Facile Synthesis of Sulfate-Intercalated CoFe LDH Nanosheets Derived from Two-Dimensional ZIF-9(III) for Promoted Oxygen Evolution Reaction

Round 1

Reviewer 1 Report

In the manuscript, Xiao et al. report a method based on etching-coprecipitation to synthesize CoFe LDH nanosheets. A 2D ZIF-9 was used as a precursor that provided the basic 2D morphology, while the additional growth of metal precipitation led to the crimped structure. Due to the increased active sites and hierarchical structure, the as-obtained catalysts show enhanced OER performance and stability. The manuscript is well organized. A few issues should be addressed before acceptance. 

1) The author mentioned that sulfate intercalation increases the interlayer spacing. A detailed analysis should be provided.

2) The synergistic effect of Co and Fe should be justified. 

3) The hierarchical structure improves the structural stability of LDH. This also needs further justification.

Author Response

In the manuscript, Xiao et al. report a method based on etching-coprecipitation to synthesize CoFe LDH nanosheets. A 2D ZIF-9 was used as a precursor that provided the basic 2D morphology, while the additional growth of metal precipitation led to the crimped structure. Due to the increased active sites and hierarchical structure, the as-obtained catalysts show enhanced OER performance and stability. The manuscript is well organized. A few issues should be addressed before acceptance.

1. The author mentioned that sulfate intercalation increases the interlayer spacing. A detailed analysis should be provided.

Response: Thank you very much for the comments. We have provided a detailed analysis about the interlayer spacing and the discussion have been added in the revised manuscript.

1) The LDH interlayer spacing was calculated using Bragg's law: d = nλ/2sinθ. It should be pointed out that the selected 2θ values correspond to the (003) crystal plane from the structure of LDH [Energy Environ. Sci. 2016, 9, 1734-1743]. Thus, the interlayer spacing was calculated to be 0.89 nm from XRD result (Fig. 2b), which is larger than that of previous reported LDH [Appl. Catal. B 2020, 272, 118959; Angew. Chem. Int. Ed. 2017, 56, 5867-5871; ChemPlusChem 2017, 82, 483-488].

2) We also supplemented high resolution transmission electron microscope (HRTEM) image of CoFe LDH nanosheets (Fig. S4) and made the corresponding analysis. Moreover, the interlayer spacing of 0.890 nm was measured and could correspond to the (003) crystal plane, which is consistent with the XRD results.

3) In order to prove the effect of sulfate on the interlayer spacing of LDH, we made another experiment. Specifically, the sample was obtained by the similar synthetic method of CoFe LDH using FeCl₂·4H₂O instead of FeSO₄·7H₂O. Unfortunately, the structure of as-obtained sample can’t be attributed to that of LDH and the nanosheets are stacked on each other from Fig. N1. So FeSO₄·7H₂O plays an important role for the successful preparation of CoFe LDH.

“The LDH interlayer spacing was calculated using Bragg's law: d= nλ/2sinθ. It should be pointed out that the selected 2θ values corresponds to the (003) crystal plane from the structure of LDH [15]. Thus, the interlayer spacing was calculated to be 0.890 nm from XRD result (Fig. 2b), which is larger than that of previous reported LDH [10-12].”

 

“Fig. 3g and S4 demonstrate HRTEM images of the nanosheet and the spacing of 0.890nm, 0.265 nm and 0.445 nm correspond to the (003), (012) and (006) plane of the LDH structure, respectively”

 

2. The synergistic effect of Co and Fe should be justified.

Response: Thanks. We have added the justification about the synergistic effect of Co and Fe, as follows:

(1) It is widely accepted that Fe³⁺ ions play an essential role to improve the electrochemical performance of Co LDH OER catalysts. For example, the strong electronic interactions between the Co and Fe elements had been demonstrated from the fact that Fe incorporation correlate with an anodic shift in the Co²⁺/Co³⁺ redox wave [J. Am. Chem. Soc. 2015, 137, 3638-3648]. Besides, Fe³⁺ can stabilize the LDH structure due to the similar ionic radii of the Co and Fe ions in the hydroxide layer [ChemSusChem 2017, 10, 156-165]. Thus, the enhanced catalytic performance could be attributed to the hierarchical structure composed of ultrathin nanosheets and synergistic effect of Co and Fe.

(2) According to XPS results, we also further discuss about the synergistic effect of Co and Fe. Interestingly, compared with our previously reported Co 2p for Co LDH, peaks of both Co 2p_3/2 and Co 2p_1/2 for CoFe LDH have a slightly positive shift [Chem. Eng. J. 2021, 414, 128784]. The result indicates that the electron cloud arrangement of Co atom could be regulated by incorporation Fe atom and the synergistic effect of the two atoms could be further justified.

 

“Interestingly, compared with our previously reported Co 2p for Co LDH, peaks of both Co 2p_3/2 and Co 2p_1/2 for CoFe LDH have a slightly positive shift [29]. The result indicates that the electron cloud arrangement of Co atom could be regulated by incorporation Fe atom and the synergistic effect of the two atoms could be further justified.”

 

 

“It is widely accepted that Fe³⁺ ions play an essential role to improve the electrochemical performance of Co LDH OER catalysts. For example, the strong electronic interactions between the Co and Fe elements had been demonstrated from the fact that Fe incorporation correlate with an anodic shift in the Co²⁺/Co³⁺ redox wave [8]. Besides, Fe³⁺ can stabilize the LDH structure due to the similar ionic radii of the Co and Fe ions in the hydroxide layer [40]. Thus, the enhanced catalytic performance could be attributed to the hierarchical structure composed of ultrathin nanosheets and synergistic effect of Co and Fe.”

 

 

3. The hierarchical structure improves the structural stability of LDH. This also needs further justification.

Response: Thanks for the construction comment. We have added the XRD pattern and SEM image for the CoFe LDH after the OER tests to further justify the structural stability of hierarchical CoFe LDH (Fig. S8). Moreover, the corresponding results and discussion were provided in the revised manuscript. as follows:

“In order to further justify the structural stability of hierarchical CoFe LDH, the XRD pattern and SEM image for the CoFe LDH were provided after the OER tests. As shown in Fig. S8a, the structure of catalyst could be ascribed to that of LDH [9]. Meanwhile, the catalyst still maintains hierarchical nanosheets after OER process (Fig. S8b). The results suggest that the hierarchical structure could improve the structural stability of LDH.”

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work, Xiao et al. develop a facile method for preparation of surface-intercalated CoFe LDH nanosheets. The synthetic strategy represents an effective approach and originate catalysts with excellent OER activity, as evidenced by very low overpotential at j₁₀ and low tafel slope. However, the article under its current state presents serious presentation flaws, since many sentences are not well expressed and the overall clearness and scientific soundness is not adequate for publication in Catalysts. Below, the points requiring a major revision are presented:

 - The manuscript needs a deep revision in English grammar and style. Some sentences are not clearly expressed, some examples are motioned below:

On pag.2 “To acquire the high performance CoFe LDH electrocatalysts, various strategies mainly focus on heteroatom doping, design of nanostructure, combing CoFe LDH with other active materials and exfoliation of CoFe LDH et al [9-12]”.

The verb is absent, and the sentence is not at all clear.

On pag. 2: “It is well-known that LDH nanosheets have abundant active sites, the rapid mass transport and the superior electron transfer ability, thereby greatly promoting catalytic properties”.

Here the use of ‘the’ is abundant, and renders the phrase not clear.

On pag. 2: “In our previous work, a novel 2D ZIF-9(III)/Co LDH was successfully synthesized controllable phase transition of 3D ZIF-9(I) and exhibited efficient OER electrocatalytic performance”.

 On pag. 2: the CoFe LDH might show extraordinary mass transport and excellent activities for OER in alkaline media.

Why ‘might show’? The catalysts demonstrate excellent activities, then I would remove might in the sentence.

 On pag. 3: “hierarchical CoFe LDH composed of ultrathin nanosheets was eventually obtained”.

Why eventually? The nanosheets are effectively obtained, then I would remove eventually.

On pag. 5: “pore-size distributions derived from the desorption branches of the isotherms for CoFe LDH illustrate that the sample is the mesoporous property”.

 On pag. 7: “The hierarchical structure composed of ultrathin nanosheets may bring about the increase of accessibly exposed active sites, responsible for enhancing the catalytic performance.

 Also in this case the sentence is not clear.

 Please check and revise all manuscript, the clearness of presentation must be improved.

-    In Fig. 5, the caption does not correspond to Figures, c and d are inverted.

-        -  On pag.7, authors claim that CoFe LDH nanosheets exhibits a significantly low overpotential of 218 mV at 10 mA cm-2, which is more positive than ZIF-9(III) (373 mV), and commercial RuO2 (285 mV).

What does it mean more positive? Please check.

 - Have the authors calculated the TOF? This would be important since it is a parameter that better reflects the intrinsic activity of a catalyst.

- Further comments about the synergistic effect of Co and Fe should be provided in the manuscript. Based also on XPS results, do the authors have some hypotheses about this?

- What is the reason for choosing a potential of 1.52V vs. RHE to carry out EIS measurements? From LSV, ZIF-9(III) is still not active at this potential.

- It would finally be useful in the conclusions that the authors add a perspective on how this type of catalyst can be incorporated in a industrially relevant scale type electrolyser.

- The authors are advised to cite recent published relevant articles focused on OER catalysts, such as Nanomaterials 2021, 11(11), 3010, and Journal of Alloys and Compounds 818 (2020) 153345.

Author Response

In this work, Xiao et al. develop a facile method for preparation of surface-intercalated CoFe LDH nanosheets. The synthetic strategy represents an effective approach and originate catalysts with excellent OER activity, as evidenced by very low overpotential at j₁₀ and low tafel slope. However, the article under its current state presents serious presentation flaws, since many sentences are not well expressed and the overall clearness and scientific soundness is not adequate for publication in Catalysts. Below, the points requiring a major revision are presented:

 

1. 1. The manuscript needs a deep revision in English grammar and style. Some sentences are not clearly expressed, some examples are motioned below:

On pag. 2 “To acquire the high performance CoFe LDH electrocatalysts, various strategies mainly focus on heteroatom doping, design of nanostructure, combing CoFe LDH with other active materials and exfoliation of CoFe LDH et al [9-12]”. The verb is absent, and the sentence is not at all clear.

Response: Thank you very much. It is right that the verb is absent due to our careless. We have corrected the sentence, as follow:

“To acquire the high performance CoFe LDH electrocatalysts, various strategies have been adopted, mainly including heteroatom doping, design of nanostructure, exfoliation of layered structure, and combing with other active materials et al [9-12].”

 

On pag. 2: “It is well-known that LDH nanosheets have abundant active sites, the rapid mass transport and the superior electron transfer ability, thereby greatly promoting catalytic properties”. Here the use of ‘the’ is abundant, and renders the phrase not clear.

Response: Thanks. We have deleted “the” to render the phrase clear, as follows:

“It is well-known that LDH nanosheets have abundant active sites, the rapid mass transport and superior electron transfer ability, thereby greatly promoting catalytic properties.”

 

On pag. 2: “In our previous work, a novel 2D ZIF-9(III)/Co LDH was successfully synthesized controllable phase transition of 3D ZIF-9(I) and exhibited efficient OER electrocatalytic performance”.

Response: Thanks. We have revised the sentence, as follows:

“In our previous work, a novel 2D ZIF-9(III)/Co LDH was successfully synthesized via controllable phase transition of 3D ZIF-9(I) and exhibited efficient OER electrocatalytic performance.”

 

On pag. 2: the CoFe LDH might show extraordinary mass transport and excellent activities for OER in alkaline media. Why ‘might show’? The catalysts demonstrate excellent activities, then I would remove might in the sentence.

Response: Thank you very much. We have removed “might” in the sentence, as follows:

“the CoFe LDH shows extraordinary mass transport and excellent activities for OER in alkaline media.”

 

On pag. 3: “hierarchical CoFe LDH composed of ultrathin nanosheets was eventually obtained”. Why eventually? The nanosheets are effectively obtained, then I would remove eventually.

Response: Thank you very much. We have removed “eventually” in the sentence, as follows:

“hierarchical CoFe LDH composed of ultrathin nanosheets was obtained”

 

On pag. 5: “pore-size distributions derived from the desorption branches of the isotherms for CoFe LDH illustrate that the sample is the mesoporous property”.

Response: As suggested, an accurate and clear description of pore size distribution analysis is presented.

“From the corresponding Barrett-Joyner-Halenda (BJH) pore-size distributions, CoFe LDH exhibits the mesoporous property.”

 

On pag. 7: “The hierarchical structure composed of ultrathin nanosheets may bring about the increase of accessibly exposed active sites, responsible for enhancing the catalytic performance.” Also in this case the sentence is not clear. Please check and revise all manuscript, the clearness of presentation must be improved.

Response: Thank you for the careful comments. Our manuscript have been checked by a native English-speaking colleague.

“Thus, the enhanced catalytic performance could be attributed to the hierarchical structure composed of ultrathin nanosheets and synergistic effect of Co and Fe.”

 

 

In Fig. 5, the caption does not correspond to Figures, c and d are inverted.

Response: Thanks. We have carefully checked the Fig 5 and exchanged caption of Fig. 5c and Fig. 5d in the revised manuscript.

On pag.7, authors claim that CoFe LDH nanosheets exhibits a significantly low overpotential of 218 mV at 10 mA cm^-2, which is more positive than ZIF-9(III) (373 mV), and commercial RuO₂ (285 mV). What does it mean more positive? Please check.

Response: Thank you for the comment. We have double-checked the sentence and it's really an inappropriate use of the phrase “more positive”. So we have described this sentence again.

 

“CoFe LDH nanosheets exhibits a lower overpotential of 218 mV at 10 mA cm^-2 than those of ZIF-9(III) (373 mV) and commercial RuO₂ (285 mV).”

 

2. 2. Have the authors calculated the TOF? This would be important since it is a parameter that better reflects the intrinsic activity of a catalyst.

Response: Thanks for the constructive suggestion. Indeed, the Turnover frequencies (TOF) is an important parameter for catalysts and we therefore have calculated the TOF. Generally, there are two approaches to determine the TOF and TOF is denoted by TOF total metal (TOF_tm) and TOF_redox. TOF_tm is calculated by using the total mass derived from inductively coupled plasma (ICP) measurements, while TOF_redox is determined by integrating the redox waves of catalysts in the polarization curves [Nanomaterials 2021, 11(11), 3010 and Chem. Mater. 2017, 29, 120-140]. In fact, TOF_redox is similar with TOF_tm if the catalyst (such as Co-based oxyhydroxides) is electrolyte-permeated and electronically conductive [Chem. Mater. 2017, 29, 120-140]. Moreover, adopting the total mass to estimate TOF is also widely considered as a common method [Chem. Eng. J. 2022, 431, 134080 and ACS Catal. 2021, 11, 7132-7143].

Therefore, the TOF of the CoFe LDH and ZIF-9(III) at different potentials were calculated using the following equation: TOF = jA/(4nF), where j (A cm^-2) is current density, A is surface area (0.5 cm²), F is the Faraday constant of 96485 C mol^-1 and n is the molar number of total metal ions. Moreover, the results and corresponding discussion were displayed in the revised manuscript, as follows:

“Indeed, the Turnover frequencies (TOF) is an important parameter for catalysts. As displayed in Fig. S7, TOF change trends for CoFe LDH and ZIF-9(III) catalysts are positively correlated with overpotential. Moreover, the TOF value of CoFe LDH is much higher than that of ZIF-9(III) at same overpotential, confirming the superior intrinsic activity. Specifically, CoFe LDH nanosheets exhibits a higher TOF values of 0.2716 s^–1 at an overpotential of 300 mV than that of ZIF-9(III) (0.0009 s^–1). The molar number (n) was calculated by based on the inductively coupled plasma-mass spectrometry (ICP-MS) results. For CoFe LDH, the content of Co and Fe is 37.23 wt. % and 27.60 wt. %, respectively. For ZIF-9(III), the content of Co and Fe is 39.98 wt. % and 2.05 wt. %, respectively.”

 

3. 3. Further comments about the synergistic effect of Co and Fe should be provided in the manuscript. Based also on XPS results, do the authors have some hypotheses about this?

Response: Thank you for the suggestion. We have added the justification about the synergistic effect of Co and Fe, as follows:

(1) It is widely accepted that Fe³⁺ ions play an essential role to improve the electrochemical performance of Co LDH OER catalysts. For example, the strong electronic interactions between the Co and Fe elements had been demonstrated from the fact that Fe incorporation correlate with an anodic shift in the Co²⁺/Co³⁺ redox wave [J. Am. Chem. Soc. 2015, 137, 3638-3648]. Besides, Fe³⁺ can stabilize the LDH structure due to the similar ionic radii of the Co and Fe ions in the hydroxide layer [ChemSusChem 2017, 10, 156-165]. Thus, the enhanced catalytic performance could be attributed to the hierarchical structure composed of ultrathin nanosheets and synergistic effect of Co and Fe.

(2) According to XPS results, we also further discuss about the synergistic effect of Co and Fe. Interestingly, compared with our previously reported Co 2p for Co LDH, peaks of both Co 2p_3/2 and Co 2p_1/2 for CoFe LDH have a slightly positive shift [Chem. Eng. J. 2021, 414, 128784]. The result indicates that the electron cloud arrangement of Co atom could be regulated by incorporation Fe atom and the synergistic effect of the two atoms could be further justified.

 

“Interestingly, compared with our previously reported Co 2p for Co LDH, peaks of both Co 2p_3/2 and Co 2p_1/2 for CoFe LDH have a slightly positive shift [29]. The result indicates that the electron cloud arrangement of Co atom could be regulated by incorporation Fe atom and the synergistic effect of the two atoms could be further justified.”

 

“It is widely accepted that Fe³⁺ ions play an essential role to improve the electrochemical performance of Co LDH OER catalysts. For example, the strong electronic interactions between the Co and Fe elements had been demonstrated from the fact that Fe incorporation correlate with an anodic shift in the Co²⁺/Co³⁺ redox wave [8]. Besides, Fe³⁺ can stabilize the LDH structure due to the similar ionic radii of the Co and Fe ions in the hydroxide layer [40]. Thus, the enhanced catalytic performance could be attributed to the hierarchical structure composed of ultrathin nanosheets and synergistic effect of Co and Fe.”

 

4. What is the reason for choosing a potential of 1.52V vs. RHE to carry out EIS measurements? From LSV, ZIF-9(III) is still not active at this potential.

Response: Thank you very much for the comment. The choice of the potential of 1.52 V vs. RHE is based on overpotential of CoFe LDH catalyst from LSV curves (Fig. 5a). It is right that ZIF-9(III) was not active at this potential. So, a new potential of 1.62 V vs. RHE was chosen to carry out EIS measurements for ZIF-9(III). The result was shown in Fig. 5d and corresponding discussion was provided in the revised manuscript, as follows:

“The CoFe LDH exhibits a smaller Rct value of 1.3 Ω cm² than ZIF-9(III) (7.94 Ω cm²), indicating the low mass transfer resistance and fast charge transfer rate for the CoFe LDH.”

 

5. It would finally be useful in the conclusions that the authors add a perspective on how this type of catalyst can be incorporated in a industrially relevant scale type electrolyser.

Response: Thanks. We have added a perspective in the conclusions in the revised manuscript, as follows:

“Moreover, the CoFe LDH catalyst could be incorporated in an industrially relevant scale type electrolyser due to the high electrocatalytic activity, stability and low cost.”

 

6. The authors are advised to cite recent published relevant articles focused on OER catalysts, such as Nanomaterials 2021, 11(11), 3010, and Journal of Alloys and Compounds 818 (2020) 153345.

Response: Thank for your suggestion. We have cited the recent published relevant literatures in our revised manuscript.

Round 2

Reviewer 2 Report

Authors have addressed all my comments in the revised version, therefore I recommend the publication of this version of manuscript to Catalysts.