Pulse Electrolysis Technique for Preparation of Bimetal Tin-Containing Electrocatalytic Materials

Round 1

Reviewer 1 Report

The manuscript describes a methodology to synthesize Pt-Sn doped materials, it is necessary to perform additional measurements and probe some facts to ensure the results are significant.

1. Figure S2 does not show XRD analysis for PtSnx, please add this measurement and analysis to compare Pt3Snx vs PtSnx

2. The description of TEM images for PtSnx+SnO2 (line 86) does not correspond to the figure caption in figure 2. Please define which one is correct.

3. Line 88, the authors mention, "This phase is probably a mixture of crystalline tin dioxide, the presence of which was confirmed by XRD" However, there is no XRD analysis for this phase.

4. Section 2.2 Electrocatalytic properties from lines 101 to 143 should be written in the Introduction section, not in the Results and discussion section.

5.- Figure 3a, which cycle is presented in this plot? The authors need to present the CV at the same scan rate without the CO saturation and 1st and 2nd cycles of CO stripping. Otherwise, the oxidation peak can be interpreted as Sn oxidation.

6.- Aiming to probe if the Sn species are not suffering from leaching, it is necessary to present CVs of the first 20 cycles at 20mV/s without CO saturation or ethanol addition.

7. Please indicate which CV cycle is presented in figure 4a. Additionally, it is necessary to perform stability tests by cycling 20 times in presence of ethanol.

Author Response

Response to Reviewer #1

Pulse electrolysis technique for preparation of bimetal tin-containing electrocatalytic materials

Alexandra Kuriganova, Marina Kubanova, Igor Leontyev, Tatiana Molodtsova and Nina Smirnova

The team of authors is grateful to the Reviewers and the Editor for such a close attention to our work. We drew attention to all these shortcomings and corrected them, made appropriate explanations, and conducted additional studies. All this allowed us to significantly improve the quality of presentation of the results of our work.

All corrections in the manuscript are highlighted in yellow.

1. Figure S2 does not show XRD analysis for PtSnx, please add this measurement and analysis to compare Pt3Snx vs PtSnx

Thank you for this remark. We have added the XRD pattern for PtSn_x material to the supplementary materials (Figure S3).

2. The description of TEM images for PtSnx+SnO2 (line 86) does not correspond to the figure caption in figure 2. Please define which one is correct.

Corrected.

3. Line 88, the authors mention, "This phase is probably a mixture of crystalline tin dioxide, the presence of which was confirmed by XRD" However, there is no XRD analysis for this phase.

Thank you for this remark. We have added the XRD pattern for PtSn_x material to the supplementary materials (Figure S3). The X-ray diffraction pattern of the synthesized PtSn_x sample contains two sets of diffraction peaks: the first of them is high-intensity broadened peaks characteristic of a face-centered Pt cell with space group Fm3m. The second set also includes broadened peaks of the tetragonal phase of tin oxide SnO₂ with space group P42/mnm. According to the results of the refinement of the X-ray diffraction pattern of the sample by the Rietveld method, the concentration of tin oxide with respect to platinum nanoparticles is 11%.

4. Section 2.2 Electrocatalytic properties from lines 101 to 143 should be written in the Introduction section, not in the Results and discussion section.

We agree with the remark. The required change has been made to the text of the manuscript.

5. Figure 3a, which cycle is presented in this plot? The authors need to present the CV at the same scan rate without the CO saturation and 1st and 2nd cycles of CO stripping. Otherwise, the oxidation peak can be interpreted as Sn oxidation.

In the plot on Figure 3a anodic parts of studied materials CVs are presented. We believe that in this representation of Figure 3a, one can better assess the effect of the presence of tin in the catalytic system on the process of carbon monoxide oxidation. However, we agree with the Reviewer that, in the presented form, the results obtained may be misinterpreted by readers. Therefore, we added figure S4 to the supplementary materials. Figure S4 demonstrates the first cycle of the CV curve, which shows the CO stripping peak, and the subsequent second cycle, in which there are no any peaks in the region of
0.4–0.9 V. An appropriate explanation and a link to Figure 4S have been added to the text.

6. Aiming to probe if the Sn species are not suffering from leaching, it is necessary to present CVs of the first 20 cycles at 20mV/s without CO saturation or ethanol addition.

This electrochemical test has been carried out. The results are shown in Figure 5S. Appropriate explanations have been added to the text.

7. Please indicate which CV cycle is presented in figure 4a. Additionally, it is necessary to perform stability tests by cycling 20 times in presence of ethanol.

Comparison of CVs presented in figure S6 was carried out after preliminary cycling of the working electrode for 20 cycles in presence of ethanol. We have added this information to the Materials and Methods section. We also provided data from the accelerated stress test in presence of ethanol (figure 4c). The rate of degradation of Pt/C and platinum-tin-based materials, which is a change in the current density of the peak of ethanol oxidation on the forward and back scan of CV under accelerated stress testing conditions for all the studied materials, has a similar character. Appropriate explanations have been made in the Results and Discussion and Materials and Methods sections.

Reviewer 2 Report

The manuscript is fascinating, as it presents in detail the platinum-tin alloy processes. However, it shows some details:

Comment #01: In all figures, tables and text, use a comma where it should be a point.

Comment #02: You must homogenize criteria since sometimes it presents the concentration in M or in mol dm^-3.

Comment #03: In line 154 you must specify with citations the formula used to calculate the electrochemically active surface area (ECSA)

Comment #04: As there are many ethanol electrooxidation papers published in the literature, the performances of the composites prepared in this study should be compared with the results reported in the literature.

Author Response

Response to Reviewer #2

Pulse electrolysis technique for preparation of bimetal tin-containing electrocatalytic materials

Alexandra Kuriganova, Marina Kubanova, Igor Leontyev, Tatiana Molodtsova and Nina Smirnova

The team of authors is grateful to the Reviewers and the Editor for such a close attention to our work. We drew attention to all these shortcomings and corrected them, made appropriate explanations, and conducted additional studies. All this allowed us to significantly improve the quality of presentation of the results of our work.

All corrections in the manuscript are highlighted in yellow.

Comment #01: In all figures, tables and text, use a comma where it should be a point.

Corrected

Comment #02: You must homogenize criteria since sometimes it presents the concentration in M or in mol dm-3.

Corrected

Comment #03: In line 154 you must specify with citations the formula used to calculate the electrochemically active surface area (ECSA)

Thank you for remark. We have added a formula for calculating the electrochemically active area of electrocatalysts in the Materials and Methods section

Comment #04: As there are many ethanol electrooxidation papers published in the literature, the performances of the composites prepared in this study should be compared with the results reported in the literature.

In Supplementary materials we have provided a table S1 comparing the results of the electrocatalytic activity of different Pt-Sn-containing materials.

Reviewer 3 Report

The author synthesized platinum-tin-containing materials by the pulsed electrolysis based on the electrochemical dispersion of platinum electrodes under the action of an alternating pulsed current in an alkaline electrolyte. The introduction of tin into the catalyst generally increases the rate of ethanol electrooxidation. However, major revisions are in need to make it more convinced.

1. Why tin was selected as the add element? Why not choose other elements? What is the exclusive advantage of the addition of tin than other elements?
2. Please provide the content of Pt and Sn in all pulsed electrolysis prepared samples, including PtSn_x + SnO₂/C, Pt/SnO₂ -C, Pt₃Sn/C. As the metal content is a critical signal for catalysts.
3. The XRD and XPS of PtSn_x + SnO₂/C, Pt/SnO₂ -C, Pt₃Sn/C should also be conducted. XRD can to some extent describe the crystallization differences among all samples, and XPS is quite important to reveal the surface chemical states of Pt, Sn, O, and C, thus, further confirm the function of tin in catalysts.
4. EDS mapping and TEM-SAED should be conducted to precisely display different chemical compositions and the corresponding parts in PtSn_x + SnO₂/C, Pt/SnO₂ -C, and Pt₃Sn/C.
5. What is the reaction mechanism of Pt/SnO₂-C for EOR? By adding tin, why Pt/SnO₂-C surpassed PtSn_x + SnO₂/C and Pt₃Sn/C exhibited the best EOR performance?
6. For EOR catalysts, stability is indispensable. It is suggested to add the stability performance of samples. Only by doing that, the catalytic performances of catalysts can be assessed in a more comprehensive way.
7. EOR catalytic performance comparison with recent publications is also suggested to further evaluate the performance of as-prepared catalysts.

Author Response

Response to Reviewer #3

Pulse electrolysis technique for preparation of bimetal tin-containing electrocatalytic materials

Alexandra Kuriganova, Marina Kubanova, Igor Leontyev, Tatiana Molodtsova and Nina Smirnova

The team of authors is grateful to the Reviewers and the Editor for such a close attention to our work. We drew attention to all these shortcomings and corrected them, made appropriate explanations, and conducted additional studies. All this allowed us to significantly improve the quality of presentation of the results of our work.

All corrections in the manuscript are highlighted in yellow.

1. Why tin was selected as the add element? Why not choose other elements? What is the exclusive advantage of the addition of tin than other elements?

Tin is known as the most active catalyst for the electrochemical oxidation of ethanol on platinum. Tin have higher oxophilicity and their surfaces have more oxygen-containing species compared to Pt and other metals [Electrochimica Acta 259 (2018) 733e741740], For example, ruthenium doped Pt electrodes do not work in the case of complete ethanol oxidation because they are unable to break the C–C bond. The incorporation of tin into a platinum catalyst changes the electrode geometric and electronic structure, providing conditions required for complete ethanol oxidation to carbon dioxide [Molecules 2021, 26, 2144]. In addition, our task was also to show the possibilities of the pulsed electrolysis method for introducing a cocatalyst for platinum in various forms: a doping element, an alloy component, and an oxide phase as a support.

2. Please provide the content of Pt and Sn in all pulsed electrolysis prepared samples, including PtSnx + SnO2/C, Pt/SnO2 -C, Pt3Sn/C. As the metal content is a critical signal for catalysts.

Pt, Pt3Sn, PtSnx loading in electrocatalysts was 20.0±2.0 wt%. Loading of SnO2 in Pt/SnO2-C electrocatalyst was 15 wt.%. According to the XRD data loading of SnO2 in PtSnx + SnO2/C was 11%. The content of Pt and Sn in all samples are presented in Materials and Methods section (3.1.Electrocatalysts preparation).

3. The XRD and XPS of PtSnx+ SnO2/C, Pt/SnO2-C, Pt3Sn/C should also be conducted. XRD can to some extent describe the crystallization differences among all samples, and XPS is quite important to reveal the surface chemical states of Pt, Sn, O, and C, thus, further confirm the function of tin in catalysts.

XRD of PtSnx + SnO2/C Pt/SnO2-C, Pt/C were added to Supplementary materials. XRD patterns of Pt/C as well as Pt/SnO₂-C sample were previously presented in our publications [Russ. J. Electrochem., 2018, 54, 561–565], [J. Appl. Electrochem. 2016, 46, 1245–1260].

Of course, XPS is a good additional tool for determining the composition and chemical state of elements in the materials under study. However, upon contact of a solid particle with an electrolyte solution, a number of processes occur that lead to surface charging: adsorption of electrolyte components on the surface of a solid body (completion of the crystal lattice); dissociation of surface groups; the transition of ions of the same sign from a solid particle to a solution, when a fragment of a group of the opposite charge remains on the surface; surface polarization (for metals, the presence of an excess/lack of electrons on the surface). Near the charged surface, the concentration of ions changes: ions of the opposite sign are drawn to the surface from the solution and ions of the same sign with the surface charge are repelled. As a result, a double electric layer is formed in the solid phase/electrolyte system. The chemical state of the surface of the solid phase will depend on the properties and composition of the electrolyte. Thus, the chemical state of the elements in the material under study as part of an electrochemical system can differ significantly from the chemical state of the elements in the materials during their study by the XPS method under ultra-high vacuum conditions. So the use of XPS will not make it possible for explain, differences in the behaviour of different materials in the same electrocatalytic process.

4. EDS mapping and TEM-SAED should be conducted to precisely display different chemical compositions and the corresponding parts in PtSnx+ SnO2/C, Pt/SnO2-C, and Pt3Sn/C.

The tin component was introduced into the composition of platinum-containing materials using different approaches. For example, to obtain materials based on an alloy of platinum and tin, Pt3Sn foil was subjected to electrochemical dispersion under pulsed electrolysis conditions. Using X-ray phase analysis, it was shown that the result of dispersion of Pt3Sn foil is the formation of nanoparticles of the same composition - Pt3Sn. The X-ray diffraction pattern of Pt3Sn/C showed no peaks characteristic of tin oxide or hydroxide phases. The Pt/SnO2-C electrocatalyst was obtained by electrochemical dispersion of platinum foil in a suspension of SnO2 and carbon black in an alkaline electrolyte. That is, we added a known amount of SnO2 to the composition of the electrocatalyst beforehand. To obtain the PtSnx+SnO2/C material, platinum foil was dispersed in the presence of tin ions (1M NaOH + 1M SnSO4 electrolyte). At the same time, the electrolyte did not contain carbon black (carbon black was added at the stage of catalytic ink preparing). As a result, we obtained not only PtSnx particles, but also the SnO2 oxide phase (the phase composition and amount of the oxide phase were determined using XRD). It was this material that was examined by TEM-SAED in order not only to additionally confirm the presence of a tin oxide phase, but also to see the distribution of PtSnx particles on the surface of SnO2 phase.

5. What is the reaction mechanism of Pt/SnO2-C for EOR? By adding tin, why Pt/SnO2-C surpassed PtSnx + SnO2/C and Pt3Sn/C exhibited the best EOR performance?

The presence of oxophilic elements, such as Sn, Ni, Ti, Ru, Rh, etc, in platinum-based materials significantly improves their electrocatalytic performance by enabling the adsorption of hydroxide ions at low overpotentials thanks to the bifunctional effect. They also change the electronic structure of the electrode by decreasing the d-band center, which weakens the adsorption of CO intermediates, so they enable both bifunctional and electronic (ligand) effects. All these results were presented in [Molecules 2021, 26, 2144]. Probably, tin oxide has a greater effect on the process of ethanol electrooxidation as a result of the greater oxophilicity of tin oxide compared to the tin component in the form of an alloy with platinum or a doping element. It has been added to the Conclusions section.

6. For EOR catalysts, stability is indispensable. It is suggested to add the stability performance of samples. Only by doing that, the catalytic performances of catalysts can be assessed in a more comprehensive way.

Thank you for your remark. We provided data from the accelerated stress test in presence of ethanol (figure 4c). The rate of degradation of Pt/C and platinum-tin-based materials, which is a change in the current density of the peak of ethanol oxidation on the forward and back scan of CV under accelerated stress testing conditions for all the studied materials, has a similar character. Appropriate explanations have been made in the Results and Discussion and Materials and Methods sections.

7. EOR catalytic performance comparison with recent publications is also suggested to further evaluate the performance of as-prepared catalysts

In Supplementary materials we have provided a table S1 comparing the results of the electrocatalytic activity of different Pt-Sn-containing materials.

Reviewer 4 Report

Pulsed electrolysis is an advantageous technique for the synthesis of platinum-based bimetallic catalysts, unlike the traditional high-temperature post-processing bottom-up methods. This manuscript reports a comprehensive study on the synthesis of three bimetallic catalysts based on tin and platinum using the pulsed electrolysis method and a study on the influence of a tin component; a dopant, an alloy or an oxide, on the catalytic activity of the bimetallic catalyst. The uniformity of the platinum phase in Pt/C, Pt/SnO2-C, and Pt3Sn/C catalysts over the surface of carbon black or carbon black + tin dioxide composite support is confirmed by TEM studies. Through STEM studies on PtSnx+SnO2, it was found that the PtSnx particles are also evenly distributed over the tin oxide phase. Further, the influence of the individual tin phase on the electrochemical oxidation of ethanol has been reported which suggested that the electrochemical activity increased in all cases than that of the Pt/C catalyst and the catalysts containing tin-oxide as support were found the most active. This is an interesting study which reports a comprehensive comparison of the tin components on the activity of the resultant platinum-tin bimetallic catalysts. The experimental work has been competently performed with a high scientific standard, and the results have been scholarly presented. The present work will surely attract the attention of a broad scientific community interested in electro-catalysis. Therefore, I recommend the publication of this work in its current form.

Author Response

Response to Reviewer #4

Pulse electrolysis technique for preparation of bimetal tin-containing electrocatalytic materials

Alexandra Kuriganova, Marina Kubanova, Igor Leontyev, Tatiana Molodtsova and Nina Smirnova

The team of authors is grateful to the Reviewers and the Editor for such a close attention to our work. We drew attention to all these shortcomings and corrected them, made appropriate explanations, and conducted additional studies. All this allowed us to significantly improve the quality of presentation of the results of our work.

We are grateful to the reviewer for the appreciation of our manuscript. We hope that in case of a positive decision by the Editorial Board, our manuscript will be useful for researchers in the field of electrocatalysis and fuel cells.

Round 2

Reviewer 1 Report

1. The figure caption for Figure S3 is incorrect. the brown tick marks correspond to Pt; meanwhile, the green tick marks correspond to SnO2. Please change the description.

Author Response

1. The figure caption for Figure S3 is incorrect. the brown tick marks correspond to Pt; meanwhile, the green tick marks correspond to SnO2. Please change the description. 

Thank you for your remark. The description for Figure S3 have been changed.

Reviewer 3 Report

Although the author has answered all questions, some investigations are missed in the revision. So, a minor revision is suggested.

1. It is suggested that the reason of choosing tin should be expressed in the introduction part.
2. Instead of in-situ electrolyte involving XPS, the dry samples’ XPS should be conducted to reveal the surface chemical states of Pt, Sn, O, and C, thus, further confirm the function of tin in catalysts.
3. No EDS mapping and TEM-SAED of PtSn_x + SnO₂/C can be found in the revised paper. Please add them in the revision.

Author Response

The team of authors is grateful to the Reviewers and the Editor for such a close attention to our work. We drew attention to all these shortcomings and corrected them, made appropriate explanations, and conducted additional studies. All this allowed us to significantly improve the quality of presentation of the results of our work.

All corrections in the manuscript are highlighted in yellow.

1. It is suggested that the reason of choosing tin should be expressed in the introduction part.

We have provided the reason of choosing tin in the introduction section and cited two more articles: [Electrochimica Acta 259 (2018) 733e741740] and [Molecules 2021, 26, 2144]

2. Instead of in-situ electrolyte involving XPS, the dry samples’ XPS should be conducted to reveal the surface chemical states of Pt, Sn, O, and C, thus, further confirm the function of tin in catalysts.

We provide 3d spectra of tin-containing materials prepared via pulse electrolysis (Figure S4). There, the presence of both an oxidized tin surface and the presence of tin in the metallic state was demonstrated, depending on the type of sample.

3. No EDS mapping and TEM-SAED of PtSn_x + SnO₂/C can be found in the revised paper. Please add them in the revision.

EDS mapping of the PtSn_x+ SnO₂/C is presented in Supplementary materials (Figure 3b). Relevant explanations are included in the text of the manuscript

The selected areas of the electron diffractograms (SAED) pattern of PtSn_x+SnO₂ material (insert on Figure 2 b) shows at least four main diffraction rings that can be indexed to the (111), (200), (220) and (311) reflections of the fcc structure of PtSn_x. Relevant explanations are included in the text of the manuscript.