Peer review of: Probing the active site in single-atom oxygen reduction catalysts via operando X-ray and electroc (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
The manuscript studies the ORR mechanism through operando X-ray absorption and electrochemical impedance spectroscopies over a new 
developed Co-N-C catalyst (pyrolyzed Vitamin B12). The operando XAS in soft XAS (s-XAS) ranges, coupled with first-principle calculations, is 
important for providing new insight into metal active-site geometry and electronic structure evolution during ORR. However, the reviewer has 
some critical concerns to the results of this work. 
1. The structure of the fresh catalyst used in this work is unclear. Characterizations such as SEM, TEM, HAADF-STEM, XPS, XRD, Raman of the 
catalysts should be provided. 
2. For the operando Co L3,2-edge NEXAFS results. The authors stated that “Co was moderately oxidized during catalytic reaction.”, however, 
the author then state that “the oxidation state of Co was maintained at nearly +2 during ORR.”, Why? Moreover, the conclusion should be 
carefully discussed in combing with the operando Co K-edge XANES results. Why there is no observed changes in the K-edge of operando 
XAS? 
3. At 0.2 V, why does the peak of operando Co XAS spectra return to its original position? The XAS spectra of the catalyst after operando 
measurement should be provided. 
4. The local environment of Co determined through fitting the EXAFS data is insufficiency. The Co-O species should be confirmed by more 
evidence. More important, why there is no changes could be observed for operando Co XAS spectra at different applied potential (1.0, 0.8. 
0.6. 0.4…)? 
5. Kinetic experiments should be performed to studies the rate-determining step, which is important to understand the intermediate Co-O 
species captured by operando XAS. 
6. Figure 3 revealed the evolution of Co2+-oxo intermediate state. Different Co2+-oxo intermediates were captured at different applied 
potential? Why? Moreover, different Co2+-oxo intermediate state will also result in the changes of Co L3,2-edge NEXAFS, how does the 
author discuss the changes of oxidation state and coordination environment of Co at the same time? 
7. The English usage should be carefully polished. Many grammatical mistakes could be found even in the Abstract. For instance, “Our results 
revealed preferential adsorption of oxygen at the Co2+ center with end-on coordination forming an oxo-like species.”, “the charge transfer 
mechanism between the catalyst and reactant enabled further realization of Co-O species formation.” Too many such kind of grammatical 
mistakes all through the manuscript should be thoroughly revised. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The manuscript presents a study of the ORR mechanism of pyrolyzed Vitamin B12 using operando X-ray absorption spectroscopy coupled with 
electrochemical impedance spectroscopy, which enables operando monitoring of the oxygen-binding site on the metal center. As it was claims 
that the results revealed preferential adsorption of oxygen at the Co2+ center with end-on coordination forming an oxo-like species. The 
topics is really interesting and would have significant impact to the catalysis research. 
 
Overall the experimental findings are solid and convincing. However, there is consistence problem between the Co K-edge EXAFS and Co L-
edge XAS conclusion. Co K-edge EXAFS reveals that the elongation of the Co–N bond was due to the distortion of the originally square planar 
Co–N4/C configuration, which arose from the displacement of the Co atom from the Co–N4/C plane induced by the oxygen-based adsorbate, 
such as O2, O−O, OH, or H2O. There is no clear evidence for the chemical bonding or adsorption of O2, O−O, OH, or H2O to Co metal center, 
as shown in figure 2(d), while Co L3-edge XAS peak in figure 3(a) presents energy shifts being assigned to the different oxygenate species 
based on the FEFF calculation. There may be O2, O−O, OH, or H2O around Co metal center, but strength of the interaction is reflected 
different from these two techniques. 
 
Also, no details on the FEFF calculation are given in regards to the pyrolyzed Vitamin B12. 
 
It is not clear to the reviewer why the authors claimed operando X-ray absorption spectroscopy coupled with electrochemical impedance 
spectroscopy. There are only a few selected potentials were set to record the Co K-edge EXAFS and Co L-edge XAS.