Fabrication and characterization of capric acid/reduced graphene oxide decorated diatomite composite phase change materials for solar energy storage

In this paper, diatomite-based composite phase change materials (DI-based CPCMs) were fabricated by the vacuum impregnation of capric acid (CA) into reduced graphene oxide decorated diatomite (rGO-DI). In the DI-based CPCMs, DI was used as the supporting material, which was first purified by thermal treatment and alkali treatment, to improve the adsorption capacity of the PCM, rGO was used to decorate the DI to improve the thermal conductivity of CPCMs. The rGO-DI could retain CA at the weight fraction of 60% without leakage. The maximum melting and freezing enthalpy of CA/rGO-DI-2 reached 106.2 J g−1 and 108.6 J g−1, respectively, and its thermal conductivity was up to 0.5226 W m−1 · K, 260.4% and 81.3% higher than pure CA and CA/DI, respectively. The CPCMs have good thermal reliability and thermal stability, and there was no chemical reaction between CA and rGO-DI. The CPCMs maintained thermal properties after 200 melting–freezing cycles. Finally, the CPCMs have potential for application in solar energy storage systems.

thermophysical properties and shape-stability and were discussed in detail. This is a comprehensive and informative research and has certain significance in improving the properties of PCMs. However, there are some problems in the manuscript that should be further explained for publication in the present form. The following points need to be noticed: 1. The language must be improved, and there are too many shorts for materials throughout the full text made it difficult to understand. 2. The article emphasizes the DI-based CPCMs aims in solar energy storage but is not reflected in the manuscript. How can this material used in solar energy storage? And there also should make a comparison between this new DI-based CPCMs and the other current materials. 3. The authors studied shape-stability of the composite PCMs by leakage tests. What was the package capacity of composite PCMs without obviously liquid leakage? For the discussion in 4.2, what's the reason for the conclusion expressed as "Thus, the impregnation ratio of DI was improved through alkali treatment, and CA/rGO-DI (60wt.%: 40wt.%) was considered the optimum combination for producing CPCM."? 4. The reasons why CA/rGO-DI CPCMs exhibited the highest thermal conductivity should be further analyzed and clarified. 5. A more comprehensive and clearly conclusion should be supplemented.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
This paper mainly reports that diatomite-based composite phase change materials (DI-based CPCMs) were fabricated by the vacuum impregnation of capric acid (CA) into reduced graphene oxide decorated diatomite (rGO-DI).Results shows that the CPCMs of this manuscript have the potential to be used in solar energy storage systems. On the whole, the experiments in the manuscript support the arguments well and this paper is reasonable in structure and expression basically. The photographs are all reasonably processed and exhibit legible information. In my opinion, this manuscript can be accepted for publication after revising. 1. In Fig. 2(c), it would be more convenient if you put 170℃ to 250℃ intervals with two vertical dashed lines. 2. In Fig. 3，Some of the pictures have shadows, so you can make them more clearly. The luminescence properties of phosphors mentioned in your title is not complete, please add more. 3. In Fig.5, please add the legend. 4. In Fig.6, If there is a standard card of a single phase of CA, please put it in the picture for comparison.

19-Nov-2018
Dear Dr Li: Title: Fabrication and characterization of capric acid/ reduced graphene oxide decorated diatomite composite phase change materials for solar energy storage Manuscript ID: RSOS-181664 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Manuscript Number: RSOS-181664 Title: Fabrication and characterization of capric acid/reduced graphene oxide decorated diatomite composite phase change materials for solar energy storage The authors fabricated a novel type of CPCMs with CA and rGO decorated diatomite using vacuum impregnation. The structure, morphology and thermal properties of CPCMs had been studied in depth. The characterization and explanation made sense. I recommend publication in Royal Society Open Science after minor revisions. Some comments are provided as follows: (1) Configuration and arrangement of figures are poor. There are so many figures in the draft manuscript. It is suggested some of the figures (e.g. Figure 6 (a) and (b), Figure 9 and 10) be combined.
(2) More details of the diatomite sample are required.
(3) The determination of the transition temperatures from the DSC thermograms should be specified (were they determined at the peak maximum ?).

Reviewer: 2
Comments to the Author(s) In this study, DI-based CPCMs with good shape stability and high thermal conductivity were well designed. The microstructures, chemical compatibility, thermal conductivity, thermophysical properties and shape-stability and were discussed in detail. This is a comprehensive and informative research and has certain significance in improving the properties of PCMs. However, there are some problems in the manuscript that should be further explained for publication in the present form. The following points need to be noticed: 1. The language must be improved, and there are too many shorts for materials throughout the full text made it difficult to understand. 2. The article emphasizes the DI-based CPCMs aims in solar energy storage but is not reflected in the manuscript. How can this material used in solar energy storage? And there also should make a comparison between this new DI-based CPCMs and the other current materials. 3. The authors studied shape-stability of the composite PCMs by leakage tests. What was the package capacity of composite PCMs without obviously liquid leakage? For the discussion in 4.2, what's the reason for the conclusion expressed as "Thus, the impregnation ratio of DI was improved through alkali treatment, and CA/rGO-DI (60wt.%: 40wt.%) was considered the optimum combination for producing CPCM."? 4. The reasons why CA/rGO-DI CPCMs exhibited the highest thermal conductivity should be further analyzed and clarified. 5. A more comprehensive and clearly conclusion should be supplemented.

Reviewer: 3
Comments to the Author(s) This paper mainly reports that diatomite-based composite phase change materials (DI-based CPCMs) were fabricated by the vacuum impregnation of capric acid (CA) into reduced graphene oxide decorated diatomite (rGO-DI).Results shows that the CPCMs of this manuscript have the potential to be used in solar energy storage systems. On the whole, the experiments in the manuscript support the arguments well and this paper is reasonable in structure and expression basically. The photographs are all reasonably processed and exhibit legible information. In my opinion, this manuscript can be accepted for publication after revising. 1. In Fig. 2(c), it would be more convenient if you put 170℃ to 250℃ intervals with two vertical dashed lines. 2. In Fig. 3，Some of the pictures have shadows, so you can make them more clearly. The luminescence properties of phosphors mentioned in your title is not complete, please add more. 3. In Fig.5, please add the legend. 4. In Fig.6, If there is a standard card of a single phase of CA, please put it in the picture for comparison. Response: Considering the reviewer's suggestion, several figures in this paper has been redrawn (Fig. 2(c), Fig. 3, Fig. 5, Fig. 6) and combined ( Fig. 9 and 10). The intensity of CA is too high, if combine Fig. 6 (a) and (b), the Fig.6 will turned the following picture.

Author's Response to Decision
Response to comment 2: More details of the diatomite sample are required.
Response: We are sorry for our negligence. We added the details of the diatomite sample in Section 3.1, Response to comment 3: The determination of the transition temperatures from the DSC thermograms should be specified (were they determined at the peak maximum?).
Response: Generally, the phase change temperature of PCMs were the onset temperature in DSC curves as the below shown, the peak temperature was corresponding the endothermic/ exothermic peak temperature.
DSC curves of pure CA.

Special thanks to you for your good comments
Reviewer #2: Response to comment 1: The language must be improved, and there are too many shorts for materials throughout the full text made it difficult to understand.
Response: The language of our manuscript has been refined and polished by a professional editing company.
Response to comment 2: The article emphasizes the DI-based CPCMs aims in solar energy storage but is not reflected in the manuscript. How can this material used in solar energy storage? And there also should make a comparison between this new DI-based CPCMs and the other current materials.
Response: Considering the reviewer's suggestion, the illustration of PCM used in solar energy storage areas has been added in "Section 2 Introduction".
We had made a comparison between the DI-based CPCMs and other DI-based CPCMs in previous manuscript (  Fig.3 we can find that the composite PCMs were agglomerate when the content of CA was higher than 60wt.%, the composite PCMs were leaked after heating under CA content of 65wt.%.
In order to indicate the package capacity of composite PCMs without obviously liquid leakage more obviously, the Fig.3 was redrawn, the mass loss of composite PCMs during the melting process was tested and the discussion in 4.2 was rewritten. The results as following: 4.2 Exudation stability of CA/DI and CA/rGO-DI CPCM The macroscopic photographs of CA/DI and CA/rGO-DI-2 CPCM with different content of CA were shown in Fig.3(a), it is obvious that the composite PCMs were agglomerate when the content of CA was 65wt.%. In order to investigate the exudation stability, CA/DI and CA/rGO-DI-1 CPCM the were heated to 80℃ for 0-2h, the residual quantity of CA/DI and CA/rGO-DI-1 CPCM were shown in Fig.3(b).
The mass loss during the heating process can be neglected when the content of CA was 55wt.% and 60wt.%. Yet significant mass loss (about 10wt.%) occurred for CA/DI and CA/rGO-DI-1 CPCM when the content of CA was 65wt.%. Thus, the optimum mass fraction of CA in composite without leakage is 60wt.%. Response to comment 4: The reasons why CA/rGO-DI CPCMs exhibited the highest thermal conductivity should be further analyzed and clarified.
Response: Considering the reviewer's suggestion, we further analyzed the reasons why CA/rGO-DI CPCMs exhibited the highest thermal