Methanol, Ethanol, and Formic Acid Oxidation on New Platinum-Containing Catalysts

Round 1

Reviewer 1 Report

This is a significant paper dealing with the electrocatalytic behavior of PtCu and Pt/SnO2 electrocatalysts  for methanol, ethanol and formic acid, and their comparison to Pt electrocatalysts.  There are several minor issues  for the improvement of the manuscript.

 

Explanation of copper oxidation on CV behavior for PtCu/C catalysts is unclear. I cannot see clearly those anodic potential regions

 

I think the authors can add an extra bar in figure 6 regarding the value of Ifmax/Ibmax ratio and correlate that criterium with the author´s results.

 

Explain the reason of choosing 0.87 and 0.60 V in order to study the chronoamperometric responses of all electrocatalysts.

 

Explain the reason for the chronoamperometric response of Figure 8b, i.e., the maximum peak and trend of plots.

 

Refer to figure caption of plots c in Figures 5, 7 and 8.

Author Response

We would like to thank the distinguished reviewers for their appreciation of the work and for the comments made. In replies to reviewers, their comments are highlighted in yellow. Changes made by us in the text of the article are also highlighted in yellow. 1. Explanation of copper oxidation on CV behavior for PtCu/C catalysts is unclear. I cannot see clearly those anodic potential regions. In this case (see lines 159-167), we refer to the literature data [34, 36, 39], which indicate the possible presence of peaks on CVA due to the anodic dissolution of copper. It is in these articles that the corresponding regions of the anode potentials are determined. When studying our catalysts, such peaks were not found, which we indicate in the text of the article: “It is known that, on CVs of PtCu / C catalysts, peaks of anodic dissolution from the intrinsic phase and from a solid solution of copper in platinum can be observed in the potential ranges of 0.25–0.45 and 0.7–0.8 V, respectively [34, 36, 39]. In this work no similar anodic maxima were observed for the studied PtCu/C catalyst, which is due to the previous dissolution of “weakly bound” copper at the stage of preliminary treatment of the PtCu/C material in acid. ” We have supplemented line 165 with a reference to Figure S1, which depicts cyclic voltammograms of surface activation. 2. I think the authors can add an extra bar in figure 6 regarding the value of Ifmax/Ibmax ratio and correlate that criterium with the author´s results. When discussing the results of this research, we largely rely on the results and conclusions of works [40, 41], in which a detailed CV study of the methanol electrooxidation on platinum was carried out. The authors of these works convincingly prove that the Ifmax/Ibmax ratio is a very dubious criterion for comparing the activity of catalysts in MOR. Therefore, we do not use the appropriate criterion when discussing the results. Specifically, we write (see lines 243-250): “Some researchers use the ratio of Ifmax /Ibmax values to assess the catalysts tolerance in the MOR. However, the results [40-41] indicate that the value of Ibmax is mostly caused not by the rate of methanol oxidation on bare platinum but by the degree of the platinum surface coverage with oxygen-containing particles. This makes the use of the Ifmax /Ibmax criterion rather doubtful in assessing the catalysts tolerance.” 3. Explain the reason of choosing 0.87 and 0.60 V in order to study the chronoamperometric responses of all electrocatalysts. In the text of the article (see lines 117 - 121) we note that “…due to the crossover of organic molecules, their fast and efficient oxidation is also of importance for the cathode catalyst of alcohol fuel cells. In this regard, the study of effective "oxygen" catalysts activity in the reactions of organic compounds oxidation at high positive potentials, close to the potential of the oxygen electrode, is also of considerable interest.“ Therefore, it was necessary to study the electrooxidation of organic reagents at the potential close to the operating potential of the oxygen electrode - 0.87 V. The potential of 0.60 V too was chosen for the chronoamperometric study both taking into account the literature data [23, 27, 31] and taking into account the results of CV. For example, in the case of methanol electrooxidation (Fig.5a) at potentials less than 0.6 V, the methanol oxidation currents are very low, which makes it difficult to reliably compare the catalyst activity. 4. Explain the reason for the chronoamperometric response of Figure 8b, i.e., the maximum peak and trend of plots. Immediately after the change in the potential of the electrode under study, that is, after the start of recording the chronopotentiogram, the state of the platinum surface and the degree of its oxidation change for some time. This affects the rate of oxidation reactions of organic substances. The initial state of the platinum surface and the rate at which a steady state of the surface is reached at a potential of 0.60 V or 0.87 V also depends on the composition of the medium in which the catalytic electrode is located. Thus, the appearance of the initial part of the chronopotentiogram is associated with a change in the oxidation state of the platinum surface, which affects the kinetics of the organic reagent oxidation reaction. The subsequent drop in the current at a potential of 0.60 V, in our opinion, is due to the gradual poisoning of the surface by intermediates, for example, CO. That is why the analysis of the current decay on chronoamperograms is used to substantiate the greater or lesser tolerance of the catalyst to the intermediate products of the conversion of organic compounds. The specificity of formic acid may also be that, due to its decomposition into CO and H2O, catalyzed by platinum, the adsorption of CO molecules can occur on platinum even before the start of chronopotentiometric measurements. It can also affect the appearance of the initial part of chronoamperograms. Note that a similar type of chronoamperometric response is described, for example, in [31]. The reasons for the presence of a maximum on the chronoamperogram are not explained by the authors. 5. Refer to figure caption of plots c in Figures 5, 7 and 8. We agree with the comment and made a clarification in figure captions of plots c in Figures 5, 7 and 8 (lines 295-296, 312-313, 353-354), as well as in the line 467.

Reviewer 2 Report

The present manuscript explores the electro-oxidation of methanol, ethanol and formic acid over three Pt-based catalysts ca PtCu/C, Pt/(SnO2/C), and Pt/C, containing 20% Pt. The Pt/(SnO2/C) catalyst performed superior compared to other catalysts for formic acid oxidation. While for methanol and ethanol oxidation the activity of Pt/(SnO2/C) was comparable to commercial PtRu/C catalyst. All the catalyst components were characterized well by using TEM, XRD, ICP, etc. The manuscript is well written and electrochemical studies are complete. I recommend the publication of the manuscript after a few minor changes.

1. The graphical abstract is not provided. A nice schematics showing the main finding of the manuscript should be added to improve the readability.
2. The introduction is a little bit longer and not focused. The past research related to current work should only be mentioned.
3. Why the electrochemical surface area of PtCu/C was low. Is it because of the reduction with sodium borohydride? What is the regular surface area of the catalyst?
4. Additionally, by using sodium borohydride is it possible to completely reduce Cu in metallic form. It might be just surface reduction. The XRD spectra of bare Cu nanoparticles should be added.
5. There are some minor corrections ml should be mL, the journal’s name should be abbreviated. The title of the articles should be either bold or completely small-case. Instead of comma for standard point should be used for the representations. 

Author Response

We would like to thank the distinguished reviewers for their appreciation of the work and for the comments made. Changes made by us in the text of the article are highlighted in yellow.

1. The graphical abstract is not provided. A nice schematics showing the main finding of the manuscript should be added to improve the readability.

We agree with the remark. The graphical abstract is provided.

    2. The introduction is a little bit longer and not focused. The past research related to current work should only be mentioned.

We understand the comment made by the reviewer. The problem, however, is that in this complex article we are comparing a) three different types of catalysts and b) the electrooxidation of three different organic substances on each of these catalysts. Therefore, it was necessary to describe the diverse results of different works related both to the use of different types of catalysts and to the oxidation of different reagents. We hope this is not a critical note and would prefer not to change the Introduction.

    3. Why the electrochemical surface area of PtCu/C was low. Is it because of the reduction with sodium borohydride? What is the regular surface area of the catalyst?

Indeed, the reduction of Cu²⁺ to Cu⁰ most effectively occurs under the action of sodium borohydride. Unfortunately, in the course of borohydride synthesis, PtCu/C catalysts are formed containing relatively large nanoparticles. The aggregation of nanoparticles is also very significant. All of this results in lower ESA values ​​compared to Pt/C. A detailed analysis of the literature data describing the ESA values ​​of PtCu/C catalysts obtained by different authors was carried out in [A.S. Pavlets, A.A. Alekseenko, N.Yu. Tabachkova, etc., A novel strategy for the synthesis of Pt-Cu uneven nanoparticles as an efficient electrocatalyst toward oxygen reduction, International Journal of Hydrogen Energy, 2021, Vol. 46, Issue 7, Pages 5355-5368. https://doi.org/10.1016/j.ijhydene.2020.11.094]. It turned out that at a platinum loading of about 20 wt%, the ESA values ​​in PtCu/C catalysts are in the range of 23 - 44 m²/g (Pt).

      4. Additionally, by using sodium borohydride is it possible to completely reduce Cu in metallic form. It might be just surface reduction. The XRD spectra of bare Cu nanoparticles should be added.

Indeed, by using sodium borohydride is it possible to completely reduce Cu in metallic form. However, if the Cu phase was formed during the combined reduction of Cu²⁺ and Pt(IV), we would see the corresponding reflections in the X-ray diffraction pattern of the PtCu / C material. However, there are no such reflections (Fig. 1, curve 3). In addition, the CV measured during the activation of Cu nanoparticles would have anodic peaks of copper dissolution, which is also not observed (Fig.1S).

      5. There are some minor corrections ml should be mL, the journal’s name should be abbreviated. The title of the articles should be either bold or completely small-case. Instead of comma for standard point should be used for the representations.

Thank you for the comment. Corrections were made. The list of references was drawn up correctly.

Round 2

Reviewer 1 Report

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