Carbonaceous FexP Synthesized via Carbothermic Reduction of Dephosphorization Slag as Hydrogen Evolution Catalyst for Water Splitting
Round 1
Reviewer 1 Report
The authors have corrected and answered all issues rised by the previous review. The work presented is of high quality and is recommended for publication. The reviewer would like to draw the attention only to one additional point. From the presented XRD spectrum, the sample obtained by high temperature carbothermal reduction presents two phases FeP and Fe2P. No obvious diffraction peak of graphitized carbon was observed and it is correctly stated that carbon may exist in amorphous phase. Nevertheless at the conclusion and electrochemical measurements the increase in performance (electrocatalytic and conductivity) is attributed to the presence of the carbon matrix. Therefore the reviewer would like the authors to clearify why they think the amorphous phase of carbon could lead to this? The reviewer rather thinks the improvement in electrocatalytic activity and conductivity is more related to the FeP biphase. The Fe2P has a lower Fermi energy. Please comment on this.
Author Response
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Reviewer 2 Report
Authors developed iron-phosphides catalysts for hydrogen evolution reaction, but additional information and analysis are required for their characterizations using XRD and EDS. These are important for supporting their results prior to conclusions. Comments and questions are the following.
1. Page 4, line 163-164: Authors mentioned EDS surface scanning showed Fe and P elements are evenly distributed and concluded FexP materials formed. They have to include the Fe and P element mapping data first in this manuscript. Concluding anything without data would be improper.
2. Figure 1. They need to show the tick numbers of X-axis in (d),(e) and (f).
3. Once they obtained the EDS spectra in Figure 1, their atomic or weight % composition would be also available. They need to provide them for supporting the similar results from ICP analysis in Table 1. If they’re different, authors need to explain the reason.
4. Figure 2, XRD: It is difficult to agree with their conclusion that FexP is FeP nanocrystalline structure. Authors analyzed the XRD patterns of FexP is very consistent with FeP, but the patterns, especially in the region between 30 and 40 degree, are not matched to the results in Refs. [21,22] and reference JCPDS data. Authors need to interpret them again. By the way, their analysis on the XRD patterns of FexP/C looks good.
5. It is recommended to calculate grain sizes by using Scherrer equation. Those would be very meaningful when compared to the particle sizes in SEM images in Figure 1.
6. How much was the double-layer capacitor (Cdl) value of Pt/C?
7. TOF: When they calculated the TOF values, it seems they used the same number of active sites for both FexP/C an FexP. If so, it may be improper as they are different in microstructure, composition and crystal phase structure as they mentioned on page 5, line 189, according to the characterization results. It is unlikely that both FexP/C and FexP have the same number of active sites per area. As they have different crystal structures, authors need to measure or calculate their numbers of active sites first. If they followed the formula in Ref. [27], what kinds of unit cells were considered for FexP/C and FexP, respectively?
8. Lines 163-164 are repeated in lines 167-169. It is recommended to rephrase the sentence to avoid any issue.
Author Response
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Round 2
Reviewer 2 Report
Authors replied reviewer’s comments and questions well and revised the manuscript according to them. Especially it is great that authors found additional JCPDS reference data (Fe3P) for FexP and included its diffraction pattern in Figure 2. Additional questions are the following.
1. Figure 1: It is good that authors added element mapping and composition data also in Figure 1. One more question is about their atomic ratios. When the weight percentages are converted to atomic compositions, the atomic ratios of Fe:P in FexP and FexP/C seem to be 2.23:1 and 0.93:1, respectively. Those are about 30% lower than the stoichiometric values of Fe3P (3:1) and FeP/Fe2P (1.5:1 for 50/50, 1.33:1 for 66:33, 1.67 for 33:66). What could be the reason for them?
2. Figure 1(b,c): EDS element mappings don’t match to their morphologies observed in SEM images. For example, the center area in (c) is denser diagonally, but weaker in P mapping. the right side area in (b) is denser, but weaker in P mapping. Does these mean the P compositions in FexP and FexP/C are not homogeneous in a few micron scales? Otherwise, any image drifting did happen during the mapping? If so, they need to measure them again.
3. Some typos are still found. It is strongly to check them all again.
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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The manuscript submitted by S. He et al. reports on interesting results regarding FexP/C catalyst for alkaline HER. Article is well written and organized, results are mostly well presented. I suggest it to be accepted for publication in Nanomaterials after following corrections are made. They are given below in the form of bullets:
- Manuscript contains occasional typos which in some places make reader confused. I suggest that authors take care of this by careful editing of these typos. Also, article needs only a slight polishing of English in some places (representative example is the very first sentence in the Abstract).
- Sentence in the Introduction (p.2 line 50-53) starting with Since the Gibbs.... needs editing.
- Authors say that dephosphorization slag contains Ni, Mn and S. Could these "impurities" impact the HER catalysis? It is well-known that Ni is very active for HER, especially in alkaline.
- How the authors controlled weight ratio of active compound (FexP) vs carbon in FexP/C catalyst? I think this is important to know, because further on it has impact on the loading of FexP during electrocatalytic measurements. Also, it influences the comparison with FeP.
- There are ICP results. for both FexP and FexP/C So can this be somehow connected with the loadings of the active compounds (FexP) in these two catalysts when studying HER?
- Also loading of Pt/C should be known. Amount of active compound is very important for such measurements.
- Authors report very low Tafel slope for Pt/C in alkaline media of 31 mV/dec and they cite two references (7 and 24) to support this. However, by checking these two citations, I can't find data supporting that Tafel slope on Pt in alkaline media is around 30 mV/dec, which is value common for Pt in acid media. Authors should double check this, and check if their Tafel slope for Pt/C is indeed around 30 mV/dec. For the Tafel slope for Pt in alkaline media , I suggest authors to check the following work: S. Štrbac et al, Electrochimica Acta 306 (2019) 18-27.
- I think it would be nice if authors add some equations or explanation about methodology for TOF calculations. Also, TOF for Pt/C would be nice to add as it provides good comparison between state-of-the-art and the new FexP/C catalyst.
- Regarding stability, authors show comparison for FexP and FexP/C. They say that FexP shows "obvious attenuation" of the activity, but they don't mention the change in overpotential at 10 mAcm-2 (which they mention for FexP/C). I think that the attenuation for FexP is not that pronounced and is only slightly higher than for FexP/C. In other words, activity loss due to the stability test is similar. Authors should provide for both catalysts the increase of overpotential at 10 mA cm-2 for proper comparison.
Reviewer 2 Report
In this manuscript, the authors prepared FexP/C electro-catalyst for the HER application. Their results show that the prepared FexP/C is composed of FeP-Fe2P mixed nanocrystals supported on amorphous carbon, which exhibited an overpotential of 145 mV for driving a cathodic current density of 10 mA cm−2 (η10). Besides, there is no obvious attenuation for their FexP/C electrode after 3000 cycles of LSV test. Above achievement seems to be just general results in the research field of HER. Furthermore, the quality of their material analysis data is poor (e.g., Figure 1, Figure 2, and Table 1). Overall, this work lacks substantial impact on the research field of HER. Also, this work falls short in providing technical innovation for preparing highly-performanced HER electro-catalysts. For these reasons, I could not be able to recommend this manuscript to be published in “Nanomaterials (5year-IF: 5.346)”.
