Article pubs.acs.org/JPCC
Hydrogen Storage Properties of Nanoconfined LiBH4−Mg2NiH4 Reactive Hydride Composites Payam Javadian,† Claudia Zlotea,*,‡ Camelia Matei Ghimbeu,§ Michel Latroche,‡ and Torben R. Jensen† †
Center for Materials Crystallography, Interdisciplinary Nanoscience Center, and Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark ‡ Institut de Chimie et des Matériaux Paris-Est, UMR 7182 CNRS UPEC, 2-8, Rue Henri Dunant, 94320 Thiais, France § Institut de Science des Matériaux de Mulhouse, UMR 7361 CNRS UHA, 15, Rue Jean Starcky 68057 Mulhouse, France S Supporting Information *
ABSTRACT: LiBH4−Mg2NiH4 reactive hydride composites have been nanoconfined into two types of mesoporous carbons: a templated carbon with ordered small pores of ∼4 nm and a carbon aerogel with pores size of ∼30 nm. In situ synchrotron X-ray diffraction has revealed the formation of the MgNi2.5B2 compound during dehydrogenation at 300 °C and 5 bar of H2 pressure. The hydrogen desorption from nanoconfined LiBH4−Mg2NiH4 shows a single-step reaction at around 300 °C, as observed by mass spectroscopy coupled with thermogravimetric analysis. A synergistic effect is suggested, which facilitates lower hydrogen release than previously reported nanoconfined systems. Effective nanoconfinement provides faster kinetics of hydrogen release. Nevertheless, LiBH4−Mg2NiH4 shows progressive loss of capacity during cycling.
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INTRODUCTION Among the most promising solid-state materials for hydrogen storage, lithium borohydride (LiBH4) has been intensively studied due to its high total gravimetric (18.5 wt %) and volumetric hydrogen density (121 kg/m3).1,2 LiBH4 may release approximately 13.8 wt % of hydrogen following the reaction LiBH4 → LiH + B + 3/2H 2
pressure at a given temperature. This concept was denoted as “reactive hydride composite” (RHC), since it combines exothermic and endothermic chemical reactions to obtain a more favorable enthalpy change.15−17 Moreover, the thermodynamic destabilization allows different reaction pathways, and therefore, this might increase the dehydrogenation/rehydrogenation kinetics. The coupled ball-milled systems LiBH4/MgH2 and LiBH4/ Mg2NiH4 show destabilized thermodynamics.13,14 The dehydrogenation of the ball-milled system LiBH4/MgH2 is a twostep reaction with a theoretical hydrogen capacity of 11.7 wt %:
(1)
However, the combination of unfavorable thermodynamics, very slow kinetics, and reduced reversibility, due to formation of more stable higher boranes, restricts its use for practical applications.3,4 Temperatures exceeding 400 °C are required for full dehydrogenation, and even more extreme conditions are needed for the rehydrogenation. Several approaches have been proposed to improve the de/ rehydrogenation properties of such a stable complex hydride: the destabilization of thermodynamic properties with an additional catalytic element, anion substitution, reactive hydride composites, and the enhancement of the dehydrogenation/ rehydrogenation properties by nanoconfinement into porous scaffolds with or without catalysts.5−12 The thermodynamic destabilization approach focuses on the combination of LiBH4 with other hydrides (MgH2 or Mg2NiH4) to form destabilized systems with tunable properties.13,14 The thermodynamic destabilization mechanism is based on the formation of new alloys or compounds during dehydrogenation. The stability of these new phases lowers the enthalpy of dehydrogenation and thus increases the equilibrium © 2015 American Chemical Society
2LiBH4 + MgH2 → 2LiBH4 + Mg + H 2 → 2LiH + MgB2 + 4H 2
(2)
Despite the improved thermodynamics, the kinetics are still sluggish, and high temperature is required to obtain reasonable dehydrogenation rates, even in the presence of a catalyst.14 The ball-milled LiBH4/Mg2NiH4 system also decomposes in a two-step pathway, and this reaction is reversible.13 The first step is consistent with the reaction given in eq 3 (with 2.5 wt % theoretical hydrogen release) Received: November 24, 2014 Revised: February 20, 2015 Published: February 20, 2015 5819
DOI: 10.1021/jp5117307 J. Phys. Chem. C 2015, 119, 5819−5826
Article
The Journal of Physical Chemistry C
subsequent handling was performed in a glovebox with purified argon atmosphere. Second, Mg2NiH4 nanoparticles have been synthesized inside the pores of both carbon scaffolds (CT and CA) following the procedure detailed in ref 31. This is based on a multistep synthesis: the formation of Ni nanoparticles inside the pores (liquid impregnation of Ni precursor and reduction in H2 flow) followed by MgH2 nanoparticles [liquid impregnation of Nicontaining carbons with dibutylmagnesium (MgBu2) organometallic precursor and hydrogenation of MgBu2]. Finally, MgH 2 reacts with Ni, forming well-dispersed Mg2 NiH4 nanoparticles at T = 300 °C and p(H2) = 20 bar. The intermediate samples are named Mg 2 NiH 4 @CA and Mg2NiH4@CT. Finally, LiBH4 is incorporated by melt infiltration into the intermediate samples containing Mg2NiH4 nanoparticles. The latter ones were ground carefully with LiBH4 (AU, 99%) until a homogeneous mixture was obtained. Each sample was then assembled and sealed in a stainless steel autoclave inside the glovebox. The autoclave was mounted on a custom-made highpressure hydrogen rig, which was evacuated and purged with helium gas before hydrogen gas was allowed to contact the sample. The samples were heated to temperatures higher than the melting point of LiBH4 (T = 300 °C, ΔT/Δt = 5 °C/min) with the temperature held constant for 30 min followed by cooling at elevated hydrogen pressures of p(H2) = 110−150 bar. To ensure a successful infiltration and to avoid bulklike properties, the final amount of confined LiBH4 + Mg2NiH4 corresponds to approximately 60% pore volume filling. This is calculated using the densities of bulk LiBH4 (ρ = 0.66 g/cm3) and Mg2NiH4 (ρ = 2.68 g/cm3) and the total pore volume of the pristine scaffolds. The ratio between LiBH4 and Mg2NiH4 was chosen to be 6:1, where Mg2NiH4 may act as a catalyst. The final chemical compositions of both samples are 34 wt % LiBH4 and 29 wt % Mg2NiH4 into CA and 22 wt % LiBH4 and 18 wt % Mg2NiH4 into CT. These samples are denoted LiBH4−Mg2NiH4@CA and LiBH4−Mg2NiH4@CT. Characterizations. Due to their air/moisture sensitivity, the samples have been stored and handled inside an Ar glovebox, and the transfer to the characterization instruments has been performed under protective atmosphere. In Situ Synchrotron Radiation Powder X-ray Diffraction. The synchrotron radiation powder X-ray diffraction (SRPXD) measurements on samples LiBH4−Mg2NiH4@CT and LiBH4−Mg2NiH4@CA were conducted at the beamline I711 at MAX-lab and the beamline P.02.1 at Petra III, respectively. The data were collected using a CCD detector and a selected wavelength of λ = 0.993 182 or 0.991 551 Å (MAX-lab) and λ = 0.2072 Å (Petra III). The samples were packed in sapphire capillary tubes (0.79 mm i.d.) and mounted on an airtight sample holder in an Ar-filled glovebox.32 The sample holder was moved from the glovebox and attached to a gas control system on the synchrotron diffractometer. Structural refinements were not attempted due to the large number of phases, the complex thermal behavior, and the insufficient quality of data. Microstructural Measurements. Microstructural observations were performed by transmission electron microscopy (TEM) with a 200 kV FEG TEM (FEI Tecnai F20 with a field emission gun, point resolution 0.24 nm and energy filtering GIF). The composites have been transferred from the Ar glovebox to the microscope with the help of an airtight sample
4LiBH4 + 5Mg 2NiH4 ↔ 2MgNi2.5B2 + 4LiH + 8MgH2 + 8H 2
(3)
whereas the second step corresponds to decomposition of MgH2. The enthalpy and entropy of the reaction 3 are ΔH = 15.4 kJ/mol H 2 and ΔS = 62.2 J/K mol H 2 . This thermodynamic destabilization is impressive despite the loss of capacity after the first cycle to about 50% of the theoretical value. Another approach consists of nanoconfinement of hydride within a porous host with or without catalyst able to increase the hydrogen release kinetics and the reversible hydrogen storage capacity at practically useful conditions.7,18−20 The porous scaffold limits the particle size distribution of the hydride via the pore size distribution and inhibits the growth and agglomeration of particles. Moreover, the nanoconfinement can limit the mobility of the desorption products, thus increasing the reversibility of the hydrogenation reaction. Enhanced desorption kinetics have been demonstrated with clear evidence of a pore-size effect on the rate of dehydrogenation: the smaller the pore size of the confining scaffold, the higher the desorption rate.7 Another important result is the gradual reduction of toxic diborane (B2H6) release with decreasing the pore size.19 Moreover, the nanoconfinement coupled with the addition of nanosized catalyst strongly improves the reversibility of LiBH4.18,21 Recently, the combination of these two approaches, thermodynamic destabilization via chemical reactions with other hydrides and nanoconfinement into scaffolds, has been demonstrated to effectively improve the dehydrogenation and the reversibility properties of LiBH4 as compared to ball-milled materials.17,22−25 The present report investigates the nanoconfinement of the RHC LiBH4/Mg2NiH4 into two mesoporous carbon scaffolds with different textural properties and pore size distributions. This is the first report, to our knowledge, on the nanoconfined reaction between LiBH4 with a transition-metal hydride complex, Mg2NiH4. The melt infiltration of LiBH4 into the scaffolds already containing well-dispersed Mg2NiH4 nanoparticles and the dehydrogenation reaction pathway were studied by in situ synchrotron powder diffraction, and the dehydrogenation properties were determined and compared to the already reported nanoconfined RHC LiBH4/MgH2.
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EXPERIMENTAL DETAILS Synthesis of Materials. The preparation of RHC LiBH4− Mg2NiH4 is performed in three main synthetic steps: the synthesis of two carbons scaffolds (carbon aerogel and carbon template), the formation of Mg2NiH4 nanoparticles inside both carbon hosts, and, finally, the melt infiltration of LiBH4 into the two carbons already containing Mg2NiH4 nanoparticles. The synthetic steps are described below. First, the resorcinol−formaldehyde carbon aerogel (CA) and the carbon template (CT) scaffolds were prepared according to previously reported procedures.26−29 The CA was activated in continuous CO2 flow at 950 °C for 6 h and shows disordered mesopores with an average size of 30 nm.30 The carbon template (CT) was obtained by a replica method starting from SBA-15 silica and displays a very well organized mesoporosity of around 3.5 nm.28,29 Both carbon scaffolds were degassed at 200−400 °C in dynamic secondary vacuum for several hours, to remove all possible contaminants at the surface of the porous hosts. All 5820
DOI: 10.1021/jp5117307 J. Phys. Chem. C 2015, 119, 5819−5826
Article
The Journal of Physical Chemistry C
Table 1. Textural Properties (SBET, Vtot, Vmicro, and θpore) of Pristine Carbon Scaffolds (CA and CT), the Intermediate Samples Mg2NiH4@CT and Mg2NiH4@CA, and the Reactive Hydride Composites LiBH4−Mg2NiH4@CT and LiBH4−Mg2NiH4@CAa
a
textural properties
CA
Mg2NiH4@CA
LiBH4−Mg2NiH4@CA
CT
Mg2NiH4@CT
LiBH4−Mg2NiH4@CT
SBET (±10) m2/g Vtot (±0.1) cm3/g Vmicro (±0.1) cm3/g θpore (±0.2) nm
2659 3.30 1.22 30
958 1.4 0.4 30
174 (470) 0.58 (1.50) 0.10 (0.3) 30
740 0.95 0.22 3.5
464 0.50 0.1 3.5
70 (117) 0.15 (0.25) 3.5
Values are relative to the total material mass, and those in brackets are relative to the mass of carbon.
holder. The sample holder was cooled to −160 °C to avoid sample decomposition by beam irradiation. Textural Properties. Textural properties have been determined by nitrogen adsorption isotherm at −196 °C using an Autosorb IQ Quantachrome instrument. The samples have been loaded inside an Ar glovebox and transferred to the instrument under protective atmosphere. Prior to measurements, the samples have been degassed under secondary vacuum at 200 °C for 12 h. The specific surface area was obtained by the Brunauer−Emmett−Teller (BET) method within the 0.05−0.25 relative pressure range, the microporous volume was determined by the Dubinin−Radushkevich (DR) equation below 0.05 relative pressure, and the total porous volume was calculated at a relative pressure of 0.97.33 The pore size distribution was determined by the Barrett−Joyner− Halenda (BJH) model using the desorption branch. Hydrogen Sorption Measurements. The stability of hydrogen release and uptake cycling was studied by Sieverts volumetric measurements (PCTpro 2000). The samples were placed in a stainless steel autoclave that was assembled and sealed in an Ar-filled glovebox and attached to the apparatus. The samples were cycled five times. The hydrogen desorption data was collected in the temperature range from 25 to 450 °C (ΔT/Δt = 5 °C/min) at p(H2) = 2−3 bar with a fixed temperature of 450 °C for 15 h until full desorption was reached. Hydrogen absorption was performed by applying a pressure of p(H2) = 176−178 bar at room temperature followed by constant heating to 400 °C (ΔT/Δt = 5 °C/min). The sample has kept at 400 °C for 10 h and finally cooled to room temperature under hydrogen pressure. Temperature-Programmed Desorption Mass Spectroscopy and Thermogravimetric Analysis. To quantify the hydrogen release capacity, thermogravimetric analysis (TGA) was conducted simultaneously with a temperatureprogrammed desorption−mass spectrometer (TPD−MS). A PerkinElmer STA 6000 was used to conduct TGA coupled with a Hiden Analytical quadrupole mass spectrometer for TPD− MS measurements at constant flow (64 mL/min) of argon (99.99%). A powdered sample (
