Direct Self-Assembly of Hierarchically Grown Rhodium Thin Films for Electrocatalytic Hydrogen Evolution Reaction
Round 1
Reviewer 1 Report
This paper describes the deposition of Rh nanostructured films on Ti foils and Ni nano foams. The deposition is obtained by AACVD. The performances obtained are very good and interesting.
I have some points that the authors should consider before the paper is published:
1 - There are no XRD of the samples deposited on Ni nanofoams
2 - The EIS analysis is not clear and should be changed and better discussed. In my opinion the EIS spectra acquired at an over-potential of 60 mV are characterized by at least 2 contributes. The first one is the RC element corresponding to the electrode + film and the second one should be the charge transfer resistance in the solution. Therefore it is more correct to fit the data with a model that describes these two precesses. The other aspect is the normalization of the data to the real area of the electrodes. There is no doubt that the Ni nano foam is characterized by a much higher geometrical and active area if compared with the Ti foil. How where the EIS spectra normalized?
3 - Lines 341: " Where, J = A cm-2, A = geometric area of the glassy carbon electrode". Where is this carbon electrode coming from?
4 -Lines 244-245: How was the amount of Rh deposited on Ni foams and on Ti foil determined?
Author Response
Comment 1 - There are no XRD of the samples deposited on Ni nanofoams.
Answer: It is difficult to record XRD patterns of the Rh films deposited on Ni foam due to the porous and mesh like structure of Ni foam. The X-rays pass through the mesh network and also include the substrate holder peaks which makes the XRD results ambiguous. However, such a problem is not seen in case of flat and closed surface of Ti foil which has the same deposit like Ni foam.
Comment 2 - The EIS analysis is not clear and should be changed and better discussed. In my opinion the EIS spectra acquired at an over-potential of 60 mV are characterized by at least 2 contributes. The first one is the RC element corresponding to the electrode + film and the second one should be the charge transfer resistance in the solution. Therefore it is more correct to fit the data with a model that describes these two processes. The other aspect is the normalization of the data to the real area of the electrodes. There is no doubt that the Ni nano foam is characterized by a much higher geometrical and active area if compared with the Ti foil. How where the EIS spectra normalized?
Answer: The respective models that were used to fit the data are included in the Figure 4(a) and (b) at page No. 7 of the revised manuscript and a sentence is added as line No. 248-249.
Comment 3 - Lines 341: " Where, J = A cm-2, A = geometric area of the glassy carbon electrode". Where is this carbon electrode coming from?
Answer: Thanks to the reviewer for highlighting this mistake. We have now made necessary correction by replacing glassy carbon with “the Ti or Ni foam” which are actually used in current work. (Line No. 284; Page No. 8 of revised manuscript)
Comment 4 -Lines 244-245: How was the amount of Rh deposited on Ni foams and on Ti foil determined?
Answer: The weights of blank Ni foam and Ti foil substrates were measured on micro balance before carrying out the deposition of Rh and the weights of these substrates were measured again after completing the deposition process. Thus the amount of Rh deposited was determined by subtracting the substrate weights before and after deposition of Rh.
Reviewer 2 Report
The authors report on the electrocatalytic activity of self-assembled Rh layers toward HER. The catalysts are deposited on Ni foam and flat Ti substrates via aerosol assisted chemical vapour deposition at low loadings. At present day, Rh is way more expensive than Pt. The high price of Rh is obviously driven by its application in catalytic converters in combustion engine based vehicles (as the authors stated in this work). Hence, one would ask, why the authors chose to use the more expensive and less active noble metal for HER compared to Pt. This makes the application of Rh in HER from the economic perspective less attractive. However, there is a realistic chance for Rh in such applications as the electro-mobility in cars is gaining traction, which will put the rhodium price (on the share market) under increased pressure. The authors needs to reshape the introduction part from this perspective (see comments below).
The obtained results are sound and well presented and merit publication. I suggest to publish this work in this journal after addressing few major revision. Following points needs attention to make this work more attractive for a broad readership:
- Typo in the abstract (line 19): "...HER performance. The Tafel slopes...."
- Reshaping the interaction part with emphasis on the Rh price, that is expected to go down with increasing electro-mobility and fuels cells based cars.
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Line 42: A broad range of approaches has been employed in single-atom catalysts to make every atom count. Hence, the authors need to cite more reference to emphasise the different strategies: i) Lu et al., Angew. Chem. Int. Ed. 2018, 57, 9382 –9386, https://doi.org/10.1002/anie.201804817; ii) Lin et al., Nano Energy 63 (2019) 103849, https://doi.org/10.1016/j.nanoen.2019.06.045; iii) Nann et al., ACS Sustainable Chem. Eng. 2017, 5 (11), 10206–10214, https://doi.org/10.1021/acssuschemeng.7b02198.
- Line 110: "...100 m cm3/min..." I'm not sure what this unit means. Either mL/min or cm3/min.
- Figure 3: It is difficult for the reader to see the difference in the colour codes for the bare substrates and the 40-min coated layers, each. I suggest to use dashed or dotted lines for the bare substrates Ti and NF.
- Bulk electrolysis (chronoamperommetry for at least 15-25 hours) of the Rh/Ti-40 and Rh/NF-40 is missing. This experiment is essential to see how these catalysts behave in terms of stability and performance over extended periods of time - especially when it comes to potential application to replace Pt. Does Rh remains stable on the flat Ti foil after bulk electrolysis? Pleased add this this part.
Author Response
Comment 1 Typo in the abstract (line 19): "...HER performance. The Tafel slopes...."
Answer: Correction has been made as suggested by reviewer. (Line No. 19 of revised manuscript).
Comment 2 Reshaping the interaction part with emphasis on the Rh price, that is expected to go down with increasing electro-mobility and fuels cells based cars.
Answer: We have now modified the introduction part of this manuscript as per reviewer suggestions to reflect on the price issues of Rhodium. These modifications can be found on page 2 and 3 of the manuscript. (Line no. 43-45 and Line no. 53-59)
Comment 3 Line 42: A broad range of approaches has been employed in single-atom catalysts to make every atom count. Hence, the authors need to cite more reference to emphasise the different strategies: i) Lu et al., Angew. Chem. Int. Ed. 2018, 57, 9382 –9386, https://doi.org/10.1002/anie.201804817; ii) Lin et al., Nano Energy 63 (2019) 103849, https://doi.org/10.1016/j.nanoen.2019.06.045; iii) Nann et al., ACS Sustainable Chem. Eng. 2017, 5 (11), 10206–10214, https://doi.org/10.1021/acssuschemeng.7b02198.
Answer: The suggested references have been added as Ref. No. 22, 24 and 25 in the revised manuscript. (Line no. 476-478 and Line no. 481-486)
Comment 4 Line 110: "...100 m cm3/min..." I'm not sure what this unit means. Either mL/min or cm3/min.
Answer: Correction have been made in the revised manuscript at page 10 line 355.
Comment 5: Figure 3, It is difficult for the reader to see the difference in the colour codes for the bare substrates and the 40-min coated layers, each. I suggest to use dashed or dotted lines for the bare substrates Ti and NF.
Answer: Figure 3 has been modified as suggested by reviewer. (Page no. 6 of revised manuscript.)
Comment 6: Bulk electrolysis (chronoamperommetry for at least 15-25 hours) of the Rh/Ti-40 and Rh/NF-40 is missing. This experiment is essential to see how these catalysts behave in terms of stability and performance over extended periods of time - especially when it comes to potential application to replace Pt. Does Rh remains stable on the flat Ti foil after bulk electrolysis? Pleased add this part.
Answer: Bulk electrolysis (chronoamperommetry) results (Figure 6) have been added on page 8-9 of the revised manuscript.(Line no. 304-320)
