J. Phys. Chem. C 2008, 112, 5651-5657
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Enhanced Hydrogen Storage Kinetics of LiBH4 in Nanoporous Carbon Scaffolds Adam F. Gross, John J. Vajo,* Sky L. Van Atta, and Gregory L. Olson HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265 ReceiVed: NoVember 20, 2007; In Final Form: January 25, 2008
Enhanced kinetics for hydrogen exchange in LiBH4 incorporated within nanoporous carbon scaffolds are described. Dehydrogenation rates up to 50 times faster than those in the bulk material are measured at 300 °C in a nanostructured hydride formed by filling a porous carbon aerogel host with LiBH4. Furthermore, the activation energy for hydrogen desorption, measured using the approach developed by Ozawa, is reduced from 146 kJ/mol for bulk LiBH4 to 103 kJ/mol for nanostructured LiBH4, and the faster kinetics result in desorption temperatures that are reduced by up to 75 °C. In addition, nanostructured hydrides exhibit increased cycling capacity over multiple sorption cycles. This work demonstrates that confinement within a porous scaffold host is a promising approach for enhancing hydrogen uptake and release in reversible light-metal complex hydrides.
Introduction Complex metal hydrides containing low-atomic-weight alkali or alkaline-earth metal cations and alanate (AlH4-), amide (NH2-), or borohydride (BH4-) covalently bonded anions have high gravimetric and volumetric capacities for hydrogen. Consequently, these materials are being extensively studied for reversible hydrogen storage applications.1-7 However, in contrast to metallic systems, ionic/covalent bonding is directional. Thus, the transition states for atomic rearrangement occur in particularly unfavorable bonding configurations. This increases the activation energies for diffusion and results in slow hydrogen sorption kinetics. Overcoming these kinetic limitations has become a critical element in the development of light-metal complex hydrides for practical storage applications. One method for facilitating enhanced reaction kinetics exploits the short diffusion distances and unique reaction environment that exist in nanoscale materials. The shorter diffusion distances for hydrogen and the other light elements within a nanoscale structure result in faster net rates for hydrogenation and dehydrogenation. However, this advantage can be lost if the size of the nanomaterial increases by sintering and agglomeration during hydrogen cycling. It is therefore necessary to develop complex metal hydride systems that not only have fast reaction rates on the initial hydrogen sorption cycle but that can also maintain fast kinetics during repeated cycles. Gutowska and co-workers recently demonstrated that the rate of hydrogen release from ammonia borane (H3NBH3), a highcapacity irreversible (chemical) hydride, could be improved by incorporating the compound into a mesoporous silica scaffold.8 They reported an increase in the dehydrogenation rate as well as a change in thermodynamics relative to that of bulk ammonia borane. In other studies, Schu¨th and co-workers9 found that NaAlH4 exhibits improved kinetics when encapsulated in porous matrices, and Balde´ et al.10 also demonstrated improved hydrogen desorption and absorption kinetics for NaAlH4 supported on surface-oxidized carbon nanofibers. In addition, ball milling lithium borohydride with mesoporous carbon (which yields LiBH4 supported on disordered carbon) or with carbon * Corresponding author. E-mail:
[email protected] nanotubes results in destabilized LiBH4 with improved dehydrogenation temperatures.11,12 The effects of porous hosts on gas-phase reactions have also recently been studied, and it was found that the reaction rate can be either enhanced or hindered depending on how the transition state is accommodated within the pore.13 Lithium borohydride is a complex hydride that is receiving considerable attention due to its high gravimetric and volumetric hydrogen storage capacities (13.6 wt % and 0.092 kg/L, respectively). It dehydrogenates and rehydrogenates according to the overall reaction LiBH4 T LiH + B + 1.5H2. Although the hydrogen content is high, the thermodynamics and rate of hydrogen exchange in LiBH4 are not favorable for reversible hydrogen storage. The thermodynamics of hydrogen storage in LiBH4 can be altered using destabilization strategies in which alloying or compound formation in the dehydrogenated state provides a reduced energy pathway for the sorption reaction.14 Although hydride destabilization can be used to reduce the reaction enthalpy (and temperature), improvements in the hydrogen exchange kinetics and cycling capacity are still required for LiBH4 to be the basis of a practical storage system. In this paper we show that the reaction rates for reversible hydrogen sorption in LiBH4 can be improved significantly by incorporating the hydride within a nanoporous carbon aerogel host. The aerogel serves as an effective structure-directing agent that restricts the particle size of the LiBH4 while providing a framework that mitigates sintering and improves the cycling capacity. Carbon aerogels were chosen because they are relatively inert and because the pore size can be synthetically adjusted,15 allowing hydrogen sorption kinetics to be correlated with the LiBH4 confinement volume. Specifically, we compare the kinetics for dehydrogenation of LiBH4 in an aerogel with LiBH4 mixed with nonporous graphite and we discuss how the nanoporous carbon pore morphology influences the dehydrogenation rates and cycling capacity. Experimental Details Carbon aerogels were synthesized using resorcinol-formaldehyde condensation.16 Aerogels with an average pore size of ∼25 nm were synthesized by mixing 20.70 g of (188 mmol)
10.1021/jp711066t CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008
5652 J. Phys. Chem. C, Vol. 112, No. 14, 2008 resorcinol (Aldrich) and 30.94 g of 36.5 wt % formaldehyde solution in water (376 mmol formaldehyde, Fluka) and 28.32 g of deionized water. Sodium carbonate (0.040 g, 0.377 mmol) was then added, and the mixture was stirred until all materials were dissolved. Aerogels with ∼13 nm average pore size were synthesized by mixing 25.88 g of (235 mmol) resorcinol and 38.68 g of 36.5 wt % formaldehyde solution in water (470 mmol formaldehyde) and 15.40 g of deionized water. Sodium carbonate (0.050 g, 0.472 mmol) was added, and the mixture was stirred until dissolved. For both samples the resorcinol/Na2CO3 ratio was 500:1, and the organic fraction was 40 wt % for the 25 nm pore size aerogel and 30 wt % for the 13 nm pore size aerogel. Once prepared, solutions were transferred to 60 mL polypropylene jars, sealed, and aged 24 h at 23 °C, 24 h at 50 °C, and 72 h at 90 °C. The samples were then cooled, cut into cubes (0.75 cm/side), and immersed in acetone to displace the water. The acetone bath was poured off and refilled twice with at least 1 h between cycles. The aerogel cubes were allowed to dry in air and then heated in a tube oven under nitrogen from room temperature to 800 °C at 2.6 °C/min and maintained at 800 °C for 6 h to pyrolyze the resorcinol-formaldehyde gel. Sample pore size, surface area, and pore volumes were characterized with N2 absorption at Micromeritics Analytical Services (Norcross, GA). Data were analyzed using the Brunner-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.17 To incorporate LiBH4 into the samples, the aerogel cubes were first dried at 400 °C under vacuum and then transferred to an argon-purged glovebox. The dry samples and LiBH4 in a quantity equal to the available pore volume in the aerogel were sealed together in a Schlenk flask. The flask was removed from the glovebox, attached to a Schlenk line, evacuated, and then back-filled with an argon atmosphere that vented to a bubbler. The samples were heated under an argon atmosphere at 280300 °C until the LiBH4 melted. The molten LiBH4 quickly wet the pores of the aerogel and within ∼1 min completely infiltrated the scaffold by capillary action. Following incorporation of the LiBH4 into the aerogel, the sample was returned to the glovebox, and excess LiBH4 on the sides of the aerogel cube was scraped away using a razor blade. Powdered forms of the aerogel samples were prepared for analysis by grinding the cubes with a mortar and pestle. Note that LiBH4 may react violently with water or oxygen and it should be maintained in an air-free environment. In addition to aerogels, activated carbon was used to evaluate the effect of very small pores on the sorption behavior. A mixture of graphite and LiBH4 served as a control sample to measure bulk behavior and to evaluate possible carbon-LiBH4 interactions in a nonporous medium. Granular activated carbon with 50 times at 300 °C compared with that of LiBH4 mixed with nonporous graphite. Moreover, the dehydrogenation rate for LiBH4 in the 25 nm aerogel is slightly slower than the rate in the 13 nm aerogel (Table 2 and Figure S5, Supporting Information), in agreement with the trends observed for the dehydrogenation temperatures and activation energies obtained from the TGA data (see Figures 4 and S2, Supporting Information). The amount of hydrogen released from bulk LiBH4 diminishes significantly with successive dehydrogenation/hydrogenation cycles.6 Consequently, we examined whether this behavior is altered by incorporation of the LiBH4 within a nanoporous scaffold. The control and LiBH4-filled carbon samples were cycled through three dehydrogenation/rehydrogenation sequences. The rehydrogenation reactions for LiBH4 in a 13 nm aerogel and LiBH4 mixed with nonporous graphite are illustrated by the hydrogen uptake vs time/temperature data in Figure S6 (Supporting Information). The hydrogen capacities measured over three sorption cycles are shown in Figure 7 where the hydrogen loss from the first dehydrogenation is normalized to one for each sample and the fraction of original capacity is shown. Although there is a decrease in cycling capacity for all samples, the magnitude of the decrease is much less for LiBH4 in the nanoporous scaffolds than in the LiBH4/graphite control sample. To attempt to compare the equilibrium hydrogen pressures from bulk LiBH4 and LiBH4 incorporated into an aerogel, long duration volumetric measurements were conducted. Pressure vs time data obtained at 300 °C are shown in Figure 8. For LiBH4 incorporated into a 13 nm aerogel, the hydrogen pressure at 300 °C reached ∼3 bar with 4.5 wt % hydrogen desorbed relative to the weight of the LiBH4. In contrast, for a similar
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Figure 7. Normalized cycling capacities for LiBH4 contained in porous scaffolds and mixed with nonporous graphite. Curve a: LiBH4 mixed with nonporous graphite. Curve b: LiBH4 contained in a 25 nm aerogel. Curve c: LiBH4 contained in a 13 nm aerogel. Curve d: LiBH4 contained in activated carbon. Capacities were measured during dehydrogenation following hydrogenation in 100 bar H2 at 400 °C for 2 h. All capacities are normalized to unity for the first cycle. The retained capacities increase for smaller pore size scaffolds.
Figure 8. Long duration volumetric hydrogen desorption for LiBH4 in porous and nonporous hosts at 300 °C. Curve a shows hydrogen desorption from LiBH4 incorporated into a 13 nm aerogel. Curve b shows desorption from LiBH4 mixed with nonporous graphite. The sample masses and desorption volumes were adjusted to prevent complete dehydrogenation and to give directly comparable desorption pressures; i.e., the final desorbed amounts were approximately equal: 4.5 wt % for curve a and 3.9 wt % for curve b. These gravimetric capacities are calculated relative to the LiBH4 weight only. The hydrogen pressure is ∼10 times higher for LiBH4 contained in the aerogel.
amount desorbed, 3.9 wt % relative to the weight of the LiBH4, the pressure rose to only 0.32 bar for LiBH4 mixed with nonporous graphite. Although the pressure in these experiments was still increasing slowly and Figure 7 indicates that dehydrogenation is not readily reversible, the results nonetheless suggest that the equilibrium hydrogen pressure for LiBH4 contained within the aerogel is ∼10 times higher than the pressure for bulk LiBH4 at ∼4 wt % desorbed and at 300 °C. The diminishing rate of dehydrogenation shown in Figure 8 appears to be limited by the buildup of hydrogen pressure (i.e., equilibrium) and not the forward reaction rate because if the hydrogen is evacuated (as described for the experiment in Figure 6), the rate of dehydrogenation increases (data not shown). In addition, the enhancement in the rate of dehydrogenation for LiBH4 in the aerogel described above is also clearly illustrated in Figure 8 by the large difference in the initial slopes for the pressure-time curves.
5656 J. Phys. Chem. C, Vol. 112, No. 14, 2008 Discussion The results summarized in Table 2 show that the incorporation of LiBH4 into porous carbon scaffolds significantly enhances the rate of dehydrogenation. Although a complete mechanistic understanding of this enhancement requires further work, several aspects of how nanoscale confinement can influence the kinetics of hydrogen desorption from LiBH4 are considered here. Incorporation into the aerogel reduces the activation energy for dehydrogenation. The lower activation energies for the filled aerogels suggest that in addition to reducing H2 diffusion distances, the incorporation of LiBH4 into a nanoporous scaffold results in a lower energy barrier pathway for dehydrogenation. If the effect of the scaffolds was to simply reduce diffusion distances, a reduction in activation energy may not be expected. If the change in reaction rate was due solely to a reduction in the activation energy, then a reduction in the activation energy of 43 kJ/mol (Table 2) would increase the Boltzmann factor (exp[-Ea/RT]) by >5000 times at 300 °C. We observe a much smaller increase, ∼50 times, indicating that both the activation energy and the pre-exponential terms for the rate of dehydrogenation are influenced by the aerogel. The results in Table 2 indicate that the rate of dehydrogenation of LiBH4 increases with decreasing scaffold pore size. For example, compared with the 25 nm aerogel, the activation energy for dehydrogenation in the 13 nm aerogel is 8 kJ/mol lower and the dehydrogenation rate at 300 °C is 1.6 times faster. Considering that both aerogels have nearly the same surface areas (within 5%) and nearly the same total micropore volumes and that the majority of the surface area is associated with the micropores, it appears that neither the surface area nor the micropores are major factors controlling the dehydrogenation rate. On the other hand, because both aerogels contain a broad distribution of pore sizes extending into the micropore regime (Figure 1) and dehydrogenation begins at lower temperatures in smaller pores, we suggest that the micropores may serve as nucleation sites for the dehydrogenation reaction. While there is a greater fraction of micropores in the 13 nm aerogel as compared with that of the 25 nm aerogel (Table 1), nucleation is likely not a limiting factor in the kinetics because at least 10% of the volume in each material is micropores. The broad diffraction features for LiBH4 incorporated in the aerogel (Figure 2) indicate that the crystalline order of the hydride within the aerogel is poor and that confinement by the pores may induce strain. These factors could certainly influence the rate of dehydrogenation. However, the effect is likely indirect because LiBH4 typically melts before dehydrogenation. For bulk LiBH4, the onset of dehydrogenation coincides with the melting point (Figure 5). This coincidence in bulk LiBH4 may result from the higher diffusion constants in liquids, which, in this case, likely increases the rate of hydrogen diffusion and the formation and growth of the dehydrogenated phase mixture (LiH + B). A similar effect may contribute to the enhanced kinetics that occur in the aerogels because differential scanning calorimetry (DSC) measurements indicate that LiBH4 in the 13 nm aerogel melts at a temperature ∼30 °C lower than that of bulk LiBH4 (Figure S7, Supporting Information). Reduced melting temperatures for materials confined within small pores have been characterized in other systems.26,27 Thus, for LiBH4 within the aerogel, the reduced melting point may increase the diffusion rates associated with dehydrogenation at lower temperatures. In contrast to the behavior of LiBH4, recent studies have shown that H3NBH3 encapsulated within mesoporous silica remains solid during dehydrogenation; i.e., dehydrogenation occurs via a solid-solid-phase transformation.8 This process
Gross et al. may destabilize the dehydrogenated products by forcing them to crystallize in a sterically confined configuration in a completely filled pore, which reduces the exothermic enthalpy difference between the initial and final states. However, molten LiBH4 in pores can diffuse away from the solid products, thereby limiting the extent to which energetic destabilization from steric confinement can occur. The origin of the loss in capacity and the differences in reversibility between bulk and filled aerogel samples (Figure 7) can be understood in terms of the formation of unreactive (or low reactivity) species during hydrogen cycling. Elemental analysis confirmed that the Li/B ratio for the filled aerogels is the same (within the measurement uncertainty of (5%) before and after the first dehydrogenation. This indicates that neither borane nor other gaseous degradation products could account for the observed loss of capacity. Because side reactions (Figures 3 and S1, Supporting Information), and significant material loss can be excluded, the loss of capacity during cycling is likely due to agglomerated boron and isolated LiH particles (see Figures 2 and 3) that have insufficient reactivity to form LiBH4 on the time scale of the hydrogenation. We propose that reversibility is improved in the aerogel samples because pores limit the size of the product agglomerates and thereby reduce diffusion distances. In addition, with smaller product agglomerates, there is likely to be increased interfacial contact between the LiH and B phases, which will facilitate rehydrogenation. Because LiH and B are solids, diffusion is especially slow and the cycling is very sensitive to the length scale of interdiffusion required to form LiBH4. Incorporation into the aerogel scaffolds reduces the gravimetric hydrogen density compared with that of pure LiBH4. For this work, LiBH4 comprises 25-45 wt % of the filled aerogels. However, for scaffolds to be used in commercial applications, higher weight fractions requiring larger total pore volumes are needed. For our synthesis technique, scaffolds with larger pore volumes generally have larger pore sizes, so there is a tradeoff between overall capacity and hydrogen desorption temperature. This can be alleviated by using porous scaffolds with small pore sizes but increased pore volumes, a condition that can be satisfied by thinner-wall structures such as those produced using solvent removal under supercritical conditions.15 Evaluation of the sorption behavior of LiBH4 in aerogels produced by this method is currently in progress. Another consideration for practical applications is reactivity of the scaffold host. For this study, carbon aerogels were chosen because they are believed to be relatively inert. However, formation of methane during hydrogen cycling is a possible scaffold degradation process. To evaluate this possibility, we analyzed the effluent from the TGA using FTIR spectroscopy. Our preliminary results (Figure S8, Supporting Information) indicate that a small amount of methane (
