Peer review of: Single-site pyrrolic-nitrogen-doped sp2-hybridized carbon materials and their pseudocapacitance (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 

In this work, Tian et al. reported their efforts on the design and synthesis of single pyrrolic N-doped 
carbon materials (SPNCMs). Their results revealed that the SPNCMs (F, 1/1) with up to 4.22 at.% 
pyrrolic nitrogen had the high specific capacitance performances in 1 M H2SO4. This is an interesting 
topic and shows some interesting results. However, some critical issues are missing. Therefore, I 
would like to recommend it for publication after the revision as follows. 
1. In the Introduction section, the authors claimed that “Apart from N-X, one or more of three other 
nitrogen centers could be considered as possible electrochemically active sites.”. To help enhance the 
readability of the paper, the proper references are needed. 
2. How much is the ratio of intensity of D/G bands before nitrogen doping? 
3. In page 5, the authors claimed that the XRD results verified the transition from amorphous F-APF 
(1/1) resins to amorphous NCMs (F, 1/1) to amorphous SPNCMs (F, 1/1). However, in page 6, the 
authors stated that “About 0.34-nm interplanar spacing corresponding to the (002) plane of graphitic 
layers confirms the disordered structure randomly stacked by small domains of microcrystalline 
graphite according to SEM (Supplementary Fig. 1b), TEM (Supplementary Fig. 1c) and HRTEM 
(Supplementary Fig. 1d) images of SPNCMs (F, 1/1).”. These two conclusions seem to be in contrast. 
The authors should check this. In addition, how did the authors observe the domains of 
microcrystalline graphite through the SEM and TEM images at such low resolution level? 
4. The authors aimed to understand the contribution of nitrogen species in N-doped carbon materials 
and they claimed that pyrrolic nitrogen species are exactly highly active pseudocapacitive sites for 
their materials. However, the O species were detected in all samples, which are confirmed by XPS 
data (Figure 3). Especially, the SPNCMs exhibited much larger oxygen content than its precursor 
(NCMs, Supplementary Table 2). Since there have been plenty of previous reports that show O-doped 
carbons as active materials. Therefore, the effects of oxygen species on the performance must be 
considered, and the O 1s XPS data should be provided. 
5. The EIS data are suggested to be fitted by using an equivalent circuit model. 
6. The authors stated that the SPNCMs (F, 1/1) had ultrahigh stability over 30,000 cycles during the 
tests. To confirm this, the SEM, TEM, HRTEM and XPS characterizations on the samples obtained after 
the stability tests are required. 
7. Some important references on the latest advances of carbon materials (for example, Chem. Soc. 
Rev., 2014, 43, 2572; Acc. Chem. Res. 2014, 47, 1186−1198; Acc. Chem. Res. 2017, 50, 2470-2478; 
Nat. Commun. 2018, 9, 1460; J. Am. Chem. Soc. 2019, 141, 10677; Nat. Commun. 2018, 9, 5309; 
Nat. Commun. 2018, 9, 3376; Angew. Chem. Int. Ed. 2018, 57, 774 –778; and Chem. Rev. 2018, 
118, 7744−7803;) are suggested to be cited in the manuscript. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The authors reported preparation method of nitrogen doped carbon materials with single XPS peak 
which was assigned as a pyrrolic nitrogen. They also reported that the amount of XPS peak can be 
concluded to correspond to the pseudocapacitance. If the assignment is reliable, the work may have a 
significance in terms of tunable controllability of specific nitrogen species in carbon materials and 
clarification of the pseudocapacitive active site. However, the conclusion strongly relies on the XPS 
assignment and the evidence of pyrrolic nitrogen species is currently weak, which does not convince 
the general reader of Nature Communications. Moreover, the redox peak assignment is ambiguous. 
Authors did not clearly mention about the “capacitance from oxygen species”. Also, the well-known 
quinone redox is not discussed. The component contribution shown in Fig. 5 should be carefully made 
by the analysis of the sample before and after the electrochemical measurements. Thus, I recommend 
the manuscript to be published if the assignment becomes reliable after the major revision. Specific 
points the authors should consider are listed below: 

(1)Assignment of pyrrolic nitrogen 
Authors assigned their observed N1s peak at 400.2-400.25 eV as pyrrolic nitrogen. This is well known 
species and thus normally the assignment is not so problematic. However, in this manuscript, this 
peak component is a key point of the main conclusion and thus authors should carefully assign it with 
enough evidence because the other type of nitrogen is known to also show the peak at similar energy 
position (and the possibility of these other nitrogen species cannot be excluded by the character of 
precursor because authors conducted the sample treatment for the precursor e.g. heating). 
Specifically, the peak at 400.2 eV is reported to appear as core level peak of pyridonic nitrogen 
(Carbon 33, 1641–1653 (1995), Energy Fuels, 8, 896–906 (1994), Fuel,78, 923–932 (1999), ACS 
Nano 8, 6856–6862 (2014).). Also, the peak at 400.7 eV is reported as hydrogenated nitrogen species 
such as hydrogenated pyridinic functionalities (J. Phys. Chem. C120, 29225−29232 (2016).). 
Therefore, the additional evidence to classify these species should be shown. For example, if the 
authors show IR spectrum with no correlation of –OH amount and N1s peak intensity, authors can 
safely avoid the mixing of pyridonic nitrogen. Also, if authors show the drastic decrease of N1s peak 
by heating at higher temperature, authors may discuss that this is not come from hydrogenated 
pyridinic nitrogen (because pyllolic nitrogen is known to be unstable against heating). These are “at 
least” required evidences, further strong and direct evidences (e.g. STM, TEM, or STEM measurements 
or other spectroscopic evidences with theoretically carculated models) are required to conclude the 
main issue of the manuscript and convince the reader of Nature Communications. 
 
(2) Redox peak 
Authors described about 0.3-0.4 V redox peak as “reversible Faradaic redox reaction of N-5 species 
and/or oxygen”. It is not clear about Faradaic redox reaction of oxygen because authors did not show 
any possible exact reaction with evidence (e.g. at least reference should be shown). It seems 
quinone/hydroquinone redox reaction. In any case, authors should discuss the redox reaction at 0.3-
0.4 V more carefully because this is a part of main message of this manuscript. 
 
(3) Fig. 5 
Authors construct Fig. 5 partly based on oxygen content of XPS peak. However, the sample character 
changes significantly after immersion the sample in electrochemical cell and CV measurements. Thus 
authors should carefully check the sample condition before and after the measurement otherwise at 
worse case it causes misleading for general readers of Nature Communications. 
 
(4) Minor point 
Authors should show the detail of XPS measurement and analysis conditions more clearly. Specifically, 
for example, pass energy used for the measurement, peak fitting process (how to determine the width 
of each peak and what is the peak width), and about charge-up correction (if any). 
 

Reviewer #3: 
Remarks to the Author: 
This study reported single pyrrolic N-doped carbon materials (SPNCMs) with a tunable nitrogen 
content from 0 to 4.22 at.% based on a combined strategy, and inferred that pyrrolic nitrogen species 
are exactly highly active pseudocapacitive sites for nitrogen-doped carbon materials. The authors try 
to set an ideal model to understand the contribution of nitrogen species in N-doped carbon materials, 
offering the principle of materials design for different energy conversions. However, the investigation 
on the experimental part is very weak (See Below). The quality of this work cannot meet the required 
standard of Nature commutation. Therefore, the reviewer cannot recommend its acceptance for 
publication. 
 
Specific comments 
 
1. The chemical structure of NCMs and SPNCMs should be further demonstrated by more physical 
characterization except XPS. 
 
2. As KOH activation was involved in the synthetic procedure, may defect structure and effects should 
also be considered for the electrochemical performance. It is hard to say the materials developed in 
this work as ideal model catalysts, particularly after the pyrolysis. 
 
3. Some of the XPS spectra are not well fitted (eg., Figure 2c and Figure 3k). The basic rules of the 
deconvolution should be obeyed. At least, the half width of the each peak for one element should be 
consistent! 
 
4. Page 2, Line 52. “The authors stated that “it seems to be an impossible mission to engineer single 
nitrogen configuration-doped carbon materials due to an inevitable thermal treatment involved in 
synthesizing almost all the nitrogen-doped carbon materials primarily via either post-synthetic or in 
situ doping methods 5,9,23.” However, Ref. 21 has already reported the synthesis of pure pyridinic N 
or graphitic N doped HOPG. Also, the Ref. 23 reported on nanoporous carbon spheres, rather than 
doping type of N. 
 
5. Some of the previous literature reported that pyridinic (pyrrolic also) N species can be transferred 
into the graphitic N during the pyrolysis, and that the pyrrolic N is not stable during the 
electrochemical catalytic process, how about the stability of the catalyst developed in this work? 
 
6. The authors stated that they reported one kind of single pyrrolic N-doped carbon materials 
(SPNCMs) as ideal model materials for supercapacitor. However, the XPS results show a high content 
of oxygen, and Raman spectra show the presence of numerous defects. Both of the oxygen functional 
groups and defects will affect the (pseudo-) capacitance, which should be distinguished from the 
contribution of the pyrrolic N. 
 
7. The graphitization degree should also be determined (e.g. by XRD), which may show a relatively 
low graphitization degree with respect to the HOPG doped by pyrrolic N only previously-reported by 
Nakamura et al. (i.e., Ref. 21). The so-called single pyrrolic N-doped carbon (SPNCM) is inferior to its 
HOPG counterparts. 
 
8. In order to claim the supercapacitance from the single pyrrolic N-doped carbon materials (SPNCMs), 
the single prydinic N-doped, and single graphitic N-doped SPNCM counterparts need to be prepared as 
references so that the oxygen and defect effects can be neglected. 
 
9. Some Refs cited in this study is not suitable, for example: 
Page 2, Line 33. “…oxygen reduction reaction (ORR)6-9, CO2 reduction reaction10-12, oxygen 
evolution reaction (OER)13-15, hydrogen evolution reaction (HER)16, nitrogen reduction reaction17-
20, and so on…”, however, the Ref. 8 focused on “Fe-Nx” active sites for ORR, while Ref. 17 studied 
on the catalysis of CO2 reduction.