Nanoconfined Magnesium Borohydride for Hydrogen Storage

(8) In the present work, the combination of SANS and in situ SAXS on the infiltrated ... Pore size distributions (PSD) were calculated based on a DFT ...
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J. Phys. Chem. C 2010, 114, 18785–18789

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Nanoconfined Magnesium Borohydride for Hydrogen Storage Applications Investigated by SANS and SAXS Sabrina Sartori,*,† Kenneth D. Knudsen,† Zhirong Zhao-Karger,‡ Elisa Gil Bardaji,‡ Jiri Muller,† Maximilian Fichtner,‡ and Bjørn C. Hauback† Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway, and Karlsruhe Institute of Technology, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany ReceiVed: June 25, 2010; ReVised Manuscript ReceiVed: September 30, 2010

Nanoscale hydride systems have recently gained increased attention for their possible energy storage applications. In the present work, nanoscale particles of Mg(11BD4)2 infiltrated into an activated carbon scaffold were studied by small-angle scattering techniques, and their behavior was compared with that of the bulk powders. Upon heating to 400 °C, under dynamic vacuum, the nanoconfined particles maintain their size distribution, and the decomposition affects only the morphology of the particle surface. On the contrary, the bulk powders showed a significant modification of both particle size and surface morphology under the same conditions. The carbon scaffold therefore serves to ensure both the desired nanoscale organization of the magnesium borohydride and stabilization in size of the incorporated material. 1. Introduction Storage of hydrogen in a safe and efficient medium is a major challenge for the introduction of hydrogen as an energy carrier. Among possible candidates considered to meet the long-term goals given by the U.S. Department of Energy (DOE) with respect to gravimetric and volumetric hydrogen capacities are solid compounds based on lightweight elements. Complex hydrides based on boron exhibit high weight capacity for hydrogen and are, therefore, considered good candidates to meet the gravimetric capacity requirements. Mg(BH4)2, for example, with 14.9 mass% hydrogen and reaction enthalpy from the decomposition typical for that of low/medium-temperature hydrides,1,2 is one of the most promising materials. Unfortunately, its high kinetic barriers lead to the main hydrogen desorption at temperatures above 300 °C.1,2 Improvements and changes in the hydrogenation/dehydrogenation performances of hydrides (such as LiBH4, Mg(BH4)2, and NaAlH4) when confined in microporous and mesoporous scaffolds have been reported.3-6 Small-angle neutron scattering (SANS) on infiltrated magnesium borohydride has indicated a characteristic size for the particles in the range of 4 nm.7 Recently, small-angle X-ray scattering (SAXS) data suggested that the integration into the scaffold of sodium alanate stabilizes the size of the particles upon heating, while the bulk powders undergo changes.8 In the present work, the combination of SANS and in situ SAXS on the infiltrated Mg(11BD4)2 and the bulk alone has been used in order to gain information about the nanoconfinement of magnesium borohydride as well as on the material behavior at different temperatures. 2. Experimental Section The investigated samples are Mg(11BD4)2 infiltrated into pretreated activated carbon (sample labeled Mg(11BD4)2/AC1), following a wet impregnation procedure, and bulk Mg(11BD4)2. * Corresponding author. E-mail: [email protected]. Tel: +47 63806388. Fax: +47 63810920. † Institute for Energy Technology. ‡ Karlsruhe Institute of Technology.

The sample preparation, together with scanning electron microscopy and X-ray diffraction results, are described elsewhere.5,7 The deuterated version of this borohydride was employed in order to obtain good contrast and low incoherent background in the SANS experiments. The SANS experiments were carried out at the JEEP II reactor at IFE, Kjeller, Norway. The q range employed in the experiments was 0.008 to 0.25 1/Å. The samples were analyzed in 1 mm Hellma quartz cuvettes under argon atmosphere. For all samples, the transmission was sufficiently high (>90%) to neglect (or disregard) multiple scattering. Standard reductions of the scattering data including transmission corrections were conducted by incorporating data collected from an empty cell, beam without cell, and blocked-beam background. The data were transformed to an absolute scale (coherent differential cross-section (dΣ/dΩ)) by calculating the normalized scattered intensity from direct beam measurements.9 In situ SAXS patterns were collected at the beamline BM26B at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.10 The wavelength was 0.95 Å, and the sample was contained in a 0.8 mm boron-silica glass capillary and heated under dynamic vacuum from room temperature (RT) to 100 °C at a constant heating rate of 20 °C/min without data acquisition and then from 100 to 400 °C at a constant heating rate of 5 °C/min. Measurements of single frames at RT and 400 °C were also recorded. The temperature was accurately registered by means of a thermocouple situated in close proximity to the sample capillary. Calibration of the scattering vector was performed by using the diffraction peak position of a silver behenate standard powder. Standard correction for background subtraction and sample transmission were applied. The 2D scattering data were transformed into 1D using a Matlab macro. Wide angle X-ray scattering (WAXS) was recorded during the SAXS acquisition by means of a Photonics CCD camera with a 2 × 8 cm opening window, situated 14 cm from the sample at angle of approximately 20°. The data obtained were integrated and put on a linear scale by means of the Fit2D program.11

10.1021/jp1058726  2010 American Chemical Society Published on Web 10/18/2010

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Figure 1. SAXS (open dots) and SANS (full dots) measurements performed at RT on AC1, Mg(11BD4)2/AC1, and bulk Mg(11BD4)2. Figure 2. Size distribution obtained from the SAXS performed at RT on AC1 and Mg(11BD4)2/AC1.

The porosity of the carbon scaffolds and the prepared Mg(11BD4)2/AC1 composites was determined by a nitrogen adsorption method and analyzed by using the BET (BrunnerEmmett-Teller) method. Physisorption isotherms were collected on a micrometrics ASAP 2020 surface area and porosity analyzer (Micromeritics Instruments. Corp.) applying N2 gas at 77 K. Prior to measurement, the samples were degassed for 6 h at 90 °C for Mg(11BD4)2/AC1 composite and at 400 °C for the desorbed Mg(11BD4)2/AC1 material, respectively. Pore size distributions (PSD) were calculated based on a DFT model (NLDFT equilibrium model assuming slit-shaped pores). 3. Results and Discussion SANS and SAXS performed at room temperature on bulk Mg(11BD4)2, Mg(11BD4)2/AC1, and the scaffold alone are shown in Figure 1. The general trend for the scattering is similar for both techniques, as expected, because they probe the same structural components but with different contrasts. The relative intensity between the three samples is also the same; for instance, the Mg(11BD4)2/AC1 sample scatters more than the scaffold alone or the bulk. Furthermore, there is a crossover between the scaffold alone and the bulk just below 0.02 1/Å in both cases. Below this q value the bulk sample shows a relatively steep rise of the SANS intensity, indicating a significant amount of large particles for the bulk sample, whereas this effect is somewhat less clear with SAXS. In the middle q range, defined between the two dotted bars of Figure 1 (around 0.02 1/Å < q < 0.1 1/Å), and in the high q range (above 0.1 1/Å), the slopes obtained with the two methods agree quite well. As demonstrated from the measured high q slopes (2.8-2.9), the scaffold alone has an open structure even at the smallest length scales. A slope beneath 3.0 in this q range indicates a mass fractal behavior, i.e., a system where the individual scatterers are small enough to be present in a large number even at this short scale (1/q). On the other hand, when the scaffold is loaded with magnesium borohydride, the measured slope increases well above 3.0 (i.e., 3.8 for SAXS), showing the conversion to a surface fractal regime, where a majority of the pores have been filled. A value for the slope of 3.8 is close to 4.0, which would represent a perfectly smooth surface. The BET measurements on Mg(11BD4)2/AC1 showed a reduced surface area of 214 m2/g compared to the value of the scaffold alone (860 m2/g). With SANS we obtain a slightly smaller value (slope of 3.4) because of the difference in SANS contrast for the Mg(11BD4)2 and AC1 with respect to the surrounding voids. The scattering

length densities of Mg(11BD4)2 and AC1 are 7.2 × 10-6 1/Å2 and ca. 3 × 10-6 1/Å2, respectively, so that the individual features of Mg(11BD4)2 are enhanced with respect to the scaffold. With SAXS there is little difference in electron density, and thus the two components are more similar from the scattering point of view. Figure 2 shows the size distribution (volume distribution) obtained by means of an indirect Fourier transform12 performed on the SAXS curves at room temperature for the AC1 scaffold alone and the Mg(11BD4)2/AC1 system. The main peak moves from about 1.5 nm (15 Å) for the AC1 to about 2.5 nm (25 Å) in the case of the infiltrated system. The particle size distribution for Mg(11BD4)2/AC1 confirms small dimensions, in the range between 1 and approximately 6 nm (10 to 60 Å), of the hydride particles when confined in the scaffold. This result is in very good agreement with the characteristic feature at about 4 nm found with our preliminary interpretations of the SANS data.7 The shift in the peak position of Figure 2 from the scaffold to the Mg(11BD4)2/AC1 could be due to the fact that the smallest pores in the porous scaffold are not filled. BET measurements comparing the pore size distribution of the carbon scaffold AC1 (majority of the pores in the range of 1 to 1.5 nm) and the Mg(11BD4)2/AC1 composite show a clear increase in intensity around 2.5 nm for the latter. Similar samples with a higher loading than the one used in the present work showed that the scaffold is partially filled by the hydride, up to approximately 92% of the total pore volume.5 A slight expansion of the scaffold when infiltrated with the hydride has been proposed in another case where wet incipient impregnation of a Ni salt complex followed by temperature treatment yielded Ni particles which could be detected directly by HR-TEM. The average particle size was approximately 4-5 nm in that case which is also a little larger than the pore size determined by the physisorption method.13 However, this point needs further study. X-ray diffraction was not useful to study this infiltrated system because of the complete disappearance of the diffraction peaks compared to the bulk Mg(11BD4)2.7 However, in addition to the information given by SANS and SAXS at room temperature, in situ SAXS measurements by heating the sample from RT to 400 °C were performed. Figure 3 displays the acquired data at RT, 100 °C, and 400 °C for the Mg(11BD4)2/AC1, together with the data for the AC1 at RT. As shown in Figure 3, in the low/ middle q range, the curves follow a trend similar to that of the AC1 alone, and with almost no differences observed as the temperature was increased to 400 °C. This indicates that when

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Figure 5. WAXS on Mg(11BD4)2 at selected temperatures. Figure 3. SAXS on AC1 at RT and on Mg(11BD4)2/AC1 at RT, 100 °C, and 400 °C.

Figure 4. (a) Values of slope parameter at high q for the in situ SAXS on Mg(11BD4)2/AC1. (b) Porod plots of the AC1 scaffold and the Mg(11BD4)2/AC1 sample at selected temperatures.

infiltrated into the scaffold there is negligible reduction/ modification in particle sizes with temperature. The only detectable changes are localized on the surface of the particles corresponding to the high q, as shown in the inset of Figure 3. A more detailed study of the change in slope parameter at high q of selected SAXS data is presented in Figure 4a. The decrease in the slope parameter from ∼3.8 to ∼3.4 suggests that heating modifies the particles toward a more rough surface. This change in the characteristics of the surface could be related to the release of hydrogen during heating, as measured previously.5 Interfaces will often appear as smooth when investigated at a sufficiently

high q-value, i.e., the scattered intensity will follow the Porod behavior, being proportional to 1/q4. The constant of proportionality is then a measure of the specific surface in the system; thus, I(q) ) K (S/V) (1/q4), where for X-rays K is a function of the electron density difference between the particles and the surroundings. By plotting Iq4 as a function of q (Figure 4b) and considering the behavior in the high q limit, changes occurring at the surface are represented. The specific surface area for the sample in question will be directly proportional to the asymptotic level in the plot. Following this argument, Figure 4b shows Porod plots of the AC1 scaffold as well as the Mg(11BD4)2/AC1 sample at selected temperatures (the same as considered in Figure 3). For the AC1 scaffold, there is no sign of a plateau at high q, showing that the internal pore surface of the scaffold is rough down to the smallest length scale accessible, as also discussed in connection with Figure 1. Importantly, when the scaffold is loaded with magnesium borohydride, a plateau appears at a q value just above 0.10 1/Å, demonstrating that the interface has changed in appearance. At length scales smaller than what corresponds to this q value, i.e., a few nanometers, the surface pores have been smoothed out due to the presence of the magnesium borohydride. No changes in specific surface can be observed upon heating from RT to 100 °C. At 400 °C, on the other hand, a significant modification has occurred, and Iq4 increases linearly with q in the high-q range. This shows that the surfaces of the particles have now become rough at these small length scales, resulting in a significantly increased specific surface area with respect to the low-temperature situation. The BET surface area confirmed an increased value from 214 to 450 m2/g after the as-prepared Mg(11BD4)2/AC1 was heated under vacuum at 400 °C for 6 h. This would be consistent with a change in the surface of the infiltrated particles during the release of hydrogen and the decomposition of the hydride. Comparison of the infiltrated system with the bulk Mg(11BD4)2 is important in order to underscore the effect of the reduction in size and the stabilization of the hydrides due to the infiltration in the scaffold. In Figure 5 selected WAXS data collected on bulk Mg(11BD4)2 are reported. At 100 °C and below, the powders contain a mixture of the R and β modification (the patterns at RT, not shown here, and at 100 °C are similar). WAXS is useful to qualitatively support the interpretation of the changes in the slope parameter seen in the medium and high q range with SAXS (Figures 6 and 7) that will be discussed below. From the in situ WAXS data the transformation from R- to β-phase is clearly observed to start at around 160 °C, and it is almost complete at 180 °C, with a slight increase in the amount of the β-phase compared to the pattern at 100 °C. From about 240 °C the WAXS shows the decomposition of the

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Figure 6. SAXS on bulk Mg(11BD4)2 at selected temperatures: 100, 200, 300, 400 °C.

Figure 7. Values of slope parameter at medium (a) and high q (b) for the in situ SAXS on bulk Mg(11BD4)2.

β-phase. The observed final phases are Mg and MgO. We do not observe any MgH2 during the decomposition. As found previously,14 it seems that the formation of MgO (also present in our case) may kinetically limit the formation and decomposition of MgH2, which appears to occur at an unusually high temperature (from around 400 °C) and therefore not visible in our study.

Sartori et al. The SAXS curves perfectly superimpose from RT to 100 °C, where no changes in the material are expected, and therefore they are not reported. Above 100 °C (Figure 6), there is a decrease in the slope parameter at medium q and high q. More detailed plots of the slope parameters in these q ranges are shown in Figure 7a and 7b, as a function of the temperature. At the temperatures where R- to β-phase transformation occurs, a decrease in the slope parameter is observed (from about 2.8 to 2.7) at medium q, and an opposite increase in the values (from 4.2 to 4.5) at high q. This change in the slope parameter visible in the temperature range 140-180 °C is likely to be associated with the crystallographic changes from the hexagonal15,16 R-phase to the orthorhombic16 β-phase, when the unit cell volume is expanded from the 3435 Å3 for the R- to 7544 Å3 for the β-phase.16 From these values one can calculate a slight variation in the density from 0.78 to the 0.76 g/cm3 for the R-phase and β-phase, respectively. The slight decrease of the slope in the medium q may therefore be explained by a less compact system due to the expansion of the unit cell. For a system with a distinct transition in electron density, one would expect a slope parameter of 4 in the high-q (Porod) regime. Instead, the present data show that the high-q slope is above 4 at low temperatures (Figure 7b). It has been shown17 that such a behavior can occur when there is a gradual instead of a welldefined transition in electron density between the particles and the surrounding medium (voids in our case). Our data therefore suggest that below ca. 250 °C the surface region of the particles is somewhat diffuse, possibly due to a gradual transition from a crystalline nanoparticle core to a less dense region near the particle surface. The reason for the increase in the slope parameter at high q from 160 to 180 °C is not clear. As shown by the WAXS data (Figure 5), the transformation to β-phase is completed at 180 °C and its decomposition starts at 240 °C. The desorption study of Nakamori et al.1 (under argon flow and a heating rate of 5 °C/min) has shown a small desorption peak between 190 and 240 °C, indicating a direct decomposition of the remaining traces of R-phase which did not undergo the R- to β-phase transformation and consequently a release of hydrogen. Furthermore, Fichtner et al.5 found that the desorption of hydrogen starts at 150 °C (in a vacuum, such as in our case, but with a heating rate of 3 °C/min compared to 5 °C/min used here). Due to the relatively low resolution in the present in situ WAXS, this option cannot be excluded. The possible release of hydrogen below 240 °C could explain the slight decrease in the slope parameter at central and high q, from 180 °C (Figure 7a and 7b). The variation in the slope parameter in Figure 7a indicates that the compactness of the mass fractal of the systems is reduced with increasing temperature, and this could be connected with the release of hydrogen. Furthermore, from the SAXS data at high q, the release of hydrogen seems to change the surface from smooth to rough (reduction in slope from about 4 toward 3). When the decomposition of the β-phase starts at 240 °C, the changes in the slope parameters becomes even more significant up to 300 °C, where only Mg and MgO are detectable. In this range, the decomposition could lead to a less compact system due to possible production of smaller particles (also, the volume for the unit cell of Mg is 46 Å3, 75 Å3 for MgO). The increase in the slope parameter at higher temperatures could be related to a reorganization of the material, possibly a change in particle size and compactness due to the coexistence of different phases. 4. Conclusions A distribution of particle size in the range of approximately 1-6 nm of nanoconfined magnesium borohydride has been

Nanoconfined Mg(BH4)2 for Hydrogen Storage demonstrated. The integration of magnesium borohydride into a carbon scaffold showed to be an efficient way of obtaining the desired nanoscale organization of this material. Upon heating, the nanoconfined particles maintain their size distribution and the decomposition affects only the morphology of particle surface. On the contrary, bulk powders showed significant modification of both particle size and surface morphology under the same conditions. We expect that the significant improved kinetics already demonstrated for nanoconfined magnesium borohydrides could be related to these size changes. Acknowledgment. Funding under the EU project NANOHy (‘Novel Nanocomposites for Hydrogen Storage Applications’, contract no. 210092) is gratefully acknowledged. The skillful assistance from the project team at the BM26B, ESRF, Grenoble, is also gratefully acknowledged. References and Notes (1) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Li, H.; Ohba, N.; Towata, S.; Zuettel, A.; Orimo, S. Phys. ReV. B 2006, 74, 045126. (2) Chlopek, K.; Frommen, C.; Leon, A.; Zabara, O.; Fichtner, M. J. Mater. Chem. 2007, 17, 3496. (3) Vajo, J. J.; Olson, G. L. Scr. Mater. 2007, 56, 829.

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18789 (4) Zhang, Y.; Zhang, W. S. A.; Wang, Q.; Suna, L. X.; Fan, M. Q.; Chu, H. L.; Sun, J. C.; Zhang, T. Int. J. Hydrogen Energy 2007, 32, 3976. (5) Fichtner, M.; Zhao-Karger, Z.; Hu, J.; Roth, A.; Weidler, P. Nanotechnology 2009, 20, 204029. (6) Lohstroh, W.; Roth, A.; Hahn, H.; Fichtner, M. Chem. Phys. Chem. 2010, 11, 789. (7) Sartori, S.; Knudsen, K. D.; Zhao-Karger, Z.; Gil Bardaji, E.; Fichtner, M.; Hauback, B. C. Nanotechnology 2009, 20, 505702. (8) Sartori, S.; Knudsen, K. D.; Roth, A.; Fichtner, M.; Hauback, B. C. Nanosci. Nanotechnol. Lett., Special Issue on Nanomaterials and Nanoscale Phenomena for Clean Energy Applications, in press. (9) Wignall, G. D.; Bates, F. S. J. Appl. Crystallogr. 1987, 20, 28. (10) Bras, W.; Dolbnya, I. P.; Detollenaere, D.; Tol, R. v.; Malfois, M.; Greaves, G. N.; Ryan, A. J.; Heeley, E. J. Appl. Crystallogr. 2003, 36, 791. (11) Hammersley, A. P. FIT2D V12.077. Internal Report ESRF-97HA02T, 1997. Internal Report ESRF-98-HA01T, 1998. (12) Svergun, I. D. J. Appl. Crystallogr. 1992, 25, 495. (13) Fichtner, M.; Roth, A.; Kuebel, C. SSHS Workshop on H Storage Materials, June 10-11, 2009, Heraklion, Greece. (14) Riktor, M. D.; Sørby, M. H.; Chlopek, K.; Fichtner, M.; Buchter, F.; Zuettel, A.; Hauback, B. C. J. Mater. Chem. 2007, 17, 1. (15) Cerny, R.; Filinchuk, Y.; Hagemann, H.; Yvon, K. Angew. Chem. 2007, 46, 5765. (16) Her, J.-H.; Stephens, P. W.; Gao, Y.; Soloveichik, G. L.; Rijssenbeek, J.; Andrus, M.; Zhao, J.-C. Acta Crystallogr. 2007, B63, 561. (17) Beaucage, G. J. Appl. Crystallogr. 1995, 28, 717.

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