Review Reports

Round 1

Reviewer 1 Report

In this review, recent status and progresses on the materials and thin film fabrication techniques used in low-temperature solid oxide fuel cells (LT-SOFCs) are discussed. A detailed introduction is provided, interpreting the significance of this work. A comprehensive review on the efficiency of fuel cells, electrolyte and electrode materials, and ceramic processing techniques are presented, followed by forward-looking conclusions. This work has important implications on the field, I therefore recommend accepting it in its present form.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

I have reviewed the article entitled “Recent status and progress of thin film fabrication techniques for low-temperature solid oxide fuel cells” by B. Horri et al submitted to Crystals / MDPI. Among the various fuel cell systems, solid oxide fuel cells are believed to have the widest potential range of applications and highest efficiencies because of their all-solid-state design and high operating temperatures. The use of high processing temperatures increases the fabrication costs while materials costs are relatively high since the use of thick rare-earth-containing compounds as electrolytes/cathodes. Alternatively, the feasibility of thin-film electrolytes (using spin coating/magnetron sputtering deposition, etc.) for SOFC has become popular which cut the costs and cell mass leading to a higher power-to-mass ratio. The authors have comprehensively reviewed SOFC design and selected suitable materials that can be fabricated entirely using low-temperature, thin film processing.

This review is timely and valid, useful to the fuel cell and energy storage communities. The presented review paper itself is written OK but the main concern of this paper and the areas that require clarification are given below.

·         Table 1, yes Fuel cells show excellent efficiencies, but the capital cost is 10 times higher than fossil fuel counterparts, justify.

·         In the PDF version, why table 1 is repeated back-to-back, please check.

·         Introduction, lines 70 – 73 can be justified through simple fuel cell equations?

·         Do the cathodes and anodes differ in the FC system listed in Table 2? Just briefly mention it.

·         Line 84, page 3, what is the term “matter” negatively referred to?

·         Why perovskite-type oxides as electrode material YSZ has not been mentioned much?

·         Why porous structures are so crucial for SOFC?

·         The key paper on SOFC and its degradation behavior must be included doi.org/10.1021/acs.energyfuels.0c02920.

·         Page 4, lines 107 – 114, Whether the interfacial resistance influences the reaction kinetics and the temperature?

·         Page 5, line 166, what is powder synthesis?

·         The primary objectives of the current review must be summarised at the end of the introduction.

·         In equation 4 delta G is explained but not delta H.

·         17 figures appear to be way over.

·         The background and other details are excellent but too extensive. Can be trimmed.

·         What material is the current collector in Figure 6?

·         Page 17, line 718, what is the cobalt? Is this superior to that of Ni's counterpart?

·         Section 3.2.2, lines 1025 – 1027, it has been mentioned that CeO2 reduces the output voltage. However, Manickam Minakshi et al (2009) reported the CeO2 dopant in MnO2 enhances the battery performance. Please include and discuss. The role of BaO and MgO and their conductivity along with the Bi2O3 and their stability (discussed in the subsequent section) has also been explained by the same authors Minakshi et al . Please extend your discussion based on these.

·         What is the synergistic effect of co-dopant materials in the host compound and has any influence on the fuel cell performance characteristics?

·         Will the deposition rate, target to substrate thickness affect the electrolyte performance?

·         Section 3.2.3, line 1130 – what is the role of Co, Bi, and Ni in the gallium sites?

·         Page 26, page 1181, what is BIMEVOX?

·         Is the reactive magnetron sputtering technique good?

·         How does anodic aluminum oxide thin-film SOFC play a role?

·         All the subtitles in section 4 like CVD, PVD, and ALD must be explained.

·         The individual coating processes must be summarised with their pros and cons.

·         Section 4.2.4 The background of the combustion process and the oxidizers such as ammonium nitrate must be referred to in the literature (doi.org/10.1016/j.mtener.2018.08.004).

·         In section 4.2.4, the temperature required for combustion and their uniqueness relative to solid-state and sol-gel must be explained from the perspective of FC application and materials synthesis.

·         Section 5, line 1986, what is GDC?

·         The suitable synthesis methods (summarised from Section 4) should be included in the conclusions.

 

·         Will the LT affect the oxygen content?

In some places/discussions, the language could be improved. However, the intended meaning is clear.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The revised version is suitable for publication.