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Stabilization of nanosized borohydrides for hydrogen storage: Suppressing the melting with TiCl3 doping Qiwen Lai, Chiara Milanese, and Kondo-Francois Aguey-Zinsou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00082 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018
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ACS Applied Energy Materials
Stabilization of Nanosized Borohydrides for Hydrogen Storage: Suppressing the Melting with TiCl3 Doping Qiwen Lai1, Chiara Milanese2 and Kondo-Francois Aguey-Zinsou1,* 1
MERLin, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia, E-mail:
[email protected] 2
Pavia Hydrogen Lab, C.S.G.I. & Department of Chemistry - Physical Chemistry Division, University of Pavia, Viale Taramelli, 16, I-27100 Pavia, Italy
Abstract
Lightweight complex hydrides, M(BH4)n (M = Li, Na, Mg and Ca, n = 1 for Li and Na, n = 2 for Mg and Ca), are believed to be promising hydrogen storage materials with extreme high hydrogen density up to 18.5 mass%. However, these materials suffer high dehydrogenation temperature, melting and reversibility problems, which exclude them from the list of practical hydrogen storage systems. Herein, borohydrides (M(BH4)n-Ti, with M = M1 or M2 and n = 1 or 2) were modified with TiCl3 via a wet chemistry approach and in some cases this led to the formation of solvent-stabilized nanoparticles. As a result of TiCl3 modification, the melting before hydrogen release was suppressed as evidenced by DSC and thermal microscopy observations. Furthermore, the hydrogen release temperature of M(BH4)n-Ti was significantly reduced. For example, the dehydrogenation temperature of NaBH4-Ti was reduced from 570 to 120 °C. Ti modification was also found to improve to some extent the reversibility of the doped materials. In particular, up to 2 mass% H2 was reversibly cycled for Ca(BH4)2-Ti at 300 °C and 9 MPa H2 pressure, in comparison to 400 °C and 70 MPa for pristine Ca(BH4)2. This study demonstrates a simple method to synthesis surfactant-free Ti-doped nanosized
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borohydrides and, by removing the melting of these materials; it provides a new path towards the stabilization of borohydride particles at the nanoscale. Keywords: Hydrogen storage; Borohydrides, Thermodynamic; Melting; Nanosizing;
1. Introduction Hydrogen is the ultimate energy carrier with potential to enable the full convergence of our current energy systems to 100% renewables. Hydrogen is clean and abundant in nature in the form of water or hydrocarbons. However, the barrier to the large-scale development of hydrogen as a universal energy carrier remains its storage in a compact and safe form. Lightweight complex hydrides, such as LiBH4, NaBH4, Mg(BH4)2 and Ca(BH4)2 are solid materials with high volumetric and gravimetric hydrogen densities (up to 18.5 mass% of hydrogen), that makes them very attractive as potential hydrogen storage materials. Within the structure of complex hydrides, hydrogen is attached to the central boron atom in the form of a complex anion via covalent bonds, which leads to very high stability of the bonded hydrogen. Consequently, one of the disadvantages of borohydrides is the poor hydrogen release paths. Borohydrides (M(BH4)n (M = Li, Na, Mg and Ca, n = 1 for Li and Na, n = 2 for Mg and Ca) decompose along various paths that can be simplified as follows:1
and
M(BH4)n → MHn + n B + 3/2 n H2
(1)
MHn + n B → MBn + 1/2 n H2
(2)
For LiBH4, 80% of the hydrogen is released at high temperatures above 380 °C and for a full hydrogen release, a temperature in excess of 700 °C is needed due to the high stability of LiH.2 The decomposition of NaBH4 at 0.1 MPa is over 500 °C, with potential formation of stable intermediates, like NaH and Na2B12H12.3 Mg(BH4)2 and Ca(BH4)2 undergo
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decomposition at a lower temperature (> 300 °C), in a more complex way and form a variety of intermediates.4 The stability of the hydrogen decomposition products remains the main problem restricting reversibility. For example, rehydrogenation of pristine LiBH4 from LiH and B requires a very high temperature and pressure (600 °C and 35 MPa).5 Similarly, reversibility of NaBH4, Mg(BH4)2 or Ca(BH4)2 are only achievable under harsh conditions.6-9 Destabilization of metal borohydrides though reaction with other hydrides is one of the most investigated approach to improve the hydrogen properties of borohydrides, and this has led to lower decomposition temperatures.10-11 However, hydrogen kinetics often remain poor and the reversibly of the hydrogen release is difficult. An alternative approach is through nanoconfinement to alter both the kinetic and thermodynamic properties of borohydrides through the reduction of particle size and confinement of nanoparticles within a porous scaffold. Incorporation of complex hydrides in carbon hosts, via melt infiltration and solvent impregnation, is among the most favored approaches. LiBH4,12 NaBH4,13 Mg(BH4)2,14 and Ca(BH4)2
15
showed significantly reduced hydrogen release temperature and improved
reversibility upon encapsulation in nanoporous carbons. However, the difficulty to fully fill porous structures as well as the reaction between complex hydrides and oxygen groups at the surface of the carbon host result in low and degrading hydrogen capacities.13 Besides, borohydrides still undergo melting before hydrogen release. It was claimed that the reduction of melting temperature upon confinement in nanoporous carbons may increase ionic diffusion rates and decrease dehydrogenation temperatures.12, 16-17 However, melting can also expel the complex hydride out of the porous structure and lead to the agglomeration of large/unconfined hydride particles. This ultimately reduces surface area, increases the dispersion of the reaction products, and thus limits the recombination of the decomposition products (i.e. elemental M and B) during hydrogen cycling. Consequently to improve the
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potential of the approach of nanosizing in leading to better hydrogen uptake and release from borohydrides their melting must be suppressed. One potential approach to suppress the melting of borohydrides is through their doping with transition metal halides. Addition of a metal salt to borohydrides has been shown to significantly improve their hydrogen desorption properties and, in some cases, modify decomposition paths.18 At a high level of doping (i.e. > 25 mol %), partial substitution can occur and lead to the formation of mixed borohydrides with alternate hydrogen properties.4, 19 For example, the doping of borohydrides with titanium-based compounds has been widely investigated due to the low electronegativity of Ti (1.54), which leads to a volatile borohydride decomposing at room temperature. Hence, it has been postulated that the combination of Ti with M(BH4)n may lead to intermediate thermodynamic stabilities.20-21 Indeed, ball milling borohydrides with Ti-based compounds has shown enhanced thermodynamic and/or kinetic properties due to the formation of more stable products like TiB2 and/or less stable intermediates like Ti(BH4)2.22-23 Among these studies, some preliminary DSC results24-25 indicate a shift in the melting of borohydrides upon Ti-doping but with some discrepancies. For example, TiCl3 and TiF3 showed opposite effects on the melting behavior of LiBH4 upon ball milling.24 TiCl3 increased the melting temperature of LiBH4 from 284 °C to 299 °C, while the addition of TiF3 led to a reduction of the melting temperature down to 264 °C. The increase in the melting point can be attributed to a partial anionic substitution of (BH4)- by Cl- upon heating leading to the more stable Li(BH4)1-xClx compound.26 Depending on the level of “pre-mixing” between LiBH4 and the metal salt, an exothermic decomposition without melting of the borohydride may also be observed as a result of its metathesis reaction with the metal halide.22
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Based on these initial findings, herein we investigated the potential of TiCl3 modification at lower levels than previously investigated on suppressing the melting of nanosized borohydrides, with the aim of stabilizing the morphology of borohydride particles at the nanoscale. This is a prerequisite before any additional functionalization towards a core-shell or nanoconfinement approach as previously discussed. Investigation was thus focused on the modification of M(BH4)n by partial reaction with titanium chloride (TiCl3) via a wet chemical approach, which unlike ball-milling can lead to the synthesis of particles with a controlled size and morphology. The physical and chemical properties of the resulting materials, including their melting behavior and reversibility, are reported. 2. Materials and Method 2.1 Preparation of materials Lithium borohydride solution (LiBH4 2.0 M in THF) and sodium borohydride (NaBH4, 98.0%), diethylene glycol dimethyl ether (99.5%, anhydrous), dibutylmagnesium (Mg(C4H9)2, 1.0 M in heptane), borane dimethyl sulphide complex (H3BS(CH3)2, 2.0 M in toluene), toluene (99.8%, anhydrous), diethyl ether (99.0%, anhydrous) and titanium chloride (TiCl3, 99.99%) were purchased from Sigma-Aldrich. Calcium chloride (CaCl2, anhydrous) was purchased from Ajax Finechem. Tetrahydrofuran (THF) was purchased as HPLC grade from Fisher Scientific and dried using a LC Technology SP-1 Solvent Purification System. All other chemicals were used as received without further purification. All operations were carried out under inert atmosphere in an Ar-filled LC-Technology glove box (
