Boosting Water Oxidation Activity via Carbon–Nitrogen Vacancies in NiFe Prussian Blue Analogue Electrocatalysts

Round 1

Reviewer 1 Report

Zhang et al. developed defect-rich Prussian blue analogues (PBA) with rich CN vacancies (D-NiFe PBA) as efficient OER electrocatalyst. The optimized D-NiFe PBA demonstrated an overpotential of 280 mV at 10 mA cm-2 and the superior stability for over 100 h in KOH electrolytes.

1. What is the origin of a peak at 1.4 V in D-NiFe PBA in Figure 5a?

 

2. “As shown in Figure 4d, D-NiFe PBA shows a symmetric signal at a magnetic field of g = 2.00, indicating the presence of V_CN in the lattice”. Explain it in detail.

 

 

Author Response

1. What is the origin of a peak at 1.4 V in D-NiFe PBA in Figure 5a?

**Thanks for your helpful suggestion, we accept it. D-NiFe PBA shows oxidation peaks at around 1.4 V in terms of the oxygen evolution in figure 5a, which are ascribed to the oxidation of Ni²⁺ to Ni³⁺. To avoid the interference of the oxidation peaks of Ni²⁺ to Ni³⁺, the overpotential at a current density of 10 mA cm⁻² is used as the benchmark for the comparison of the OER activity [1,2], and the corresponding descriptions have been added in the Results and discussion section (Page 12, lines 240-244).

Figure 5. (a) LSV curves of D-NiFe PBA

1. Yi, X., He, X., Yin, F., Chen, B., Li, G., & Yin, H. Amorphous Ni–Fe–Se hollow nanospheres electrodeposited on nickel foam as a highly active and bifunctional catalyst for alkaline water splitting. Dalton Trans, 2020, 20, 6764-6775.
2. Patil, S. J., Chodankar, N. R., Hwang, S. K., Rama Raju, G. S., Huh, Y. S., & Han, Y. K. (2022). Fluorine Engineered Self‐Supported Ultrathin 2D Nickel Hydroxide Nanosheets as Highly Robust and Stable Bifunctional Electrocatalysts for Oxygen Evolution and Urea Oxidation Reactions. Small, 2022, 7, 2103326.

 

2. “As shown in Figure 4d, D-NiFe PBA shows a symmetric signal at a magnetic field of g = 2.00, indicating the presence of V_CNin the lattice”. Explain it in detail.

**Thanks you for your good comment, wherein, the EPR signal is originated from the unpaired electrons of the Ni³⁺ (t_2g⁶e_g¹) species, suggesting that the generation of CN vacancies in the presence of the more oxidation state of Ni [1], and the corresponding descriptions have also been added in the Results and discussion section (Page 11, lines 227-229).

1.Yi, X., He, X., Yin, F., Chen, B., Li, G., & Yin, H. Amorphous Ni–Fe–Se hollow nanospheres electrodeposited on nickel foam as a highly active and bifunctional catalyst for alkaline water splitting. Dalton Trans, 2020, 20, 6764-6775.

Reviewer 2 Report

In this manuscript, the authors introduce defects into the Prussian blue analogue (PBA) catalysts for the water oxidation. Although detailed study has been carried out on the oxygen evolution reaction, there are still some questions need to be addressed, especially on the characterizations of materials. Therefore, I think the manuscript can be published after revision.

1.       It is somehow difficult to understand CN vacancy – both C and N are disappeared after reduction? How the authors calculate V-CN content in Table S1? Also, the authors please show the SEM and TEM-mapping results of catalysts before the reduction process.

2.       From the XPS results (Fig. 4a-c), the oxidation states of Ni and Fe are changed and it leads to make a formation (hydroxide or oxyhydroxide) on the catalysts surface before the OER. Please the authors give an information of O 1s spectra.

3.       The impedance measurements were conducted at 1.524 V (vs. RHE). In case of NiFE PBA (before reduction catalyst), it does not start OER performance at that potential.

4.       Please describe EPR techniques in Experimental part.

Author Response

1. It is somehow difficult to understand CN vacancy – both C and N are disappeared after reduction? How the authors calculate V-CN content in Table S1? Also, the authors please show the SEM and TEM-mapping results of catalysts before the reduction process.

**Thanks for your kind advice, we accept it. Since FeC₆ and NiN₆ octahedra are bridged through CN ligand, the decreased C/Fe or N/Ni atomic ratio can reflect the V_CN concentration. However, due to the large deviation of the C content derived from XPS, the surface C/Fe atomic ratios of various samples are not accurate. Therefore, we use the N/Ni atomic ratio rather than the C/Fe atomic ratio to determine the V_CN contents. The N/Ni atomic ratios of NiFe PBA and D-NiFe PBA were 3.3 and 2.9, respectively. Therefore, we can obtain the V_CN contents of D-NiFe PBA are 24% (Table S1). In addition, we suppose the SEM and TEM-mapping results before the reduction process, as shown in the below figure S1.

 

Figure S1 (a) SEM images of NiFe PBA (b) TEM element mapping D-NiFe PBA of Ni, Fe, N, and C.

2. From the XPS results (Fig. 4a-c), the oxidation states of Ni and Fe are changed and it leads to make a formation (hydroxide or oxyhydroxide) on the catalysts surface before the OER. Please the authors give an information of O 1s spectra.

Figure S2. O 1s spectra of NiFe PBA and D-NiFe PBA.

**Thanks for your helpful suggestion. The O 1s XPS spectra of the NiFe PBA and D-NiFe PBA was presented in Figure.S2. The peaks centered at the binding energies of 529.9, 531.44, and 533.05 eV can be assigned to the oxygen–metal bonds (M = Ni or Fe), hydroxyl groups, and absorbed molecular water molecules, respectively. The intensity of M-O peak increased after NaBH₄ reduction, indicating surface M–O bond formation induced by NaBH₄ reduction, and the corresponding descriptions have also been added in the Results and discussion section (Page 11, lines 119-225).

3. The impedance measurements were conducted at 1.524 V (vs. RHE). In case of NiFE PBA (before reduction catalyst), it does not start OER performance at that potential.

**Thanks you for good suggestion, we accept it., Actually, the electrochemical impedance spectrum (EIS) of the NiFe PBA was tested at a potential of 1.674 V vs. RHE (amplitude 5 mV) in 1 M KOH solution over a frequency range from 100 mHz to 100 kHz. The corresponding descriptions have also been added in the electrochemical Characterization section (Page 7, lines 147-150).

4. Please describe EPR techniques in Experimental part.

**Thank you for your good suggestion. EPR spectra were obtained from a JEOL JES-FA300 EPR spectrometer (9065.8 MHz, X band, 300 K), and the corresponding descriptions have also been added in the Physical characterization section (Page 6, lines 128-130).

 

Reviewer 3 Report

The development of electrocatalysts for oxygen evolution reaction (OER) is important for hydrogen production. In this paper, the authors report on their success in synthesizing defective Prussian blue analogues (PBA) with rich CN vacancies (D-NiFe PBA) as efficient OER electrocatalysts. The D-NiFe PBA had an overpotential of 280 mV at 10 mA cm-2 and superior stability for over 100 hours in KOH electrolytes. The formation of CN vacancies in NiFe PBA effectively inhibited the loss of Fe active sites and promoted the reconstruction of NiFe oxygen (hydroxide) active layers in the OER process, leading to improved electrocatalytic activity and stability. This work presents a feasible approach for the widespread application of defect engineering in PBA electrocatalysts. I’m supportive of the publication of this paper, if the following questions can be addressed.

 

• Can the authors provide more information on how the formation of CN vacancies in NiFe PBA improves the electrocatalytic activity and stability of the catalyst?

 

• It is stated that the Tafel slope of D-NiFe PBA is much smaller than that of NiFe PBA, indicating improved OER kinetics. The authors should use ECSA-normalized Tafel slope to claim on the intrinsic OER activity. 

• The electrocatalytic performance of D-NiFe PBA is compared to that of IrO2. Can you discuss the potential implications of this comparison for future research and development in the field of OER electrocatalysts?

• The stability of D-NiFe PBA is demonstrated to be superior to that of NiFe PBA, with the catalyst remaining active for over 100 hours. Can you discuss the potential practical applications of this high level of stability in real-world settings? How about the stability at other potential?

Author Response

1. Can the authors provide more information on how the formation of CN vacancies in NiFe PBA improves the electrocatalytic activity and stability of the catalyst?

**Thank you very much for your kind advice. As we know that, the anion exchange between [Fe(CN)₆]^3- and OH^- in NiFe PBA during OER can lead to Fe leaching and occur rapid degradation of the catalytic activity, while Fe in D-NiFe PBA could be incorporated into the active species to form Fe-OH bonds and to improve activity and stability. The energy dispersive X-ray spectroscopy (EDX) in Figure S8 analysis showed that most iron species in the D-NiFe PBA structure had been well-retained after OER. In contrast, NiFe PBA lost most of the Fe species after 24 h of continuous reaction. These results are in good agreement with XRD and IF-IR data of D-NiFe PBA after 24 h OER reaction., which fully demonstrated that the present catalyst possess excellent catalytic activity and stability in OER. The corresponding descriptions have also been added in the Results and discussion section (Page 15, lines 285-289).

 

Figure S8.EDX spectra of NiFe PBA and D-NiFe PBA before and after 24 h OER test

 

It is stated that the Tafel slope of D-NiFe PBA is much smaller than that of NiFe PBA, indicating improved OER kinetics. The authors should use ECSA-normalized Tafel slope to claim on the intrinsic OER activity.

**Thank you very much for your kind advice! To demonstrate the origin of the activity, the ECSA was determined by measuring the capacitive current associated with double-layer charging from the scan rate dependence of the CVs (Figure. S2). The capacitance currents shown in Figure 5c were obtained from the corresponding CV curves. The Cdl of D-NiFe PBA was calculated to be 11.26 mF cm^-2, which is almost 28 times as high as that of NiFe PBA (0.39 mF cm^-2). The result suggests that D-NiFe PBA exhibit superior intrinsic activity. the corresponding descriptions in the Results and discussion section (Page 13, lines 253-259).

2. The electrocatalytic performance of D-NiFe PBA is compared to that of IrO2. Can you discuss the potential implications of this comparison for future research and development in the field of OER electrocatalysts?

**Thank you for your helpful suggestion. Compared with IrO₂, the D-NiFe PBA has lower catalytic overpotential, cheap and easy to obtain. In future practical applications, it can reduce the consumption of electrical energy in electrolytic hydrogen production and reduce the cost of industrial hydrogen production.

3. The stability of D-NiFe PBA is demonstrated to be superior to that of NiFe PBA, with the catalyst remaining active for over 100 hours. Can you discuss the potential practical applications of this high level of stability in real-world settings? How about the stability at other potential?

**Thank you for your nice suggestion. The improved stability of the catalyst in the real environment will favor for electrolysis of hydrogen generation in water, such as Yang et al. produced CuFe Prussian blue analogue (PBA) nanocubes as efficient precatalysts for OER catalysis by a self-sacrificial templating method, and notably, OER can achieve stable currents of more than 300 h and 150 h, significantly improve catalytic stability [1]. Constructing PBA frames with defects can improve the stability for catalysts. The long-term stability of D-NiFe PBA is also expected to be a candidate material for commercial electrocatalytic hydrogen production [2]. We anticipate that the stability of D-NiFe PBA could also be achievable in other PBAs, which thus opens up the possibilities for exploring new applications of PBAs beyond OER catalysis. such as electrochemical Sensing, Zn–Air Batteries, sodium battery and so on [8-10]. For the assessment of the electrolytic water stability performance of reconstituted catalysts, chrono-potential measurements are generally used, so the multi-step chrono-current method was not employed.

1.Zhao, Y., Jia, X., Chen, G., Shang, L., Waterhouse, G. I., Wu, L. Z., ... & Zhang, T. (2016). Ultrafine NiO nanosheets stabilized by TiO2 from monolayer NiTi-LDH precursors: an active water oxidation electrocatalyst. J. Am. Chem. Soc, 2016, 20, 6517-6524.

2.Yu, Z. Y.; Duan, Y.; Liu, J. D.; Chen, Y.; Liu, X. K.; Liu, W.; Ma, T.; Li, Y.; Zheng, X. S.; Yao, T.; Gao, M. R.; Zhu, J. F.; Ye, B. J.; Yu, S. H., Unconventional CN vacancies suppress iron-leaching in Prussian blue analogue pre-catalyst for boosted oxygen evolution catalysis. Nat. Commun, 2019, 10, 2799.

Round 2

Reviewer 2 Report

In the revised manuscript the authors have taken into account and implemented all the reviwer's suggestions and the revised manuscript, in my view, can now be published in Colloids and Interfaces.