Manipulating dehydrogenation kinetics through dual-doping Co3N electrode enables highly efficient hydrazine oxidation assisting self-powered H2 production

Replacing sluggish oxygen evolution reaction (OER) with hydrazine oxidation reaction (HzOR) to produce hydrogen has been considered as a more energy-efficient strategy than water splitting. However, the relatively high cell voltage in two-electrode system and the required external electric power hinder its scalable applications, especially in mobile devices. Herein, we report a bifunctional P, W co-doped Co3N nanowire array electrode with remarkable catalytic activity towards both HzOR (−55 mV at 10 mA cm−2) and hydrogen evolution reaction (HER, −41 mV at 10 mA cm−2). Inspiringly, a record low cell voltage of 28 mV is required to achieve 10 mA cm−2 in two-electrode system. DFT calculations decipher that the doping optimized H* adsorption/desorption and dehydrogenation kinetics could be the underlying mechanism. Importantly, a self-powered H2 production system by integrating a direct hydrazine fuel cell with a hydrazine splitting electrolyzer can achieve a decent rate of 1.25 mmol h−1 at room temperature.

The authors proposed a facile two-step strategy to synthesize P,W-codoped cobalt nitride nanowire array material. They characterized the structure and morphology by a number of techniques such as TEM, SEM, XPS, and XRD. They further demonstrated the potential application as bifunctional electrocatalyst for hydrazine oxidation and hydrogen evolution. The catalytic mechanism has been studied by density function theory calculation. This work is publishable in Nature Communicaiotn after addressing the following concerns: (1) In Figure 5a, the comparison is not fair because of different thermodynamic potentials of OWS (1.23 V) and OHzS (-0.33 V). I suggest the comparison of faraday efficiencies and overpotentials.
(2) A table should be given to compare the activity of HzOR and HER with the state-of-the-art literature.
(3) What is the efficiency from hydrazine to hydrogen for self-powered H2 production system? The authors should compare it with other hydrogen generation systems.
(4) The authors described that their catalyst has a "Pt-like" activity. They may need to compare the activity of HER with commercial Pt/C. Further, DFT calculation of HER free energy for Pt should also be given for comparison. (5) Hydrazine is a highly toxic chemical. Therefore, the authors should discuss further the potential problems and challenges for the large-scale applications of as-proposed self-powered H2 production system. NWA/NF. Specifically, W-Co 3 N NWA/NF required working potentials of -48, 65 and 239 mV to achieve anodic current density of 10, 200 and 600 mA cm −2 . Meanwhile, to reach anodic current density of 10, 200 and 600 mA cm −2 , P-Co 3 N NWA/NF needed working potentials of -47, 67 and 191 mV, while the corresponding values for PW-Co 3 N NWA/NF were only -55, 27 and 127 mV, respectively. As for HER, PW-Co 3 N NWA/NF still owned the best HER activity among all the doped samples. These results indicated that dual doping of P and W is able to improve the electrochemical activity more effectively than the solo doping of P or W. The related material characterizations and electrochemical performance measurements have also been added to Main Text and Supplementary Information part of the revised manuscript. Please see  2. All the results were obtained using related materials (PW co-precursor, Co 3 N, Ni foam) to the current co-doped catalyst, it would be good to use some other known catalysts such as Pt to provide an extra external reference.
Our response: We really appreciate the reviewer for the professional suggestion. According to your suggestion, the HzOR and HER catalytic performance of commercial Pt/C (20 wt.%) with the identical mass loading of 2 mg cm -2 to that of PW-Co 3 N NWA/NF sample have been investigated as comparison and the results were also added to Fig. 4a-b, Fig. 5a-b, Supplementary Fig. 16 and Supplementary Fig. 19-20 of the revised manuscript, which were also provided below for your convenience. As indicated, our PW-Co 3 N NWA/NF exhibited much better electrocatalytic activity for HzOR compared with that of Pt/C in 1.0 M KOH/0.1 M N 2 H 4 .
Specifically, Pt/C required working potentials of 14 and 223 mV to achieve anodic current density of 10 and 200 mA cm −2 . For HER, the overpotential of PW-Co 3 N NWA/NF is 9 mV larger than that of Pt/C (32 mV) at 10 mA cm -2 , while the Tafel slope of PW-Co 3 N NWA/NF is 40 mV dec -1 , which is also comparable to commercial Pt/C (31 mV dec -1 )The electrochemical performance of known benchmark Pt/C as an extra external reference further proved the exciting HzOR and HER performance of PW-Co 3 N NWA/NF. The related discussions are also added to the main text of the revised manuscript. Please see Lines 10 and 21, Page 13; Lines 3 and 22-23, Page 15; Line 13, Page 16.  Polarization curves of PW-Co 3 N NWA/NF before and after CV testing of 1000 and 5000 cycles.
The inset is corresponding Nyquist plots. (d) The chronoamperometric test recorded at overpotential of 92 mV. different materials; CV curves of (b) PW-Co 3 N NWA/NF, (c) W-Co 3 N NWA/NF, (d) P-Co 3 N NWA/NF, (e) Co 3 N NWA/NF, (f) Ni foam, (g) PW-Co-precursor and (h) Pt/C in the double layer capacitive region at the scan rates from 10 mV to 100 mV s -1 . Figure 20. Nyquist plots of PW-Co 3 N NWA/NF, W-Co 3 N NWA/NF, P-Co 3 N NWA/NF, Co 3 N NWA/NF, Ni foam, PW-Co-precursor and Pt/C obtained at overpotential of 100 mV for HER. The inset is the enlarged view of Nyquist plots of PW-Co 3 N NWA/NF, W-Co 3 N NWA/NF, P-Co 3 N NWA/NF, Co 3 N NWA/NF, Ni foam, PW-Co-precursor and Pt/C.

Supplementary
3. On page 13, can the authors comment on why the electrochemical surface area for PW-Co 3 N is so much bigger or have more number of accessible active sites?
Our response: We thank the reviewer for the good question. Indeed, the electrochemical surface area of PW-Co 3 N NWA/NF is nearly two times larger than that of pure Co 3 N NWA/NF. The possible reasons for the much enhanced electrochemical surface area can be illustrated as bellows: Firstly, the P/W co-doping could effectively modulate the electronic structure of Co 3 N confirmed by the experimental data based on the XPS and XANES results ( Fig. 2 and Fig. 3 in the revised manuscript) together with the added DFT calculation on charge density difference analysis ( Fig. 8h-i in the revised manuscript). It can be concluded that the electron density around Co and N atoms can be effectively modulated after doping, which could contribute to the catalytic performance of PW-Co 3 N. Secondly, the hydrogen absorption kinetics can be modified to more thermoneutral and the H 2 O adsorption is further facilitated after doping according to the DFT calculation ( Fig. 8e and g in the revised manuscript), which can further enhance the catalytic activity (Nat. Commun. 2019, 10, 1743Angew. Chem. Int. Ed. 2019, 58, 11903-11909).
In short, the much enhanced electrochemical surface area can be attributed to the doping induced electronic structure modulation and hydrogen absorption kinetics manipulation, which have also been observed in previous literatures (Adv. Mater. 2019, 31, 1901174;Angew. Chem. Int. Ed. 2018, 57, 5076 -5080;Adv. Funct. Mater. 2017, 27, 1704169). 4. The authors cited "best reported values" in many places. It would be much more informative if they listed a few concrete values and systems.
Our response: We thank the reviewer for the constructive suggestion, which is very important for us to further improve the quality of our manuscript. According to your suggestion, we added two Tables (Supplementary Table 2 and Table 3) to compare the HzOR and HER performance of PW-Co 3 N NWA/NF and recently reported literatures with more detailed information such as materials system, detailed overpotential/working potential values and electrolyte compositions.
The relevant discussions have also been added and highlighted in the main text of the revised manuscript (Lines 16-18, Page 13; Lines 16-19, Page 15). We also provided the Tables below for your convenience. 5. On page 16, it mentioned "energetically favorable to replace surface Co atoms …". Please provide those energetic data.

Supplementary
Our response: We thank the reviewer for the good suggestion. According to your suggestion, we provided the energetic data of two different models as Supplementary Fig. 26 of the revised manuscript, which is also provided below for your convenience. As can be seen, the total energy of system after surface substitution of 4 Co with 4 W atoms at Co 3 N (001) plane is -328.12 eV with no visible structural distortion, which is consistent with the XRD results before and after P/W doping. Contrastively, there will be serious structural distortion for the system with 4 W doped at the subsurface of Co 3 N (001) planes where the N atoms of second layer totally deviate from its original position, as shown in Supplementary Fig. 26b. This is not reasonable since the crystal structure can be well maintained after W doping according to the XRD results, although the total energy (-333.76 eV) is lower compared to the system with surface doping model. Based on these results, we adopt the optimal structure of surface doping model during our calculation.

Supplementary Figure 26. Model simulation of 4 W atoms and 1 P atom substituted Co 3 N.
(a) Model simulation of 4 W atoms substituted surface Co sites and (b) substituted subsurface Co sites, before (left) and after (right) geometric optimization; The balls in yellow, pink, green and purple represent Co, W, P and N atoms, respectively.
6. While the hydrogen adsorption free energy becomes less negative in the co-doped case, it is still far from the conventional optimal value of 0. Even pure Ni has free energy of ~-0.3 eV. It is not obvious from Fig. 7b, why the catalyst has favorable HER activity.
Our response: We thank the reviewer for the professional question. In order to further understand the possible reasons for the favorable HER activity of the PW-Co 3 N NWA/NF, we conducted the extra DFT calculations on d-band center analysis, the water absorption energy and charge density difference analysis. The related results are added to Fig. 8 in the revised manuscript, which are also provided below for your convenience. As indicated, the d-band center of pure Co 3 N (-1.41 eV) notably shifts down to -1.54 eV after P/W doping (Fig. 8f), which clearly demonstrates that hydrogen desorption is promoted besides the decreasing of hydrogen absorption energy. In addition, the adsorption energy of H 2 O (ΔE H2O ) on Co 3 N (001) and PW-Co 3 N (001) are -0.31 and -0.56 eV (Fig. 8g)  Our response: We really appreciate the reviewer for the good suggestion. According to your suggestion, we have modified the color scheme of Fig. 8 in the revised manuscript.
8. I would recommend performing some charge analysis, in the form of calculating Bader charges or obtaining charge density difference plot, to understand what is the impact of codoping on Co 3 N.
Our response: We thank the reviewer for the good suggestion. According to your suggestion, we performed charge density difference plot of PW-Co 3 N, as shown below. As can be seen, the 9. Significant justification is needed for the co-doping calculation model. Why the Cotermination unit cell is adopted? What is the current W coverage? Why using 4 W but only one P?
Why only surface substitution is used, but not subsurface ones?
Our response: We really appreciate the reviewer's professional questions. We are sorry that we didn't provide clear instruction about the reason why we adopted the Co-termination unit cell as the model for DFT calculation. In fact, when we performed the DFT calculation, these two different types of unit cells, i. e. Co-terminated and N-terminated were both tried. As shown in Supplementary Fig. 27, the distance between N2H4 molecular and the nearest neighboring N atom is 5.39 Å, which is much larger than the distance of 1.98 Å between N 2 H 4 and Co atom.
This means that the N atom terminated unit cell is not favorable for N 2 H 4 adsorption, which is inconsistent with our experimental results. Therefore, we adopted the Co-termination unit cell as The W coverage is confirmed according to the atomic ratios from EDS and ICP-AES results, which is provided below. In experiment, the P: W: Co is about 1: 3.50: 36.9 calculated from EDS and ICP-AES measurements. In DFT calculation, the atom number for P, W and Co is 1, 4 and 32 in order to be close to the experimental results. The related data were added as Regarding the reason to adopt the surface substitution model during the DFT calculation, we confirmed this according to the calculated energetic data of two different models (See Figure   S26 below). As can be seen from Supplementary Fig. 26a, the total energy of system after surface substitution of 4 Co with 4 W atoms at Co 3 N (001) plane is -328.12 eV with no visible structural distortion, which is consistent with the XRD results before and after P/W doping.
Contrastively, there will be serious structural distortion for the system with 4 W doped at the subsurface of Co 3 N (001) planes where the N atoms of second layer totally deviate from its original position, as shown in Supplementary Fig. 26b. This is not reasonable since the crystal structure can be well maintained after W doping according to the XRD results, although the total energy (-333.76 eV) is slightly lower to the system with surface doping model. Based on these results, we adopt the optimal structure of surface doping model during our calculation. Co sites, before (left) and after (right) geometric optimization; The balls in yellow, pink, green and purple represent Co, W, P and N atoms, respectively.

Reviewer #2 (Remarks to the Author):
This manuscript by Liu and co-workers described P, W co-doped cobalt nitride nanowire arrays for catalytic hydrazine oxidation and hydrogen evolution. In general, the resulted nanocomposites demonstrated quite impressive activities for both reactions, which allowed the integration of a direct hydrazine fuel cell with a hydrazine splitting electrolyzer for self-powered H 2 production. However, the manuscript contains substantial unproven or unnecessary claims and its overall quality does not warrant its suitability for publication in leading journals such as Nature Commun.
Our response: Thank you very much for your precious time on reviewing our manuscript. We have carefully studied your comments and conducted a series of additional experiments with related discussions according to your suggestions. Also, we tried our best to answer your questions point-to-point, which are provided below.
1. The fabrication method is quite routine, and there are many hundreds of similar works to decorate various electrochemically active materials onto Ni foams for various electrocatalytic reasons such as OER, HER, CO 2 RR, N 2 RR, and HzOR. And this work is no exception.
Our response: We thank the reviewer for the comment.
We would like to highlight the novelty and significance of our work in the following aspects:  Table 2 for the detailed information, which is also provided below for your convenience. In addition, as a demonstration for potential applications, we built a "self-powered" H 2 production system using DHzFC driven OHzS based on PW-Co 3 N NWA/NF for both HzOR and HER catalysts, where a decent H 2 production rate of 1.25 mmol h −1 can be achieved. It should be emphasized that the reported H 2 production rate is achieved with the DHzFC working at room temperature with electrolyte composed of 1 M KOH/0.5 M hydrazine and with the two-electrode OHzS electrolyzer working with the 1 M KOH/0.5 M hydrazine electrolyte. The power density of the DHzFC is highly related with the working temperature and the H 2 production rate is also dependent on the concentration of hydrazine in the electrolyte (Adv. Mater. 2015, 27, 2361-2366. The reported performance in our work regarding the maximum power density and the H 2 production rate obtained in the self-power system are superior under the same or close conditions of working temperature and electrolyte components. More detailed explanations are provided regarding the response to the reviewer's comment (4).
(iv) Last but not the least, it is meaningful to disclose the possible origin of the enhanced catalytic activity after doping by density functional theory (DFT) calculations. In our work, the calculation results indicate that the P/W doping can not only manipulate the dehydrogenation kinetics for HzOR, but also simultaneously modulate the hydrogen absorption energy. Moreover, in the revised manuscript, we further performed the charge density difference analysis, combined with experimental results including XPS and XANES characterizations (Fig. 2), which show that the P/W doping can also modulate electronic structure of Co 3 N. Additionally, the added FT and WT of EXAFS characterizations (Fig. 3) in the revised manuscript verify that the doped W substitutes Co site and bonds with P and N. In short, these results can provide a meaningful reference for future research on related material systems. 2. The structure characterization is very poor. There is no discussion of how and where P and W species locate within the Co 3 N lattice. HRTEM imaging and X-ray absorption seem necessary to address this critical issue. The authors did not even provide the chemical composition of their materials system. Without all these key information, what is the reliability for their DFT models?

Supplementary
Our response: We thank the reviewer for the important questions. In order to tackle these questions, we have added aberration corrected HAADF-STEM measurements for PW-Co 3 N NWA/NF with atomic resolution and XAS characterizations (both XANES and EXAFS) in order to further strengthen the structure characterizations.
The atomically resolved HAADF-STEM images are added as Fig. 1h-k in the revised manuscript, which is also provided below for your convenience. As can be seen in Fig. 1h and i, the W atoms can be seen as bright dots in Co 3 N lattice due to Z-contrast in HAADF-STEM image, since the atomic number of W is significantly larger than that of Co. However, the brightness of W atoms is not so obvious due to the relatively thick Co 3 N substrate, especially when viewing along the zone axis. We further tilted the TEM holder to make the specimen off the zone axis, so that the lattice of Co 3 N cannot be clearly seen, and then the brightness of W showed up (Fig. 1j).
The X-ray absorption (XAS) results were added to Fig. 2 and Fig. 3 in the revised manuscript, which are also provided below for your convenience. As indicated, the XANES results for Co L-edge and N K-edge show a positive shift for the Co L 3 /L 2 peaks and a negative shift for N K-edge peaks after P/W doping, which is consistent with the XPS results and can further verify the existed charge transfer due to the P/W doping. This phenomenon is also consistent with the DFT calculation results on charge density difference analysis (Provided as Fig. 8h in the revised manuscript). Moreover, observed from FT of the Co K edge EXAFS spectra (Fig. 3b), the intensity of Co-Co peak decreases obviously in PW-Co 3 N, suggesting the success doping of P/W. Besides, the FT of W L 3 -edge EXAFS spectra (Fig. 3f) declare completely different coordination environment of W in PW-Co 3 N compared with W foil and WO 3 , which further proves that the W is successfully doped into Co 3 N. We also performed FT-EXAFS fitting to get insight on the coordination environment of doping W in PW-Co 3 N, as shown in Fig. 3g, the fitting results are summarized in Supplementary Table 1, which are also provided below for your convince. The fitting results indicate clearly W-N bond, W-P bond and W-Co bond, which confirms that the doped W substitutes Co sites and bonds with P and N.
Moreover, the WT of W L 3 -edge contour plots of PW-Co 3 N (Fig. 3h) exhibit the typical W-Co bond (centered about 7.5 Å -1 ) and Co-N bond (centered about 6.0 Å -1 ), which further supports that W substitutes Co site. The related discussions have also been added to the main text of the manuscript. Please see Lines 7-15, Page 8; Page 17-23, Page 10; Lines 1-23, Page 11; Lines 8-14, Page 12 of the revised manuscript.     As a conclusion, the most important scientific contribution of our work is the first report demonstrating the notable effect of heteroatom doping of transition metal nitride can largely improve its HzOR activity, and simultaneously enhance the HER activity in certain degree. The theoretical DFT calculation combined with systematic experimental data disclose the fundamental origin of the enhanced performance. We are confident that our work regarding the P/W doped Co 3 N nanowire array electrode possesses sufficient novelty regarding the reported strategy for simultaneous enhancement for HzOR and HER activity, as well as significant scientific contribution regarding the fundamental understanding over the structure-property relationship based on the combined DFT calculation and experimental data.
4. By claiming "a record high", "outperforms most of the literatures", "is one of the best reported values", "is superior compared to the reported values", "the exciting potential for highly efficient H 2 productions", "possesses benchmark electrocatalytic activity", "is superior compared to stateof-the-art values", the authors should at least provides sufficient literatures for direct comparison (such as a table?). Unfortunately, the authors did not even conduct control experiments using standard electrode materials such as Pt/C, which makes the data less convincing. For instance, the H 2 evolution rate of 1.25 mmol/h is much smaller than the reported 9.95 mmol/h in ref. 15.
The observed maximum power density of 46.3 mW/cm 2 as a direct hydrazine fuel cell is actually among the lowest in this field! Several hundreds of milliwatts are quite easy to achieve.
Our response: We thank the reviewer for the comment. Firstly, it is necessary to clarify that the maximum power density of the DHzFC is highly related with the working temperature and the H 2 evolution rate in two-electrode OHzS electrolyzer is highly dependent on the hydrazine respectively, in order to provide direct comparison with sufficient literatures. Besides, the HzOR and HER performance of commercial Pt/C have been collected. Please see Fig. 4a-b, Fig. 5a-b, Supplementary Fig. 16 and Supplementary Fig. 19-20 in the revised manuscript, which are also provided below for your convenience. For HzOR, the activity of PW-Co 3 N NWA/NF notably outperforms that of Pt/C, which requires working potentials of 14 and 223 mV to achieve anodic current density of 10 and 200 mA cm −2 . For HER, the overpotential of PW-Co 3 N NWA/NF is 9 mV larger than that of Pt/C (32 mV) at 10 mA cm -2 with comparable Tafel     5. The possible contribution of Ni foam should be noticed. The authors should be aware that nickel compounds such as Ni 2 P show very good electrocatalytic activity in this system.

Our response:
We thank the reviewer's professional question. It is true that nickel compounds like Ni 2 P has been reported possessing excellent catalytic activity for both HzOR and HER (Angew. Chem. Int. Ed. 2017, 56, 842-846). However, in our PW-Co 3 N NWA/NF sample, the XRD results ( Supplementary Fig. 2) indicated that there is no signal for CoP or Ni 2 P phase.
Moreover, the atomic ratio in PW-Co 3 N/NF for P: W: Co is confirmed to be 1: 3.50: 36.9 by Ni foam annealed at 420 o C for 2 h under NH 3 atmosphere. The existence of phosphorus and nitride in the prepared material (denoted as PN/NF) was confirmed via EDS spectrum (added as Supplementary Fig. 21). The as-obtained PN-Ni foam exhibits much poorer HzOR and HER performance compared with that of PW-Co 3 N NWA/NF (Added as Supplementary Fig. 22 7. Many typos exist, such as "the mixer of H2 and O2", "a ultrasmall", ...

Our response:
We thank the reviewer for pointing out these errors for us. We have carefully rechecked our manuscript and corrected the grammatical errors and typos, which are highlighted in the main text of the revised manuscript.

Reviewer #3 (Remarks to the Author):
The authors proposed a facile two-step strategy to synthesize P, W-codoped cobalt nitride nanowire array material. They characterized the structure and morphology by a number of techniques such as TEM, SEM, XPS, and XRD. They further demonstrated the potential application as bifunctional electrocatalyst for hydrazine oxidation and hydrogen evolution. The catalytic mechanism has been studied by density function theory calculation. This work is publishable in Nature Communications after addressing the following concerns: Our response: We thank the reviewer for the encouraging comments on our manuscript. It is greatly appreciated for the reviewer to provide constructive suggestions, which is quite helpful and significant for us to further improve the quality of the manuscript. In the revised manuscript, we have conducted a series of additional experiments and provided related discussions following your comments and suggestions.
(1) In Figure 5a, the comparison is not fair because of different thermodynamic potentials of OWS (1.23 V) and OHzS (-0.33 V). I suggest the comparison of faraday efficiencies and overpotentials.
Our response: We thank the reviewer for the professional suggestion. As you suggested, we calculated the corresponding overpotentials in Fig. 6b of the revised manuscript considering the thermodynamic cell voltage of 1.23 V for OWS and -0.33 V for OHzS. Specifically, it only requires the overpotentials (compared to the theoretical value of -330 mV) of 358, 428, 501 and 607 mV in OHzS system to reach current densities of 10, 50, 100 and 200 mA cm -2 (Fig. 6b), respectively, while much higher (compared to the theoretical value of 1230 mV) overpotential of 350, 530, 650 and 869 mV are required in the case of OWS to obtain the same current density.
The results proved that our OHzS system not only demands less electric energy but also exhibits the feasible kinetics considering the thermodynamic potentials for H 2 production. The Faraday efficiency of OWS is measured to be about 94% (Fig. 6d), which is slightly lower than that of OHzS system (96%). The related discussions have also been added to the main text of the revised manuscript. Please see Line 22, Page 17 and Lines 9-15, Page 18 in the revised manuscript. (2) A table should be given to compare the activity of HzOR and HER with the state-of-the-art literature.

Response:
We thank the reviewer for the good suggestion. According to your suggestion, we added two tables (Supplementary Table 2 and Table 3 of the revised manuscript) to compare our results with recently reported state-of-the-art literatures for HzOR and HER activities, respectively, which are also provided below for your convenience. (3) What is the efficiency from hydrazine to hydrogen for self-powered H 2 production system?
The authors should compare it with other hydrogen generation systems.
Our response: We thank the reviewer for the constructive suggestion. Following your suggestion, we have calculated the efficiency from hydrazine to hydrogen in our self-power H 2 production system. The total efficiency is calculated according to the following equation: Total Efficiency (TE, %) = N H 2 /2N DHzFC *FE OHzS * 100 % Where N H 2 is the amount (mol) of produced hydrogen, N DHzFC is the amount (mol) of consumed hydrazine from the electrolyte of DHzFC and FE is the Faraday efficiency (%) of OHzS. The amount (mol) of the consumed hydrazine in the DHzFC were carefully measured by a UV-vis spectrophotometric method proposed by Watt and Chrisp (Adv. Mater. 2019, 31, 1902709;ACS Catal. 2019, 9, 7311-7317;Anal. Chem. 1952, 24, 2006-2008. The color reagent is the mixed solution of 1.0 g p-(dimethylamino) benzaldehyde, 50 mL ethanol and 5 mL of 0.12 M HCl.
Meanwhile, firstly, 50 μL of electrolyte from DHzFC after reaction was added into measuring flask and DIW was also added into the same measuring flask till 500 mL in total volume. Then 1 mL of the above solution from the measuring flask was mixed with 1 mL color reagent and 4 mL of 0.12 M HCl. After standing at room temperature for 20 min, the UV-vis spectrum of the solution was collected. The concentration-absorbance curves were calibrated using standard hydrazine solution in a series of concentrations (as indicated in Supplementary Fig. 24a-b).
Following this strategy, the total efficiency is calculated to be about 45.8 % in our system, which is outstanding compared to other hydrogen generation system, such as mechanical energy driven self-power system ( (4) The authors described that their catalyst has a "Pt-like" activity. They may need to compare the activity of HER with commercial Pt/C. Further, DFT calculation of HER free energy for Pt should also be given for comparison.
Our response: We thank the reviewer for the good suggestion. According to the suggestion, we added the HER performance of commercial 20 wt.% Pt/C loaded on Ni foam with the same mass loading of 2 mg cm -2 (Fig. 5a, b in the revised manuscript). As indicated below, the overpotential of PW-Co 3 N NWA/NF is only 9 mV larger than that of Pt/C (32 mV) at 10 mA cm -2 , and the Tafel slope of PW-Co 3 N NWA/NF is comparable to benchmark Pt/C (31 mV dec -1 ). The related discussions were added in the main text of the revised manuscript (Please see Lines 16   We also provide the content below for your convenience: Compared with gaseous hydrogen or carbon monoxide, hydrazine has the advantage of more convenient transportation and storage as a liquid fuel at ordinary temperatures. However, it is necessary to state that hydrazine is a highly toxic chemical, which may be a challenging issue for the large-scale applications (Energy Environ. Sci. 2011, 4, 1255-1260. This is also the major reason that we choose to build our self-powered system working at room temperature and low hydrazine concentration. The text now appears on page 21. "it is energetically favorable to replace surface Co atoms in Co3N with W atoms and subsurface N atoms with P atoms" Please provide the substitution energy of W to Co and the substitution energy of N by P (by comparing the total energy of difference systems and adopting proper reference for W, Co, N and P).
I would recommend publication of the manuscript if the requested information is added.
Reviewer #2 (Remarks to the Author): In general, the authors have substantially improved their manuscript by conducting in-depth studies of their electrocatalysts, and the reviewer recommends the acceptance of this work to our journal. However, I still believe it is not necessary to overclaim their results by selectively narrowing the comparison to their own advantages. For instance, in the Supplementary  2010,195,4135, the maximum power reaches ~1000 mW cm-2 with an OCV of 1.75 V, which is much much better than the observed 46.3 mW cm-2 and 0.98 V in the present work. The authors might argue their data were collected under different conditions such as at room temperature. This is because most fuel cells are designed to work only under specific conditions, such as 80 C. Besides, the authors weakened the fact that their PW-Co3N NWA/NF sample has the highest capacitance (which is directly relevant to their surface areas), more than twice of that of Co3N NWA/NF, and nearly two orders of magnitude larger than Ni foam and PW-Co-precursor. How to provide a fair comparison may need protocols or standards, which are yet to be established for this community. I remind the editors to notice a recent discussion by Martin Pumera at

Reviewer #1 (Remarks to the Author):
The authors have addressed most of the previous comments, except for 5. On page 16, it mentioned "energetically favorable to replace surface Co atoms …". Please provide those energetic data. The text now appears on page 21. "it is energetically favorable to replace surface Co atoms in Co 3 N with W atoms and subsurface N atoms with P atoms". Please provide the substitution energy of W to Co and the substitution energy of N by P (by comparing the total energy of difference systems and adopting proper reference for W, Co, N and P). I would recommend publication of the manuscript if the requested information is added.
Our response: We really appreciate the reviewer's valuable suggestions and support on publication of our work! According to your suggestion, we provided the energetic data of the two different models in the revised Supplementary Fig. 26, which is also provided below for your convenience. In our model, there are three Co atom layers in Co 3 N (001) surface. We calculated 4W atoms substituted for 4 Co atoms in different Co layers of Co 3 N (001) surface. After geometric optimization, as shown in Supplementary Fig. 26a, total energy of 4 W substituted for 4 Co atom at first Co layers of Co 3 N (001) surface are -330.180 eV, and its structure shows no significant distortion. Contrastively, there will be serious structural distortions for the system with 4 W doped at second and third Co layers of Co 3 N (001) planes where the N atoms of second layer totally deviate from its original position, as shown in Supplementary Fig. 26b Besides, the authors weakened the fact that their PW-Co 3 N NWA/NF sample has the highest capacitance (which is directly relevant to their surface areas), more than twice of that of Co 3 N NWA/NF, and nearly two orders of magnitude larger than Ni foam and PW-Co-precursor. How to provide a fair comparison may need protocols or standards, which are yet to be established for this community. I remind the editors to notice a recent discussion by Martin Pumera at ACS Nano entitled "Will Any Crap We Put into Graphene Increase Its Electrocatalytic Effect?", which unfortunately criticized the massive irresponsible researches regarding doping and codoping.
Our response: We really thank the reviewer's valuable comments and support on the publication of our work! According to suggestion, we change the words/phrases such as "best", "record high" to "decent", "competitive" or "remarkable" in both Introduction (Page 6) and Result (Page 19) parts in order to avoid unnecessary overclaim, which are highlighted in the revised manuscript.
We totally agree with the reviewer's opinion that "how to provide a fair comparison may need protocls or standards, which are yet to be established for this community", since it will be quite difficult to illustrate the experimental data to readers. For instance, as the work mentioned above by the reviewer (J. Power Sources 2010, 195, 4135), the maximum power density can achieve as high as 1 W cm-2 under their conditions by integrating 20 wt.% H 2 O 2 in 5 wt.% H 2 SO 4 solution as oxidizer and 10 wt.% N 2 H 4 in 15 wt.% NaOH solution at 80 o C. In our work, we adopt the room temperature as working environment is based on the concept that the fewer demand of external equipment could make the extend application of "self-power" system be more satisfied to the need of actual large-scale application thus avoid the heating equipment (it is only the proof-of-concept for sure). Besides, the lower working temperature could reduce the risk of highly toxic hydrazine.
As for the comparison standards, we agree that fair comparison standards for this community need the joint efforts of all the researches in this field including us obviously, and in our present work, the comparison standards are referring to several recent reported literatures, where the prepared materials on Ni foam is used as electrode directly and the current density is calculated via the geometric area of materials (J. Am. Chem. Soc. 2019, 141, 7537−7543;Nat. Commun. 2018, 9, 924;Nat. Commun. 2018, 9, 4531;Angew. Chem. Int. Ed. 2018, 57, 7649-7653;Angew. Chem. Int. Ed. 2017, 56, 842-846;). As a conclusion, it is worth for us to reiterate that the most important scientific contribution of our work is the first report demonstrating the notable effect of heteroatom doping of transition metal nitride can largely improve its HzOR activity, and simultaneously enhance the HER activity in certain degree. Importantly, the DFT calculation combined with systematic experimental data unravels the fundamental origin of the enhanced performance. We are confident that our work regarding the P/W doped Co 3 N nanowire array electrode possesses sufficient novelty regarding the reported strategy for simultaneous enhancement for HzOR and HER activity, as well as significant scientific contribution regarding the fundamental understanding over the structure-property relationship based on the combined DFT calculation and experimental data.

Reviewer #1 (Remarks to the Author):
The author provided additional structural figures and total energy for the structures. They did not address my previous questions for calculating substitution energy. The total energy does not support their claim on page 21 that "it is energetically favorable to replace surface Co atoms in Co 3 N with W atoms and subsurface N atoms with P atoms". They would need to perform calculations to compare energy difference to find out about whether it is "energetically favorable to replace". For example, using the following equation: E(W-doped Co 3 N) -E(Co 3 N) + E(Co metal) -E(W metal). If they just want to express where the substitution would be in the slab, they can rewrite that sentence. Even there, please be cautious about using "energetically favorable", because the W on the surface is higher in energy than in the subsurface ones.
Our response: We really thank the reviewer's valuable suggestions. It is true that our statement of "energetically favorable" may be inappropriate under this circumstance. According to your suggestion, we have re-written the sentence as "Combined with the theoretical calculation and experimental result regarding the structural stability after doping, it is reasonable to use the model where the surface Co atoms in Co 3 N are replaced by W atoms and subsurface N atoms are replaced by P atoms, as indicated in Fig. 8a-d, Supplementary Fig. 26 and Supplementary Note 2.", which is also highlighted in the revised manuscript (Page 21).