The synergistic effect of Hf-O-Ru bonds and oxygen vacancies in Ru/HfO2 for enhanced hydrogen evolution

Ru nanoparticles have been demonstrated to be highly active electrocatalysts for the hydrogen evolution reaction (HER). At present, most of Ru nanoparticles-based HER electrocatalysts with high activity are supported by heteroatom-doped carbon substrates. Few metal oxides with large band gap (more than 5 eV) as the substrates of Ru nanoparticles are employed for the HER. By using large band gap metal oxides substrates, we can distinguish the contribution of Ru nanoparticles from the substrates. Here, a highly efficient Ru/HfO2 composite is developed by tuning numbers of Ru-O-Hf bonds and oxygen vacancies, resulting in a 20-fold enhancement in mass activity over commercial Pt/C in an alkaline medium. Density functional theory (DFT) calculations reveal that strong metal-support interaction via Ru-O-Hf bonds and the oxygen vacancies in the supported Ru samples synergistically lower the energy barrier for water dissociation to improve catalytic activities.

5. All the spin orbit coupling originated peaks in XPS need to be assigned. 6. Authors saying that "owing to the larger Ru nanoparticle size of VO-Ru/HfO2-P, which decreases the number of Ru-O-Hf bonds" in the line number of '119-120'. This decrease in number of Ru-O-Hf, needs to be supported by some experimental evidences.
7. Did authors consider the ligand field (from PVP and Oleylamine's coordination) effect while calculating the charge density profile given in Figure 4h? 8. From the theoretical studies the role of 'oxygen vacancy' is not fully understandable. Authors should give a look into it.
Reviewer #3 (Remarks to the Author): In the manuscript "The Synergistic Effect of Hf-O-Ru Bonds and Oxygen Vacancies in Ru/HfO2 for Enhanced Hydrogen Evolution", Li and co-authors have reported their study on the HER activity of Ru nanoparticles supported by HfO2, They discovered that both HfO2 and the oxygen vacancies (Vo) play a crucial role in the increase of the HER activity.
The manuscript is clear and well-written. The methodology clearly explained. I recommend the paper for publication after a few comments have been addressed.
1) The authors say that Vo play an important role for OER. What is the Vo concentration in the calculations? Is there an effect of the concentration as well?
2) Are the Vo localised on the surface of the nanocluster or deeper in the material?
3) Are the Vo participating in the adsorption of water and its splitting or they participate only by perturbing the electron distribution?

Reviewer #1
In this manuscript the authors have synthesized different types of Ru/HfO2 electrocatalysts for the hydrogen evolution reaction (HER) in alkaline electrolyte. The results suggest that the HER activity can be affected by the formation of the Ru-O-Hf bonds and by the presence of oxygen vacancies. However, there are some problems in the manuscript. The authors should address the following issues before the manuscript can be considered for publications in any journal.
1. Similar to many recent electrocatalysis papers, the authors attempt to combine the experimental HER results with XANES/EXAFS characterization and DFT calculations.
This combined approach can be quite useful if it is done with expertise. However, in the current manuscript the three components are poorly integrated. Below are several examples: -It appears that the XANES/EXAFS results were not measured under in-situ HER conditions. If the measurements were measured after the samples were exposed to air without reduction treatment, the Ru would be oxidized, making the comparison of Ru oxidation state meaningless. Furthermore, the EXAFs analysis was not very well done.
For example, in Table S1 for VO-Ru/HfO2-P, the coordination numbers are 11.0 for Ru-M and 4.8 for Ru-O, making the total coordination for Ru to be 15.8. That is physically impossible because the maximum coordination number of Ru is 12.
Response: 1) We appreciate your insightful comments. As the reviewer remarked, insitu XAS analysis is a powerful technique to observe the electrocatalytic behavior during the reaction and compare the characteristics before and after the reaction. Therefore, we conducted operando XAS analysis under in-situ HER conditions. For operando XAS analysis, home-made operando three electrode cell system, which consist of platinum counter electrode, Hg/HgO reference electrode, and electrocatalysts loaded working electrode, with polyimide film windows were employed. The operando XAS results were also collected, after conducting chronoamperometry (CA) test at specific voltage in 1.0 M KOH electrolyte for 12 h. "The Fig. 3h and i show the operando Ru K-edge XANES spectra and corresponding Fourier-transformed (FT) magnitudes in operando Ru K-edge EXAFS spectra of VO-Ru/HfO2-OP before and after CA testing at -0.039 V (vs. RHE) for 12 h. Evidently, both the XANES and EXAFS are similar with the initial open-circuit voltage (OCV) ones when the applied potential returned to OCV after long term CA testing, indicating the high stability." This content has been added to page 13 of manuscript.
"In situ and operando XAS analysis of VO-Ru/HfO2-OP. In order to monitor the electronic state of the Ru active sites during the HER, potential-dependent Ru K-edge XAS measurements were performed using a home-made operando three electrode cell system. Three important peaks labeled as pre-edge peak (1s → 4d transition), white line peak (1s → 5p transition), and maximum peak (1s → 5p transition multiple scattering), are obviously changed along with the applied potentials. The variation can be more clearly discerned from the differentiated Ru K-edge XANES intensity (In th -I1 st ) in detached. As evidenced by the variations of pre-edge peaks and maximum peaks, the structural change occurred from the ex-situ sample to the OCV stage, which corresponds to the electrode activation process. Moreover, the structural change was occurred continuously during the HER, but it is reversibly and stable. These above results indicate that the VO-Ru/HfO2-OP is flexible with respect to structural distortions and the reversible redox reaction of Ru, resulting in high catalytic activity as well as stability. The bonds of Ru-O-Hf and Ru-Ru were contracted and stretched during reaction, and the FT peaks intensity were increased and decreased, as more clearly presented in Figs. 4f and g. Impressively, the change frequency of interatomic distances relative to Ru-O-Hf and Ru-Ru is high, however, the variation of interatomic distance changes is low, effectively demonstrating the flexible structure of VO-Ru/HfO2-OP and highly stable of it during the alkaline hydrogen electrocatalysis, which is consistent with the results of operando XANES spectra. Besides, the change frequency of FT peak intensity is high, but it is low for variation of intensity. This is due to the fast adsorption and desorption   2) We thank the reviewer for reading the manuscript so carefully and apologize for our mistake. We found out a typing error in Supplementary   2. This question is a general concern for many of the HER manuscripts. There have been thousands of published papers on HER. Is it worth to publish more papers on this particular topic, especially in high impact journals? The authors claim that "Importantly, Ru costs approximately half of Pt, and hence, such catalysts are economically viable".
Such statement is misleading. Ru is less abundant than Pt. Therefore, Ru would most likely become more expensive than Pt if Ru is used in large scale applications.
Replacing one precious metal with another precious metal, especially involving complicated synthesis procedures, would not solve any hurdles related to catalyst cost.

Response:
We appreciate your comments. Based on your suggestions, we re-wrote the abstract and introduction, our original idea is whether or not Ru nanocomposite with a large band gap metal oxide, which is larger than TiO2 (3.2 eV), can be tuned into a highly efficient HER electrocatalyst. If like this, we can distinguish the activity of Ru nanoparticles from the substrates, because some substrates such as heteroatom dopedcarbon, Ni foam, they themselves have some HER activity. For these cases, it is hard to know how much activity is from Ru nanoparticles, and how much is from substrates.
The statement of "replacing platinum with Ru is because Ru is much cheaper than Pt/C" was also cancelled. We re-emphasized the advance of the work in abstract and introduction. Please see the abstract and introduction in the revised manuscript.
3. This is a relatively minor issue. The authors made extensive discussion about the shift in the XPS peak position for Ru. Similar to the concern made for XANES/EXAFS described above, Ru would be oxidized upon exposure to air, making the comparison meaningless. Furthermore, the authors appear to compare the Ru peak positions for catalysts with different particle size of Ru. The authors should check XPS literature on the "final state effect" to understand that the XPS position can also be affected by the particle size.

Response:
We very appreciate your comments. We agree with the reviewer's opinion that the Ru would be oxidized upon exposure to air, so we deleted the relevant description. Moreover, as suggested by the reviewer, we have checked the literature and reinterpreted the relationship between XPS position and particle size based on the final state effect.
"The XPS of VO-Ru/HfO2-OP depicts a Ru 3d3/2 peak, which shows a significant shift to a higher binding energy relative to that of bulk Ru (Fig. 2a). This positive core level shifts involved in the smaller metal clusters supported on less conductive substrates can be interpreted by final state effects. 23,24 As the final state of the photoemission process, the positive hole can be less efficiently screened, leading to a positive core level shift with decreasing particle size. 25 .

Reviewer #2
The itself has no catalytic activity. Therefore, the catalytic activity is only originated from VO-Ru/HfO2-OP. In addition, in our experiments, the 5 wt % Nafion solution was used as the binder.
3. Authors can perform the electrochemical HER activity comparison of VO-Ru/HfO2-OP materials with various oleyl amine to PVP ratio.

Response:
We appreciate your comments. The effect of different ratio of O (oleylamine) to P (PVP) on the HER catalytic activity was explored. We varied the ratio of O to P from 2 : 50 to 8 : 50, and the VO-Ru/HfO2-OP materials prepared with the ratio of O to P of 4 : 50 exhibited the best activity with the lowest overpotential to output the benchmark current density of 10 mA cm -2 . These above results have been added to page 14 of the manuscript, and the Supplementary Fig. 19 has been added to the supporting information.
Supplementary Fig. 19 The polarization curves of the catalysts prepared with different ratio of O (oleylamine) to P (PVP).
4. It is suggested to provide the equivalent circuit diagram for EIS outcomes.

Response:
We appreciate your comments. The equivalent circuit used for simulating the Nyquist plots in Fig. 3e and the corresponding electrochemical impedance parameters have been added to supporting information.
Supplementary Fig. 15 Equivalent circuit was used for simulating the Nyquist plots in Fig. 3e. Rs, Rct and CPE represent the solution resistance, the charge transfer resistance and constant phase element, respectively.

Supplementary Table 2
Electrochemical impedance parameters obtained simulating the Nyquist plots to the equivalent circuit model in Supplementary Fig. 15. 5. All the spin orbit coupling originated peaks in XPS need to be assigned.

Response:
We appreciate your comments. All the XPS peaks for C 1s + Ru 3d spectra have been reassigned. "The XPS of VO-Ru/HfO2-OP depicts a Ru 3d3/2 peak, which shows a significant shift to a higher binding energy relative to that of bulk Ru (Fig. 2a).
This positive core level shifts involved in the smaller metal clusters supported on less conductive substrates can be interpreted by final state effects 23,24 . As the final state of the photoemission process, the positive hole can be less efficiently screened, leading to a positive core level shift with decreasing particle size 25 . Thus, the size of Ru cluster in VO-Ru/HfO2-OP is much smaller than that of bulk Ru. In contrast, VO-Ru/HfO2-P show a negative shift of 0.4 eV compared to that of VO-Ru/HfO2-OP, owing to the larger Ru cluster size of VO-Ru/HfO2-P. The binding energy for Ru 3d3/2 of VO-Ru/HfO2-O is located in the middle of VO-Ru/HfO2-OP and VO-Ru/HfO2-P, demonstrating that the Ru cluster size in VO-Ru/HfO2-O is between those of VO-Ru/HfO2-OP and VO-Ru/HfO2-P. effect while calculating the charge density profile given in Figure 4h?
Response: We very appreciate your comment. We did not consider the ligand field (from PVP and Oleylamine's coordination) effect while calculating the charge density profile. The reasons are as follows: Preparation of VO-Ru/HfO2-OP was conducted in two continuous steps. First, a modified polyol process with oleylamine and polyvinylpyrrolidone as structure-directing agents was employed to prepare pristine Ru/HfO2-OP. Second, the pristine Ru/HfO2-OP was annealed at 750 o C under a H2/Ar atmosphere for 2 h to obtain VO-Ru/HfO2-OP. In the first step, when the reaction was completed, the resultant products were fully washed with ethanol and cyclohexane. The aim of careful washing is to remove the ethylene glycol, PVP and Oleylamine that absorbed on the catalysts. After this step, most of the organic matter has been removed, even if there is a small amount residue, it will be pyrolyzed into carbon in the second step of high temperature calcination (750 o C for 2 h). Based on the above reasons, we think the influence of the coordination field effect could be negligible.
8. From the theoretical studies the role of 'oxygen vacancy' is not fully understandable.
Authors should give a look into it.

Response:
We appreciate your comment. To better understand the effect of oxygen vacancies on alkaline HER, we conduct further investigations of the influences of oxygen vacancies. First, the oxygen vacancy localized on the surface of HfO2 is the most stable as revealed by total energy based DFT calculation, as shown in Supplementary Fig. 25. Second, the computed adsorption energy of H2O is -0.21 eV for HfO2(001) and -0.25 eV for VO-HfO2 ( Supplementary Fig. 31), indicating that the ignorable effects of Vo on the adsorption of water. However, the computed adsorption energy of H2O for Ru3/HfO2 is -0.78 eV, and the water adsorption energy of VO-Ru3/HfO2 is -0.89 eV. These results suggest that the VO do not directly participate in the adsorption of water but play a primary role in perturbing the electron distribution of Ru cluster. Third, the energy barrier of water dissociation for Ru/HfO2 is 0.65 eV. For the case of VO-Ru/HfO2-OP, the energy barrier of water dissociation is 0.62 eV. It is even reduced to 0.54 eV for V2O-Ru/HfO2-OP (Supplementary Table 3