Hierarchical Design of Homologous NiCoP/NF from Layered Double Hydroxides as a Long-Term Stable Electrocatalyst for Hydrogen Evolution
, Lei Bai 1, Manman Duanmu 1, Lixin Wang 3,*, Haifeng Dong 1,*, Xiujuan Qin 1 and Guangjie Shao 1,2,*
Round 1
Reviewer 1 Report
The manuscript "Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution" reports HER electrocatalytic activity of NiCoP in form of nanowires on NiCoP nanosheets grown on nickel foam. Although authors didn’t provide detailed investigation and discussion about the effects of these nanostructures on the improvement of electrocatalytic stability and activity, manuscript contains sufficient electrochemical and microscopic data to support conclusions about catalytic activity and stability. Before publishing authors should do some minor improvements, listed below:
1. Terminology in lines 218-219 should be corrected; combination of two succesive steps is mechanism, not another step .
2. Terminology in lines 222-223 should be corrected; there are not two interfaces, so charge transfer cannot be "between interfaces", only at the interface between electrode and electrolyte.
3. In lines 228-229 authors claim that "the reduction of overpotential is closely related to the improvement of material conductivity". However, they didn't provide any measurement of the conductivity of the material itself. Rct is resistance to heterogenous charge transfer, not the resistance of the material.
4. Use of Pt counter electrode is strongly discouraged in experiments with noble-metal-free elecrtocatalysts. Authors should be aware of this, although it is not pronounced in alkaline as it is in acidic media (Sci Rep. 2022; 12: 9368).
English must be improved, especially in the title ("long-term electrocatalyst" sounds very strange) and Materials and Methods part.
Author Response
Guangjie Shao
College of Environmental and Chemical Engineering
Yanshan University
438 Hebei West Avenue,
Qinhuangdao 066004,
P.R. China
Tuesday, August 8, 2023
Dear Editors and Reviewers:
Thank you for your letter and for the Reviewer's comments concerning our manuscript entitled “Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution” (No.: catalysts-2536681). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have carefully read all the comments and made revisions to our manuscript accordingly. Revised portions are marked in yellow on the manuscript. We have also made modifications to improve the quality of the language in the manuscript. The main corrections in the manuscript and the responses to your comments are as follows:
Response to reviewers:
Reviewer #1:
The manuscript "Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution" reports HER electrocatalytic activity of NiCoP in form of nanowires on NiCoP nanosheets grown on nickel foam. Although authors didn’t provide detailed investigation and discussion about the effects of these nanostructures on the improvement of electrocatalytic stability and activity, manuscript contains sufficient electrochemical and microscopic data to support conclusions about catalytic activity and stability. Before publishing authors should do some minor improvements, listed below:
- Terminology in lines 218-219 should be corrected; combination of two successive steps is mechanism, not another step.
Response: Thanks for your careful review. We are very sorry for our inaccurate representation. The terminology used in the sentence in lines 218-219 (now lines 228-234) has been corrected in the revised manuscript.
Lines 232-237 of Page 6-7 “As shown in Figure 4b, the Tafel slope of NiCoP@NiCoP/NF-160 (84.2 mV dec-1) is lower than NiCoP@NiCoP/NF-120 (97.5 mV dec-1), NiCoP@NiCoP/NF-140 (96.5 mV dec-1), NiCoP@NiCoP/NF-180 (336.2 mV dec-1) and NF (146.4 mV dec-1), while only higher than Pt/C (23.6 mV dec-1), indicating that the HER mechanism of NiCoP@NiCoP/NF-160 is Volmer-Heyrovsky step, while the HER mechanism of Pt/C is Volmer-Tafel step, and their rate-determining steps are Heyrovsky step and Tafel step respectively [50].”
- Terminology in lines 222-223 should be corrected; there are not two interfaces, so charge transfer cannot be "between interfaces", only at the interface between electrode and electrolyte.
Response: Thanks for your careful review. Once again, we are very sorry for our misstatement. The terminology used in the sentence in lines 222-223 (now lines 236-238) has been corrected in the revised manuscript.
Lines 240-242 of Page 7 “The charge transfer resistance (Rct) is mainly the charge transfer resistance at the interface between the electrode and the electrolyte.”
- In lines 228-229 authors claim that "the reduction of overpotential is closely related to the improvement of material conductivity". However, they didn't provide any measurement of the conductivity of the material itself. Rct is resistance to heterogenous charge transfer, not the resistance of the material.
Response: Thanks for your careful review. We are very sorry again for our inaccurate expression. The terminology used in the sentence in lines 228-229 (now lines 243-245) has been corrected in the revised manuscript.
Lines 246-248 of Page 7 “Compared with the control samples, NiCoP@NiCoP/NF-160 still has the smallest Rct, which proves that the reduction of overpotential is closely related to the improvement of charge transfer ability.”
- Use of Pt counter electrode is strongly discouraged in experiments with noble-metal-free electrocatalysts. Authors should be aware of this, although it is not pronounced in alkaline as it is in acidic media (Sci Rep. 2022; 12: 9368).
Response: Thanks for the reviewer’s professional comments. In the research and development process of carbon-based electrode and Pt electrode which is more suitable for catalytic reaction, Prof. Chang Hyuck Choi et al., Prof. Gregory Jerkiewicz et al., and Benjamin Martindale all have conducted in-depth research and analysis. The deposition of Pt will affect the performance evaluation of the catalyst, but in the course of long-term use, the carbon electrode will be corroded, which will also affect the performance evaluation of the catalyst. Carbon-based counter electrodes (CEs) may be more appropriate in alkaline solution but, most importantly, in cases where CO does not bind to the work electrode material. However, the negative effects of Pt deposition and carbon corrosion products can also be effectively avoided based on the use of reaction cells such as two-cell H-type electrolyzers. These important results show that there is no simple one-size-fits-all solution when it comes to selecting components for electrocatalytic test devices. In the test process, although we did not emphasize this content in the experimental part, in the actual test process, due to avoid the greater impact of Pt deposition, we have been using the H-type three-cell electrolyzer. In addition, under this system, we also replaced Pt CE with graphite CE for testing, and no significant changes in the results of major material properties were found. In summary, we choose to use Pt electrodes for testing, and under the premise of current universality, it is also easy to compare the performance with other excellent works (J. Am. Chem. Soc. 2022, 144, 3411; Small 2022, 2107974; J. Mater. Chem. A, 2017, 5, 15940). The results for LSV curves, Tafel plot and EIS plot using a graphite electrode as CE are shown below. In future work, we will consider using graphite rods directly as counter electrodes, thank you again for your correction and sincere suggestion.
Figure R1. a) iR-corrected LSV polarization curves, b) Tafel slope and c) EIS plot of different catalysts tested by the carbon-based counter electrode (CE).
We tried our best to improve the manuscript and made changes in the manuscript. We list the changes and marked them in yellow in the revised manuscript. We carefully reviewed the language section of the manuscript and made appropriate modifications.
We appreciate for Editors/Reviewers' warm work earnestly and hope that the revision will meet with approval. Once again, thank you very much for your comments and suggestions.
Best regards,
Sincerely yours,
Guangjie Shao
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this work, the authors have investigated the Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution. This reaction is of interest. This study is executed very well and this manuscript describes interesting findings for the electrocatalytic properties of the proposed systems. I suggest that this manuscript is published after the authors address the following:
1) The authors claimed the Ni metal particles in addition to NiCoP@NiCoP/NF from XRD patterns but the role of Ni particles was explained in the manuscript.
2) Throughout the manuscript the authors discussed about the composite rather than the individual counterpart. However, I would suggest the authors to incorporate the electrocatalytic hydrogen generation of individual counterparts such as nanowires and nanosheets of NiCoP also. Hence, the article will have a broad readership. On the other hand, the authors can claim the importance of the composite material.
3) From the XPS study, I am able to see the minor amount of NiP and CoP. Precisely, the authors need to consider the minor amount of NiP and CoP on catalytic performance.
4) The authors should compare the electrocatalytic performance of NiCoP with morphologies.
5) There are several typos the authors should correct before publication.
English should be improved.
Author Response
Guangjie Shao
College of Environmental and Chemical Engineering
Yanshan University
438 Hebei West Avenue,
Qinhuangdao 066004,
P.R. China
Tuesday, August 8, 2023
Dear Editors and Reviewers:
Thank you for your letter and for the Reviewer's comments concerning our manuscript entitled “Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution” (No.: catalysts-2536681). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have carefully read all the comments and made revisions to our manuscript accordingly. Revised portions are marked in yellow on the manuscript. We have also made modifications to improve the quality of the language in the manuscript. The main corrections in the manuscript and the responses to your comments are as follows:
Response to reviewers:
Reviewer #2:
In this work, the authors have investigated the Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution. This reaction is of interest. This study is executed very well and this manuscript describes interesting findings for the electrocatalytic properties of the proposed systems. I suggest that this manuscript is published after the authors address the following:
1) The authors claimed the Ni metal particles in addition to NiCoP@NiCoP/NF from XRD patterns but the role of Ni particles was explained in the manuscript.
Response: Thanks for the reviewer’s comments. After carefully examining the content of the manuscript, we found that we do not have a clear statement about nickel particles. About XRD patterns, the strong diffraction peaks of Ni originate from the nickel foam substrate. We are sorry for any possible misleading made to the reviewers and readers. We have made minor modifications to the relevant content. The below sentence has been revised in the revised manuscript.
Lines 145-149 of Page 4 “As shown in Figure S1, except that 44.5 °, 51.8 ° and 76.4 ° correspond to the three strong peaks of Ni (111), (200) and (222) (PDF#87-0712) from NF substrate, the diffraction peaks at 41.0 °, 44.9 °, 47.6 ° and 54.4 ° correspond to the (111), (201), (210) and (300) crystal planes of NiCoP (PDF#71-2336), which proves that ternary NiCoP is successfully synthesized.”
2) Throughout the manuscript the authors discussed about the composite rather than the individual counterpart. However, I would suggest the authors to incorporate the electrocatalytic hydrogen generation of individual counterparts such as nanowires and nanosheets of NiCoP also. Hence, the article will have a broad readership. On the other hand, the authors can claim the importance of the composite material.
Response: Thanks for the reviewer’s valuable comments. We conducted supplementary tests on the electrochemical hydrogen evolution of single-component NiCoP (Figure S1), and added specific explanations in the relevant sections of the article. Figure S1 and the below sentences have been added to the revised manuscript.
Figure S1. SEM images of a) NiCoP nanosheets and b) NiCoP nanowires. c) LSV curves and d) Tafel plots of NiCoP nanosheets and NiCoP nanowires.
Lines 134-139 of Page 3-4 “We also prepared control samples for one-step hydrothermal-phosphating. Figures S1a and b show the SEM images of NiCoP nanosheets and NiCoP nanowires obtained through a one-step hydrothermal process, respectively. It can be seen that the NiCoP after one-step hydrothermal-phosphating exhibits distinct sheet or wire structures, providing a foundation for the establishment of composite structures.”
3) From the XPS study, I am able to see the minor amount of NiP and CoP. Precisely, the authors need to consider the minor amount of NiP and CoP on catalytic performance.
Response: Thanks for the reviewer’s valuable comments. Due to the absence of relevant diffraction peaks in XRD, it can be determined that NiP and CoP are extremely small amount or exist in amorphous form. Due to the extremely small amount of these possible substances, we did not discuss its role in composite materials. The existence of amorphous materials is usually accompanied by the generation of defects, and they have better corrosion resistance compared to crystalline materials (Adv. Mater. 2022, 34, 2110631; Nat. Commun. 2019, 10, 5692.). This may be one of the reasons why the material exhibits good activity and stability. We have also added relevant explanations in the revised manuscript.
Lines 171-173 of Page 5 “Although XPS results suggest the presence of a small amount of nickel/cobalt phosphide in the sample, the XRD results indicate that the main component responsible for the catalytic role is the NiCoP phase.”
Lines 277-280 of Page 8 “The long-term stability of the material is not only attributed to the establishment of the composite structure but may also be related to the small amount of amorphous nickel/cobalt phosphides detected by XRD and XPS characterization, thereby improving the corrosion resistance of the material.”
4) The authors should compare the electrocatalytic performance of NiCoP with morphologies.
Response: Thanks for the reviewer’s valuable comments. We compared the morphology and electrocatalytic performance of NiCoP through supplementary characterization and testing in Figure S6. The morphology and performance of the single phase NiCoP are consistent with expectations. The below sentences have been added in the revised manuscript.
Figure S1. SEM images of a) NiCoP nanosheets and b) NiCoP nanowires. c) LSV curves and d) Tafel plots of NiCoP nanosheets and NiCoP nanowires.
Lines 223-224 of Page 6 “Compared with single phase NiCoP, NiCoP composite structure obviously has lower overpotential (Figure S1c).”
Lines 237-239 of Page 7 “Similarly, compared to single phase NiCoP, NiCoP composite structures also exhibit faster reaction kinetics (Figure S1d).”
5) There are several typos the authors should correct before publication.
Response: Thanks for the reviewer’s considerable reminding. We checked the full text for typos and corrected them in the revised manuscript. Some of the changes are listed below:
Line 18 of Page 1: conduction-----transfer
Line 41 of Page 1: substitute-----alternatives
Line 62 of Page 2: has-----displays
Line 83 of Page 2: micro morphology-----micromorphology
Line 86 of Page 2: of-----in
Line 87 of Page 2: establishment-----establishments
Line 92 of Page 2: anion exchange technology-----anion-exchange technology
Line 96 of Page 2: in-situ grown-----in situ grown
Line 98 of Page 3: phosphate using-----phosphated by
We tried our best to improve the manuscript and made changes in the manuscript. We list the changes and marked them in yellow in the revised manuscript. We carefully reviewed the language section of the manuscript and made appropriate modifications.
We appreciate for Editors/Reviewers' warm work earnestly and hope that the revision will meet with approval. Once again, thank you very much for your comments and suggestions.
Best regards,
Sincerely yours,
Guangjie Shao
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript "Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution" introduced a simple method for the synthesis of non-precious electrocatalysts, consisting of ternary transition metal phosphide for high-performance HER catalysts in alkaline medium. Although the NiCoP nanowire@NiCoP nanosheet on nickel foam composite demonstrates its good catalytic activities for HER in an alkaline condition, many points are controversial. So, I recommend it for publication in this journal after some corrections. Issues are followed.
1. The authors claim the long-term stability of the heterogeneity of crystal structure, which looks a little doubtful. Since composites of transition metal derivatives and transition metals are usually degraded with time in an alkaline medium please give some more explanation for this point.
2. From the TEM images, there are no obvious crystal planes in the as-prepared NiCoP nanowire@NiCoP nanosheet samples, but in the XRD spectrum, there are obvious diffraction patterns attributed to the NiCoP@NiCoP crystal. So what’s the reason?
3. I recommend that the authors provide a reason why the NiCoP@NiCoP-160 is optimal for good HER activity.
4. Authors must provide XPS of the sample after stability measurement to know the chemical stability of the NiCoP@NiCoP-160 sample.
5. Since the authors have checked the electrochemical surface areas of different Cu2MoS4/Ti3C2Tx catalysts, I think they can get more useful information from the normalized current density by electrochemical surface area (Nat. Mater. 2015, 14, 1245; ACS Catal. 2020, 10, 1511; Mater. Today Phys. 2018, 7, 121). Based on this normalization, they can identify whether the outstanding overall electrode activity is from the increased surface area, or from the high intrinsic catalytic activity.
6. In the introduction part, it is suggested to discuss more backgrounds and structures of ternary transition metal derivatives and phosphides (Nickle phosphide, copper-nickel nitride) for electrocatalytic HER. There are recent studies (ACS Sustainable Chem. Eng. 2020, 8, 46, 17116–17123; ACS Sustainable Chem. Eng. 2023, 11, 13, 5229–5237, should be cited in the introduction part.
Author Response
Guangjie Shao
College of Environmental and Chemical Engineering
Yanshan University
438 Hebei West Avenue,
Qinhuangdao 066004,
P.R. China
Tuesday, August 8, 2023
Dear Editors and Reviewers:
Thank you for your letter and for the Reviewer's comments concerning our manuscript entitled “Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution” (No.: catalysts-2536681). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have carefully read all the comments and made revisions to our manuscript accordingly. Revised portions are marked in yellow on the manuscript. We have also made modifications to improve the quality of the language in the manuscript. The main corrections in the manuscript and the responses to your comments are as follows:
Response to reviewers:
Reviewer #3:
The manuscript "Hierarchical design of homologous NiCoP/NF from LDH as a long-term electrocatalyst for hydrogen evolution" introduced a simple method for the synthesis of non-precious electrocatalysts, consisting of ternary transition metal phosphide for high-performance HER catalysts in alkaline medium. Although the NiCoP nanowire@NiCoP nanosheet on nickel foam composite demonstrates its good catalytic activities for HER in an alkaline condition, many points are controversial. So, I recommend it for publication in this journal after some corrections. Issues are followed.
- The authors claim the long-term stability of the heterogeneity of crystal structure, which looks a little doubtful. Since composites of transition metal derivatives and transition metals are usually degraded with time in an alkaline medium please give some more explanation for this point.
Response: Thanks for the reviewer’s valuable comments. In long-term electrode reactions, transition metal ions will dissolve from the cathode to the liquid electrolyte, leading to a decrease in material lifespan (Science 2020, 369, 140). But the long-term stability in there refers to the potential stability displayed under constant current output, which represents the overall performance of the material during the testing process, i.e. the total dynamic evolution of the structure. As for the more specific structural dynamic evolution that can be revealed by more accurate ex-situ and in-situ characterization besides HRTEM, XRD, and XPS, we will conduct experiments and analysis in more in-depth work in the future, and the long-term stability here is only used to simply guide the performance stability exhibited by the material like the published work, Small 2022, 2204255; J. Mater. Chem. A, 2022, 10, 4181.
- From the TEM images, there are no obvious crystal planes in the as-prepared NiCoP nanowire@NiCoP nanosheet samples, but in the XRD spectrum, there are obvious diffraction patterns attributed to the NiCoP@NiCoP crystal. So what’s the reason?
Response: Thanks for the reviewer’s comments. First, low-magnification TEM can not observe obvious lattice fringes due to insufficient resolution. The low-resolution TEM image in Figure 1c was used to reflect a relatively independent microstructure. Then, the XRD spectrum was used to investigate the NiCoP@NiCoP crystal types. The combination of the two realizes the progression from geometric structure analysis to crystal structure analysis.
- I recommend that the authors provide a reason why the NiCoP@NiCoP-160 is optimal for good HER activity.
Response: Thanks for the reviewer’s valuable comments. We have added a description of the reasons for the good HER activity of optimal sample in the revised manuscript.
Lines 293-298 of Page 8 “The improvement of HER activity of the catalyst is closely related to the number of accessible active sites, the intrinsic activity, the efficiency of charge and mass transfer and the stability displayed by active substances. After being optimized by temperature and P introduction, the homologous NiCoP/NF catalyst has a more suitable electronic structure, more stable structure characteristics, and higher conductivity, thereby demonstrating good HER activity.”
- Authors must provide XPS of the sample after stability measurement to know the chemical stability of the NiCoP@NiCoP-160 sample.
Response: Thanks for the reviewer’s valuable comments. We supplemented the XPS characterization of the sample after stability testing and presented the results in Figure 5. An explanation of relevant content has been added to the manuscript. The Figure 5 and the below sentences have been revised and added in the revised manuscript.
Figure 5. The a) SEM, b) TEM, c) XRD images and d-f) XPS spectra of NiCoP@NiCoP/NF-160 after 500 h stability test.
Lines 289-293 of Page 8 “From the XPS spectra after stability, it can also be seen that there is no significant change in the chemical state of the material before and after testing (Figures 5d-f). It is worth noting that the P-Metal peaks appeared in the XPS of P 2p with a significant intensity, which further proved that the absence of P-Metal peaks before testing was caused by surface oxidation of the sample.”
- Since the authors have checked the electrochemical surface areas of different Cu2MoS4/Ti3C2Tx catalysts, I think they can get more useful information from the normalized current density by electrochemical surface area (Nat. Mater. 2015, 14, 1245; ACS Catal. 2020, 10, 1511; Mater. Today Phys. 2018, 7, 121). Based on this normalization, they can identify whether the outstanding overall electrode activity is from the increased surface area, or from the high intrinsic catalytic activity.
Response: Thanks for the reviewer’s valuable comments. We supplemented the LSV curves of several samples normalized by ECSA in Figure S5 to compare the improvement in intrinsic activity of the materials. The Figure S5 and below sentences have been added in the revised manuscript.
Figure S6. ECSA-normalized polarization curves of NiCoP@NiCoP/NF-120, NiCoP@NiCoP/NF-140, NiCoP@NiCoP/NF-160 and NiCoP@NiCoP/NF-180.
Lines 263-268 of Page 7 “To investigate the intrinsic activity of catalysts, we performed ECSA normalization on the LSV curves (Figure S6). The results indicate that the NiCoP@NiCoP/NF-160 exhibits higher intrinsic catalytic activity compared to the control samples. Based on the above results, the joint improvement of intrinsic activity, accessible active sites and charge transfer ability lead to a significant decrease in the HER overpotential of the catalyst.”
- In the introduction part, it is suggested to discuss more backgrounds and structures of ternary transition metal derivatives and phosphides (Nickle phosphide, copper-nickel nitride) for electrocatalytic HER. There are recent studies (ACS Sustainable Chem. Eng. 2020, 8, 46, 17116–17123; ACS Sustainable Chem. Eng. 2023, 11, 13, 5229–5237, should be cited in the introduction part.
Response: Thanks for the reviewer’s valuable comments. We have carefully read these articles. These are papers of great scientific significance and that are related to our manuscript. We have added the references in the revised manuscript and provided a brief description to improve the quality of the manuscript. We have added the references (Ref. 20 and Ref. 21) in the revised manuscript and provided a brief description to improve the quality of the manuscript. The below sentences have been added in the revised manuscript.
Lines 64-70 of Page 2 “Researchers often use various methods, such as constructing hetero-interfaces or doping heteroatoms, to expose more active centers and achieve flexible and adjustable electronic structures of the catalysts. Jeon et al. used continuous 3-dimensional (3D) nanopatterning to synthesize Ni2P-coated 3D Ni composite nanostructures with Cu atom-doped Ni3N [20,21]. The prepared catalyst exhibits good HER performance due to optimized Gibbs free energy of H* (ΔGH*), fast electron transfer channels and highly exposed active sites.”
We tried our best to improve the manuscript and made changes in the manuscript. We list the changes and marked them in yellow in the revised manuscript. We carefully reviewed the language section of the manuscript and made appropriate modifications.
We appreciate for Editors/Reviewers' warm work earnestly and hope that the revision will meet with approval. Once again, thank you very much for your comments and suggestions.
Best regards,
Sincerely yours,
Guangjie Shao
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript can be published in its present form as the authors satisfied all the comments.
