Density Functional Theory Study of Nitrogen Reduction to Ammonia on Bilayer Borophene

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The current presentation is incomplete as the free energy curves in Figure 3 should include free N2(g) as the starting point of all three pathways. If N2(g) is lower in free energy than *N-N, it is free energy of the former that determines the potential of the initial electrochemical step otherwise it is determined by the latter.

 

The free energy of *N-N relative N2(g) is also important for determining the competition with HER. If H binding is stronger than *N-N the surface will be poisoned by H. This needs to be discussed and evaluated.

 

The free energy of *N-N relative N2(g) for the different binding conformations should also be reported in Figure 4, as it determines the equilibrium between the conformations.

 

The overpotential seems to be incorrectly calculated. Since the overall reaction is exergonic, the overpotential should be higher not lower than the limiting potential.

Author Response

Reviewer: 1

Comments:
(1) The current presentation is incomplete as the free energy curves in Figure 3 should include free N2(g) as the starting point of all three pathways. If N2(g) is lower in free energy than *N-N, it is free energy of the former that determines the potential of the initial electrochemical step, otherwise it is determined by the latter.

(2) The free energy of *N-N relative N2(g) is also important for determining the competition with HER. If H binding is stronger than *N-N the surface will be poisoned by H. This needs to be discussed and evaluated.

(3) The free energy of *N-N relative N2(g) for the different binding conformations should also be reported in Figure 4, as it determines the equilibrium between the conformations.

(4) The overpotential seems to be incorrectly calculated. Since the overall reaction is exergonic, the overpotential should be higher not lower than the limiting potential.

Author Reply:

We appreciate your time and effort for the valuable feedback on the manuscript. The corresponding reply is as follows:

(1) The free energy evolution includes free gaseous N2 as the starting point, which is also shown in Figure 3 in the manuscript. The potential of the initial electrochemical step was also discussed accordingly.

 

Figure 1. Gibbs free-energy evolution of N2 reduction through (a) distal, (b) alternating and (c) enzymatic pathways at different applied potentials.

Changes: Page 6, lines 210-211

(2) It is also reasonable to discuss the side reaction, since HER may influence the catalysts’ performance and lead to by-product generation. We have tested neighbouring sites for the assessment of selectivity and plot the results as below. It can be seen that the proton binding on top and hollow sites is not favoured as the free energy value is higher than zero. And the proton adsorption on bridge does not block the nitrogen adsorption, whose value is far below zero, indicating the poor HER performance. Thus, the selectivity of the catalysts towards ammonia is also excellent.

 

Figure 2. The Gibbs free energy evolution for proton adsorption on top, bridge and hollow sites.

Changes: Page 8, lines 255-263

Besides, the selectivity of the catalysts is also important to the assessment of the performance. A good catalyst should produce the desired products without many by-products. Thus, we evaluate the HER performance on the bilayer borophene. As shown in Figure 5, the adsorption energy of protons are far away from zero, indicating poor HER performance. Thus, the bilayer borophene also possesses high selectivity.

(3) Thanks for the professional suggestion. We have attached the corresponding energy change in the Figure 4 for clarity.

Figure 4. (a) The possible end-on and side-on configurations of the adsorption of N2 with corresponding adsorption energy on the bilayer borophene.

Changes: Page 7, lines 226-227

 

(4) As suggested, we have modified the overpotential values according to the equation in the computation section.

Changes: Page 3, lines 122

 

 

Changes: N/A

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Finding an alternative process for ammonia synthesis that could replace the energy-intensive Haber–Bosch method is undoubtedly a major scientific challenge. Therefore, the subject of this manuscript is of clear relevance and interest to both the scientific community and broader society. The author proposes a bilayer borophene system as a catalyst to reduce the reaction overpotential.

However, the manuscript only partially support its claims. The presented thermodynamic analysis is incomplete, as it neglects the crucial effects of the electrolyte environment and electrode potential. These can be readily accounted for using well-established tools such as VASPsol, JDFTx, Environ, and others — many of which are freely available. Furthermore, the study only considers a low-coverage regime. In practice, adsorption free energies are strongly influenced by the presence of multiple adsorbates. This should be explicitly acknowledged, as a proper investigation would require identifying the system’s resting state under realistic operating conditions. Is there any evidence that omitting these effects is a correct strategy for assessing the efficiency of a catalyst for ammonia synthesis? The author is strongly encouraged to present well-founded arguments in support of this.

Another critical omission is the lack of free energy calculations for the Transition States (TS) of the reaction’s elementary steps. Kinetics is essential to determine whether a catalytic electrochemical process can compete with the Haber–Bosch process in terms of energy efficiency. Can the author argue why omitting the TS calculations can still lead to useful conclusions about the efficiency of a catalyst for ammonia synthesis?

Additionally, the quality of the figures, especially Fig.2, is very poor, making it difficult to extract meaningful information such as energy values and the stoichiometry of the reaction steps. A clearer and more informative presentation of results is recommended, for example, following the format used in DOI: 10.1039/c1cp22271f. Ball-and-Stick representations of key reaction intermediates should also be provided as Supporting Information to help clarify structural details. Tables with calculated reactions and the evaluated energy contributions will also help understand the content of the research and the followed approach. At the beginning of pag. 5, something seems to be missing in the text.

Finally, there are many similar studies in the literature about doped Transition Metal borophene in NRR. To demonstrate the robustness of the claims, the Gibbs free energy changes (ΔG) of the process should be compared and discussed in light of these previous studies.

The consistency between the manuscript content and the associated references should be carefully checked. 

 

Author Response

 

Reviewer: 2

Comments:
Finding an alternative process for ammonia synthesis that could replace the energy-intensive Haber–Bosch method is undoubtedly a major scientific challenge. Therefore, the subject of this manuscript is of clear relevance and interest to both the scientific community and broader society. The author proposes a bilayer borophene system as a catalyst to reduce the reaction overpotential.

However, the manuscript only partially support its claims. The presented thermodynamic analysis is incomplete, as it neglects the crucial effects of the electrolyte environment and electrode potential. These can be readily accounted for using well-established tools such as VASPsol, JDFTx, Environ, and others — many of which are freely available. Furthermore, the study only considers a low-coverage regime. In practice, adsorption free energies are strongly influenced by the presence of multiple adsorbates. This should be explicitly acknowledged, as a proper investigation would require identifying the system’s resting state under realistic operating conditions. Is there any evidence that omitting these effects is a correct strategy for assessing the efficiency of a catalyst for ammonia synthesis? The author is strongly encouraged to present well-founded arguments in support of this.

Another critical omission is the lack of free energy calculations for the Transition States

Author Reply:

1. Thank you very much for the comment. The electrolyte environment is indeed complicated and thus the solvation effect was studied using VASPsol package. The solvation effect was examined as attached In Figure 1. It can be seen that the catalytic performance of the electrocatalysts is still good when the solvation effect is considered. Thus, the selection of the promising candidate remains unchanged. The solvent effect is discussed in page 10 in the manuscript and the plot of Gibbs free energy change is also attached for reference.

 

Figure 1. (a) The comparison of free energy evolution considering solvation effects on the bilayer borophene.

The current analysis is conducted under constant electron conditions, which is quite different from the realistic constant potential conditions. Based on constant-potential method, we utilized JDFTx to compare the adsorption energy difference with varied applied potentials.

 

Figure 2. (a) The N2 adsorption energy in horizontal and enzymatic manners with varied potential on the bilayer borophene.

With the increase of applied potentials, the adsorption energy increases accordingly, no matter what types of adsorption configurations.

It is understandable that the existence of other adsorbates may influence the catalytic performance of the catalysts. The types and number of multiple adsorbates could change the reaction pathway and intermediate configurations. However, the goal of this study is to assess the potential performance of bilayer borophene on NRR process, which focus less on such complicated situations. Besides, it is not reasonable and quite challenging to construct the models with so many adsorption sites and configurations of various adsorbates. There are no experimental supports about the types and configurations of adsorbates on the slab to guide the construction of the models. It is not arguable that the adsorbate existence is quite a meaning topic to catalysts’ performance, however, this deeper investigation needs more experimental evidence to promote the understanding of the reaction process.

We appreciate your thoughtful words. The activation barrier is one of the vital factors which may influence the reaction process. However, it is widely reported that there exist the Brønsted–Evans–Polanyi (BEP) relations between the calculated activation energy and reaction energy (ACS Appl Mater Interfaces. 2025 May 7;17(18):26491-26500; J. Phys. Chem. C 2008 112 (5), 1308–1311; Journal of Catalysis 2004 222(1), 206-217), which provides an efficient way to estimate kinetic parameters using the thermodynamic descriptors. Thus, the free energy is solely considered to evaluate the catalysts’ performance with acceptable accuracy, which also significantly reduces the computation load and facilitates the fast screening of promising catalysts.

Changes: N/A

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The author has not adequately addressed all reviewer comments, therefore, the publication must be delayed. Below are the specific concerns that remain unresolved:

• Condition of the Present Study

The author should explicitly state in the manuscript that the study considers a low coverage regime in order to reduce the complexity of the simulations and the associated computational cost. Additionally, the author should clearly mention in the manuscript that reaction barriers were not calculated. Instead, the study assumes the Brønsted–Evans–Polanyi (BEP) relationship between activation energy and reaction energy.

• Solvation Effect and Related Data

The sentence “The solvation effect, according to the previous reports, can be ignored since its influence on the onset potential change is less than 0.1 eV [64, 65].” remains in the manuscript and should be removed since  references [64] and [65] do not support this claim. Additionally, the author performed a partial analysis of the solvent effect using Vaspsol on the promising reaction pathway. THESE RESULTS SHOULD BE EXPLICITLY DISCUSSED IN THE MANUSCRIPT, AND THE CONTENT FROM FIGURE 1 OF THE CATALYSTS-3617713-COVERLETTER.PDF FILE SHOULD BE INTEGRATED INTO FIGURE 3 OF THE MAIN MANUSCRIPT. Note that the author’s assertion in the cover letter—that “the solvent effect is discussed on page 10 of the manuscript”—is incorrect, as that page primarily lists references and does not contain any relevant technical discussion.

Furthermore, the author’s response lacks technical details regarding the Vaspsol calculations. Specifically, it is unclear whether the calculations involved only single-point SCF evaluations on geometries relaxed in vacuum, or if the geometries were consistently relaxed in the presence of the solvent. Additionally, the manuscript does not specify the dielectric constant used, nor does it provide a rationale for its selection. This information is essential and must be included in Section 2 (Computational Methods) of the manuscript.THIS INFORMATION MUST BE INCLUDED IN SECTION 2 (COMPUTATIONAL METHODS) OF THE MANUSCRIPT.

• JDFTx Calculations and Surface Adsorption

The JDFTx results are indeed interesting, although consistency would have been improved if these calculations were performed using Vaspsol, in line with the rest of the study. The results indicate that the free adsorption energy of N₂ remains positive across the entire potential range [-0.5 V, +0.5 V], suggesting that under these conditions, bilayer borophene would have limited N₂ adsorption. The estimated surface coverage would likely fall below a few percentage points relative to the gas phase, significantly reducing catalytic activity. The report trend show a slight reduction at negative voltage electrode.  The author could verify whether further modulation of the electrode potential or other thermodynamic parameters could suppress N₂ adsorption to zero. ANYWAY A DISCUSSION OF THE TECHNICAL ASPECTS OF THE JDFTX CALCULATIONS IS NECESSARY, AND FIGURE 2 FROM THE CATALYSTS-3617713-COVERLETTER.PDF SHOULD BE INCORPORATED INTO FIGURE 4 OF THE MANUSCRIPT, WITH APPROPRIATE COMMENTARY ADDED IN THE RESULTS SECTION.

• Concluding Statement

Given the current findings, the manuscript’s final claim—“Our theoretical analysis demonstrates that the bilayer borophene can be an efficient NRR catalyst and provides a cost-efficient process for ammonia production”—is overstated. A more appropriate and evidence-aligned conclusion would be: “Our theoretical analysis provides preliminary evidence that bilayer borophene could serve as an efficient NRR catalyst under specific electrochemical conditions, potentially enabling a cost-effective process for ammonia production.”

Author Response

Comments:
(1) Condition of the Present Study

The author should explicitly state in the manuscript that the study considers a low coverage regime in order to reduce the complexity of the simulations and the associated computational cost. Additionally, the author should clearly mention in the manuscript that reaction barriers were not calculated. Instead, the study assumes the Brønsted–Evans–Polanyi (BEP) relationship between activation energy and reaction energy.

(2) Solvation Effect and Related Data.

The sentence “The solvation effect, according to the previous reports, can be ignored since its influence on the onset potential change is less than 0.1 eV [64, 65].” remains in the manuscript and should be removed since  references [64] and [65] do not support this claim. Additionally, the author performed a partial analysis of the solvent effect using Vaspsol on the promising reaction pathway. THESE RESULTS SHOULD BE EXPLICITLY DISCUSSED IN THE MANUSCRIPT, AND THE CONTENT FROM FIGURE 1 OF THE CATALYSTS-3617713-COVERLETTER.PDF FILE SHOULD BE INTEGRATED INTO FIGURE 3 OF THE MAIN MANUSCRIPT. Note that the author’s assertion in the cover letter—that “the solvent effect is discussed on page 10 of the manuscript”—is incorrect, as that page primarily lists references and does not contain any relevant technical discussion.Furthermore, the author’s response lacks technical details regarding the Vaspsol calculations. Specifically, it is unclear whether the calculations involved only single-point SCF evaluations on geometries relaxed in vacuum, or if the geometries were consistently relaxed in the presence of the solvent. Additionally, the manuscript does not specify the dielectric constant used, nor does it provide a rationale for its selection. This information is essential and must be included in Section 2 (Computational Methods) of the manuscript.THIS INFORMATION MUST BE INCLUDED IN SECTION 2 (COMPUTATIONAL METHODS) OF THE MANUSCRIPT.

(3) JDFTx Calculations and Surface Adsorption

The JDFTx results are indeed interesting, although consistency would have been improved if these calculations were performed using Vaspsol, in line with the rest of the study. The results indicate that the free adsorption energy of N₂ remains positive across the entire potential range [-0.5 V, +0.5 V], suggesting that under these conditions, bilayer borophene would have limited N₂ adsorption. The estimated surface coverage would likely fall below a few percentage points relative to the gas phase, significantly reducing catalytic activity. The report trend show a slight reduction at negative voltage electrode.  The author could verify whether further modulation of the electrode potential or other thermodynamic parameters could suppress N₂ adsorption to zero.

ANYWAY A DISCUSSION OF THE TECHNICAL ASPECTS OF THE JDFTX CALCULATIONS IS NECESSARY, AND FIGURE 2 FROM THE CATALYSTS-3617713-COVERLETTER.PDF SHOULD BE INCORPORATED INTO FIGURE 4 OF THE MANUSCRIPT, WITH APPROPRIATE COMMENTARY ADDED IN THE RESULTS SECTION.

(4) Concluding Statement

Given the current findings, the manuscript’s final claim—“Our theoretical analysis demonstrates that the bilayer borophene can be an efficient NRR catalyst and provides a cost-efficient process for ammonia production”—is overstated. A more appropriate and evidence-aligned conclusion would be: “Our theoretical analysis provides preliminary evidence that bilayer borophene could serve as an efficient NRR catalyst under specific electrochemical conditions, potentially enabling a cost-effective process for ammonia production.”

Author Reply:

We appreciate your professional guidance for the improvements on the manuscript. The modifications have been made and is summarized as follows:

(1) The statements about low coverage regime and BEP relationship were added to the manuscript as suggested.

Changes: Page 6, lines 221-225

It should be noted that the study was conducted considering a low coverage regime for reducing the complexity of the simulations and computational cost. Besides, the free energy is solely con-sidered to evaluate the catalyst performance because it is widely reported there exist the Brønsted–Evans–Polanyi (BEP) relations between the calculated ac-tivation energy and reaction energy [74], which is also assumed here to reduce the computation cost.

(2) The solvation statements in the computation section were replaced with more modelling details. And the results with illustration was also added.

Changes: Page 6, lines 208-212; Page 6, lines 220-221

The solvation effect was also studied, which shows that the catalytic performance of the electrocatalysts is still good.

(3) Thanks for the professional suggestion. The computation details, figure and related discussion were added to the manuscript.

Changes: Page 3, lines 130-137; Page 7, lines 228-235; Page 7, lines 258-262

To consider the potential effects, the chemical-potential-dependent thermody-namic study was conducted within the framework of JDFTx [71]. The grand free energy was obtained with the constant-potential method (CPM) ranging from -0.5 V to 0.5 V. The linear polarizable continuum model (PCM) was adopted, where the charge-asymmetry corrected, local-response, nonlocal-cavity solvation model (CAN-DLE) was utilized to simulate the aqueous environment composed of 1 M KF [72]. All the calculations were carried out with plane-wave cutoff and charge density cutoff of 20 Ha and 100 Ha, respectively [73]. The PBEsol pseudopotentials were used with the default exchange functional.

As shown in Figure 4d, the external potentials, ranging from -0.5 V to 0.5 V, can have a dramatic effect on the adsorption behaviour of reactant molecules, where the decreased potential can increase the N2 adsorption in both horizontal and enzymatic manners. It can be expected that lower value of potentials (< 0.5 V) could enhance the N2 adsorp-tion for further reduction.

(4) The concluding statement is much more precise and professional. We appreciate the suggestion a lot. And the related sentence has been replaced as suggested.

Changes: Page 8, lines 282-284

Our theoretical analysis provides preliminary evidence that bilayer borophene could serve as an efficient NRR catalyst under specific electrochemical conditions, potentially enabling a cost-effective process for ammonia production.

Author Response File: Author Response.pdf