Enhancement of Catalytic Activity and Stability of La_0.6Ca_0.4Fe_0.7Ni_0.3O_2.9 Perovskite with ppm Concentration of Fe in the Electrolyte for the Oxygen Evolution Reaction

Round 1

Reviewer 1 Report

This paper reports on the catalytic activity and stability of La0.6Ca0.4Fe0.7Ni0.3O2.9 for oxygen evolution reaction. The authors have found that their samples consist of porous hollow spherical particles with diameters ranging between 200 and 1200 nm, with reconstructed surfaces. As importantly, they have demonstrated that the addition of 1 ppm of Fe into alkaline electrolyte led to a stable oxyhydroxide interface with the host perovskite material, vital to the catalytic activity.

The main article is well-written, and I would have recommended the paper for publication if Figure S1 had not been missing from the supplementary information. In the main text, the authors promise Figure S1 to contain the XRD data and the accompanying refinement. However, I could not see this figure in the supplementary information. Given this information is important to the identification of the synthesized material, I recommend the authors add this missing figure and resubmit their paper for reassessment. Finally, it would be nice if the authors can also mention more recent advances in perovskite phase for catalysis as presented here: Cu-Modified SrTiO3 Perovskites Toward Enhanced Water–Gas Shift Catalysis: A Combined Experimental and Computational Study, https://doi.org/10.1021/acsaem.0c02371

Author Response

We thank the Reviewer for pointing out the missing Figure. We have now added Figure S1 to the Supplementary information. We also added the suggested reference to the Introduction section. 

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper entitled “Enhancement of catalytic activity and stability of the 2 La0.6Ca0.4Fe0.7Ni0.3O2.9 perovskite with ppm concentration of Fe3 in electrolyte for the oxygen evolution reaction” is an original work. The data and figures in the work are quite interesting. The structure and flow of research paper is logical and intuitive. However, there are a number of instances where some corrections are required. I would recommend for a major revision before it is published. 

 

1. The introduction section is not well written. It should clearly justify the material selection, and the novelty of this work. In the introduction authors did not cover the benefits of La0.6Ca0.4Fe0.7Ni0.3O2.9 perovskite-based materials over other oxide-based materials for catalytic activity. For instance, ZnO–CuO nanocomposite is a very attractive material for catalytic activity. Authors are suggested to cover this part and refer the following article “ZnO–CuO nanocomposites with improved photocatalytic activity for environmental and energy applications, Journal of Electronic Materials 47 (11), 6731-6745”.
2. In the EDX analysis authors mentioned that “This impurity, however, is washed out with aqueous electrolyte allowing us to conduct the electrochemical measurements solely on the perovskite phase.” However, they did not show any evidence of their claim.
3. In TEM analysis, authors have shown the crystal plan orientation. Does the orientation have any impact on the catalytic activity. Moreover, authors mentioned that “additional (oxy)hydroxide layer on host (perovskite) structure is responsible for enhanced OER activity and prevents deep structural changes of the perovskite particles.” What is the reason behind it? Why does additional (oxy)hydroxide layer on host (perovskite) structure incur enhanced OER activity?
4. In the manuscript I have found many grammatical/typographical mistakes. Please have a careful look and correct those in the revised submission.

Author Response

1 Response: We modified the Introduction section to justify our selection of the perovskite material:

“In this work, we report on the enhanced OER catalytic activity of Ca-doped Ni/Fe-mixed perovskite with the additional presence of Fe ions in the electrolyte. This material was demonstrated to possess promising OER activity due to the beneficial effect of Ca doping, which decreases the formation energy of the oxygen vacancies [23]. In this work, we search for strategies to further increase the activity and long-term stability of this material under the OER conditions.”

The article which the Reviewer suggests to cite it surely very interesting, but it refers to photocatalysis rather than electrocatalysis. We have recently established a research group on photocatalytic water splitting and, as soon as first results are published, we will definitely cite this highly relevant article.

2 Response: In our previous work on this material, we demonstrated that CaO impurity is washed out after soaking the material in alkaline solution (ACS Catal. 2021, 11, 8338−8348, Figure 1). We cite this previously reported result to support our claim.

“Small particles with apparent Ca excess were also observed with EDX compositional mapping and attributed to the CaO impurity phase. This impurity, however, is washed out when soaking the sample in alkaline electrolyte [23], which allowed us to conduct the electrochemical measurements solely on the perovskite phase.”

3 Response: Recently, the phenomenon of restructuring of the perovskite surface with the formation of active (oxy)hydroxide layers, the activity of which is mainly due to the accumulation of Fe from the alkaline electrolyte, has been substantiated in Refs [1],[2]. We discuss these results in the second and third paragraphs of the Introduction section.

The orientation of the surface of the (oxy)hydroxides might have an effect on the OER activity. However, since these layers are formed spontaneously and contain both amorphous and crystalline particles, it is not possible to conclude on the effect of orientation on the catalytic activity under the experimental conditions of our study.

4 Response: We tried our best to correct the grammatical/typographical mistakes in the revised manuscript.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript concerns with Enhancement of catalytic activity and stability of the La0.6Ca0.4Fe0.7Ni0.3O2.9 perovskite with ppm concentration of Fe in electrolyte for the oxygen evolution reaction. The work is certainly interesting and well-presented. However, I am summarizing below some of my concerns regarding the manuscript:

1. Similar research idea or work is published recently, such as https://pubs.acs.org/doi/10.1021/acscatal.1c00796 , https://pubs.acs.org/doi/10.1021/jacs.0c08959

What is the novelty and how the Author distinguishes his work and fit for communication?

2. The previous studies show that 6Ca0.4Fe0.7Ni0.3O2.9 perovskite are stable materials for OER, but we cannot confirm the stability level (Increased or Decreased) without comparing with the results of without using Fe in electrolyte. Did the author perform comparison test separately for confirmation that the presence of Fe in electrolyte supports better stability? In general, it is hard to clarify that Fe is in support to increase stability. (https://pubs.acs.org/doi/10.1021/jacs.0c08959). Overall, the manuscript is well presented.

Author Response

Response: The first paper mentioned by the Reviewer (10.1021/acscatal.1c00796) is our previous work on the Ca-doped Ni/Fe-mixed perovskite, where we report high OER activity due to unique morphology of hollow spheres and improved catalytic performance due to Ca-doping. In our new article we report that the activity of the La_0.6Ca_0.4Fe_0.7Ni_0.3O_2.9 perovskite can be further increased upon the addition of trace amounts of Fe ions to the electrolyte.

The second paper mentioned by the Reviewer (10.1021/jacs.0c08959) is indeed the study which motivated us to examine the effect of Fe on the OER activity of Ca-doped Ni/Fe-mixed perovskite. In the study by the group of N. Markovic they showed the beneficial effect of Fe_aq on the activity and stability of OER on La_1–xSr_xCoO₃ perovskite. In our work we report on the similar electrocatalytic enhancement of the OER on a different perovskite La_0.6Ca_0.4Fe_0.7Ni_0.3O_2.9, which is a novel result, and supports the generality of the recently proposed and justified mechanism for dynamic Fe dissolution/re-deposition as a main source of OER enhancement for transition-metal based catalysts under the alkaline conditions. Additionally, we provide information on the structure and thickness of oxyhydroxide layers on the La_0.6Ca_0.4Fe_0.7Ni_0.3O_2.9 surface, which is a novel result.

Since the combination of a unique morphology and catalytic enhancement by Fe_aq results in a superior electrocatalytic activity and stability of the La_0.6Ca_0.4Fe_0.7Ni_0.3O_2.9 material, we believe this is an important result, which deserves fast publication. This is why we intend to publish this work in the communication format.

2 Response: We have now added Figure S2 to the supplementary information, which demonstrates the difference.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors have responded adequately. The paper can be published now.

Reviewer 2 Report

Authors made significant changes in the revised version. The manuscript can be accepted in its present form.

Reviewer 3 Report

The authors revised the manuscript well by following reviewers' comments. Therefore, I accept the publication of this paper.