Influence of Sintering Temperature on the Structural, Morphological, and Electrochemical Properties of NiO-YSZ Anode Synthesized by the Autocombustion Route

Round 1

Reviewer 1 Report

1. In the experiment part, is NO₃ written wrong in the equation? Please confirm and revise it.
2. In conductivity measurements, please give the activation energy after fitting the Arrhenius curve, indicating at what temperature the anode catalytic performance is better after sintering.
3. In the cell test, the open-circuit voltage of YSZ anode supported single cell is bigger than 1 V at 800℃, why is your open-circuit voltage less than 1 V?

Author Response

1. In the experiment part, is NO₃written wrong in the equation? Please confirm and revise it.

Response: Thank you for the positive feedback. We have revised the chemical equation.

2. In conductivity measurements, please give the activation energy after fitting the Arrhenius curve, indicating at what temperature the anode catalytic performance is better after sintering.

Response: Thank you for your comments. The calculated activation energies for samples sintered at 1200 ºC, 1300 ºC and 1400 ºC were 0.398 eV, 0.365 eV and 0.161eV, respectively. The lowest activation energy is observed for the sample sintered at 1400 ºC. We also incorporated values of activation energy in the manuscript.

3. In the cell test, the open-circuit voltage of YSZ anode supported single cell is bigger than 1 V at 800 ℃, why is your open-circuit voltage less than 1 V?

Response: Thank you for your comments. Low OCV can originate because of multiple factors including gas leakage due to poor sealing, structural defects of the electrolyte layer, and low electrode kinetics. In the current project, the probable cause of the low OCV can be attributed to the potential leakage of gases across the electrolyte due to the existence of any pinhole in the thin electrolyte as also highlighted elsewhere. Additionally, low OCV may also be a result of lack of gas equilibration in the microporous structures of the electrodes as pointed out by a group of researchers [31][32].

Reviewer 2 Report

In this manuscript, the authors prepared NiO-YSZ anodes for use in SOFC and investigated the influence of sintering temperature in the preparation of them. I have carefully read this manuscript and I think that this paper shows well-organization and detailed and systematic characterization. Therefore, in my opinion, there are some minor points before considering it for publication in Metal.

1. As shown in Figure 3, the NiO-YSZ powders prepared at different temperatures show different morphology. The authors described the change in the porous structure with the different temperatures based on the SEM images. However, in order to more directly explain the change in porosity by temperature, I think that BET measurement should be performed.
2. In the electrochemical performance, I am considering that all samples prepared at 1200-1400 ^oC exhibit a lower open-circuit voltage (OCV) than that of the existing SOFC system reported in other literature (about 1.05-1.20). What is the reason why the OCV is relatively low in the system? And it is needed to describe in the manuscript.

Author Response

As shown in Figure 3, the NiO-YSZ powders prepared at different temperatures show different morphology. The authors described the change in the porous structure with the different temperatures based on the SEM images. However, in order to more directly explain the change in porosity by temperature, I think that BET measurement should be performed.

Response: Thank you for your comments. The SEM micrographs shown in figure 3 are of low resolution and do not reflect the proper morphology symmetry. Unfortunately, it was not possible for the authors to access BET currently at home institution. We even tried other institutions but there is long waiting list due to COVID pandemic and winter vacations. We respect your comments and apologize for this shortcoming; however, in our future publications, we will take care of this comment and will definitely include BET.

In the electrochemical performance, I am considering that all samples prepared at 1200-1400 ^oC exhibit a lower open-circuit voltage (OCV) than that of the existing SOFC system reported in other literature (about 1.05-1.20). What is the reason why the OCV is relatively low in the system? And it is needed to describe in the manuscript.

Response: Thank you for your comments. Low OCV can originate because of multiple factors including gas leakage due to poor sealing, structural defects of the electrolyte layer and low electrode kinetics. In current project, the probable cause of the low OCV can be attributed to the potential leakage of gases across the electrolyte due to existence of any pinhole in the thin electrolyte as also highlighted elsewhere. Additionally, low OCV may also be a result of lack of gas equilibration in the microporous structures of the electrodes as pointed out by group of researchers [31][32].

Reviewer 3 Report

The manuscript concerns the implementation of combustion synthesis of the zirconia-based composite. The material was characterized in terms of phase composition and electrochemical performance. Generally, the text is arranged with logical progression. However, the manuscript has some serious flaws. The remarks are listed below:

1. abstract contains information about TGA analysis: “TGA analysis showed the weight loss of as-prepared materials for different temperature ranges.” I suggest to remove this sentence or to add information about the conclusions of this experiment. The information given as it is gives no relevant information.
2. Last sentence of the introduction part is: “The electrochemical performance of NiO-YSZ using the co-precipitation method was studied.” whereas in the presented studies no co-precipitation method was utilized.
3. page 3, equation (1) – the equation is incorrect. The stoichiometric coefficients are different as those used in the described experiment and flow chart (Fig. 1).
4. at 150°C the product was dried not calcined.
5. Page 5: “The comparison shows that with the increase in sintering temperature, the structure becomes denser and denser due to an increase in the agglomeration of NiO-YSZ anode.” with the increase of sintering temperature the sintering process results in densification of the material.
6. 3 – in my opinion the SEM imaging should be corrected. The mapping of elements should be carried out to present the distribution of Ni in the composite body. It would be also advisable to make the images in BSE mode. If the NiO was detected by XRD it should be also visible in the image. Additionally, the Authors claim that bodies sintered in temperature of 1400 °C contain Ni (“Also, at 1400°C, NiO is reduced into Ni-particles”) – better imaging could prove this statement.
7. Page 7 – After synthesis there is still a lot of nitrate and organic residues (“the weight loss may be due to evaporation and decomposition of water, nitrates, hydroxide, excessive ethylene glycol from the dried gel, dehydration of gel and combustion of polymeric gel.”) Why wasn’t the powder calcined prior anode fabrication? Is the presence of the reactants residues beneficial? The residues decompose during sintering, which cause the decrease in sintering ability of the powder and decrease of mechanical performance of the sinters. Is it not a matter of concern in this studies? what are the conclusion of the TGA analysis?
   Besides, considering the scaling up of the technology it should be taken into account that the amount of the residues may vary in a significant degree. Thus the sintered bodies will exhibit totally different properties (for example: porosity).
8. Page 7 – “the weight loss may be due to evaporation and decomposition of water, nitrates, hydroxide, excessive ethylene glycol from the dried gel, dehydration of gel and combustion of polymeric gel.” in the experimental part no ethylene glycol was mentioned. What was the source of water if the powder after synthesis was dried? Where did the polymeric gel come from?

In general, I have doubts if the title is correct. The synthesized material is usually related to as the NiO-YSZ, but in some parts of the text it said that this is a cermet (Ni-YSZ). What is the state of nickel in this material? There are some inconsistencies also in the research results. For instance, it is claimed that the material sintered at temperature of 1400°C contains metallic nickel, however there is no evidence for such. Additionally, the Authors mentioned that NiO is reduced during the SOFC working cycle. The nature of the nickel derived phases should be better explained and more consistent.

Author Response

abstract contains information about TGA analysis: “TGA analysis showed the weight loss of as-prepared materials for different temperature ranges.” I suggest to remove this sentence or to add information about the conclusions of this experiment. The information given as it is gives no relevant information.

Response: Thank you for your comments. We removed it.

Last sentence of the introduction part is: “The electrochemical performance of NiO-YSZ using the co-precipitation method was studied.” whereas in the presented studies no co-precipitation method was utilized.

Response: Thank you for your comments. We have corrected it. We were working parallel on two manuscripts in which one material was synthesized by co-precipitation and other by auto-combustion, therefore by mistake it was written as co-precipitation rather than auto combustion in the current manuscript.

page 3, equation (1) – the equation is incorrect. The stoichiometric coefficients are different as those used in the described experiment and flow chart (Fig. 1).

Response: Thank you for your comments. We have revised the chemical equation 1.

at 150°C the product was dried not calcined.

Response: Thank you for your comments. We have corrected it.

Page 5: “The comparison shows that with the increase in sintering temperature, the structure becomes denser and denser due to an increase in the agglomeration of NiO-YSZ anode.” with the increase of sintering temperature the sintering process results in densification of the material.

Response: Thank you for your comments. We have corrected the structure of the sentence.

3 – in my opinion the SEM imaging should be corrected. The mapping of elements should be carried out to present the distribution of Ni in the composite body. It would be also advisable to make the images in BSE mode. If the NiO was detected by XRD it should be also visible in the image. Additionally, the Authors claim that bodies sintered in temperature of 1400 °C contain Ni (“Also, at 1400°C, NiO is reduced into Ni-particles”) – better imaging could prove this statement.

Response: Thank you for your comments. The SEM micrographs shown in figure 3 are of low resolution and do not reflect the proper morphology. Unfortunately, it was not possible for the authors to access FESEM currently at home institution. We even tried other institutions but there is long waiting list due to COVID pandemic and winter vacations. We respect your comments and apologize for this shortcoming; however, in our future publications, we will take care of this comment and will definitely include high resolution micrographs. Furthermore, the sentence regarding reduction of NiO to Ni is corrected because they cannot be reduced on their own even at high temperature until unless some gas is not provided

Page 7 – After synthesis there is still a lot of nitrate and organic residues (“the weight loss may be due to evaporation and decomposition of water, nitrates, hydroxide, excessive ethylene glycol from the dried gel, dehydration of gel and combustion of polymeric gel.”) Why wasn’t the powder calcined prior anode fabrication? Is the presence of the reactants residues beneficial? The residues decompose during sintering, which cause the decrease in sintering ability of the powder and decrease of mechanical performance of the sinters. Is it not a matter of concern in this studies? what are the conclusion of the TGA analysis? Besides, considering the scaling up of the technology it should be taken into account that the amount of the residues may vary in a significant degree. Thus the sintered bodies will exhibit totally different properties (for example: porosity).

Response: Thank you for your comments. We have corrected the sentence because no ethylene glycol is used. However during auto-combustion process formation of gel takes place just before combustion that combust upon further heating. The TGA analysis was used to optimize minimum sintering temperature so that no reaction takes place during cell operation.

We never thought of calcination before sintering in view of mentioned aspects regarding affect of residues during sintering temperature and in our future projects we will make calcination necessary before the sintering. Thanking you for giving us new dimension.

Page 7 – “the weight loss may be due to evaporation and decomposition of water, nitrates, hydroxide, excessive ethylene glycol from the dried gel, dehydration of gel and combustion of polymeric gel.” in the experimental part no ethylene glycol was mentioned. What was the source of water if the powder after synthesis was dried? Where did the polymeric gel come from?

Response: Thank you for your comments. We have corrected it because no ethylene glycol is used during the combustion process. Although, samples were dried before TGA but humidity in ambient environment can introduce some contents of water in the samples.

In general, I have doubts if the title is correct. The synthesized material is usually related to as the NiO-YSZ, but in some parts of the text it said that this is a cermet (Ni-YSZ). What is the state of nickel in this material? There are some inconsistencies also in the research results. For instance, it is claimed that the material sintered at temperature of 1400°C contains metallic nickel, however there is no evidence for such. Additionally, the Authors mentioned that NiO is reduced during the SOFC working cycle. The nature of the nickel derived phases should be better explained and more consistent.

Response: Thank you for your comments. The material characterizations prior to electrochemical testing are based on NiO state. During electrochemical testing, the cell undergoes reduction phase therefore the term cermets is used thereafter. We have addressed the inconsistencies and in future we will be more careful about it.

Round 2

Reviewer 3 Report

Thank you for the answer. The manuscript is now more clear.

However, I have still some remarks concerning the general equation of autocombustion reaction. According to the equation the ratio Y:Zr is 1:1, whereas in reality it is 8:92. Maybe it would be easier to write the x, y, n, m etc. stoichiometric coefficients.

Additionally, in the combustion reaction products H2 and O2 are listed, which in those conditions would probably react with each other. According to other works on the combustion synthesis the products are: N2, CO2, H2O (here is some review on the synthesis: A. Varma et.al.: "Solution Combustion Synthesis of Nanoscale Materials" (DOI: 10.1021/acs.chemrev.6b00279)).

Author Response

Response: Thank you for your comments. The new hypothesized chemical equation based on reviewer’s comments for the synthesis of 8YSZ through auto combustion is given in the attached file. Given the expertise of reviewer, if reviewer consider that it is not suitable we can remove the chemical equation from the manuscript.

Author Response File: Author Response.pdf