Peer review of: Reactant friendly hydrogen evolution interface based on di-anionic MoS2 surface (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The observation of a particularly high catalytic activity in a modified MoS2 structure is a potentially 
interesting finding. The authors propose a structural model and a catalytic mechanism to explain the 
observed high activity supported by a large number of spectroscopic measurements such as EXAFS, 
XPS, Raman, FTIR, NMR. While credit is due to the authors for the systematic spectroscopic 
investigations, I don’t think that the proposed structure is unambiguously determined by the provided 
experimental data, as claimed by the authors. There are a series of omissions they need to clarify 
before one can judge whether the proposed structure is most likely to be responsible for the observed 
activity. 
 
1. It would be desirable to provide structural data (SEM, TEM) also on the final material, exhibiting the 
highest activity, not only the intermediate Ru-MoS2. For instance, to exclude that the final material is 
more disordered than the precursor materials, as the increased activity could come alone form the 
increased disorder (e.g. edge to surface ratio). 
 
2. The authors ignore in their discussions some highly relevant literature that claims the oxygen 
substitution of S, instead of OH (Nat. Chem. 10, 1246 (2018), Nat. Comm. 10, 3382, (2019)). While I 
personally think both O and OH substitutions are plausible, the simplistic energetic argument of the 
authors that OH has a higher binding energy is not sufficient to explain their claim because S 
vacancies are highly reactive, and they would bind most available species including O, O2 and OH. 
What synthesis conditions make OH groups more abundant than other O containing groups for 
saturating the S vacancies in this case? 
 
3. The fact that OH groups can be identified by FTIR, does not directly imply that they are the 
substitutional groups. OH groups can be present in various configurations on the sample (e.g. on the 
edges). The authors only show the presence of OH groups by FTIR on Pd, Ru-MoS2-xOHy samples. 
What about the FTIR spectra of other samples, no OH signature? 
 
4. A rather technical question: in the XPS spectra of Suppl. Fig.18 for supporting the increasing 
oxygen incorporation with Pd addition, why do the peaks of adsorbed water and surface oxygen 
increase with increasing Pd content? 
 
I consider that although the manuscript contains a large amount of potentially useful spectroscopic 
and catalytic data, the proposed structure and therefore catalytic mechanism is not supported well-
enough to be suitable for publication in Nature Communications. 
 
 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The present work deals with the adaptation of MoS2 with OH, Pd and Ru for the hydrogen evolution 
reaction. The work has very interesting points but also weak points. The concept itself reminds of a 
similar concept introduced by Markovic (DOI: 10.1038/NMAT3313) which is also cited in the here 
presented manuscript. Here, reactivity trends for the HER were established, indicating an OHad-M 
interaction in close proximity to a noble metal that dictates the HER reaction rate. In the here 
described manuscript, Pd and Ru as noble metals are in close proximity to Mo-OH/S. Recently, also 
the surface oxophilicity was used as parameter for the HER activity 
(https://www.sciencedirect.com/science/article/pii/S1388248119300062) An Cu(I)-OHad---OH2 
activated complex was found to be essential for lowering the energy barrier for water dissociation. A 
similar point is shown here. The novelty of the here presented work is not completely clear, the 
authors also reported a similar paper previously (https://www.nature.com/articles/s41467-018-
04501-4). 
 
 
Major points: 
It is still not clear what the role of MoS2 is. The title and abstract suggests that MoS2 is the catalyst. 
It is claimed that the performance is the highest ever reported for MoS2-based catalysts in acidic 
media. However, MoS2 is not MoS2 anymore (“-OH anions molecularly replace S sites at the interface 
in a controllable manner can create a reactant benign interface.”). It is also questionable if it is really 
the catalysts. Especially Pd is extremely active towards the HER/HOR; Pd can be even found in some 
cases on the top of the volcano. Intrinsically very active HER catalysts are used to replace S by OH 
making it look suspicious that MoS2/OH should be the real catalyst. The paper suggests that MoS2 is 
active (e.g. “We show that the HER activity of the final catalyst exhibits highest kinetic performance 
exceeding the existed MoS2 based material in both acidic and alkaline environments.”). Later in the 
manuscript, it is proposed that Pd-S* is the active specie (“In acidic solution, the protons migrate to 
the energetically preferred Pd-S* sites (step II1) (the asterisk denotes the adsorption site) and are 
reduced by one electron to produce Pd-S*-H (step III1). Finally, another proton from an adjacent 
H+(H2O)n reacts with the first Had to generate H2 (step Ⅳ1-Ⅴ1), in a Heyrovsky mechanism.”). Pd 
itself is already on top of the volcano with higher current exchange densities than the values reported 
here, why would the H-adsorption take place on the S* sites? In the previously mentioned paper from 
Markovic, H* is still adsorbed on the noble metal, not on MO(O)H (DOI: 10.1038/NMAT3313). A 
control experiment could be to poison the Pd (and Ru)surface and check the activity once more to 
systematically exclude the possibilities. 
 
A similar point concerns the catalytically active surface area. Different materials are compared with 
different amounts of noble metals present. From Figure S14a, it is not surprising that the HER activity 
increases with higher Pd amount. We all know how hard the surface area determination can be. 
However, please use electrochemical tests to provide estimates of the real surface area (e.g. CO 
stripping, HUPD, capacitance measurements, redox transitions, etc.), especially of Pd. 
 
The exchange current density lies with 2.884 mA cm-2 much lower compared to state of the art 
materials such as Pt (216 mA/cm2Pt;). It is surprising, that the authors claim a comparable activity to 
Pt despite the large (order of magnitudes) differences. 
Additionally, it is advised to measure the exchange current density another time as e.g. proposed by 
Zheng and coworkers to separeate kinetic from mass transport influences 
(http://jes.ecsdl.org/content/162/14/F1470.abstract?ijkey=03f257db84f70cf8dc5f6285868bb4969884
a928&keytype2=tf_ipsecsha) 
 
Please specify the electrochemical reaction conditions: specifically, counter electrode, reference 
electrode and the material of the electrochemical cell (e.g. glass, Teflon, etc.) 
 
“We therefore carried out a prolonged test for 100 h, where results show an overall decay of only 16 
mV (Supplementary Fig. 21b), even better than the Pt based catalysts” 
For the OER it was recently shown that bubble blockage of the active surface corresponds mainly to 
the loss of activity (http://jes.ecsdl.org/content/166/8/F458.abstract). The same could be true here as 
the HER is, similar to the OER, not mass transport limited suggesting that the comparison with bare Pt 
might not be valid as the surface oxophilicity effects the bubble nucleation and detachment. 
 
“The stability of the catalyst in acidic solution is primarily concerned, as the Ru-OH is suspicious to 
dissolution under the attack of hydronium ions.” 
It is not clear why Ru-OH should be prone to dissolution. Should Ru not be reduced and be 
thermodynamic stable at these potentials and pH values? 
 
Figure 2: LSVs in KOH: it seems that reductive processes before HER are still taking place, especially 
in KOH. For Pd/Ru-MoS-OH it seems that the capacitive current is much higher compared to the other 
samples suggesting that the observed effect might be a simple surface area effect. Especially since the 
used scan rate was only 5 mV s−1. 
 
Minor points: 
It is surprising that the LSVs to these high current densities do not result in bubble formation and that 
no forced convection was used to remove gas bubbles. Depending on the hydrophilicity/hydrophobicity 
of the samples, the effective surface is changed drastically, leading to different geometric-based 
activities. It is suggested to repeat the measurements under rotation to have a defined diffusion layer. 
The HER specific activities in H2SO4 for noble metals is lower compared to non-adsorbing electrolytes 
such as HClO4. Please repeat the LSV/CV experiments in HClO4 to have a fair comparison with bare 
noble metals. 
 
“Meanwhile, if Ru bonded defects sites (SVs) are formed, they readily captures the nucleophilic -OH 
species owning to their thermodynamic favorable formation energy of -4.01 eV compared to other 
species” 
This might be true during the synthesis. Why is Ru-OH not reduced under HER conditions? 
 
Terminology. Please make the abbreviation of SV uniform: “is if Ru bonded defects sites (SVs)”; 
“S vacancies (SVs)” 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
Currently, the correlative investigations of electrocatalysts for water splitting process have become 
one of the hottest areas in view of rather serious energy and environmental crisis. In this work, the 
authors have performed the detailed experimental and theoretical investigations on the HER catalytic 
activity of di-anionic MoS2 material and the related mechanism behind water splitting. This work is 
interesting. However, the following questions should be addressed before its publication on the Nature 
Communication: 
(1) In this work, the theoretical model related to Ru-doped MoS2 is adopted to perform the DFT 
calculation. However, it is not comprehensive considering only the effect of Ru-doping, in view of the 
fact that doping Pd also has an important effect on the formation of vacancy and the adsorption of OH. 
Therefore, it is suggested that the co-doping model by Ru and Pd should be constructed to perform 
the correlative DFT calculations. 
(2) According to the authors’ description, we can understand that -OH group occupies the vacancy on 
the surface of Pd,Ru-MoS2-xOHy, and brings a high HER catalytic activity. However, it is puzzling why 
the vacancy (SV) still exists in HER mechanism diagrams of Figure 4. Here, it is suggested that the 
authors should provide a reasonable explanation for it, as well as the model in Figure 3b. 
(3) In this work, the PDOS of O and H is calculated to confirm the existence of non-covalent bonding 
between OH and H2O. Here, it's more reasonable that the authors should re-plot this picture by using 
the data only related to the H and O involved in hydrogen bonding instead of all the H and O atoms. 
(4) For the convenience of readers, the authors should add the corresponding energy values in the 
Supplementary Figure 25. Additionally, the caption of Figure S25a should be corrected since it does 
not match the corresponding graph. 
(5) Moreover, the computational details about ΔGH* in Supplementary Materials should be deleted 
since the authors do not perform the related calculations.