Cathode Catalyst PdAgCo/C for Optimal Performance of the Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

     In the manuscript the authors studied the enhancement of ammonia fuel cell performance using Pd (palladium) and Co (cobalt) doped cathode catalysts. It could be a good paper if well written. However, the structure of the manuscript is weak. Section 2 and 3 should be switched with each other. In addition, the writing quality of the manuscript needs to be improved. A few discussions are not convincible. There are also typos that need to be fixed. Some examples are listed below. Major revision and further review are needed.

1. P1, L21. “15wt% PdAgCo/C (15wt% PdAg and 85wt% C)” in the abstract.

Comment: 15wt% PdAg and 85wt% C, 15wt%+ 85wt% =100wt%, then what is the weight percent of Co?

2. P2, L61. “and the chances corrosion”.

Comment: Grammar issue.

3. P2, L63-66. “including the sluggish electrochemical reaction kinetics due to the lower operating temperature, subsequently pitching a strong requirement for an effective electrocatalyst that would promote the ammonia oxidation reaction(AOR) [7], thus enhancing the overall performance of the fuel cell.”.

Comment: A little clumsy. Need to improve the writing.

4. P3, L109. “1634”.

Comment: It is 1632 in Fig. 1.

5. P3, L111. “C=C peak at 1544”.

Comment: It is 1632 in Fig. 1.

6. P3, L111. “C=O peak at 1256”.

Comment: It is 1750-1705 in Fig. 1.

7. P5, Fig. 4.

Comment: Fig. 4 should be introduced before being discussed.

8. P5, L139. “Fig. 6(a)”.

Comment: Should be “Fig. 4(a)”.

9. P5, L146. “Figure 5.”.

Comment: Figure 5 should be introduced before being discussed.

10. P6, L163. “As shown in Figure 6, with the adding NH4OH can improved the ORR activity compare to only adding KOH.”.

Comment 1: Grammar issue. Not a complete sentence.

Comment 2: Figure 6 should be introduced before being discussed.

11. P6, L175. “as shown in Table 4.”.

Comment: Table 4 should be introduced before being discussed.

12. P7, L191. “as shown in Table 5.”.

Comment: Table 5 should be introduced before being discussed.

13. P7, L193. “concluding the optimal performance at 110°C.”.

Comment: This statement is not convincible. If one increase operating temperature to 120°C, one may see that the power density will also increase. Thus, one can say that the optimal performance is not 110°C.

14. P8, L196. “as shown in Table 6.”.

Comment: Table 6 should be introduced before being discussed.

15. P8, L193. “best power density performance, reaching 40 mW/cm,”.

Comment: This statement is not convincible. If one increase the cathode humidification temperature to 100°C, one may see that the power density will also increase. Thus, one can say that the optimal performance is not 100°C.

16. P8, L209. “shown in Figure 7 and Table 7”.

Comment: Figure 7 and Table 7 should be introduced before being discussed.

17. P8, L213. “as in Table 8.”.

Comment: Table 8 should be introduced before being discussed.

18. P8, L222. “as shown in Figure 8 and Table 8.”.

Comment: Should be “as shown in Figure 8 and Table 9.”. Figure 8 and Table 9 should be introduced before being discussed.

18. P9, L230. “by Figure 9 and Table 9.”.

Comment: Should be “by Figure 9 and Table 10.”. Figure 9 and Table 10 should be introduced before being discussed.

19. P9, L232. “Figure 9.”.

Comment: Should be “Figure 7”.

20. P10, L236. “Figure 10.”.

Comment: Should be “Figure 8”.

21. P10, L240. “Figure 11.”.

Comment: Should be “Figure 9”.

22. P11, L279. “Figure 12.”.

Comment: Should be “Figure 10”.

23. P12, L285. “Figure 12.”.

Comment: Should be “Figure 10”.

24. P13, L352. “13(a).”.

Comment: Should be “11(a)”.

25. P13, L354. “Figure 13.”.

Comment: Should be “Figure 11”.

 

Comments on the Quality of English Language

English is good. But there are some grammar issues that need to be addressed. There are also typos that need to be fixed.

Author Response

In the manuscript the authors studied the enhancement of ammonia fuel cell performance using Pd (palladium) and Co (cobalt) doped cathode catalysts. It could be a good paper if well written. However, the structure of the manuscript is weak. Section 2 and 3 should be switched with each other. In addition, the writing quality of the manuscript needs to be improved. A few discussions are not convincible. There are also typos that need to be fixed. Some examples are listed below. Major revision and further review are needed.

 

P1, L21. “15wt% PdAgCo/C (15wt% PdAg and 85wt% C)” in the abstract.

Comment: 15wt% PdAg and 85wt% C, 15wt%+ 85wt% =100wt%, then what is the weight percent of Co?

Ans. It is revised to “15wt% PdAgCo/C (15wt% PdAgCo and 85wt% C)” at L21.

 

P2, L61. “and the chances corrosion”.

Comment: Grammar issue.

Ans. It is revised to “the risk of corrosion” at L61.

 

P2, L63-66. “including the sluggish electrochemical reaction kinetics due to the lower operating temperature, subsequently pitching a strong requirement for an effective electrocatalyst that would promote the ammonia oxidation reaction(AOR) [7], thus enhancing the overall performance of the fuel cell.”.

Comment: A little clumsy. Need to improve the writing.

Ans. The sentence is improved “this challenge arises from the sluggish electrochemical reaction kinetics at lower operating temperatures, which underscores the strong need for an effective electrocatalyst to promote the ammonia oxidation reaction (AOR)[7] and thereby enhance the overall performance of the fuel cell.” at L63-66.

 

P3, L109. “1634”.

Comment: It is 1632 in Fig. 1.

Ans. It is revised to 1632 cm^-1 at L224.

 

P3, L111. “C=C peak at 1544”.

Comment: It is 1632 in Fig. 1.

Ans. It is revised to 1632 cm^-1 at L226.

 

P3, L111. “C=O peak at 1256”.

Comment: It is 1750-1705 in Fig. 1.

Ans. It has been revised to 1750-1705 cm^-1 at L226.

 

P5, Fig. 4.

Comment: Fig. 4 should be introduced before being discussed.

Ans. Because Reviewer 4 recommend change the chapter 3 to chapter 2. So, the sequence is change. “In Figure 6(a)” is put at the front of the sentence at L251. Also “Figure 6(b)” is changed to front sentence at L256.

 

P5, L139. “Fig. 6(a)”.

Comment: Should be “Fig. 4(a)”.

Ans. It has been revised to Figure 6(a) at L251.

 

P5, L146. “Figure 5.”.

Comment: Figure 5 should be introduced before being discussed.

Ans. Figure 8 has changed to the position at L259.

 

P6, L163. “As shown in Figure 6, with the adding NH4OH can improved the ORR activity compare to only adding KOH.”.

Comment 1: Grammar issue. Not a complete sentence.

Ans. It has been revised to “As shown in Figure 6, adding NH₄OH enhances the ORR activity relative to KOH alone.” at L279.

 

Comment 2: Figure 6 should be introduced before being discussed.

Ans. The sentence “As shown in Figure 6,” is added at L277.

 

P6, L175. “as shown in Table 4.”.

Comment: Table 4 should be introduced before being discussed.

Ans. The sentence “As shown in Table 4,” is added at L289.

 

P7, L191. “as shown in Table 5.”.

Comment: Table 5 should be introduced before being discussed.

Ans. The sentence “Table 5 shows” is added at L306.

 

P7, L193. “concluding the optimal performance at 110°C.”.

Comment: This statement is not convincible. If one increase operating temperature to 120°C, one may see that the power density will also increase. Thus, one can say that the optimal performance is not 110°C.

Ans. The sentence has been revised to “concluding the optimal performance at 110°C among temperature from 80°C to 110°C.”. at L317.

 

P8, L196. “as shown in Table 6.”.

Comment: Table 6 should be introduced before being discussed.

Ans. The sentence “Table 6 shows” is added at L326.

 

P8, L193. “best power density performance, reaching 40 mW/cm,”.

Comment: This statement is not convincible. If one increase the cathode humidification temperature to 100°C, one may see that the power density will also increase. Thus, one can say that the optimal performance is not 100°C.

Ans. The statement has revised to “the ammonia fuel cell has better power density performance, reaching 40 mW/cm².” at L317.

 

P8, L209. “shown in Figure 7 and Table 7”.

Comment: Figure 7 and Table 7 should be introduced before being discussed.

Ans. It has revised to “Figure 9 and Table 7 shows that…” at L326.

 

P8, L213. “as in Table 8.”.

Comment: Table 8 should be introduced before being discussed.

Ans. It has revised to “Table 8 shows…” at L330.

 

P8, L222. “as shown in Figure 8 and Table 8.”.

Comment: Should be “as shown in Figure 8 and Table 9.”. Figure 8 and Table 9 should be introduced before being discussed.

Ans. It has revised to “In Figure 10 and Table 9,” at L335.

 

P9, L230. “by Figure 9 and Table 9.”.

Comment: Should be “by Figure 9 and Table 10.”. Figure 9 and Table 10 should be introduced before being discussed.

Ans. It has revised to “In Figure 9 and Table 10,” at L345.

 

P9, L232. “Figure 9.”.

Comment: Should be “Figure 7”.

Ans. It maintain “Figure 9” at L349.

 

P10, L236. “Figure 10.”.

Comment: Should be “Figure 8”.

Ans. It maintain “Figure 10” at Line353.

 

P10, L240. “Figure 11.”.

Comment: Should be “Figure 9”.

P11, L279. “Figure 12.”.

Comment: Should be “Figure 10”.

P12, L285. “Figure 12.”.

Comment: Should be “Figure 10”.

P13, L352. “13(a).”.

Comment: Should be “11(a)”.

P13, L354. “Figure 13.”.

Comment: Should be “Figure 11”.

Ans. The sequence of Figures haves already changed and revised due to the recommend of reviewer 4. The end of is Figure 11 and Table 10.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this work the ammonia fuel cell performance using a PdAgCo cathode catalyst has been invstigated. This is an interesting work, which can be accepted after the following point is addressed. In Fig.6 the electrochemical oxygen reduction reaction (ORR) activity of the Pd/C catalyst has to be reported for comparison.

Author Response

In this work the ammonia fuel cell performance using a PdAgCo cathode catalyst has been investigated. This is an interesting work, which can be accepted after the following point is addressed. In Fig.6 the electrochemical oxygen reduction reaction (ORR) activity of the Pd/C catalyst has to be reported for comparison.

Comment: We have added the Pd/C catalyst in the manuscript. Suggesting that PdAgCo/C can be one of the ORR-catalyst among Pt/C, Ag/C, and Pd/C with the following sentence “As shown in Figure 8, cathode catalyst loading is setting at 0.5 mg/cm², electrolyte 3M KOH and 3M KOH+3M NH₄OH, rotation rate at 1600 rpm, scan rate = 10 mV/s on rotating disk electrode (RDE). Adding NH₄OH enhances the ORR activity relative to KOH alone. For PdAgCo/C, E₁/₂ is 0.48 V in 3 M KOH and 0.52 V in 3 M KOH + 3 M NH₄OH, suggesting that PdAgCo/C can act as a Pt-free ORR catalyst, offering an alternative to Pt/C, Ag/C, and Pd/C.”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study develops a novel 15wt% PdAgCo/C cathode catalyst to enhance the performance of Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells (AEM-DAFCs). Building upon a baseline Ag/C cathode, doping with Pd and Co yielded PdAgCo/C, significantly increasing power density to 74 mW/cm². Further optimization was achieved by employing a high-performance 40wt% PtIr/C anode catalyst and optimizing cathode catalyst loading, reaching 172 mW/cm². Comprehensive physicochemical (FTIR, SEM-EDS, XRD) and electrochemical (ORR activity) characterizations confirm catalyst synthesis and elucidate the enhanced performance attributed to the synergistic effects of Pd, Ag, and Co. This work demonstrates PdAgCo/C as a promising, high-performance cathode catalyst for low-temperature ammonia fuel cells.
I recommend the manuscript for publication after the authors respond to comments and make clarifications to the study.

1. In my opinion, the disadvantage of the work is limited novelty/benchmarking. While PdAgCo/C is novel in this context, the manuscript lacks sufficient comparison with state-of-the-art ORR catalysts for AEMFCs or DAFCs beyond Pt/C and its own baselines (Ag/C, PdAg/C). How does PdAgCo/C compare to other promising non-Pt catalysts (e.g., Fe-N-C, other alloys) reported in the literature, especially concerning cost? Claims like "better than conventional catalysts" need stronger benchmarking support.

2. The rationale for the specific Pd:Ag:Co ratio (implicitly 1:1:1 based on precursors?) is not provided. Why this composition?

3. The claim that Co doping improves "electrochemical stability" (Introduction, Conclusions) is not substantiated by any stability tests.

4. The manuscript is missing Figures 7 and 8.

5. Performance Benchmarking: How does the peak power density of 332 mW/cm² achieved with PdAgCo/C and PtIr/C compare to other recently reported high-performing AEM-DAFC systems using different catalysts (especially non-Pt group metals)? Please add a comparative discussion in the Results or Discussion section.

6. Section 3.2.2 mentions washing modified carbon black to neutrality. How was neutrality confirmed (pH measurement)? For the PdAgCo/C synthesis, were the precursors (K₂PdCl₄, AgNO₃, CoCl₂) added simultaneously? What was the pH during reduction? These details impact particle size/distribution.

Author Response

This study develops a novel 15wt% PdAgCo/C cathode catalyst to enhance the performance of Alkaline Anion Exchange Membrane Direct Ammonia Fuel Cells (AEM-DAFCs). Building upon a baseline Ag/C cathode, doping with Pd and Co yielded PdAgCo/C, significantly increasing power density to 74 mW/cm². Further optimization was achieved by employing a high-performance 40wt% PtIr/C anode catalyst and optimizing cathode catalyst loading, reaching 172 mW/cm². Comprehensive physicochemical (FTIR, SEM-EDS, XRD) and electrochemical (ORR activity) characterizations confirm catalyst synthesis and elucidate the enhanced performance attributed to the synergistic effects of Pd, Ag, and Co. This work demonstrates PdAgCo/C as a promising, high-performance cathode catalyst for low-temperature ammonia fuel cells.

I recommend the manuscript for publication after the authors respond to comments and make clarifications to the study.

 

1. In my opinion, the disadvantage of the work is limited novelty/benchmarking. While PdAgCo/C is novel in this context, the manuscript lacks sufficient comparison with state-of-the-art ORR catalysts for AEMFCs or DAFCs beyond Pt/C and its own baselines (Ag/C, PdAg/C). How does PdAgCo/C compare to other promising non-Pt catalysts (e.g., Fe-N-C, other alloys) reported in the literature, especially concerning cost? Claims like "better than conventional catalysts" need stronger benchmarking support.

Ans. We compare different ORR catalysts for AEMFCs or DAFCs. Then, compare non-Pt catalysts Fe-N-C to our study(PdAgCo/C) as the following:

Table 11 shows the AEMFCs and DAFCs comparison of cathode catalyst between non-Pt group metals and Pt group metals for state-of-the-art ORR catalysts. Non-Pt group metals can indeed achieve higher performance than Pt group metals by enhance the loading or higher temperature. In the research field of DAFCs, non-Pt catalysts such as Fe-N-C have the power density of 107 mW/cm² while Pt/C have the power density of 158.6 mW/cm². Compare to Pt/C catalyst, the Fe-N-C catalyst can reach even lower ammonia water concentration at same loading.

2. The rationale for the specific Pd:Ag:Co ratio (implicitly 1:1:1 based on precursors?) is not provided. Why this composition?

Ans. The sentence is added at L25 with following “Through the EDS analysis, the weight ratio of Pd, Ag and Co are 6.01%, 6.30% and 6.01% respectively. After calculate, the ratio of Pd:Ag:Co is 1:1:2.”.

 

 

3. The claim that Co doping improves "electrochemical stability" (Introduction, Conclusions) is not substantiated by any stability tests.

Ans. The stability test is finished and shown in Figure 12 at L378-380.

 

4. The manuscript is missing Figures 7 and 8.

Ans. The manuscript can be reorganized after revised.

 

5. Performance Benchmarking: How does the peak power density of 332 mW/cm² achieved with PdAgCo/C and PtIr/C compare to other recently reported high-performing AEM-DAFC systems using different catalysts (especially non-Pt group metals)? Please add a comparative discussion in the Results or Discussion section.

Ans. For PtIr/C catalyst, it usually applied on the anode. As question from number one,  we compared the results base on cathode catalyst which is this paper aim at (results are shown in Table 11).

 

6. Section 3.2.2 mentions washing modified carbon black to neutrality. How was neutrality confirmed (pH measurement)? For the PdAgCo/C synthesis, were the precursors (K₂PdCl₄, AgNO₃, CoCl₂) added simultaneously? What was the pH during reduction? These details impact particle size/distribution.

Ans. The sentence of pH measurement “Neutrality is confirmed by pH measurement (DOGGER, D9UL-PH5011).” is added at Line284. The mixed precursors (K₂PdCl₄, AgNO₃, CoCl₂) solution dropwise into the basic simultaneously, reducing carbon slurry under stir and keep pH value at 11.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This paper experimentally demonstrates the performance enhancement of an AEM-based direct ammonia fuel cell (DAFC) using a PdAgCo/C catalyst. The results may provide valuable insights to researchers in the fields of fuel cells and catalysts. However, the manuscript requires several critical improvements before it can be considered for publication in Catalysts. My detailed comments are as follows:

1. The introduction should include the theoretical background relevant to the study of the PdAgCo/C catalyst.
2. For better readability, the experimental methods (Chapter 3) should be presented before the results and discussion (Chapter 2).
3. Abbreviations should be defined only once at first mention in the manuscript and used consistently thereafter (e.g., open-circuit voltage (OCV)).
4. Durability evaluation results are needed. For example, long-term continuous operation tests of 100 hours or more would help verify performance retention.
5. All performance data are currently presented as single measurements; the average and standard deviation from replicate experiments should be reported. In addition, error bars should be included in the figures.
6. The theoretical interpretation of the improved catalytic performance is insufficient. In particular, the effects of Pd and Co doping on electronic structure and reaction mechanisms are not adequately explained. Additional analyses, such as XPS, TEM, and EIS, are recommended to clarify the origin of the doping effect.
7. The optimal conditions for catalyst composition and content should be identified.

Author Response

This paper experimentally demonstrates the performance enhancement of an AEM-based direct ammonia fuel cell (DAFC) using a PdAgCo/C catalyst. The results may provide valuable insights to researchers in the fields of fuel cells and catalysts. However, the manuscript requires several critical improvements before it can be considered for publication in Catalysts. My detailed comments are as follows:

 

1.The introduction should include the theoretical background relevant to the study of the PdAgCo/C catalyst.

Ans. We have added the theoretical background of the PdAgCo/C catalyst in following sentence from L90-101 with citations. “Though there are many promising electrocatalysts, the PdAgCo/C was chosen due to its versatility and potential properties. The significant catalytic activity of Pd, tolerance of Ag towards ammonia and electronic structure modification for enhanced catalytic activity upon doping a conductive transition metal like cobalt, altogether is to be reaped for an improved fuel cell performance[18-20]. Nanoparticles Pd rich shell and Ag/C rich core showed a mass activity almost 1.5 times than that of Pd/C in alkaline medium[21]; Pd/O binding energy playing a significant role in ORR activity. The surface modification of Pd due to Ag and Co make it less favourable for adsorption and oxidation of ammonia [22]. The role of an electropositive transition metal like Cobalt in downshift of d band centre of Pd atoms due to compressive strain and ligand effect which subsequently leads to enhanced ORR catalytic activity that may even match up to the activity of precious metals if optimized [23].”

 

 

2.For better readability, the experimental methods (Chapter 3) should be presented before the results and discussion (Chapter 2).

Ans. The sequence of experimental methods is present before experimental methods.

 

3.Abbreviations should be defined only once at first mention in the manuscript and used consistently thereafter (e.g., open-circuit voltage (OCV)).

Ans. “Pd and Ag catalyst” is revised at L161, “NH₄OH” is delete at table 4 at L304 and “Cobalt and palladium, the cyclic voltammetry” is revised at conclusion part at L364, L367, L370 and L376.

 

4.Durability evaluation results are needed. For example, long-term continuous operation tests of 100 hours or more would help verify performance retention.

Ans. We finish the durability test of PdAgCo. “In Figure 12, the durability is use DAFC test station (FCED-PD50) for 100 cycles. The test condition is PtIr/C catalyst as anode, PdAgCo/C catalyst as cathode, same loading of 2 mg/cm², PiperION-A20-HCO3 TP-85 as alkaline anion exchange membrane, 3M NH4OH+3M KOH as anode fuel and pure oxygen as cathode fuel. Results shows that after 100 cycles, the power density is drop 40% from 332 mW/cm² to 200 mW/cm².” at L374-380.

5.All performance data are currently presented as single measurements; the average and standard deviation from replicate experiments should be reported. In addition, error bars should be included in the figures.

Ans. For experiments, we have done at least three times. The Figure and the data on the manuscript is the final result which mean is the average result (the deviation) already. In order to let reviewer or reader directly see through, we put it as final result without error bar and standard deviation.

 

6.The theoretical interpretation of the improved catalytic performance is insufficient. In particular, the effects of Pd and Co doping on electronic structure and reaction mechanisms are not adequately explained. Additional analyses, such as XPS, TEM, and EIS, are recommended to clarify the origin of the doping effect.

Ans. The DAFC cell performance by using the presented catalysts is our main target. Those additional analyses are the basis from our literature reviews. Thus, these are not showed in this manuscript. 

 

7.The optimal conditions for catalyst composition and content should be identified.

Ans. In this study, the ratio of PdAgCo is 1:1:2. It was added as question 2 from reviewer 3.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

     In the revised manuscript the authors addressed all my concerns. The structure and writing quality of the manuscript are improved. But some figures and tables are not well introduced before being discussed. For example, in p11, L340. “Figure 9 and Table 7 shows that PdAgCo/C shows a better power density of 74”. This is discussion, not introduction. Besides, there are also typos that need to be fixed. An example is listed below. Minor revision is needed.

1. P2, L63. “, this challenge”.

Comment: Grammar issue. “,”Should be “.”.

Author Response

Comment: In the revised manuscript the authors addressed all my concerns. The structure and writing quality of the manuscript are improved. But some figures and tables are not well introduced before being discussed. For example, in p11, L340. “Figure 9 and Table 7 shows that PdAgCo/C shows a better power density of 74”. This is discussion, not introduction. Besides, there are also typos that need to be fixed. An example is listed below. Minor revision is needed.

1. P2, L63. “, this challenge”. Comment: Grammar issue. “,”Should be “.”.

Response:

1. The sentence “Figure 9 and Table 7 show that PdAgCo/C has a better power density of 74 mW/cm²,” is revised in p11, L340-L341.

 

2. The sentence “, this challenge” is revised to “. This challenge” in p2, L63

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors answered the questions and clarified the new data in the Manuscript. The article may be published

Author Response

Comments and Suggestions for Authors

The authors answered the questions and clarified the new data in the Manuscript. The article may be published.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have carefully revised the manuscript according to the referees’ comments. In my opinion, this manuscript could be accepted for publication in Catalysts.

 

Author Response

Comments and Suggestions for Authors

The authors have carefully revised the manuscript according to the referees’ comments. In my opinion, this manuscript could be accepted for publication in Catalysts.