Peer review of: Ruthenium anchored on carbon nanotube electrocatalyst for hydrogen production with enhanced (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The manuscript by Mahmood, Baek and co-workers deals with the development of stable and highly 
active HER cathodes based on Ru NPs onto multiwalled carbon-nanotubes and their inclusion in an 
overall water splitting device for the generation of H2 in alkaline media at high Faradaic efficiency. The 
authors claim that the reported Ru@MWCNTs system shows excellent performance (in terms of 
overpotentials, specific activities and TOF values) in HER at both acidic and alkaline conditions, 
outperforming commercial Pt/C. This is professionally and convincingly demonstrated in the 
manuscript by benchmarking the reported system (and the reference Pt/C) by well-stablished 
electrochemical methods. The care with that the authors tackle this benchmarking by tracking Ru 
active sites and ECSA by Cu-UPD methods is remarkable, not common for supported systems like the 
one reported The interest of Ru-based systems for HER, particular in alkaline conditions where they 
show by far more stable than Pt-based systems, is not new and has been recently reviewed (see for 
instance ChemSusChem 2019, 12, 2493-2514 / ACS Catal. 2019, DOI: 10.1021/acscatal.9b02457). 
The preparation of Ru NPs stabilized onto carbon-based supports from the reductive decomposition of 
pre-arranged RuCl3 has also been reported by similar synthetic methods (see for instance: Angew. 
Chem. Int. Ed. 2018, 57, 5848). Thus, the manuscript does not contribute with new concepts to the 
research field from these points of view. However, additionally to lower overpotential values, the 
reported Ru@MWCNTs system outperform Pt/C in terms of intrinsic activity of the active sites in both 
acidic and alkaline media (TOFs, exchange current density). This unusual fact, together with the 
practical inclusion of the reported cathodes in overall water splitting devices of almost quantitative 
Faradaic efficiency and very high H2 production per consumed power (superior to that of the reference 
Pt/C anode), makes the manuscript susceptible to be published in Nature Communications after 
considering the following points: 
 
1) The authors should clarify if the surface oxidation state of the Ru-based NPs evolve with time when 
exposed to air. Is the XPS analysis provided in Figure 3 of the Supplementary Information obtained 
after air-exposition of the hybrid system? The 3d5/2 peak at 280.4 eV observed (Figure 3, 
supplementary info) could in fact correspond to a mixed Ru/RuO2 phase (see BE for Ru and RuO2 in 
Surf. Interface Anal. 2015, 47, 1072-1079). Did the authors check the deconvolution of this peak by 
fitting a mixture of Ru and RuO2? Echem analysis is carried out under N2 atmosphere. Does this have 
anything to do with the potential evolution of the Ru metal NPs under air conditions? The Ru/RuO2 
surface ratio and its evolution with time under air and under turnover conditions is key in Ru-based 
systems for HER and should be carefully analysed. 
2) The comparison of overpotentials (Figure 3, g-h) and TOFs (Figure 4e) with related/representative 
reported systems is incomplete. In terms of overpotentials, the following relevant references should 
be included: Nano Energy 2019, 58, 1-10 & ACS Catal. 2018, 8, 11094-11102). Given key role of Ru-
based systems in HER catalysis, for a fair comparison of the here reported system a set of relevant 
Ru-based electrocatalysts studied both under acidic and alkaline conditions should be included in the 
TOF analysis together with the data for Pt/C and 1st-row based electrocatalysts. As exposed above, 
these data have been recently reviewed in ChemSusChem 2019, 12, 2493-2514 / ACS Catal. 2019, 
DOI: 10.1021/acscatal.9b02457. Particularly, the following systems show TOF data competitive with 
the values reported by the authors and merit inclusion in Figure 4e: 
- Acidic conditions: ACS Catal. 2018, 8, 5714 / J. Am. Chem. Soc. 2018, 140, 2731 / ACS Catal. 2018, 
8, 11094 / Adv. Mater. 2018, 40, 1800047 / J. Mat. Chem. A 2018, 6, 2311. 
- Alkaline conditions: Nano Energy 2019, 58, 1-10 / Adv. Mater. 2018, 40, 1800047 / Angew. Chem. 
Int. Ed. 2018, 57, 5848 / ACS Appl. Mater. Interfaces 2017, 9, 3785. 
 
Minor points: 
 
1) Repetition should be avoided between Figure captions and manuscript text. An example is found for 
instance in the caption of Figure 1. 
2) Page 2, second paragraph: the media where Pt shows poor electrochemical stability (alkaline 
conditions) should be detailed. 
3) Transferring the numeric scale bars from Figure captions to TEM images would ease the reading of 
the manuscript. 
4) The stability analysis through repetitive LSV cycle in acidic conditions whould be added to the 
Supplementary Information (Figure S10). 
5) The applied potential value should be noted in Figure S10 as well as in the manuscript text 
description. 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
Comments: 
This paper reports on an efficient but simple method of synthesizing a HER catalyst, consisting of 
ruthenium (Ru) nanoparticles uniformly deposited on the surface of multi-wall carbon nanotubes 
(MWCNTs). This catalyst shows excellent performance for HER. I would like to recommend its 
acceptance for publication in this journal after the following major revisions: 
1. The Ru content should be provided by ICP measurement. 
2. The particle size distribution of the nanomaterials should be provided in Figure 2c. 
3. The Ru-based electrocatalysts for HER were reported in previous literatures. In particular, what are 
the difference and innovation of this paper compared to those (Nat. Nanotech. 2017, 12, 441; Chem. 
Eur. J. 2019, 25, 8579-8584, Nat. Commun. 2019, 10, 631)? 
4. There are many minor mistakes in the References, I just listed some as below but suggest the 
authors to check throughout the References and may find more: 
1). Page 15, ACS Catalysis 4, 1693-1705 (2014). 
Comment: please replace “ACS Catalysis” by “ACS Catal.” 
2). Page 15, the Hydrogen Evolution Reaction at All Ph Values. Angew. Chem. Int. Ed. 56, 11559-
11564 (2017). 
Comment: please replace “All Ph Values” by “All pH Values” 
3) Page 15, Mahmood J. et al. An Efficient and Ph-Universal Ruthenium-Based Catalyst for the 
Hydrogen Evolution Reaction. Nat. Nanotechnol. 12, 441 (2017). 
Comment: please replace “Ph-Universal” by “pH-Universal”. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
In this paper, Kweon and his co-workers have reported their work entitled "Ruthenium@carbon 
nanotube electrocatalyst with superior Faradaic efficiency for hydrogen production". The hybrid sample 
of ultrathin Ru nanoparticles on MWCNT (Ru@MWCNT) was prepared through a simple reduction 
method with the aid of sodium borohydride (NaBH4). Fundamental morphology, structure, catalytic 
performance and stability of the material in both alkaline and acidic media were studied. Ru@MWCNT 
displays outstanding HER performance in both acidic and alkaline solutions in almost all aspects such 
as the overpotential at a current density of 10 mA cm-2, Tafel slope, the exchange current density, 
charge transfer resistance, ECSA, and stability, TOF, Faradaic efficiency, as well as various specific 
activities, etc., obviously superior to benchmark Pt/C. Although the author provides a relatively 
complete study on the Ru@MWCNT materials for hydrogen evolution in both acidic and alkaline 
solution, the paper does not provide sufficiently the technical-novelty or scientific achievement in the 
development of Ru-based HER catalysts. First, the preparation method by the reduction method with 
the help of NaBH4 has been widely used in previous works, for instance Adv. Energy Mater. 2018, 
1801698, Nature nanotechnology 12.5 (2017): 441. Second, only some basic characterization 
techniques were used which cannot provide insights in the materials. The author employed 
thiocyanate ions (–SCN) to identify the active sites on the Ru@MWCNT. In order to provide the in-
depth understanding about the catalyst for HER, the authors should pay more attention to more 
advanced characterization techniques such as Operando XPS or X-ray absorption spectroscopy (XAFS) 
or DFT calculations to reveal the real active sites. Lastly but most importantly, recently, there are 
many similar literatures about the Ru-C-based materials as HER catalysts in pH-universal electrolytes 
(Energy Environ. Sci., 2018, 11, 800; Adv. Energy Mater. 2018, 1801698; J. Mater. Chem. A, 2018, 6, 
2311; Adv. Mater. 2018, 30, 1803676; Sustainable Energy Fuels, 2017, 1, 1028; J. Mater. Chem. A, 
2017, 5, 25314; Chem. Commun., 2018, 54, 13076; Catal. Sci. Technol., 2017, 7, 4964 etc.). 
Considering the insufficient novelty of this study, the manuscript was not suitable to be published in 
Nature Communications. 
 
 
Reviewer #4 (Remarks to the Author): 
 
In the manuscript, the authors claimed a Ruthenium@carbon nanotube electrocatalyst for HER with 
superior faradaic efficiency. Yet, there are several points that the authors should be noticed clearly. 
1.So many Ru based catalysts for HER have been reported. This work doesn’t demonstrate their 
novelty in the material design. 
2. As for the synthetic route, this work doesn’t show its advantages compared to other noble-metal 
based catalysts which shown in Supp. Table S2,3. 
3. The electrochemical investigation could not satisfy the performance explanation, further theoretical 
explanation should be involved. 
In summary, I think the novelty and quality of this work do not reach the level of Nature Commun.