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Article

Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage

by
Chulaluck Pratthana
1 and
Kondo-Francois Aguey-Zinsou
2,*
1
MERLin Group, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
2
MERLin Group, School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(9), 4742; https://doi.org/10.3390/app12094742
Submission received: 7 April 2022 / Revised: 1 May 2022 / Accepted: 5 May 2022 / Published: 8 May 2022
(This article belongs to the Special Issue Recent Development and Application of Hydrogen Production and Storage Materials)
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Abstract

:
LiAlH4 and NaAlH4 are considered to be promising hydrogen storage materials due to their high hydrogen density. However, their practical use is hampered by the lack of hydrogen reversibility along with poor kinetics. Nanosizing is an effective strategy to enable hydrogen reversibility under practical conditions. However, this has remained elusive as the synthesis of alanate nanoparticles has not been explored. Herein, a simple solvent evaporation method is demonstrated to assemble alanate nanoparticles with the use of surfactants as a stabilizer. More importantly, the roles of the surfactants in enabling control over particle size and morphology was determined. Surfactants with long linear carbon chains and matching the hard character of alanates are more prone to lead to the formation of small particles of ~10 nm due to steric hindrance. This can result in significant shifts in the temperature for hydrogen release.

Graphical Abstract

1. Introduction

Hydrogen is considered as a sustainable energy carrier owing to the near-zero carbon emission that is possible through its use. However, methods to enable the safe and efficient storage of hydrogen with high density remain a major obstacle for its widespread use [1,2,3]. The U.S. Department of Energy has outlined a technical target for the on-board hydrogen storage system of 5.5 mass % H2 by 2025 [4]. A range of materials including metal-organic frameworks, carbon nanostructures, liquid organic hydrogen carriers, metal hydrides, and complex metal hydrides have been proposed as promising hydrogen storage materials [5,6,7,8,9]. In particular, NaAlH4 and LiAlH4 have garnered immense attention due to their high theoretical hydrogen storage capacity (7.4 and 10.5 mass% H2, respectively) and relatively low decomposition temperature (<300 °C) [10,11]. Unfortunately, the poor hydrogen sorption kinetics of these materials and the lack of hydrogen reversibility in particular for LiAlH4 remain severe drawbacks when considering any applications [12,13].
Typically, the regeneration of LiAlH4 from the decomposition products would require an extreme pressure, more than 1000 MPa at 170 °C, due to the thermodynamic constraints [12,14,15]. In contrast, the regeneration of NaAlH4 from NaH and Al is easier, but still remains sluggish without appropriate modifications [16]. Extensive efforts have shown that the kinetics, thermodynamics, and reversibility of NaAlH4 can be significantly improved by adding catalysts such as transition metal halides [17,18], lanthanide oxides [19], titanium compounds [20,21], or carbon materials [22,23]. Unfortunately, the positive catalytic effect on reversibility observed with NaAlH4 have been difficult to translate to LiAlH4 due to the exothermic nature of hydrogen release from pristine LiAlH4 [24]. An alternative approach to improve the hydrogen storage properties is through nanosizing. Several studies have shown significant improvements in hydrogen sorption properties upon nanosizing of hydrides [25,26]. For example, Balde et al. synthesized NaAlH4 nanoparticles (<20 nm) supported on carbon nanofibers and achieved a low desorption temperature of less than 160 °C [27]. In this case, reducing the particle size of NaAlH4 to the nanoscale leads to a modification of NaAlH4 thermodynamics due to their increased surface area and number of hydrogen adsorption sites [28,29]. Additionally, smaller hydride particles induce shorter diffusion paths, which results in faster hydrogen kinetics and improved reversibility [29,30].
Currently, the simplest approach to synthesize nanoparticles of alanates is through confinement within the porosity of high surface area materials and carbons in particular, via melt infiltration or solvent impregnation [29,30,31,32]. For example, the nanoconfinement of LiAlH4 in high surface area graphite led to drastic improvements with an onset hydrogen desorption occurring at 135 °C instead of 200 °C and rehydrogenation observed at 300 °C under 7 MPa of hydrogen pressure [30]. Similarly, Zhou et al. reported on an improved cycling stability of NaAlH4 upon confinement in mesoporous carbons [33]. Unfortunately, these nanoconfinement approach suffers from significant drawbacks including the dead weight and volume of the scaffold and the difficulties in filling the pores. This inevitably reduces the practical hydrogen storage capacity to <1 mass%, which is far below the practical requirements [25,29,30].
An alternative strategy is through the synthesis and stabilization of the isolated nanoparticles in a core-shell manner as previously reported [34,35]. In this case, LiAlH4 nano-core was synthesized with Ti acting as a shell to facilitate the hydrogen uptake and release and retain the nanofeatures of LiAlH4 upon dehydrogenation [34]. In this core-shell approach, control over the physical and chemical properties of the nano-core alanate is important as it is expected not to only dictate the temperature at which hydrogen can be released, but also determine to some extent the hydrogen thermodynamics and kinetics [36]. To date, it is believed that particle size may play a role in shortening the hydrogen diffusion path, but the interfacial energy and specific surface planes may also determine the hydrogen properties of nanosized hydride [25,37,38,39]. In this respect, solvent evaporation offers simple means to enable control over the morphology and structure of nanosized alanate particles [34,40]. In solvent evaporation method, the solubilized hydride precursor is forced to precipitate with the help of stabilizing agents into various nanoparticles of controlled morphology and size. We recently demonstrated the potential of this synthetic approach toward the control growth of LiBH4 and NaBH4 nanoparticles and reported on the interlink between morphology/nanostructure control and hydrogen properties [37,40,41].
Herein, we report on similar investigations on the alanate family instead of the borohydrides, with the aim to understand the broadness of the observations made with LiBH4 and NaBH4. In particular, we report on the effect of various surfactants in controlling the final morphology of LiAlH4 and NaAlH4. Our study reveals that surfactants with a hard character and longer carbon chains can strongly bind to the surface of alanates particles and this leads to the stabilization of smaller particles. In addition, we found that the hydrogen release behavior of the nanosized alanates varies as a function of particle size. More remarkably, the hydrogen desorption temperature of LiAlH4 was found to decrease as a function of the particle size and the amine and thiol-surfactant based head groups induced a small hydrogen release within the lower temperature range of 65 to 160 °C. It is thus apparent that the size, morphology, and H2 release behavior of LiAlH4 and NaAlH4 can be controlled through the use of surfactants.

2. Materials and Methods

All experiments were performed under inert atmosphere in an argon filled glove box (O2 and H2O < 1 ppm) provided by LC technology.
LiAlH4 1.0 M in diethyl ether and anhydrous diethyl ether were purchased from Sigma Aldrich and used as received. Tetrahydrofuran (THF) was purchased as HPLC grade from Fisher Scientific and dried using a LC Technology SP-1-Solvent Purification System. Sodium aluminum hydride (NaAlH4, 90%) was purchased from Sigma-Aldrich, and purified by dissolving in large amounts of THF and recrystallizing from the filtrate solution. Hexylamine (HXA, 99%), dodecylamine (DDA, ≥99%), octadecylamine (ODA, ≥99%), heptanethiol (HTT, 98%), dodecanethiol (DDT, ≥98%), octadecanethiol (ODT, 98%), tetra-n-butylammonium bromide (TBAB, ≥98%), tetra-n-octylammonium bromide (TOAB, 98%), tetra-n-decylammonium bromide (TDAB, ≥99%) and tridecylic acid (TDA, ≥98%) were purchased from Sigma-Aldrich.

2.1. Synthesis of MAlH4 (M = Li and Na) via Solvent Evaporation Using Surfactants

LiAlH4 and NaAlH4 are highly soluble in THF. However, LiAlH4 forms a strong adduct with THF and is difficult to dry even under vacuum. Therefore, diethyl ether was used for LiAlH4, while the NaAlH4 surfactant stabilized particles were synthesized in THF. A 10 mL solution of 1 mmol alanate was prepared by dissolving appropriate amounts of NaAlH4 and LiAlH4 in their respective solvent. In the case of LiAlH4, the solution was prepared by adding 1 mL of LiAlH4 solution (1.0 mM in diethyl ether) into 9 mL of diethyl ether. For NaAlH4, 54 mg of purified NaAlH4 was dissolved in 10 mL of THF. To a solution of alanate, the desired concentration of surfactant (0.1–10 mM) was added and stirred at 500 rpm for 1 h at room temperature. The resulting solution was then placed on a Schlenk line under vacuum (2 mbar) to remove all remaining solvent. For comparison, the pristine materials without any surfactant were also prepared under the same conditions. The material prepared are denoted as MAlH4-X-N (where M = Li or Na, X = HXA, DDA, ODA, HTT, DDT, ODT, TBAB, TOAB, TDAB, TBPB, or TDA, N = 0.1, 1, 5, 10 is the surfactant concentration in mM).

2.2. Material Characterisation

The morphologies, particle size distribution, energy dispersive x-ray spectroscopy (EDS) analysis and elemental mapping were performed on a Philips CM200 operated at 200 kV. The materials were dispersed in cyclohexane followed by ultrasonication for a few seconds before being dropped onto a carbon-coated copper grid and dried in an argon filled glovebox. The prepared copper grid was transferred to the TEM in a quick manner to minimize air exposure. To verify that the observed particles were the stabilized-alanate nanoparticles, elemental mapping was carried out on the representative LiAlH4-DDT-1 and NaAlH4-DDT-1 nanoparticles (Figure S1).
The crystalline nature of the material was determined by X-ray Diffraction (XRD) using a Philips X’pert Multipurpose XRD system operated at 40 mA and 45 kV with a monochromated Cu Kα radiation (λ = 1.541 Å) from 20° to 70°. The materials were placed on the stainless-steel holder and protected against oxidation from air by a Kapton foil.
Infrared analysis was carried out on a Bruker Vertex 70V equipped with an MCT-detector and a Harrick-Scientific Praying Mantis Diffuse Reflectance Infrared Fourier Transform Spectroscopy accessory. The materials were mixed with KBR in the glove box and then loaded in an air-tight chamber fitted on the Praying Mantis. The spectra were recorded at room temperature from 3500 to 400 cm−1 with 124 scans and a resolution of 1 cm−1.
Hydrogen desorption profiles were acquired by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) in conjunction with Mass Spectrometry (MS)-OmniStar instrument (Pfeiffer). The measurements were conducted under argon flow at 20 mL min−1 from 20 to 500 °C with a heating rate of 10 °C min−1. Masses between m/z = 2 and 100 were followed by MS.

3. Results and Discussion

3.1. Effects of Surfactant in Directing the Size and Morphology of Alanate Nanostructures

The length of surfactants’ carbon chain can play a significant role in controlling particle size during solvent evaporation. Our work on NaBH4 and LiBH4 revealed that longer linear surfactant chains lead to smaller nanoparticles due to the larger steric hindrance, whereas a reverse trend was found with particles stabilized with quaternary ammonium bromides [40,41]. The effects of carbon chain length were thus investigated to determine whether the trends observed with borohydrides also apply to alanates. This was carried out by using linear surfactants with increasing chain length such as hexylamine (CH3(CH2)5NH2), dodecylamine (CH3(CH2)11NH2), and octadecylamine (CH3(CH2)17NH2). For comparison, a quaternary ammonium bromide (R4NBr) such as TBAB ([CH3(CH2)3]4NBr), TOAB ([CH3(CH2)7]4NBr) and TDAB ([CH3(CH3)9]4NBr) were also investigated. It should be noted that TBAB, TOAB, and TDAB are not soluble in diethyl ether. Therefore, the effect of these three surfactants was only investigated with NaAlH4.
Upon solvent evaporation, spherical-like nanoparticles were observed (Figure 1), and the average particle size of both LiAlH4 and NaAlH4 was found to decrease with increasing alkyl chain lengths. For example, with increasing chain length from 6 (HXA) to 18 (ODA) carbons, the particle size of LiAlH4 decreased from 394 to 53 nm. Switching the surfactant from a linear amine to a thiol with different alkyl chain lengths revealed a similar trend, in agreement with the general observations that larger steric hindrance occurs with longer linear carbon chains (Figure 2 and Figure S15, Supporting information) [42,43].
In contrast, stabilization of NaAlH4 with a quaternary ammonium bromide showed a reverse trend (Figure 2b and Figure S16). NaAlH4 particles of 9 ± 3 nm were obtained upon stabilization with TBAB, whereas with TOAB (alkyl chain with 8 carbon atoms instead of 4 for TBAB), NaAlH4 particles with an average size of 106 ± 17 nm were observed (Figure 2b and Figure S16). With TDAB (i.e., 10 carbons) NaAlH4 particles growth restriction failed (Figure S16) owing to the strong intrinsic steric hindrance of the surfactant which result in a low packing density of the surfactant at particle’s surface and thus poor NaAlH4 particle stabilization [44,45]. Besides, the interaction of NaAlH4 surface with the surfactants may be further weaken by the strong surfactant-surfactant interaction [46]. As such, poorer stabilizing ability was observed with longer R4NBr chains.
Interestingly, it can be observed that the ability of amine-based surfactants in leading to small particles and thus better stabilization effects is superior to that of thiol-based surfactants (Figure 2). Several studies have shown that the strength of the interaction between the surface of stabilized particles and surfactant’s head group may affects the growth of nanoparticles [47,48]. According to the Pearson’s Hard and Soft Base (HSAB) principle, chemical substances can be categorized into hard, soft, and borderline character. The affinity between Lewis acids and Lewis bases would be stronger if they have similar hard or soft properties [24,49]. Based on this, Li+, Na+, and AlH4- can be considered as hard acids [24]. Therefore, LiAlH4 and NaAlH4 are expected to coordinate stronger with surfactants carrying a hard base head group. To investigate this, the effect of surfactants displaying hard and soft properties with the same carbon chain length, i.e., dodecylamine (RNH2), dodecanethiol (RSH), and tridecylic acid (RCOOH) were investigated. Generally, the relatively small non-polarizable donors such as N, O, and F hold a hard character, while the soft character corresponds to the polarizable donors such as P, S, Cl. Based on these considerations, dodecylamine (RNH2), tridecylic acid (RCOOH), and dodecanethiol (RSH) were considered to be hard base, hard acid, and soft base, respectively.
The average particle size observed upon LiAlH4 and NaAlH4 evaporation with surfactants of hard and soft properties are summarized in Figure 3 and Figure S17. The significant differences observed in particle size were taken as indirect evidence that the functionality of the surfactant head group affected the size of LiAlH4 and NaAlH4 morphology. For example, LiAlH4 stabilized by DDA (RNH2), TDA (RCOOH), and DDT (RSH) have an average particle size of 64 ± 29, 179 ± 40, and 188 ± 11 nm, respectively. Considering that LiAlH4 is a hard acid, it would tend to bind more strongly to –NH2 and –COOH (of hard character) compared to –SH which is a soft base. As such, LiAlH4 nanoparticles observed by TEM are smaller with the –NH2 and –COOH functionalities (Figure S17).
A similar trend was observed with NaAlH4, where the formation of small NaAlH4 particles is favored with hard surfactants (i.e., dodecylamine (DDA), and tridecylic acid (TDA)). Again, strong coordination of NaAlH4 with –NH2 and –COOH was found to better hinder the growth and agglomeration of the NaAlH4 nanoparticles as compared to the weaker NaAlH4/SH based surfactant interaction.
It is noteworthy that the reducing ability of LiAlH4 and NaAlH4 may inevitably affect the size restriction efficiency [50,51]. LiAlH4 is capable of reducing carboxylic acids into primary alcohols while amines remain unreacted [52]. Potential reaction between LiAlH4 and TDA are given below in Equation (1) [51,53,54].
LiAlH4 + RCOOH → RCH2OH + 2LiOH + AlHx
In contrast, although NaAlH4 is capable of reducing COOH functions, NaAlH4 is a milder reducing agent as compared to LiAlH4 [51]. By XRD, no by-product from the potential reaction of TDA with the alanates was observed. This may be due to the small amount of the resulting products, or their amorphous nature (Figures S18 and S19). Upon FTIR analysis, –CO (1105 and 1124 cm−1) and –OH (3264 and 3322 cm−1) stretching vibrations corresponding to a primary alcohol were observed in LiAlH4-TDA and NaAlH4-TDA. This suggested that the COOH functionality in TDA was fully reduced by both LiAlH4 and NaAlH4, as the C=O stretching of carboxylic acid was not observed (Figure 4) [40,41]. It is likely that the initial reduction of the carboxylic function of the surfactant, into an –OH will result in a different stabilization mechanism of the alanate, and thus different particle sizes. For example, it has been reported that the binding energy between Li+ and the various functional groups follows the order of OH < NH2 < COOH [55]. Therefore, TDA is expected to lead to the formation of smaller particles than DDA due to the stronger affinity of Li+ to –COOH as compared to –NH2. However, the reverse was observed (Figure 3), owing to the reduction of carboxylic groups to alcohols. As such, LiAlH4 particles synthesized with TDA would inevitably be larger than that of DDA due to the reduction of COOH into OH.
The concentration of the surfactants can also be expected to affect the nucleation and growth process of LiAlH4 and NaAlH4 particles, owing to the concentration dependent organizational structures of the surfactant in solution and associated critical micelle concentration (CMC). As shown in Figure 5, which summarized the evolution of particle size as determined by TEM upon changing the surfactant concentration (Figures S3–S14), a relationship can be drawn between the surfactants’ concentration and particle size. For the amine stabilized LiAlH4, it can be seen that particle size decreases with an increase in surfactant concentration, with the smallest particles obtained at a surfactant concentration of 10 mM. A similar trend was observed with thiol-stabilized LiAlH4 with the exception of LiAlH4-ODT. LiAlH4 stabilized with ODT exhibited a different trend where at 10 mM, the average particle size increased from 22 ± 6 to 200 ± 78 nm. This suggested that a concentration of 10 mM is far exceeding the CMC of ODT in diethyl ether. Under these conditions LiAlH4 particles would start to agglomerate to form larger particles. Indeed, it was reported that surfactants with longer carbon chains would display lower CMC than the ones with shorter chains [56]. Therefore, ODT would have lower CMC than DDT. Additionally, it has been previously reported that the CMC decreases as the size of surfactants’ head group increases [57,58]. Based on these considerations, ODT which has a larger head group can be assumed to have lower CMC than ODA with smaller head group.
A similar correlation of particle size and surfactant concentration was observed with thiol-stabilized NaAlH4 such as DDT and ODT (Figure 5d and Figures S12–S14) where a spherical-like particles were produced. Upon increasing the concentration of surfactant from 0.1 mM to 1 mM, a significant reduction in average particle size was observed (e.g., from 108 ± 25 to 26 ± 6 nm). Nevertheless, heptanethiol (HTT) failed to restrict the particle size of NaAlH4, possibly due to the low steric hindrance. For the NaAlH4 stabilized by HXA, DDA, and ODA (Figure 5c and Figures S8–S11), the size of the nanostructure decreases when the concentration increases from 0.1 to 1 mM. Unfortunately, it was not possible to determine the size of these particles at a 10 mM surfactant concentration because of the severe particles’ agglomeration (Figures S9–S11).
When comparing the size of LiAlH4 and NaAlH4 nanoparticles, it can be seen that the stabilization effects (i.e., smaller particles) are more pronounced in NaAlH4 than with LiAlH4. This can be explained by: (1) the higher reactivity of LiAlH4 with the surfactants, e.g., DDT, ODT, and TDA as compared to NaAlH4; and the higher amount of possible by-product (e.g., LiOH formed during the reaction between LiAlH4 and the surfactants), which may potentially weaken the interaction with the surfactant; (2) differences in the evaporation rates of the solvent (THF and diethyl ether boiling point is 66 °C and 34.6 °C respectively), which may affect the growth of nanoparticles due to differences in nucleation rate and; (3) the tendency of NaAlH4 to bind to THF, and this may affect the growth of NaAlH4 nanoparticles by possibly preventing further Ostwald ripening [59,60,61].
Interestingly, different morphologies were observed when increasing the concentration of TDA (Figure 6). For TDA-stabilized LiAlH4 and NaAlH4, the sphere-like particles evolved into a rod-like structure at a concentration above 5 mM. The same phenomenon has been observed with TDA-stabilized NaBH4 where spherical nanoparticles transformed into bar-like morphologies at increasing TDA concentrations [37]. Indeed, TDA has been proposed to have two CMCs and this would lead to variations in morphologies [41,62]. Considering this, we propose that the self-assembly of TDA at its first CMC value would lead to a spherical micelle, and the nanorod is possibly the result of a side-to-side attachment of alanate nucleic or the aggregation of surfactants into larger rod-like micelles at the second CMC value [62,63,64].
A morphological evolution upon increasing the surfactant concentration to 10 mM was also observed in NaAlH4-HXA, NaAlH4-DDA, and NaAlH4-ODA. In this case, the nanoparticles severely agglomerated into a thread-like network (Figures S9–S11). This may be due to the entanglements and interaction among the various micelles structure at high concentration. Indeed, extremely small nanoparticles possess a high surface free energy, therefore the particles may aggregate into a thread-like network to minimize the excess free energy by reducing the number of end caps [65,66,67].
As observed from TEM analysis (Table S1 and Figures S3–S8), the optimum surfactant concentration to produce the smallest LiAlH4 nanoparticles is ~10 mM. Therefore, LiAlH4 stabilized with 10 mM of surfactants was further investigated. On the other hand, NaAlH4 particles stabilized by HXA, DDA, and ODA were severely agglomerated at a surfactant concentration of 10 mM. Hence, NaAlH4 stabilized with 1 mM of surfactants was further investigated as well-defined nanoparticles were produced at this concentration.

3.2. Structural Properties

XRD analysis was used to determine any change in the alanate crystalline structure upon surfactant stabilization (Figures S18 and S19). XRD analysis revealed that all of the synthesized materials retained the same crystalline structure as their pristine counterpart. Notably, some broadening of peaks and the reduction in the crystalline sizes (Table S2) compared to the bulk materials were observed and this would be consistent with the formation of smaller particles size upon surfactant stabilization. However, neither the surfactant nor any by-products were detected by XRD.
The synthesized materials were further analyzed by FTIR to confirm the presence of surfactants and any compositional changes that were not visible by XRD. All of the as-synthesized LiAlH4 and NaAlH4 nanostructures displayed a typical Al-H stretching and bending between 1600 and 1800 cm−1 and between 700 and 900 cm−1, respectively (Figure 7 and Figure 8) [68,69]. The existence of the surfactants was confirmed by the C–H stretching modes observed between 3000 and 2840 cm−1. These peaks correspond to the alkyl chain in the surfactants [41]. For LiAlH4 and NaAlH4 stabilized by HXA, DDA, and ODA, the broad stretching vibrational between 3360 and 3200 cm−1 and between 1250 and 1020 cm−1 were assigned to the –NH and –CN stretching of the amine group, respectively. For NaAlH4 stabilized by HTT, DDT, and ODT, small vibrations corresponding to the –SH stretching of thiol group were observed at 2574, 2576, and 2578 cm−1, respectively [41]. Surprisingly, there is no evidence of the thiol stretching mode between 2600 and 2550 cm−1 in the thiol stabilized LiAlH4 particles. This indicates that the –SH group may have reacted with LiAlH4 to form a thioalkyl derivative as shown in reaction (2) [70]. Also, the AlH4- stretching modes of the LiAlH4-HTT-10, LiAlH4-DDT-10, and LiAlH4-ODT-10 between 1600 and 1800 cm−1 were broader as compared to the pristine material, owing to the formation of a LiAlH4-thiol complex [70].
LiAlH4 + 4RSH → LiAlH4−n(SR)n + nH2
For LiAlH4-TDA, the peaks at 3264 and 1124 cm−1 were attributed to O–H and C–O vibration of a primary alcohol. In addition, no C=O stretching mode was observed. This further proved the reduction of the carboxylic group upon the stabilization of LiAlH4 with TDA as per reaction (1). Similarly, TDA stabilized NaAlH4 displayed stretching modes at 3322 and 1105 cm−1 attributed to the alcohol group. For NaAlH4-TBAB, the peak at 1443 cm−1 was assigned to the tetra butyl groups of TBAB [71].
It is noteworthy that the shift in C–H vibrational frequencies can be used to determine the conformational order of the surfactant molecules on the surface of the alanate particles [72]. Comparing the C–H asymmetric (2912–2929 cm−1) and symmetric (2849–2861 cm−1) stretching modes of the surfactant stabilized alanate particles with that of the surfactants in a solution, it was found that the C–H vibrational modes of LiAlH4 and NaAlH4 stabilized by HXA and DDA all shifted to higher frequencies [40]. This was taken as an indication for an increase in carbon chains ‘gauche defects, where the surfactants are in a disordered arrangement or liquid-like structure. As for LiAlH4-ODA, NaAlH4-DDT, NaAlH4-TBAB, and NaAlH4-TDA, the CH2 stretching observed at a lower wavenumber, suggests a decrease of gauche conformers and thus highly ordered alkyl surfactant chains along an all-trans conformer’s structure [73,74,75]. Therefore, it is likely that HXA and DDA are organized in a disordered liquid-like structure at the surface of the alanate particles. In contrast, ODA, TBAB, DDT, and TDA would form an ordered surfactant’s structure at the surface of the alanate particles.
These results reveal that the size and morphology of alanate particles can be tuned upon an appropriate choice of surfactant. Surfactants with a hard character such as amine and carboxylic acid provide a stronger steric hindrance, which in turn leads to smaller particles. However, unlike the other surfactants, the amine-based surfactants remained unreacted and are most likely the best choice in stabilizing alanate nanoparticles.

3.3. Hydrogen Desorption Properties

LiAlH4 and NaAlH4 nanomaterials, i.e., LiAlH4 stabilized by 10 mM DDA and 1 mM DDA, respectively, were further investigated by TGA/DSC and MS to determine any influence of the surfactants’ binding strength on the hydrogen desorption profiles. For pristine LiAlH4, melting occurs at 182 °C. In contrast, surfactants stabilized LiAlH4 melted at lower temperatures between 166 and 169 °C depending on the surfactant (Figure 9). This indicates an influence of particle size reduction on the melting behavior of LiAlH4 akin to previous observations with metal nanoparticles [76,77,78]. To our surprise, such a reduction in melting point for the surfactants stabilized NaAlH4 particles followed two separate trends related to the nature of the surfactant (i.e., of hard or soft character). For hard surfactants including DDA, TDA, and TBAB, the melting point decreases as a function of particle size similar to that of LiAlH4. In contrary, for the soft thiol-stabilized NaAlH4 particles, the overall melting point was decreased by ~8 °C with no clear correlation to the particle size. Compared to LiAlH4, the reduction in melting point of NaAlH4 is less pronounced. A similar phenomenon was reported with nanoconfined NaAlH4 where only a ~13 °C decrease in the apparent melting temperature was observed when NaAlH4 was confined within the 35 nm porosity of metal-organic frameworks [79]. This suggests that the melting point of NaAlH4 only marginally reduces despites a significant reduction in particle size.
The desorption profile of the as-synthesized LiAlH4 nanoparticles still occurs in three steps. However, a correlation between the particle size with the dehydrogenation of LiAlH4 can be seen from the first major decomposition step of LiAlH4 into Li3AlH6 (Figure 10). All of the surfactant stabilized LiAlH4 particles decomposed at lower temperatures as compared to pristine LiAlH4 (218 °C). Notably when the particle size was reduced below 10 nm, the temperature required to decompose LiAlH4 decreased by 21 °C. Furthermore, the dehydrogenation of LiAlH4 was found to roughly correlate with its mean particle size. As such, this can be taken as an indication that a change in particle size can eventually leads to the improve hydrogen release properties for LiAlH4 [80]. Unfortunately, further decomposition of Li3AlH6 and LiH did not show a similar trend, and this is most likely due to the premature decomposition of the surfactants which in turn contribute to the loss of nanosizing effects (Figure S29). Nevertheless, it is not certain that the reduction in particle size may be the sole contributor to the improved dehydrogenation temperatures as the interaction between LiAlH4 and surfactants may also affect the hydrogen releasing mechanism.
Remarkably, all of the amine-stabilized LiAlH4 started to release hydrogen from 75 °C (Figure S23). Indeed, many LiAlH4-NH2 systems have been reported to induce hydrogen release at low temperatures [81,82], owing to the Hδ+ (amine) and Hδ− (hydride) interaction [83]. For LiAlH4 stabilized by thiol bases surfactants, a modest amount of hydrogen was observed between 65 to 160 °C prior to the major decomposition of LiAlH4 into Li3AlH6 at ~200 °C (Figure 11 and Figure S24). This was attributed to the reaction between the remaining thiol group and LiAlH4 [70]. Similarly, for LiAlH4-TDA-10, the initial mass loss observed by TGA and the hydrogen desorption peak at 151 °C (Figure S22) may be the result of the interaction between LiAlH4 and the alcohol group (from the reduction of –COOH) as per reaction (3) [84].
4LiAlH4 + 12ROH → LiAlH4 + 3LiAl(OR)4 + 12H2
In contrast to LiAlH4, for the surfactant stabilized NaAlH4 particles, no significant reduction in the first decomposition step was observed (Figure 12) and this was similar for the second decomposition step (Figure S30). Interestingly, hydrogen release peak was observed at about 330 °C for all of the nanosized NaAlH4. This may be the result of an early decomposition of Na3AlH6 induced by the decomposition of the surfactants and traces of THF at the same temperature. Nevertheless, the majority of Na3AlH6 decomposed into NaH and Al between 376 and 396 °C (Figures S20 and S30) whereas the bulk material released hydrogen at 372 °C. The increase in dehydrogenation temperature at this stage may be due to the impurities from thermal decomposition of the surfactants which may further stabilized Na3AlH6.
In contrast to nanosized LiAlH4, no clear trend was obtained when comparing the dehydrogenation temperature of NaAlH4 as a function of the particle size within the current experimental work. This is possibly due to the fact that the surfactants decompose prematurely and/or do not effectively stabilize the NaAlH4 nanostructures upon heating. NaAlH4 infiltrated within nanoporous carbon with a pore size of 2–3 nm decomposes at ~185 °C [16]. At this temperature, the surfactant stabilized NaAlH4 are already in a molten state (Figure 9b).
In this work, it can be seen that the use of surfactants enables an effective stabilization of alanate nanoparticles as well as triggers a low temperature hydrogen release-although limited because of the morphology degradation during hydrogen release. The low dehydrogenation temperature observed in this study is believed to be the combination result of nanosizing and the destabilization through the alanate-surfactant interaction [40,41]. It is also worth mentioning that the effect of nanosizing alone is not enough to enable rapid hydrogen release at near room temperature for both alanates. As evidenced in a well-nanoconfined LiAlH4 and NaAlH4, although the materials were encapsulated within a 2–5 nm porosity, the dehydrogenation event still occurs at above 100 °C [16,29,30,31,32]. However, we believe that an effective control of the particle size provides ground for the controlled synthesis of core-shell based nanostructures where the nanostructures can be further tuned and stabilized within a metallic shell to improve their hydrogen desorption properties along their hydrogen reversibility.

4. Conclusions

LiAlH4 and NaAlH4 nanoparticles were successfully synthesized via a facile solvent evaporation method using various types of surfactants as a stabilizer. Increasing the surfactant concentration to 10 mM resulted in the smallest average particle size between 10 to 200 nm. Following the HSAB principle, it was found that surfactants with matching hard character exhibited the best stabilizing effect and displayed a stronger binding to the particle surface which eventually leads to the formation of smaller particles. This study also revealed that the size of alanates nanoparticles can be controlled through an appropriate choice of surfactants. Surfactants with longer linear carbon chains would favour the growth of smaller nanoparticles due to the strong steric hindrance, whereas surfactants with higher intrinsic steric hindrance would result in larger nanoparticles. Also, surfactants with thiol and amine head groups enable the release of hydrogen at lower temperature due to their thermal reactivity with LiAlH4. In addition, the reduction in particle size and the interaction with surfactant were found to enhance the hydrogen desorption properties of LiAlH4. In contrast, no clear trend was obtained when comparing the dehydrogenation temperature of NaAlH4 as a function of particle size. As such, it can be concluded that the successful stabilization of alanate nanoparticles is not sufficient to enable adequate hydrogen storage properties. Agglomeration of the alanate nanoparticles, likely to occur upon decomposition or melting of surfactants during heating for hydrogen release, should be restrained. In this respect, future work on the nanostructuring of core-shell alanates is still required to further stabilize and restrict the alanate particles to withstand the high temperature during hydrogen cycling. Nevertheless, it is necessary to understand the effect of surfactants on the surface state of the alanate nanoparticles, i.e., binding strength and redox potential, in order to enable a homogenous shell growth. To this aim, the material which can reduce the reduction kinetics of alanates and has an average size of 20–30 nm will be considered for the core-shell synthesis.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app12094742/s1. Figure S1: Typical TEM images of pristine LiAlH4 and NaAlH4, Figures S2–S16: Typical TEM images of surfactant stabilized LiAlH4 and NaAlH4, Figure S17: Elemental mapping of stabilized LiAlH4 and NaAlH4, Figures S18–S20: XRD of LiAlH4 and NaAlH4 nanostructures, Figures S21–S28: TGA/DSC-MS of as-synthesized LiAlH4 and NaAlH4 nanoparticles, Figure S29: Decomposition temperature of LiAlH4 particles, Figure S30: Decomposition temperature of NaAlH4 nanoparticles, Table S1: Particle size of all synthesized materials, Table S2: Crystallite size of all synthesized material, Table S3: Decomposition temperature of the investigated surfactants, Table S4: FTIR vibration of the investigated surfactants and Table S5: Gravimetric hydrogen capacities of all synthesized materials.

Author Contributions

C.P. carried out all of the experimental work which was conceived and designed with K.-F.A.-Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article or supplementary material.

Acknowledgments

Financial supported by UNSW Internal Research Grant program is gratefully acknowledged. We appreciate the use of instruments in the Mark Wainwright Analytical Centre at UNSW.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 04742 g001 550
Figure 1. Typical TEM images of (a–c) LiAlH4 and (d–f) NaAlH4 stabilised by (a,d) HXA, (b,e) DDA, and (c,f) ODA at 1 mM.
Figure 1. Typical TEM images of (a–c) LiAlH4 and (d–f) NaAlH4 stabilised by (a,d) HXA, (b,e) DDA, and (c,f) ODA at 1 mM.
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Applsci 12 04742 g002 550
Figure 2. Change in average particle sizes of (a) LiAlH4 stabilized by RNH2 and RSH and (b) NaAlH4 stabilized by RNH2, RSH, and R4NBr with an increase in carbon chain length (surfactant concentration = 1 mM). Error bars represent standard deviation of the mean particle size. Typical corresponding TEM images are summarized on Figures S3–S16.
Figure 2. Change in average particle sizes of (a) LiAlH4 stabilized by RNH2 and RSH and (b) NaAlH4 stabilized by RNH2, RSH, and R4NBr with an increase in carbon chain length (surfactant concentration = 1 mM). Error bars represent standard deviation of the mean particle size. Typical corresponding TEM images are summarized on Figures S3–S16.
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Figure 3. The average particle size of LiAlH4 and NaAlH4 stabilized by DDA (RNH2), TDA (RCOOH), and DDT (RSH) at 1 mM. Error bars represent standard deviation of the mean particle size (Figure S17).
Figure 3. The average particle size of LiAlH4 and NaAlH4 stabilized by DDA (RNH2), TDA (RCOOH), and DDT (RSH) at 1 mM. Error bars represent standard deviation of the mean particle size (Figure S17).
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Figure 4. FTIR spectra of (a) LiAlH4-TDA-10 in black and (b) NaAlH4-TDA-1 in red.
Figure 4. FTIR spectra of (a) LiAlH4-TDA-10 in black and (b) NaAlH4-TDA-1 in red.
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Figure 5. The change in the average particle size of (a) LiAlH4 stabilized with –RNH2 (HXA, DDA, and ODA), (b) LiAlH4 stabilized with –RSH (HTT, DDT, and ODT), (c) NaAlH4 stabilized with –RNH2 (HXA, DDA, and ODA), and (d) NaAlH4 stabilized with –RSH (DDT and ODT) with an increase in surfactant concentration. Error bars represent standard deviation of the mean particle size.
Figure 5. The change in the average particle size of (a) LiAlH4 stabilized with –RNH2 (HXA, DDA, and ODA), (b) LiAlH4 stabilized with –RSH (HTT, DDT, and ODT), (c) NaAlH4 stabilized with –RNH2 (HXA, DDA, and ODA), and (d) NaAlH4 stabilized with –RSH (DDT and ODT) with an increase in surfactant concentration. Error bars represent standard deviation of the mean particle size.
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Figure 6. Morphological evolution of TDA-directed formation of (a–d) LiAlH4 nanostructures and (e–h) NaAlH4 with an increase in concentration.
Figure 6. Morphological evolution of TDA-directed formation of (a–d) LiAlH4 nanostructures and (e–h) NaAlH4 with an increase in concentration.
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Figure 7. FTIR spectra of LiAlH4 with various surfactants investigated at 10 mM.
Figure 7. FTIR spectra of LiAlH4 with various surfactants investigated at 10 mM.
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Figure 8. FTIR spectra of NaAlH4 with various surfactants investigated at 1 mM.
Figure 8. FTIR spectra of NaAlH4 with various surfactants investigated at 1 mM.
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Figure 9. Evolution of the melting point as function of the particle size for: (a) LiAlH4 and (b) NaAlH4 stabilized with the investigated surfactants (the red dotted lines are a guide to the eye).
Figure 9. Evolution of the melting point as function of the particle size for: (a) LiAlH4 and (b) NaAlH4 stabilized with the investigated surfactants (the red dotted lines are a guide to the eye).
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Figure 10. Relation between the first decomposition stage of LiAlH4 and particle size of LiAlH4 (the red dotted lines are a guide to the eye).
Figure 10. Relation between the first decomposition stage of LiAlH4 and particle size of LiAlH4 (the red dotted lines are a guide to the eye).
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Figure 11. (a) TGA/DSC and (b) MS of LiAlH4-HTT. The number in the box corresponds to the decomposition of: 1 LiAlH4 to Li3AlH6, 2 Li3AlH6 to LiH, and 3 LiH to metallic Li.
Figure 11. (a) TGA/DSC and (b) MS of LiAlH4-HTT. The number in the box corresponds to the decomposition of: 1 LiAlH4 to Li3AlH6, 2 Li3AlH6 to LiH, and 3 LiH to metallic Li.
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Figure 12. Relation between the first decomposition stage of NaAlH4 and particle size of NaAlH4 (the red dotted lines are a guide to the eye).
Figure 12. Relation between the first decomposition stage of NaAlH4 and particle size of NaAlH4 (the red dotted lines are a guide to the eye).
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Pratthana, C.; Aguey-Zinsou, K.-F. Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage. Appl. Sci. 2022, 12, 4742. https://doi.org/10.3390/app12094742

AMA Style

Pratthana C, Aguey-Zinsou K-F. Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage. Applied Sciences. 2022; 12(9):4742. https://doi.org/10.3390/app12094742

Chicago/Turabian Style

Pratthana, Chulaluck, and Kondo-Francois Aguey-Zinsou. 2022. "Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage" Applied Sciences 12, no. 9: 4742. https://doi.org/10.3390/app12094742

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