Scandium Doping Effect on a Layered Perovskite Cathode for Low-Temperature Solid Oxide Fuel Cells (LT-SOFCs)

Round  1

Reviewer 1 Report

I think it is an interesting article and very well carried  out. My only comment is that it would be nice to determine the cell  parameters of CGO and the different PBCs after the compatibility tests.  It might indicate some reaction not detected by simple visual inspection  of the XRD patterns

Author Response

Point 1

I  think it is an interesting article and very well carried out. My only  comment is that it would be nice to determine the cell parameters of CGO  and the different PBCs after the compatibility tests. It might indicate  some reaction not detected by simple visual inspection of the XRD  patterns

Response 1

Thanks  for your comment. As you mentioned, it would be better to determine the  cell parameters of GDC and the cathode materials. There is a small  difference of lattice parameters between a sample sintered 1150 oC and  the other sample sintered 950 oC. The difference in lattice parameters  can be considered to the difference in crystallinity due to the  different sintering conditions rather than due to the chemical reaction  between the electrolyte and the cathode material.

Author Response File: Author Response.pdf

Reviewer 2 Report

I suggest the publication of this manuscript after the following minor revisions:

1) Section 2:

Move  the following sentence (row 81 pag 2) “For the preparation of cathode  slurry, all pre-calcined electrode powders were blended with an organic  binder (Heraeus V006) to form slurries” when you describe the cells  preparation. Moreover, you should introduce some details about the  solvent, binder, dispersant and plasticizer used for the GDC suspension.

The  iodometric titration used for the estimation of O content must be  described in detail, what is the error of this analytic method?

2) Section 3:

Could  the lower ASR observed by Sc doping be related to other parameters such  as more compatibility between GDC and PBCSc in terms of thermal  expansion coefficient?

Author Response

Point 1. 

Move  the following sentence (row 81 pag 2) “For the preparation of cathode  slurry, all pre-calcined electrode powders were blended with an organic  binder (Heraeus V006) to form slurries” when you describe the cells  preparation. Moreover, you should introduce some details about the  solvent, binder, dispersant and plasticizer used for the GDC suspension.

Response 1. 

Thanks  for your comments. That sentence would be moved to the cell fabrication  part. Additionally, we added the introduction to the GDC suspension  solution.

We have moved that sentence in the manuscript as follows:

[Experimantal section]

GDC  suspension was prepared by dispersing GDC powder in the solution of  2-butanone and ethanol with a small amount of polyvinyl butyral as a  binder, plasticizers such as polyalkylene glycol and butyl benzyl  phthalate, and triethanolamine as a dispersant at a ratio of 1:10.  The GDC suspension solution was drop-coated to the Ni-GDC anode  support, then followed by drying in air and co-sintering at 1400 oC for 5  hours. For the  preparation of cathode slurry, all pre-calcined electrode powders were  blended with an organic binder (Heraeus V006) to form slurries. The cathode slurries were screen-printed on the surface of the electrolyte with an active cathode area of 0.36 cm2.

Point 2.

The  iodometric titration used for the estimation of O content must be  described in detail, what is the error of this analytic method?

Response 2. 

We have added the detail sentence describing the iodometric titration.

We have added the detail sentence in the manuscript as follows:

[Experimantal section]

The iodometric titration was performed to measure the oxygen contents at room temperature. The  samples were dissolved in HCl under inert gas atmosphere to prevent the  oxidation of the I- ions. The solution was titrated with sodium  thiosulfate solution. Regarding the error of this analytic method, the  standard deviation values of the samples are calculated.

[Results section]

The oxygen contents (5+δ) of samples at room temperature were determined by the iodometric titration. Average  δ values of PBCO and PBCSc at room temperature were 0.6833, 0.6367 with  standard deviation values of 8.916 × 10-3 and 9.251 × 10-3,  respectively. In the temperature range of 100 ~ 900 oC, the Sc3+ substitution into PBCO leads to relatively decreases the oxygen content.

Point 3. 

Could  the lower ASR observed by Sc doping be related to other parameters such  as more compatibility between GDC and PBCSc in terms of thermal  expansion coefficient?

Response 3.

This  is a good point, the electrocatalytic performance of single cells can  be affected by the thermal expansion coefficient. In this study, we have  carefully observed the interface between the cathode and electrolyte by  the SEM measurement. The delamination between them was not observed  from the cells used in this work, indicating that the Sc-doped PBCO and  GDC electrolyte are mechanically compatible.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper is well written and the results are well presented. The authors need to clarify the following aspects:

Page 6 Line 184, the fitting method for obtaining R2 and R3 are not presented in the paper, please specify.

Fig.  6: OCV value is low for IV curves. At 550 C with 3% humidified H2, the  theoretical value is ~1.14 V. Can the authors explain the much lower  OCV? Also, OCV for fig.6(a) is lower than those of fig.(b) and (c)

The  high power density of PBCSc and its corresponding ohmic resistances  from impedance spectra look unreal to me, For example from Figure S2b,  ohmic at 550 C is 0.03-0.04 ohm cm2. Assuming all the ohmic reistance is  from electrolyte (which is not the case), for 10 micron GDC electrolyte  and 0.36 cm2 active area, the electrical conductivity of GDC in this  paper is ~0.07-0.09 S/cm. This value is ~1 order of magnitude higher  than the reported electrical conductivity of GDC. Can the authors  explain?

Author Response

Point 1.

Page 6 Line 184, the fitting method for obtaining R2 and R3 are not presented in the paper, please specify.

Response 1

Thanks for your comments. We have added details of the fitting method.

We have added wording in the manuscript as follows:

[Experimantal section]

~  Impedance spectra of the symmetrical cell in the air were measured  under 400 ~ 600 oC under OCV in a frequency range of 1 mHz to 500 kHz  with 10 mV AC perturbation with  curve fitting using software EC-lab. Impedance spectra of the samples  were fitted to equivalent circuits shown in Fig.5 (a).

Point 2.

Fig.  6: OCV value is low for IV curves. At 550 C with 3% humidified H2, the  theoretical value is ~1.14 V. Can the authors explain the much lower  OCV? Also, OCV for fig.6(a) is lower than those of fig.(b) and (c)

Response 2.

This  is a good point. As the reviewer mentioned, the single cell having a  GDC electrolyte generally shows lower OCV than that of theoretical value  due to the Ce ion reduction from Ce4+ to Ce3+, resulting in the  conduction of electron. [1]

The  value of OCV can be slightly changed depending on the electrocatalytic  activity of the electrode. [2] In our research, the difference of OCV  between the single cells are less than only 5 % at 450 oC as shown in  the table below. This is could be originated from the different  electrocatalytic activities of Sc-doped PBCOs.

 [Reference]

  1.  Chiodelli, G.; Malavasi, L. Electrochemical open circuit voltage (OCV) characterization of SOFC materials. Ionics. 2013, 19, 1135-1144

            2.  Noren, D.A.; Hoffman, M.A. Clarifying the Butler-Volmer equation and  related approximations for calculating activation losses  in solid oxide fuel cell models. J. Power Sources. 2005, 152, 175-181
Point 3.

The  high power density of PBCSc and its corresponding ohmic resistances  from impedance spectra look unreal to me, For example from Figure S2b,  ohmic at 550 C is 0.03-0.04 ohm cm2. Assuming all the ohmic resistance  is from electrolyte (which is not the case), for 10 micron GDC  electrolyte and 0.36 cm2 active area, the electrical conductivity of GDC  in this paper is ~0.07-0.09 S/cm. This value is ~1 order of magnitude  higher than the reported electrical conductivity of GDC. Can the authors  explain?

Response 3.

Thank you for valuable comment. As you can see in Figure S2b, ohmic resistances at 550 oC are 0.06 ~ 0.08 Ω cm2. These values of ohmic resistance were also reported in other literatures using the GDC as an electrolyte. [1, 2]

 [Reference]

1. Yoo,  S.; Jun, A.; Ju, Y.-W.; Odkhuu, D.; Hyodo, J.; Jeong, H.Y.; Park, N.;  Shin, J.; Ishihara, T.; Kim, G. Development of double-perovskite  compounds as cathode materials for low-temperature solid oxide fuel  cells., Angew. Chem. Int. Ed. 2014, 53, 13064–13067

         2. Yoo, S.; Kim, J.; Song, S.Y.; Lee, D.W.; Shin, J.; Ok, K.M.; Kim, G.  Structural, electrical and  electrochemical  characteristics of La0.1Sr_0.9Co_1−xNb_xO_3−δ as a cathode material for intermediate  temperature solid oxide fuel cells, RSC Adv., 2014, 4, 18710

Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript deals with an interesting way to improve electrochemical properties of the double perovskite PrBaCo₂O_5+δ, i.e by doping with Sc.

It is acceptable for publication, although some minor weak points should be addressed before. In particular:

There  is some confusion in the identification of samples and the amount of  Sc. It looks like it was planned to explore the general composition PrBaCo_2-xSc_xO_5+δ,  with x=0, 0.1, 0.2 but, due to the change in structure with increasing  Sc content, a different label (SP-PBCSc) was introduced for x=0.2.

I would recommend to better explain the meaning of "Sc content" in table 1, which might be sufficient to remove ambiguity.

An  indication of the error on the measurement of Oxygen stoichiometry  (from both iodometric titration and TG) should be given in order to  support significance of the difference between curves plotted in figure  3a.

The  statement "PBCSc shows smaller electrical conductivity than PBCO due to  the disturbance of electron conduction, which originated from the  substitution of constant valence state Sc3+ ion for multivalent Co ion"  is not clear and should be reformulated. Electron hopping between  cations through 2p orbitals of Oxygen is negatively affected by Sc  orbitals. Authors are encouraged to better describe this phenomenon to  explain the reduced conductivity in Sc-doped material.

English should be revised as there are some errors all along the text.

Author Response

Point 1.

There  is some confusion in the identification of samples and the amount of  Sc. It looks like it was planned to explore the general composition  PrBaCo2-xScxO5+δ, with x=0, 0.1, 0.2 but, due to the change in structure  with increasing Sc content, a different label (SP-PBCSc) was introduced  for x=0.2.

I would recommend to better explain the meaning of "Sc content" in table 1, which might be sufficient to remove ambiguity.

Response 1. 

Thanks for your comments. We have added the detail explanation about the ‘Sc content’ in the manuscript.

We have added wording in the manuscript as follows:

[Results section]

Crystal structures and the calculated lattice parameters were summarized in Table 1. PrBaCo_1.8Sc_0.2O_5+δ is labeled with SP-PBCSc because of its different crystal structure with PrBaCo_1.8Sc_0.2O_5+δ. The samples for Sc3+ ≤ 0.1 represent the layered perovskite phase.

Point 2.

An  indication of the error on the measurement of Oxygen stoichiometry  (from both iodometric titration and TG) should be given in order to  support significance of the difference between curves plotted in figure  3a.

Response 2.

Oxygen  stoichiometry in Figure. 3a is based on the oxygen contents obtained by  iodometric titration and the weight change measured by  thermogravimetric analysis (TGA). Therefore, it would be better to show  the deviation of the measured oxygen stoichiometry at room temperature  in the manuscript than to add the indicator of error in Figure. 3a. We  have added the deviation of the iodometric titration results in the  manuscript.

We have added wording in the manuscript as follows:

[Results section]

The oxygen contents (5+δ) of samples at room temperature were determined by the iodometric titration. Average  δ values of PBCO and PBCSc at room temperature were 0.6833, 0.6367 with  standard deviation values of 8.916 × 10-3 and 9.251 × 10-3,  respectively. In the temperature range of 100 ~ 900 oC, the Sc3+ substitution into PBCO leads to relatively decreases the oxygen content.
Point 3.

The  statement "PBCSc shows smaller electrical conductivity than PBCO due to  the disturbance of electron conduction, which originated from the  substitution of constant valence state Sc3+ ion for multivalent Co ion"  is not clear and should be reformulated. Electron hopping between  cations through 2p orbitals of Oxygen is negatively affected by Sc  orbitals. Authors are encouraged to better describe this phenomenon to  explain the reduced conductivity in Sc-doped material.

Response 3.

In  the case of the Co-rich cathode materials, the electron transport  between B site cations in the cathode material lattice is intervened by  2p oxygen orbitals, known as the Zerner double-exchange. As you  mentioned, constant valence state of Sc3+ reduces the overlapping  between B site cations with the oxygen. We have added the more detail  explanation in the manuscript as follows:

We have added wording in the manuscript as follows:

[Results section]

PBCSc  shows smaller electrical conductivity than PBCO due to the disturbance  of electron conduction, which originated from the substitution of  constant valance state Sc3+ ion for multivalent Co ion [24, 40, 41]. Substitution  of the Sc3+ reduces the overlapping between multivalent Co ion with the  oxygen 2p orbitals and decreases the electron transport between the  multivalent cobalt cations, which is known as Zener double exchange.

Author Response File: Author Response.pdf

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