Methods to Improve the First Hydrogenation of the Vanadium-Rich BCC Alloy Ti16V60Cr24
Round 1
Reviewer 1 Report
The authors present the results of systematic studies of different ways to improve the hydrogenation kinetics of the vanadium-rich BCC alloy Ti-V-Cr. The key point is a thorough comparison of the effects of cold rolling, ball milling, Zr addition, and air exposure on the hydrogenation properties of the Ti16V60Cr24 alloy. This work continues previous investigations of the same group related to hydrogen storage materials. I believe that the results obtained in this work are of potential interest to the hydrogen storage community, in particular, to readers of the Hydrogen. However, a certain revision of the manuscript appears to be necessary. The authors should address the following questions and concerns.
1) Figure 3 and the related discussion. It is not quite clear, why the secondary phase for the sample with Zr addition is not seen in the XRD pattern. The structure of this secondary phase appears to be important, considering its role as "a gateway for hydrogen".
2) Figure 4. The nature of the curves between the experimental points should be clarified. Was there any fitting, or the curves just represent guides to an eye? The same also applies to Figure 6.
3) Figure 5 and the related discussion. As in my comment 1), the problem here is the absence of any traces of the secondary phase for the hydrogenated sample with Zr addition. One may expect several Bragg peaks from this phase in XRD patterns.
4) List of references. Ref. 5 is not complete. Something wrong is also with Ref. 15. In particular, the lines "Doctorate, D.U.;", "Sciences, E.N.;", "Mat, N." look, as if they were the author names.
Author Response
Thank you for your comments. Please see below the answers to all your comments.
- Figure 3 and the related discussion. It is not quite clear, why the secondary phase for the sample with Zr addition is not seen in the XRD pattern. The structure of this secondary phase appears to be important, considering its role as "a gateway for hydrogen".
Answer: The fraction of the secondary phase seen in the SEM is relatively low (7%). Thus, the Bragg peaks of this phase will have small intensities. It should also be considered that, because the crystallite size of the BCC phase is quite small, the peaks are quite broad. We expect to have similar crystallite size for the secondary phase. Thus, a combination of low peak intensities and their high width makes the secondary phase practically invisible in XRD spectra. However, we are currently performing synchrotron radiation measurements to highlight that secondary phase as the resolution is better in synchrotron than in laboratory XRD.
- Figure 4. The nature of the curves between the experimental points should be clarified. Was there any fitting, or the curves just represent guides to an eye? The same also applies to Figure 6.
Answer: To make the curve more readable only one in ten data points is shown. Therefore, for small t the curve is not linking the two symbols but actually all data between these two points indicated with a symbol. We used a logarithm scale in order to highlight the difference of incubation times.
- Figure 5 and the related discussion. As in my comment 1), the problem here is the absence of any traces of the secondary phase for the hydrogenated sample with Zr addition. One may expect several Bragg peaks from this phase in XRD patterns.
Answer: The answer to this question is basically the same as to question 1. We are now performing in-situ synchrotron diffraction during dehydrogenation. Hopefully, it will show the presence of the secondary phase.
- List of references. Ref. 5 is not complete. Something wrong is also with Ref. 15. In particular, the lines "Doctorate, D.U.;", "Sciences, E.N.;", "Mat, N." look, as if they were the author names.
Answer: The mistakes are corrected.
Reviewer 2 Report
The work reports a study on Ti16V60Cr24, a vanadium-rich BCC type alloy, with the use of several different preparation methods, including the addition of 4% of V, with the aim of improving the activation process, that is to say the first hydrogenation of the sample.
The work is well organized and develops consequently from detailed microstructural characterization (elemental/phase abundance, lattice parameters) to kinetics and thermodynamic properties of the analyzed materials.
The introduction is short but conveys the message, is interesting and it describes the motivation and sets the backgrounds, with reference to relevant literature and similar studies.
The experimental part is alright, it contains all the necessary details.
Results are presented in succession in a logical way and the discussion, although lean, includes some links and between the different results and data and also few correlations with literature. The way the matter is developed is correct and linear, it can be easily understood. Some sentences may require few more details and some minor corrections, see the details listed in the following lines.
The conclusions are resuming nicely the main points of the work and close the paper appropriately, summarizing with a bullet point list. Writing style is OK, some minor English language mistakes require attention.
The manuscript gave me a positive impression, I think it can be considered for publication after revisions, if the others reviewers agrees. It is not a comprehensive work including numerous different compositions and/or attempts in several operating conditions, but it delivers very effectively the findings and it shows the successful improvement of activation of the alloy.
However, some corrections and amendments are strongly recommended:
- On line 82 "It is known that..." represents a sentence that should be avoided in scientific literature, without the support of proper references. True, that information is elementary compared to other findings, but citing a manual, handbook, or phase diagrams supporting the statement could help.
- Fig. 2 shows BSE micrographs of samples. While it made sense in Fig.2 to show the different phase compositions, here I do not understand why SE were not used, to underline the morphology. Please replace the figures with SE micrographs, if available, or explain/comment the choice.
- Expert crystallographers will not need this, but peraphs adding the Miller indices in the XRD patterns will help the other readers.
- Of course the mechanical treatments will not influence the thermodynamic properties of the alloy, so I understand the choice not to measure the PCIs, but the section 3.4 would benefit from some curves and the plot of the alloy without 4% Zr, for comparison. If not available, data from literature could be used directly in the graphs or at least cited and commented in the text.
- The slope in the PCI plateaus is very marked, please comment about the choice for the point to extrapolate the pressure level and build the van't Hoff plot. It is my impression that error bars in Fig. 7 would be recommendable.
Author Response
Thank you for your comments. Below are the specific answer.
- On line 82 "It is known that..." represents a sentence that should be avoided in scientific literature, without the support of proper references. True, that information is elementary compared to other findings, but citing a manual, handbook, or phase diagram supporting the statement could help.
Answer: A reference related to that has been added. It is now on line 89.
- Fig. 2 shows BSE micrographs of samples. While it made sense in Fig.2 to show the different phase compositions, here I do not understand why SE were not used, to underline the morphology. Please replace the figures with SE micrographs, if available, or explain/comment the choice.
Answer: We do not show SE because we think it does not bring any new information. In SE the contrast between the matrix and secondary phase is very weak. Regarding the morphology, the sample was polished. Therefore, there is no real topological information to see. In some alloys, the different phases may have different mechanical properties and thus one phase could be grinded more than the other and it could be seen by SE. But it is not the case here.
- Expert crystallographers will not need this, but peraphs adding the Miller indices in the XRD patterns will help the other readers.
Answer: Miller indices have been added.
- Of course, the mechanical treatments will not influence the thermodynamic properties of the alloy, so I understand the choice not to measure the PCIs, but the section 3.4 would benefit from some curves and the plot of the alloy without 4% Zr, for comparison. If not available, data from literature could be used directly in the graphs or at least cited and commented in the text.
Answer: Thank you for this remark. The equilibrium pressure and enthalpy reported in this paper are in agreement with the ones found by Okada’s group. This is discussed in lines 202 to 204.
- The slope in the PCI plateaus is very marked, please comment about the choice for the point to extrapolate the pressure level and build the van't Hoff plot. It is my impression that error bars in Fig. 7 would be recommendable.
Answer: Each corresponding pressure is chosen by considering the half-desorbed capacity of each PCI curve. This detail is given in the experimental section.
Reviewer 3 Report
The article devoted to the problem relevant for "hydrogen (green) energecity", namely, the creation of devices for storage, transportation and safe extraction of hydrogen.
The article is impeccable in terms of stylistics, language and presentation of the received regularites. At the same time, when reading it, some questions and comments arise:
1. What methods (devices) were used to obtain curves of Fig. 6 and 7? (There is nothing about this in the section “Materials and Methods”).
2. In “Conclusion” it would be nice to compare the Hydrogen Capacy alloys of vanadium with other materials suggested for the creation of hydrogen storage facilities in hydride form.
Author Response
Thank yopu for your comments. Below are the answers.
- What methods (devices) were used to obtain curves of Fig. 6 and 7? (There is nothing about this in the section “Materials and Methods”).
Answer: The description of the kinetic measurement has been added to the experimental section.
- In “Conclusion” it would be nice to compare the Hydrogen Capacity alloys of vanadium with other materials suggested for the creation of hydrogen storage facilities in hydride form.
Answer: This detail is not covered here because the emphasis is mainly on the improvement of the kinetic. But we would consider that for a later study which will focus more on storage capacity.
