Investigation of Pore Size on the Hydrogen Evolution Reaction of 316L Stainless Steel Porous Electrodes
, Luis Olmos 2, Melina Velasco-Plascencia 1, Héctor J. Vergara-Hernández 1,*, Julio C. Villalobos 1, Mario Misael Machado López 1 and Juan Manuel Salgado López 3Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this work, the authors investigate the effect of different pore sizes on the hydrogen evolution performance of 316L stainless steel electrodes. Among these, the electrode fabricated with 100-200 μm pore formers exhibited the highest HER activity. The authors believe that the enhanced HER originates from its higher active surface area. However, the electrochemical analysis of the material requires further enhancement, as some issues remain insufficiently clarified. Accordingly, I think this work can be accepted after revisions. Here are some questions which should be also carefully addressed.
1. Samples sintered under different conditions should be assigned specific abbreviations instead of repeatedly emphasizing the sintering conditions for differentiation.
2. Why does the catalytic activity of the sample with sintering conditions of 300–400 μm decrease from the best to the worst with increasing NaOH concentration?
3. Corrosion resistance should be further evaluated through corrosion potential and stability tests, followed by analysis of the impedance curves.
4. The electrochemical active surface area can be further analyzed by testing the Cdl through cyclic voltammetry.
5. The superscripts and subscripts mentioned in the text are not indicated. Please check carefully.
6. The reference format is inconsistent, with some missing publication years, page numbers, and article titles. Please review and correct carefully.
Author Response
RESPONSE TO REVIEWER 1:
In this work, the authors investigate the effect of different pore sizes on the hydrogen evolution performance of 316L stainless steel electrodes. Among these, the electrode fabricated with 100-200 μm pore formers exhibited the highest HER activity. The authors believe that the enhanced HER originates from its higher active surface area. However, the electrochemical analysis of the material requires further enhancement, as some issues remain insufficiently clarified. Accordingly, I think this work can be accepted after revisions. Here are some questions which should be also carefully addressed.
- Samples sintered under different conditions should be assigned specific abbreviations instead of repeatedly emphasizing the sintering conditions for differentiation.
R: The authors changed the names of the samples to make the results and discussion more legible and easier to understand. WS for the sample without salts; S100 for the sample 100-200 µm; S200 for the sample 200-300 µm; and S300 for the sample 300-400 µm. This designation is described in the Experimental Method.
- Why does the catalytic activity of the sample with sintering conditions of 300–400 μm decrease from the best to the worst with increasing NaOH concentration?
R: This effect on the decrease of catalytic activity of the 300-400 μm sample as de NaOH concentration increases was discussed on the manuscript in section 2.4 Linear Sweep Voltammetry Analysis.
- Corrosion resistance should be further evaluated through corrosion potential and stability tests, followed by analysis of the impedance curves.
R: The authors agree with the reviewer, corrosion resistance is an important parameter to analyze the performance of the electrodes in anodic processes. However, the aim of this research work is to analyze the effect of the porous size and the effect of former porous particles on the hydrogen evolution reaction which is a cathodic process. However, it is important to let you know that the authors have made the corrosion and stability test for operation conditions, but these results or analysis are considered for other research paper.
Authors include the evidence of the potentiodynamic curves that support this statement.
- The electrochemical active surface area can be further analyzed by testing the Cdl through cyclic voltammetry.
R: The electrochemical active area was analyzed by Electrochemical Impedance Spectroscopy, specifically calculated the Roughness Factor (Rf) which is reported to have a high accuracy on determining the electrochemical active area. The authors include the results of the values of Cdl obtained for the adjustment of the EEC in table 2.
- The superscripts and subscripts mentioned in the text are not indicated. Please check carefully.
R: According to the observation of the reviewer, the manuscript was checked, and the superscripts and subscripts were indicated in the text.
- The reference format is inconsistent, with some missing publication years, page numbers, and article titles. Please review and correct carefully.
R: The reference format was corrected, and the missing information was included.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript investigates an interesting, often understudied, aspect of catalysis porosity. The material studied is also of interest due to being a low-cost alternative to more traditional materials.
There are several aspects of the paper that need to be improved before publication.
+Figure 2 specifies the relative density is shown without error bars but throughout the paper and in the abstract there are statments like the samples have "similar relative density values" when the value range from 85% to 60%. If these values are simular then they should be within error of eachother.
+It is not clear if the pore are truely empty or if the salt used still exists within the pore but due to the differance in electron density would make the pore appear empty by xray tomography. This concern is further exaserbated by the vague explanation of the tomography data treatment.
+ The differance in voxel size between the sets of samples is clearly needed, but without having hte resolution to see pores of less than 8 um it is not possible to make some of the claims following figure 5. It is still possible that the samples with salts have both large pores that do not exist in the blank while also having the small pores that exist in the blank.
+The trend of specific surface area with pore former seems counter intutive. As pore volume increases from a pristine steel to large pore there should be a low vlaue of with a 'perfect' steel sample, then a maxium as pores increase surface area, then begin to descrease as the pores no longer increase the surface area. What is shown in table 1 is only the decrease after the maxium. This implies that the blank is far from the idealized steel. While not incorrect is it counter intutive. A statement explaining why the blank with a low pore size as could be made has the highest surface area would be appreacated.
+CV's of the samples would be appeacated. There is no spectroscopic proof that the samples are truely only steel after the pore former was removed. CV's showing that each sample have the have CV curves would indicate that the catalyst are preforming the same reaction.
+Several of the figures are of very low quality specifically, 2, 5, 8 and 9. The issues range from low DPI, to small font sizes. Please also use color blind friendly color schemes.
+Please review the document carefully mistakes like line 201 should not appear.
Author Response
RESPONSE TO REVIEWER 2:
The manuscript investigates an interesting, often understudied, aspect of catalysis porosity. The material studied is also of interest due to being a low-cost alternative to more traditional materials.
There are several aspects of the paper that need to be improved before publication.
- Figure 2 specifies the relative density is shown without error bars but throughout the paper and in the abstract there are statments like the samples have "similar relative density values" when the value range from 85% to 60%. If these values are simular then they should be within error of each other.
R: Similar values are referred to the sample produced with the salts of different size. This part was clarified in the manuscript.
- It is not clear if the pore are truely empty or if the salt used still exists within the pore but due to the differance in electron density would make the pore appear empty by xray tomography. This concern is further exacerbated by the vague explanation of the tomography data treatment.
R: Pores are empty because the salt elimination is achieved in a previous step, in where after the heat treatment to eliminate the salts, the sample is weighted to ensure that all salt particles were vanished. Besides, the sintering temperature is wide large in comparison to the vaporization temperature of salts (180°C vs 1260°C)
- The differance in voxel size between the sets of samples is clearly needed, but without having hte resolution to see pores of less than 8 um it is not possible to make some of the claims following figure 5. It is still possible that the samples with salts have both large pores that do not exist in the blank while also having the small pores that exist in the blank.
R: The interparticle pores exists in the highly porous samples, however, the numerical simulations shown that fluid pass rather through the largest pores that provides a better flow paths. Therefore, the quantity of liquid passing throughout the interparticle pores is negligible in comparison to the large interconnected pores. Then, the samples fabricated with the salts were analyzing from this assumption. However, the images are not included in the paper because we consider that they can give a confusion of the readers.
- The trend of specific surface area with pore former seems counter intutive. As pore volume increases from a pristine steel to large pore there should be a low vlaue of with a 'perfect' steel sample, then a maxium as pores increase surface area, then begin to descrease as the pores no longer increase the surface area. What is shown in table 1 is only the decrease after the maxium. This implies that the blank is far from the idealized steel. While not incorrect is it counter intutive. A statement explaining why the blank with a low pore size as could be made has the highest surface area would be appreacated.
R: The specific surface follows the correct trend, since it is reported in the literature that the specific surface area increases with decreasing particle size. Making an analogy, in this work the pore size determines the specific surface, therefore, as the pore size diminishes the specific surface increases, which is the values reported in Table 1.
- CV's of the samples would be appeacated. There is no spectroscopic proof that the samples are truely only steel after the pore former was removed. CV's showing that each sample have the have CV curves would indicate that the catalyst are preforming the same reaction.
R: The authors consider that the reviewer’s comment is not clear at all, because is not indicated which means by the term “CV”, and also is not indicated the line number or section where the reviewer’s comment refers. The authors understand CV such as cyclic voltammetry and the response in focused on this technique. Electrochemical Impedance Spectroscopy was carried out to evaluate Rf in order to infer the active catalytic area instead of cyclic voltammetry (CV) to evaluate the Cdl values as analyzed in Table 2. The EIS technique is used to determine the electrochemical active area, which is directly related to the interface steel-electrolyte, furthermore, the fitting of the EEC is widely used to explain the mechanisms in the HER. It is reported that the EEC used in the manuscript describes two processes: one related to the hydrogen evolution reaction (CPE1-R2) and the porosity (CPE2-R3).
- Several of the figures are of very low quality specifically, 2, 5, 8 and 9. The issues range from low DPI, to small font sizes. Please also use color blind friendly color schemes.
R: According to the reviewer’s comment, the quality and DPI of the figures 2, 5, 8 and 9 were improved for a better appreciation of the information presented.
- Please review the document carefully mistakes like line 201 should not appear.
R: Line 201 was deleted, and the authors carefully revised the document to avoid these mistakes.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors' answers fully address my concerns, therefore I recommend publishing this work in Catalysts.
Author Response
The authors thank for their comments and recommendations.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have made significant improvements to the paper but there are still lingering concerns about the manuscript.
Chief among the concerns is the chemical structure of the materials. While the authors have used several techniques to verify aspects like surface area and chemically active areas, there is no evidence that the reaction is exclusively HER for the porous samples. For this reason, CV curves were suggested, as opposed to impedance, where side reactions, such as any reaction with embedded pore formers or iron nitrides, would interfere with the interpretation. Any technique that would verify the sample composition would be equally valuable such as XPS, FTIR, ICMS, AAS, or anything similar.
If it is not possible to perform any verification that the porous materials are only steel the authors need to provide some citation to give reason to believe their method of pore formation produces pure steel without excess carbide, nitride, or oxide.
The method for sample preparation involves adding 30% salt contamination into 316L steel then without any post-analysis the reader is to believe the sample follows the same chemistry as unaltered 316L steel. While it is very likely there is no salt left over the authors do need some evidence to support this claim.
Figure one has faint pink text free applications like Inkscape are well suited to remove this.
Author Response
RESPONSE TO REVIEWER 2
REVIEWER 2
- The authors have made significant improvements to the paper but there are still lingering concerns about the manuscript.
Chief among the concerns is the chemical structure of the materials. While the authors have used several techniques to verify aspects like surface area and chemically active areas, there is no evidence that the reaction is exclusively HER for the porous samples. For this reason, CV curves were suggested, as opposed to impedance, where side reactions, such as any reaction with embedded pore formers or iron nitrides, would interfere with the interpretation. Any technique that would verify the sample composition would be equally valuable such as XPS, FTIR, ICMS, AAS, or anything similar.
If it is not possible to perform any verification that the porous materials are only steel the authors need to provide some citation to give reason to believe their method of pore formation produces pure steel without excess carbide, nitride, or oxide.
The method for sample preparation involves adding 30% salt contamination into 316L steel then without any post-analysis the reader is to believe the sample follows the same chemistry as unaltered 316L steel. While it is very likely there is no salt left over the authors do need some evidence to support this claim.
R: First of all, the authors agree with the reviewer, the formation of iron nitrides would interfere with the HER interpretation and the contribution of the porous size with the catalytic activity of the samples. This interpretation was carried out by an author’s mistakes in the manuscript redaction, which indicates that the salt particles were eliminated at 500 °C, which indeed, is a temperature where nitrides can be produced. However, the real temperature used to eliminate the salt particles were 180 °C; the authors indicated the correct temperature in the manuscript, and the explanation of the difference temperatures processes are indicated as follows:
The process to generate porous materials was previously published [1][2], the temperature to eliminate the salts is 180 °C and the pores are empty because the salt elimination is achieved at this temperature, and the sample is weighed to ensure that all salt particles vanished. It is important to clarify that nitriding temperature was above 723 K [3], this is the reason why the authors consider that there is no nitriding during the electrode manufacturing process. At 180 °C, there is no nitriding process formation, for the reason the authors did not consider a change of composition in the surface of the stainless steel and the authors consider enough the Electrochemical Impedance Spectroscopy to evaluate the electrochemical active area as function of porous size and its effect on HER which is the principal aim of this manuscript. Also, it is important to consider that the Cr2O3 passive layer in stainless steel which is a stable oxide and protects stainless steel even though to change of chemical composition on the steel matrix.
[1]. Cabezas-Villa, J. L., Olmos, L., Bouvard, D., Lemus-Ruiz, J., & Jiménez, O. (2018). Processing and properties of highly porous Ti6Al4V mimicking human bones. Journal of Materials Research, 33(6), 650-661.
[2]. Olmos, L.; Bouvard, D.; Cabezas-Villa, J.L.; Lemus-Ruiz, J.; Jiménez, O.; Arteaga, D. Analysis of Compression and Permeability Behavior of Porous Ti6Al4V by Computed Microtomography. Metals and Materials International 2019, 25, 669–682, doi:10.1007/S12540-018-00223-W/METRICS.
[3]. Christiansen, T., & Somers, M. A. (2005). Low temperature gaseous nitriding and carburising of stainless steel. Surface Engineering, 21(5-6), 445-455.
- Figure one has faint pink text free applications like Inkscape are well suited to remove this.
R: Figure 1 was edited from original plot to avoid watermarks and included in the manuscript in section 2.1 sintering.
Author Response File: Author Response.pdf
