Electrocatalytic Ni-Co Metal Organic Framework for Efficient Urea Oxidation Reaction

Round 1

Reviewer 1 Report

In the present manuscript, NiCo bimetallic organic framework materials were synthesized using ultrasound and hydrothermal methods while mono-metallic cobalt and nickel nanomaterials were synthesized by hydrothermal method. Four metal organic framework materials were characterized by SEM and XRD analysis and then tested in the urea oxidation reaction. The stability of bimetallic organic framework materials was investigated as well as the urea degradation.

Some considerations:

What is the Ni/Co molar ratio in NiCo-UMOFs  and NiCo-MOFs ?

Line 203: the introduction of Co component also enhances the overall electrocatalytic activity of the material. It is not the conclusion of SEM image and it is recommended to exclude this part of the sentence.

It is recommended to present diffractograms of Ni-MOFs and CoMOFs in Fig. 2.

The XPS measurements are recommended in order to investigate the chemical states of various bonded elements at the surface of the synthesized NiCo-based MOFs.

 What are the specific surface areas and porosity of the NiCo-UMOFs  and NiCo-MOFs?

It is necessary to mention Fig. 3b, 3c, and 3d in the text of the manuscript.

Line 229: the electrocatalytic activity of Ni-MOFs is lower but higher than that of Co-MOFs. Please rephrase the sentence?!

 

Explain the role of Co and Ni oxides in UOR.

 Minor editing of the English language is recommended.

Author Response

Reviewer #1: In the present manuscript, NiCo bimetallic organic framework materials were synthesized using ultrasound and hydrothermal methods while mono-metallic cobalt and nickel nanomaterials were synthesized by hydrothermal method. Four metal organic framework materials were characterized by SEM and XRD analysis and then tested in the urea oxidation reaction. The stability of bimetallic organic framework materials was investigated as well as the urea degradation.

Some considerations:

1. What is the Ni/Co molar ratio in NiCo-UMOFs and NiCo-MOFs?

Reply: Thanks for your kind suggestion. It is our neglect not to provide the Ni/Co molar ratio in NiCo-UMOFs and NiCo-MOFs. We have detected the Ni and Co element of NiCo-UMOFs (20 mg) and NiCo-MOF (20 mg) detected by inductively coupled plasma (ICP). As we can see in Table R1, the content of Ni and Co element in NiCo-UMOFs were 413mg/L and 379mg/L respectively, while 1.15mg/L and 925mg/L in NiCo-MOFs. According to these results, the Ni/Co molar ratio in NiCo-UMOFs and NiCo-MOFs were calculated as 1.1 : 1 and 1 : 4.5 respectively. The NiCo-UMOFs prepared using ultrasonic method was more favorable to the coordination of Ni, and large amounts of Ni facilitated the electrocatalytic urea oxidation reaction.

Table R1. The content of Ni and Co element of NiCo-UMOFs and NiCo-MOF detected by ICP.

Sample	Ni (mg/L)	Co(mg/L)
NiCo-UMOFs	413	379
NiCo-MOFs	130	584

 

 

2. Line 203: the introduction of Co component also enhances the overall electrocatalytic activity of the material. It is not the conclusion of SEM image and it is recommended to exclude this part of the sentence.

Reply: Thanks for your kind suggestion. We have deleted the sentence“but the introduction of Co component also enhances the overall electrocatalytic activity of the material”(please see the content in Page 5, line 208).

3. It is recommended to present diffractograms of Ni-MOFs and Co-MOFs in Fig. 2.

Reply: Thanks for your kind suggestion. It is our neglect not to present diffractograms of Ni-MOFs and Co-MOFs in Fig. 2. We have supplemented XRD measurements of Ni-MOFs and Co-MOFs, the results are shown in Fig. R2. The strong diffraction peaks appear in Ni-MOFs at 2θ degree of about 45º, which is attributed to nickel peak. The XRD pattern of Co-MOFs exhibited weaker peaks at 2θ degree of 33.5°. NiCo-UMOFs and NiCo-MOFs have similar diffraction peaks to Ni-MOFs, indicating that the introduction of Co did not affect the structure of Ni-MOF, and NiCo-UMOFs is successfully prepared.

The relevant contents have been added in the revised manuscript, and were marked red (please see the content in Page 6, Figure 2, line 216-220).

 

Fig. R1. XRD parttern of Co-MOFs, Ni-MOFs, NiCo-MOFs and NiCo-UMOFs.

4. The XPS measurements are recommended in order to investigate the chemical states of various bonded elements at the surface of the synthesized NiCo-based MOFs.

Reply: Thanks for your kind suggestion. We have added the XPS measurements of NiCo-UMOFs and the results are displayed in Fig. R2. The survey spectrum (Fig. R2A) demonstrates the presence of Ni, Co, O, and C elements in NiCo-UMOF. In the Ni 2p spectrum (Fig. R2B), the characteristic peaks at 855.7 and 873.8 eV attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively, with the two satellite peaks at 861.6 and 880.1 eV [1], indicating Ni exist in Ni^3+/2+ forms in the NiCo-UMOFs. In the Co 2p spectrum (Fig. R2C), the characteristic peaks located at 781.1 and 797.4 eV are corresponding to Co 2p_3/2 and Co 2p_1/2, and the two satellite peaks were centered at 785.2 and 802.6 eV [2], revealing Co ions exist mainly in Co²⁺ forms in the synthesized NiCo-UMOFs. The C 1s spectrum (Fig. R2D) was divided into two peaks at 284.6 and 288.3 eV, which are corresponding to the bonds of C-C=C and O-C=O, respectively [3]. The O 1s spectrum (Fig. R2E) were indexed into two peaks, which attributed to the metal- oxygen bonds M-O-M (531.0 eV) in the metal oxides and the oxygen in -OH (532.8 eV) [4]. Thus, XPS results verify the formation of the nickel and cobalt phthalic acid phases.

The relevant contents have been added in the revised manuscript, and were marked red (please see the content in Page 6, Figure 3, line 221-234).

Fig. R2. XPS spectra of NiCo-UMOFs.: (A) Survey, (B) Ni 2p, (C) Co 2p, (D) C 1s, and (E) O 1s.

 

References

[1]. Veeramani, V.; Madhu, R.; Chen, S. M.; Sivakumar, M.; Hung, C. T.; Miyamoto, N.; Liu, S. B. NiCo₂O₄-decorated porous carbon nanosheets for high-performance supercapacitors. Electrochim. Acta 2017, 247, 288-295. https://doi.org/10.1016/j.electacta.2017.06.171

[2]. Wang, X. Y.; Liu, B. B.; Li, J.; Zhai, Y. Y.; Liu, H. Q.; Li, L.; Wen, H. R. Conductive 2D metal-organic framework (Co, NiCo, Ni) nanosheets for enhanced non-enzymatic detection of urea. Electroanalysis 2021, 33, 1484-1490. https://doi.org/10.1002/elan.202060586

[3]. Zhang, T.; Du, J.; Xi, P.; Xu, C. Hybrids of cobalt/iron phosphides derived from bimetal-organic frameworks as highly efficient electrocatalysts for oxygen evolution Reaction. ACS Appl. Mater. Interfaces 2017, 9, 362-370. https://doi.org/10.1021/acsami.6b12189

[4]. Wang, Q.; Wang, Q.; Xu, B.; Gao, F.; Gao, F.; Zhao, C. Flowershaped multiwalled carbon nanotubes@nickel-trimesic acid MOF composite as a high-performance cathode material for energy storage. Electrochim. Acta 2018, 281, 69-77. https://doi.org/10.1016/j.electacta.2018.05.159

5. What are the specific surface areas and porosity of the NiCo-UMOFs and NiCo-MOFs?

Reply: Thanks for your kind suggestion. The specific surface areas and porosity of NiCo-UMOFs and NiCo-MOFs were tested using the BET multi-point method (BET) and N₂ adsorption--desorption. As shown in Table R2, NiCo-UMOFs showed larger specific surface areas and pore sizes than that of NiCo-MOFs. Further, the adsorption and desorption curves are shown in Fig. R3, an obvious IV isotherm for NiCo-UMOFs and NiCo-MOFs can be observed, which refers to the presence of a mesoporous structure. More catalytic active sites can be exposed due to the larger specific surface areas and pore size, which facilitates the electrocatalytic urea reaction. Thus, NiCo-UMOFs showed the better catalytic performance of UOR.

 

Table R2. BET data of NiCo-UMOFs and NiCo-MOFs.

Sample	SBET (m2/g)	Pore volume (cm3/g)	Average pore diameter size (nm)
NiCo-UMOFs	10.45	0.03	11.39
NiCo-MOFs	8.01	0.04	7.51

 

Fig. R3. N₂ adsorption-desorption isotherms of NiCo-UMOFs and NiCo-MOFs.

6. It is necessary to mention Fig. 3b, 3c, and 3d in the text of the manuscript.

Reply: Thanks for your kind suggestion. It is our neglect not to mention Fig. 3b, 3c, and 3d in the text. We have added the relevant description, please see the content in Page 7, line 248, 253, and 256. We also adjusted the position of the paragraphs to make the descriptions of Figure 4 are all on top of the picture.

7. Line 229: the electrocatalytic activity of Ni-MOFs is lower but higher than that of Co-MOFs. Please rephrase the sentence?!

Reply: Thanks for your valuable comments. The sentence “the electrocatalytic activity of Ni-MOFs is lower but higher than that of Co-MOFs.” was changed into “both the electrocatalytic activity of Ni-MOFs and Co-MOFs are lower than that of NiCo-MOFs.” (please see the content in Page 7, line 248)

8. Explain the role of Co and Ni oxides in UOR.

Reply: Thanks for your valuable comments. The role of Co and Ni oxides in UOR was reported in previous works [5-8]. More research focused on Ni-based catalysts such as nickel oxides in the research of UOR due to their cheapness and good catalytic performanc. Ni²⁺ is oxidized to NiOOH in alkaline media, which is considered to be the active site for urea oxidation. The main interaction between bridging O^2- and Ni²⁺ is e^--e^- repulsion, while Co²⁺ can act on the bridging O^2- through π-donation. When Ni²⁺ is coupled with Co²⁺, the e^--e^- repulsion between O^2- and Ni²⁺ can enhance the π-donation of Co-O, resulting in partial charge transfer from Ni²⁺ to Co²⁺. Thus, Co can enhance the activity of nickel oxides related to shifting the Ni^2+/3+ redox wave, indicating strong synergistic metal-metal electronic interactions between Ni and Co. Our XPS results in Fig. R2. showed Ni existed in Ni^3+/2+ forms in NiCo-UMOFs, which proved this point. Moreover, the synergistic effect between Ni and Co weakens the strength of the C-N bond in the urea molecule and promotes complete oxidation. In addition, due to the stronger catalytic CO oxidation ability of the Co element, catalyst poisoning can be effectively alleviated. Thus, NiCo-UMOFs showed the better UOR performance due to the elevated Ni oxidation state caused by the electron transfer from Ni to Co, the weakness of the strength of the C-N bond in the urea molecule and the stronger catalytic CO oxidation ability.

References

[5]. Li, B.; Song, C.; Rong, J.; Zhao, J.; Wang, H. E.; Yang, P.; Ye, K.; Cheng, K.; Zhu, K.; Yan, J.; Cao, D.; Wang, G. A new catalyst for urea oxidation: NiCo₂S₄ nanowires modified 3D carbon sponge. J. Energy Chem. 2020, 50, 195-205, https://doi.org/ 10.1016/j.jechem.2019.12.018.

[6] Nangan, S.; Ding, Y.; Alhakemy, A. Z.; Liu, Y.; Wen, Z. Hybrid alkali-acid urea-nitrate fuel cell for degrading nitrogen-rich wastewater. Appl. Catal. B Environ. 2021, 286, 119892, https://doi.org/10.1016/j.apcatb.2021.119892.

[7]. Zhao, S.; Wang, Y.; Dong, J.; He, C. T.; Yin, H.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L. Ultrathin metal-organic framework nanosheets for electrocatalytic oxygen evolution. Nat. Energy 2016, 1, 16184, https://doi.org/10.1038/nenergy.2016.184.

[8]. Lin, Z. M.; Xiang, L. Z. Supercapacitor characteristics of Co(OH)₂ synthesized by deposition transformation. Chin. J. Inorg. Chem. 2002, 18, 513-517. https://doi.org/ 10.1007/s11581-016-1829-4

9. Minor editing of the English language is recommended.

Reply: Thanks for your kind suggestion. We have revised the English language of our manuscript carefully and highlighted changes in red. Some corrections were listed but not limited as followed：

1. “urea polluted” was changed into “urea-polluted” (please see the Page 1, line 37).
2. “proved” was changed into “proven” (please see the Page 2, line 56).
3. “high oxidation state” was changed into “a high oxidation state” (please see the Page 2, line 69).
4. “it is” was deleted (please see the Page 3, line 120).
5. “is” was changed into “was” (please see the Page 3, line 121).
6. “for 3-5 times” was changed into “3-5 times” (please see the Page 3, line 124).
7. “the” was added (please see the Page 3, line 127).
8. “to” was changed into “as” (please see the Page 3, line 127).
9. “nickel cobalt” was changed into “nickel-cobalt” (please see the Page 7, line 264).
10. “reduced” was changed into “decreased” (please see the Page 9, line 319).

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Authors,

The subject of the study is interesting and topical, with scientific and practical importance.

The introduction is presented correctly, in accordance with the subject. Numerous scientific articles, in concordance to the topic of the study, were consulted.

Methodology of the study was clearly presented, and appropriate to the proposed objectives.

The obtained results are important and have been analyzed and interpreted correctly, in accordance with the current methodology.

The Discussion chapter is missing.

The results are not analyzed and discussed in relation to other studies in the field. No bibliographic source is cited in the "Results and discussion" chapter.

The scientific literature, to which the reporting was made, is recent and representative in the field.

Some suggestions and corrections were made in the article.

The following aspects are brought to the attention of the authors.

1.

Chapter title

Page 3, row 97

"2. Materials and Methods" instead of “2. Methods”

Please check Instructions for Authors, and Microsoft Word template, Processes journal

 

2.

The Discussion chapter is missing.

However, Instructions for Authors, and Microsoft Word template, Processes journal, recommend a distinct chapter "4. Discussion"

The “Results” chapter is considered “3. Results and Discussion”

The results are presented appropriately.

The results are not analyzed and discussed in relation to other studies in the field. No bibliographic source is cited in the "Results and discussion" chapter.

It is recommended that the obtained results, very valuable by the way, be discussed in relation to other studies in the field.

This involves bibliographic sources, and this will modify the content of the References chapter, with new bibliographic sources.

 

3.

References

According to Instructions for Authors, and Microsoft Word template, Processes journal

“Author 1, A.B.; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.”

“Include the digital object identifier (DOI) for all references where available.”

e.g.

Page 10, rows 328 - 329 

“Rollinson, A.N; Jones, J.; Dupont, V.; Twigg, M.V. Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply. Energy Environ. Sci. 2011, 4(4), 1216-1224. http://dx.doi.org/10.1039/c0ee00705f”

Instead of

“Rollinson, A. N; Jones, J.; Dupont, V.; Twigg, M. V., Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply. Energy & Environmental Science 2011, 4(4), 1216-1224.”

 

It is recommended to check the entire References chapter, and correct where necessary.

 

4.

Other minor corrections were suggested in the article

 

Comments for author File: Comments.pdf

Author Response

Reviewer #2: The subject of the study is interesting and topical, with scientific and practical importance. The introduction is presented correctly, in accordance with the subject. Numerous scientific articles, in concordance to the topic of the study, were consulted. Methodology of the study was clearly presented, and appropriate to the proposed objectives. The obtained results are important and have been analyzed and interpreted correctly, in accordance with the current methodology. The Discussion chapter is missing. The results are not analyzed and discussed in relation to other studies in the field. No bibliographic source is cited in the "Results and discussion" chapter. The scientific literature, to which the reporting was made, is recent and representative in the field.

Some suggestions and corrections were made in the article. The following aspects are brought to the attention of the authors.

1. Chapter title Page 3, row 97 "2. Materials and Methods" instead of “2. Methods”

Please check Instructions for Authors, and Microsoft Word template, Processes journal.

Reply: Thanks for your kind suggestion. We check the Instructions for Authors, and Microsoft Word template of Processes journal, it is our neglect not to use the right template. The “2. Methods” was changed into "2. Materials and Methods", and the description of Materials used in this work was added in “2.1. Materials”. (please see the content in Page 3, line 97-103).

2. The Discussion chapter is missing. However, Instructions for Authors, and Microsoft Word template, Processes journal, recommend a distinct chapter "4. Discussion" The “Results” chapter is considered “3. Results and Discussion” The results are presented appropriately. The results are not analyzed and discussed in relation to other studies in the field. No bibliographic source is cited in the "Results and discussion" chapter. It is recommended that the obtained results, very valuable by the way, be discussed in relation to other studies in the field. This involves bibliographic sources, and this will modify the content of the References chapter, with new bibliographic sources.

Reply: Thanks for your valuable comments. The “Results and Discussion” chapter is changed into “Results”, and the “Discussion” chapter is added. We compared performance for electrocatalytic urea oxidation of NiCo-UMOFs with some reported Ni and Co-based catalysts, as summarized in Table R1. It is obvious from the table that NiCo-UMOFs achieved an onset potential of 0.32 V (vs. Ag/AgCl), which is lower than the other catalysts, even some composite catalysts. Moreover, NiCo-UMOFs achieved a higher current density of 13 mA cm^-2 toward UOR, which is also competitive among these UOR catalysts. From the LSV test in KOH and urea solution, the UOR electrocatalytic performance of Ni-Co bimetallic materials is better than Co-MOFs and Ni-MOFs, indicating the doping Co facilitates the UOR. From SEM images, NiCo-UMOFs presented a two-dimensional thin structure with a smooth surface, so it possessed both the characteristics of the bulk MOFs and a high percentage of exposed metal active sites on the surface, facilitating more interactions between active sites and urea and thus enhancing the UOR performance of NiCo-UMOFs. So, the good UOR performance of NiCo-UMOFs could be attributed to the cooperation of Co and Ni, the rich exposed catalytic sites, and huge surface area.

The relevant contents have been added in the revised manuscript, and were marked red (please see the content in Page 10, Table 1, line 327-342).

Table R1. Comparison of different catalysts for UOR performances.

Catalysts	Onset potential (V vs. Ag/AgCl)	Current density (mA/cm-2)	References
Ni(OH)2NS@NW	0.73	26	[1]
Ni/NiO nanosheets	0.38	10	[2]
NiO	0.33	20	[3]
FeCo2O4@Co3O4	0.42	10	[4]
NiO/Fe3O4@chitosan	0.45	35.4	[5]
Co dendrites	0.35	21	[6]
NiCo-UMOFs	0.32	13	This work

References

[1].  Yue, X.; Yao, S.; Li, Y.; Zhu, W.; Zhang, W., Wang, R., Wang, J., Huang, L., Zhao, D.; Wang, J., Surface engineering of hierarchical Ni(OH)₂ nanosheet@nanowire configuration toward superior urea electrolysis. Electrochimica Acta 2018, 268, 211-217. https://doi.org/10.1016/j.electacta.2018.02.059

[2].  Ji, X. Y.; Zhang, Y. X.; Ma, Z.; Qiu, Y. F., Oxygen Vacancy-rich Ni/NiO@NC Nanosheets with Schottky Heterointerface for Efficient Urea Oxidation Reaction. ChemSusChem 2021, 13, 5004-5014. https://doi.org/10.1002/cssc.202001185

[3].  Alex, C.; Shukla, G.; John, N. S., Introduction of surface defects in NiO with effective removal of adsorbed catalyst poisons for improved electrochemical urea oxidation. Electrochimica Acta 2021, 385, 138425. https://doi.org/ 10.1016/j.electacta.2021.138425

[4].  Gao, S. Q.; Fan, J. C.; Xiao, G. C.; Cui, K. X; Wang, Z. H.; Huang, T.; Tan, Z. C.; Niu, C. Q.; Luo, W. B.; Chao, Z. S., Synthesis of FeCo₂O₄@Co₃O₄ nanocomposites and their electrochemical catalytical performaces for energy-saving H₂ prodcution. Hydrogen Energy 2023, 48, 17147-17159. https://doi.org/ 10.1016/j.ijhydene.2023.01.196

[5].  Mahmoud, A.; Hefnawy, S. S.; Medany, R. M.; El-Sherif, S. A.; Green synthesis of NiO/Fe₃O₄@chitosan composite catalyst based on graphite for urea electro-oxidation. Materials Chemistry and Physics 2022, 290, 126603. https://doi.org/ 10.1016/j.matchemphys.2022.126603

[6]. Mohammad, A. A.; Hussain A.; Olabi, A. G., Enhancing the performance of direct urea fuel cells using Co dendrites Enas Taha Sayed, Applied Surface Science 2021, 555, 149698. https://doi.org/ 10.1016/j.apsusc.2021.149698

3. References. According to Instructions for Authors, and Microsoft Word template, Processes journal “Author 1, A.B.; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.” “Include the digital object identifier (DOI) for all references where available.” e.g. Page 10, rows 328-329. “Rollinson, A.N; Jones, J.; Dupont, V.; Twigg, M.V. Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply. Energy Environ. Sci. 2011, 4(4), 1216-1224. http://dx.doi.org/10.1039/c0ee00705f” Instead of “Rollinson, A. N; Jones, J.; Dupont, V.; Twigg, M. V., Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply. Energy & Environmental Science 2011, 4(4), 1216-1224.” It is recommended to check the entire References chapter, and correct where necessary.

Reply: Thanks for your kind suggestion. The Reference 1 was changed into “Rollinson, A.N; Jones, J.; Dupont, V.; Twigg, M.V. Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply. Energy Environ. Sci. 2011, 4(4), 1216-1224. http://dx.doi.org/10.1039/c0ee00705f”

We have also checked and corrected the formatting of entire references, please see the Reference in Page 11-13.

4. Other minor corrections were suggested in the article.

Reply: Thanks for your kind suggestion. We have corrected the manuscript in line with your suggestion, and the modification content is as follows：

1. The point behind 10% [3] was added (please see the Page 1, line 32).
2. “^[9,10]” was changed into “[9,10]” (please see the Page 1, line 38).
3. “etc. [19]” was changed into “etc [19].” (please see the Page 2, line 54).
4. “Methods” was changed into “Materials and Methods” (please see the Page 3, line 97).
5. “;” was changed into “.” (please see the Page 3, line 137).
6. “Fig. 5a.” was changed into "Figure 5a"; “Fig. 5b.” was changed into “Figure 5b” (please see the Page 9, line 318, 321).

Author Response File: Author Response.pdf

Reviewer 3 Report

The modern approach to solving environmental problems and problems, associated with alternative energy, is to solve these problems within one approach. This circumstance largely relates to the processing of urea into environmentally friendly substances, on the one hand, and the production of hydrogen as fuel, on the other. For this purpose, catalytic technologies are widely used, in which the vast majority use precious metals. In order to reduce the cost of technology, a lot of research is aimed at finding alternative materials. Among the latter, transition metals, in particular Ni, have the corresponding potential. At the same time, to enhance the process of catalytic oxidation of urea using Ni, various approaches are used to select the appropriate morphology of the material, its structure, component composition, etc. However, approaches associated with the use of systems based on Ni and MOFs are very limited, despite their potential attractiveness for the above urea recycling process. In this regard, this work makes contribution to fill this gaps in terms of expanding the understanding of the possibility of replacing expensive catalysts based on precious metals with cheaper, but competitive Ni and Co. Synthesizing different MOF structures with both Ni and CO, as well as Ni and Co separately the authors have found the optimal synthetic strategy for the most efficient systems for UOR. Namely, among the used synthesized ultrasound (NiCo-UMOFs) and hydrothermal (NiCo-MOFs, Ni-MOFs and Co-MOFs) methods, the former is found to be the most efficient. It is found that this is attributed to the specific 2D morphology of the NiCo-UMOF structure, enabling as high as 45% of urea removal.

The strength of the work: Detail and controlled sample preparation, wide set of characterization tools enabling variety of data regarding the state of the sample and its electrocatalytic performance, their detail interpretation and analysis. Quite high applied relevance.

The weakness: The work would gain if for better understanding of the effect of the charge transfer within Ni-Co-MOF, affecting the catalytic activity of Ni, the analysis of Ni 2p photoelectron line shifts and shapes would be useful. Moreover that the authors claim in Section 2.4 the XPS as one of the characterization tools.

In general, the manuscript is scientifically sound with the appropriate design to address the issues under consideration. It is clear, relevant for the field and presented in a well-structured manner. The manuscript provides sufficient details that support the conclusions. Referencing is quite comprehensive and up-to-dated.

The work is suitable for publications in Processes in its present form.

Author Response

Reviewer #3: The modern approach to solving environmental problems and problems, associated with alternative energy, is to solve these problems within one approach. This circumstance largely relates to the processing of urea into environmentally friendly substances, on the one hand, and the production of hydrogen as fuel, on the other. For this purpose, catalytic technologies are widely used, in which the vast majority use precious metals. In order to reduce the cost of technology, a lot of research is aimed at finding alternative materials. Among the latter, transition metals, in particular Ni, have the corresponding potential. At the same time, to enhance the process of catalytic oxidation of urea using Ni, various approaches are used to select the appropriate morphology of the material, its structure, component composition, etc. However, approaches associated with the use of systems based on Ni and MOFs are very limited, despite their potential attractiveness for the above urea recycling process. In this regard, this work makes contribution to fill this gaps in terms of expanding the understanding of the possibility of replacing expensive catalysts based on precious metals with cheaper, but competitive Ni and Co. Synthesizing different MOF structures with both Ni and CO, as well as Ni and Co separately the authors have found the optimal synthetic strategy for the most efficient systems for UOR. Namely, among the used synthesized ultrasound (NiCo-UMOFs) and hydrothermal (NiCo-MOFs, Ni-MOFs and Co-MOFs) methods, the former is found to be the most efficient. It is found that this is attributed to the specific 2D morphology of the NiCo-UMOF structure, enabling as high as 45% of urea removal.

The strength of the work: Detail and controlled sample preparation, wide set of characterization tools enabling variety of data regarding the state of the sample and its electrocatalytic performance, their detail interpretation and analysis. Quite high applied relevance.

The weakness: The work would gain if for better understanding of the effect of the charge transfer within Ni-Co-MOF, affecting the catalytic activity of Ni, the analysis of Ni 2p photoelectron line shifts and shapes would be useful. Moreover that the authors claim in Section 2.4 the XPS as one of the characterization tools.

In general, the manuscript is scientifically sound with the appropriate design to address the issues under consideration. It is clear, relevant for the field and presented in a well-structured manner. The manuscript provides sufficient details that support the conclusions. Referencing is quite comprehensive and up-to-dated.

The work is suitable for publications in Processes in its present form.

Reply: Thanks for your kind suggestion. We have added the XPS measurements of NiCo-UMOFs and the results are displayed in Fig. R1. The survey spectrum (Fig. R1A) demonstrates the presence of Ni, Co, O, and C elements in NiCo-UMOF. In the Ni 2p spectrum (Fig. R1B), the characteristic peaks at 855.7 and 873.8 eV attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively, with the two satellite peaks at 861.6 and 880.1 eV [1], indicating Ni exist in Ni^3+/2+ forms in the NiCo-UMOFs. In the Co 2p spectrum (Fig. R1C), the characteristic peaks located at 781.1 and 797.4 eV are corresponding to Co 2p_3/2 and Co 2p_1/2, and the two satellite peaks were centered at 785.2 and 802.6 eV [2], revealing Co ions exist mainly in Co²⁺ forms in the synthesized NiCo-UMOFs. The above results proved the existence of electron transfer from Ni to Co, the introduction of Co was related to shifting the Ni^2+/3+ redox wave and enhanced the activity of Ni [3]. The C 1s spectrum (Fig. R1D) was divided into two peaks at 284.6 and 288.3 eV, which are corresponding to the bonds of C-C=C and O-C=O, respectively [4]. The O 1s spectrum (Fig. R2E) were indexed into two peaks, which attributed to the metal- oxygen bonds M-O-M (531.0 eV) in the metal oxides and the oxygen in -OH (532.8 eV) [5]. Thus, XPS results verify the formation of NiCo-UMOFs and the introduction of Co was related to shifting the Ni^2+/3+ redox wave.

The relevant contents have been added in the revised manuscript, and were marked red (please see the content in Page 6, Figure 3, line 221-234).

Fig. R1. XPS spectra of NiCo-UMOFs.: (A) Survey, (B) Ni 2p, (C) Co 2p, (D) C 1s, and (E) O 1s.

 

References

[1]. Veeramani, V.; Madhu, R.; Chen, S. M.; Sivakumar, M.; Hung, C. T.; Miyamoto, N.; Liu, S. B. NiCo₂O₄-decorated porous carbon nanosheets for high-performance supercapacitors. Electrochim. Acta 2017, 247, 288-295. https://doi.org/10.1016/j.electacta.2017.06.171

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have responded to all the questions and the revised manuscript is now accepted.