Enhancing Hydrogen Storage Kinetics and Cycling Properties of NaMgH₃ by 2D Transition Metal Carbide MXene Ti₃C₂

Round 1

Reviewer 1 Report

The authors report in this manuscript the results of some experiments on the generation of H2 from solid NaMgH3 compound, using the 2D Ti3C2 compound as a kind of catalyst. Although the subject is of current interest for hydrogen storage, there are a few issues with this manuscript:

i) I find the term 'MXenes' not well defined and rather obscur. It represents a kind of degradation of scientific terminology. In the orginal paper quoted, the authors did not propose such a term. The title needs to be modified.

ii) Authors tends to claim their compounds being 'very promising' for this and for that... For NaMgH3, I do not see it as promising for H2 storage, because it gives only <5 wt% of H2..., even with the presence of Ti3C2. The authors need to make a brief survey of the literature on NaMgH3 to justify their claim.

ii) At the time being, if I undertsand correctly, the wt% is >20% for other compounds. So why should you consider on a compound having only a wt% H2 <5%.?

iii) The role of Ti3C2 in the reactions is not clear: the author use the term 'doping'... How can you dope it into NaMgH3? Does it play the role of a catalyst? If it is catalyst, how about its catalytic process in both directions?

iv) an important issue in H2 storage is the regeneration of the starting materials. Such a process for NaMgH3 is not well demonstrated in this manuscript.

 

Author Response

Dear Editor and Reviewers:

We are very glad to hear from you about our manuscript (1339375). We thank you very much for the valuable review comments. In the revised manuscript, we have made corrections and modifications and highlighted them. We have carefully checked and improved the English writing in the revised manuscript. The response to each individual comment has been described in detail below:

 

Responding to the review comments:

 

Reviewer #1: The authors report in this manuscript the results of some experiments on the generation of H₂ from solid NaMgH₃ compound, using the 2D Ti₃C₂ compound as a kind of catalyst. Although the subject is of current interest for hydrogen storage, there are a few issues with this manuscript:

1. i) I find the term 'MXenes' not well defined and rather obscur. It represents a kind of degradation of scientific terminology. In the orginal paper quoted, the authors did not propose such a term. The title needs to be modified.

 

Response:The general formula of MXenes is M_n+1X_nT_x (n =1–3), where M represents a transition metal, X is carbon and/or nitrogen and T_x stands for the surface terminations[1]. Meanwhile, the title of this manuscript has been modified as: Enhancing hydrogen storage kinetics and cycling properties of NaMgH₃ by lamellar-structure 2D transition metal cardide Mxene Ti₃C₂.

 

2. ii) Authors tends to claim their compounds being 'very promising' for this and for that... For NaMgH₃, I do not see it as promising for H₂ storage, because it gives only <5 wt% of H₂..., even with the presence of Ti₃C₂. The authors need to make a brief survey of the literature on NaMgH₃ to justify their claim.

 

Response: As the GdFeO₃-type perovskike (space group P_nma) hydride, NaMgH₃ has received considerable attention for its high gravimetric and volumetric hydrogen densities (6 wt.% and 88 kg/m³) and possesses a considerable theoretical thermal storage gravimetric density (2881 kJ/kg) with reversible two-step de/re-hydrogenation process[2-8]:

NaMgH₃ → NaH + Mg + H₂                                  (1)

NaH + Mg → Na + Mg + 1/2 H₂                               (2)

Especially, NaMgH₃ is considered as a potential thermal energy medium for Concentration Solar Power (CSP) because of the high thermodynamic stability[9]. In particular, as a suitable candidate for Thermal Energy Storage (TES), NaMgH₃ has several obvious advantages of the high hydrogen storage capacity, low and flat hydrogen de/absorption pressure and plateau, negligible hysteresis, high thermal storage density and low cost of raw materials[10]. In addition, de/re-hydrogenation kinetics performances and cycling properties are also important parameters to determine whether it is available to TES. However, the further applications to thermal energy storage are limited by sluggish de/re-hydrogenation kinetic performances and dramatical degradation of cycling properties of NaMgH₃.

 

iii)  At the time being, if I undertsand correctly, the wt% is >20% for other compounds. So why should you consider on a compound having only a wt% H₂ <5%.?

 

Response: Thanks for the reviewer`s comment. There are indeed many other higher hydrogen storage capacity compounds, such as LiBH₄ with 18.5 wt.%, MgH₂ with 7.6 wt.% and NaAlH₄ with 5.6 wt.%[11, 12]. Although the reversible hydrogen storage capacity of NaMgH₃ is about 6 wt.%, it exhibits other engineering feasibility in heat storage, accompanying with the reversible hydrogen storage process[13]. From pressure–composition isotherm (PCI) measurements, the enthalpy, ΔH(des), and entropy, ΔS(des), of desorption for Eq. (1) were determined as 86.6 kJ·mol⁻¹ H₂ and 132.2 J·mol⁻¹·H₂·K⁻¹. The enthalpy and entropy of desorption for Eq. (2) correspond to those for pure NaH(ΔH(des) =116 kJ·mol⁻¹·H₂and ΔS(des)=168.2 J·mol⁻¹·H₂·K⁻¹). The theoretical gravimetric heat storage capacity of Eqs. (1) and (2) is 2881 kJ·kg⁻¹ (Eq. 1 = 1721 kJ·kg⁻¹, Eq. 2 = 1160 kJ·kg⁻¹).

NaMgH₃ → NaH + Mg + H₂                                       (1)

NaH + Mg → Na + Mg + 1/2 H₂                                    (2)

Therefore, NaMgH₃ would be an ideal thermal storage medium during the hydrogen storage process. However, the further applications to thermal energy storage are limited by sluggish de/re-hydrogenation kinetic performances and dramatical degradation of cycling properties of NaMgH₃. In this work, the 2D MXene Ti₃C₂ with a unique lamellar-structure was introduced into NaMgH₃ for enhancing its de/re-hydrogenation kinetics and cycling behaviors for the first time. It is confirmed that the de/re-hydrogenation kinetics of NaMgH₃ can be improved by doping with Ti₃C₂, and the Ti₃C₂ could disperse the agglomerated NaMgH₃ particles homogeneously, decrease the activation energy of NaMgH₃ and prevent Na from separating Mg during the cycles. Moreover, the thermal storage density for NaMgH₃-7 wt.% Ti₃C₂ is evaluated to be 2562 kJ/kg.

 

1. iv) The role of Ti₃C₂ in the reactions is not clear: the author use the term 'doping'... How can you dope it into NaMgH₃? Does it play the role of a catalyst? If it is catalyst, how about its catalytic process in both directions?

 

Response: Thanks for the reviewer`s comment. First, the Ti₃C₂ was doped into NaMgH3 by milling with stainless steel balls of which the weight ratio to composites was 40:1 under 10 bar H₂ for 12 h at 400 rpm using a planetary ball mill, according to the composition proportion of NaMgH₃-x wt.% Ti₃C₂ (x= 0, 3, 5, 7 and 9). Second, it can be confirmed by SEM and EDS results that the lamellar-structure Ti₃C₂ is present in the NaMgH₃-x wt.% Ti₃C₂ composites in Fig. 5. Based on the fact that the significantly reducing of operating temperature and superior enhancement of dehydrogenation kinetics properties in the Ti₃C₂ doped sample, it can be proposed that the Ti₃C₂ plays the role of catalyst to improve the dehydrogenation kinetics properties of NaMgH₃ hydride, which is in agreement with our previous papers in Ti₃C₂-doped Mg(BH₄)₂[14]and sodium alanates systems[15]. Finally, the main contribution of this study is proving that the 2D MXene Ti₃C₂ with a unique lamellar-structure can improve the dehydrogenation kinetics properties of NaMgH₃ with a relatively high level of reversibility, and confirming the thermal storage density of up to 2562 kJ/kg for Ti₃C₂ doped NaMgH₃ composite. The detailed microstructure information of Ti₃C₂ can not be easily revealed after re/dehydrogenation process due to the small amounts of Ti₃C₂ additive, the catalytic mechanism is undiscovered at present. We hope the catalytic mechanism of NaMgH₃-x wt.% Ti₃C₂ composite can be revealed by X-ray absorption near-edge structure (XANES) spectrum in the future.

 

1. iv) an important issue in H₂ storage is the regeneration of the starting materials. Such a process for NaMgH₃ is not well demonstrated in this manuscript.

 

Response:Thanks for the reviewer`s comment. The regeneration and reversibility of NaMgH₃ system are very important. According to the reverse reactions of Eq.(1) and Eq.(2) in the revised manuscript, the starting materials of Na and Mg can be easily rehydrogenated into NaH and NaMgH₃ in term of thermodynamics. However, the reversible hydrogen storage of NaMgH₃ is limited by sluggish de/re-hydrogenation kinetic performances. In this manuscript, it is demonstrated that the poor cycling property of NaMgH₃ is improved by Ti₃C₂, with 4.6 wt.% H₂ capacity remained after 5 cycles. Particularly, indicated by XRD, SEM and EDS characterizations, the lamellar-structure Ti₃C₂ can homogeneously separate aggregated NaMgH₃ particles and prevent Na from separating Mg.

 

 

References:

 

[1] Anasori, B.; Lukatskaya, M.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, 1-17.

[2] Reshak, A.H. NaMgH₃ a perovskite-type hydride as advanced hydrogen storage systems: Electronic structure features. Int. J. Hydrogen Energy 2015, 40, 16383-16390.

[3] Ikeda, K.; Kogure, Y.; Nakamori, Y.; Orimo, S. Reversible hydriding and dehydriding reactions of perovskite-type hydride NaMgH₃. Scripta Mater. 2005, 53, 319-322.

[4] Bouamrane, A.; Laval, J.P.; Soulie, J.P.; Bastide, J.P. Structural characterization of NaMgH₂F and NaMgH₃. Mater. Res. Bull. 2000, 35, 545-549.

[5] Pottmaier, D.; Pinatel, E.R.; Vitillo, J.G.; Garroni, S.; Orlova, M.; Baró, M.D.; Vaughan, G.B.M.; Fichtner, M.; Lohstroh, W.; Baricco, M. Structure and Thermodynamic Properties of the NaMgH₃ Perovskite: A Comprehensive Study. Chem. Mater. 2011, 23, 2317-2326.

[6] Bouhadda, Y.; Fenineche, N.; Boudouma, Y. Hydrogen storage: Lattice dynamics of orthorhombic NaMgH₃. Phys. B: Condens. Matter. 2011, 406, 1000-1003.

[7] Wang, Z.; Tao, S.; Li, J.-J.; Deng, J.-Q.; Zhou, H.; Yao, Q. The Improvement of Dehydriding the Kinetics of NaMgH₃ Hydride via Doping with Carbon Nanomaterials. Metals 2017, 7, 9.

[8] Wu, H.; Zhou, W.; Udovic, T.; Rush, J.; Yildirim, T. Crystal Chemistry of Perovskite-Type Hydride NaMgH₃ : Implications for Hydrogen Storage. Chem. Mater. 2008, 20, 2335-2342.

[9] Sheppard, D.A.; Paskevicius, M.; Buckley, C.E.J.C. ChemInform Abstract: Thermodynamics of Hydrogen Desorption from NaMgH₃ and Its Application as a Solar Heat Storage Medium. Chem. Mater. 2011, 23, 4298-4300.

[10] Poupin, L.; Humphries, T.D.; Paskevicius, M.; Buckley, C.E. A thermal energy storage prototype using sodium magnesium hydride. Sustain. Energy Fuels 2019, 3, 985-995.

[11] Yartys, V.A.; Lototskyy, M.V.; Akiba, E.; Albert, R.; Antonov, V.E.; Ares, J.R.; Baricco, M.; Bourgeois, N.; Buckley, C.E.; Bellosta von Colbe, J.M., et al. Magnesium based materials for hydrogen based energy storage: Past, present and future. Int. J. Hydrogen Energy 2019, 44, 7809-7859.

[12] He, T.; Pachfule, P.; Wu, H.; Xu, Q.; Chen, P. Hydrogen carriers. Nat. Rev. Mater. 2016, 1, 1-17.

[13] Sheppard, D.A.; Humphries, T.D.; Buckley, C.E. Sodium-based hydrides for thermal energy applications. Appl. Phys. A 2016, 122, 406.

[14] Zheng, J.; Cheng, H.; Xiao, X.; Chen, M.; Chen, L. Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH₄)₂ composited with 2D MXene. Int. J. Hydrogen Energy 2019, 44, 24292-24300.

[15] Jiang, R.; Xiao, X.; Zheng, J.; Chen, M.; Chen, L. Remarkable hydrogen absorption/desorption behaviors and mechanism of sodium alanates in-situ doped with Ti-based 2D MXene. Mater. Chem. Phys. 2020, 242, 122529.

Author Response File: Author Response.docx

Reviewer 2 Report

In this work, the hydrogen storage kinetics and cycling properties of NaMgH3 doped with Ti3C2 lamellar-structure 2D MXene were well studied. It was found that the unique lamellar-structure of Ti3C2 can separate the agglomerated NaMgH3 particles homogeneously and decrease the energy barriers of two-step reaction of NaMgH3 (114.08 and 139.40 kJ/mol). The manuscript may be accepted after minor revision. The authors should address the following queries:
 1. The authors wrote “To some extent, the Ti3C2 can relieve the degradation of dehydrogenation capacity, because of the improved dehydrogenation kinetics.”, any explanation?

2. The authors stated that “Ti3C2 dramatically enhances the de/re-hydrogenation kinetics and cycling performances of NaMgH3, making it more suitable for the application of TES”. Please give the relationship between the de/re-hydrogenation kinetics and cycling performances of NaMgH3 and its application in TES.

3. In this manuscript, 3/5/7/9 wt.% Ti3C2 were doped into NaMgH3. Please give the reason for the selection of these doping ratios.

4. There are a few grammatical mistakes and typo errors, which should be modified.

5. Please double check references.

Author Response

Dear Editor and Reviewers:

We are very glad to hear from you about our manuscript (1339375). We thank you very much for the valuable review comments. In the revised manuscript, we have made corrections and modifications and highlighted them. We have carefully checked and improved the English writing in the revised manuscript. The response to each individual comment has been described in detail below:

 

Responding to the review comments:

 

Reviewer #2: In this work, the hydrogen storage kinetics and cycling properties of NaMgH₃ doped with Ti₃C₂ lamellar-structure 2D MXene were well studied. It was found that the unique lamellar-structure of Ti₃C₂ can separate the agglomerated NaMgH₃ particles homogeneously and decrease the energy barriers of two-step reaction of NaMgH₃ (114.08 and 139.40 kJ/mol). The manuscript may be accepted after minor revision. The authors should address the following queries:

 

1. The authors wrote “To some extent, the Ti₃C₂ can relieve the degradation of dehydrogenation capacity, because of the improved dehydrogenation kinetics.”, any explanation?

 

Response: Thanks for the reviewer`s comment. It is well known that the faster kinetics the material has, the more hydrogen it releases within a fixed time. Hence, the improved dehydrogenation kinetics would relieve the degradation of dehydrogenation capacity indicated by the content of hydrogen released in a certain period, e.g. 20min.

 

2. The authors stated that “Ti₃C₂ dramatically enhances the de/re-hydrogenation kinetics and cycling performances of NaMgH₃ making it more suitable for the application of TES”. Please give the relationship between the de/re-hydrogenation kinetics and cycling performances of NaMgH₃ and its application in TES.

 

Response: Thanks for the reviewer`s comment. It was reported that a system for TES application should have several advantages, such as faster kinetics, high energy storage density, and good cycling performance[1]. Therefore, it is reasonable to deduce that the enhancement in de/re-hydrogenation kinetics and cycling performance of NaMgH₃ contributes to its application in TES. Ref.[1] has been added in the revised manuscript.

 

3. In this manuscript, 3/5/7/9 wt.% Ti₃C₂ were doped into NaMgH₃. Please give the reason for the selection of these doping ratios.

 

Response: Thanks for the reviewer`s comment. Generally, the doping ratio of catalyst should be controlled within a certain proportion to play its role better. Here, doping less than 3 wt.% Ti₃C₂ into NaMgH₃ could not work effectively due to its excessive low amount. On the other hand, the doping ratio of over 9 wt.% would inhibit catalysis of Ti₃C₂, owing to reduction in its specific surface area caused by agglomerating. Correspondingly, in the manuscript, the best catalysis is obtained at the doping ratio of 7 wt.% instead of 9 wt.%. Thus, appropriate Ti₃C₂-doping ratios of 3/5/7/9 wt.% were adopted and studied in this work.

 

4. There are a few grammatical mistakes and typo errors, which should be modified.

 

Response: Thanks for the reviewer`s comment. The grammatical mistakes and typo errors have been modified in the revised manuscript.

 

5. Please double check references.

 

Response: Thanks for the reviewer`s comment. The references have been checked carefully.

 

 

References:

 

[1] Aydin, D.; Casey, S.P.; Riffat, S. The latest advancements on thermochemical heat storage systems. Renew. Sust. Energ. Rev. 2015, 41, 356-367.

 

Author Response File: Author Response.docx