potential system to storage hydrogen keeping in mind that the decomposition reaction occurs in one stage with a relatively low temperature starting from 240 °C. The hydrogen rate is the higher among boron mixtures in the temperature range 250
350 °C. To note is Ca(BH4)2+B X = 0.8 with a hydrogen rate 8.3  10
3 bar/min that is higher comparing 1392

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The Journal of Physical Chemistry C to the corresponding LiBH4+B (X = 0.8); nevertheless, the most of the decomposition takes place around 350 °C. In the issue of hydrogen storage, the use of boron mixtures of NaBH4 at any molar ratios seems to be a prohibitive strategy, mostly due to the inhibition effect of the boron on the decomposition rate.

’ CONCLUSIONS In this Article, the effect of the addition of amorphous boron on hydrogen rate, after the melting point, for the thermal decomposition of borohydrides of lithium, sodium, and calcium was investigated; pure borohydrides and boron mixtures were investigated by means of manometric measurements under 1 bar H2 and nonisothermal conditions. The highest rate was found for pure borohydrides ∼8  10
3 bar/min. In general, the addition of boron affects the hydrogen rate, at any molar ratios, lowering its value, and, in some cases, decomposition temperature shift is found. For NaBH4+B, the boron almost suppressed the decomposition rate, whereas for Ca(BH4)2+B, any important variation is shown. Moreover, in LiBH4+B (X = 0.8), the decomposition reaction occurred in one stage in the decomposition temperature range 250
350 °C, which is lower compared with temperatures of the other boron mixtures. Therefore, the mixture of amorphous boron and LiBH4 provided a reasonable compromise between hydrogen rate and decomposition temperature for a potential hydrogen storage material. This study was applied to borohydrides of lithium, sodium, and calcium and will be extended to other light metal borohydrides and various heating rates, which may have an enhancement in hydrogen release rate. ’ ASSOCIATED CONTENT

bS

Supporting Information. Temperatures that are obtained in this work and comparison with data report ed in literature for borohydride of lithium, sodium, and calcium. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: fl[email protected].

’ ACKNOWLEDGMENT This work was financially supported by the European Commission contract numbers MRTN-CT-2006-032474 Hydrogen Network. ’ REFERENCES (1) Brown, H. C. Hydroboration: With Supplement, Nobel Lecture, December 8, 1979; Benjamin/Cummings Pub. Co., Inc., Advanced Book Program: Reading, MA, 1980. (2) Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283–1315. (3) Eberle, U.; Felderhoff, M.; Schuth, F. Angew. Chem., Int. Ed. 2009, 48, 6608–6630. (4) R€onnebro, E. Curr. Opin. Solid State Mater. Sci. 2011, 15, 44–51. (5) Bogdanovic, B.; Felderhoff, M.; Streukens, G. J. Serb. Chem. Soc. 2009, 74, 183–196 . (6) George, L.; Saxena, S. K. Int. J. Hydrogen Energy 2010, 35, 5454–5470. (7) Jain, I. P.; Jain, P.; Jain, A. J. Alloys Compd. 2010, 503, 303–339.

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’ NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on December 13, 2011. Reference 22 has been revised. The corrected version was published on December 21, 2011.

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