Progress in the Development of Electrodeposited Catalysts for Direct Liquid Fuel Cell Applications

Round 1

Reviewer 1 Report

In this manuscript, the developments in the electrodeposition of Pt and Pd-based catalysts on various supports with conventional supporting electrolytes are summarized. Such a method of preparation for the electrocatalyst is very effective at improving the performance of electrochemical reactions in DLFCs, with Pt-based for direct alcohol-based FCs and Pd-based for direct formic acid FCs. Remarkable progress has been observed in producing shape-controlled Pt, Pd-based catalysts via template free approach. With a better understanding, electrodepositing metal catalysts with different shapes and structures on metal support could be a potential way forward for their commercialization. The end of this review also outlined the major challenges to focus on in the future, which is intended to potentially benefit the research community working in this field.

I consider the content of this manuscript will definitely meet the reading interests of the readers of the Applied Sciences journal. Therefore, I suggest giving a minor revision and the authors need to clarify some issues or supply some more data to enrich the content.

1. Abstract and Introduction

• For the title, ‘direct liquid fuel cells’ should be mentioned, since this review is not so generalized for all types of fuel cells. The same applies to the Keywords.

• Line 34, ‘Hence, these continue to serve as potential energy conversion devices and contribute to the renewable energy sector [1].’

   Since the journal does not only focus on batteries or energy, it is suggested that the description of this part can be properly expanded. For example, what is the difference between DLFC to the traditional PEMFC/DMFC? What is the problem with renewable energy? ‘The power/energy generation is not always continuous, that is, intermittent. This opens spatial and temporal gaps between the availability of the energy and its consumption by end-users’ [Electrochimica Acta 309 (2019): 311-325]. In order to address these issues, it is necessary to develop suitable energy storage systems for the power grid. That is why, to ensure the stability of energy supply based on renewable energy sources, the energy storage system is needed [Electrochimica Acta 336 (2020): 135646].

• Line 78, ‘While the quest for low-cost electrodes for fuel cells has been a focus of attention, extensive efforts have been made to investigate electrodeposited noble metals as fuel cell electrodes.’

    This sentence makes certain confusion. I consider it is not to use ‘electrodeposited noble mteals’ as electrodes but to employ the ‘carbon electrode decorated with electrodeposited noble mteals’ as the electrodes.

• At the end of the Introduction, I should point out that pay attention to the grammar and spelling throughout the manuscript. For example, in Line 100, ‘Pd-based nanoparticles are regarded as an ideal alternative catalystsfor oxygen reduction reactions (ORR).’ It should be ‘an ideal catalyst’ or ‘ideal catalysts’, but not ‘an ideal catalysts’.

2. Progress in the electrodeposition of carbon-supported Pt and Pd catalysts

• Pay attention to whether the use of prepositions and verb collocations are accurate. For example, in Line 149, should be ‘ might be influenced by the Pt specific surface area’. Line 161, ‘devoted to’

• Line 208, ‘Fuel cell polarization studies showed that the MEA containing the pulsed electrodeposited cathode was higher than that of the conventional Pt/C electrodes.’

It is not clear at all, what is higher than that of the conventional electrodes? Which property? It should be further clarified.

• Line 228, it should be ‘2.2 Palladium – a possible alternative to platinum’.

In addition, Line 230, ‘However, the fuel crossover through the Nafion membrane and the costs with increased Pt metal catalyst loadings and methanol are the two major obstacles affecting the performance of the cell drastically. ’ It is not difficult to understand the fuel crossover through the membrane. But why ‘the costs with increased Pt metal catalyst loadings and methanol’? Why for DMFC catalyst loadings need to be higher than other fuel cell systems? And why does DMFC also increase the cost of methanol, which is quite cost-effective as described previously?

• Line 287, in the whole paragraph, ‘SWNT’should be ‘SWCNT’, since they are carbon nanotubes.

• Line 303, ‘because of its excellent electrical property, chemical stability’. The expression of juxtaposition should be 'A, B, and C', not 'A, B, C'. It is suggested that the authors check whether the juxtaposition relationship of the full text is in the correct form.

• Line 334, for carbon fibers (CFs), it is correct to describe it as ‘a similar 3D structure support’. However, there are other important benefits for CFs being used as electrodes, including the larger specific surface area and better three-dimensional network structures, as well as high chemical stability and electrical conductivity. The significant structural transformations of CFs occur during the carbonization step of the production process [International Journal of Energy Research 44.5 (2020): 3839-3853].

• The second part is a purely written description, too long and boring, without representative experimental results and chart display.It seems that it is just a list of some existing experimental results. Appropriate modifications are recommended to enhance the readability and linearity of the results. This also applies to Part 3.

3. Electrodeposition on non-carbon substrates

• Line 359, ‘The authors demonstrated that the electrodeposition conditions influenced the morphological characteristics. size distribution of the Pd nanostructureswill have possible applications in electrocatalysis and nanotechnology.’

The bold part is very confusing. If they are two sentences, it should be ‘The size distribution of .....’

• Line 391, it should be ‘good dispersion’, not ‘well dispersion’.

4. Shape control – A way forward

• Line 471, ‘Li et al.[68] aimed at improving the performance of PEMFC ORR cathode by minimizing the Pt loading in the electrode.’It should be ‘improving the ORR performance of the PEMFC cathode....’ or ‘improving the performance of PEMFC cathode ORR... ’.

• Line 597, ‘Besides, the development of Pd structures directly on carbon(graphene, wood apple carbon, carbon nanotubes, Vulcan) with varying morphologies such as planar, microspheres, tubular, mesoporous, etc.) coated carbon displayed a 3-4-fold increase in electrocatalytic activity in comparison to conventional Pd deposit.’ This sentence is very confusing. The Pd structures are formed on carbon or coated carbon? What is the relation between carbon and coated carbon, or only coated carbon should be used?

• Line 636, ‘3-4 catalytic activity enhancement ’should be ‘3-4 times enhancement’.

• Line 680, ‘He et [93] attempted to...’

5. Fuel Cell Polarization

• Line 787, The sentence ‘For DLFCs to function effectively, optimal electrocatalysts are’ is not completed yet.

• Line 837, ‘Multiple efforts were made to optimize the various Ti mesh-based MEA topologies for use in passive DMFCs.’

6. Other comments

• How about the technique of Electrospray Deposition? I consider this is also related to electrodeposition, and it is definitely different from the conventional methods. I suggest also covering this topic briefly [ACS Applied Energy Materials 4.8 (2021): 7394-7404].

• For electrodeposition, it seems Pt-Fe is not mentioned much (more Ti is mentioned). This part should also be introduced briefly [The Journal of physical chemistry c 115.5 (2011): 2483-2488].

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The review is exhaustive manuscript about the synthesis of fuel cell catalysts through electrodeposition for direct liquid fuel cells (DLFCs). There are 113 references to support this long review that provides an overview of the performances of electrodeposited Pt and Pd based catalysts. Similar to many reviews, this work presents the evolution and advances that have been reported in the literature over the years. This approach is certainly useful but it would be important to highlight in a more exhaustive way that many of the works mentioned are carried out in a three-electrode cell and not in a real fuel cell where the performance of the catalyst can be very different. Although the section “Summary and Future Challenges” highlights this point, the issue is almost underestimated in the first part of the review. Especially the first part, although very exhaustive of references, appears as a long list of works and lacks critical aspects and personal evaluations that can help future researchers in the synthesis of catalysts by electrodeposition. As a consequence, this part is somehow not homogeneous and difficult to read. Adding figures could make the text easier for the reader. As examples:

The section “Progress in the electrodeposition of carbon supported Pt and Pd catalysts” section 1.1 lacks figures that could help the reader follow the text more effectively.

Pag 5-6 lines 243-269 please review this part which appears as a "copy-paste" of Ref [36]. Reference [36] shows a schematic figure of the production of the catalytic layer on graphite, I suggest to use a similar scheme and revise the text accordingly.

Similar consideration for pag 9 lines 440-446: the anisotropic growth (not “anistropic”-line 444) of Pd should be supported by a Figure (XRD or FESEM), to help the reader understand the phenomenon.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Hi, Authors,

I think the overall quality of the paper is good, and I recommend to publish this paper in Applied Sciences. If possible, authors could add some measured EIS data (e.g. electrochemical surface area, activation loss) in the table for comparisons. 

Thank you. 

Author Response

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Author Response File: Author Response.pdf