Peer review of: Engineering unsymmetrically coordinated Cu-S1N3 single atom sites with enhanced oxygen reduc (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
In this work the authors present a CuNxCy catalyst containing sulphur atoms and showing an 
enhanced oxygen reduction reaction activity than the sulphur-free compounds. The morphology and 
active site structure are mainly investigated by electron microscopy techniques, X-ray absorption 
spectroscopy and DFT calculations. This is an interesting system, but there are a number of puzzling 
issues: 
1) We know that X-ray absorption spectroscopy is a bulk technique, so how the authors can exclude 
the formation of copper sulphide species coexisting with CuNx sites, rather than a new compound with 
formula CuNxSy? This doubt is also fostered by the fact that the Cu-S bond length does not change 
under working conditions, and if CuS clusters are very small, they cannot be detected by TEM. 
2) For the same reason of the previous point, authors cannot claim that a Cu+1 is formed and it is the 
active centre. We know that under applied potential CuNC forms very small Cu nanoparticles 
(DOI:10.1038/s41467-018-02819-7; DOI: https://doi.org/10.1002/anie.201907994, even if this is 
the extreme case of very low potential applied ). This means that if a small fraction of Cu+2 is 
reduced to metallic copper, the average oxidation state will be lower than 2+. It is important to stress 
that EXAFS is not able to highlight a Cu-Cu contribution if the metallic fraction is small. Still, a peak 
around 2.5 Å appears in the FT-EXAFS spectrum at 0.75 V, and it is not necessary to find a peak 
exactly at same position of metallic copper to have a metallic-based cluster with maybe a different 
nature than that of crystalline copper. 
3) Therefore, on the basis of the issue 1) and 2) the DFT model proposed by the authors may not be 
valid. 
Other minor issues: 
4) Why for the estimation of the oxidation number of the catalyst, only Cu foil and CuO references are 
used (figure 16 of supplementary materials)? This kind of determination requires attention, because 
the oxidation state (related to the XAS edge position) is not absolute, and it depends on the specific 
compound. 
5) Why XAS spectra are just recorded between 0.75 and 1.05 V? 
6) Is the in-situ XAS spectrum reversible when you come back to the OCV potential? 
7) I personally do not appreciate images like Figure 1e with red circles around hypothetical sites. In 
many parts of these pictures the spots overlap and the hypothetical single-atoms site is not clear. 
8) The authors completely forget to mention a lot of literature from other worldwide research groups, 
and not only from the restricted geographical area of most of the authors in the bibliography of this 
manuscript. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The authors report on the synthesis, structural and electrochemical characterisation of Cu-N-C and 
Cu-S-N-C materials for oxygen reduction reaction in alkaline electrolyte. The main novel claim is the 
coordination of Cu by both S and N atoms, not only by N atoms. This seems to improve significantly 
the ORR activity in alkaline medium of single-atom Copper by tuning the binding of oxygen 
adsorbates. While demonstrating that the single Cu atoms are coordinated with a mix of N and S 
atoms versus only N atoms is not an easy task, the authors give convincing evidence for this. The ORR 
activity in alkaline is at par or higher than state of art Fe-N-C catalysts 
 
Major comments: 
 a) According to authors, the S L-edge XANES spectrum shows only S-C bonds apparently and no S-Cu 
signal (suppl. fig 15). Should there not be a specific signal for direct S-Cu binding in this spectrum, if 
the claim of CuS1N3 coordination is correct? 
 
b) The key data that directly supports the Cu-S binding is the EXAFS spectrum at Cu K edge, figure 
S17. However, it might also be interpreted as superimposed signal from single atom Cu (eg CuN4) 
and a small amount of Copper sulfide particles in the sample. Did the authors verify on many locations 
by STEM that no such particles are present? 
 
c) A puzzling issue is that the calculated XANES signal for CuS1N3 is still quite different from the 
experimental XANES spectrum (Suppl. Figure 20b). The experimental spectrum looks in first 
approximation maybe more similar to that for crystalline CuS. Could the authors compare the 
experimental XANES spectrum to that of CuS and other copper sulfide structures in one graph , And 
try if linear combination fitting of the experimental spectrum with the calculated spectrum for CuN4 
and experimental spectra for CuS and/or Cu2S gives a possible fit as well, or impossible fit ? 
 
d)On page 6, line 136-138, the authors write that “in-situ environmental microscopic studies suggest 
that the permeation of sulfur plays an important role for etching the carbon frameworks”. 
It is not clear why and how sulfur would do that. Without addition of sulfur, ZIF-8 (with or without 
addition of copper salt) leads to high BET area N-C material (e.g. figure S5 shows this, with Cu-
ISA/NC having BET of 1377 m2/g, only slightly lower than with sulfur addition). From figure S11, a 
hole inside the nanozif-8 pyrolyzed particles is seen for S-N-C, but barely seen for Cu/SNC in figure 
S13 
 
e) The synthesis of S-Cu-ISA/SNC is not fully described in the Methods section on pg 24 and cannot 
be easily reproduced by others at the moment (eg what amount of sulfur, of Cu-ZIF-8? what size of 
sulfur particles, etc), what was the ramping rate during heating ? 
 
f) could it be possible to perform Sulfur K-edge xanes and exafs measurements, and compare a 
sample where Cu-S bonds are expected to exist, to another sample where no such bond is believed to 
exist (only S-C or S-N) ? 
 
g) last but not least, could the authors comment on the ORR activity in acid medium ? does it also 
compete with SACs of type Fe-N-C ? 
 
 
Minor: 
 
Pg 5 line 99: the acronyms used at that location have not yet been explained for Cu-ISA/SNC and Cu-
ISA/NC. In particular, how is Cu-ISA/SNC distinct from S-Cu-ISA/SNC? they both contain sulfur...to 
ease the reading, this should be shortly desribed ther, with reference to full details in Methods section 
 
Pg 6 line 134 : « N2 absorption-desorption” => adsorption-desorption 
 
Pg 7, line 144 :the atomistic dispersion => atomic dispersion 
 
Figure caption of figure S9 is unclear: “size distribution of single –atom Cu” …the title says that these 
are single atoms Cu, while the data is supposed to demonstrate that. Maybe the title could be “The 
size distribution of Cu signal detected by EDS-HRTEM” ? 
 
 
Reviewer #3: 
Remarks to the Author: 
The manuscript, entitled " Engineering unsymmetrical isolated Cu-S1N3 atomic interface 
for boosting oxygen reduction reaction ", reported that a N, S co-coordinated Cu based single atom 
electrocatalyst shows an excellent ORR performance. The authors claimed that this kind of asymmetric 
coordinate structure offers a more suitable free energy and favorable adsorption abilities for 
intermediates. The work is of significance and novelty in exploring the underlying catalytic mechanism 
of single metal atomic site such as Cu-S1N3 coordination structure. The experiments are reasonable, 
and the results are convincing. So I recommend this work to be published in Nature Communications. 
Before it can be accepted by the journal, there are a few questions needed to be clarified. 
 
1. In Fig 1e, the scale bar should be provided. 
 
2. The ICP data of Zn species should be given to demonstrate its absence in the whole sample. If it 
exists, a further demonstration of its function in ORR performance should be clarified, as some recent 
work reported Zn-N2 active sites may facilitate O-O bond activation (Nature Communications, 2019, 
DOI: 10.1038/s41467-019-10622-1). 
 
3. The XPS spectrums of S and N species should be provided to analyze the type of these two 
elements in sample, which can help the rationality of DFT model. 
 
4. Different kinds of S-M- S1N3 ISA/SNC (Mn, Fe, Co, Ni) samples have been synthesized successfully. 
How about their ORR performance as compared to Cu based N, S co-coordinated structure? 
 
5. The asymmetry of the electronic redistribution discussed in Adv Mater 2019, 1805581 may offer 
help for discussion in this work.