Article pubs.acs.org/JPCC
Hydrogen Dynamics in Nanoconfined Lithiumborohydride Arndt Remhof,*,† Philippe Mauron,† Andreas Züttel,† Jan Peter Embs,‡ Zbigniew Łodziana,§ A. J. Ramirez-Cuesta,∥ Peter Ngene,⊥ and Petra de Jongh⊥ †
Empa, Swiss Federal Institute for Materials Science and Technology, Hydrogen and Energy, CH-8600 Dübendorf, Switzerland Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI, Switzerland § The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, PL-31-342 Kraków, Poland ∥ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom ⊥ Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, NL-3584 CA Utrecht, The Netherlands ‡
ABSTRACT: Lithiumborohydride (LiBH4) contains 18.5 wt % hydrogen and exhibits a structural phase transition (orthorhombic→ hexagonal) at 381 K, which is associated with a large increase in hydrogen and lithium mobility in the solid. Confining metal hydrides in a nanoporous matrix may change the hydrogen desorption kinetics and reversibility, and influence phase equilibria. The hydrogen mobility in nanoconfined LiBH4 was studied using inelastic and quasielastic neutron scattering. Confinement in nanoporous carbon leads to a greater anion mobility and a reduced activation energy of 8 kJ/mol at room temperature as compared to 17.3 kJ/mol in bulk LiBH4. In the nanoconfined phase, the mobility resembles that of the high-temperature bulk phase, and no distinct phase transition was observed. However, a substantial fraction of the hydrogen is immobile, leading to effectively reduced anion dynamics as compared to the bulk high-temperature phase. We tentatively attribute these effects to lattice distortions due to the finite pore size, and to thermally induced stress leading to a loss in long-range order and an increase in dynamical disorder, as supported by first principle calculations. LiBH4 is an ionic crystal, consisting of (Li)+ cations and (BH4)− anions.4 LiBH4 melts at 550 K. It undergoes a solid− solid structural phase transition at 381 K, from an orthorhombic low-temperature structure (Pnma) to a hexagonal high-temperature structure (P63mc).5,6 The hightemperature phase is associated with dynamical disorder, high [Li]+ translational mobility, rotational jumps of the [BH4]− anions in the terahertz range, and strong lattice anharmonicities.5−11 Within the low-temperature phase, the sharply defined structural phase transition at 381 K is preceded by a continuous increase in hydrogen and lithium mobility, which can be interpreted as a second-order mobility phase transition interrupted by a first-order structural phase transition.12,13 The practical use of LiBH4 as a hydrogen storage material is hindered by the high temperatures required to release the hydrogen. For bulk LiBH4, considerable amounts of hydrogen are released only after reaching the melting temperature. Furthermore, hydrogen release kinetics are poor, and reversibility of the hydrogen desorption is limited.14,15 There have been many efforts to activate LiBH4, to enhance the reaction kinetics, and to improve the reversibility. Also, the potential use of LiBH4 as a solid-state electrolyte is hindered by
I. INTRODUCTION The conversion and the storage of energy will become increasingly important in the future. Finite reserves of fossil energy carriers and growing environmental concerns lead to an increased use of so-called renewable energy sources such as solar or wind energy. In contrast to the fossil energy carriers, the renewables deliver energy fluxes. A future society therefore requires synthetic energy carriers to store and transport energy. Hydrogen has a chemical energy density of 121 MJ/kg (lower heating value), exceeding that of gasoline by a factor of 2.5, and would therefore be an ideal energy carrier. At ambient conditions, 1 kg of hydrogen requires a volume of 11 m3. The storage of hydrogen mainly requires the reduction of the volume, by either compression, liquefaction, adsorption to high surface area materials, or chemical bonds. Chemical hydrides reach volumetric hydrogen densities of more than 140 kg/L, exceeding the density of liquid hydrogen by a factor of 2.1 High volumetric densities combined with high gravimetric densities are realized in aluminum- and boron-based complex hydrides of the type M(AlH4)n and M(BH4)n, where M is a metal and n is the metal’s valence. Because of its high hydrogen content of 18.5 wt %, lithiumborohydride, LiBH4, is one of the most discussed lightweight complex hydrides.2 It is also considered as a potential solid-state electrolyte for lithium-ion batteries and fuel cells, due to its high lithium ion conductivity of σ = 1 × 10−3 S/cm in the hexagonal phase at 393 K.3 © 2013 American Chemical Society
Received: November 8, 2012 Revised: January 30, 2013 Published: February 6, 2013 3789
dx.doi.org/10.1021/jp311064d | J. Phys. Chem. C 2013, 117, 3789−3798
The Journal of Physical Chemistry C
Article
the low Li+ conductivity at room temperature. Efforts to increase room-temperature Li+ mobility include formation of LiBH4−LiX (X = Cl, Br, I) solid solutions16 or (BH4)(NH2)based structures.17 For LiBH4, nanoconfinement in a carbon scaffold leads to a strong improvement of the dehydrogenation and rehydrogenation kinetics as compared to the bulk.18,19 The reversibility could be further enhanced by adding small amounts of additives such as Ni.20 Additionally, nanoconfinement can influence the thermodynamical stability of metal hydrides as demonstrated on the example of NaAlH4.21,22 Although the beneficial effects on hydrogen storage properties are clear, still little is understood about the exact nature of the impact of nanoconfinement. Confining an active phase in nanoporores reduces the size of the crystallites and often leads to the absence of long-range X-ray crystallinity, which will affect the structural and hence physichemical properties. In addition, the interface between the active phase and carbon materials is reported to have an important effect, which might either be electronic of nature or related to clamping or physical confinement. In the present study, we investigate the hydrogen dynamics in nanoconfined LiBH4, prepared by melt infiltration into porous high-surface-area-graphite by means of quasielastic (QENS) and inelastic (INS) neutron scattering. Neutrons may exchange energy and momentum with the sample due to the excitation or de-excitation of phonons (collective modes of displacement of atoms from their equilibrium positions), magnons (changes in the collective electron spin arrangements in the lattice), or any other internal degree of freedom of the sample. In an INS spectrum, the excitations appear as discrete peaks, well separated from the elastic line. As compared to other spectroscopic techniques such as Raman or infrared spectroscopy, INS has the advantage that it is especially sensitive to hydrogen and that the optical selection rules do not apply. All vibrations are in principle accessible.23 QENS refers to the limiting case of inelastic neutron scattering, where the energy transfers are small as compared to the incident energy. QENS studies the broadening of the elastic line, originating from the energy transfer that occurs if the target atoms undergo translational or rotational diffusion during the scattering process. In conjunction with the large sensitivity to hydrogen, QENS is a suitable method to study the motion of hydrogen in solids.24,25 The observed changes in hydrogen dynamics as a result of nanoconfinement are discussed in the light of reduced length scales and distortions induced by the nanoconfinement, also based on density functional calculations.
Goebel mirror, selecting the Cu Kα radiation with a wavelength of λCu−Kα = 1.54 Å. Representative samples were measured in glass capillaries (diameter 1 mm, wall thickness 0.01 mm), sealed under argon atmosphere. Nitrogen physisorption measurements were performed at 77 K using a Micromeritics ASAP2020. BJH pore size distributions were calculated from the adsorption branch of the isotherm. Quasielastic and inelastic neutron scattering (QENS and INS) measurements were carried out using the time-of-flight (TOF) neutron spectrometer FOCUS located at the continuous spallation source SINQ at the Paul Scherrer Institute, Switzerland26,27 and the TOSCA spectrometer of the ISIS facility of the Rutherford Appleton Laboratory in Didcot, UK.28,29 The spectrometers are complementary. TOSCA measures energy loss covering a large energy range. It can therefore be applied at low temperatures. FOCUS mainly measures the energy gain and allows angular resolved measurements. At FOCUS, the samples were loaded in lead sealed, double walled-hollow cylindrical containers. The diameter of the cylindrical container was 10 mm, and the wall distance (i.e., the sample space) was 1 mm. Incident neutrons were prepared with a wavelength of λi = 4 Å, corresponding to Ei = 5.11 meV and vi = 989 m/s. The scattered intensity I(2θ,t) is recorded as a function of scattered angle 2θ and time-of-flight t. Data reduction is carried out using the data analysis and visualization environment “DAVE”,30 to convert the instrument specific I(2θ,t) into the scattering function S(Q,ω). Thereby, the scattered intensity is expressed as a function of the momentum transfer ℏQ = ℏki − ℏkf, where ki and kf are the incident and scattered wave vectors, and as a function of the energy transfer ℏω = Ei − Ef, where Ei and Ef are the incident and scattered neutron energies, respectively. At TOSCA, the sample was mounted into a flat aluminum sample holder of 1 mm thickness, and the raw data were converted into S(Q,ω) using the standard routines available at ISIS. In the present case, the signal is dominated by the (very) large incoherent scattering cross section σinc of hydrogen as compared to those of lithium and boron. Therefore, in first approximation, we attribute all scattered intensity to the incoherent scattering of hydrogen. Hence, the contributions of other species, coherent scattering, and multiphonon events are neglected. In this case, the measured intensity is proportional to the incoherent scattering function Sinc(Q,ω), which is related to the vibrational density-of-states, g(Q,ω):23,31
II. METHODS a. Experimental Section. A series of samples was produced by melt infiltration of LiBH4 (KatChem) into high surface area porous carbon (HSAG-500, Timcal Ltd., single point total pore volume 0.66 cm3 g−1, BET surface area 500 m2 g−1, broad pore size distribution with an average pore size of 10 nm). Samples containing 15, 25, and 40 wt % of LiBH4 were prepared following a literature procedure.20 Pure carbon, pure LiBH4, and a physical mixture of carbon and 25 wt % LiBH4 were used as reference samples. To avoid the strong neutron absorption of 10B, 11B enriched (99 mass %) LiBH4 was used. The samples were protected from air and always handled in glove boxes filled with purified argon or helium. Structural characterization was carried out by X-ray diffraction (XRD), using a Bruker D8 diffractometer with a
(1)
g (Q , ω) =
ℏω Sinc(Q , ℏω)[coth(ℏω/2kBT ) ± 1]−1 Q2
In the following, we denote the experimentally determined density-of-states g(Q̅ ,ω), derived from the measured, Qaveraged scattering function as the generalized density of stated (GDOS). The Bose−Einstein statistics correction for the thermal population is given by (coth((ℏω)/(2kBT)) ± 1). The positive sign applies to neutron energy loss and the negative sign to neutron energy gain. Because of the low absorption cross section of carbon, absorption and multiple scattering can be neglected. At FOCUS, the energy resolution defining the width of the elastic line equals ΔE = 0.2 meV at the settings used. Data acquisition and treatment were carried out as described earlier:9 the spectra were binned in a range of Q values of 0.5 Å−1 < Q 3790
dx.doi.org/10.1021/jp311064d | J. Phys. Chem. C 2013, 117, 3789−3798
The Journal of Physical Chemistry C
Article
110 °C). However, that does not mean that the high-temperature LiBH4 phase is simply stabilized by the nanoconfinement to be present also at room temperature. First calculations performed indicate not only that LiBH4 is a rather soft material among the other alkali metal borohydrides, that is, having small elastic constants, but also that stressinduced lattice expansion might lead to higher mobilities within the low-temperature phase. Furthermore, the nanoconfined LiBH4 shows some peculiarities, such as a mobile and an immobile fraction of hydrogen atoms. Fundamental understanding of how nanoconfinement leads to high atom mobilities, and by which experimental parameters this can be tuned, is highly relevant to develop novel nanostructured materials for applications under dynamic conditions, such as reversible hydrogen storage and Li-ion batteries, in which high ion and atom mobilities are crucial.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +4158 765 4369. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Financial support from the Korea Research Council of Fundamental Science and Technology and by a grant from Switzerland through the Swiss Contribution to the enlarged European Union is gratefully acknowledged. Z.L. acknowledges CPU allocation at PL-Grid Infrastructure. We are grateful to Timcal Ltd. Switzerland for providing the high surface area graphite. P.d.J. and P.N. acknowledge financial support from NWO-Vidi (Netherlands) and the NWO-ACTS Sustainable Hydrogen Program.
■
REFERENCES
(1) Schlapbach, L.; Züttel, A. Hydrogen-storage materials for mobile applications. Nature (London) 2001, 414, 353−358. (2) Saldan, I. A prospect for LiBH4 as on-board hydrogen storage. Cent. Eur. J. Chem 2011, 9, 761−775. (3) Matsuo, M.; Nakamori, Y.; Orimo, S.-I.; Maekawa, H.; Takamura, H. Lithium superionic conduction in lithium borohydride accompanied by structural transition. Appl. Phys. Lett. 2007, 91, 224103. (4) Buchter, F.; Łodziana, Z.; Remhof, A.; Mauron, Ph.; Friedrichs, O.; Borgschulte, A.; Züttel, A.; Filinchuk, Y.; Palatinus, L. Experimental charge density of LiBD4 from maximum entropy method. Phys. Rev. B 2011, 83, 064107. (5) Soulie, J. Ph.; Renaudin, G.; Cerny, R.; Yvon, K. Lithium borohydride LiBH4: I. Crystal structure. J. Alloys Compd. 2002, 346, 200−205. (6) Hartman, M. R.; Rush, J. J.; Udovic, T. C.; Bowman, R. C.; Hwang, S. J. Structure and vibrational dynamics of isotopically labeled 3797
dx.doi.org/10.1021/jp311064d | J. Phys. Chem. C 2013, 117, 3789−3798
The Journal of Physical Chemistry C
Article
(26) Mesot, J.; Janssen, S.; Holitzner, L.; Hempelmann, R. FOCUS: Project of a space and time focussing time-of-flight spectrometer for cold neutrons at the spallation source SINQ of the Paul Scherrer Institute. J. Neutron Res. 1996, 3, 293−310. (27) Janssen, S.; Mesot, J.; Holitzner, L.; Furrer, A.; Hempelmann, R. FOCUS: a hybrid TOF-spectrometer at SINQ. Physica B 1997, 234− 236, 1174−1176. (28) Parker, S. F.; Carlile, C. J.; Pike, T.; Tomkinson, J.; Newport, R. J.; Andreani, C.; Ricci, F. P.; Sacchetti, F.; Zoppi, M. TOSCA: a world class inelastic neutron spectrometer. Physica B 1998, 241−243, 154− 156. (29) Colognesi, D.; Celli, M.; Cilloco, F.; Newport, R. J.; Parker, S. F.; Rossi-Albertini, V.; Sacchetti, F.; Tomkinson, J.; Zoppi, M. TOSCA neutron spectrometer: The final configuration. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 64−66. (30) Azuah, R. T.; Kneller, L. R.; Qiu, Y.; Tregenna-Piggott, P. W. L.; Brown, C. M.; Copley, J. R. D.; Dimeo, R. M. DAVE: A comprehensive software suite for the reduction, visualization, and analysis of low energy neutron spectroscopic data. J. Res. Natl. Inst. Stand. Technol. 2009, 114, 341−358. (31) Squires, G. L. Introduction to the Theory of Thermal Neutron Scattering; Cambridge University Press: Cambridge, 1978. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (33) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953−17979. (34) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758−1775. (35) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169−11186. (36) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (37) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558−561. (38) Hagemann, H.; Gomes, S.; Renaudin, G.; Yvon, K. Raman studies of reorientation motions of [BH4]− anions in alkali borohydrides. J. Alloys Compd. 2004, 363, 129−132. (39) Tomkinson, J.; Waddington, T. C. Inelastic neutron scattering from the alkali metal borohydrides and calcium borohydride. J. Chem. Soc., Faraday Trans. 2 1976, 72, 528−538. (40) Martelli, P.; Remhof, A.; Borgschulte, A.; Ackermann, R.; Strässle, T.; Embs, J. P.; Ernst, M.; Matsuo, M.; Orimo, S. I.; Züttel, A. Rotational motion in LiBH4/LiI solid solutions. J. Phys. Chem. A 2011, 115, 5329−5334. (41) Dmitriev, V.; Filinchuk, Y.; Chernyshov, D.; Talyzin, A. V.; Dzwilewski, A.; Andersson, O.; Sundqvist, B.; Kurnosov, A. Pressuretemperature phase diagram of LiBH4: Synchrotron x-ray diffraction experiments and theoretical analysis. Phys. Rev. B 2008, 77, 174112. (42) Nye, J. F. Physical Properties of Crystals; University Press: London, 1957. (43) Chernyshov, D.; Bosak, A.; Dmitriev, V.; Filinchuk, Y.; Hagemann, H. Low-lying phonons in NaBH4 studied by inelastic scattering of synchrotron radiation. Phys. Rev. B 2008, 78, 172104. (44) Zhang, X. D.; Hou, Z. F.; Jiang, Z. Y.; Hou, Y. Q. Elastic properties of MBH4 (M=Na, K, Rb, Cs). Physica B 2011, 406, 2196− 2199. (45) Nelson, J. B.; Riley, D. P. The thermal expansion of graphite from 15°C. to 800°C.: part I. Experimental. Proc. Phys. Soc. 1945, 57, 477−486. (46) Rude, L. H.; Groppo, E.; Arnbjerg, L. M.; Ravnsbæk, D. B.; Malmkjær, R. A.; Filinchuk, Y.; Baricco, M.; Besenbacher, F.; Jensen, T. R. Iodide substitution in lithium borohydride, LiBH4−LiI. J. Alloys Compd. 2011, 509, 8299. (47) Borgschulte, A.; Gremaud, R.; Kato, S.; Stadie, N. P.; Remhof, A.; Züttel, A.; Matsuo, M.; Orimo, S. I. Anharmonicity in LiBH4−LiI induced by anion exchange and temperature. Appl. Phys. Lett. 2010, 97, 031916.
(48) Shane, D. T.; Corey, L. R.; McIntosh, C.; Rayhel, L. H.; Bowman, R. C., Jr.; Vajo, J. J.; Gross, A. F.; Conradi, M. S. LiBH4 in carbon aerogel nanoscaffolds: An NMR study of atomic motions. J. Phys. Chem. C 2010, 114, 4008. (49) Verkuijlen, M. H. W.; Ngene, P.; de Kort, D. W.; Barré, C.; Nale, A. C.; van Eck, E. R. H.; van Bentum, P. J. M.; de Jongh, P. E.; Kentgens, A. P. M. Nanoconfined LiBH4 and enhanced mobility of Li+ and BH4− studied by solid-state NMR. J. Phys. Chem. C 2012, 116, 22169−22178.
3798
dx.doi.org/10.1021/jp311064d | J. Phys. Chem. C 2013, 117, 3789−3798
