Low-Temperature Reversible Hydrogen Storage Properties of LiBH4

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Low-Temperature Reversible Hydrogen Storage Properties of LiBH4: A Synergetic Effect of Nanoconfinement and Nanocatalysis Jie Shao, Xuezhang Xiao, Xiulin Fan, Liuting Zhang, Shouquan Li, Hongwei Ge, Qidong Wang, and Lixin Chen* State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: LiBH4 has been loaded into a highly ordered mesoporous carbon scaffold containing dispersed NbF5 nanoparticles to investigate the possible synergetic effect of nanoconfinement and nanocatalysis on the reversible hydrogen storage performance of LiBH4. A careful study shows that the onset desorption temperature for nanoconfined LiBH4@MC-NbF5 system is reduced to 150 °C, 225 °C lower than that of the bulk LiBH4. The activation energy of hydrogen desorption is reduced from 189.4 kJ mol−1 for bulk LiBH4 to 97.8 kJ mol−1 for LiBH4@MCNbF5 sample. Furthermore, rehydrogenation of LiBH4 is achieved under mild conditions (200 °C and 60 bar of H2). These results are attributed to the active Nbcontaining species (NbHx and NbB2) and the function of F anions as well as the nanosized particles of LiBH4 and high specific surface area of the MC scaffold. The combination of nanoconfinement and nanocatalysis may develop to become an important strategy within the nanotechnology for improving reversible hydrogen storage properties of various complex hydrides.

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INTRODUCTION Hydrogen, as a chemical energy carrier, is widely regarded as a potential cost-effective, renewable, and clean energy alternative to fossil fuel, especially in the transportation sector. The development of safe hydrogen storage technology with a high energy density is a key prerequisite for the widespread usage of hydrogen mobile applications.1 Recently, intensive interest has been focused on complex hydrides,2−8 especially for lithium borohydride (LiBH4), owing to its high gravimetric and volumetric hydrogen capacities (18.5 wt % and 121 kg H2 m−3).9 Unfortunately, the high desorption temperature (above 400 °C), sluggish kinetics, and undesirable rehydrogenation conditions (350 bar of H2 at 600 °C) appear to limit its practical application as an on-board hydrogen storage medium.10 It is worthwhile to study strategies to improve the hydrogen sorption kinetics and reversibility in complex metal hydrides. To date several approaches, like partial cation/anion substitution,11−13 catalyst doping,14,15 and reactant destabilization,16,17 have been proposed to ease these problems. However, desorption and rehydrogenation temperatures are still too high. A recent promising strategy to significantly accelerate the reaction kinetics is to reduce the particle size and form nanomaterials.18 By ball milling or a solvent evaporation process, nanoscale LiBH4 was formed with enhanced hydrogen desorption kinetics.19 Nanoconfinement into a mesoporous scaffold can be used to restrain the particle growth and agglomeration during desorption/absorption cycles.20−22 Thereby, kinetic enhancement and improved cycling stability were achieved. Furthermore, nanoconfinement can even alter © XXXX American Chemical Society

the thermodynamical stability of metal hydrides as has been demonstrated by theoretical calculations and experiments.23,24 In the case of LiBH4, Vajo’s group demonstrated that the desorption temperature of LiBH4 was lowered by 75 °C, and relatively mild absorption conditions of 100 bar of H2 and 400 °C were obtained when incorporated into a carbon scaffold.25 Nevertheless, relying solely on this method is still difficult to achieve the rapidly reversible hydrogen absorption and desorption under mild conditions. In our previous work, we demonstrated that the hydrogen storage properties of LiBH4 can be substantially improved by introducing NbF5 through the in situ formation of active Nbcontaining catalysts and the function of F− anions.26−28 More recently, Heyn et al.29 and Jensen et al.30 both found hydrogen−fluorine substitution in metal borohydrides, which provides a destabilization of metal borohydrides and may facilitate hydrogen uptake. Herein, we report a remarkable enhancement of the reversibility of hydrogen storage performance in LiBH4 by a synergetic effect of nanoconfinement and NbF5 addition. By comparing the hydrogen de/absorption kinetics with those of bulk LiBH4, nanoconfined LiBH4, and NbF5-doped LiBH4, we will show how NbF5 can improve the hydrogen storage properties of nanoconfined LiBH4. Moreover, in order to acquire detailed information about the kinetics of the reactions, we further investigated the kinetic mechanism of Received: March 29, 2014 Revised: May 7, 2014

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Figure 1. TEM (a) image of MC-NbF5 specimen, SEM (b, c) images of nanoconfined LiBH4@MC-NbF5 specimen shot by gallium ion beam, EDX maps of B (d), C (e), Nb (f), and F (g) to the area in the red square (b), and the corresponding quantitative analyses of elements for the selected areas shown in the inset.

methods, see Figure S2). In order to maximize the infiltration amount of LiBH4, different pore volume fractions of LiBH4 infiltration amounts have been conducted (see Figure S1), and the optimal infiltration amount of LiBH4 we chose is 85 vol %. The premelted sample is hereafter referred to as LiBH4@MCNbF5. Preparation of Control Samples. The infiltrated LiBH4 into MC and a ball-milled mixture of LiBH4 with NbF5 were prepared as control samples. The infiltration procedure of LiBH4@MC is consistent with that of the LiBH4@MC-NbF5. While 10 wt % NbF5 and LiBH4 were ball milled via a planetary QM-3SP4 for 3 h. The ball-to-powder ratio was 40:1 with a milling speed of 400 rpm. Hereafter, the ball-milled sample is labeled as LiBH4-NbF5. Characterization. X-ray diffraction (XRD) analyses were performed on an X’Pert Pro X-ray diffractometer (PANalytical, The Netherlands) with Cu Kα radiation at 40 kV and 40 mA. During the sample transfer and scanning, a lab-built argon filled container was applied to prevent the samples from air and moisture. The morphology of the sample was observed using scanning electron microscopy (SEM, Hitachi SU-70) equipped with an energy dispersive X-ray spectroscopy (EDX, HORIBA X-Max). An internal view of the specimen (∼12 × 10 × 1 μm3) was prepared by a focus ion beam technique (FIB) using a FEI QUATA 3D. The specimen was shot by gallium ion beam with the energy of 30 kV. The microstructure was further examined by transmission electron microscopy (TEM, JEOL JEM-1230 working at 120 kV). Special caution had been taken to prevent the H 2 O/O2 contamination during the measurements. Physisorption isotherms were collected using a Quantacrome Autosorb-1-C system with nitrogen gas at 77 K. Prior to measurement, the samples were degassed for 24 h at 120 °C. The surface areas were calculated by the Brunauer−Emmett− Teller (BET) method. The pore size distributions were derived from the adsorption branches of the isotherms using the

the LiBH4@MC-NbF5 system. At the end of the article, the active species and their catalytic mechanisms are discussed.

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EXPERIMENTAL SECTION Reagents. Commercially available LiBH4 (95%, Acros Organics), NbF5 (98%, Aldrich), NbB2 (325 mesh, Aldrich), tetraethyl orthosilicate (TEOS, 98%, Aldrich), and Pluronic surfactant P123 (Mw 5800, Aldrich) were used as received for this research. All sample handling was performed in an MBraun glovebox maintained under a pure argon atmosphere with 0.99; their reaction behaviors can be reasonably interpreted by the threedimensional phase boundary controlled model and the threedimensional diffusion controlled model, respectively. According to the rate constants derived from Figure S9, the apparent

2LiBH4 + NbHx → 2LiH + NbB2 + (3 + x /2)H 2 ΔG298, x = 1 = −113.97 kJ 48

(1)

Kinetic Mechanism of the LiBH4@MC-NbF5 Composite. To identify the kinetics mechanism for the dehydrogenation of confined LiBH4, we also measured the isothermal dehydrogenation kinetic curves of LiBH4@MC and LiBH4@ F

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activation energy, Ea, of the hydrogen desorption can be calculated using the Arrhenius equation K = A exp(−Ea/RT) (2). The results are presented in Figure S10 and Table 2. It is

of hydrogen storage property enhancement by nanoconfinement and catalysis is very complicated and not sufficiently understood. Further research on this topic is needed.

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Table 2. Apparent Activation Energies (Ea) for As-Prepared Samples by Different Methods

CONCLUSIONS In this work, we have reported a simple strategy to significantly improve the hydrogen storage properties of LiBH4, by combining the NbF5 catalyst doping and the nanoconfinement. The results show that the onset desorption temperature for nanoconfined LiBH4@MC-NbF5 system reduces to 150 °C, 225 °C lower than that of the bulk LiBH4. More importantly, rehydrogenation of LiBH4 is achieved under mild conditions (200 °C and 60 bar of H2), which are the lowest conditions reported to date. The activation energy of hydrogen desorption is reduced from 189.4 kJ mol−1 for bulk LiBH4 to 97.8 kJ mol−1 for the LiBH4@MC-NbF5 sample, indicating a reduced kinetic barrier. All these have demonstrated that there is a favorable synergetic effect between nanoconfinement and NbF5 addition. A full understanding of this synergetic effect may help to significantly enhance the reversible hydrogen storage properties of LiBH4. The combination of nanoconfinement and functionalized nanoporous scaffolds may develop to become an important protocol within the nanotechnology for improving properties of variety complex hydrides.

Ea (kJ mol−1) sample LiBH4 LiBH4@MC LiBH4@MC-NbF5

Arrhenius

Kissinger

140.9 ± 6.2 102.0 ± 3.8

189.4 ± 3.9 143.1 ± 3.7 97.8 ± 3.7

found that Ea is only 102.0 ± 3.8 kJ mol−1 for hydrogen desorption from LiBH4@MC-NbF5, which is drastically lower than that of LiBH4@MC (Ea = 140.9 ± 6.2 kJ mol−1). In addition, we use Kissinger’s method to further investigate the apparent activation energy.52 Experimental data collected under different heating rates of 3.5, 5, 6.5, 8, and 10 °C min−1 were fit using the Kissinger equation ln(β/Tp2) = −Ea/RTp + ln(AR/Ea) (3), and the Ea was calculated from the slope of the fitting line. As shown in Figure 6c and Table 2, the data derived from Kissinger’s method are very close to the value that determined by Arrhenius equation. The Ea for hydrogen release of LiBH4@ MC-NbF5 is 91.6 kJ mol−1 lower than that of bulk LiBH4 (Ea = 189.4 ± 3.9 kJ mol−1), indicating that the Ea is significantly affected by nanoconfinement and nanocatalysis, which thus enable superior kinetics under the same conditions.25,53,54 Discussion of the Enhanced Hydrogen Storage Properties. On the basis of the above observations, the results show that the hydrogen desorption kinetics, reversibility, and cycling stability are all improved considerably by confining LiBH4 into the NbF5 loaded MC scaffold as compared to the LiBH4@MC sample. This is evident that the combination of nanoconfinement and NbF5 addition does bring a favorable synergistic catalytic effect. It is well-known that the practical catalytic efficiency relies not only on the intrinsic activity of the species but also on the distribution of the catalyst particles.15,55 The more homogeneously the catalyst particles disperse and the more tightly the catalyst bind to the reactants, the better catalytic effect that is obtained. This is the reason why the ballmilling method is usually effective for the mixing of materials and catalyst particles, and nanosized particles are much more efficient in catalysis than microsized particles.46,56 In this present work, NbF5 nanoparticles are incorporated into the MC matrix with small pore size (3.8 nm) and high surface area, resulting in a well distribution and strong contact with reactants. Meanwhile, the NbHx and F-containing catalytic species formed during the LiBH4 infiltration are also confined in the nanopores, which are small enough and provide a huge number of active sites for the catalytic decomposition of LiBH4. They may also facilitate the dissociation and recombination of hydrogen molecules on their surface and the atomic hydrogen diffusion along the grain boundaries and inside the grains. Furthermore, the unstable intermediate NbHx will mostly convert to NbB2 heterogeneous nucleation agent in the subsequent desorption cycles, which substantial improves the cycle stability of the LiBH4@MC-NbF5 system.46,47 More importantly, additional NbF5 doping can further modify the thermodynamics of nanoconfined LiBH4 the function of F anions, which has been theoretically and experimentally demonstrated in the study of LiBH4,28,57,58 enabling hydrogen cycle under the desirable conditions. In general, the mechanism

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ASSOCIATED CONTENT

S Supporting Information *

DSC curves for samples infiltrated with different pore volume fractions of LiBH4. SEM and TEM images of as-prepared mesoporous carbon scaffold; pore size distributions for the MC and the loaded LiBH4@MC-NbF5 composite from the BET results; SEM and EDX maps of MC-NbF5 specimen; XRD patterns of as-prepared pure MC, MC-NbF5, and ball-milled LiBH4-NbF5; long duration volumetric hydrogen desorption for nanoconfined LiBH4 at 300 °C under 3.60 bar of H2; hydrogen cyclic desorption curves for LiBH4@MC and LiBH4@MCNbF5 at 260 °C under 0.02 bar of H2; time dependence of kinetic modeling equations f(α) and Arrhenius plots for the desorption kinetics of LiBH4@MC and LiBH4@MC-NbF5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86 571 8795 1152; Fax +86 571 8795 1152 (L.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for this research from the National High Technology Research & Development Program of China (2012AA051503), from the National Basic Research Program of China (2010CB631300), from the National Natural Science Foundation of China (51171173 and 51001090), from the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), and from the Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). G

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dx.doi.org/10.1021/jp503127m | J. Phys. Chem. C XXXX, XXX, XXX−XXX