Self-Standing Hierarchical Porous Nickel-Iron Phosphide/Nickel Foam for Long-Term Overall Water Splitting
Round 1
Reviewer 1 Report
This paper reports the study Electrolytic water splitting is a promising path for production of clean hydrogen when combined with green electric power, such as photovoltaic and wind power. However, the high current water electrolysis is mainly dependent on utilization of Pt, Ru and other expensive materials. While the transition metal-based catalysts still need improvement in electrocatalytic activity and stability. Here, we present preparation of economic and scalable electrode materials, NiFeP/NF, with hierarchical porous structure for overall water splitting as both anode and cathode. An overall potential of 1.85 V for the current density of 100 mA cm-2, and a long lifetime of 700 h, were achieved by NiFeP/NF as both anode and cathode. The nanostructures of the composite phosphides were investigated and the spent elelctrode after long term electrolysis were characterized to investigate the long-term failure mechanism of the phosphides. Surface shedding and reconstruction theories were proposed for the failure of NiFeP/NF cathode and anode in long term electrolysis, respectively. Furthermore, TiO2 coating was proved to be an efficient strategy to elongate the life time of the phosphide electrodes, which shows a slow current decline rate of 0.49 mA·cm-2h-1. The review results are interesting and informative, but some data were not well presented. Details are listed below.
1) The authors synthesized NiFeP/NF and NiFeP/TiO2/NF. However, data are insufficient to describe the structure of this sample. To confirm the correct alloy structure, cross-sectional compositional line profiles and EDS mapping data must be added.
2) The author carried out the HER, OER by the synthesized catalyst. However, the improved activity should be explained by adding comparative data on the electrochemical catalytic activity of general catalyst Pt/C.
3) The authors synthesized NiFeP/TiO2/NF and NiFeP/NF. The author presented the compositional analysis of metals weight. However, synthesized sample metals weight is not accurate after reaction. It is necessary to add data by analyzing it with ICP-AES and EDS.
Comments for author File: 
Comments.pdf
This paper reports the study Electrolytic water splitting is a promising path for production of clean hydrogen when combined with green electric power, such as photovoltaic and wind power. However, the high current water electrolysis is mainly dependent on utilization of Pt, Ru and other expensive materials. While the transition metal-based catalysts still need improvement in electrocatalytic activity and stability. Here, we present preparation of economic and scalable electrode materials, NiFeP/NF, with hierarchical porous structure for overall water splitting as both anode and cathode. An overall potential of 1.85 V for the current density of 100 mA cm-2, and a long lifetime of 700 h, were achieved by NiFeP/NF as both anode and cathode. The nanostructures of the composite phosphides were investigated and the spent elelctrode after long term electrolysis were characterized to investigate the long-term failure mechanism of the phosphides. Surface shedding and reconstruction theories were proposed for the failure of NiFeP/NF cathode and anode in long term electrolysis, respectively. Furthermore, TiO2 coating was proved to be an efficient strategy to elongate the life time of the phosphide electrodes, which shows a slow current decline rate of 0.49 mA·cm-2h-1. The review results are interesting and informative, but some data were not well presented. Details are listed below.
1) The authors synthesized NiFeP/NF and NiFeP/TiO2/NF. However, data are insufficient to describe the structure of this sample. To confirm the correct alloy structure, cross-sectional compositional line profiles and EDS mapping data must be added.
2) The author carried out the HER, OER by the synthesized catalyst. However, the improved activity should be explained by adding comparative data on the electrochemical catalytic activity of general catalyst Pt/C.
3) The authors synthesized NiFeP/TiO2/NF and NiFeP/NF. The author presented the compositional analysis of metals weight. However, synthesized sample metals weight is not accurate after reaction. It is necessary to add data by analyzing it with ICP-AES and EDS.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Review attached
Comments for author File: Comments.pdf
Author Response
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Reviewer 3 Report
The article entitled "Self-standing hierarchical porous nickle-iron phosphide/nickel foam for long-term overall water splitting" is focused on obtaining an electrode material for the water splitting reaction, capable, firstly, of serving as both an anode and a cathode, and secondly, of remaining and being stable for a long (200-700 hours) of continuous operation. It can be concluded that these tasks were successfully solved by the authors of the work. At the same time, cheap substrates that do not contain noble metals were used, and the efficiency, nevertheless, turned out to be very good, perhaps inferior to the test sample from Ru-Ir-Pt only in stability. The manuscript leaves a good impression, the authors examined their electrocatalyst in detail, for example, using a complex method to obtain a FIB sample. To increase the stability, the authors even deliberately violated the initial coherent logic of their work: a TiO2 nano-network was added to the sample (and this sample was no longer studied in such detail) but in the end, the desired stability was obtained, and, consequently, higher prospects in the applied aspect. Understanding this, the manuscript has only lot of minor remarks, it is as follows:
1. Great respect to the authors for the fact that they provide schemes that facilitate the understanding of the work. To improve on Figure 1 in the last sample, it is necessary to increase the font of the explanatory inscriptions and highlight the phosphorus layer more clearly (now the layer is similar to the third sample).
2. Section 2.4. The description is very unclear and confusing. What and where is “L shape”? Was the sample a square with 5 mm sides? Why is the thickness 2.5mm if the thickness of the original sample is 1mm (Line 121)? How it goes with Figure 2c. It is better to give a scheme here.
3. Section 2.5 and 2.6. There is no indication of the equipment manufacturer.
4. Line 46. The word “rate" is not applicable in this context.
5. Page 6. Check again which of the Figures is 2 and which is 3.
6. Figure 2c. Add a ruler or scale to the photo.
7. Line 257 and in other places too. The observed process is not just catalytic, but electrocatalytic.
8. Figure 7. You should increase the font or the shapes themselves.
9. Line 249-251. “…According to the theory of the influence of bimetallic phosphides on the adsorption energy of reactants, the adsorption energy of H on the two metal phosphates is positive and negative, respectively.” This statement should be supported by a reference.
10. Line 293. “…After cathodic HER catalysis… ”. The phrase is constructed incorrectly.
11. Line 301 and below. The explanation that hydrogen bubbles destroy the sample is incomprehensible. The shape of the bubbles depends on and is obtained under the pressure of the external environment (that is, here from the electrolyte) and mechanical friction of the bubbles on the electrode should be minimal. It would be nice to give a link here if this is not the case. On the other hand, it is widely known that many metals are capable of strongly absorbing hydrogen (hydrogenating), which leads to embrittlement of metals (this is a known problem) and destruction of the structure. Even more effective than when the surface is oxidized on an opposite electrode. Over the long period of the experiment and the huge volume of hydrogen released (as the authors note, twice as much as oxygen), such an effect can play a major role.
12. The designation NiFeP/TiO2/NF (Line 335 and elsewhere) and is not correct. The TiO2 layer (more precisely, the network) does not lie under the NiFeP, but is located on top of it. And since there is no complete shielding (this is only a network), we can assume that NiFeP and TiO2 lie in the same layer. That is, the best spelling (and more understandable for readers) will be: NiFeP-TiO2/NF or
TiO2-Ni Fe P/NF.
13. Figures 5 and 8. This is an optional remark to improve the work of the authors in the future. It is also recommended to carry out mapping of elements. This would finally remove all questions about changing samples.
14. Figure 9. It is recommended for better perception to reduce the number of values on the X axis for (a) and (b) and on the Y axis for (a).
15. This is an optional remark to improve the work of the authors in the future. According to Figure 9, stability for most of the samples studied occurs somewhere around 140 hours. It would be interesting to see figures like 5 and 8 for samples after 1 hour, 10, 50, 140 hours. The authors could get an excellent and beautiful dynamic picture of the change in the state of the surface.
16. In Conclusion (Line 365) what is :”scalable preparation method”?
17. The authors should once again carefully proofread the entire manuscript to search for annoying typos, for example, ”stratagy” (Line 362).
18. The list of references is not designed according to MDPI rules. It is unclear why duplicate references numbers? The DOI index does not contain any of the references. What is the [J] at the end of the title of each article?
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Good!!
Good!!
