Accelerated Electrons Transfer and Synergistic Interplay of Co and Ge Atoms (111 Crystal Plane) Activated by Anchoring Nano Spinel Structure Co₂GeO₄ onto Carbon Cloth Composite Electrocatalyst for Highly Enhanced Hydrogen Evolution Reaction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work is devoted to the preparation and research of electrocatalysts based on cobalt germanate. It is shown that catalysts with Co2GeO4 particles fixed on the surface of a carbon cloth have high activity in the hydrogen evolution reaction in combination with good stability under reaction conditions. The presented materials can be of interest to a broad auditory of the researchers working in a field of electrocatalysis and heterogeneous catalysis in general. The materials were fully characterized by a range of modern appropriate methods, their activity was measured with a combination of the electrochemical techniques and corresponding discussion of the properties and structure of materials is provided. From my point of view this manuscript is appropriate for consideration in Catalysts journal however, some serious improvements need to be made before final decision. 

1)  Since the Co2GeO4-based materials were intensively studied previously authors should comment in detail on method of the materials' preparation: if it is original conception of the preparation designed by authors the chemical reaction behind the preparative route should be disclosed, else if the authors used the method reported previously then corresponding references and points of modification should be given.

Also, a lot of Co2GeO4-based electrocatalysts were prepared and studied, including materials for HER. Authors should present a tabulated data comparing the performance of previously reported materials and their catalysts

2) The yields of the CGOx materials prepared via hydrothermal reaction should be presented

3) The quality of the XRD data is low: as it can be seen from TEM data, the Co2GeO4 have a good crystallinity, but the reflections depicted in XRD patterns are weak and partially obscured by a noise. This is most likely due to the small amount of samples taken for analysis. It is very strange, especially for the case of pure CGOx materials because the starting reagents are cheap and of common availability. To bring more straightforward proofs of the phase purity of the prepared materials, authors should repeat the XRD data collection with more substantial quantity of the samples.

4) The DFT computation were performed by authors to afford a theoretical analysis of the HER process on the prepared catalysts. However, these data are not consistent with the experimental findings presented by authors. The authors chose as a starting model a surface containing germanium and cobalt atoms in a partially oxidized state, which contradicts the experimental data presented in the work. After all, according to the XPS, cobalt on the surface is in the only state corresponding to the form of cobalt 2 + and germanium in the form of four-valent germanium. No other states, especially those assumed in the model of states with free valences, are observed on the surface (according to the XPS data). This model, moreover, contradicts the usual pattern expected for the surface of oxides in aqueous solutions with terminal oxygen atoms rather than metals. Thus, the given calculation data should be questioned, the authors should argue for their proposed starting model or replace it with a more realistic one. 

5) Line 320 "Additionally, the O1s peak of all the CGOx@CC samples (except CGO6@CC) exhibited two discernible peaks at about 530 and 532 eV (Figure 5d and Figure S3c), which might correspond to two different Co-O and Ge-O bonds in the molecular structure of Co2GeO4" 

It is very strange assumption because in the crystal structure of Co2GeO4 all oxygen atoms are connected to both Co and Ge atoms! Please rewrite.

6) Line 328 "This mutual movement implied a gradual change in the chemical bonds between cobalt/germanium and oxygen atoms during the hydrothermal reaction process, which might be a manifestation of gradual maturation and regularity of the spinel Co2GeO4 crystal." 

This is a very vague statement, please try to explain the observed phenomena within the framework of traditional chemical concepts.

7)  Line 445 "It is well-established that DFT calculations aid in comprehending the elementary reaction processes and their governing steps in the overall HER mechanism." Such a phrase should be confirmed by corresponding refferences.

 

Comments on the Quality of English Language

Some phrases are not correctly formulated, please rewrite.

For example,

the phrase "DFT technology" should be changed to just "DFT" or "DFT methods"

The following phrase "the theoretical catalytic ability of spinel Co2GeO4 crystal" also looks strange. What is theoretical catalytic ability? The sentence needs to be rephrased.

Author Response

Reviewer 1:

1) Since the Co2GeO4-based materials were intensively studied previously authors should comment in detail on method of the materials' preparation: if it is original conception of the preparation designed by authors the chemical reaction behind the preparative route should be disclosed, else if the authors used the method reported previously then corresponding references and points of modification should be given.

Reply: Thank you very much for your comments. The references, corresponding modifications and optimization of the synthesis sections have been included in the revised manuscript, as detailed in section 2.1 “The synthesis process of catalytic materials”.  

 

Also, a lot of Co2GeO4-based electrocatalysts were prepared and studied, including materials for HER. Authors should present a tabulated data comparing the performance of previously reported materials and their catalysts

Reply: Thank you very much for your comments. The research results on hydrogen evolution for Co2GeO4-based electrocatalysts have been added to Table 1, and the discussion in Table 1 has also been revised.

 

2) The yields of the CGOx materials prepared via hydrothermal reaction should be presented.

Reply: Thank you very much for your comments. The corresponding description for yield of CGOx materials has been added in section 2.1 “The synthesis process of catalytic materials”.

 

3) The quality of the XRD data is low: as it can be seen from TEM data, the Co2GeO4 have a good crystallinity, but the reflections depicted in XRD patterns are weak and partially obscured by a noise. This is most likely due to the small amount of samples taken for analysis. It is very strange, especially for the case of pure CGOx materials because the starting reagents are cheap and of common availability. To bring more straightforward proofs of the phase purity of the prepared materials, authors should repeat the XRD data collection with more substantial quantity of the samples.

Reply: Thank you very much for your comments. We fully agree with your opinion on the XRD curve. We conducted a new experiment on the materials and found that it might not be a problem with the samples’ amount and cleanliness, but a problem with the instrument. It is a domestic old instrument, with the model XD-6 polycrystalline X-ray diffractometer (Beijing Puxi, made in China), and its background noise is relatively high. But we believe that this does not seriously affect its identification of the crystal type.

 

4) The DFT computation were performed by authors to afford a theoretical analysis of the HER process on the prepared catalysts. However, these data are not consistent with the experimental findings presented by authors. The authors chose as a starting model a surface containing germanium and cobalt atoms in a partially oxidized state, which contradicts the experimental data presented in the work. After all, according to the XPS, cobalt on the surface is in the only state corresponding to the form of cobalt 2 + and germanium in the form of four-valent germanium. No other states, especially those assumed in the model of states with free valences, are observed on the surface (according to the XPS data). This model, moreover, contradicts the usual pattern expected for the surface of oxides in aqueous solutions with terminal oxygen atoms rather than metals. Thus, the given calculation data should be questioned, the authors should argue for their proposed starting model or replace it with a more realistic one.

Reply: Thank you very much for your comments. We agree that the crystal model of DFT really does not show the valence states of atoms such as Ge, Co, and O. But, in the DFT calculation process, we first need to calculate and optimize its structure and valence charges distribution before we could calculate its catalytic properties for hydrogen evolution. Thus, throughout the entire DFT process, the valence state of atoms should be equivalent to the distribution of valence charges, which affects the catalytic calculation results. On the other hand, due to the strict periodic structure required for DFT calculations, it is not possible to arbitrarily add other atoms to construct models for (111) crystal plane like other theoretical calculations, but only strictly cleaved from theoretical crystal models. So, DFT calculations can only explain the trend and theoretical catalytic performance of a certain reaction from an atomic perspective, and cannot simulate more complex catalytic system (like solution system with large computational complexity) like typical molecular dynamics. Therefore, we believe that the existing models can demonstrate the advantages and catalytic mechanism of Co₂GeO₄ in hydrogen evolution to some extent.

 

5) Line 320 "Additionally, the O1s peak of all the CGOx@CC samples (except CGO6@CC) exhibited two discernible peaks at about 530 and 532 eV (Figure 5d and Figure S3c), which might correspond to two different Co-O and Ge-O bonds in the molecular structure of Co2GeO4". It is very strange assumption because in the crystal structure of Co2GeO4 all oxygen atoms are connected to both Co and Ge atoms! Please rewrite.

Reply: Thank you very much for your comments, we completely agree with your opinion. The corresponding discussion has been modified, as detailed in section “3.3 XPS”.

 

6) Line 328 "This mutual movement implied a gradual change in the chemical bonds between cobalt/germanium and oxygen atoms during the hydrothermal reaction process, which might be a manifestation of gradual maturation and regularity of the spinel Co2GeO4 crystal." This is a very vague statement, please try to explain the observed phenomena within the framework of traditional chemical concepts.

Reply: Thank you for your comments. We have optimized the corresponding discussion, as shown in section “3.3 XPS”.

 

7) Line 445 "It is well-established that DFT calculations aid in comprehending the elementary reaction processes and their governing steps in the overall HER mechanism." Such a phrase should be confirmed by corresponding refferences.

Reply: Thank you for your comments. The corresponding literatures have been added to the revised manuscript.

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "Accelerated Electrons Transfer and Synergistic Interplay of Co and Ge Atoms (111 Crystal Plane) Activated by Anchoring Nano Spinel Structure Co2GeO4 onto Carbon Cloth Composite Electrocatalyst for Highly Enhanced Hydrogen Evolution Reaction" has interesting results and valuable data. This study presents a novel technique for investigating a highly efficient composite electrocatalyst that combines transition metal with carbon material to increase HER activity. The introduction is sufficient, and the references are relevant to the content. The authors provided a comprehensive explanation of both the technique and theoretical treatment. The results are presented properly, and they support the conclusion. so, I recommended that the paper can be accepted for publication in its present form.

  Comments on the Quality of English Language

Moderate the English Language is required.

Author Response

Reviewer 2:

The manuscript "Accelerated Electrons Transfer and Synergistic Interplay of Co and Ge Atoms (111 Crystal Plane) Activated by Anchoring Nano Spinel Structure Co2GeO4 onto Carbon Cloth Composite Electrocatalyst for Highly Enhanced Hydrogen Evolution Reaction" has interesting results and valuable data. This study presents a novel technique for investigating a highly efficient composite electrocatalyst that combines transition metal with carbon material to increase HER activity. The introduction is sufficient, and the references are relevant to the content. The authors provided a comprehensive explanation of both the technique and theoretical treatment. The results are presented properly, and they support the conclusion. so, I recommended that the paper can be accepted for publication in its present form. Moderate the English Language is required.

Reply: Thank you very much for your recognition and assistance with our manuscript. The English of the manuscript has been optimized.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The appropriate work was performed by the authors on the manuscript in order to enhance the quality and most points were solved. However before final acceptance, in my opinion, the DFT calculation part should be corrected. Authors correctly argue that the charge balanced models should be used in DFT optimization. For oxides in aqueous media, the surface is terminated with oxygen in several forms: O (oxo-group), OH (hydroxo group) and H2O (aqua). Therefore, balancing the charge with appropriate quantity of protons, authors can construct a realistic starting model of the Co2GeO4 particles terminated with oxygen-containing groups and corresponding to the points of zero charge (PZC) of the oxide. Then such model will be constructed, authors should repeat the calculations and provide renewed results. In other case, I do not recommend including the DFT data in the manuscript as they are in deep contradiction with current knowledge about oxide's surface in aqueous media and also with experimental data of authors. 

Author Response

Thank you for the reviewer’s comments. Out of respect for the reviewer's comments and after careful consideration, we have decided to move the DFT calculation section of manuscript to the supporting materials.

Academically, we still have different opinions on the second comments from the reviewer #1,

Firstly, as I mentioned in my previous feedback, DFT calculation, as a popular calculating method under molecular level, need tightly ordered and periodic molecular structures. Therefore, DFT calculations all start from a well-ordered unit cell with a periodic structure. As we know, in crystallography, all the crystal structures have strict cell shapes and space groups, and cannot be arbitrarily constructed or added with atoms or groups. Therefore, the reviewer's suggestion of constructing the initial structure by adding atoms themselves is not feasible in DFT calculations.

Secondly, regarding this manuscript, I agree that catalyst materials undergo surface modification in solution. However, the current purpose of DFT hydrogen evolution calculations is to theoretically explore the pathways and trends of entire HER catalytic process. With current computational capabilities, it is impossible and unnecessary for us to simulate the effects of all real-world factors on the theoretical catalytic process.

Thirdly, I have listed some recent literatures on DFT hydrogen evolution calculations (metallic oxide) as attachments, all of which have used methods similar to mine for constructing computational models.

1. Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction. Nature communications, |https://doi.org/10.1038/s41467-020-14462-2;
2. Platinum single-atom catalyst coupled with transition metal/metal oxide heterostructure for accelerating alkaline hydrogen evolution reaction. Nature communications, https://doi.org/10.1038/s41467-021-24079-8;
3. Lattice oxygen activation and local electric field enhancement by co-doping Fe and F in CoO nanoneedle arrays for industrial electrocatalytic water oxidation. Nature communications, https://doi.org/10.1038/s41467-024-45320-0;
4. Nanoporous cobalt-doped AlNi3/NiO architecture for high performing hydrogen evolution at high current densities. Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2024.04.009;
5. O-vacancy-rich and heterostructured Cu/Cu2O/NiO@NiCu foam self-supported advanced electrocatalyst towards hydrogen evolution: An experimental and DFT study, Chemical Engineering Science, https://doi.org/10.1016/j.ces.2023.119026;
6. Co-enhanced Pd-based catalysts for direct synthesis of hydrogen peroxide: Insights from DFT studies and experimental verification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2024.175265;
7. Active Co@CoO core/shell nanowire arrays as efficient electrocatalysts for hydrogen evolution reaction, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2021.132226.