The Challenge of Achieving a High Density of Fe-Based Active Sites in a Highly Graphitic Carbon Matrix

Round 1

Reviewer 1 Report

This paper deals with a very detailed physicochemical study regarding the Fe-based active sites useful as catalysts for ORR in acid and alkaline conditions.

Author Response

we thank the reviewer for his/her positive comments .

Reviewer 2 Report

In their manuscript, Li et al. Report on the achievement of an  high density of Fe-based active sites in a graphitic carbon matrix. The degree of innovation is good, as well as the potential impact. The paper is generally well written. However, some amendments are necessary before publication.

1)It is impossible to visualize experimental points in Figure 3. The marker is nearly invisible.

2)When discussing about the reduction of Pt content in electrodes, the authors miss to cite some milestones on Pt-based alloys [1-6].

3)ORR is introduced without any citation. The addition in the bibliography of some review papers could be useful for non-expert readers [7-9].

[1]Surface segregation and oxidation of Pt3Ni(111) alloys under oxygen environment, Catal. Today 260 (2016) 3.

[2]The Pt-enriched Pt3Ni alloy surface and its excellent catalytic performance in hydrolytic hydrogenation of cellulose, ChemSusChem 7 (2014) 1415 

[3]Unveiling the Oxidation Processes of Pt3Ni(111) by Real-Time Surface Core-Level Spectroscopy, ChemCatChem 8 (2016) 713.

[4]Role of Transition Metal in Fast Oxidation Reaction on the Pt3TM (111) (TM = Ni, Co) Surfaces, Adv. Energy Mater. 3 (2013) 1257.

[5]Layered PtTe2 Matches Electrocatalytic Performance of Pt/C for Oxygen Reduction Reaction with Significantly Lower Toxicity, ACS Sustainable Chemistry & Engineering 6 (2018) 7432.

[6]Tailoring the Surface Chemical Reactivity of Transition-Metal Dichalcogenide PtTe2 Crystals, Adv. Funct. Mater. 28 (2018) 1706504.

[7]Understanding catalytic activity trends in the oxygen reduction reaction, Chem. Rev. 118 (2018) 2302.

[8]A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells, J. Mater. Chem. A 5 (2017) 1808.

[9]Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction, Chem. Soc. Rev. 44 (2015) 2168.

Author Response

see attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

The present manuscript reports a study describing the preparation of an innovative family of “Pt-free” electrocatalysts for the oxygen reduction. The manuscript then makes important efforts to sort out which moieties are involved in the active sites for the ORR at various pH values. The manuscript suffers from important shortcomings, that must be addressed before its publication can be recommended on “Catalysts”.

·       The first important shortcoming of the manuscript is that the authors do not try to quantify the number of the active sites, nor their relative activity in the different conditions (i.e., upon AW, and at different values of pH). Thus it is quite difficult to correlate reliably the trends observed in the electrochemical performance as determined by RDE measurements with the physicochemical properties. In the revised version of the manuscript, the authors must first quantify the bulk wt% of Fe, N and, if possible, O in the materials, e.g., by ICP-AES and microanalysis. Then, they must use these results together with the outcome of the advanced physicochemical characterizations (e.g., Moessbauer, XANES, TEM) to at least estimate qualitatively the relative amounts of iron oxide and FexN moieties in the ICM-FePhen3 and ICM-FePhen3-AW. The impact of the AW treatment in the relative amounts of active sites must then be discussed in the revised version of the manuscript, and correlated with the RDE performance. It is also highly advised that the authors carry out N2 physisorption analyses on both ICM-FePhen3 and ICM-FePhen3-AW; the information on the surface area must then be correlated (at least qualitatively) with all the other results to gauge the relative intrinsic activity of iron oxide and FexN moieties in the different measurement conditions.

·       The second important shortcoming of the manuscript is that there is no information whatsoever on the durability of the materials. This has very important implications. For instance, are the peaks observed in Figure 1(d) at ca. 0.7 V vs. RHE stable upon cycling? If not, this indicates that the measurement itself is affecting the physicochemical properties of the ECs, reducing the relevance of the physicochemical characterizations and the validity of the interpretations provided by the authors, with a particular reference to the prevalence of iron oxides.

·       The authors must also provide a broader overview of the literature, comparing their synthetic approach with other avenues proposed by other authors using different precursors. The following references could be taken into consideration, highlighting similarities and differences, placing particular emphasis on the sources used to provide the N and metal atoms to the ECs: Adv. Funct. Mater. 28, #1803973 (2018); Chem. Mater. 30, 2651–2659 (2018); Electrochim. Acta 280, 149–162 (2018); ACS Appl. Mater. Interf. 9, 32840-32850 (2017).

Minor points

·       The authors must better organize the manuscript, discussing the results corresponding to a given figure close to the figure itself. For instance, the results of Fig. 2(c) and Fig. 2(d) are discussed at the end of the manuscript.

·       The legend of Figure 1(d) must be improved: the distinction between the results obtained at pH = 1 and pH = 13 is not indicated clearly.

·       The labels indicating the assignment of the Raman peaks must be added to Figure 2(b) to improve readability.

·       Page 9, line 275. The authors must provide a clear explanation to the acronym DFT.

Author Response

We thank the reviewer for the constructive comments,

see attached file for our point by point responses and actions

Author Response File: Author Response.pdf