Aluminium doping composite metal-organic framework by alane nanoconfinement: Impact on the room temperature hydrogen uptake
Graphical abstract
Introduction
Hydrogen is considered as the clean future fuel. However, its use for energy applications requires a compact and cheap way of storage [1], [2], [3]. The challenge with hydrogen is that it is a low-density gas and it is difficult to efficiently store enough hydrogen on-board with an autonomy of 500 km (e.g. 5 kg of hydrogen stored) [1]. The United States department of energy (DOE) has set the targets for onboard hydrogen storage for 2017 as 5.5 wt.% and they have given an ultimate target of 7.5 wt.% of hydrogen in a storage system, including tank and valves, and the filling time should not exceed 5 min [4]. A practical solution to circumvent the problem of storing hydrogen would be to find a storage material that can readily take up and release large amounts of hydrogen at ambient conditions. Moreover, the thermal properties of the storage material have to match the operation conditions of a fuel cell, which means that the temperature necessary to release the hydrogen from the storage material should not exceed the working temperature of the fuel cell device. None of the solid state materials currently used for hydrogen storage reach the required storage densities for an onboard application. New strategies for storage systems are then necessary to fulfill the specific requirements, and the combination of different storage systems may provide a possible solution to store sufficiently high amounts of hydrogen.
Metal-organic frameworks (MOFs) are a new class of highly crystalline porous materials consisting of metal ions or metal clusters and organic ligands as linkers. MOFs possess very low densities, huge surface area values and adjustable pore size which make them useful for a wide range of applications including gas storage, separation, catalysis, drug delivery and sensing [4], [5], [6], [7], [8], [9], [10], [11]. MOFs have attracted the attention of researchers in recent years in the field of hydrogen storage because of their high specific surface areas, tunable pore sizes and functionalized pore walls [12], [13]. However the higher hydrogen uptake capacities reported in MOFs were mainly at cryogenic temperatures only, unfortunately the room temperature hydrogen uptake capacities in MOFs are quite low and needs to be improved [14], [15]. Researchers have tried different modification strategies such as framework catenation, impregnation and organic linker functionalization in MOFs to enhance their hydrogen uptake capacities [16]. Doping of MOFs with various carbonaceous materials like carbon nanotubes (CNTs), activated carbon (AC), graphite nanofibres (GNF), graphite oxide (GO) etc. have given them an enhanced composite performance with unusual mechanical and hydrophobic properties [17], [18], [19], [20]. Moreover these hybrid composite MOFs are more thermally stable and have better moisture resistance compared to bare MOFs [17] which contain large void spaces not beneficial for retention of small molecules like hydrogen at ambient conditions owing to weak dispersive forces. The formation of composite increases the dispersive forces in MOFs via the presence of carbon materials inside the pores [18], [19]. The new pore space available in the composite results in increased dispersive forces, and contributes to enhance adsorption capacity at both cryogenic and ambient conditions. Theoretical and experimental studies have shown that lithium ion doping was also effective in enhancing the hydrogen adsorption capacity in MOFs [20], [21], [22], [23]. Lithium was largely used by researchers for doping as it is light weight and it can effectively donate electron density to the organic linkers in MOFs [24]. Our previous studies have shown that hydrogen adsorption capacity at ambient temperature in MOFs was further enhanced by the combined modification by carbon nanotubes or activated carbon incorporation coupled to lithium doping [25], [26].
Aluminium hydride (AlH3) known as alane is a covalent binary hydride which is an attractive medium for onboard hydrogen storage applications with 10.1 wt.% hydrogen and a density of 1.48 g mL−1 [27]. Alane has three polymorphs (α, β and γ) in which α-AlH3 is the most stable polymorph. The measured enthalpy of formation of alane is approximately −10 kJ mol−1 which makes reversible hydrogen storage difficult in alane as large amount of energy are needed to regenerate the hydride after decomposition [28]. Complex hydrides like LiAlH4, alanates (NaAlH4, Mg(AlH4)2) have high gravimetric hydrogen capacities but they all have kinetics and reversibility issues due to their complex nature and formation of multiple phases after decomposition [29]. Nanostructuring is a generally accepted method for enhancing hydrogen sorption kinetics in hydrides. Nanoparticles have shown significantly different properties as compared to bulk materials. Reducing the size of particles to nano region increases the surface area of the reactants, nanoscale diffusion distances and number of atoms in the grain boundaries [30], [31], [32], [33]. Thus the nanostructuring of hydrides improves the hydrogen uptake, facilitates its release and, enhance the reaction kinetics. The thermodynamics may also be improved. Ball milling is the most common method used for the preparation of hydride nanoparticles [34]. However, there are some drawbacks concerning this method. The crystallite size of the bulk material was reduced mechanically during high energy ball milling up to less than 10–15 nm only; further reduction in size may not be possible; moreover ball milling often contaminates the sample with traces of metal from vials or balls. There can also be the risk of agglomeration in ball milled samples that is the nanosized particles may grow into larger particles up on several hydrogen uptake and release cycles [35], [36].
Another recent approach for the preparation of particles less than 10 nm involves the use of high surface area porous scaffold or matrix for the confinement of nanosized hydrides [37], [38]. This new approach limits the particle size of the hydride to the pore size of the scaffold material, which helps to produce smaller particles than obtained by ball milling. As the nanoparticles of the hydride are confined within the pores, particle growth and agglomeration may be hindered and also limits the mobility of the decomposition products, and keep them in intimate close contact. Thus nanoconfinement may improve hydrogen absorption properties of metal hydrides. Researchers have shown that nanoconfining ammonia borane in mesoporous silica improved its hydrogen desorption properties, and this was the first ever research work reported on this topic [39]. Later, various light metal hydrides were nanoconfined inside nanoporous scaffold materials [40], [41], [42], [43], [44], [45], [46]. Hydrogen storage kinetics of LiBH4 was enhanced by incorporating it inside nanoporous carbon scaffolds [40]. Zeolite templated carbon was also used by the researchers for nanoconfining LiBH4 [41]. Reversible hydrogen storage was demonstrated in NaAlH4 nanoconfined within the nano-pores of titanium-functionalized metal–organic framework MOF-74(Mg) [42]. MgH2 has been studied by researchers for hydrogen storage enhancement using nanoconfinement procedure [43]. Another complex hydride system like LiBH4 and multiple phase systems like LiBH4+MgH2 were also nanoconfined inside various porous scaffolds to study enhancement in their hydrogen sorption properties [44], [45]. Banach et al. have loaded alane nanoparticles inside ZIF-8 framework by solution infiltration method and showed the framework structure was not affected by the loading of alane [46]. As per our knowledge there was no literature on hydrogen sorption studies in aluminium doped MOF materials synthesised by alane nanoconfinement. These observations encouraged us to synthesize alane and to nanoconfine it in activated carbon incorporated MIL-101 composite metal-organic framework using solution impregnation procedure to produce aluminium doped composite MOF samples and to study the hydrogen sorption properties of the new material at ambient temperature up to 100 bar pressure.
Section snippets
Materials and methods
Chromium (III) nitrate (Cr(NO3)3·9H2O), 1,4-benzenedicarboxylic acid (H2BDC), acetic acid (C2H4O2), activated carbon (NORIT-RB3), nitric acid (HNO3), hydrogen peroxide (H2O2), ammonium fluoride (NH4F), anhydrous aluminium chloride (AlCl3), lithium aluminium hydride (LiAlH4) were supplied by Sigma-Aldrich and were used as received without any purification. Tetrahydrofuran (THF), purified by the solvent purification system (MBRAUN) was used for synthesis. All the synthesis procedures for alane
Results and discussion
The PXRD patterns of MIL-101, AC-MIL-101 and aluminium doped AC-MIL-101 samples (Fig. 1) were in good agreement with published results in literature [48]. This indicates that the framework structure and crystallinity of MIL-101 was not affected by activated carbon incorporation and alane nanoconfinement [25]. The FTIR spectra of MIL-101, AC-MIL-101 and aluminium doped MOF samples are shown in Fig. 2. The infrared frequency vibrational band at 1635 cm−1 indicates the presence of adsorbed water
Conclusions
Aluminium nanoparticles were finely doped into activated carbon incorporated MIL-101 composite MOF with an original procedure by alane nanoconfinement using solution impregnation technique. The hydrogen uptake capacity of MIL-101 was greatly enhanced by both activated carbon incorporation and aluminium doping. A small amount of doped aluminium (925 ppm), multiplied by more than three the hydrogen uptake capacity of the composite MIL-101 material at 298 K up to 100 bar. Unfortunately, excessive
Acknowledgments
The authors acknowledge the “Délégation Générale de l’Armement (DGA)” for financial support, Dr Elise Provost for her help concerning the characterizations by Atomic Absorption Spectroscopy, Dr Jean François Hochepied for the access to the PXRD apparatus and, Dr Anthony Chesnaud and Dr Alain Thorel for TEM analysis.
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