Reviewers' Comments: Reviewer #1 (Remarks to the Author) The manuscript reports the synthesis of a series of Mo2C@NPC-rGO hybrid HER electrocatalysts by employing the precursor of PMo12 (H3PMo12O40)-PPy/rGO nanocomposite with different initial addition contents of PMo12. Also, the mono Mo2C@NPC and commercial 20% Pt-C electrocatalyst materials were compared in the manuscript. The results of HER activity evaluation showed that the Mo2C@NPC-rGO (2.2) displayed the superior HER electrocatalytic activity (a low onset overpotential of 0 mV (vs. RHE), a small Tafel slope of 33.6 mV dec-1, and 34 mV overpotential at 10 mA cm-2 current density), which is comparable to the commercial 20% Pt-C electrocatalysts. All the electrocatalysts were well characterized by the XRD, TEM, SEM, XPS, and N2 sorption. Meanwhile, the DFT calculations were performed to explain the reason of the amazing HER activities of the Mo2C@NPC-rGO electrocatalysts. All the analysis results obtained in this study are reasonable and interesting. However, the discussions for the N and P doping into the electrocatalysts were not in details. Several other several key points listed below need to be addressed before it can be considered for publication. I recommend acceptance of this paper for publication in "Nature Communications" after the following major modifications. 1. In this manuscript, there are many abbreviations, some of them were not well illustrated, for example, the Mo2C@NPC, readers are easily confused by the NPC. Illustration should be given when it appeared in the main text for the first time. 2. The different onset potential values may be obtained due to the different analysis methods, the author should give the details of how to derive the onset potential. 3. The author didn't give any information about the formation of pyridinic-N, graphitic-N, and P-C, i.e., heteroatoms (N, P) doping into the carbon structure. There is no direct evidence proving the heteroatoms (N, P) doping into the Mo2C structure. 4. In this work, the rGO may only be a good support for the dispersion of PMo12 during the carbonization process. How to confirm there are no heteroatoms (N, P) doping into the rGO structure during the carbonization process with the high temperature ? If they exists in the structure, there could be some contribution to the high HER activity. 5. In the "Methods" section, the author described that the pore size distributions were measured by the Barret-Joyner-Halenda (BJH) model, which is the classic analysis method for mesoporous materials (pore size 2 nm). However, the author adopted the DFT method to obtain the pore size distributions data (Figure S5) without proving details and illustration to connect these two. 6. The Brunauer-Emmett-Teller (BET) surface areas of electrocatalysts were measured in this manuscript, the analysis method should be added in the "Methods" section. 7. As we known, the Mo2C may change to other phases with the modulation of carbonization temperature, it is more useful to compare the HER activities under the same Mo2C phase. 8. The long-term stability testing experiment of Mo2C@NPC electrocatalyst should be provided for comparison. Reviewer #2 (Remarks to the Author)

The paper describes MoC nano particles decorating graphene nano sheets. The material is thoroughly evaluated as hydrogen evolution catalysts. All the data appear sound and the conclusions are supported by the datasets. The research is novel and contributes significantly to a highly-competitive area. The text may need a little English editing, but in general all apiarist to be in place for a great publication. The supplementary material teaches enough detail and all looks/reads great. No request for any review.

Responses to the Referees' Comments Referee: 1 Comments to the Author The manuscript reports the synthesis of a series of Mo2C@NPC-rGO hybrid HER electrocatalysts by employing the precursor of PMo12 (H3PMo12O40)-PPy/rGO nanocomposite with different initial addition contents of PMo12. Also, the mono Mo2C@NPC and commercial 20% Pt-C electrocatalyst materials were compared in the manuscript. The results of HER activity evaluation showed that the Mo2C@NPC-rGO (2.2) displayed the superior HER electrocatalytic activity (a low onset overpotential of 0 mV (vs. RHE), a small Tafel slope of 33.6 mV dec-1, and 34 mV overpotential at 10 mA cm-2 current density), which is comparable to the commercial 20% Pt-C electrocatalysts. All the electrocatalysts were well characterized by the XRD, TEM, SEM, XPS, and N2 sorption. Meanwhile, the DFT calculations were performed to explain the reason of the amazing HER activities of the Mo2C@NPC-rGO electrocatalysts. All the analysis results obtained in this study are reasonable and interesting. However, the discussions for the N and P doping into the electrocatalysts were not in details. Several other several key points listed below need to be addressed before it can be considered for publication. I recommend acceptance of this paper for publication in "Nature Communications" after the following major modifications. 1. In this manuscript, there are many abbreviations, some of them were not well illustrated, for example, the Mo2C@NPC, readers are easily confused by the NPC. Illustration should be given when it appeared in the main text for the first time. Response: Thanks for the referee’s nice suggestion. We have checked and illustrated these abbreviations in the main text. 2. The different onset potential values may be obtained due to the different analysis methods, the author should give the details of how to derive the onset potential. Response: Thanks for the referee’s helpful suggestion. The hydrogen-evolving reaction (HER) is given in equation (1):

2H+ + 2e- H2, Ereduction= 0.0 V (1) We have shown the 0.0 V potential and added the details in the main text (Figure 4a, purple line). Beyond 0.0 V, the cathodic current rises sharply, corresponding to catalytic H2 evolution. This is the same as the literatures reported (Nat. Commun. 6, 5982 (2015); Nano Lett. 14, 1228-1233 (2014); J. Am. Chem. Soc., 137, 6983-6986 (2015)). 3. The author didn't give any information about the formation of pyridinic-N, graphitic-N, and P-C, i.e., heteroatoms (N, P) doping into the carbon structure. There is no direct evidence proving the heteroatoms (N, P) doping into the Mo2C structure. Response: Thanks for the referee’s nice comment. On the basis of X-ray photoelectron spectroscopy (XPS) data, the deconvolution of N1s energy level signals for Mo2C@NPC/NPRGO revealed the peaks at 398.6 and 401.3 eV, which were assigned to pyridinic and graphitic N,

respectively (Fig. 1a). From Fig Fig. 1b, it can be seen that the P2p peak at about 133.5 was attributed to P-C bonding. Besides, the high-resolution Mo 3d XPS revealed that the peak at 228.8 eV was attributable to Mo2+, stemming from Mo2C. In parallel, as a consequence of surface oxidation, the peaks at 232.05 and 235.2 were attributable to MoO3 and that at 232.7 eV was assignable to MoO2 (Fig. 1c). From the high-resolution transmission electron microscopy image (HRTEM) (Fig. 1d), the Mo2C nanoparticles were encapsulated by the RGO-supported carbon shells. As the referee pointed that where the heteroatoms (N, P) doped into the materials is also the problem we expect to make it clear. According to the elemental binding energy, the N1s (397.05 eV) and P2p (129.8 eV) peaks attributed to N-Mo and P-Mo, respectively, were not found from the XPS high-resolution N 1s, and P 2p spectra of Mo2C@NPC/NPRGO. So we speculate that the heteroatoms (N, P) were not almost doped into the Mo2C structure. This is similar to the literatures reported (J. Am. Chem. Soc., 137, 15753-15759 (2015); Angew. Chem. Int. Ed. 54, 10752-10757 (2015); Adv. Mater. Nano, 27, 4234-4241 (2015)). From energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Fig. 2a), we found that the heteroatoms (N, P) were uniformly dispersed. In view of the aforementioned considerations, we believe that the heteroatoms (N, P) were distributed homogeneously over the RGO and carbon shells. The Mo2C@NPC-rGO catalyst is renamed Mo2C@NPC/NPRGO. Meanwhile, we have revised the expressions in the main text.

Fig. 1 XPS high-resolution scans of (a) N 1s, (b) P 2p, (c) Mo 3d electrons and (d) HRTEM image of Mo2C@NPC/NPRGO. Scale bar: 2 nm.

Fig. 2 (a-b) EDX elemental mapping of N, and P of Mo2C@NPC/NPRGO. Scale bar: 50 nm.

4. In this work, the rGO may only be a good support for the dispersion of PMo12 during the carbonization process. How to confirm there are no heteroatoms (N, P) doping into the rGO structure during the carbonization process with the high temperature? If they exists in the structure, there could be some contribution to the high HER activity. Response: Thanks for the referee’s nice suggestion. As illustrated for the third question, we believe that the heteroatoms (N, P) were codoped into the carbon and RGO structure, contributing to the high HER activity. Theoretical calculations based on density functional theory reveal that the highly active sites for the HER stem from the pyridinic N, as well as the C atoms, in the graphene. All those are discussed in the main text. 5. In the "Methods" section, the author described that the pore size distributions were measured by the Barret-Joyner-Halenda (BJH) model, which is the classic analysis method for mesoporous materials (pore size 2 nm). However, the author adopted the DFT method to obtain the pore size distributions data (Figure S5) without proving details and illustration to connect these two. Response: Thanks for the referee’s helpful suggestion. We have corrected the error in the “Methods” section. Namely, the pore size distributions were measured by using the nonlocalized density functional theory method (NLDFT). 6. The Brunauer-Emmett-Teller (BET) surface areas of electrocatalysts were measured in this manuscript, the analysis method should be added in the "Methods" section. Response: Thanks for the referee’s nice suggestion. We have added the analysis method in the “Methods” section. 7. As we known, the Mo2C may change to other phases with the modulation of carbonization temperature, it is more useful to compare the HER activities under the same Mo2C phase. Response: Thanks for the referee’s helpful comment. The referee is very experienced. The different phases of Mo2C may be obtained with the modulation of carbonization temperature. In the main text, the POMs-PPy/RGO was carbonized 700, 900, and 1100 oC, respectively. For comparison, we further carbonized the POMs-PPy/RGO composite at 800, and 1000 oC, respectively (denoted as Mo2C@NPC/NPRGO-800 and Mo2C@NPC/NPRGO-1000). The PXRD peaks of Mo2C@NPC/NPRGO-900, Mo2C@NPC/NPRGO-1100 are identical with those of

Mo2C@NPC/NPRGO-800, Mo2C@NPC/NPRGO-1000. They were broad and exhibited low intensity because of the smaller crystallites of Mo2C or Mo2C coated with amorphous carbon shells (Fig. 3). This is similar to that of the literature reported (Angew. Chem. Int. Ed. 53, 7310-7315 (2015), Fig. 4). We still obtained the alfa phase of Mo2C (Fig. 3). In others words, other phases of Mo2C were not obtained in our work, which may be related to the structure and composition of precursor. Therefore, we investigated and discussed the HER performance of the alfa phase of Mo2C carbonizing at different temperature in our work. We gratefully acknowledge the helpful suggestion of the referee. In future work, we will try to investigate the effect of phases of Mo2C on the HER performance.

Fig. 3 PXRD patterns of different catalysts derived from POMs-PPy/RGO with different carbonization temperature.

Fig. 4 PXRD pattern α-MoC1-x/AC catalyst.

8. The long-term stability testing experiment of Mo2C@NPC electrocatalyst should be provided for comparison. Response: Thanks for the referee’s nice suggestion. We have provided the long-term stability testing experiment of Mo2C@NPC electrocatalyst, which was added in the main text and Supplementary Figure 10.

Supplementary Figure 10. (a-b) Polarization curves of Mo2C@NPC and Mo2C@NPC/NPRGO initially and after 1000 cycles, respectively. (a-b) Inset: Time-dependent current density curve of Mo2C@NPC and Mo2C@NPC/NPRGO under a static overpotential of 296 and 48 mv, respectively. Referee: 2 Comments to the Author The paper describes MoC nano particles decorating graphene nano sheets. The material is thoroughly evaluated as hydrogen evolution catalysts. All the data appear sound and the conclusions are supported by the datasets. The research is novel and contributes significantly to a highly-competitive area. The text may need a little English editing, but in general all apiarist to be in place for a great publication. The supplementary material teaches enough detail and all looks/reads great. No request for any review. Response: Thanks for the referee’s nice suggestion. The written English has been improved and edited the text again.

Reviewer #1 (Remarks to the Author)

The authors have addressed my questions and concerns. And I think I am fine with the acceptance for the manuscript