Electrochemical, Structural and Morphological Characterization of Hydrothermally Fabricated Binary Palladium Alloys PdCo and PdNi
Round 1
Reviewer 1 Report
Please find attached comments
Comments for author File: 
Comments.pdf
Author Response
Reviewer 1
1) Explanation was given explaining why the binary palladium alloys could potentially be used as electrodes in capacitors. Mainly based on its ability to facilitate higher rates of redox reactions. This is essential in hybrid electrochemical capacitors where faradic electrochemical processes are believed to increase specific capacitance and working voltage from charge accumulation
2) An explanation was given to explain why a complete DSSC was not fabricated. Evaluation of the DSSC using an iodine electrolyte although useful for comparison purposes to other values in literature is not ideal. The iodine electrolyte is perhaps the most problematic component in the DSSC, since it is volatile, corrosive to all the other elements in the DSSC as well as being difficult to use because of its liquid nature. Besides research towards finding a possible solution to the expensive platinum counter electrode perhaps the biggest initiative is to find a replacement for the iodine electrolyte. This work mainly entails characterization of the developed samples to evaluate their suitability as counter electrodes. Furthermore, the DSSC is composed of 4 main parts which all work in sync to produce the most efficient results thus parameters such as power conversion efficiency, fill factor, open circuit voltage and short circuit current although important do not explain anything to the effect of the counter electrode but rather explain the functions of the dye and TiO2photoanode in electron generation. In my opinion the function of the counter electrode is determined by its electrochemical properties.
3) Reference was added to line 54 in order to specific the components of the PdCo based DSSC.
4) Figures 4 and 6 were corrected accordingly.
5) Figure 5 was removed.
Author Response File: Author Response.pdf
Reviewer 2 Report
This manuscript deals about the fabrication and characterisation of palladium alloys materials PdCo and PdNi. Overall, the manuscript is well written, the fabrication methodology is well described and conducted, and the new materials are extensively and well characterised by different techniques. I believe that this work, in is whole, can be of interest for the readership of Metals. Thus, I would tentatively recommend this work for publication in Metals. However, the authors must answer/justify the points raised below, especially point 2.
1) Such new alloys materials seek to replace (or compete with) the rare and expensive palladium metal commonly used as counter electrode in dye-sensitized solar cell (DSSC) or capacitors. In the introduction, it is well and extensively explained for DSSC applications; however, nothing is said about capacitors (neither in the abstract). The first time it is mentioned appears at the end of the manuscript (line 202), and only one line in the conclusion is written about it (it is concluded that the materials should not work properly for capacitors). More importantly, it is not clear what is the potential interest for capacitor applications after reading the whole manuscript. This point should be better explained / clarified in the introduction section and main text.
2) My major concern about this work is that their new materials were not tested in real (or similar) conditions for practical use in DSSC (and capacitors) to asses their efficiency and competitiveness (and if they do work properly). Thus, the term “for use as counter electrodes in dye sensitized solar cells or capacitors” that appears in the title and keywords, is not relevant in my opinion. For instance, efficiencies in operative DSSC/capacitors devices with their materials should be compared with those made with standard Pt electrode and/or other Pt alloys previously reported. This was not quantified/investigated in the present study. Maybe alternatively, electrochemical study could be conducting using electrolyte commonly used in DSSC with similar composition of redox couple (I-/I3-), and as well compared to Pt CE.
Minor points:
- Line 54: “The PdCo based DSSC had an efficiency of 6.44% and (…)”
First, a reference should be added to the referred efficiency. Next, the DSSC system the authors are referring to, should be more detailed since the counter electrode is not the only elements constituting the DSSC: which is the kind of sensitizer? Is it a solid-state DSSC or using liquid electrolyte?, etc… Is there any comparison with a traditional DSSC made with Pt CE only, so one can judge the difference in efficiency between the two?
- section 3.3 (SEM analysis): “Figure 3a-d” are cited many times in this section. (I suppose) it should be “Figure 4”.
- Figure 4: At first sight, it is almost impossible to understand what is the difference between the figures. Magnification scale is hardly legible (“50 and 10 micrometres”); letters are too small, and should be written bigger in each figures and/or in the footnote. Moreover “a-d” letters are missing in the pictures.
- In the section “Electrochemical Analysis” (which should be 3.4 not 3.3): Figure 5 and comments in the text line 143-144. In my opinion, it does not bring any useful information to the reader (especially photograph on the left).
- Figure 6 should be improved (quality and legibility). It should be mentioned in the footnote that CVs a-b were measured at different scan rates and detailed values in the figures must be magnified (value are too small, illegible). Figure 6c: I guess this is the comparison of the CVs at 10 mV/s of figure a,b. It must be indicated more clearly.
Author Response
Reviewer 2
1.Reference was added detailing the components of the PdCo DSSC with power conversion efficiency of 6.44%.
2.An explanation was given why the amounts used
for the synthesis were chosen Primarily which has to do with work from Zhang et
al which describes how the rate of reduction in Pd alloys potentially increases
with low palladium content. Ref 17
3.FTIR bands in Fig 3 were explained
4.Reference was added to explain particle aggregation from magnetic behaviour
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper deals with the develpopment of palladium alloys aimed to replace platinum at teh counter electrode in DSCs. In the introduction, the authors comment on the fact that 'platinum is an expensive precious metal'. Ths is true, but in the current market (February 2019), the price of palladium is actually higher than platinum, see for example :https://www.lppm.com/data/#c=pd&y=2019&t=daily
The fact that metal prices fluctuate and that Pd is not necessarily a cheaper option to Pt should be mentioned. The fact that the authors are trialling alloys containing first row d-block metals is a strong selling point of their work.
I recommend acceptance after some revision.
Line 55. The statement that 'The PdCo based DSSC had an efficiency of 6.44%" needs more information. What was the dye and the electrolyte? And what was the comparable PCE with Pt (if these data are available)?
Line 65 and 66: The amounts of materials are given in grams. ON what graounds were these masses chosen? Is the molar ratio of Co (or Ni) : Pd to be considered?
Caption to Fig. 1 - it would be sensible to refer forward to Fig. 4 so that the details of the SEM image can be found by the reader.
Line 93: 'Crystallographic structures' is not strictly correct since the structure is not determined by powder XRD. Please rephrase.
The sentences on lines 117-119 are oddly phrased. I am not convinced by section 3.2. Are the authors suggesting that ethanol remains in the sample after drying ? What gives rise to the bands at 1000, 675 and 560 cm-1? Are the IR data really helpful in screening samples?
Line 137. A reference should be given for the proposal that aggregation of Pd alloys may arise from magnetic behaviour.
I cannot comment in detail on the electrochemical part of the paper.
Author Response
Reviewer 3
1.An explanation was given as to why PdNi and PdCo are advantageous than other binary alloys. These include the reduction in susceptibility to corrosion , cheapness since cobalt and nickel are readily available as well as the increased catalytic capability with reduced mass loading of the expensive palladium metal. Palladium being a good electrocatalyst as platinum is not required in large amounts, this has led to development of core shell nanoparticles in which the more reactive palladium makes up the shell and effects maximum contact with the electrolyte whilst the cheaper elements like nickel and cobalt make up the core. However, this work does not entail fabrication of core shell structure because of cost effect as well as synthesis difficulties.
2. Reasons were more clearly given as to why PdNi is deemed more effective than PdCo. Mainly explained by the high reduction current density which explains how the catalyst affects the rate of the reduction reaction. The peak to peak potential difference which is inversely proportional to the rate of reduction reaction is another parameter explained. This value is low for PdNi as compared to PdCo. Furthermore, to evaluate the effect of the counter electrode to electron transfer the charge transfer resistance is explained. Similar to the reduction current density and peak to peak potential difference the charge transfer resistance gives an indication of how that particular electrode affects electron transfer from the counter electrode to the electrolyte. In this case PdNi clearly has lower resiatance at 0.345Ωcm-2as compared to 0.372 for PdCo.
3. Grammatical and other corrections to figures were done.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments to the Author
This manuscript reports hydrothermally fabricated binary palladium alloys PdCo and PdNi for use as counter electrodes in dye sensitised solar cells or capacitors. I recommend it accepted for publication after some major revision. The comments are as follows:
Except for the electrical property and conductivity, what is the advantage of your PdCo and PdNi system compared with other binary metal alloys?
Does the amount of nickel and cobalt precursor solutions have an effect in its catalytic activity?
According to the author's research results, PdNi exhibits a better performance than PdCo. The reason should be illustrated more clearly.
The font size for the x and y-axis of all figures is very small and can’t be seen clearly. Also the size should be consistent and matched with the other figures. Please check it carefully.
The Figure number stated in line 2, Page 4 doesn’t match with figure number listed in below. Please check it carefully.
An obvious grammatical error appears in line 5 and 8, page 8, please check it carefully.
Author Response
Reviewer 4
An explanation was given to the FTIR bands in Figure 3.
Round 2
Reviewer 1 Report
It is acknowledge the effort of the authors to address the suggestions and comments of the reviewers
