Surfactant Induced Synthesis of LiAlH₄ and NaAlH₄ Nanoparticles for Hydrogen Storage

Round 1

Reviewer 1 Report

Figure 1 shows the 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. How did the author prove the particles are LiAlH4 or NaAlH4? How about the stability of LiAlH4 or NaAlH4 under the E-beam in the TEM?

DSC measurements for decomposition of LiAlH4 and NaAlH4 should be added.

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. an one-step approach towards hydrogen production, storage and transportations as the new method via Li (Na) BH4 regeneration via using Mg based alloys as the new topics for hydrogen-energy process chain and hydrogen economics published in ‘Adv. Energy Mater., 2017, 1700299’, ‘Angew. Chem. Int. Ed., 59(2020)8623–8629’.

Using hydrogen energy is the most effective strategy to deal with the problems of energy exhaustion and the environmental pollution. More references published in 2021and 2022 should be summarized for the development of hydrogen energy.

Author Response

We thank the Reviewer for the constructive comments.

1) Figure 1 shows the 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. How did the author prove the particles are LiAlH4 or NaAlH4? How about the stability of LiAlH4 or NaAlH4 under the E-beam in the TEM?

Authors’ reply: We thank the reviewer for this comment. We have done the elemental mapping on the representative LiAlH₄-DDT-1 and NaAlH₄-DDT-1 nanoparticles to confirm the existence of both alanates and the stabilising surfactant (Figure S1). Due to the instrument limitations, Li could not be detected. However, the overlapping of Al and S signals indicates that the nanoparticles consist of both LiAlH₄ and dodecanethiol. For NaAlH₄-DDT-1, elemental mapping revealed a close vicinity of Na and Al and the homogenous distribution of the additional S at the particles. This implies that NaAlH₄ particles were indeed stabilised by dodecanethiol.  We have added this figure in the supplementary section and updated in the manuscript on page 3.

Both LiAlH₄ and NaAlH₄ are relatively unstable under the electron beam, e.g. Lei et al. Int. J. Hydrog. Energy, 42.20 (2017) 14144-14153 and Beattie et al. Int. J. Hydrog. Energy, 34.22 (2009) 9151-5156.  During TEM imaging, we used a low magnification and reduced the beam intensity to minimise the decomposition of the alanate nanoparticles. Moreover, all the images were acquired with short exposure of the beam.  

2) DSC measurements for decomposition of LiAlH4 and NaAlH4 should be added.

Authors’ reply: DSC measurements for all materials have been provided in the supplementary section (Figure S21-S28) together with the TGA plot. The DSC plots are marked in blue.

3) 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. An one-step approach towards hydrogen production, storage and transportations as the new method via Li (Na) BH4 regeneration via using Mg based alloys as the new topics for hydrogen-energy process chain and hydrogen economics published in ‘Adv. Energy Mater., 2017, 170212’, ‘Angew. Chem. Int. Ed., 59(2020)8623–8629’.

Authors’ reply: Thanks for the reviewer’s comment, but we do not understand this comment. We have read the suggested papers, however we found that these two papers are focusing on the hydrolysis and regeneration of NaBH₄. We have mentioned different promising hydrogen storage materials and cited the suggested paper in the main text as reference 6 and 9.

4) Using hydrogen energy is the most effective strategy to deal with the problems of energy exhaustion and the environmental pollution. More references published in 2021 and 2022 should be summarized for the development of hydrogen storage material.

Authors’ reply: We thank the reviewer for this suggestion. More references from 2020-2022 have been added as listed below.

1 Zhu, Y.; Ouyang, L.; Zhong, H.; Liu, J.; Wang, H.; Shao, H.; Huang, Z.; Zhu, M. Closing the loop for hydroge storage: facile regeneration of NaBH₄ from its hydrolytic product. Angewandte Chemie 2020, 132, 8701-8707.

2 Sazelee, N.A.; Ismail, M. Recent advances in catalyst-enhanced LiAlH₄ for solid-state hydrogen storage: A review. International Journal of Hydrogen Energy 2021, 46, 9123-9141, doi:https://doi.org/10.1016/j.ijhydene.2020.12.208.

3 Yuan, Z.; Zhang, D.; Fan, G.; Chen, Y.; Fan, Y.; Liu, B. N-doped carbon coated Ti₃C₂ MXene as a high-efficiency catalyst for improving hydrogen storage kinetics and stability of NaAlH₄. Renewable Energy 2022, 188, 778-787, doi:https://doi.org/10.1016/j.renene.2022.02.068.

4 Ren, Z.; Zhang, X.; Huang, Z.; Hu, J.; Li, Y.; Zheng, S.; Gao, M.; Pan, H.; Liu, Y. Controllable synthesis of 2D TiH₂ nanoflakes with superior catalytic activity for low-temperature hydrogen cycling of NaAlH₄. Chemical Engineering Journal 2022, 427, 131546, doi:https://doi.org/10.1016/j.cej.2021.131546.

5 Chowarot, C.; Siriwongrungsona, V.; Hongrapipat, J.; Pang, S.; Messner, M. Characterization of deposited Ti-doped lithium aluminium hydride thin film using dip coating method. Journal of Physics: Conference Series 2022, 2175, 012015, doi:10.1088/1742-6596/2175/1/012015.

6 Tena-García, J.R.; Casillas-Ramírez, A.; Guerrero-Ortiz, R.; Poiré de la Cruz, D.R.; Suarez-Alcantara, K. LiAlH₄–ZrCl₄ mixtures for hydrogen release at near room temperature. International Journal of Hydrogen Energy 2022, doi:https://doi.org/10.1016/j.ijhydene.2022.02.028.

7 Jeong, U.; Kim, H.; Ramesh, S.; Dogan, N.A.; Wongwilawan, S.; Kang, S.; Park, J.; Cho, E.S.; Yavuz, C.T. Rapid access to ordered mesoporous carbons for chemical hydrogen storage. Angewandte Chemie 2021, 133, 22652-22660.

Reviewer 2 Report

The manuscript is a compilation of huge work on the synthesis and characterization of LiAlH₄ and NaAlH₄ nanoparticles. The overall merit is very high and must be published after minor changes, listed below in order of importance:

1. Authors achieved really small particle sizes that reduce the hydrogen release temperature, (mostly for LiAlH₄). However, hydrogen release temperature is still “high” for many accepted applications. Authors can comment on what would be the “ideal” particle size to match the hydrogen release at near room temperature, let’s say 25-75°C?
2. DSC/TGA plots are very interesting and at least some of them should be presented in the manuscript, for example, the LiAlH₄-HTT material.
3. Which is the theoretical hydrogen content of the materials, considering the complex hydride mass plus the surfactant mass (after dried)?
4. Authors can comment on the solid dehydrogenation products, are they the classical LiH/Al and NaH/Al or they were influenced by the presence of surfactants?
5. For the conclusion section, in the opinion of the authors, among the tested surfactants, which one would you recommend for further work? at which concentration? which other experimental conditions are important to consider?

Author Response

We thank the Reviewer for the constructive comments

1) Authors achieved really small particle sizes that reduce the hydrogen release temperature, (mostly for LiAlH₄). However, hydrogen release temperature is still “high” for many accepted applications. Authors can comment on what would be the “ideal” particle size to match the hydrogen release at near room temperature, let’s say 25-75°C?

Authors’ reply: We thank the review for the suggestion. The effect of nanosizing alone cannot result in a rapid decomposition of both LiAlH₄ and NaAlH₄ at 25-75 °C. As evidenced in the nanoconfined NaAlH₄ and LiAlH₄, although the materials were confined within the porosity of less than 3 - 5 nm, the major decomposition still occurs at relatively high temperature (126 – 200 °C), e.g. Gao et al. J. Phys. Chem. C 114.10 (2010), 4675-4682 and Cho et al. ACS nano 15.6 (2021) 10163-10174. This has been added to the main text page 15-16 (Line 457-461). Moreover, the low temperature hydrogen release is believed to be the result of a combination of small particles and destabilisation through the stabilising effects associated with the particles (Line 455-457).

2) DSC/TGA plots are very interesting and at least some of them should be presented in the manuscript, for example, the LiAlH₄-HTT material.

Authors’ reply: We thank reviewer for the suggestion. The TGA/DSC and MS of LiAlH₄-HTT have been added in the main text (Figure 11).  

3) Which is the theoretical hydrogen content of the materials, considering the complex hydride mass plus the surfactant mass (after dried)?

Authors’ reply: We have already included the calculated theoretical hydrogen content, taken into the consideration of the surfactant mass in the supplementary section (Table S5). From the table above, it can be seen that the actual mass loss observed by the TGA is higher than the theoretical H₂ content. The additional mass loss is believed to be due to the decomposition of the surfactants and remaining solvent traces as evidenced by the MS (Figure S22-S28).

4) Authors can comment on the solid dehydrogenation products, are they the classical LiH/Al and NaH/Al or they were influenced by the presence of surfactants?

Authors’ reply: We thank the reviewer for the suggestion. To verify the decomposition products of surfactant-stabilised LiAlH₄ and NaAlH₄, the representative LiAlH₄-DDT-1 and NaAlH₄-DDT-1 were heated to 450 °C and analysed with XRD. As shown in Figure S20, the only decomposition products of NaAlH₄-DDT-1 is NaH and Al. Therefore, we can assume that the decomposition path of the surfactant-stabilised NaAlH₄ still follows the conventional route. On the other hand, diffraction peaks corresponding to Al were detected upon heating of LiAlH₄-DTT-1 to 450 °C. The existence of LiH in the decomposition is not confirmed as the diffraction peaks of LiH and Al generally overlap. In bulk LiAlH₄, LiH starts to decompose at ~435 °C, therefore we believe that LiH in the surfactant-stabilised LiAlH₄ will also decompose in 400-450 °C ranges. This has been added to the manuscript (Line 407-410), with additional details in the supplementary section (Figure S20).

5) For the conclusion section, in the opinion of the authors, among the tested surfactants, which one would you recommend for further work? at which concentration? which other experimental conditions are important to consider?

Authors’ reply: Thank the reviewer for the comment. Within the scope of this current study, we cannot yet determine which material is most suitable for the synthesis of core-shell structure. In the constructing of core-shell structure, alanate nanoparticles will be encapsulated within the metallic shell (e.g. Ti) through the reduction reaction. In the coating process, it is critical to control the reduction rate of the Ti precursor in order to facilitate the nucleation/growth and deposition of metallic shell on the nanoparticle surface. As both alanates are known for their strong reducing ability, the reduction rate may be too fast and thus result in homogeneous nucleation of Ti. To take this into account, it is necessary to determine the reduction rate of Ti by each surfactant-stabilised alanate. In this context, the material which has the slowest reduction kinetics will be considered, and this will be the scope of future work. Moreover, alanate nanoparticles with the size of ~20-30 nm is preferred as it may be too difficult to deposit a thin layer of Ti on an extremely small particles. This has been discussed in the manuscript on page 16 (Line 487-491).

Reviewer 3 Report

Chulaluck et al demonstrated the synthesis of LiAlH4 and NaAlH4 nanoparticles with the use of surfactants as a stabiliser. They discussed the role the roles of the surfactants in enabling control over particle size and morphology. The observed the lowered dehydrogenation temperature of LiAlH4 and NaAlH4 compared to the bulk samples. The results could be interesting to the hydrogen storage community. However, there are some discrapencies which need to be claried.

 

1. Thoroughout the manuscript, the measured particle sizes do not refer to the LiAlH4 and NaAlH4 adducts that contain surfactants as a stabiliser, expect bulk LiAlH4 and NaAlH4.

 

2. The dehydrogenation of LiAlH4 and NaAlH4 adducts may be determined by both size the interaction with surfactants. Therefore, the results in Figures 11 and 12 could not tell the relation between particle sizes and the dehydrogenation temperature.

 

3. It is confusing that the crystal structures of LiAlH4 and NaAlH4 do not change when containing certain amount of surfacants. What is the interaction between LiAlH4 and NaAlH4, and surfacants?

Author Response

We thank the Reviewer for the constructive comments

1) Throughout the manuscript, the measured particle sizes do not refer to the LiAlH4 and NaAlH4 adducts that contain surfactants as a stabiliser, expect bulk LiAlH4 and NaAlH4.

Authors’ reply: We thank the reviewer for the comment. In this work, we use TEM analysis to visualise the size and morphology of the as-synthesised materials. However, the surfactants generally cannot be observed by the traditional TEM due to the poor scattering contrast and complex staining process, i.e. Zhang et al. Front. Chem. 7(2019) 242. To obtain the “true” size of the surfactant-stabilised nanoparticles, atomic force microscope (AFM) can be used. Generally, the particle size obtained by AFM would be considerably larger than that of TEM due to convolution induced by tip shape. Nevertheless, the tip geometries can be extracted from the nanoparticle widths by the use of appropriate geometrical models, i.e. Warr, G. Curr Opin Colloid Interface Sci. 5.1 (2000)88-94 and Van Cleef et al. J. Microsc. 181.1(1996)2-9. However, the determination of surfactant layer is not within the scope of this study. This work serves as a prerequisite for the nanoengineering of the core-shell structure. Indeed, the surfactant layer will later be replaced by the metallic coating, therefore the size of only alanate nanoparticles is concerned. As such, we think that TEM analysis is sufficient for this work.   

2) The dehydrogenation of LiAlH₄ and NaAlH₄ adducts may be determined by both size the interaction with surfactants. Therefore, the results in Figures 11 and 12 could not tell the relation between particle sizes and the dehydrogenation temperature.

 Authors’ reply: We thank the reviewer for the comment. The surfactant has indeed affected the dehydrogenation temperature due to the reactions discussed in the main text. Nevertheless, the nanosizing effect also cannot be overlooked. We have modified the main text accordingly to clarify this point (Line 413-416).

3) It is confusing that the crystal structures of LiAlH4 and NaAlH4 do not change when containing certain amount of surfactants. What is the interaction between LiAlH4 and NaAlH4, and surfactants?

Authors’ reply: For complex hydrides, it is commonly observed that the crystal structure of the surfactant-stabilised material would remain the same (see Reference 25, 26, 28, 31, and 32 in the manuscript). Rather than intercalate into the layered crystal structure and modify the lattice structure, it is likely that the surfactants only bind at the surface of alanate. In fact, the current crystal structure of LiAlH₄ (space group P2₁/C) is relatively stable and requires relatively high pressure (> 3 GPa) to induce phase transformation.  The DFT results also show that the P2₁/C crystal structure possesses the lowest energy, therefore LiAlH₄ would naturally occur as crystalline solid with the P2₁/C space group, i.e. Keyan et al. AIP Adv. 10.2(2020) 025030. Similarly, NaAlH₄ would undergo phase transformation at 6.43 GPa. At ambient conditions, NaAlH₄ generally crystallizes in the tetragonal structure (space group (I4₁/a), i.e. Vajeeston et al. Appl. Phys. Lett. 82.14 (2003)2257-2259. Again, this is the thermodynamically favourable phase for NaAlH₄ at ambient conditions.

Round 2

Reviewer 1 Report

It is true that all the images were acquired with short exposure of the beam and  low magnification and reducing the beam intensity to minimise the decomposition of the alanate nanoparticles are necessary.  During TEM imaging, it is still to keep it stable.