Solvent-free microwave synthesis of ultra-small Ru-Mo2C@CNT with strong metal-support interaction for industrial hydrogen evolution

Exploring a simple, fast, solvent-free synthetic method for large-scale preparation of cheap, highly active electrocatalysts for industrial hydrogen evolution reaction is one of the most promising work today. In this work, a simple, fast and solvent-free microwave pyrolysis method is used to synthesize ultra-small (3.5 nm) Ru-Mo2C@CNT catalyst with heterogeneous structure and strong metal-support interaction in one step. The Ru-Mo2C@CNT catalyst only exhibits an overpotential of 15 mV at a current density of 10 mA cm−2, and exhibits a large turnover frequency value up to 21.9 s−1 under an overpotential of 100 mV in 1.0 M KOH. In addition, this catalyst can reach high current densities of 500 mA cm−2 and 1000 mA cm−2 at low overpotentials of 56 mV and 78 mV respectively, and it displays high stability of 1000 h. This work provides a feasible way for the reasonable design of other large-scale production catalysts.

(RuMo2C@CNT), its catalytic properties are not sufficient to meet and/or to exceed the state-of-theart electrocatalysts towards hydrogen evolution reaction (HER) in alkaline environment. This observation is very likely related to the lack of knowledge in the field of electrochemistry. The referee is very surprised about the extremely poor HER performance of pure Pt/C which is actually wellestablished and frequently reported (e.g. see Nature Communications volume 11, Article number: 1278 (2020) for Pt/C data). In addition, the authors have analyzed their electrochemical data with a minimum effort. Excellent and very good electrochemical data for HER/HOR are typically shown including Butler-Volmer plots, exchange current density with normalization of metal mass and number of catalytically active sites as well as charge transfer coefficient to get a deeper insight into the kinetics and mechanism of HER. Furthermore, the determination of the ECSA via underpotential deposition of Cu is completely wrong. In Figure S16 and S28, the anodic peaks of a Cu monolayer appear at the same potentials like the redox potential of Cu/Cu2+. The authors mainly observed a bulk dissolution of metallic Cu and this fact explains the high ECSA values. In Figure S32, the Nyquist plot starts for all materials nearly at Z´=0 Ohm. This is an observation which can not be explained by the presented data. It is also frustrated to see that the authors did not make enough effort to measure their materials in a proper manner. The Raman, XRD, EDX and other data are presented in a poor quality.
Reviewer #3 (Remarks to the Author): Electrocatalytic hydrogen evolution reaction (HER) by splitting water has become an effective method for the sustainable production of H2. It is highly desirable and imperative to develop new HER electrocatlysts with low-cost and high-performance. Herein, the authors reported a simple, fast and solvent-free microwave pyrolysis method for the synthesis of ultra-small (3.5 nm) Ru-MxC@CNT (M=Mo, Co, Cr) catalyst with heterogeneous structure and strong metal-support interaction in one step. The fabricated Ru-Mo2C@CNT catalyst exhibits a low overpotential of 15 mV at a current density of 10 mA cm-2 , and exhibits a large TOF value up to 57.8 s -1 under an overpotential of 100 mV. This paper is interesting, and I recommend this paper can be accepted after following revisions: 1. The author claims that the synthesis of Ru-Mo2C@CNT, however, in Figure 1, only one Ru-Mo2C nanoparticle is seen in Figure 1c, indicating that the Ru-Mo2C nanoparticles are not ubiquitous on CNT. So the TEM image with more Ru-Mo2C nanoparticles should be provided. This is crucial for this paper. 2. For Ru-Mo2C@CNT, how about the content of Ru or Mo2C on the catalytic activity and stability for HER? The authors should provide more information. 3. How about the long-term durability of Ru-Mo2C@CNT for HER in alkaline media? As shown in Figure  4f, the test time of durability is too short, and the authors should measure the durability of HER for at least 50 hours. 4. How do you prove the structure stability of Ru-Mo2C@CNT for long-term durability of HER？Please provide some evidences for structure stability of Ru-Mo2C@CNT after durability test. 5. The loading of Ru-Mo2C on CNTs should be provided for Figure 4, and the effect of the loading of Ru-Mo2C on CNTs on the catalytic activity and stability should be studied. 6. For the data shown in Figure 5g, to maintain the current density at 500 mA cm-2, the overpotential should be provided. 7. Why does Ru-Mo2C have higher catalytic performance than the other Ru-Co3C@CNT, Ru-Cr23C6@CNT? Please provide some explains in the paper. 8. Some relevant references about hydrogen evolution electrocatalysis may be considered to be cited, such as Angew. Chem. Int. Ed. 2017, 56, 2960Angew. Chem. Int. Ed. 2017, 56, 8120;J. Am. Chem. Soc. 2018, 140, 5118. 9. For supplementary Figure 21, the TEM image of Ru-Co3C@CNT does not show the existence of heterojunction, please provide another TEM.

Dear Reviewers,
Thank you for your precious time to constructive comments on our manuscript titled "Solvent-free microwave synthesis of ultra-small Ru-Mo 2 C@CNT with strong metal-support interaction for industrial alkaline hydrogen evolution reaction" (Manuscript ID: NCOMMS-20-48819) for Nature Communications. We sincerely appreciate your opinions and confirmation of our work. Accordingly, we have supplied the corresponding response and revision based on the comments. We sincerely hope that our responses will fully address your concerns about our work.
To Reviewer 1: General Comment: The manuscript "Solvent-free microwave synthesis of ultra-small Ru-Mo 2 C@CNT with strong metal-support interaction for industrial alkaline hydrogen evolution reaction" describes the fabrication of Ru-Mo alloy nanoparticles attached to carbon nanotubes through the decomposition of Mo and Ru of carbonyls in a household microwave oven. The group characterizes these nanoparticle-decorated CNTs to show the bonding and incorporation of the nanoparticles. Electrochemical analysis in strong basic solutions showed superior hydrogen generation compared to catalysts from similar research. The manuscript was easy to follow and gave a clear picture of the aims and results.
This manuscript is another step in a long line of studies on alloy nanoparticles bound to CNTs for catalysis purposes. That being said, the chemical composition of the catalyst is novel, and the electrochemical activity of these structures show notable improvement compared to past publications. Therefore, I recommend publishing this manuscript once the authors have considered these comments and made the corresponding minor modifications.
Author Reply: Thanks for your positive comments and support for our work. To further improve the quality of this manuscript as well as address your concerns, we have revised our manuscript as your suggestions. In addition, we also supplied a point-by-point response as follows. We wished the revised manuscript can fulfill your high requirements for the publication in Nature Communications.
Question 1: Many of the figures show a detailed study of the catalyst that brings great insight to its properties and how it fairs compared to catalysts from similar publication. However, there is a large amount of overlap in the data shown in many of the graphs. These redundant representations might be distracting to the reader. I recommend moving some of these figures to the supplementary information section of this manuscript. raw data is represented in black, whereas in plot c, it is red. Using consistant representation in graphs is highly advised. The XPS signal to noise ratio for the CNT is much higher than the other samples, and it seems that the same characterization conditions were not used for all samples. I recommend the authors to revisit the data and confirm its soundness.
Author Reply 4: Thank you for reviewer's helpful suggestion. The retested data and the corrected plot c have been shown in Figure R1. In addition, we re-characterized the XPS data of CNTs ( Figure R1a) using the same test conditions as the other catalysts in the article. Question 5: EDX shows a wide view of a CNT decorated with nanoparticles. To better show the presence of the elements in question in these particles, it is highly recommended to include a closeup image of one or few nanoparticles with overlapping element colormaps. This would provide a more effective visual representation of the distribution.
Author Reply 5: Thanks for your kind comments and suggestion. As shown in Figure R2, in order to better show the distribution of each element in the catalyst, we re-tested the EDX mapping of Ru-Mo 2 C@CNT, and provided the element color map of the simultaneous presence of Ru and Mo elements. Author Reply: Many thanks for taking time to access our manuscript and providing thoughtful feedbacks. The reviewer's comments were helpful to improve the quality of our manuscript. Based on your valuable comments, we have supplied the detailed.
Question 1: Although the authors have prepared a new material (Ru-Mo 2 C@CNT), its catalytic properties are not sufficient to meet and/or to exceed the state-of-the-art electrocatalysts towards hydrogen evolution reaction (HER) in alkaline environment. This observation is very likely related to the lack of knowledge in the field of electrochemistry.
Author Reply 1: Thank you for your helpful comment. This new catalyst (Ru-Mo 2 C@CNT) we designed and synthesized (the overpotential only is 15 mV at 10mA cm -2 ) becomes among the best HER electrocatalysts in alkline solution (Supplementary Table 4). More importantly, when the catalyst Ru-Mo 2 C@CNT was loaded on the Ni foam, the overpotential exhibited at large currents of 500 mA cm -2 and 1000 mA cm -2 is the lowest in the reported literature (Figure 4h). In terms of the simplicity of the synthesis method, the environmental friendliness of the process, the high yield and the high catalytic activity for HER, the research of Ru-Mo 2 C@CNT is of great significance in practical applications.

Question 2:
The referee is very surprised about the extremely poor HER performance of pure Pt/C which is actually well-established and frequently reported (e.g. see Nature Communications volume 11, Article number: 1278 (2020) for Pt/C data).
Author Reply 2: Thanks for your valuable comment. We repurchased a new commercial Pt/C from Aladdin. And we re-provide the LSV curve ( Figure R1a), overpotential at 10 mA cm -2 ( Figure R1c) and 20 mA cm -2 ( Figure R2), exchange current density (Figure R1c), Tafel slope ( Figure R1b) and TOF ( Figure R1e) data of commercial Pt/C. Among them, the LSV curve of commercial Pt/C shows its better electrochemical HER activity. The overpotential of commercial Pt/C at 10 mA cm -2 and 20 mA cm -2 are 33 mV and 60 mV, respectively, and the Tafel slope is 44 mV dec -1 , which are similar to the performance in the literature (Nat. Commun. 2020, 11, 1278. In addition, the exchange current density of commercial Pt/C is 2.23 mA cm -2 , and the TOF value at 100 mV shows 8.45 s -1 . The comparison with the properties of the catalyst synthesized in this article is shown in the figure below.

Manuscript Revision:
We changed the original sentences to "Correspondingly, Ru-Mo 2 C@CNT catalyst showed lower overpotential than commercial Pt/C (33 mV), which further indicated that Ru-Mo 2 C@CNT had an excellent electrocatalytic HER activity." and "In 1.0 M KOH solution, Ru-Mo 2 C@CNT only showed a low Tafel slope of 26 mV dec -1 (Figure 3b), which was much lower than Pt/C (44 mV dec -1 ), Ru@CNT (76 mV dec -1 ) and Mo 2 C@CNT (197 mV dec -1 ), which implies that the Tafel-volmer mechanism is present in the catalytic process, and the electrochemical desorption of H 2 is the rate determining step." in the revised manuscript (please see 2-3 lines and 7-9 lines (the red-label part) of Page 9 in the revised manuscript). The Figure R1 and Figure  Question 3: In addition, the authors have analyzed their electrochemical data with a minimum effort. Excellent and very good electrochemical data for HER/HOR are typically shown including Butler-Volmer plots, exchange current density with normalization of metal mass and number of catalytically active sites as well as charge transfer coefficient to get a deeper insight into the kinetics and mechanism of HER.
Author Reply 3: Thanks for reviewer's valuable comment and suggestion. We obtained the exchange current density of the catalyst by extrapolating the Tafel diagram. As shown in Figure R1c and R3, the exchange current density has been normalized by the electrochemical surface area of the supported nanocatalyst of this series of materials, and the result shows that the exchange current density of Ru-Mo 2 C@CNT is 4.4 mA cm -2 , which is greater than other catalysts in this article. In order to compare the exchange current density of materials in other literatures more clearly, the exchange current density of the catalyst Ru-M x C@CNT (M=Mo,Co,Cr) and other materials in the literatures are listed in the Table R1.
In addition, we calculated the exchange current density (j 0 ) with normalization of catalyst mass of the catalyst Ru-Mo 2 C@CNT in this paper is 440 mA cm -2 mg -1 , which is also greater than the commercialized Pt/C (223 mA cm -2 mg -1 ).
Moreover, the number of active sites (n) of the catalyst is calculated by the following formula. n = * . As shown in Figure R4, the catalyst Ru-Mo 2 C@CNT has the largest number of active sites, which is 2.1*10 -3 mol g -1 Ru , which is larger than commercial Pt/C (1.6*10 -3 mol g -1 Pt ). The number of active sites for other catalysts are shown in Figures R4 and R5.
Due to the limitation of experimental conditions, we cannot carry out the HOR reaction of the material, nor can we obtain a complete Butler-Volmer plots and charge transfer coefficient. However, from the Tafel slope and the above exchange current density and the number of active sites of the catalysts, we deeply explained the reaction kinetics and mechanism of HER from a certain aspect, which also shows that the catalyst has excellent electrocatalytic HER activity. Figure R3. Comparison of overpotential changes at 10 mA cm -2 and exchange current density of Ru-Co 3 C@CNT, Ru-Cr 23 C 6 @CNT and Ru-Mo 2 C@CNT catalysts.  Figure R5. The active sites using the Cu-UPD method of Ru-Mo 2 C@CNT, Ru-Cr 23 C 6 @CNT and Ru-Co 3 C@CNT. mA cm -2 , respectively. Obviously, the intrinsic catalytic activity of Ru-Mo 2 C@CNT with a small-sized Ru-Mo 2 C heterostructure is the best, and it exhibits the best HER performance in alkaline media." has been added in the revised manuscript (please see 14-17 lines (the red-label part) of Page 9 in the revised manuscript). And the corresponding sentence "The exchange current densities of Ru-Co 3 C@CNT and Ru-Cr 23 C 6 @CNT are also extrapolated from Tafel, which are 2.9 and 3.8 mA cm -2 , respectively." has also been added in the revised manuscript (please see 12-13 lines (the red-label part) of Page 11 in the revised manuscript). The Figure R3 has been corrected in the Manuscript (please see Supplementary Figure 28c). The Table   R1 has been given in the Supporting Information (please see Supplementary Table 4). The references (ACS Catal. 2016, 6, 1929-1941ChemSusChem, 2018ChemSusChem, , 11, 2388ChemSusChem, -2401 have been added in the revised manuscript as Reference 56 and 57 on page 23.
Question 4: Furthermore, the determination of the ECSA via underpotential deposition of Cu is completely wrong. In Figure S16 and S28, the anodic peaks of a Cu monolayer appear at the same potentials like the redox potential of Cu/Cu 2+ . The authors mainly observed a bulk dissolution of metallic Cu and this fact explains the high ECSA values.
Author Reply 4: Thanks for your valuable suggestion. We retested the Cu UPD (Figures R4 and R6) of the catalysts and corrected the ECSA. The ECSA values are calculated by formulas ECSA (cm 2 metal /g metal )= × . As shown in Figures R5 and R7, after recalculation, the ECSA of Ru-Mo 2 C@CNT is 97.6 m 2 g -1 Ru , this result indicates that the electrochemically active area of the catalyst is larger than some other materials reported in the literature.   Manuscript Revision: The corresponding sentence "The ECSA of Ru-Mo 2 C@CNT is 97.6 m 2 g -1 Ru , which is larger than Pt/C (73.8 m 2 g -1 ), Ru@CNT (69.1 m 2 g -1 ) and Mo 2 C@CNT (42.8 m 2 g -1 ) (Supplementary Figure 18)." has been corrected in the revised manuscript (please see 21-23 lines (the red-label part) of Page 9 in the revised manuscript). The corresponding sentence "When evaluating HER electrocatalysts, the turnover frequency (TOF) and the overpotential at 10 mA cm -2 respectively reveal the intrinsic activity of the catalyst and the potential for practical applications. According to the estimated number of active sites, the TOF value of each active site of Ru-Mo 2 C@CNT, Pt/C, Ru@CNT and Mo 2 C@CNT in alkaline electrolyte was calculated (Supplementary Figures 18)." has been corrected in the revised manuscript (please see 2-5 lines (the red-label part) of Page 10 in the revised manuscript). And the corresponding sentence "By calculation, the ECSA of Ru-Co 3 C@CNT and Ru-Cr 23 C 6 @CNT catalysts were 76.2 and 88.1 m 2 g -1 Ru , respectively (Supplementary Figure 30c). The TOF was obtained by the same method, as shown in Supplementary Figure 28d, Ru-Co 3 C@CNT, Ru-Cr 23 C 6 @CNT and Ru-Mo 2 C@CNT all showed a large TOF value, which was 10.3, 9.2 and 21.9 s -1 under the overpotential of 100 mV, respectively." have been corrected in the revised manuscript (please see 15-19 lines (the red-label part) of Page 11 in the revised manuscript).
The Figure R4, Figure R5, Figure R6 and Figure Figure S32, the Nyquist plot starts for all materials nearly at Z´=0 Ohm. This is an observation which can not be explained by the presented data.
Author Reply 5: Thanks for your kind suggestion. After retesting, the Nyquist diagrams of all materials almost start around Z´= 2.1 ohms, which indicates that the solution resistance (or contact resistance) Rs is small.  "Besides, the electrochemical impedance spectroscopy (EIS) (Supplementary Figure 32) showed that both Ru-Co 3 C@CNT (24.1 Ω) and Ru-Cr 23 C 6 @CNT (22.4 Ω) also had smaller impedance values, indicating that such materials had higher charge transfer rates and easier HER reaction kinetics." in the revised manuscript (please see 13-15 lines (the red-label part) of Page 10 and 1-3 lines (the red-label part) of Page 12 in the revised manuscript).
The Figure R7 and Figure R8 have been corrected in the Supporting Information, respectively (please see Supplementary Figure 21 and Supplementary Figure 32.).
Question 6: It is also frustrated to see that the authors did not make enough effort to measure their materials in a proper manner. The Raman, XRD, EDX and other data are presented in a poor quality.

To Reviewer 3
General Comment: Electrocatalytic hydrogen evolution reaction (HER) by splitting water has become an effective method for the sustainable production of H 2 . It is highly desirable and imperative to develop new HER electrocatlysts with low-cost and high-performance. Herein, the authors reported a simple, fast and solvent-free microwave pyrolysis method for the synthesis of ultra-small (3.5 nm) Ru-MxC@CNT (M=Mo, Co, Cr) catalyst with heterogeneous structure and strong metal-support interaction in one step. The fabricated Ru-Mo 2 C@CNT catalyst exhibits a low overpotential of 15 mV at a current density of 10 mA cm -2 , and exhibits a large TOF value up to 57.8 s -1 under an overpotential of 100 mV. This paper is interesting, and I recommend this paper can be accepted after following revisions.
Author Reply: Thank you for the valuable confirmation of our work. We highly appreciate your efforts in reviewing our work and giving valuable comments. Based on your valuable comments, we have supplied the detailed.
Question 1: The author claims that the synthesis of Ru-Mo 2 C@CNT, however, in Figure 1, only one Ru-Mo 2 C nanoparticle is seen in Figure 1c, indicating that the Ru-Mo 2 C nanoparticles are not ubiquitous on CNT. So the TEM image with more Ru-Mo 2 C nanoparticles should be provided. This is crucial for this paper.
Author Reply 1: Thanks for your helpful comment and suggestion. To show that the presence of Ru-Mo 2 C nanoparticles on CNTs is universal, TEM images with more Ru-Mo 2 C nanoparticles are added and shown in Figure R1d. Author Reply 2: Thanks for reviewer's valuable suggestion. After experimental testing, we found that when the content ratio of Ru and Mo 2 C is 2:1, Ru-Mo 2 C@CNT has the best HER catalytic activity ( Figure R2 a-b). Although the content ratio of Ru and Mo 2 C is different, these catalysts all have excellent electrochemical stability (Figure R 2c).

Manuscript Revision:
We changed the original sentence to "Based on the 1:1 ratio of metal and support, the best content ratio of Ru: Mo 2 C was explored. Among them, the initial reactants Ru 3 (CO) 12 and Mo(CO) 6 with mass ratios of 1:2, 1:1, and 2:1 were placed in a microwave oven for reaction." (please see 1-2 lines (the red-label part) of Page 6) and "Electrochemical hydrogen evolution tests were performed on three samples of different proportions in 1.0 M KOH solution, and we found that the electrochemical performance was the best when the Ru:Mo 2 C element content ratio was 2:1 (Supplementary Figure 5)." (please see 4-7 lines (the red-label part) of Page 6) in the revised manuscript.
The Figure R2 has been given in the Supporting Information (please see Supplementary Figure 5

.)
Question 3: How about the long-term durability of Ru-Mo 2 C@CNT for HER in alkaline media? As shown in Figure 4f, the test time of durability is too short, and the authors should measure the durability of HER for at least 50 hours.
Author Reply 3: Thanks for reviewer's valuable comment and suggestion. We have retested the electrochemical stability of the catalyst in 1.0 M KOH electrolyte for 100 hours, and the results also prove that Ru-Mo 2 C@CNT has good long-term durability.

Manuscript Revision:
We changed the original sentence to "The current-time (i-t) test (Figure 4f) showed that the current density remained almost constant for 100 hours." in the revised manuscript (please see 6-7 lines of Page 12 in the revised manuscript).
The Figure R3 has been given in the Manuscript (please see Supplementary Figure 28f.) Question 4: How do you prove the structure stability of Ru-Mo 2 C@CNT for long-term durability of HER ？Please provide some evidences for structure stability of Ru-Mo 2 C@CNT after durability test.
Author Reply 4: Thanks for your helpful suggestion. In the supporting information of the article, we provide the SEM, TEM and XRD data of the Ru-Mo 2 C@CNT catalyst after the HER long-term stability test to prove that the catalyst has good structural stability. In order to further prove the excellent stability of this catalyst, we have added its XPS test. As shown in Figure R4, after the HER reaction, the peaks of each element basically did not change.

Manuscript Revision:
We changed the original sentence to "In addition, the SEM and TEM images of Ru-Mo 2 C@CNT after the long-term stability test showed no change in the morphology of the material, and the XRD and XPS images further reflected that the structure of this material did not change ( Supplementary Figures 35-36)." in the revised manuscript (please see 10-12 lines of Page 13 in the revised manuscript).
The Figure R4 has been given in the Supporting Information (please see Supplementary Figure 36.)

Question 5:
The loading of Ru-Mo 2 C on CNTs should be provided for Figure 4, and the effect of the loading of Ru-Mo 2 C on CNTs on the catalytic activity and stability should be studied. Figure R5, the ratio of Ru-Mo 2 C to CNT is 1:1, that is, 5 mg of metal elements are loaded on 5 mg of CNT. In addition, we studied the effects of different Ru-Mo 2 C loads on the catalytic activity and stability of CNTs, as shown in Figure R5. The results showed that the catalyst had the best catalytic activity when the contents of Ru-Mo 2 C and CNTs were the same. When the content of Ru-Mo 2 C was lower than that of CNTs, the number of active sites decreased and the catalytic activity decreased. When the content of Ru-Mo 2 C was higher than that of CNTs, too much load blocked part of the active sites, and the catalytic activity decreased slightly. Therefore, only when the content of Ru-Mo 2 C was the same as that of CNTs, the active sites were most fully exposed and the catalytic activity was the best. In addition, the catalysts with different Ru-Mo 2 C loading loads have good electrochemical stability.  Figure 4g, to maintain the current density at 500 mA cm -2 , the overpotential should be provided.

Author Reply 5: Thanks for your valuable suggestions. For the catalyst in
Author Reply 6: Thanks for your helpful suggestion. As shown in Figure R6, we have supplemented the chronopotentiometric test of the Ru-Mo 2 C@CNT catalyst supported on Ni foam at 500 mA cm -2 . The results also prove that the Ru-Mo 2 C@CNT material has excellent electrochemical stability required by the industry. Figure R6. The chronopotentiometric curve of the Ru-Mo 2 C@CNT electrode tested at a constant current density of 500 mA cm -2 for 500 h.

Manuscript Revision:
The corresponding sentence "In addition, as shown in Supplementary Figure 33, the chronopotentiometric curve of the Ru-Mo 2 C@CNT electrode was tested at a constant current density of 500 mA cm -2 for 500 h. And this result further prov that the Ru-Mo 2 C@CNT material has excellent stability." has been added in the revised manuscript (please see 6-8 lines (the red-label part) of Page 13 in the revised manuscript).
The Figure R6 has been given in the Supporting Information (please see Supplementary Figure 33.) Question 7: Why does Ru-Mo 2 C have higher catalytic performance than the other Ru-Co 3 C@CNT, Ru-Cr 23 C 6 @CNT? Please provide some explains in the paper.

Author Reply 7:
Thanks for your kind comments. According to the analysis of the peaks in the 3p orbital of Ru in the XPS data of Ru-Mo 2 C@CNT, Ru-Co 3 C@CNT and Ru-Cr 23 C 6 @CNT, it can be obtained by adding the Mo element, the binding energy of Ru element moved to a higher position by nearly 0.31 eV, and after adding Co and Cr elements, the binding energy moved 0.46 eV and 0.19 eV, respectively. After a lot of investigation literature, we found that proper electron transfer on the catalyst surface can promote the electrocatalytic performance of the material (Angew. Chem. Int. Ed. 2021, 60, 4110-4116;Angew. Chem. Int. Ed. 2014, 53, 122-126.). For example, the literature [Adv. Mater. 2020, 32, 2005433] shows that the Ru 3p binding energy of RuMo nanoalloy-embedded 2D porous carbon (2DPC-RuMo) nanosheets slightly shifts to higher binding energy ~0.3 eV, the electronic structure is adjusted to the most suitable position, which further indicates that proper electron transfer is beneficial to promote alkaline

Ru-Mo 2 C@CNT
HER Catalytic activity. In addition, the surface of the 2DPC-RuMo catalyst has the best Gibbs free energy of intermediates corresponding to the chemically adsorbed H*. Moreover, after a lot of research literature, we found that both the doping of Mo atoms [Adv. Mater. 2020, 32, 2005433] and the formation of Mo 2 C [Adv. Funct. Mater. 2019, 29, 1901217] can provide an easier water dissociation process. Therefore, compared with Ru-Co 3 C@CNT and Ru-Cr 23 C 6 @CNT, Ru-Mo 2 C@CNT has greater advantages in regulating electronic structure and promoting water dissociation, and therefore its electrocatalytic HER activity is higher.  Int. Ed. 2017, 56, 2960Angew. Chem. Int. Ed. 2017, 56, 8120;J. Am. Chem. Soc. 2018, 140, 5118.) have been added in the revised manuscript as Reference 9, 15 and 6 on Page 18 and page 19.
Question 9: For supplementary Figure 21, the TEM image of Ru-Co 3 C@CNT does not show the existence of heterojunction, please provide another TEM.
Author Reply 9: Thanks for your comments. In order to prove the existence of heterojunction in Ru-Co 3 C@CNT catalyst, we added HRTEM images with heterojunction, and further proved the successful preparation of Ru-Co 3 C@CNT catalyst (Figure R7). At the same time, we also added HRTEM images of Ru-Cr 23 C 6 @CNT with heterojunction ( Figure R8). Figure R7. HRTEM image of Ru-Co 3 C@CNT. Figure R8. HRTEM image of Ru-Cr 23 C 6 @CNT.

Manuscript Revision:
The Figure R7 and Figure R8 have been given in the Supporting Information, respectively (please see Supplementary Figure 22d and Supplementary Figure 25d.).
In the end, we would like to express our thanks to the precious time of the reviewers and the editor. We sincerely wish that our point-to-point response and the revised manuscript can address your concerns and satisfy your requirements for publications. We would be grateful if we have the chance to share our work with readers of Nature Communications.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): Thank you for considering the comments, I also thank reviewers 2 and 3 for bringing up very important observations about this paper. This paper has brought insight into a novel material for HER, and showed how it fared against similar materials. With the corrections applied, I believe this paper is suitable for publishing.
Reviewer #2 (Remarks to the Author): The revised manuscript reported by Wu et al. addressed most of the comments by the reviewer. The presented reference material (Pt/C) is now in the well-known range.
Page 9, line 12/13: Some statements are not correct in view of the referee. The referee cannot agree with the proposed rate determining step. The following statement needs revision or more explanation about the Volmer-Tafel versus Volmer-Heyrovsky. Why is the H2 desorption an electrochemical step?
The preparation of MWCNT in the Methods Section is removed in the present version. Please explain.
In the view of catalytic improvement by support / metal interaction this information should not be missing.
Furthermore, it's hard for the readership to figure out which the real catalyst loading (e.g. wt.% metal) was applied. Please calculate the values and provide the information within the manuscript.
The referee is wondering about the minimum effort to analyze the data by using We highly appreciate your kind consideration and review on our paper entitled "Solvent-free microwave synthesis of ultra-small Ru-Mo2C@CNT with strong metal-support interaction for industrial alkaline hydrogen evolution reaction" . We have carefully revised the manuscript and responded all the raised valuable comments by three reviewers. We have also highlighted the changes in manuscript and supporting information with red color. Thanks to the valuable suggestions, our manuscript could be significantly improved. We response to the reviewers' comments point by point and highlight the changes in the revised manuscript. The list of changes and our responses to three reviewers' comments are provided as follows.
Dear Reviewers, Thank you for your precious time to constructive comments on our manuscript titled "Solvent-free microwave synthesis of ultra-small Ru-Mo2C@CNT with strong metal-support interaction for industrial alkaline hydrogen evolution reaction" (Manuscript ID: NCOMMS-20-48819) for Nature Communications. We sincerely appreciate your opinions and confirmation of our work. Accordingly, we have supplied the corresponding response and revision based on the comments. We sincerely hope that our responses will fully address your concerns about our work.

To Reviewer 1:
General Comment: Thank you for considering the comments, I also thank reviewers 2 and 3 for bringing up very important observations about this paper. This paper has brought insight into a novel material for HER, and showed how it fared against similar materials. With the corrections applied, I believe this paper is suitable for publishing.
Author Reply: Thank you for your precious support and appreciation of our work. Your kind comments are very supportive to our present work and our future works in this field. We sincerely hope that our work will deliver a novel research to the electrocatalyst design in the future.

To Reviewer 2:
General Comment: The revised manuscript reported by Wu et al. addressed most of the comments by the reviewer. The presented reference material (Pt/C) is now in the well-known range.
Author Reply: Thank you for the valuable confirmation of our work. We highly appreciate your efforts in reviewing our work and giving valuable comments. Based on your valuable comments, we have supplied the detailed.
Question 1: Page 9, line 12/13: Some statements are not correct in view of the referee. The referee cannot agree with the proposed rate determining step. The following statement needs revision or more explanation about the Volmer-Tafel versus Volmer-Heyrovsky. Why is the H2 desorption an electrochemical step?
Question 2: The preparation of MWCNT in the Methods Section is removed in the present version. Please explain. In the view of catalytic improvement by support / metal interaction this information should not be missing.
Author Reply 2: Thanks for your valuable comment. I have added its specific experimental steps to the manuscript.

Manuscript Revision:
The corresponding sentence "Preparation of MWCNT. Disperse 50 mg MWCNT powder in a mixed solution with a concentration of H2SO4: HNO3=3:1 for ultrasonic treatment for 1-2 h, and then expose it to 1 M HCl for ultrasonic treatment for 30 minutes. Finally, filter the acidified CNT, wash it with ionized water until pH = 7, and dry it at 60℃ for 12 h." has been added in the revised manuscript (please see 1-5 lines (the red-label part) of Page 15 in the revised manuscript).
Question 3: Furthermore, it's hard for the readership to figure out which the real catalyst loading (e.g. wt.% metal) was applied. Please calculate the values and provide the information within the manuscript.
Author Reply 3: Thanks for reviewer's valuable comment and suggestion. In order to enable readers to get a clearer view of the metal content in the catalyst, we have carried out a detailed description of the catalyst content in the manuscript.

Manuscript Revision:
The corresponding sentence "And it was estimated by ICP-AES results that the loading of Ru-Mo2C in the catalyst Ru-Mo2C@CNT is about 19 wt%." has been added in the revised manuscript (please see 7-8 lines (the red-label part) of Page 6 in the revised manuscript). The corresponding sentence "(catalyst loading is 0.14 mg cm -2 , equals to a Ru-Mo2C loading of ca. 0.03 mg cm -2 )" has also been added in the revised manuscript (please see 20 line (the red-label part) of Page 8 in the revised manuscript). And the corresponding sentence "(When the Ru-Mo2C loading amount in the catalyst supported on NF is 0.95 mg cm -2 , the catalyst Ru-Mo2C@CNT achieved the industrial current densities of 500 mA cm -2 and 1000 mA cm -2 at low overpotentials of 56 mV and 78 mV (Figure 4d), respectively.)" has also been added in the revised manuscript (please see 3-4 lines (the red-label part) of Page 13 in the revised manuscript).

Question 4:
The referee is wondering about the minimum effort to analyze the data by using Butler-Volmer equation in a careful way. Numerous calculations are not clear or based on highly distributed values. Please show the Butler-Volmer plots, discuss the transfer coefficient for HER/HOR and explain the calculated values.
Author Reply 4: Thanks for your valuable suggestion. We supplemented the Butler-Volmer plots of HER/HOR, and obtained the symmetry factor and exchange current density of Ru-Mo2C@CNT and commercial Pt/C according to the Koutecky-Levich equation and Butler-Volmer equation. As shown in R1 and R2, the transfer coefficients of Ru-Mo2C@CNT and commercial Pt/C are 0.45 ( Figure R1a) and 0.47 (Figure R2a), respectively. All curves can be installed in the range of 0.4-0.6, indicating that the branches of HOR and HER have good symmetry. The j0 values obtained from linear fitting of micropolarization regions (Figure R1b and R2b) are consistent with the values of j0 obtained from Bulter-Volmer fitting. And, it is almost consistent with the results obtained by tafel slope extrapolation before. where j is the measured current density, η is the overpotential, R is the universal gas constant, T is the temperature, and F is Faraday's constant. Therefore, the exchange current density (j0) can be obtained from the slope of the linear fitting of j-η curve in micropolarization regions. = 0.62nFD 2/3 υ -1/6 C0ω 1/2 in which F is the Faraday constant (96 485 C mol -1 ), n is the number of electrons involved in the oxidation reaction, C0 is the H2 concentration in solution, D is the diffusion coefficient of the reactant (cm 2 s -1 ), υis the viscosity of the electrolyte (cm 2 s -1 ), andωis the rotation speed (rpm).
The Figure R1 and Figure R2 have been given in the Supporting Information, respectively. (please see In the end, we would like to express our thanks to the precious time of the reviewer. We sincerely wish that our point-to-point response and the revised manuscript can address your concerns and satisfy your requirements for publications. We would be grateful if we have the chance to share our work with readers of Nature Communications.
Sincerely yours, Lei Wang