ARTICLE pubs.acs.org/JPCC
2LiBH4MgH2 in a ResorcinolFurfural Carbon Aerogel Scaffold for Reversible Hydrogen Storage Rapee GosalawitUtke,†,‡,* Thomas K. Nielsen,§ Klaus Pranzas,† Ivan Saldan,† Claudio Pistidda,† Fahim Karimi,† Daniel Laipple,† Jørgen Skibsted,|| Torben R. Jensen,§ Thomas Klassen,† and Martin Dornheim† †
Institute of Materials Research, Materials Technology, HelmholtzZentrum Geesthacht, D21502 Geesthacht, Germany School of Chemistry, Institute of Science, Suranaree University of Technology, NakhonRatchasima 30000, Thailand § Center for Energy Materials, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK8000 Aarhus C, Denmark Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK8000 Aarhus C, Denmark
)
‡
ABSTRACT: The reactive hydride composite of 2LiBH4MgH2 has been melt infiltrated in a resorcinolfurfural (RFF) carbon aerogel scaffold. Dried aerogel of RFF, further pyrolyzed to obtain a carbon aerogel scaffold, is prepared by CO2 supercritical drying, where time consumption is significantly lower than the normal procedures of solvent exchange and drying under ambient conditions. On the basis of scanning electron microscopy energy dispersive X-ray spectroscopy (SEMEDS) mapping, the complex hydrides are homogeneously dispersed in the nanometer scale both inside the nanopores and over the surface of the RFF carbon aerogel. Synchrotron radiation powder X-ray diffraction (SRPXD) and Raman results reveal the reaction mechanisms during melt infiltration, de- and rehydrogenation of this system, as well as the differences from the previous studies of the nanoconfined 2LiBH4MgH2 in a resorcinolformaldehyde (RF) carbon aerogel. Thermogravimetric and hydrogen titration measurements reveal a significant improvement in dehydrogenation kinetics of 2LiBH4MgH2RFF as compared with the bulk 2LiBH4MgH2 system. For instance, an approximate single-step dehydrogenation together with almost 100% of the total hydrogen storage capacity is accomplished within 6 h during the first dehydrogenation, while the bulk material performs clearly two-step reaction and requires ∼30 h (at T = 45 °C and p(H2) = 34 bar). Moreover, the gravimetric hydrogen storage capacity in the range of 4.24.8 wt % (∼1011.2 wt % H2 with respect to the hydride content) is maintained over four dehydrogenation and rehydrogenation cycles.
1. INTRODUCTION Lithium borohydride (LiBH4) has been extensively studied as a promising hydrogen storage material because of its high gravimetric hydrogen storage capacity of 18.5 wt % H2.1 Nevertheless, unfavorable kinetics and/or thermodynamics for dehydrogenation and rehydrogenation (e.g., high desorption temperatures above 380 °C and slow rehydrogenation at 600 °C under 150 bar H2) and in some cases the release of toxic diborane (B2H6) during decomposition have diminished its practical applications as a hydrogen storage medium.25 To improve the above characteristics, several approaches, such as catalytic doping6,7 and Reactive Hydride Composites (RHCs),2,812 have been explored. The addition of metal halides and oxides (Tiisopropoxide, TiCl3, ZrCl2, VCl3, MgCl2, TiO2, SiO2, and V2O3) to LiBH4 were carried out to improve the hydrogen release and uptake kinetics as well as to decrease the dehydrogenation temperature.6,7,1315 In the case of RHCs, mixtures of hydrogen-containing compounds and borohydrides have been considered. For example, r 2011 American Chemical Society
the LiBH4MgH2 system showed a reduction in de-/rehydrogenation enthalpy due to the formation of MgB2 upon dehydrogenation. Moreover, the required temperature and pressure conditions are milder with respect to pure LiBH4. However, all limitations associated with practical applications have not been overcome yet; thus, further improvements are still in progress. In this context, nanoengineering on the basis of reducing the particle size of the metal hydrides is currently considered as an attractive strategy to improve the hydrogen release and uptake kinetics. Fichtner and de Jongh et al. have shown that specific properties (enhanced surface interactions, faster kinetics, increased number of defects, and modified phase transformations) were remarkably obtained by decreasing the size of the hydride phases to the nanometer scale, leading to an improved dehydrogenation Received: September 13, 2011 Revised: November 29, 2011 Published: December 01, 2011 1526
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The Journal of Physical Chemistry C behavior as compared with the bulk materials.16,17 High-energy or reactive ball milling has been reported as an effective method for reducing the dimension of the crystallites of the designed metal hydrides to the nanoscale; however, the accurate particle sizes for the hydride phases could not be controlled.1820 Moreover, hydrogen release and uptake cycles at elevated temperatures favor agglomeration and sintering effects, resulting in a loss of the nanoscale-related properties.12,21,22 Recently, the use of foreign nanoscale structure-directing agents, such as mesoporous silica and carbon scaffold materials, has been developed as a host medium for not only the confinement of the complex hydrides but also preserving their nanosctructure during cycling. Several reports have revealed that nanoconfinement of metal hydrides facilitates enhanced kinetics for reversible hydrogen storage relative to the bulk materials.2325 Gutowska et al. showed that nanoconfinement of NH3BH3 in mesoporous silica lowered the activation barrier from 184 to 67 kJ/mol.26 However, Mosegaard et al. found that LiBH4 reacts with silicon dioxide, forming Li2SiO3 or Li4SiO4, even with relatively low amounts of SiO2.15 Therefore, several studies have focused on the confinement of metal hydrides in chemically inert microporous and mesoporous carbon scaffold materials.27,28 LiBH4 has been incorporated into an activated carbon (AC) scaffold by means of solvent impregnation, resulting in lowering dehydrogenation temperature of ΔT = 150 °C with respect to bulk LiBH4.29 For NaAlH4 confined in an ordered nanoporous carbon scaffold, it was found that not only a kinetic improvement was obtained but also an enhanced cycling stability was achieved; e.g., a high-capacity retention of >80% after 15 release and uptake cycles was obtained.30 Polycondensation of resorcinolformaldehyde (RF) aerogel has been considered as a carbon precursor for carbon scaffold materials preparation due to their unique properties, such as controllable mass densities, continuous porosities, large pore volumes, and high surface areas.31 Nanoconfinement of MgH2 in RF carbon aerogel was performed by infiltrating a dibutylmagnesium solution (MgBu2) and hydrogenating MgBu2 to MgH2.32,33 Dehydrogenation kinetics of the confined hydride was improved with respect to the bulk material, and it was found that smaller pores mediated a faster desorption rate by the size reduction of MgH2.29 Subsequently, Nielsen et al. prepared the nanoconfined 2LiBH4MgH2 composite in RF carbon aerogel, using the MgBu2 precursor and wet impregnation of molten LiBH4.23 Furthermore, direct melt infiltration of the ball-milled 2LiBH4MgH2 in RF carbon aerogel was also successfully performed.34 Both systems of the nanoconfined 2LiBH4MgH2 in RF carbon aerogel showed an improvement of not only the hydrogen desorption kinetics but also the degree of reversibility, stability, and possibly the thermodynamics. In the previous work,23,34 the carbon aerogel of the resorcinol formaldehyde (RF) precursor was prepared via several procedures of (i) polycondensation; (ii) solvent exchange from water to a low boiling point solvent (e.g., acetone); and (iii) drying in ambient condition. On the basis of steps (ii) and (iii) mentioned above, more than a week was required to accomplish a dried gel for carbonization. In this work, not only a new material of resorcinolfurfural (RFF) aerogel was used but also a new technique of CO2 supercritical drying was carried out for carbon aerogel preparation. By using the CO2 supercritical drying technique, time consumption for dried gel preparation decreased significantly; that is, instead of more than a week the dried gel was achieved within 12 h. Moreover, the weight ratio of composite
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hydride (2LiBH4MgH2):carbon aerogel increased from 1:2 (in the previous work34) to 1.5:2 to enhance hydrogen storage capacity of the system. After carbonization, the carbon aerogel scaffold obtained from RFF aerogel was nanoconfined with a bulk 2LiBH4 MgH2 composite via direct melt infiltration. The infiltration of the 2LiBH4MgH2 composite in the carbon aerogel scaffold has been confirmed by scanning electron microscopyenergy dispersive X-ray spectroscopy (SEMEDS) mapping together with the focused ion beam (FIB) technique and small angle X-ray scattering (SAXS). The reaction mechanisms during de-/rehydrogenation were determined and compared with the previous report of nanoconfined 2LiBH4MgH2 in the RF carbon aerogel scaffold34 by means of in situ synchrotron radiation powder X-ray diffraction (SRPXD) and nuclear magnetic resonance (NMR) spectroscopy. Desorption kinetics and cycling efficiencies of the nanoconfined 2LiBH4MgH2 have been evaluated by simultaneous differential scanning calorimetrythermogravimetric analysismass spectroscopy (DSCTGMS) and the Sievert-type method.
2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. The resorcinolfurfural (RFF) carbon aerogels were prepared according to previously published procedures.35 The aerogel was synthesized by mixing 5.0 g of resorcinol (99%, Aldrich), 11.20 mL of furfural (>98.0%, Fluka), 0.09 g of NaOH (>99.99%, Sigma-Aldrich), and 12.5 mL of ethanol (99.8%, Labscan) in a beaker with continuous stirring. The mixture was stirred until a homogeneous solution was obtained. The solution was decanted into a 50 mL glass bottle and sealed with a lid. The mixtures were aged at 80 °C in an oil bath for 4 days and afterward cooled in air to room temperature. Subsequently, CO2 supercritical drying was performed as reported elsewhere.36 The organic gels were wrapped in a filter paper and placed in a 4 L cylindrical stainless steel vessel. Supercritical CO2 was delivered using a high-pressure diaphragm pump, and it was introduced from the top of the vessel at a constant flow rate (100200 g/min). Temperature was maintained constantly at 40 °C using an oil heating jacket. The drying was completed after 12 h, and the gel was removed from the vessel. The dried gels were pyrolyzed in a tube furnace under nitrogen flow by heating the gels to 900 °C (5 °C/min) and dwelling at 900 °C for 5 h. The furnace was switched off, and the samples were slowly cooled in the furnace to room temperature. Finally, the obtained gels were dried at 400 °C under vacuum for 5 h, giving the samples denoted as RFF carbon aerogel. The powders of LiBH4 (>95.0%, Sigma-Aldrich) and MgH2 (Alfa Aesar GmbH & Co KG) in the molar ratio of 2:1 were milled for 5 h using a Spex 8000 M Mixer Mill, placed in a glovebox with an argon atmosphere, using stainless steel vials and balls (10 mm) with a ball-to-powder weight ratio (BPR) of 10:1. The mixture of RFF carbon aerogel and bulk 2LiBH4MgH2 at a weight ratio of 2:1.5 was ground in the mortar, denoted as mortarmixed 2LiBH4MgH2RFF. Melt infiltration was prepared using a Sievert-type apparatus by heating the powder sample of the mortar-mixed 2LiBH4MgH2RFF to 350 °C (10 °C/min) under 60 bar H2 and dwelling at 350 °C for 1 h. 2.2. Characterizations. The nanoporous RFF carbon aerogel was characterized by gas absorption and desorption using a Nova 2200e surface-area and pore-size analyzer from Quantachrome. The aerogel was degassed at 200 °C under vacuum for several hours, prior to the measurements. A full absorption and desorption 1527
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The Journal of Physical Chemistry C isotherm was measured in the pressure range from 0 to 1 p/p0 at liquid nitrogen temperatures with nitrogen gas as the adsorbent. Data were analyzed using the t-plot method,37,38 the Brunner EmmetTeller (BET) method,39 and the BarrettJoyner Halenda (BJH) method, and the total volume was calculated from a single point at p/p0 ∼1.40 Small-angle X-ray scattering (SAXS) measurements were performed at the HASYLAB Synchrotron, at beamline BW4 in the research laboratory of DESY, Hamburg, Germany. Data were recorded with a sample-to-detector distance of 1.042 and 13.40 m using an X-ray wavelength of 1.381 Å. Each scattering pattern was detected by a MAR 165 CCD detector (2048 2048 pixels, pixel size 79.1 μm 79.1 μm). The powder samples were mounted tightly between thin Kapton films inside the sample holder in a purified argon glovebox. The raw data were evaluated by the fit2d and Gnom45 programs to obtain pore size distribution of the RFF carbon aerogel before and after melt infiltration. In situ synchrotron radiation powder X-ray diffraction (SR PXD) experiments were carried out at the MAX II Synchrotron, beamline I711 in the research laboratory of MAX-Lab, Lund, Sweden.41 Each powder diffraction pattern was detected by a MAR165 CCD detector with a selected X-ray wavelength of 0.94608 Å. The powder samples were filled in airtight sapphire capillaries in a purified argon glovebox. Heating was applied by a tungsten wire placed under the capillary, whereas the temperature was controlled by an external PID regulator and a thermocouple inserted into the powder bed as shown elsewhere.42 On the basis of the temperature controller, the deviation between the real and measured temperatures was observed at elevated temperature. Melt infiltration was investigated by heating the powder sample of the mortar-mixed 2LiBH4MgH2RFF sample to 350 °C (10 °C/min) under 60 bar H2, followed by dwelling at 350 °C for 30 min and cooling to room temperature. Continuously, dehydrogenation was investigated by heating the infiltrated powder sample to 450 °C (10 °C/min) under 34 bar H2, keeping it at this temperature for 1 h and cooling it to room temperature. Rehydrogenation was carried out by heating the sample to 450 °C (10 °C/min) under 130 bar H2, using a dwelltime of 1 h at 450 °C, and subsequent cooling to room temperature. Scanning electron microscopy (SEM) was performed by using an Auriga from Zeiss, Germany. Energy dispersive X-ray spectroscopy (EDS) mapping was carried out using an apparatus from EDAX Inc., USA. Smart SEM and EDS Genesis programs were used for morphological studies and elemental analysis, respectively. The powder sample was deposited onto the sample holder using silver glue (in n-butylacetate). The sample was coated by palladiumgold sputtering with a current of 30 mA for 30 s under vacuum. An internal view of the specimen was prepared by the focused ion beam technique (FIB) using a Canixon from Orsay Physics, France. The specimen was shot by a gallium ion beam with the energy of 30 kV. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) were carried out simultaneously by Netzsch STA 409 in a purified argon glovebox. The infiltrated powder samples prepared by a Sievert-type apparatus and bulk 2LiBH4 MgH2 were heated from room temperature (20 °C) to 550 °C with a heating rate of 5 °C/min under an argon flow of 50 mL/min. The relative fraction of hydrogen in the exhaust gas was continuously measured in a Hiden HPR-20 QIC mass spectrometer (MS). 11 B magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded on a Varian INOVA-400 spectrometer (9.39 T), using a home-built X-{1H} double-resonance
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Table 1. Texture Parameters of the Pristine Nanoporous RFF Carbon Aerogel Scaffolda sample
BETb(m2/g)
microareac(m2/g)
Vtot (cm3/g)
RFF carbon aerogel
484.33
458.15
0.21
scaffold a c
Degass temperature was 200 °C, 524 h. b BET region 00.1 p/p0. t-plot region, 0.20.5 p/p0.
MAS NMR probe for 5 mm od rotors. The NMR experiments were performed at ambient temperatures and employed air-tight end-capped zirconia rotors packed with the sample in a purified argon-filled glovebox. The 11B isotropic chemical shifts are given in parts per million (ppm) relative to neat F3B 3 O (CH2CH3)2. Dehydrogenation, rehydrogenation, and cycle efficiencies were studied with a carefully calibrated Sievert-type apparatus (PCTPro-2000 from Hy-Energy LLC). The powder samples (∼120 mg) were filled in a high-pressuretemperature vessel and transferred to the Sievert-type apparatus. Dehydrogenation was performed on the infiltrated powder sample at 425 °C (5 °C/min) under 3.4 bar H2. For rehydrogenation, the dehydrogenated powder sample was heated to 425 °C (5 °C/min) and p(H2) ∼ 140 bar, where it was kept for 12 h. For comparison, bulk 2LiBH4 MgH2 was also dehydrogenated (T = 425 °C and p(H2) = 34 bar) and rehydrogenated (T = 425 °C and p(H2) ∼ 140 bar) by similar procedures.
3. RESULTS AND DISCUSSION 3.1. Melt Infiltration. A chemically inert carbon aerogel scaffold of RFF with a surface area and a total pore volume of 484.33 m2/g and 0.21 cm3/g, respectively, was used in this study (Table 1). To preliminarily confirm the nanoconfinement of 2LiBH4MgH2 in RFF carbon aerogel, SAXS and SRPXD experiments of mortar-mixed 2LiBH4MgH2RFF were carried out before, after, and during direct melt infiltration. From Figure 1, RFF carbon aerogel shows two different pore size distributions in nano- and micrometer ranges together with the peaks at 5.5 nm and 1 μm. It is found that the scattering intensity of 5.5 nm pores decreases after melt infiltration, while that of 1 μm pores is not significantly changed (Figure 1). This suggests nanoconfinement of the composite hydrides in RFF carbon aerogel after melt infiltration. The SRPXD results also indicate nanoconfinement of the composite hydrides and the reaction mechanism during melt infiltration. Figure 2 reveals characteristic Bragg reflections of LiBH4 and MgH2 at room temperature together with broad regions in the 2θ range of 1015°, corresponding to graphite like structural features within the RFF carbon aerogel.43 The polymeric phase transformation from o-LiBH4 to h-LiBH4 and the melting of h-LiBH4 are observed at ∼117 and 285 °C, respectively. By increasing the temperature to ∼335 °C, the decomposition of MgH2 is detected together with the formation of Mg and MgO (Figure 2). The formation of MgO could be due to the small amount of oxygen in the capillary during SRPXD experiments. After melt infiltration and cooling, the LiBH4 diffraction peaks appear broader and weaker, relative to those from mortar-mixed 2LiBH4MgH2RFF. This suggests that some of the LiBH4 is amorphous and some is nanocrystalline (neglecting possible strain).27 The average crystallite size of the nanoconfined LiBH4 after melt infiltration is calculated by the Scherrer formula,20 using the fwhm parameter of bulk LiBH4 as an internal 1528
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Figure 1. SAXS patterns of the mortar-mixed 2LiBH4MgH2RFF and its infiltrated powder sample.
Figure 3. SEM micrograph of the infiltrated powder sample of 2LiBH4MgH2RFF, that is, specimen shot by gallium ion beam (A), boron mapping mode (B), magnesium mapping mode (C), and quantitative analysis of the elements in the internal area (D).
Figure 2. SRPXD spectra of the mortar-mixed 2LiBH4MgH2 RFF during melt infiltration under 60 bar H2 with the temperature program of (i) heating to 350 °C (10 °C/min); (ii) dwelling at 350 °C for 30 min; and (iii) cooling to room temperature.
standard for the instrumental broadening and neglecting possible stress and strain. A crystallite size of ∼28 nm is found, which is larger than the 5.5 nm pore size of the RFF carbon aerogel, revealed by the SAXS results. This implies that LiBH4 not only is confined in the small nanopores but also occupies larger pores of the RFF carbon aerogel. On the basis of the previous work of the nanoconfined 2LiBH4MgH2 in RF aerogel (31 nm pore size),34 there are some differences in reaction mechanisms during melt infiltration as compared with the present studies. For example, the SRPXD results during melt infiltration of the nanoconfined 2LiBH4MgH2 in the RF carbon aerogel sample showed that not only nanoconfinment of LiBH4 was successfully obtained but also partial dehydrogenation of both LiBH4 and MgH2 was observed, indicated by the formation of MgB2. In the case of nanoconfinement in RFF carbon aerogel, SRPXD patterns (Figure 2) revealed the broader peaks of LiBH4 after melt infiltration together with complete dehydrogenation of MgH2 to Mg; however, there was no MgB2 formed during melt infiltration. This suggests nanoconfinement of LiBH4 in RFF carbon aerogel without dehydrogenation.
To further clarify the state of the composite hydrides with RFF carbon aerogel, SEMEDS mappings were carried out. The infiltrated sample was shot by a gallium ion beam using the FIB technique (Figure 3A) leading to the exposure of an internal area with ∼5 μm depth (see the red square). The homogeneous dispersion of boron (from LiBH4) and magnesium (from MgH2) atoms in nanometer scale is observed inside and on the surface of the aerogel (Figure 3BC). On the basis of SAXS, SRPXD, and SEMEDS mapping results, it is found that the hydride composite is not only nanoconfined inside the porous structures of the carbon aerogel but also disperses homogeneously on the surface. Figure 3D exhibits a significant amount of Mg from nanoconfined and surface-occupied MgH2 as well as carbon (C) from the RFF carbon aerogel. For boron (B) (from LiBH4), its signal is considerably adjacent to that of C. On the basis of the limitation of EDS experiments, which is not sensitive to the light elements, B is not considerably detected. However, the infiltration of LiBH4 was confirmed by SAXS, SRPXD, and DSCTGMS (the results are shown and discussed in Section 3.2) results. In the case of oxygen (O) and gallium (Ga), they are obtained from oxidation of complex hydrides and FIB technique, respectively. 3.2. Dehydrogenation and Rehydrogenation. The hydrogen capacity of the composite hydride 2LiBH4MgH2 is 11.6 wt % according to eq 18,11,44 2LiBH4ðlÞ þ MgH2ðsÞ T 2LiHðsÞ þ MgB2ðsÞ þ 4H2ðgÞ ð1Þ The nanoconfined composite of 2LiBH4MgH2RFF, thus, has a theoretical hydrogen storage capacity of 4.97 wt %, corresponding to 11.6 wt % H2 with respect to hydride content. For preliminary studies, dehydrogenation behaviors of bulk 2LiBH4 MgH2 and an infiltrated sample of 2LiBH4MgH2 in RFF carbon aerogel were determined using simultaneously performed DSCTGMS measurements. For the bulk material, the polymeric phase transformation from o-LiBH4 to h-LiBH4 (peak A) and melting of h-LiBH4 (peak B) of the bulk material are observed at 116.7 and 291.7 °C, respectively (DSC curve of Figure 4A). 1529
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Figure 5. SRPXD spectra of infiltrated powder sample of 2LiBH4 MgH2RFF (from Figure 2) during dehydrogenation under 34 bar H2 with the temperature program of (i) heating to 450 °C (10 °C/min) and (ii) dwelling at 450 °C for 1 h.
Figure 4. Simultaneously performed DSCTGMS measurements of bulk 2LiBH4MgH2 (A) and infiltrated powder sample of 2LiBH4 MgH2RFF (B).
Thereafter, dehydrogenation of MgH2 (peak C) is observed as an endothermic peak at 304 °C, while those of LiBH4 (peaks D1, D2, and D3) are separated into three endothermic peaks at 429.2, 451.7, and 520 °C. These dehydrogenation peaks are consistent with the hydrogen signal (m/e = 2) obtained from the MS results. In the case of the hydrogen storage capacity, the TG thermogram shows clearly a two-step reaction together with a small dehydrogenation after 500 °C, corresponding to peaks C, D1 + D2, and D3 together with 10 wt % H2 (TG thermogram of Figure 4A). For the infiltrated 2LiBH4MgH2 in the RFF carbon aerogel sample, peaks A and B are observed at 108.7 and 278.6 °C, respectively (DSC curve of Figure 4B). It should be noted that the peak temperatures of peaks A and B from infiltrated 2LiBH4 MgH2 in the RFF carbon aerogel sample are shifted considerably to lower values (i.e., ΔT = 8 and 13 °C, respectively) as compared to the bulk material. This agrees with the previous reports for LiBH4 confined within small pores.20 Thereafter, two endothermic peaks (C and D) are observed at 372 and 440.5 °C, respectively, as well as a hydrogen signal (MS results), which is in accordance with the dehydrogenation of MgH2 and LiBH4, respectively (Figure 4B). Moreover, a hydrogen storage capacity of ∼3.1 wt % (∼7.2 wt % H2 with respect to hydride content) is achieved from the TG thermogram. This inferior amount of hydrogen released as compared to the theoretical value (4.97 wt % H2) could be due to the partial dehydrogenation of composite hydrides during the ball-milling process. Furthermore, the complete dehydrogenation of MgH2 to Mg during melt infiltration (Figure 2) could also cause the reduction of hydrogen amount released during the dehydrogenation step. Therefore, to confirm the actual hydrogen storage capacity of the 2LiBH4MgH2 RFF system, the de-/rehydrogenation in several cycles were
carried out (the results and discussion are shown in Figures 8 and 9 and Section 3.3). Moreover, it should be noted that peak D of infiltrated 2LiBH4MgH2 in the RFF carbon aerogel sample shifts meaningfully to lower temperature as compared to peaks D2 and D3 of the bulk material (ΔT = 11 and 79.5 °C, respectively). Furthermore, the dehydrogenation of 2LiBH4MgH2 RFF is approximately a single-step reaction (TG thermogram in Figure 4B), which is almost similar to the previous report of nanoconfined 2LiBH4MgH2 in RF aerogel.34 Although peaks C and D of the 2LiBH4MgH2RFF system do not completely combine into one peak as in the previous work,34 the DSC curve (Figure 4B) shows the continuous dehydrogenation of LiBH4 instantly after MgH2. This is proven by partial overlap between two endothermic peaks (C and D in DSC curve) and broad hydrogen signal (MS results). It also confirms the significant improvement of the dehydrogenation kinetics of the 2LiBH4MgH2 RFF system over the bulk material. Furthermore, the reaction mechanisms during dehydrogenation were studied by SRPXD experiments. The sample was heated to 450 °C (10 °C/min) under a hydrogen pressure of 34 bar and dwelled at 450 °C for 1 h. From Figure 5, the diffraction peaks of o-LiBH4, Mg, and MgO are detected at room temperature. Thereafter, phase transformation (from o-LiBH4to h-LiBH4) and melting of h-LiBH4 are observed at ∼109 and 277 °C, respectively, which is consistent with the DSC results (Figure 4B). Moreover, the diffraction peaks of MgH2 are temporarily found in the temperature range of 280288 °C. This could be residual MgH2, which did not dehydrogenate during melt infiltration. Diffraction peaks from MgB2 arise at ∼380 °C, whereas those of Mg considerably decrease (Figure 5), indicating that molten LiBH4(l) reacts with Mg(s) and forms MgB2(s). This phenomenon is similar to bulk 2LiBH4MgH2.45 In the case of LiH, the other theoretical dehydrogenated product of LiBH4, its diffraction peaks are not observed. This could be due to the fact that nanosized LiBH4 (∼28 nm calculated by the Scherrer formula using the fwhm parameter from SRPXD results in Figure 2) dehydrogenated and produced nanoparticle of LiH, which could not be detected by SRPXD. Interestingly, LiBH4 starts to release 1530
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Figure 6. SRPXD spectra of the dehydrogenated powder sample (from Figure 5) during rehydrogenation under 130 bar H2 with the temperature program of (i) heating to 450 °C (10 °C/min); (ii) dwelling at 450 °C for 1 h; and (iii) cooling to room temperature.
hydrogen (at 380 °C, where MgB2 is generated) almost immediately after dehydrogenation of MgH2 (at 372 °C as shown in the DSC curve in Figure 4B), relevant to the partial overlap between dehydrogenation peaks of MgH2 and LiBH4 together with the broad hydrogen signal (DSC and MS results in Figure 4B). It should be noted that dehydrogenation of LiBH4 in the present studies was clearly separated from melt infiltration clarified by the formation of MgB2 only during dehydrogenation. In the case of the previous studies of the nanoconfined 2LiBH4MgH2 in RF carbon aerogel,34 dehydrogenation of both LiBH4 and MgH2 had been observed since melt infiltration, and it continuously increased during dehydrogenation as shown as the enhancement of MgB2 diffraction intensity (SRPXD results). However, incomplete reaction between MgH2 and LiBH4, evidenced by residual Mg after dehydrogenation, was observed in both studies of nanoconfinement in RF and RFF carbon aerogels. In the next step, the reversibility of the 2LiBH4MgH2RFF system was confirmed by performing rehydrogenation in SRPXD. The dehydrogenated powder sample was heated to 450 °C under hydrogen pressure and dwelled for 1 h (10 °C/min, p(H2) = 130 bar). Figure 6 reveals diffraction peaks of Mg and MgB2 at room temperature. Afterward, the diffraction peaks of Mg slightly decrease at about 220 °C and completely disappear at 300 °C, leading to the formation of MgH2. However, at about 400 °C, the dehydrogenation of MgH2 is observed, and it yields regaining of Mg. This spectacle of Mg and MgH2 during rehydrogenation is relevant to the previous studies of nanoconfined 2LiBH4MgH2 in RF aerogel.34 Thereafter, the intensity of Mg peaks diminishes continuously and disappears during isothermal conditions (at 450 °C), resulting in the recovery of MgH2 observed during the cooling state. In the case of MgB2, its diffraction peaks decrease gradually and vanish at about 400 °C, while a new phase mentioned as an unknown phase arises (Figure 6). The formation of o-LiBH4, which is observed by its corresponding peaks at room temperature, is accomplished. These results confirm the reversibility of the 2LiBH4MgH2RFF hydrogen storage system. MgO is found throughout the experiment. For further investigation on the amorphous contents after rehydrogenation of the 2LiBH4MgH2RFF sample, 11B MAS NMR was employed since this method detects crystalline as well as amorphous phases
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Figure 7. 11B MAS NMR spectrum (9.4 T) of the central-transition region for the rehydrogenated powder sample of 2LiBH4MgH2RFF. The spectrum employed a spinning speed of νR = 10.0 kHz, a short excitation pulse (τp = 0.5 μs), a 4 s repetition delay, and 824 scans.
with high sensitivity for 11B and allows distinction of different borohydride phases. The 11B MAS NMR spectrum in Figure 7 includes two centerband resonances, where the dominating resonance at 41.2 ppm originates from LiBH4.46 This resonance from LiBH4 contains 92% of the centerband intensity, demonstrating a nearly complete rehydrogenation. The low-intensity resonance at 15.1 ppm is ascribed to a B12H12 species, reflecting a small degree of LiBH4 decomposition. With respect to the tentative assignment which follows earlier studies of the NMR peak at 15 ppm as well as our system including Mg (from MgH2) and Li (from LiBH4), this B12H12 phase could be attributed to MgB12H12 and/or LiB12H12.47,48 Consequently, the results from SRPXD and 11B NMR strongly suggest that the rehydrogenated products are LiBH4, MgH2, MgB12H12, and/or LiB12H12 and an unknown phase. On the basis of the previous studies,34 different reaction mechanisms and rehydrogenated products were accomplished. For example, there were only LiBH4 and MgH2 detected after rehydrogenation of the sample of nanoconfined 2LiBH4MgH2 in RF carbon aerogel, while the system of RFF carbon aerogel showed not only LiBH4 and MgH2 but also MgB12H12 and/or LiB12H12 and the unknown phase. Moreover, the present studies show that although MgB12H12 and/or LiB12H12 are mentioned as byproducts of rehydrogenation nearly complete rehydrogenation, revealed by 92% centerband intensity of LiBH4 (NMR results), is obtained. 3.3. Kinetics, Reversibility, and Hydrogen Storage Reproducibility. Four hydrogen release and uptake cycles were performed in the Sievert-type apparatus to confirm kinetic improvement and reversibility as well as hydrogen storage reproducibility of the 2LiBH4MgH2RFF hydrogen storage system. As shown in Figure 8A, 2LiBH4MgH2RFF releases 3.4 wt % H2 after 6 h during first dehydrogenation. This is in agreement with the hydrogen storage capacity of 3.1 wt % from the TG thermogram (Figure 4B). The first rehydrogenation was continuously performed at 425 °C under 140 bar H2 for 12 h. It results in the significant improvement in hydrogen content released during the second dehydrogenation of 4.8 wt %, which is 11.2 wt % H2 with respect to the hydride content, after 25 h. In the case of the second and third rehydrogenation, the same conditions as in the first cycle were used. Figure 8A shows the hydrogen storage capacity of 4.3 and 4.2 wt % (∼10 wt % H2 as compared to complex hydride content) after 28 h during the third and fourth dehydrogenations, respectively. It should be noted that the hydrogen 1531
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Figure 9. Normalized hydrogen desorption profiles (data from Figure 8(A) and (B)).
Figure 8. Hydrogen desorption profiles and cycling efficiency of 2LiBH4MgH2RFF (A) and bulk 2LiBH4MgH2 (B) at 425 °C under 34 bar H2.
contents released during the second, third, and fourth cycles are approximately in accordance with the theoretical value of 4.97 wt % H2. The greater hydrogen content achieved from the second, third, and fourth dehydrogenations as compared to the first cycle could be due to the dehydrogenation of MgH2 during melt infiltration and the ball-milling process as mentioned previously (Section 3.2). Moreover, all four dehydrogenations of 2LiBH4MgH2 RFF perform approximately single-step reaction which is consistent to the TG thermogram. The inset shows that among all cycles the first dehydrogenation exhibits the best kinetics, while those of the second, third, and fourth are comparable (Figure 8A). This spectacle could be explained by the incomplete formation of MgB2 after the first dehydrogenation of 2LiBH4MgH2RFF (Section 3.2 and Figure 5), which was confirmed by the residual Mg. Vajo et al.8 and Barkhordarian et al.11 showed that the formation of MgB2 upon dehydrogenation could enhance hydrogen sorption properties of bulk 2LiBH4MgH2. Therefore, the Mg atoms, which were not converted entirely to be MgB2 after the first dehydrogenation, could cause the sluggish kinetics in the next cycle. In addition, the formation of MgO during melt infiltration (Figure 1) could also result in the loss of kinetic properties. For comparison, the bulk 2LiBH4MgH2 was also investigated using the Sievert-type method under the same temperature and pressure conditions as in 2LiBH4MgH2RFF for both dehydrogenation and rehydrogenation. From Figure 8B, the twostep dehydrogenation, referring to the dehydrogenation of MgH2 and the subsequent decomposition of LiBH4 together with 10.6 wt % H2, is obtained after 30 h during the first dehydrogenation. For the second dehydrogenation, 10 wt % H2 is accomplished after 28 h. The inset shows the slight improvement in hydrogen desorption kinetics of the second cycle over the first
ones. On the basis of titration and DSCTGMS experiments, it is confirmed that the 2LiBH4MgH2RFF system showed almost single-step dehydrogenation, while the bulk 2LiBH4MgH2 was found to be an obviously two-step reaction. The shift of the desorption temperatures to lower values suggests the significant improvement in kinetic properties of the 2LiBH4MgH2RFF system over the bulk material, although the hydride composite was not totally confined in the porous structures of the RFF carbon aerogel as in the previous work.34 Another explanation could be a catalytic activity of carbon itself, which could favor the kinetic improvement as previously reported.49,50 However, the hydrogen sorption properties of the complex hydrides, based on kinetics and storage capacity, were not significantly considerable, when carbon powder was added as a catalyst (by ball-milling or simply physical mixing). Therefore, in this study the nanoconfinement of 2LiBH4MgH2 in the RFF carbon aerogel itself should play an important role for the kinetic improvement more than a potential catalytic effect of carbon. Moreover, ∼96% reproducibility of the hydrogen storage capacity is accomplished over four cycles (excluding the first dehydrogenation) of 2LiBH4 MgH2RFF, while that of the bulk material is ∼90%. To compare the kinetic improvement of the 2LiBH4MgH2 RFF system over the bulk material, normalized dehydrogenation plots were considered. As shown in Figure 9, it is clearly seen that the dehydrogenation kinetics of the 2LiBH4MgH2RFF for all cycles are considerably faster than those of the bulk material. For example, the 2LiBH4MgH2RFF sample releases almost 100% of the hydrogen within 6 h during the first dehydrogenation, whereas the bulk material requires ∼30 h for a similar hydrogen fraction (Figure 9). Interestingly, after the first cycle, the 2LiBH4MgH2RFF sample provides approximately constant kinetics as shown as the comparable dehydrogenation rate during the second, third, and fourth cycles. Moreover, the hydrogen storage capacity in the range of 4.24.8 wt % (almost 100% reproducibility) of 2LiBH4MgH2RFF over four hydrogen release and uptake cycles is considerably higher than those of both nanoconfined 2LiBH4MgH2 prepared either via direct melt infiltration in 31 nm aerogel (i.e., 3.6 wt % H2)34 or via impregnation of MgBu2 and continuous LiBH4 melt infiltration in 21 nm aerogel (i.e., 3.63.9 wt % H2).23 In the present studies, it should be noted that although the hydrogen storage capacity is increased by enhancing the amount of composite hydride it does not affect the superior kinetics of the system over the bulk material, in accordance with the previous systems.23,34 1532
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4. CONCLUSIONS Direct melt infiltration of the 2LiBH4MgH2 complex hydride in resorcinolfurfural (RFF) carbon aerogel was carried out for hydrogen storage material preparation. By using a new technique of CO2 supercritical drying, a significant decrease in time consumption for dried gel preparation was obtained. SRPXD, SAXS, and SEMEDS mapping results confirmed that both LiBH4 and MgH2 were homogeneously dispersed in the nanometer scale both inside the porous structure and over the surface of the RFF carbon aerogel. From DSC and MS results, the dehydrogenation LiBH4 approached to that of MgH2 for the 2LiBH4MgH2RFF system, while that of bulk material was clearly separated. This resulted in approximately single-step dehydrogenation observed in the TG thermogram and titration results of 2LiBH4MgH2RFF, and it suggested the considerable kinetic improvement as compared to the bulk material. This was in agreement with the previous work, where the kinetic properties were also improved after nanoconfinement of the composite hydride in the RF carbon aerogel;34 however, different reaction mechanisms based on melt infiltration, dehydrogenation, and rehydrogenation were determined. The reversible hydrogen storage capacity in the range of 4.24.8 wt % (1011.2 wt % H2 with respect to hydride content) was preserved over four hydrogen release and uptake cycles. Therefore, this work proved that simply melt infiltration of complex hydrides in the nanoporous structure of RFF carbon aerogel could provide significant improvement in hydrogen desorption kinetics. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to acknowledge the financial support from the Research, Development and Engineering (RD&E) fund through The National Nanotechnology Center (NANOTEC), The National Science and Technology Development Agency (NSTDA), Thailand (P-11-00991) to Suranaree University of Technology. We are also thankful to the Alexander von Humboldt Foundation, Germany. We would like to thank Mr. Uwe Lorenz for kind help in SEM-EDS-mapping measurements. We acknowledge Dr. Jans Perlich and Dr. Yngve Cerenius for the access to beam time at BW4 beamline (DESY, Hamburg, Germany) and I711 beamline (MaxLab, Lund, Sweden), respectively. The use of the facilities at the Instrument Centre for SolidState NMR Spectroscopy, Aarhus University, sponsored by the Danish Natural Science Research Councils, is acknowledged along with the Danish Council for Independent Research Natural Sciences (FNU) for equipment grants. ’ REFERENCES (1) Yang, J.; Sudik, A.; Wolverton, C. J. Phys. Chem. C 2007, 111, 19134. (2) Mauron, Ph.; Buchter, F.; Friedrichs, O.; Remhof, A.; Bielmann, M.; Zwicky, C. N.; Z€uttle, A. J. Phys. Chem. B 2008, 112, 906. (3) Orimo, S.; Nakamori, Y.; Kitahara, G.; Miwa, K.; Ohba, N.; Towata, S.; Z€uttle, A. J. Alloys Compd. 2005, 404406, 427. (4) Ravnsbaek, D. B.; Filinchuk, Y.; Cerny, R.; Jensen, T. R. Z. Kristallogr. 2010, 225, 557. (5) Kostka, J.; Lohstroh, W.; Fichtner, M.; Hahn, H. J. Phys. Chem. C 2007, 111, 14026.
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