Influence of Stoichiometry on the Hydrogen Sorption Behavior in the

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Influence of Stoichiometry on the Hydrogen Sorption Behavior in the LiF−MgB2 System Ivan Saldan,*,† Rapee Gosalawit-Utke,† Claudio Pistidda,† Ulrike Bösenberg,† Matthias Schulze,‡ Torben R. Jensen,§ Klaus Taube,† Martin Dornheim,† and Thomas Klassen†,‡ †

Institute of Materials Research, Materials Technology, Helmholtz-Zentrum Geesthacht, Max-Planck Str. 1, D-21502 Geesthacht, Germany ‡ Institute of Materials Technology, Helmut-Schmidt-University, Holstenhofweg Str. 85, D-22043 Hamburg, Germany § Center for Materials Crystallography, iNANO and Department of Chemistry, Åarhus University, Langelandsgade 140, DK-8000 Åarhus C, Denmark ABSTRACT: Chemical reactions between LiF and MgB2 with different molar ratios of 1:1 and 4:1 under hydrogen atmosphere were studied by high-pressure differential scanning calorimetry (HP-DSC) and in situ synchrotron radiation power X-ray diffraction. Hydrogen sorption properties of the composites were evaluated using a Sievert’s type apparatus. After hydrogenation only LiBH4 and MgF2 are found as the main products. However, DSC characterization showed multistep events related to LiBH4 that might be explained by different phases or some intermediates.

1. INTRODUCTION With a theoretical content of 18 wt % of hydrogen, LiBH4 is one of the solids with the highest hydrogen capacities per weight known today and therefore a desirable compound for hydrogen storage. During the decomposition to LiH and B up to 13.8 wt % H2 can be released according to1 2LiBH 4 → LiH + B + 3H2 (1)

wt % of H2. Extrapolation of isothermal measurements predicted an equilibrium pressure of 1 bar at approximately ∼225 °C, with an estimated total reaction enthalpy of ∼40.5 kJ/mol H2.2 Strong influences of the reaction conditions on the reaction pathway and cycling kinetics have been observed.11,12 Separate decomposition of LiBH4 and MgH2 was observed at higher temperatures and low pressures (T ≥ 450 °C and P ≤ 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 took place at T ≈ 400 °C and a hydrogen pressure of ∼5 bar.11 Another approach to destabilize LiBH4 could be by partial substitution of hydrogen atoms inside the [BH4]− anion. Recently, such a partial substitution in a complex hydride was reported to occur in the case of Na3AlH6.13,14 Because of the smallest difference in covalent radius between H and F atoms (0.31 and 0.57 Å, respectively), from the crystallographic point of view fluorine should be the best candidate. In a recent work15 thermodynamic modeling of solid solutions between LiBH4 and LiBF4 has been performed and the quantummechanical data of mixed compounds were used to derive the ΔH, ΔS, and ΔG at T = 298 K and P = 1 bar for the reaction

The equilibrium temperature for reaction 1 is about 400 °C at a pressure of 1 bar H2. The actually required desorption temperature is much higher due to kinetic constraints. The high thermodynamic stability hampers the practical application of pure LiBH4. This can only be overcome by tuning the thermodynamic properties. In addition, sluggish kinetics and irreversibility are the next hurdles to overcome for a future application of the material. One possible approach is the formation of a new compound in an exothermic process during the overall endothermic desorption reaction, thus lowering the total reaction enthalpy.2−11 For composites of two high capacity hydrides, the total gravimetrical hydrogen capacity remains high. One prominent example is the class of reactive hydride composites (RHCs), where a high-capacity hydride (e.g., MgH2) and a borohydride are used to produce a new compound (MgB2). For LiBH4 this can be expressed as 2LiBH 4 + MgH 2 ↔ 2LiH + MgB2 + 4H2 (2)

(1 − a /16)LiBH 4 + (a /16)LiBF4 → (1/4)Li 4B4H16 − aFa (3) Received: December 21, 2011 Revised: February 11, 2012 Published: February 21, 2012

Mechanically milled mixtures of LiBH4 with MgH2 with molar ratio of 2:1 have been shown to reversibly store more than ∼8 © 2012 American Chemical Society

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dx.doi.org/10.1021/jp212322u | J. Phys. Chem. C 2012, 116, 7010−7015

The Journal of Physical Chemistry C

Article

where a is the number of F ions which substitute for H inside the unit cell (0 ≤ a ≤ 16). The main conclusion was that ΔH values for all compositions are positive indicating (i) that this substitution indeed would lead to a destabilization of borohydrides by F substitution and (ii) that this new compound would be metastable only and thus difficult to form. In another theoretical work16 the decomposition reactions of F substituted LiBH4 were calculated by firstprinciples calculations. Possible decomposition of Li8B8H32−xFx and formation of Li8H8−xFx (x ≤ 4) compounds has been shown by the following reaction Li8B8H32 − xFx → Li8H8 − xFx + 8B + 12H2

hydrogen flow of 20 mL/min was used to obtain a constant pressure under hydrogen absorption of 50 and 5 bar during desorption. In situ SR-PXD was performed at the I711 beamline at the MAX II synchrotron in Lund, Sweden.20 A Mar165 chargecoupled detector (CCD) was exposed for 20 s with selected Xray wavelengths of 1.07200 or 0.94608 Å for different measurements. The samples were encapsulated airtight in sapphire capillaries to be installed in a special in situ SR-PXD cell.21 Milled samples were heated with 5 °C/min rate from room temperature up to 390 °C and kept at isothermal conditions for 5 h. After complete hydrogen desorption, the samples were heated with a rate of 10 °C/min up to 390 °C, followed by a further 1 h isothermal period and then fast cooling down to room temperature. All handling and preparation of materials took place in a glovebox with continuously purified argon atmosphere. Oxygen and moisture contents were less than 1 ppm.

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Reaction enthalpies with zero-point energy correction at x = 0, 1, 2, 3, and 4 have been calculated as 60.9, 47.6, 36.6, 22.5, and 13.0 kJ/mol H2, respectively. In 1960 Messer and Mellor synthesized a LiH−LiF solid solution near the melting temperature but partial decomposition into separate phases took place during cooling.17 Therefore the crystals, grown by the Czochralski method were cooled in liquid nitrogen just after removal from the melt;18 the composition of mixed crystal was confirmed by X-ray analysis and exciton energy spectra. On the basis of synchrotron radiation power X-ray diffraction (SRPXD) and attenuated total reflectance Fourier transform infrared spectroscopy results the formation of LiF1−xHx and LiBH4−yFy was observed during hydrogenation first in an experimental work.19 A significant kinetic destabilization with respect to pure LiBH4 was shown for hydrogen absorption− desorption of a LiF−MgB2 composite with a molar ratio of 2:1. It was explained by the formation of MgF2 and LiBH4−yFy during hydrogenation process, where on the one hand formation of MgF2 (more stable than MgH2) is the driving force for hydrogen absorption and on the other hand LiBH4−yFy (less stable than LiBH4) improves hydrogen desorption. Understanding the reaction mechanism between LiF and MgB2 under hydrogen pressure is the key to H→F partial substitution inside the [BH4]− anion. Together with different reaction conditions the molar ratio between these reagents might be responsible for changing in thermodynamics and kinetics. In the present work, the effect of molar ratio on hydrogenation and hydrogen storage capacity is investigated in detail.

3. RESULTS AND DISCUSSION 3.1. Influence of Stoichiometry on Hydrogen Sorption of LiF−MgB2 Composites with Different Molar Ratios of 1:1 and 4:1. To study the influence of stoichiometry on the hydrogen sorption of LiF−MgB2 system we have conducted two approaches: double excess of MgB2 and LiF regards to the stoichiometric system (2:1). So, LiF−MgB2 composites with molar ratios of 1:1 and 4:1 have been prepared the same way. For the (1:1) LiF−MgB2 system three hydrogenation/ dehydrogenation cycles have been performed (Figure 1). Faster kinetics for hydrogen absorption as well as for desorption were observed with respect to the (2:1) mixture.19

2. EXPERIMENTAL METHODS LiF (99,99%, Sigma Aldrich) and MgB2 (>96%, Alfa Aesar) powders were selected to prepare the mixtures with molar ratios of 1:1; 2:1 and 4:1, respectively. These mixtures were ball-milled for 5 h using a Spex 8000 M Mixer Mill under argon atmosphere. Stainless steel balls of 10 mm in diameter and a ball to powder ratio of 10:1 were used. Hydrogen sorption measurements were carried out in a carefully calibrated Sievert’s type apparatus (HERA, Quebec, Canada). Milled mixtures were hydrogenated under 60 bar of hydrogen pressure and at 390 °C in a special high pressure/ high temperature sample holder. Hydrogen desorption was performed at 5 bar back pressure of hydrogen and 420 °C, after the previous absorption was fully completed. High-pressure differential scanning calorimetry (HP-DSC) was performed using a Netzsch DSC 204 HP Phoenix located in a dedicated glovebox. By use of 5 °C/min heating and cooling rate, all samples were heated from about ∼25 to 530 °C and after that cooled down to room temperature again. A

Figure 1. Hydrogen absorption at 390 °C under 60 bar H2 pressure and desorption at 420 °C and 5 bar for LiF−MgB2 composite with molar ratio of 1:1. There are three complete hydrogenation/ dehydrogenation cycles. 7011



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first and second desorption 4.6 and 3.5 wt % H2 during ∼70 h, respectively (Figure 2b). As in the other measurements described above, the slope of the first absorption curve is different than that of the next cycles. Concluding, a lower hydrogen capacity and no improvements in reversibility for the (4:1) LiF−MgB2 system compared to the (2:1) LiF−MgB2 have been observed. 3.2. Effect of Molar Ratio between LiF and MgB2 under DSC Characterization. For all LiF−MgB2 composites prepared with different molar ratios of 1:1, 2:1, and 4:1 HPDSC measurements during hydrogen sorption have been performed (Figure 3). Because of high pressure during

Because of the excess of MgB2 in (1:1) LiF−MgB2 composite, the theoretical gravimetric hydrogen capacity is approximately 5.4 wt % H2. For the first, second and third absorptions approximately 5.3, 5.2, and 5.1 wt % H2 have been obtained in ∼7, 6, and 5 h, respectively (Figure 1a); in good agreement with the desorption measurements (∼5 wt % H2 released after ∼32, 24, and 20 h, respectively (Figure 1b)). In addition, the slope of the first absorption curve is different than that for the next cycles. In the case of absorption, curves corresponding to the second and third cycles are parallel to each other. For the first, second, and third desorption only differences in incubation time have been observed. This means that during the first cycle some activation processes took place and during the following cycle reaction kinetics were improved with almost constant hydrogen capacity. Similar kinetic effects have been shown already for LiBD4−MgD2 mixtures and explained as another path-way for hydrogen desorption in the case of MgD2rich sample.12 Our proposed reaction mechanism of hydrogen sorption will be discussed in section 3.3. In conclusion for the (1:1) and (2:1) LiF−MgB2 systems, a compromise between improved kinetics and good reversibility in the (1:1) or high hydrogen capacity in the (2:1) has to be found. Two cycles have been performed for the (4:1) LiF−MgB2 system (Figure 2). Similar behavior of hydrogen sorption/

Figure 3. HP-DSC investigation for the LiF−MgB2 system during hydrogen absorption (a) and desorption (b). Peak position (A) corresponds to LiBH4 phase transformation, (B) melting/solidification of LiBH4, (C) hydrogen absorption, (D) decomposition of LiBH4. Hydrogen flow of 20 mL/min was used to obtain a constant pressure under hydrogen absorption of 50 and 5 bar during desorption.

absorption (50 bar of H2; Figure 3a) the DSC data were more scattered compare with that during hydrogen desorption (5 bar of H2; Figure 3b). Hydrogen flow of 20 mL/min was used to obtain a constant pressure during heating/cooling in case of absorption or desorption. All peak positions correspond to the temperature regions defined in,19 but their intensity were different due to the changing in stoichiometry between LiF and MgB2. At the first hydrogen absorption during heating we observed a broad exothermic peak starting at approximately 310 °C, which was not finished in the investigated temperature range. Under cooling at about ∼430−460 °C we found a more distinct peak (Figure 3a). Most probably upon cooling exothermic solidification at ∼250−270 °C and the polymorphic

Figure 2. Hydrogen absorption at 390 °C under 60 bar H2 pressure and desorption at 420 °C and 5 bar for LiF−MgB2 composite with molar ratio of 4:1. There are two complete hydrogenation/ dehydrogenation cycles.

desorption curves during cycling was observed as for the (2:1) mixture in ref 19: after first cycle kinetics becomes faster while total hydrogen capacity decreases. Moreover, because of LiF excess in the (4:1) LiF−MgB2 system the theoretical hydrogen capacity is only ∼5.1 wt % H2. During the first and second absorptions, approximately 4.7 and 3.6 wt % H2 have been obtained in ∼16 and 12 h, respectively (Figure 2a) and for the 7012



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phase transformation in region of ∼90−110 °C of LiBH4 took place. In all three mixtures the corresponding peaks at around 100 °C were split into two. Increasing peak shifts to lower temperatures were observed for all DSC events under cooling in the order: (2:1) LiF−MgB2; (1:1) LiF−MgB2; (4:1) LiF− MgB2 (Figure 3a). However, during the following hydrogen desorption, these peak positions had the same values (Figure 3b) during heating. At ∼430−460 °C, the expected decomposition signal of LiBH4 is visible again in the form of a double peak in all investigated mixtures. The intensity of this event decreased in the order (2:1) LiF−MgB2, (1:1) LiF− MgB2, and (4:1) LiF−MgB2 and thus could be related to the hydrogen storage capacity. As hydrogen desorption was completed, no further exothermic peaks were found during cooling. The observed doubling of peaks might be explained by multistep processes which are more clearly visible for multicomponent systems (in our case it is composite) than for pure compound (pure LiBH4 phase). These multistep processes were well pronounced at all the DSC events related to LiBH4: phase transformation, melting point and decomposition. The difference of DSC behavior between (2:1) LiF−MgB2 and (2:1) LiH−MgB27−9 composites can be explained by the LiF phase. From the thermodynamic point of view (ΔfH° for LiF and LiH are −616.0 and −90.5 kJ/mol, respectively22), and hydrogen absorption of LiF−MgB2 composites should result into a more stable compound than MgH2. 3.3. Reaction Mechanism based on in situ SR-PXD. Since in situ SR-PXD measurements for the (2:1) LiF−MgB2 were recently published elsewhere,19 in the present work data for LiF−MgB2 composites with molar ratios of 1:1 and 4:1 are shown. To understand the reaction mechanisms during the hydrogenation/dehydrogenation of the (1:1) LiF−MgB2 composite, fresh as milled and three times cycled samples (after the third dehydrogenation) have been prepared. In both cases the results show the appearance of crystalline MgF2 as the main product of the hydrogenation (Figure 4). Because of melting/solidification of LiBH4 (∼ 250−270 °C; see paragraph 3.2) no diffraction peaks were visible at 390 °C (Figure 4a) though small peaks with positions similar to that of LiBH4 were visible after fast cooling down to room temperature (Figure 4b). For the milled (1:1) LiF−MgB2 sample, the first hydrogen absorption started before reaching the final/maximum temperature of 390 °C, presumably because of the effect of MgB2 excess (red diffraction pattern in Figure 4a). During 5 h in the isothermal period the LiF peaks disappeared completely while some MgB2 remained. Its maximum peak intensity decreased to the half. As it was shown in19 a small shift of the LiF peaks in the temperature region of 350−380 °C was observed (see red polygonal line in Figure 4). This can be explained by the thermal expansion of LiF (34.4 × 10−6 K−1 at 300 K22), because under the following cooling, the peak positions returned back to their original positions. Approximately around ∼100 °C under cooling the orthorhombic phase of LiBH4 appeared. No parallel reflexes of possibly H→F substituted material were detected (Figure 4b). The appearance of some small signals of pure Mg in the dehydrogenated sample suggests some loss of boron in the course of the reaction. This could be due to possible chemical reactions like formation of amorphous boron, Li2B12H12 or volatile B2H6. In all these three cases B would be trapped and cannot participate in further hydrogen sorption. Taking into account all experiments made for the (1:1) LiF−

Figure 4. In situ SR-PXD for LiF−MgB2 composite with molar ratio of 1:1 at hydrogen absorption under 60 bar H2 and 390 °C. X-ray wavelength of 1.07200 Å was used for the milled sample (a) and 0.94608 Å for the sample after third desorption (b).

MgB2 system, the following major pathway for the reversible hydrogen reaction can be proposed LiF + MgB2 + 2H2 ↔ LiBH 4 +

1 1 MgF2 + MgB2 2 2 (5)

In situ SR-PXD data for the (4:1) LiF−MgB2 composite of an as-milled sample and after the second dehydrogenation are shown in Figure 5. For the as-milled sample the first hydrogen absorption started only at maximum temperature (red diffraction pattern in Figure 5a). During 5 h in the isothermal period MgB2 almost disappeared, while LiF remained present. But its peak intensity decreased to a half. The same shift for the LiF peaks in the temperature region of 350−380 °C was observed as in the (1:1) LiF−MgB2 composite (see red polygonal line in Figure 5). The LiBH4 peak intensity was quite low and no LiBH4−xFx compounds could be detected. The appearance of Mg precipitations was observed in the cycled samples, too (Figure 5b). Taking into account all experimental data of the (4:1) LiF−MgB2 system, the following major reaction pathway for the reversible hydrogen reaction is proposed 4LiF + MgB2 + 4H2 ↔ 2LiBH 4 + MgF2 + 2LiF 7013

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(ΔfH° for MgB2 and LiF are −92.0 and −616.0 kJ/mol, respectively22). 4 The observed doubling of peaks during DSC characterizations might be explained by different phases or some intermediates. The pronounced double peaks are related to LiBH4 events: phase transformation, melting point, and decomposition. 5 In comparison to the mechanochemical method employed in this study, it is reasonable to study another (e.g., wet chemical) approach to synthesize fluorine substituted compounds.

AUTHOR INFORMATION

Corresponding Author

*Phone:+47 6380-6079. Fax: +47 6381-0920. E-mail: ivan_ [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The corresponding author is very thankful to Dr. Y. Cerenius for good management at the I711beamline in MAX-Lab. The research leading to these results has received funding from the European Community’s Seventh Framework Programme FP 7/ 2007-2013 under Grant No. 226943-FLYHY.

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REFERENCES

(1) Schlapbach, L.; Züttel, A. Nature 2001, 414, 353−358. (2) Vajo, J.; Skeith, S.; Mertens, F. J. Phys.Chem. B 2005, 109, 3719− 3722. (3) Vajo, J.; Olson, G. L. Scr. Mater. 2007, 56, 829−834. (4) Vajo, J.; Salguero, T. T.; Gross, A. F.; Skeith, S. L.; Olson, G. L. J. Alloys Comp. 2007, 446−447, 409−414. (5) Vajo, J. Curr. Opin. Solid State Mater. Sci. 2011, 15, 52−61. (6) Barkhordarian, G.; Klassen, T.; Dornheim, M.; Bormann, R. J. Alloys Comp. 2007, 440, L18−L21. (7) Dornheim, M.; Doppiu, S.; Barkhordarian, G.; Bösenberg, U.; Klassen, T.; Gutfleisch, O.; Bormann, R. Scr. Mater. 2007, 56, 841− 846. (8) Bösenberg, U.; Kim, J. W.; Gosslar, D.; Eigen, N.; Jensen, T. R.; Bellosta Von Colbe, J. M.; Zhou, Y.; Dahms, M.; Kim, D. H.; Günther, R. Acta Mater. 2010, 58, 3381−3389. (9) Bösenberg, U.; Vainio, U.; Pranzas, K.; Bellosta Von Colbe, J. M.; Goerigk, G.; Welter, E.; Dornheim, M.; Schreyer, A.; Bormann, R. Nanotechnology 2009, 20, 204003. (10) Nielsen, T. K.; Bösenberg, U.; Gosalawit, R.; Dornheim, M.; Cerenius, Y.; Besenbacher, F.; Jensen, T. R. ACS Nano 2010, 4, 3903− 3908. (11) Bösenberg, U.; Ravnsbæk, D. B.; Hagemman, H.; D’Anna, V.; Bonatto Minella, Ch.; Pistidda, C.; Wouter van Beek; Jensen, T. R.; Cerenius, Y.; Gutfleisch, O.; Bormann, R.; Dornheim, M. J. Phys. Chem. C 2010, 114, 15212−15217. (12) Price, T. E. C.; Grant, D. M.; Legrand, V.; Walker, G. S. J. Hydrogen Energy 2010, 35, 4154−4161. (13) Eigen, N.; Bösenberg, U.; Bellosta von Colbe, J.; Jensen, T. R.; Cerenius, Y.; Dornheim, M.; Klassen, T.; Bormann, R. J. Alloys Comp. 2009, 477, 76−80. (14) Brinks, H. W.; Fossdal, A.; Hauback, B. C. J. Phys. Chem. C 2008, 112, 5658−5661. (15) Corno, M.; Pinatel, E.; Ugliengo, P.; Baricco, M. J. Alloys Comp. 2011, 509, 679−683. (16) Yin, L.; Wang, P.; Fang, Zh.; Cheng, H. Chem. Phys. Let. 2008, 450, 318−321. (17) Messer, Ch. E.; Mellor, J. J. Phys. Chem. 1960, 64, 503−505. (18) Polenko, A. N.; Tabachnic, M. E.; Cholakh, S. O.; O’ConnelBronin, A. A. Phys. Status Solidi 1993, 179, 411−418.

Figure 5. In situ SR-PXD for LiF−MgB2 composite with molar ratio of 4:1 at hydrogen absorption under 60 bar H2 and 390 °C. X-ray wavelength of 1.07200 Å was used for the milled sample (a) and 0.94608 Å for the sample after second desorption (b).

4. CONCLUSIONS In the present work the influence of stoichiometry on hydrogen sorption in LiF−MgB2 systems was studied. For the LiF−MgB2 composites with different molar ratios of 1:1; 2:1, and 4:1 the following general conclusions can be drawn: 1 The changes in stoichiometry of the LiF−MgB2 systems influence their hydrogen sorption reactions. Excess of MgB2 improves kinetics and reversibility though the hydrogen storage capacity is only ∼5 wt % H2. Excess of LiF leads to no improvements neither in kinetics nor in reversibility and the value of hydrogen capacity does not reach even 5 wt % H2. 2 Some enhancement of hydrogen absorption kinetics and at the same time impediment for desorption kinetics is observed in the LiF−MgB2 systems compared to the LiH−MgB2, most probably, because of the appearance of thermodynamically more stable compound than MgH2, i.e., MgF2 (ΔfH° for MgF2 and MgH2 are −1124.2 and −75.3 kJ/mol, respectively22). Changing in stoichiometry of the studied LiF−MgB2 composites suggests that the reaction mechanisms should be similar and the different molar ratios influence hydrogen storage capacity, only. 3 After hydrogenation of the LiF−MgB2 composites with different molar ratios of 1:1; 2:1, and 4:1, the main reaction products are LiBH4 and MgF2. The order from the slowest to the fastest kinetics is (4:1) < (2:1)19 < (1:1). A probable reason might be the amount of MgB2 in the LiF−MgB2 mixtures, which is less stable than LiF 7014



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(19) Gosalawit, R.; Bellosta von Colbe, J. M.; Dornheim, M.; Jensen, T. R.; Cerenius, Y.; Bonatto, Ch. M.; Peschke, M.; Bormann, R. J. Phys. Chem. C 2010, 114, 10291−10296. (20) Cerenius, Y.; Ståhl, K.; Svesson, L.; Ursby, T.; Oskarsson, Å.; Albertsson, J. J. Synchrotron Rad. 2000, 7, 203−208. (21) Ravnsbæk, D.; Mosegaard, L.; Jørgensen, J. E.; Jensen, T. R.. In Energy materials − advances in characterization, modelling and application; Andersen, N. H., Eldrup, M., Hansen, N., Juul Jensen, D., Nielsen, E. M., Nielsen, S. F., Sørensen, B. F., Pedersen, A. S., Vegge, T., West, S. S., Eds.; Proceedings of the 29th Risø international symposium on materials science, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Risø, Denmark, 2008. (22) Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1993−1994.

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Influence of Stoichiometry on the Hydrogen Sorption Behavior in the

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