MOF-Derived Electrocatalysts for High-Efficiency Hydrogen Production via Water Electrolysis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Title: MOFs-Derived Electrocatalysts for High-Efficiency Hydrogen Production via Water Electrolysis
The present report, three mono- and bimetallic metalorganic framework (MOFs)-derived electrocatalysts— Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs— were synthesized. The synthesized materials were checked Electrochemical Hydrogen production, that the Fe/Mn-MOFs catalyst exhibited superior performance, achieving an overpotential of 1362.8 mV and a Tafel slope of 59.6 mV·dec⁻¹, alongside the largest electrochemical active surface area (ECSA). But, Fe/Co-MOFs showed moderate catalytic activity. Stability tests revealed that Fe/Mn-MOFs retained 92.3% of its initial current density after 50 h of continuous operation, highlighting its excellent durability for the oxygen evolution reaction (OER). However, there still concern need to be addressed by authors.
Comments to authors 
1.    Fig. 1 In the schematic synthetic diagram authors should provide clear details like temperature and what they added in the mixtures.
2.    The EDX spectra and values for light elements (e.g., C, H, N) are inaccurate and need to be done with other analytical techniques
3.    Usually cobalt is more active than Mn then why Fe/Mn-MOFs poor activity compare than Fe/Co-MOFs
4.    In the experimental conditions authors need to provide the purity percentage of chemicals 
5.    In the PXRD there are some small humps are the authors need to find and discuss, and too provide the JCPDS card number. Also, use symbols in X axis of XRD
6.    I suggest authors should provide experimental conditions to each experiment’s
7.    Stability of catalysts need to prove before and after the experiment’s, also catalysts surface oxidation states need to confirmed

Author Response

Response Letter

The authors would like to thank the editor and all the referees for their insightful comments and constructive suggestions. We have carefully considered each of them and the manuscript has been revised accordingly. Please note that all the changes made during the revision are highlighted in red in the revised manuscript. Below, we give our itemized response to each of the comments and suggestions.

Response to Reviewer 1:

The present report, three mono- and bimetallic metalorganic framework (MOFs)-derived electrocatalysts— Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs— were synthesized. The synthesized materials were checked Electrochemical Hydrogen production, that the Fe/Mn-MOFs catalyst exhibited superior performance, achieving an overpotential of 1362.8 mV and a Tafel slope of 59.6 mV·dec⁻¹, alongside the largest electrochemical active surface area (ECSA). But, Fe/Co-MOFs showed moderate catalytic activity. Stability tests revealed that Fe/Mn-MOFs retained 92.3% of its initial current density after 50 h of continuous operation, highlighting its excellent durability for the oxygen evolution reaction (OER). However, there still concern need to be addressed by authors.

Authors’ reply: We thank the referee for his/her nice summary of our work and constructive comments raised by referee which are extremely valuable for improving the manuscript.

1. Fig. 1 In the schematic synthetic diagram authors should provide clear details like temperature and what they added in the mixtures.

Authors’ reply: We thank the referee for technical questions raised by the referee which are addressed below. We have carefully revised the schematic synthesis diagram to incorporate additional experimental details in the Figure 1 in the revised manuscript. As suggested, the reaction temperature for each step and the exact components added to the mixtures are now clearly labeled in the modified figure.

Figure 1.  Schematic illustration of the synthesis of Fe-MOFs, Fe/Mn-MOFs, and Fe/Co/Mn-MOFs catalysts.

2. The EDX spectra and values for light elements (e.g., C, H, N) are inaccurate and need to be done with other analytical techniques

Authors’ reply: We greatly appreciate your careful review and suggestions to improve the accuracy of our analytical data. Regarding the concerns about the EDX spectra and values for light elements (C, H, N), we acknowledge that EDX indeed has limitations in precisely characterizing these elements due to its lower sensitivity and potential interferences from the matrix. To address this, we have conducted supplementary X-ray photoelectron spectroscopy (XPS) measurements, which provide higher resolution and more reliable quantitative analysis for elemental composition. The XPS results have been used to thoroughly validate the elemental distribution and contents, providing more robust evidence for the study.The revised data (now included in Figure 4) confirm the consistent presence and chemical states of these elements.

Paragraph 3 in Section 3.1:

XPS was performed to probe the electronic structure and bimetallic interactions (Figure 4). The survey spectra confirmed the presence of Fe, Mn/Co, O, C, and N, with atomic ratios matching the synthetic feed (Fe:Mn/Co = 1:1). In Fe/Mn-MOFs, the Fe 2p3/2 peak at 711.8 eV shifted ~0.6 eV lower than in Fe-MOFs, indicating electron delocalization from Mn²⁺ to Fe³⁺, which optimizes OH⁻ adsorption [27]. The Mn 2p3/2 peak at 641.5 eV suggested hybridized Mn²⁺ states, enhancing charge transfer kinetics. Conversely, Fe/Co-MOFs showed negligible shifts in Fe 2p and Co 2p peaks, implying weaker synergistic effects. The O 1s spectrum of Fe/Mn-MOFs featured a higher hydroxyl component (531.5 eV), aligning with its larger electrochemical active surface area (ECSA). These results validate that the Fe³⁺–Mn²⁺ interaction via electron-withdrawing NO₂-BDC ligands boosts OER activity by modulating electronic structure and active site accessibility.

Figure 4. X-ray Photoelectron Spectroscopy (XPS) Analysis of Fe/Mn-MOFs, Fe/Co-MOFs, and Fe-MOFs: Elemental Composition and Bimetallic Electronic Interactions

Reference:

27. Cheng, Y.; Luo, Y.; Zheng, Y.; Pang, J.; Sun, K.; Hou, J.; Wang, G.; Guo, W.; Guo, X.; Chen, L., Self-supporting one-dimensional ZnFe-BDC for electrocatalysis oxygen evolution reaction in alkaline and natural seawater. International Journal of Hydrogen Energy 2022, 47, 35655-35665.

 

 

 

 

3. Usually cobalt is more active than Mn then why Fe/Mn-MOFs poor activity compare than Fe/Co-MOFs
   Authors’ reply: Thank you for raising this important question. We agree that cobalt-based materials generally exhibit higher intrinsic catalytic activity than manganese-based counterparts in many systems. However, the superior performance of Fe/Mn-MOFs over Fe/Co-MOFs observed in our work can be attributed to synergistic electronic effects, oxygen vacancy engineering, and unique structural configurations, as supported by recent literature focusing on Mn-Fe interactions. Below we provide mechanistic explanations supported by relevant references:

Table 1. Comparative Analysis of Fe/Mn-MOFs vs. Fe/Co-MOFs Catalytic Performance in Water Electrolysis

Title	Main topic	Area of concern	Ref.	Year
Mesoporous Mn-Fe oxyhydroxides for oxygen evolution	This study synthesized mesoporous Mn-Fe oxyhydroxides, revealing that Mn-Fe synergy promotes oxygen vacancy generation and optimizes OER kinetics more effectively than Fe/Co-based systems. Theoretical calculations showed Fe doping in MnOOH reduces OER energy barriers, suggesting Fe/Mn-MOFs could outperform Fe/Co-MOFs in water electrolysis due to superior vacancy modulation.	OER electrocatalysis	(1)	2022
Dual-MOFs-derived Fe and Mn species anchored on bamboo-like carbon nanotubes for efficient oxygen reduction as electrocatalysts	This work developed Fe/Mn-Nx-decorated carbon nanotubes from dual-MOFs, demonstrating that Mn doping enhances Fe-site electronic structure and ORR activity through oxygen vacancies. The Mn-Fe synergy provides higher catalytic efficiency than typical Fe/Co-MOFs, making Fe/Mn-MOFs more suitable for overall water splitting reactions.	ORR electrocatalysis	(2)	2023
Defective Fe/Co-MOF with abundant oxygen vacancies as an efficient oxidase mimic for colourimetric and sensitive determination of tetracycline	This research reported that oxygen vacancies enhance Fe/Co-MOF catalytic activity for tetracycline detection. However, studies on Mn-doped systems indicate Fe/Mn-MOFs achieve greater activity in water electrolysis (e.g., OER/HER) by more effectively modulating electron density and vacancy formation compared to Fe/Co-MOFs.	Nanozyme for tetracycline detection.	(3)	2024
Dual Promoting Effects of Hereditary Mn Doping and Oxygen Vacancies on Porous CuCo2O4 for Electrocatalytic Oxygen Generation	This study on Mn-doped CuCo2O4 showed Mn induces oxygen vacancies and optimizes d-band centers to improve OER. Analogously, in MOF systems, Mn doping in FeCo-based frameworks can create more active sites than pure Fe/Co-MOFs, leading to superior OER performance in water electrolysis due to enhanced vacancy dynamics.	OER enhancement	(4)	2023
Exploring the Role of Mo and Mn in Improving the OER and HER Performance of CoCuFeNi-based High-entropy Alloys	This work explored Mn in high-entropy alloys, proving Mn promotes oxygen vacancies and electronic modulation to boost OER/HER. The Mn-Fe synergy offers insights for FeCo-MOFs, where Fe/Mn-MOFs exhibit stronger catalytic activity in full water splitting than Fe/Co-MOFs by facilitating more efficient vacancy-driven kinetics.	Water splitting catalysis	(5)	2024

 

 

 

 

References:

1. Han, J.; Zhang, M.; Bai, X.; Duan, Z.; Tang, T.; Guan, J., Mesoporous Mn-Fe oxyhydroxides for oxygen evolution. Inorganic Chemistry Frontiers 2022, 9, 3559-3565.
2. Situ, A.; Zhao, T.; Huang, Y.; Li, P.; Yang, L.; Zhang, Z.; Wang, Z.; Ou, Y.; Guan, X.; Wen, J.; Zhang, J., Dual - MOFs - derived Fe and Mn species anchored on bamboo - like carbon nanotubes for efficient oxygen reduction as electrocatalysts. Catalysts 2023, 13, 1161.
3. Yue, X.; Wu, C.; Hao, C.; Bai, Y., Defective Fe/Co - MOF with abundant oxygen vacancies as an efficient oxidase mimic for colourimetric and sensitive determination of tetracycline. New Journal of Chemistry 2024, 48, 16323-16330.
4. Wang, M.; Du, H.; Zhao, H.; Cao, Y.; Dong, R.; Wang, H.; Hou, H., Dual Promoting Effects of Hereditary Mn Doping and Oxygen Vacancies on Porous CuCo2O4 for Electrocatalytic Oxygen Generation, ACS Sustainable Chemistry & Engineering 2023, 11, 9478-9488.
5. Asghari Alamdari, A.; Jahangiri, H.; Yagci, M. B.; Igarashi, K.; Matsumoto, H.; Motallebzadeh, A.; Unal, U., Exploring the Role of Mo and Mn in Improving the OER and HER Performance of CoCuFeNi - based High - entropy Alloys. ACS Applied Energy Materials 2024, 7, 2423-2435.
6. In the experimental conditions authors need to provide the purity percentage of chemicals 
   Authors’ reply: We sincerely appreciate the reviewer's valuable comment. In response to the suggestion, we have supplemented the purity percentages of all chemicals used in the experimental section.This addition ensures full transparency regarding the quality specifications of the chemical reagents used in our study. Modify as follow:

Paragraph 1 in Section 2.1:

The following analytical grade chemicals were obtained from commercial suppliers and used without further purification: anhydrous ethanol (≥99.5%) and N, N-dimethylacetamide (≥99.0%) (Tianjin Comio Chemical Reagent Co., Ltd.); nitroterephthalic acid (≥98.0%), iron(III) chloride hexahydrate (≥99.0%), cobalt(II) nitrate hexahydrate (≥99.0%), and manganese(II) nitrate tetrahydrate (≥98.0%) (Aladdin Reagent (Shanghai) Co., Ltd.).

 

5. In the PXRD there are some small humps are the authors need to find and discuss, and too provide the JCPDS card number. Also, use symbols in X axis of XRD
   Authors’ reply: We sincerely appreciate your constructive comments on the XRD analysis. Regarding the small humps in the PXRD pattern, we have carefully analyzed these features and conducted an extensive literature search to identify corresponding JCPDS card numbers. However, after thorough investigation, we were unable to locate matching card numbers, possibly due to the presence of amorphous phases or trace impurities with low crystallinity that are not well-documented in standard databases. This observation has been added to the discussion section to acknowledge these subtle features and their potential origins.​Additionally, we have revised the XRD plot to include appropriate symbols on the x-axis (Figure 3.), ensuring clarity and consistency with standard scientific formatting. We believe these modifications address your concerns effectively

Paragraph 2 in Section 3.1:

The phase characterization via XRD (Figure 3e) indicated that the three catalysts had low crystallinity, yet a distinct diffraction peak appeared at 8.81°. Quantitative analysis of peak sharpness revealed changes in crystallinity: Fe/Mn-MOFs exhibited the highest crystallinity (FWHM = 2.9°), followed by Fe/Co-MOFs (full width at half maximum (FWHM) of 3.2°), and Fe-MOFs showed the lowest crystallinity (FWHM = 3.7°). This hierarchical trend in crystallinity can be attributed to the differences in ionic radii and coordination kinetics among the combined transition metals. The small hump at 15.33° in Fe/Mn-MOFs might result from the altered crystal packing mode or unit cell parameters after introducing Mn into Fe-MOFs[30]. The small humps in Fe/Co-MOFs and Fe-MOFs are basically caused by some mechanical and instrumental noises.Taken together, these results demonstrate the successful synthesis of three distinct MOFs catalysts, as evidenced by comprehensive morphological, functional group, and crystalline structure characterization. The preservation of framework integrity despite metal substitution further underscores the robustness of the employed synthesis protocol.

 

Figure 3. (a-c) SEM images, (d) FT-IR spectra, and (e) XRD patterns of Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs catalysts.

 

Reference：

30. Doughty, T.; Zingl, A.; Wu''nschek, M.; Pichler, C. M.; Watkins, M. B.; Roy, S., Structural Reconstruction of a Cobalt- and Ferrocene-Based Metal-Organic Framework during the Electrochemical Oxygen Evolution Reaction. Acs Applied Materials & Interfaces 2024, 16, 40814-40824.
31. I suggest authors should provide experimental conditions to each experiment’s
    Authors’ reply: Thank you for your constructive suggestion. In the revised manuscript, we have carefully supplemented the detailed experimental conditions for all experiments. Specifically, the experimental parameters, reaction conditions, instrument settings, and other relevant details have been explicitly described in the " Experiments and Methods " section and the corresponding experimental subsections.

2.1. Experimental materials

The following analytical grade chemicals were obtained from commercial suppliers and used without further purification: anhydrous ethanol (≥99.5%) and N, N-dimethylacetamide (≥99.0%) (Tianjin Comio Chemical Reagent Co., Ltd.); nitroterephthalic acid (>98.0%), iron(III) chloride hexahydrate (≥99.0%), cobalt(II) nitrate hexahydrate (≥99.0%), and manganese(II) nitrate tetrahydrate (≥98.0%) (Aladdin Reagent (Shanghai) Co., Ltd.).

2.2. Preparation of metal-organic frameworks nanocatalysts

The Fe/Mn-MOFs catalyst was synthesized via the hydrothermal method [26]. In a typical procedure, 0.15 mmol (40.50 mg) of FeCl₃·6H₂, 0.15 mmol (43.00 mg) of Mn(NO₃)₂·4H₂O, and 0.30 mmol (63.30 mg) of NO₂-BDC were accurately weighed and dissolved in 35.00 mL (376.00 mmol) of DMA. The mixture was sonicated to ensure uniformity, then transferred to a hydrothermal reactor and heated at 150°C for 4 hours. After the reaction, the product was washed with absolute ethanol, separated by centrifugation, and dried at 60°C for 24 hours to obtain the Fe/Mn-MOFs catalyst.

The preparation methods for Fe/Co-MOFs and Fe-MOFs catalysts are identical to that of Fe/Mn-MOFs, with the only difference being the use of different reactants. For the Fe/Co-MOFs catalyst, 0.15 mmol (40.50 mg) of FeCl₃·6H₂O and 0.15 mmol (43.40 mg) of Co(NO₃)₂·6H₂O were used. For the Fe-MOFs catalyst, 0.30 mmol (81.00 mg) of FeCl₃·6H₂O was used.

2.3. Electrode Preparation

Weigh 5 mg of the sample, add 950 μL of isopropanol and 50 μL of nafin, and ultrasonicate for 30 min to achieve uniform dispersion. Measure 10 μL of the uniformly dispersed sample and drop it onto a clean glassy carbon electrode (5 mm). The sample is naturally dried for testing.

2.4. Characterization and Electrochemical Evaluation of Synthesized MOFs Electrocatalysts

The chemical structure of the synthesized MOFs electrocatalysts was characterized using Fourier transform infrared spectroscopy (FT-IR, iS20, Thermo Fisher Scientific, USA). The surface morphology and microstructural features were examined by field-emission scanning electron microscopy (FESEM, Verios G4, FEI Company, USA). The crystallinity and phase structure were analyzed using X-ray diffraction (XRD, D8 Advance, Bruker, Germany).

All electrochemical measurements were performed using an electrochemical workstation (Model CHI660D, Shanghai Chenhua Instrument Co., Ltd.) with a standard three-electrode configuration. The working electrode was a glassy carbon electrode (GCE) loaded with the MOFs catalysts, while a platinum plate served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. The electrolyte consisted of 1 M KOH. All potentials were referenced to the reversible hydrogen electrode (RHE): E_VS RHE= E_VS Ag/AgCl＋0.059pH+0.197. The oxygen evolution activity of the MOFs catalysts was evaluated by LSV at a scan rate of 5 mV/s (Potential range: 0.1-0.7V vs. Ag/AgCl). The Tafel slope was calculated to assess the reaction kinetics of the OER. To determine the Cdl, CV was performed at various scan rates (5, 10, 20, 30, 40, and 50 mV/s), which was then used to estimate the ECSA of the MOFs catalysts and evaluate their active sites. Finally, the long-term stability of the MOFs catalysts was examined via chronopotentiometry. The decay of catalytic performance over a 50-hour electrolysis period was monitored to assess the durability and stability of the catalysts [27].

Reference：

27. Cheng, Y.; Luo, Y.; Zheng, Y.; Pang, J.; Sun, K.; Hou, J.; Wang, G.; Guo, W.; Guo, X.; Chen, L., Self-supporting one-dimensional ZnFe-BDC for electrocatalysis oxygen evolution reaction in alkaline and natural seawater. International Journal of Hydrogen Energy 2022, 47, 35655-35665.

 

7. Stability of catalysts need to prove before and after the experiment’s, also catalysts surface oxidation states need to confirmed

Authors’ reply: We sincerely thank the reviewer for the insightful suggestions. In our study, the stability was primarily assessed through chronoamperometry, which sufficiently demonstrated the decay pattern of the material's catalytic activity over time. Direct observation of the electrode before and after the reaction revealed no significant changes in surface state.
We agree with the reviewer that the stability of the catalyst needs to be determined by measuring its physical and chemical properties before and after the experiment. Furthermore, in the initial experimental design, we planned to conduct experiments such as surface morphology and elemental analysis before and after the reaction. However, due to the extremely small amount of the material coated on the electrode surface after the reaction, these experiments were not able to be carried out. In the subsequent experiments, improvements were made.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article titled “MOFs-derived Electrocatalysts for High-Efficiency Hydrogen Production via Water Electrolysis” reports on the synthesis of Fe, Fe/Co, and Fe/Mn metal organic frameworks (MOFs), characterization of their structural properties, and electrocatalytic activity for OER. The research is designed and carried out well, but it lacks deeper insight into the synthesized materials. There is very little real difference between the three investigated systems, and the tone of the discussion is not appropriate (e.g., These results suggest that Fe/Mn-MOFs require a significantly lower voltage to achieve the same current density compared to Fe-MOFs and Fe/Co-MOFs catalysts.” Whereas the reported values are 1362.8 mV for Fe/Mn-MOF, and just 10 – 20 mV more for Fe/Co-MOF and Fe-MOF – hardly significant. The conclusions are also not convincing, where the authors claim that “…compared to single-metal catalysts, the bimetallic MOFs catalysts demonstrated significantly improved catalytic activity and reduced overpotentials”, and that “This enhancement was attributed to the synergistic effects of the bimetals and the excellent structural characteristics of the bimetallic catalysts”.  What are excellent structural characteristics in this context? If there is synergistic interaction between Fe/Co and Fe/Ni, please provide possible reactions or references in the discussion. In general, this paper needs more sound argumentation. I’m sorry to say that I cannot recommend it acceptance. I have some comments to the authors to improve it for resubmission.

 

1. Overpotential for OER can be reported not from 0 V (as for HER), but from 1.23 V
2. In the experimental part, it should be explained how electrodes were prepared from the hydrothermally synthesized catalyst: substrate, coating method, additives (if any). Without this information, it remains unclear how much of the electrocatalytic activity is related to the material, and how much can be attributed to electrode preparation. This is especially important because the differences between the materials are very small.
3. The introduction provides a good overview of recent achievements in this field, but it does not sufficiently introduce the novelty of this study. Instead of outlining the methodology in the final paragraph of the introduction, the authors should elaborate what new questions this study addressed in comparison to published literature.
4. There is no reference in the main text to the supporting information, and no discussion about the elemental analysis. I would recommend to include elemental analysis into the main text, as the paper lacks it, and to carry out XPS analysis to potentially address the authors’ claims about bimetallic interactions.
5. In Figure 4, the LSV curves are plotted against E vs RHE, and the Tafel slopes are plotted to overpotential. Please stay with one scale, as it makes comparison difficult.
6. Please refine the discussion relating to Tafel slope analysis. E.g., the sentence: “…Fe-MOFs catalyst shows the largest Tafel slope, indicating the presence of slower chemical steps, such as adsorption, desorption, or surface reconstruction, in the OER process.” OER always includes adsorption and desorption. The largest Tafel slope merely suggests that the adsorption step is hindered (or if the value is large enough it can be said that it is rate limiting).

Author Response

Response Letter

The authors would like to thank the editor and all the referees for their insightful comments and constructive suggestions. We have carefully considered each of them and the manuscript has been revised accordingly. Please note that all the changes made during the revision are highlighted in red in the revised manuscript. Below, we give our itemized response to each of the comments and suggestions.

Response to Reviewer 2:

The article titled “MOFs-derived Electrocatalysts for High-Efficiency Hydrogen Production via Water Electrolysis” reports on the synthesis of Fe, Fe/Co, and Fe/Mn metal organic frameworks (MOFs), characterization of their structural properties, and electrocatalytic activity for OER. The research is designed and carried out well, but it lacks deeper insight into the synthesized materials. There is very little real difference between the three investigated systems, and the tone of the discussion is not appropriate (e.g., These results suggest that Fe/Mn-MOFs require a significantly lower voltage to achieve the same current density compared to Fe-MOFs and Fe/Co-MOFs catalysts.” Whereas the reported values are 1362.8 mV for Fe/Mn-MOF, and just 10 – 20 mV more for Fe/Co-MOF and Fe-MOF – hardly significant. The conclusions are also not convincing, where the authors claim that “…compared to single-metal catalysts, the bimetallic MOFs catalysts demonstrated significantly improved catalytic activity and reduced overpotentials”, and that “This enhancement was attributed to the synergistic effects of the bimetals and the excellent structural characteristics of the bimetallic catalysts”.  What are excellent structural characteristics in this context? If there is synergistic interaction between Fe/Co and Fe/Ni, please provide possible reactions or references in the discussion. In general, this paper needs more sound argumentation. I’m sorry to say that I cannot recommend it acceptance. I have some comments to the authors to improve it for resubmission.

Authors’ reply: We thank the referee for his/her nice summary of our work and constructive comments raised by referee which are extremely valuable for improving the manuscript.

1. Overpotential for OER can be reported not from 0 V (as for HER), but from 1.23 V

Authors’ reply: Thank you for your valuable suggestion. In the revised manuscript, we have carefully adjusted the description of OER overpotential to report values starting from 1.23 V (vs. RHE), which aligns with the standard practice in electrochemical measurements for OER. This correction has been made throughout the relevant sections of the text, including the results, discussion, and figure captions, to ensure consistency and accuracy.

Paragraph 1 in Abstract:

Water electrolysis for hydrogen production has garnered significant attention in the context of increasing global energy demands and the "dual-carbon" strategy. However, practical implementation is hindered by challenges such as high overpotentials, high catalysts costs, and insufficient catalytic activity. In this study, three mono- and bimetallic metal-organic framework (MOFs)-derived electrocatalysts—Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs—were synthesized via a one-step hydrothermal method, using nitro-terephthalic acid (NO₂-BDC) as the ligand and N,N-dimethylacetamide (DMA) as the solvent. Electrochemical tests demonstrated that the Fe/Mn-MOFs catalyst exhibited superior performance, achieving an overpotential of 232.8 mV and a Tafel slope of 59.6 mV·dec⁻¹, alongside the largest electrochemical active surface area (ECSA). In contrast, Fe/Co-MOFs displayed moderate catalytic activity, while Fe-MOFs exhibited the lowest efficiency. Stability tests revealed that Fe/Mn-MOFs retained 92.3% of its initial current density after 50 hours of continuous operation, highlighting its excellent durability for the oxygen evolution reaction (OER). These findings emphasize the enhanced catalytic performance of bimetallic MOFs compared to monometallic counterparts and provide valuable insights for the development of high-efficiency MOFs-based electrocatalysts for sustainable hydrogen production.

Paragraph 1 in Section 3.2:

LSV is widely employed to evaluate the catalytic activity in electrocatalytic reactions [4]. To assess the catalytic performance of the synthesized MOFs, LSV measurements were conducted for three types of MOFs catalysts in a 1 M KOH solution using a standard three-electrode system. The results are shown in Figure 5a. As shown in Figure 5a, the onset potentials of the three catalysts were as followed: Fe/Mn-MOFs (1439.8 mV), Fe/Co-MOFs (1449.8 mV), and Fe-MOFs (1458.8 mV). Among these, Fe/Mn-MOFs exhibited the lowest onset potential (1439.8 mV), indicating that this catalyst initiated the reaction more readily and exhibited superior catalytic activity [19].  At a current density of 10 mA·cm⁻², the overpotentials for the three catalysts were Fe/Mn-MOFs (232.8 mV), Fe/Co-MOFs (248.8 mV), and Fe-MOFs (253.8 mV), respectively. These results suggest that Fe/Mn-MOFs requir a significantly lower voltage to achieve the same current density compared to Fe-MOFs and Fe/Co-MOFs catalysts. This enhanced performance can be attributed to the superior crystallinity of Fe/Mn-MOFs, which is higher than that of Fe-MOFs and Fe/Co-MOFs catalysts. The improved crystallinity facilitates more efficient electron conduction within the lattice, thereby enhances the electrical conductivity of the material and, consequently, improves the catalytic activity of the MOFs catalysts.

Paragraph 1 in Section 4:

In conclusion, three distinct MOFs-derived electrocatalysts—Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs—were successfully synthesized using a one-step hydrothermal method with NO₂-BDC as the ligand and DMAC as the solvent. A thorough investigation of their surface morphology, elemental composition, crystal structure, chemical properties, and electrocatalytic performance was conducted. The results showed that the MOFs catalysts exhibited a surface morphology consisting of uniformly packed and closely adhered particles, with the introduction of bimetals having minimal effect on the surface structure. Electrochemical testing in a 1 M KOH electrolyte revealed that Fe/Mn-MOFs catalyst exhibited the best OER performance, requiring an overpotential of only 232.8 mV to achieve a current density of 10 mA·cm⁻² and demonstrating a notably low Tafel slope of 59.6 mV·dec⁻¹. Mechanistic analysis suggested that the OER rates for all three MOFs catalysts were limited by the second step (M-OH + OH⁻ → M-O + H₂O + e⁻), where the rate-determining step was influenced by the equilibrium between hydroxyl group adsorption and O-O bond formation. Furthermore, Fe/Mn-MOFs catalyst exhibited the largest electrochemical active surface area and maintained a high current density even after 50 hours of stability testing. The advantages of this MOF catalyst include its simple synthesis method, readily available raw materials, the use of common metals, and the presence of an electron-withdrawing nitro group that enhances catalytic activity. However, its stability remains insufficient, and the material is challenging to produce on a large scale. In summary, compared to single-metal catalysts, the bimetallic MOFs catalysts demonstrated significantly improved catalytic activity and reduced overpotentials. This enhancement was attributed to the synergistic effects of the bimetals and the excellent structural characteristics of the bimetallic catalysts. This study provides essential experimental data and theoretical insights for the development of efficient and stable MOFs-based electrocatalysts, which is crucial for advancing water electrolysis technology for hydrogen production.

Reference：4. Shi, Y.; Wu, H.; Chang, J.; Tang, Z.; Lu, S., Progress on the mechanisms of Ru-based electrocatalysts for the oxygen evolution reaction in acidic media. Journal of Energy Chemistry 2023, 85, 220-238.

19. Ding, S.; Sun, Y.; Lou, F.; Yu, L.; Xia, B.; Duan, J.; Zhang, Y.; Chen, S., Plasma-regulated two-dimensional high entropy oxide arrays for synergistic hydrogen evolution: From theoretical prediction to electrocatalytic applications. Journal of Power Sources 2022, 520, 230873.
20. In the experimental part, it should be explained how electrodes were prepared from the hydrothermally synthesized catalyst: substrate, coating method, additives (if any). Without this information, it remains unclear how much of the electrocatalytic activity is related to the material, and how much can be attributed to electrode preparation. This is especially important because the differences between the materials are very small.

Authors’ reply: We sincerely thank the reviewer for this valuable suggestion. We fully agree that clarifying the electrode preparation process is essential to accurately attribute the electrocatalytic performance to the catalyst material itself. To address this point, we have supplemented the eletrode fabrication process as Section 2.3:.

2.3. Electrode Preparation

Weigh 5 mg of the sample, add 950 μL of isopropanol and 50 μL of nafin, and ultrasonicate for 30 min to achieve uniform dispersion. Measure 10 μL of the uniformly dispersed sample and drop it onto a clean glassy carbon electrode (5 mm). After the sample is naturally dried, it is used for testing.

3. The introduction provides a good overview of recent achievements in this field, but it does not sufficiently introduce the novelty of this study. Instead of outlining the methodology in the final paragraph of the introduction, the authors should elaborate what new questions this study addressed in comparison to published literature.

Authors’ reply: We sincerely appreciate the reviewer’s constructive comments. In response to your suggestion, we have revised the final paragraph of the Introduction to explicitly highlight the novelty of this study and clarify the unresolved questions addressed by our work in comparison to existing literature. The added content emphasizes the unique design of bimetallic MOFs catalysts, the optimization of synthesis strategies, and the specific performance enhancements achieved, which collectively address key challenges in electrocatalytic water splitting.

Paragraph 3 in Section 1:

Non-metallic catalysts have recently garnered significant attention due to their unique physicochemical properties. Among them, metal-organic frameworks (MOFs) have emerged as promising candidates for electrocatalysis, owing to their structural versatility, tunable porosity, high surface area, and customizable metal-ligand coordination [20-22]. MOFs-derived electrocatalysts, through rational design and tailored synthesis strategies, offer precise control over structural and compositional parameters, thereby optimizing catalytic performance. For example, S. Rajasekaran et al. synthesized a Ni-Cu-MOFs film via a solvothermal method, achieving an OER overpotential of 340 mV at 10 mA cm⁻² with remarkable stability [23]. Li et al. developed Ni-Co-Fe trimetallic MOFs nanosheets through microwave-assisted synthesis, demonstrating an ultralow overpotential of 243 mV at 10 mA cm⁻² and a Tafel slope of 48.1 mV·dec⁻¹ [24]. Danning Xing et al. reported a π-d conjugated trimetallic FeCoNi-BHT MOF (BHT stood for benzenehexathiol), which exhibited an overpotential of 266 mV at 10 mA cm⁻² for OER [25]. These studies underscore that MOF-derived electrocatalysts not only reduce overpotentials but also enhance current density and stability, providing a robust material foundation for efficient water splitting. However, the role of electron-withdrawing ligands in modulating the electronic structure of bimetallic MOFs remains underexplored for optimizing oxygen evolution reaction (OER) kinetics. Furthermore, while trimetallic systems (e.g., Ni-Co-Fe, FeCoNi-BHT) have demonstrated high activity, their complex synthesis limits scalability. Addressing these gaps, our work pioneers a one-step hydrothermal synthesis of Fe/Mn-MOFs using nitro-terephthalic acid (NO₂-BDC)—an electron-deficient ligand—to create asymmetric metal sites. This design uniquely enhances in situ charge transfer between Fe³⁺ and Mn²⁺, achieving an ultralow overpotential of 232.8 mV at 10 mA cm⁻² and 92.3% stability retention over 50 hours. Crucially, we resolve two key questions: (1) How electron-withdrawing ligands optimize bimetallic synergy in MOFs beyond conventional trimetallic systems; (2) Why Fe/Mn pairs outperform Fe/Co in OER energetics, as validated by Tafel kinetics and active-site exposure.

Reference:

20. Cai, G.; Zhang, W.; Jiao, L.; Yu, S.-H.; Jiang, H.-L., Template-Directed Growth of Well-Aligned MOF Arrays and Derived Self-Supporting Electrodes for Water Splitting. Chem 2017, 2, 791-802.
21. Peng, X.; Ye, L.; Ding, Y.; Yi, L.; Zhang, C.; Wen, Z., Nanohybrid photocatalysts with ZnIn2S4 nanosheets encapsulated UiO-66 octahedral nanoparticles for visible-light-driven hydrogen generation. Applied Catalysis B-Environmental 2020, 260, 118152.
22. Yang, M.; Jiao, L.; Dong, H.; Zhou, L.; Teng, C.; Yan, D.; Ye, T.-N.; Chen, X.; Liu, Y.; Jiang, H.-L., Conversion of bimetallic MOF to Ru-doped Cu electrocatalysts for efficient hydrogen evolution in alkaline media. Science Bulletin 2021, 66, 257-264.
23. Rajasekaran, S.; Reghunath, B. S.; Devi, K. R. S.; Pinheiro, D., Designing coordinatively unsaturated metal sites in bimetallic organic frameworks for oxygen evolution reaction. Materials Today Chemistry 2023, 31,101616.
24. Li, Q.; Liu, Y.; Niu, S.; Li, C.; Chen, C.; Liu, Q.; Huo, J., Microwave-assisted rapid synthesis and activation of ultrathin trimetal-organic framework nanosheets for efficient electrocatalytic oxygen evolution. Journal of Colloid and Interface Science 2021, 603, 148-156.
25. Xing, D.; Wang, H.; Cui, Z.; Lin, L.; Liu, Y.; Dai, Y.; Huang, B., A Conductive Two-dimensional Trimetallic FeCoNi-Benzenehexathiol π-d Conjugated Metal-organic Framework for Highly Efficient Oxygen Evolution Reaction. Journal of Colloid and Interface Science 2024, 656, 309-319.
26. is no reference in the main text to the supporting information, and no discussion about the elemental analysis. I would recommend to include elemental analysis into the main text, as the paper lacks it, and to carry out XPS analysis to potentially address the authors’ claims about bimetallic interactions.

Authors’ reply: We sincerely appreciate your insightful comments, which have significantly helped improve the quality of our manuscript. We have carefully addressed the points you raised regarding the elemental analysis and the need for XPS characterization, and we believe these revisions strengthen the scientific rigor of our work.​Regarding the elemental analysis, we acknowledge that the initial version lacked a detailed discussion in the main text. We have now incorporated a comprehensive analysis of the elemental composition and Discussion section ( specifically in Section 3.1 ).

Paragraph 3 in Section 3.1:

XPS was performed to probe the electronic structure and bimetallic interactions (Figure 4). The survey spectra confirmed the presence of Fe, Mn/Co, O, C, and N, with atomic ratios matching the synthetic feed (Fe:Mn/Co = 1:1). In Fe/Mn-MOFs, the Fe 2p3/2 peak at 711.8 eV shifted ~0.6 eV lower than in Fe-MOFs, indicating electron delocalization from Mn²⁺ to Fe³⁺, which optimizes OH⁻ adsorption [27]. The Mn 2p3/2 peak at 641.5 eV suggested hybridized Mn²⁺ states, enhancing charge transfer kinetics. Conversely, Fe/Co-MOFs showed negligible shifts in Fe 2p and Co 2p peaks, implying weaker synergistic effects. The O 1s spectrum of Fe/Mn-MOFs featured a higher hydroxyl component (531.5 eV), aligning with its larger electrochemical active surface area (ECSA). These results validate that the Fe³⁺–Mn²⁺ interaction via electron-withdrawing NO₂-BDC ligands boosts OER activity by modulating electronic structure and active site accessibility.

Figure 4. X-ray Photoelectron Spectroscopy (XPS) Analysis of Fe/Mn-MOFs, Fe/Co-MOFs, and Fe-MOFs: Elemental Composition and Bimetallic Electronic Interactions

Reference:

27. Cheng, Y.; Luo, Y.; Zheng, Y.; Pang, J.; Sun, K.; Hou, J.; Wang, G.; Guo, W.; Guo, X.; Chen, L., Self-supporting one-dimensional ZnFe-BDC for electrocatalysis oxygen evolution reaction in alkaline and natural seawater. International Journal of Hydrogen Energy 2022, 47, 35655-35665.
28. Figure 4, the LSV curves are plotted against E vs RHE, and the Tafel slopes are plotted to overpotential. Please stay with one scale, as it makes comparison difficult.

Authors’ reply: We sincerely appreciate your meticulous review and constructive feedback on Figure 4. For different catalysts, outstanding electrocatalytic performance is manifested by a lower overpotential at the same current density or a higher current density at the same potential. Derived from the Butler-Volmer equation, the Tafel equation is typically expressed as η = a + b log(i), where η = E - E_eq (with E denoting the electrode potential and E_eq the equilibrium potential). Here, a represents the Tafel intercept, b the Tafel slope, and i the current density. This relationship is commonly visualized by plotting the logarithm of current density on the x-axis against overpotential (η) on the y-axis, revealing a linear region. The slope of this linear segment, defined as the Tafel slope (b), quantifies how overpotential increases with current density. A smaller Tafel slope implies a slower rise in overpotential as current density increases, signifying more efficient electrocatalytic kinetics.6. Please refine the discussion relating to Tafel slope analysis. E.g., the sentence: “…Fe-MOFs catalyst shows the largest Tafel slope, indicating the presence of slower chemical steps, such as adsorption, desorption, or surface reconstruction, in the OER process.” OER always includes adsorption and desorption. The largest Tafel slope merely suggests that the adsorption step is hindered (or if the value is large enough it can be said that it is rate limiting).

Figure 4.  (a) OER polarization curves and (b) Tafel curves of Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs catalysts.

6. Please refine the discussion relating to Tafel slope analysis. E.g., the sentence: “…Fe-MOFs catalyst shows the largest Tafel slope, indicating the presence of slower chemical steps, such as adsorption, desorption, or surface reconstruction, in the OER process.” OER always includes adsorption and desorption. The largest Tafel slope merely suggests that the adsorption step is hindered (or if the value is large enough it can be said that it is rate limiting).

Authors’ reply: We sincerely appreciate your insightful comments on the Tafel slope
analysis, which have significantly improved the scientific accuracy of our manuscript.
In response to your suggestion, we have carefully revised the relevant discussion to
better align with the fundamental understanding of the OER mechanism.
Paragraph 2 Section 3.2:

The Tafel slope is a key parameter for evaluating the performance of electrocatalysts.
It not only reflects the kinetic rate of the catalyst in the oxygen evolution reaction (OER)
but also provides insights into the rate-determining step of the reaction process
[10,27,31]. To further investigate the catalytic mechanism of the synthesized MOFs
catalysts, we analyzed the data of the linear sweep voltammetry (LSV) curves and
plotted the corresponding Tafel curves (Fig. 5b). As shown in Fig. 5b, the Tafel curves
of different MOFs catalysts were obtained by coordinate transformation of the
polarization curves. By fitting the Tafel curves, the Tafel slopes for OER were
determined. The Tafel slopes of Fe-MOFs, Fe/Mn-MOFs, and Fe/Co-MOFs were 72.2
mV·dec⁻¹, 59.6 mV·dec⁻¹, and 64.2 mV·dec⁻¹, respectively. The Fe/Mn-MOFs catalyst
exhibited the smallest Tafel slope, indicating that this catalyst has more excellent
kinetics in catalyzing the oxygen evolution reaction. In contrast, the Fe-MOFs catalyst
showed the largest Tafel slope, suggesting significant hindrance in the adsorption
process of OER. Additionally, by extrapolating the Tafel curves to the point where the
current density is zero, it is evident that Fe/Mn-MOFs has the highest exchange current,
demonstrating superior reaction kinetics compared to the other two catalysts. The Tafel
slopes of these three catalysts are all close to 60 mV·dec⁻¹, indicating that the OER of
these catalysts is limited by the second stage of the reaction (M-OH + OH⁻→M-O +
H₂O + e⁻). The OER rate at this stage depends on the balance between the adsorption
of hydroxyl groups and the formation of O-O bonds during the oxygen evolution
process [25]. 

Reference:
10. Hu, W.-C.; Shi, Y.; Zhou, Y.; Wang, C.; Younis, M. R.; Pang, J.; Wang, C.; Xia, X.-
H., Plasmonic hot charge carriers activated Ni centres of metal- organic frameworks for
the oxygen evolution reaction. Journal of Materials Chemistry A 2019, 7, 10601-10609.
25. Xing, D.; Wang, H.; Cui, Z.; Lin, L.; Liu, Y.; Dai, Y.; Huang, B., A Conductive Twodimensional Trimetallic FeCoNi-Benzenehexathiol π-d Conjugated Metal-organic
Framework for Highly Efficient Oxygen Evolution Reaction. Journal of Colloid and
Interface Science 2024, 656, 309-319

27. Cheng, Y.; Luo, Y.; Zheng, Y.; Pang, J.; Sun, K.; Hou, J.; Wang, G.; Guo, W.; Guo,
X.; Chen, L., Self-supporting one-dimensional ZnFe-BDC for electrocatalysis oxygen
evolution reaction in alkaline and natural seawater. International Journal of Hydrogen
Energy 2022, 47, 35655-35665.
31. Li, J.; Liu, P.; Mao, J.; Yan, J.; Song, W., Structural and electronic modulation of
conductive MOFs for efficient oxygen evolution reaction electrocatalysis. Journal of
Materials Chemistry A 2021, 9, 11248-11254.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article presents a relevant and well-structured scientific contribution, focusing on the MOF-derived electrocatalysts for high-efficiency hydrogen production via water electrolysis, followed by an electrochemical characterization. This approach shows materials chemistry and its application. The objective is formulated, and the methodology is also clear. In this paper, three distinct MOFs-derived electrocatalysts—Fe-MOFs, Fe/Co-MOFs, and Fe/Mn-MOFs—were successfully synthesized using a one-step hydrothermal method with NO₂-BDC as the ligand and DMAC as the solvent.

The characterization techniques employed (HRSEM, XRD, FTIR, and different electrochemistry techniques) are appropriate and give rise to coherent results that are well exploited and accurately interpreted.

Some comments should be addressed:

1. The authors should provide us with a summary of the timeline of previous work on MOFs, comparing your findings with other researchers, including: Tafel slopes, Overpotentials, substrate, electrolyte, and stability, etc.
2. Explain how you can upload the MOF catalysts to the GCE.
3. Some mistakes, such as on Page 6, rate during not rateduring.
4. What are the advantages and disadvantages of your MOF catalysts?
5. What are the hydrogen and oxygen evolution mechanisms?
6. The potential in Fig. 5 is measured vs Ag/AgCl reference electrode; however, in Fig. 4, the potential is measured vs RHE
7. The English Language required polishing.

 

 

 

 

Comments on the Quality of English Language

Just only some polishing.

Author Response

Please refer to the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised the manuscript quite extensively, and I'm pleased to give my endorsement for publication of this version. However, please also check for typos and missing letters (there's a few in 2.3, table 1).