Experimental Procedures & First Results of an Innovative Solid Oxide Fuel Cell Test Rig: Parametric Analysis and Stability Test

Round 1

Reviewer 1 Report

Dear Authors,

The tile the paper is aiming to present findings on “Experimental procedures & first results of an innovative Solid Oxide Fuel Cell test rig: parametric analysis and stability test” which is a catchy thing because SOFC are devices suitable to address the environmental and climate change, issues associated with fossil fuel based electric power generation.

From abstract we learn the test testing is done for all-ceramic compact SOFC.

The introduction provides sufficient background information for readers not in the immediate field to understand the problem. However, I would like to make some remarks. As I progress in reading it makes me think that I am dealing with a review paper, rather than some research results of author's own work.

After a long introduction, line 29-87, we find out what this paper actually means to us. The way it is presented, in my opinion, suggests rather a dissertation / master thesis etc.

I think the paragraph “The paper is organized as follows……”- line 88- 99, should be reformulated and replaced earlier in introduction.

In my opinion, the numbering “1.1 Solid Oxide Fuel Cells – brief technology overview” of the only paragraph has no place. The story could be more flowing.

Overall, the introduction suggests a general review, and does not understand the specific problem that the authors want to solve / address.

For this reason, I suggest approaching the introduction in accordance with the problem to be developed.

In “Table 1. Fuel Cell technologies overview. Advantages and disadvantages at system and laboratory level” I would replace the bullets symbol to more discreet one.

Figure 3. Experimental SOFC test bench setup scheme from “2. Materials and Methods / 2.1. Experimental Setup & Procedures” contains 2 (two) pictures that have no explanations I suggest making a separate Figure with the necessary explanations if authors want to include them in this article.

Line 239-249, starting with  “In this work …..” I suggest to be included in the introduction.

The paragraph from line 255 to 273 I would recommend rather than a bulleted list, a paragraph with sentences.

The “start-up and reduction procedure” in my opinion is “Experimental campaign design”

In “2.2. Experimental campaign design” large parts I consider does not belong to this section, for example the paragraph starting with line 332 to 361; and should be revised and included perhaps in a different section.  

Between the “3. Results & Discussion” and “5. Conclusions” I do not see the section 4.

Author Response

The tile the paper is aiming to present findings on “Experimental procedures & first results of an innovative Solid Oxide Fuel Cell test rig: parametric analysis and stability test” which is a catchy thing because SOFC are devices suitable to address the environmental and climate change, issues associated with fossil fuel based electric power generation.

From abstract we learn the test testing is done for all-ceramic compact SOFC.

1. The introduction provides sufficient background information for readers not in the immediate field to understand the problem. However, I would like to make some remarks. As I progress in reading it makes me think that I am dealing with a review paper, rather than some research results of author's own work.

Authors reply: The authors thank the reviewer for the valuable comment.

The Introduction has been deeply reorganized, shortened and revised in order to provide a more concise overview of the background/context followed by a very brief description of the technology and its characteristics.

New lines 59-71 have been added to address the specific problem of the paper (electrochemical characterisation of SOFCs under different operating conditions) and novelty proposed by the authors (need of specific test rigs for testing such as the all-ceramic SOFC holder).

Ultimately the outline of the paper is provided to successfully guide the reader throughout the sections.

After a brief and concise introduction (as described above) the rest of the background information is provided in two separate subsections

• 1 Solid Oxide Fuel Cells – brief technology overview
• 2 Solid Oxide Fuel Cells – laboratory scale considerations

In section 1.1 the basic operating principles and literature performance trends are provided, this is necessary to have as a reference (e.g. Nernst equation and overvoltages to explain temperature effects) when discussing the results in Section 3.

Section 1.2 is thought to provide practical considerations regarding SOFC testing at laboratory scale. The authors believe that this is also an important aspect to show in order to understand the experimental setup and procedures.

The authors hope that in this way the introduction has been improved in a concise and to the point way which can be acceptable for the reviewer. The addition of the two subsections allow the reader to decide whether to go into more specific background in sections 1.1 or 1.2 or to directly proceed to section 2.

2. After a long introduction, line 29-87, we find out what this paper actually means to us. The way it is presented, in my opinion, suggests rather a dissertation / master thesis etc.

Authors reply: The introduction has been revised, see comment above

3. I think the paragraph “The paper is organized as follows……”- line 88- 99, should be reformulated and replaced earlier in introduction.

Authors reply: The outline of the paper (lines 88-99) has been moved earlier in the introduction in order to better navigate the reader throughout the sections of interest, see comment above

4. In my opinion, the numbering “1.1 Solid Oxide Fuel Cells – brief technology overview” of the only paragraph has no place. The story could be more flowing.

Authors reply: In the new version of the manuscript two subsections 1.1 and 1.2 have been added to the introduction section 1.

(see comment above)

In section 1.1 the basic operating principles and literature performance trends are provided, this is necessary to have as a reference (e.g. Nernst equation and overvoltages to explain temperature effects) when discussing the results in Section 3.

Section 1.2 is thought to provide practical considerations regarding SOFC testing at laboratory scale. The authors believe that this is also an important aspect to understand the experimental setup and procedures.

The authors believe that both subsections are important and necessary to correctly address the subsequent Methodology, Results & Discussion sections of the paper.

5. Overall, the introduction suggests a general review, and does not understand the specific problem that the authors want to solve / address.

Authors reply: The introduction has been deeply reorganized, shortened and revised, see comment above.

New lines 59-71 have been added to address the specific problem of the paper (importance of electrochemical characterisation of SOFCs under different operating conditions) and novelty proposed by the authors (need of specific test rigs for testing such as the all-ceramic SOFC holder).

6. For this reason, I suggest approaching the introduction in accordance with the problem to be developed.

Authors reply: See comment above

7. In “Table 1. Fuel Cell technologies overview. Advantages and disadvantages at system and laboratory level” I would replace the bullets symbol to more discreet one.

Authors reply: The bullet symbol has been modified accordingly.

8. Figure 3. Experimental SOFC test bench setup scheme from “2. Materials and Methods / 2.1. Experimental Setup & Procedures” contains 2 (two) pictures that have no explanations I suggest making a separate Figure with the necessary explanations if authors want to include them in this article.

Authors reply: All elements of Figures 4 and 5 have been explained and referenced in the text to introduce such pictures

9. Line 239-249, starting with  “In this work …..” I suggest to be included in the introduction.

Authors reply: As suggested by the reviewer, Lines 239-249 have been included earlier in the introduction.

The introduction has been deeply reorganized, shortened and revised, see comment above.

10. The paragraph from line 255 to 273 I would recommend rather than a bulleted list, a paragraph with sentences.

Authors reply: Lines 255-273 have been reworded in paragraphs instead of bulleted list. Each paragraph has been kept as a bullet in order to help the reader understand at a first glance the different phases.

11. The “start-up and reduction procedure” in my opinion is “Experimental campaign design”

Authors reply: As suggested by the reviewer the “start-up and reduction procedure” has been included in the Section 2.2 “Experimental campaign design”

12. In “2.2. Experimental campaign design” large parts I consider does not belong to this section, for example the paragraph starting with line 332 to 361; and should be revised and included perhaps in a different section.  

Authors reply: An additional subsection “2.3. Parameter calculation – Utilization Factor and Area Specific Resistance” has been added for what does not concern experimental procedures (i.e. line 332 to 361 as suggested by the reviewer, regarding UF and ASR parameter calculation) .

13. Between the “3. Results & Discussion” and “5. Conclusions” I do not see the section 4.

Authors reply: Thank you for pointing this out, this was a mistake in the title numbering. The conclusion section has been renumbered to 4. Conclusions.

Reviewer 2 Report

The present manuscript deals the “Experimental procedures & first results of an innovative Solid Oxide Fuel Cell test rig: parametric analysis and stability test”. The authors have reported detailed procedural analysis for SOFC test rig and first experimental testing results. In addition, they performed the stability test up to 140h under constant current load. This is a well written manuscript and I recommend to publish this paper after addressing following issues.

1) Please always mention (in the text as well as in the figures) the unit of I in (A.cm-2), P in (W.cm-and R in (Ω.cm2). Likewise, it will be easy to compare these data with the literature.

2) The authors have mentioned the “long term stability test up to 140 h”, in fact this in not the long term. Long term means at least up to 1000h. Please change it to short term stability test. Is there any specific reason to select 0.3 A.cm-2 for the stability test? This is indeed too low for practical application.

3) Line 107-108; “The denomination of said category of fuel cells is related to the electrolyte, which is composed of a solid-state cermet composite”. The electrolyte is not cermet, instead, the anode is composed of cermet. Please correct the sentence.

4) Line 129-130; GDC layer is only used for LSCF and not for LSM. The GDC layer is not used to improve the catalytic action. It is used to avoid the reactivity between LSCF and YSZ electrolyte. Please modify the sentences and add proper reference.

5) Line 141-142; instead of single repeating unit please write single cell.

Author Response

• Please always mention (in the text as well as in the figures) the unit of I in (A.cm-2), P in (W.cm-and R in (Ω.cm2). Likewise, it will be easy to compare these data with the literature.

Authors reply: The whole manuscript is revised (text and figures) to mention always surface-specific units of I in (A.cm-2), P in (W.cm-2)-and R in (Ω.cm2).

Furthermore the data visualization has improved and uniformed to those units.

Results are discussed in specific units and Table 3 has been added to compare the used testing conditions and results with examples of comparable literature

• The authors have mentioned the “long term stability test up to 140 h”, in fact this in not the long term. Long term means at least up to 1000h. Please change it to short term stability test. Is there any specific reason to select 0.3 A.cm-2 for the stability test? This is indeed too low for practical application.

Authors reply: The authors agree with the reviewers comment and have modified “long-term stability” to “short-term stability” throughout the paper, both in the text and in the Figures.

Regarding the current loading of the short-term stability test, the authors agree that 1 A (i.e. 0.3 A/cm2) is quite a low current load, even though in ref. [24] Bertoldi et al. (2017) a similar current load of 0.4 A/cm2 has been used at short stack level, with comparable cell type and manufacturer. Such current was chosen for two reasons:

• The first reason is merely precautionary, being the first step for this experimental setup the intention of the authors was to validate the ceramic cell holder & test rig rather than to push the fuel cell performance to higher current densities (0.5-1 A/cm2) which are more comparable with practical applications. The interest was more in verifying that the cell performance is stable in the used test bench throughout the test (140 h) and no intrinsic performance loss is seen (which, since the cell is operated in pure hydrogen, would be attributable only to the test bench setup). The authors acknowledge this limitation and have added a sentence in the manuscript to mention this (new lines 333-336). More work will be done in order to follow up on this aspect, increasing the current loading and extending the gas supply to other mixtures and contaminants other than pure H2. Likewise, the voltage threshold of 750 mV is quite conservative and can be reduced in future tests.
• Secondly, the authors are carrying out parallel test in another laboratory facility (ENEA C.R. Casaccia, Rome) where similar tests are being carried out at a load of 1 A (in this case the current collector is only 2 cm2 so the current density is 0.5 A/cm2). Therefore, the choice of the loading of 1 A is also related to being able to benchmark the cell performance with a different test bench, under same operating conditions in absolute value. However, the authors will take advantage of the comment by the reviewer and will repeat the short-term stability tests also for 0.5 A/cm2 in order have comparable datasets, which will be object of future work.

• Line 107-108; “The denomination of said category of fuel cells is related to the electrolyte, which is composed of a solid-state cermet composite”. The electrolyte is not cermet, instead, the anode is composed of cermet. Please correct the sentence.

Authors reply: The sentence has been corrected as follows “The denomination of said category of fuel cells is related to the electrolyte, which is composed of a ceramic oxide”

• Line 129-130; GDC layer is only used for LSCF and not for LSM. The GDC layer is not used to improve the catalytic action. It is used to avoid the reactivity between LSCF and YSZ electrolyte. Please modify the sentences and add proper reference.

Authors reply: Lines 129-130 have been modified according to the reviewer comment as follows : “an oxide (Gadolinia Doped Ceria “GDC”, Yttria Doped Ceria “YDC”) interlayer is usually added to avoid the reactivity between LSCF and YSZ electrolyte and avoid the diffusion of cathode compounds towards the electrolyte [27–29].”

References [29] and [30] were added to support this statement, while reference [28] is left since it already was mentioned in relation to the avoidance of migration of cathode compounds into the YSZ electrolyte.

• Line 141-142; instead of single repeating unit please write single cell.

Authors reply: The sentence has been corrected as suggested

Reviewer 3 Report

Dear Authors, 

Thank you for submitting your work to Energies. A few suggestions:

1) Expand your literature survey, and include some value of previous researchers' performance.

2) Change some of your x-axis from A to mA/cm^2, which helps people to compare.

3) No Section 4 before the section 5 Conclusion

4) Check your format and writing.

Author Response

• Expand your literature survey, and include some value of previous researchers' performance.

Authors reply: As suggested by the reviewer, the literature survey has been extended by adding the following references:

[23]       Aravind PV, Cavalli A, Patel HC, Recalde M, Saadabadi A, Tabish AN, et al. Opportunities and Challenges in Using SOFCs in Waste to Energy Systems. ECS Trans 2017;78:209–18. doi:10.1149/07801.0209ecst.

[29]       Railsback J, Choi SH, Barnett SA. Effectiveness of dense Gd-doped ceria barrier layers for (La,Sr)(Co,Fe)O 3 cathodes on Yttria-stabilized zirconia electrolytes. Solid State Ionics 2019;335:74–81. doi:10.1016/j.ssi.2019.02.020.

[30]       Kim SJ, Choi GM. Stability of LSCF electrode with GDC interlayer in YSZ-based solid oxide electrolysis cell. Solid State Ionics 2014;262:303–6. doi:10.1016/j.ssi.2014.01.001.

[62]       Cavalli A, Kunze M, Aravind P V. Cross-influence of toluene as tar model compound and HCl on Solid Oxide Fuel Cell anodes in Integrated Biomass Gasifier SOFC Systems. Appl Energy 2018;231:1–11. doi:10.1016/j.apenergy.2018.09.060.

[63]       Cavalli A, Bernardini R, Del Carlo T, Aravind PV. Effect of H2S and HCl on solid oxide fuel cells fed with simulated biosyngas containing primary tar. Energy Sci Eng 2019;7:2456–68. doi:10.1002/ese3.434.

[64]       Barelli L, Bidini G, Cinti G, Gallorini F, Pöniz M. SOFC stack coupled with dry reforming. Appl Energy 2017;192:498–507. doi:10.1016/j.apenergy.2016.08.167.

The additional literature has added throughout the paper and not in the introduction following the advice of reviewer #1 which has suggested to shorten the introduction rather than to extend it. From this a reorganization of the introduction section has been done.

Table 3 has been added in order to perform a comparative summary between some examples of examined literature and the present work in terms of SOFC type, Testing conditions, Technique, Used housing in order to compare numerical operating conditions and results.

• Change some of your x-axis from A to mA/cm^2, which helps people to compare.

Authors reply: The whole manuscript is revised (text and figures) to mention always surface-specific units of I in (A.cm-2), P in (W.cm-2)-and R in (Ω.cm2).

Furthermore the data visualization has improved and uniformed to those units.

Results are discussed in specific units and Table 3 has been added to compare the used testing conditions and results with examples of comparable literature

• No Section 4 before the section 5 Conclusion

Authors reply: Thank you for pointing this out, this was a mistake in the title numbering. The conclusion section has been renumbered to 4. Conclusions.

• Check your format and writing.

Authors reply: The manuscript has been checked by a native English speaker. The authors have double checked the manuscript in terms of format and have corrected format issues at the best of their capabilities. The authors believe that the format aspects will be dealt in more detail in a further proof/typesetting stage.