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by
  • Chengwei Fan 1,
  • Fuhe Le 1,* and
  • Xiaoyu Dong 1,*
  • et al.

Reviewer 1: Anonymous Reviewer 2: Anonymous

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The main problem with the manuscript is that is unconvincing. The authors 
try to explain the catalytic activity based on their XPS study and the previously published papers, but without explanation how the literature research is connected to the catalyst they developed. Specifically, the authors need to explain the following:
(1) How the increase in the Co valency is beneficial to the HER? 
(2) How is the highly oxidative oxygen species beneficial to the HER?
(3) How was the deconvolution of O 1s XPS signal carried out? Why is the oxygen band deconvoluted into four peaks when only two are clearly visible? Could a minor change in the fitting parameters produce a reasonable fit without the peak around 529 eV, which authors believe to play a crucial role in HER? 
(4) Why is Fig. S7 obtained in such a limited range of merely 100 mV? Is the catalyst to electrochemical sweeping beyond the potentials of 0.33 V and up to what upper potential limit?  What is the open circuit potential, and does a prolonged exposure to electrolyte without voltage applied affect the catalytic activity? A CV of the catalyst in an extended potential region would be helpful.

Comments on the Quality of English Language

The manuscript should be reviewed by an English literate. There are several sentences that cannot be understood and must be rephrased, like Line 115: "...conductive to increasing...." and line 157: "...conductive to enhancing..." Avoid adjectives like "most excellent" (line 88). 

 

Author Response

The main problem with the manuscript is that is unconvincing. The authors try to explain the catalytic activity based on their XPS study and the previously published papers, but without explanation how the literature research is connected to the catalyst they developed. Specifically, the authors need to explain the following:

Comment1: How the increase in the Co valency is beneficial to the HER? 

Response1: Thank you for pointing out this issue. We have conducted a more in-depth discussion on the issue that increasing the valence state of cobalt is beneficial to HER.The corresponding description has been provided below:

“…In cubic perovskites, Co3+ ions generally adopt a low-spin electronic configuration (t2g6eg0). In comparison, enhancing the cobalt valence state effectively reduces the electron density around Co3+ sites, favoring the formation of Co4+ (t2g4eg1)[34]. Relative to Co3+, Co4+ possesses more unpaired electrons, which significantly enhances the electronic conductivity of the catalyst. As known, efficient electron transfer throughout the electrode is essential to achieve high current densities for electrocatalytic reactions [8, 35]. Therefore, superior electrical conductivity represents a key prerequisite for high-performance electrocatalysts…”

Comment2:  How is the highly oxidative oxygen species beneficial to the HER?

Response2: Thank you for pointing out this issue. We have further elaborated on how the highly oxidative oxygen species contribute favorably to the HER process. The corresponding description has been provided below:

“…Notably, previous studies have demonstrated that surface O22−/O− species are closely associated with oxygen vacancies in perovskite catalysts. The non-metal oxygen sites such as oxygen vacancies and lattice oxygen can serve as favorable active sites for the adsorption of hydrogen intermediates and enhance electronic conductivity, thus playing a critical role in boosting the HER kinetics...”

Comment3:  How was the deconvolution of O 1s XPS signal carried out? Why is the oxygen band deconvoluted into four peaks when only two are clearly visible? Could a minor change in the fitting parameters produce a reasonable fit without the peak around 529 eV, which authors believe to play a crucial role in HER? 

Response3: Thank you for your valuable suggestion.

1) How was the deconvolution of O 1s XPS signal carried out?

Response:The O 1s XPS spectrum was deconvoluted using a standard peak-fitting procedure with Gaussian–Lorentzian mixed functions and a Shirley background subtraction, which is widely accepted for transition metal oxide systems.

2) Why is the oxygen band deconvoluted into four peaks when only two are clearly visible?

 Response: Although only two main features appear visually, high-resolution fitting reveals four chemically distinct oxygen species that correspond to different surface coordination environments: lattice oxygen (M–O), oxygen vacancies (~529 eV), hydroxyl groups (M–OH), and adsorbed water / surface oxygenates.

3) Could a minor change in the fitting parameters produce a reasonable fit without the peak around 529 eV, which authors believe to play a crucial role in HER? 

Response: These components are chemically reasonable and consistent with many reported HER electrocatalysts. Fitting without the peak at ~529 eV would lead to a poorer fit with larger residual errors. More importantly, the peak at ~529 eV is not arbitrarily assigned but is consistent with the oxygen vacancies environment that has been widely demonstrated to facilitate water adsorption and optimize H* adsorption energy and improve electrical conductivity, thus playing a key role in enhancing HER activity. Minor adjustments of fitting parameters do not eliminate the necessity of this peak for a physically meaningful and statistically reliable fit.

Commtet4: Why is Fig. S7 obtained in such a limited range of merely 100 mV? Is the catalyst to electrochemical sweeping beyond the potentials of 0.33 V and up to what upper potential limit?  What is the open circuit potential, and does a prolonged exposure to electrolyte without voltage applied affect the catalytic activity? A CV of the catalyst in an extended potential region would be helpful.

Response4: 1) Why is Fig. S7 obtained in such a limited range of merely 100 mV?

Response: The CV curves in Fig. S7 are the most conventional method for testing the double-layer capacitance (Cdl). When using the CV method to test Cdl, it must be carried out in the non-Faradaic reaction region, and the potential window should not be too large; generally, a 100 mV test interval is selected. This is determined by the basic principle of testing Cdl using the CV method. The specific principle is as follows:

The double-layer charging and discharging current ic consists of two parts, as shown in equation (1): one part is the charging and discharging of the double layer when the electrode potential changes, thereby changing the charge state of the electrode/solution interface, which is the first term in (1); the other part is the change in the double-layer current caused by the change in capacitance Cdl, which is the second term in (2). During linear scanning, since the scanning rate is constant, ic ≠ 0 during the potential scanning process.

 

   （1）

 

For the second term in (1), the double-layer capacitance Cdl generally changes with the change of the electrode surface state. However, when the potential scanning range is relatively small, the charge state of the interfacial double layer can be approximately considered unchanged, so the corresponding double-layer capacitance Cdl also remains unchanged. In this case, the second term on the right side of the equal sign in equation (1) is zero and can be ignored. In addition, the scanning speed of linear scanning is constant, so the first term in equation (1) is a fixed value. Thus, the double-layer current ic remains unchanged, and equation (1) can be rewritten as equation (2).

 

  （2）

From equation (2), it can be known that ic is proportional to v, where v is the potential scanning rate. When plotting the double-layer current against the scanning speed, a straight line is obtained, and the slope of the straight line is the double-layer capacitance Cdl.

2)  Is the catalyst to electrochemical sweeping beyond the potentials of 0.33 V and up to what upper potential limit?

Response: It can exceed 0.33 V (vs. RHE), but it must not be in the Faradaic reaction region. For the HER, it is generally chosen to be in the no-Faradaic region slightly exceeding 0 V (vs. RHE). If the voltage is too high, other reactions may generally occur.

3) What is the open circuit potential, and does a prolonged exposure to electrolyte without voltage applied affect the catalytic activity? A CV of the catalyst in an extended potential region would be helpful.

Response: The open circuit potential (OCP) of the catalyst was measured to be ~70 mV and prolonged exposure to the electrolyte without applied potential showed no obvious negative effect on the catalytic activity.

According to the above Cdl testing principle, A CV of the catalyst in an extended potential region is unhelpful to obtaining accurate Cdl values.

Comment5: The manuscript should be reviewed by an English literate. There are several sentences that cannot be understood and must be rephrased, like Line 115: "...conductive to increasing...." and line 157: "...conductive to enhancing..." Avoid adjectives like "most excellent" (line 88). 

Response5: Thank you for this critical comments on the quality of english language. We have already had professionals polish the entire manuscript. The manuscript has been updated accordingly.

“…The BPCF0.2 exhibited an irregular block-like structure with pores of varying sizes. Such porous architecture effectively increases the accessible surface area between the catalyst and electrolyte, shortens the mass/charge transport pathways, and thus significantly boosts the electrocatalytic activity…”；“…This distinctive electronic structure of BPCF0.2 favors the enhancement of electronic conductivity in the perovskite oxide, further optimizing its electrocatalytic activity and synergistically complementing the beneficial effect of elevated Co valence states on the overall catalytic performance...”

Reviewer 2 Report

Comments and Suggestions for Authors

This paper summarizes the synthesis, state analysis, and electrochemical property evaluation of Fe-doped Ba-Pr-Co perovskite-type oxides for highly efficient and low-cost electrochemical hydrogen evolution reaction (HER). There is no doubt that HER occupies a crucial position as a solution to energy problems considering environmental impact. Replacing expensive precious metals with inexpensive and abundant transition metals is a logical approach. The significant changes in electrochemical properties due to Fe doping are highly intriguing.

However, the conclusions stated in the Conclusion section raise questions. Lines of 334-336 state, “Fe doping resulted in the formation of highly oxidized Co, leading to an increase in oxygen vacancies.” Typically, when the oxidation state increases, wouldn't the concentration of negatively charged O2- ions increase to compensate for the charge?

This point lies at the core of this paper and requires careful reconsideration.

Author Response

Comment: This paper summarizes the synthesis, state analysis, and electrochemical property evaluation of Fe-doped Ba-Pr-Co perovskite-type oxides for highly efficient and low-cost electrochemical hydrogen evolution reaction (HER). There is no doubt that HER occupies a crucial position as a solution to energy problems considering environmental impact. Replacing expensive precious metals with inexpensive and abundant transition metals is a logical approach. The significant changes in electrochemical properties due to Fe doping are highly intriguing.

However, the conclusions stated in the Conclusion section raise questions. Lines of 334-336 state, “Fe doping resulted in the formation of highly oxidized Co, leading to an increase in oxygen vacancies.” Typically, when the oxidation state increases, wouldn't the concentration of negatively charged O2- ions increase to compensate for the charge?

This point lies at the core of this paper and requires careful reconsideration.

Response: Thank you for pointing out this issue. We rechecked our XPS peak fitting results and reconfirmed the content of each oxygen species. It was found that the proportion of lattice oxygen (O2-) did increase after Fe doping, and we have revised this result. The manuscript has been updated accordingly.

“…The non-metal oxygen sites such as oxygen vacancies and lattice oxygen can serve as favorable active sites for the adsorption of hydrogen intermediates and enhance electronic conductivity, thus playing a critical role in boosting the HER kinetics [8, 27]. Quantitative analysis indicates that the relative content of oxygen vacancies remains essentially unchanged, while the lattice oxygen content in BPCF0.2 reaches 24.6%, which is higher than the 19.3% observed in BPC (Table S2). This difference in non-metal oxygen sites-related species content is consistent with the trend of HER activity observed for the two samples, further verifying the positive correlation between oxygen vacancies and catalytic performance...”

 “…Fe doping effectively regulated the oxidation states of Co sites in the perovskite, facilitating the formation of high-valence Co ions and increasing the concentration of lattice oxygen within the BPCF0.2…”

 

Table S2. XPS peak deconvolution of O 1s in BPC and BPCF0.2.

Catalysts

H2Oads

−OH/O2

O22−/O−

O2−

BPC

4.9%

71.4%

4.5%

19.3%

BPCF0.2

4.7%

66.7%

4.0%

24.6%

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for answering my questions. I have no further inquiries.

Reviewer 2 Report

Comments and Suggestions for Authors

Thank the authors for their thoughtful responses. It is confirmed that appropriate revisions have been made.