Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater

Round 1

Reviewer 1 Report

I have read the work submitted by Zhang et al to Processes / MDPI. The electrochemical water splitting (desalinating seawater) via hydrogen evolution reaction on cathodes and oxygen evolution reaction on anodes is a promising pathway to convert sustainable electricity from solar, water, and wind into chemical energy. Recently, transition metals and their alloys have been extensively investigated to have high catalytic activities due to their unique structural design, superior stability, excellent functional feature, and superior mechanical performance. However, most of the reported high-entropy alloys focus on studying the compositional design and microstructure, and mechanical properties of materials. There are relatively few studies on electrochemical performance and their suitability as catalysts. In the submitted work, the authors have taken a high entropy alloy with optimised surface and examined the unique catalytic performance for hydrogen evolution reaction. The manuscript is written well supporting with neat characterization, however, the areas that require clarification are given below.

My specific points are:

·         Considering the costs of materials and fabrication process as well as bulk catalysis, does the high entropy alloy has potential practical applications than most nanostructured HER catalysts.

·         Are the demonstrated catalytic performances illustrate that the activities are originated from the intrinsic nature of the metal cations in the high - entropy alloys due to the synergistic reaction of elements?

·         Minakshi et al has published significant amount of works on nickel metal oxides and molybdates for various applications. The key papers on the ternary metal oxide reported for XPS analysis for various elements such a Ni 2p, Co 2p, and Mo 3d XPS analysis must be included in the appropriate (XPS, and Nyquist plots) section of the results and discussion (Section 3).

·         Are these materials or facile charge transfer and electron transport during the electrochemical reactions?

·         What is the rationale to mix both acid (NaCl) and basic solutions (KOH) as electrolytes for the OER test?

 

·         OER performances of HEAs and other reported electrocatalysts in acidic electrolytes can be either included in the existing Table 1 or add a new table.

How the current work differs from that of similar work published in ACS Materials Letters (2019, 1, 5, 526 0 533) please justify  doi.org/10.1021/acsmaterialslett.9b00414.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Reviewers Report

Manuscript ID: processes-2130782

Title: Surface modified CoCrFeNiMo high entropy alloys for oxygen evolution reaction in alkaline seawater

Authors have successfully fabricated metal salt-free fabrication catalysts composed of CoCrFeNiMo layered double hydroxide array self-supported on CoCrFeNiMo high entropy alloy substrate and tested for the oxygen evolution reaction (OHR) from seawater. The obtained catalysts exhibit improved catalytic performances for OER in artificial seawater electrolyte. It appears that highly porous morphology with a large surface area of the working electrode ensures a large number of active metal sites for OER and efficient mass transfer.

This study has a high potential to be cited in large. However, I was not able to find supplement material along with Figures marked as S. This is a significant problem.

I would highly recommend that Editorial Office consider this manuscript for publication but I have to inspect the supplement material.

 

1.      Introduction

This section is correctly written, and supported with up-to-date references.

Line 91            "In 1.0 m KOH, the improved …" it should be capitalized M.

2.      Materials and Methods

All experimental procedures were explained in detail.

Line 170 and 174        Relevant references should be introduced for  Equations 1 and 2.

3.      Results

 

The results obtained are professionally conducted and interpreted.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

 

The author reported a one-pot facial synthesis of high-entropy catalysts self-supported CoCrFeNiMo-LDHs with large surface area through the corrosion method and the application as an electrocatalyst for oxygen evolution reaction in alkaline artificial seawater solution composed of 0.1 M KOH+0.5 M NaCl. Particularly, sample named ‘LDHs-3 M-12 h’, preparing in 3 M NaOH solution at 130℃ with 12 h treatment duration showed a low overpotential of 260.1, 294.3, and 308.4 mV at current densities of 10, 50, and 100 mA cm^-2, respectively without obvious stability decay over 24 hr under a constant overpotential. The results are somehow interesting and particularly good job on construction, chemical composition, and surface chemical states characterization upon the stability analysis. However, this manuscript cannot be recommended for publication at this stage. Here are some suggestions for the improvement needed.

 

1) For the synthesis of catalysts CoCrFeNiMo-LDHs, the growth process needs more elucidation.

Why the released ions simultaneously combined with OH‾ and grafting on the substrate rather than start the nucleation themselves? Also, how does the ‘grafting to’ growth give out nanosheets? Why not nanoparticles and/or nanocubes?

 

2) Structural characterization.

Porous structure characterization like BET is needed to build up the structure-function correlations?

Also, will that be comparable to ECSA results here?

 

3) What are the roles of other elements like Co, Cr and Ni since the active sites for OER are solely Mo and Fe?

 

 

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The revised manuscript is OK to publish.

Author Response

We appreciate for your support for our work.