Novel Functional Materials of Hydrogen Storage B₂₀N₂₄: A First-Principles Calculation

Round 1

Reviewer 1 Report

The MS titled «Novel Functional Materials of hydrogen storage B₂₀N₂₄: A First principles calculation» presents an interesting and promising discovery in the field of materials science. The authors propose a new N-rich BN which is found to be stable under ambient conditions and exhibits semiconducting properties with a low direct band gap. The authors back their findings with first-principles calculations, including phonon dispersion spectra and born stability criteria. Overall, this article is well-written and provides valuable insights into the potential applications of B₂₀N₂₄. The research presented has important implications for the development of new materials with advanced properties and should be of interest to a wide range of researchers in the field of materials science.

I have a number of suggestions that might improve the readability of the MS before publication.

Major points:

1. I would like to draw attention to one of the most interesting parts of the article, which is presented rather succinctly. Specifically, the authors describe the structure searching process that led to the discovery of the B₂₀N₂₄ polymorph (P. 2, see the last paragraph of the introduction), which involved conducting research on carbon structures and then using cage-like configurations with B and N atoms. This methodology provides important insights into the discovery process and illustrates the structural similarities between carbon and BN allotropes, which could have broader implications for materials science. Is this the authors’ method or has this method been demonstrated with another class of compounds? If so, please give the references.

2. P. 2, see the first paragraph of the Results and Discussion: While the selection of allotropes based on low energy is a common practice in materials science research, I would like to understand the criteria used to select only six out of thousands of novel carbon structures. The authors could provide more details on the selection process to give readers a better picture of how they narrowed down their candidates, potentially leading to more transparent and reproducible research in the field.

3. See the first paragraph on P.4: The authors used the Mehl method to calculate the elastic constants. However, no reference or explanation of this method was provided in the article. This could be problematic for readers who are not familiar with the Mehl method and may need to understand it in order to fully understand the research methodology and results. What strains were used to determine the elastic constants?

4. See the first paragraph on P.5: It is essential that almost all part of the text should be moved to the introduction.

5. The authors did not provide a clear explanation for the significant change in the energy of formation of the system when the number of hydrogen molecules changed from 10 to 11. This information is crucial for readers to fully understand the implications of the research.

6. Although the authors briefly mention some theoretical implications of their findings, I believe that a more in-depth analysis is necessary for a full understanding of the research. Further discussion of the results is essential to contextualise the significance and relevance of the B₂₀N₂₄ polymorph and to provide suggestions for future experiments and research. Therefore, I recommend that the authors revise Results and Discussion section to more fully unpack the implications of their results.

7. Some references could be added to introduction:

https://doi.org/10.1021/acs.jpcc.2c02749

https://doi.org/10.1016/j.matchemphys.2020.123245

My conclusion is that this MS represents a promising discovery, but it would benefit from a major revision. Several of the comments I have made need to be addressed, either by clarifying or correcting misunderstandings, or by making revisions where necessary. In addition, I believe that the paper would be improved by providing more detailed information, discussion and perspectives on the proposed B₂₀N₂₄ polymorph. Although the article is well written and easy to read, it lacks the depth necessary to fully understand the significance of the findings.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly, and detailed corrections are listed below point by point:

Question1: I would like to draw attention to one of the most interesting parts of the article, which is presented rather succinctly. Specifically, the authors describe the structure searching process that led to the discovery of the B20N24 polymorph (P. 2, see the last paragraph of the introduction), which involved conducting research on carbon structures and then using cage-like configurations with B and N atoms. This methodology provides important insights into the discovery process and illustrates the structural similarities between carbon and BN allotropes, which could have broader implications for materials science. Is this the authors’ method or has this method been demonstrated with another class of compounds? If so, please give the references.

Respond: Thank you for your advice.

As previously studies shown, structure prediction of one system can be executed via various methods, such as structural search program, simulated annealing10,11, basin hopping14,15, metadynamics16,17, evolutionary metadynamics18,19, and so on. Besides, based on the structural similarities among different systems, more and more novel structures are predicted by changing the atoms of revealed structure which are synthesized or proposed into some other system to study. Carbon and BN are isoelectronic structure, many phases of the two systems sharing the same configurations. For example, diamond and cubic BN, hexagonal diamond and wurtzite BN, carbon nanotube and BN nanotube, graphene and BN nanoribbons, and so on. Moreover, Cco-C8 structure of carbon allotrope are predicted as a candidate for cold-compressed graphite. Crystals sharing the same framework between different systems not only exist in carbon and BN compound, but also can be found in some other classes. Several crystal structures, like rocksalt-type BN are proposed as a high-pressure phase of BN compound, which sharing the framework of rocksalt crystal [31-32]. Thereafter, z-BN with the same framework with Cco-C8 is proposed as a novel phase for BN compound. Therefore, it is viable to change a carbon structure into B and N atoms alternatively during BN structure prediction process.

It is effective to change the atoms of a known structure with other elements to construct novel structures of this system, and this method have excuted in many classes, such as carbon, BN, Nacl, Si, B-C-N compounds and so on. See 4^th paragraph in P. 2 and Ref. 14 and 15.

Question 2: P. 2, see the first paragraph of the Results and Discussion: While the selection of allotropes based on low energy is a common practice in materials science research, I would like to understand the criteria used to select only six out of thousands of novel carbon structures. The authors could provide more details on the selection process to give readers a better picture of how they narrowed down their candidates, potentially leading to more transparent and reproducible research in the field.

Respond: Thank you for your advice. The more details are added in the revised manuscript.

Secondly, according to the geometries of carbon structures, we have selected some of the special configurations. During this process, these structures with odd rings are throw away to avoid forming high energy B-B/N-N bonds in the atoms changing process subsequently. Therefore, some cage-like carbon structure and composed of even-membered rings are retained. Finally, carbon atoms are replaced with B and N atoms alternatively to avoid B-B/N-N bonds to search for stable nonstoichiometric B-N polymorphs.

See 2nd paragraph in P. 3.

Question 3: See the first paragraph on P.4: The authors used the Mehl method to calculate the elastic constants. However, no reference or explanation of this method was provided in the article. This could be problematic for readers who are not familiar with the Mehl method and may need to understand it in order to fully understand the research methodology and results. What strains were used to determine the elastic constants?

Respond: Thank you. The details of the elastic constants calculations are listed in 1st paragraph in P. 3 in the revised manuscript.

The elastic properties are predicted with CASTEP code. In this process, a unit cell was adopted. The elastic constants Cij are derived based on the strain-stress relationship (Hooke’s law), which is within the range of elastic deformation, a finite strain is applied to the optimized structure, and the applied strain and the resulting stress are obtained. The maximum of applied strain amplitude of 0.3% and number of steps for each strain of 9 of this calculation. To verify the accuracy of our calculations, we calculated the lattice parameters and bond length diamond and cubic BN. The results show the lattice parameters of diamond and cubic BN is a= 3.528 Å and a= 3.580 Å, and the bond length of C-C and B-N bond is 1.528 Å and 1.550 Å, respectively, which are close to the results derived from experiments (3.567 Å and 1.545 Å for diamond, 3.615 Å and 1.565 Å for cubic BN). Therefore, we supposed the calculations in this paper are feasible.

Question 4: See the first paragraph on P.5: It is essential that almost all part of the text should be moved to the introduction.

Respond:Thank you very much for your advice. We have moved that paragraph into the introduction part. See 3rd paragraph in P. 2 in the revised manuscript.

Question 5: The authors did not provide a clear explanation for the significant change in the energy of formation of the system when the number of hydrogen molecules changed from 10 to 11. This information is crucial for readers to fully understand the implications of the research.

Respond:Thank you for your suggestion, it is very useful. The explanations are depicted in 1st paragraph in P. 7.

We supposed that this is because of the more the hydrogen molecules in this BN structure, the framework of nH₂@B₂₀N₂₄ complex gets more and more deformation. That B-N bonds are expanded, thereby inducing the high energy of B-N chemical bonds, also the repulsive between H atoms are increased as the distance of H atoms are closer, that for to attain the equilibrium state for the complex. Therefore, the energy of the nH₂@B₂₀N₂₄ complexes are raised.

Question 6:Although the authors briefly mention some theoretical implications of their findings, I believe that a more in-depth analysis is necessary for a full understanding of the research. Further discussion of the results is essential to contextualise the significance and relevance of the B₂₀N₂₄ polymorph and to provide suggestions for future experiments and research. Therefore, I recommend that the authors revise Results and Discussion section to more fully unpack the implications of their results.

Respond:Thanks. We have revised the results and discussion section. See 2^nd paragraph in P. 8.

As we have mentioned above, owing to the high chemical stability of the B-N compounds, it is a good candidate for hydrogen storage. And owing to the cage-like configuration of B₂₀N₂₄ structure, the storage ability is excellent. As we known, B-N materials are synthesized compounds. These materials are widely used in industries, most of B-N compounds are semiconductors with wide band gap, thus they also used in electronics. In recent years, more and more B-N materials are predicted with help of computational materials science. The studies of the novel structure of one class, not only widen the crystal structure information of it, but also can reveal more possible physical properties in theoretical. For one thing, in terms of structural configurations, the results in this work, provide a useful method in structural prediction, not only in B-N compounds, but also in any other classes. For another, we provide a new way to design functional material. For example, the cage-like structure can be a candidate in hydrogen storage, a fully sp³-hybridzed carbon or BN structure may be a good candidate of superhard materials.

Question 7 Some references could be added to introduction:

https://doi.org/10.1021/acs.jpcc.2c02749

https://doi.org/10.1016/j.matchemphys.2020.123245

Respond: The references have been added to the introduction. See 1st paragraph in P. 2.

Reviewer 2 Report

I have a few points about the presentation that should be addressed prior to publication. The paper by Zhao et al. deals with ab initio investigations of B₂₀N₂₄ polymorph as promising for use in electronic devices and hydrogen storage. 

The manuscript is appropriately structured and comprehensible as well. Terminology is defined and used consistently. The figures are valid and readable. Terminology is defined and used in a consistent way.

 My arguments in favor of a major revision:

1.      In its current form, the work does not seem relevant, and the scientific results do not seem convincing. The concepts are not explained enough according to the current understanding in the field. Previously performed relevant work has not been adequately discussed.

Referring to the articles of 2002 and 2005, the manuscript's authors assure that "In recent years, extensive studies showed that porous BN polymorphs and BN cages exhibit promising application in hydrogen storage[36-37]. For example, studies showed that BN nanotubes can store hydrogen as much as 2.6 wt.% [36], and the B₃₆N₃₆ cage can reach to 4 wt.% [37]. Considering the cage-like structural feature and low density of B₂₀N₂₄, it might be a candidate in hydrogen storage."

However, according to the results of Sun et al. [ref 37 in the manuscript],  only "at zero temperature, up to 18 hydrogen molecules can be stored inside a B₃₆N₃₆ cage corresponding to a gravimetric density of 4 wt %". At room temperature (T = 300 K), that "high weight percentage hydrogen storage cannot be achieved in B−N cage structures and thus these materials may not be good for practical applications" (because hydrogen is found to be escaping the cage).

Thus, the manuscript's authors suggest that B₂₀N₂₄ may be used in hydrogen storage at a capacity of ~6.8 wt.% without any study of the behavior of their new material at room temperature. As a result, the conclusions are confirmed only by the results of DFT calculations at a temperature of 0 K - under such circumstances, it is not surprising that when the B-N bonds in the B₂₀N₂₄ cage are broken after it is filled with 20 hydrogen molecules, these H₂ molecules remain in the cell.  Will these H₂ molecules remain in the cell at room temperature?

The work requires additional research.

2.   It would be beneficial to provide answers to the following essential questions:

Does this novel material have thermal stability?

How do the geometry and electronic structure of the cage change as more hydrogen molecules are stored?

Is this novel material suitable for practical applications?

3.      It would be helpful to check the accuracy of the chosen computational method, for example, by calculating the binding energy and bond length of H₂, as well as the geometry and binding energy of the B₂₀N₂₄ cage.

4.      It would be helpful to explain the (physical/chemical) meaning of the negative energy of formation.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly, and detailed corrections are listed below point by point:

1. In its current form, the work does not seem relevant, and the scientific results do not seem convincing. The concepts are not explained enough according to the current understanding in the field. Previously performed relevant work has not been adequately discussed.

Referring to the articles of 2002 and 2005, the manuscript's authors assure that "In recent years, extensive studies showed that porous BN polymorphs and BN cages exhibit promising application in hydrogen storage[36-37]. For example, studies showed that BN nanotubes can store hydrogen as much as 2.6 wt.% [36], and the B36N36 cage can reach to 4 wt.% [37]. Considering the cage-like structural feature and low density of B20N24, it might be a candidate in hydrogen storage."

However, according to the results of Sun et al. [ref 37 in the manuscript], only "at zero temperature, up to 18 hydrogen molecules can be stored inside a B36N36 cage corresponding to a gravimetric density of 4 wt %". At room temperature (T = 300 K), that "high weight percentage hydrogen storage cannot be achieved in B−N cage structures and thus these materials may not be good for practical applications" (because hydrogen is found to be escaping the cage).

Thus, the manuscript's authors suggest that B₂₀N₂₄ may be used in hydrogen storage at a capacity of ~6.8 wt.% without any study of the behavior of their new material at room temperature. As a result, the conclusions are confirmed only by the results of DFT calculations at a temperature of 0 K - under such circumstances, it is not surprising that when the B-N bonds in the B₂₀N₂₄ cage are broken after it is filled with 20 hydrogen molecules, these H₂ molecules remain in the cell.  Will these H₂ molecules remain in the cell at room temperature?

The work requires additional research.

Respond: Thank you very much for your advice. First-principles calculations are calculated based on ambient conditions. The formation energies of nH₂@B₂₀N₂₄ complex as a function of the number of H₂ stored and configurations of nH₂@B₂₀N₂₄ complex as more and more H₂ molecules stored are calculated and discussed in the manuscript (See Fig.5 and 6 and P.6-7 in the revised manuscript). However, it is necessary to further study the capacity of the H₂ in the B₂₀N₂₄ for practical application, but it is very difficult for our team in present situation. We will do the relevant research if the condition is permitted.

2. It would be beneficial to provide answers to the following essential questions:

Does this novel material have thermal stability?

How do the geometry and electronic structure of the cage change as more hydrogen molecules are stored?

Is this novel material suitable for practical applications?

Respond: Thank you for your question. First-principles calculations are calculated based on ambient conditions. Normally, the thermal stability of one structure is a relative concept comparing to some other stable or metastable structure. Therefore, we calculated the energetic stability of B₂₀N₂₄ cage, and some other B-N compounds for comparison. (See Figure 2). Besides, the thermal stability and practical applications can be studied via molecular dynamics, and considering the calculations costs (time and fund), we can not afford a specific result. We are shamed about this. Sorry. We will do the relevant research if the condition is permitted.

3. It would be helpful to check the accuracy of the chosen computational method, for example, by calculating the binding energy and bond length of H2, as well as the geometry and binding energy of the B20N24 cage.

Respond: Thanks. In the revised manuscript we listed the lattice parameters and bond length of diamond and cubic BN to demonstrate the feasibility of our calculations. The calculated results of the two crystals are close to the results derived from experiment. Therefore, we supposed that the calculations in this work are feasible. See the end of 1^st paragraph in P. 3.

4. It would be helpful to explain the (physical/chemical) meaning of the negative energy of formation.

Respond: Thank you for your suggestion. The meaning of the negative energy of formations are explained in the revised manuscript. See the red part of 1^st paragraph in P6.

Reviewer 3 Report

In my opinion, the manuscript is not suitable to be published in Crystals due to several serious problems. My further comments are as follows:

1. Similar studies for B24N24 were published in Zhanlin Ma, Yan Zhang, Fei Li, Hongshan Chen, Comparative study of H2 adsorption on B24N24, Al24N24 and B12Al12N24 clusters, Computational Materials Science, Volume 117, 2016, Pages 71-75, and Sinan Sayhan, Armağan Kinal, Computational investigation and comparison of hydrogen storage properties of B24N24 and Al24N24 nanocages, International Journal of Hydrogen Energy, Volume 42, Issue 20, 2017, Pages 14166-14180.

2. The study of carbon allotropes do not indicate any reasonable information about the structures of boron nitride.

3. Such cage-like BN systems were not obtained up to now, because they are rather impossible to be formed in real world conditions. What experimental methods are required for synthesis of fullerene like BN systems?

1. The manuscript is very poorly written. Even the abstract is difficult to be read, e.g., 'dubbed' is very informal, 'Electronic properties calculations show that B20N24 exhibits a semiconducting feature with a 0.87 eV direct band gap derived from HSE06 functional, which is much lower than many other B-N polymorphs.' is unclear / incorrect.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly, and detailed corrections are listed below point by point:

Question 1: Similar studies for B₂₄N₂₄ were published in Zhanlin Ma, Yan Zhang, Fei Li, Hongshan Chen, Comparative study of H₂ adsorption on B₂₄N₂₄, Al₂₄N₂₄ and B₁₂Al₁₂N₂₄ clusters, Computational Materials Science, Volume 117, 2016, Pages 71-75, and Sinan Sayhan, Armağan Kinal, Computational investigation and comparison of hydrogen storage properties of B₂₄N₂₄ and Al₂₄N₂₄ nanocages, International Journal of Hydrogen Energy, Volume 42, Issue 20, 2017, Pages 14166-14180.

The study of carbon allotropes do not indicate any reasonable information about the structures of boron nitride.

Respond: The structure of B₂₄N₂₄ and B₂₀N₂₄ is very different, so it is very necessary to investigate the B₂₀N₂₄ and its capacity of hydrogen storage.

Question 2: Such cage-like BN systems were not obtained up to now, because they are rather impossible to be formed in real world conditions. What experimental methods are required for synthesis of fullerene like BN systems?

Respond: The calculation is in the relative ideal condition and can predict the result of the problem not solved via experiment, so it is the leader of the practice.

Comments on the Quality of English Language

Question 3: The manuscript is very poorly written. Even the abstract is difficult to be read, e.g., 'dubbed' is very informal, 'Electronic properties calculations show that B20N24 exhibits a semiconducting feature with a 0.87 eV direct band gap derived from HSE06 functional, which is much lower than many other B-N polymorphs.' is unclear / incorrect.

 Respond: The manuscript has been polished via a specialized agency.

Round 2

Reviewer 1 Report

Thanks to the authors for their work on the manuscript. It looks more attractive now and suitable for publication in Crystals.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly. Thank you very much for your support.

Author Response File: Author Response.docx

Reviewer 2 Report

Unfortunately, the article still contains misleading statements. For example, "Specifically, owing to its cage-like framework, B20N24 may be used in hydrogen storage at capacity of ~6.8 wt.%." I did not find any mention in the text that B20N24 may be used in hydrogen storage at the capacity of ~6.8 wt.% only at 0 K. I did not find any mention in the text that at room temperature (T = 300 K), such a high weight percentage of hydrogen storage is unlikely to be achieved  and then these materials may not be suitable for practical applications as hydrogen storage. I did not find any information in the manuscript that more research is needed to ensure this material is suitable for practical applications as hydrogen storage.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly, and detailed corrections are listed below point by point:

Question 1: Unfortunately, the article still contains misleading statements. For example, "Specifically, owing to its cage-like framework, B20N24 may be used in hydrogen storage at capacity of ~6.8 wt.%." I did not find any mention in the text that B20N24 may be used in hydrogen storage at the capacity of ~6.8 wt.% only at 0 K. I did not find any mention in the text that at room temperature (T = 300 K), such a high weight percentage of hydrogen storage is unlikely to be achieved  and then these materials may not be suitable for practical applications as hydrogen storage. I did not find any information in the manuscript that more research is needed to ensure this material is suitable for practical applications as hydrogen storage.

Respond: Thank you very much for your question. Notably, we should clear that the first-principles calculations are based on the ambient conditions, which the results are derived under 0 K and 0 GPa. Therefore, the hydrogen storage ability of B₂₀N₂₄ structure in this study is at 0 K. Considering the vibrations of atoms, periodic boundary condition and some other conditions during the calculations at 0 K and 300 K, we supposed that the hydrogen storage capacity of B₂₀N₂₄ may not as high as 6.8 wt.%. The hydrogen storage capacity of this B-N compounds at room temperature or any other temperature can be derived via molecular dynamics. Normally, it may be lower than the result at 0 K. Theoretically, the actual capacity of B₂₀N₂₄ at 300 K need more calculations to reveal. In this work, the B₂₀N₂₄ structure is proposed as a novel compound of B-N system, and can be viewed as a candidate for hydrogen storage during its highly chemical inertness and cage-like structure. Please see 2^nd paragraph in P.8.

Reviewer 3 Report

Although the materials are completely artificial, the journal is OA and the scientific community will evaluate the impact of your work.

In my opinion, words like 'dubbed' should not be present in the scientific text, but I'm not a language editor.

Author Response

Dear Reviewer,

Thank you very much for your careful investigation throughout the whole text and useful comments.

We have modified the manuscript accordingly.

Question 1: In my opinion, words like 'dubbed' should not be present in the scientific text, but I'm not a language editor.

Respond: Thank you very much for your advice. We have modified the manuscript. Please see 1^st  paragraph in P. 1.

Author Response File: Author Response.docx