Morphological Effect on the Surface Activity and Hydrogen Evolution Catalytic Performance of Cu2O and Cu2O/rGO Composites
, Ankai Wang 2, Shengli Zou 2, Karin Y. Chumbimuni-Torres 2 and Lei Zhai 1,2,3,*
Round 1
Reviewer 1 Report
In this study, the authors synthesize different Cu2O particles with different morphologies for catalyzing acidic HER. They find that a higher 111/100 facets ratio on Cu2O shows a higher oxidation resistance and better HER performance. However, some important issues must be solved to strengthen the conclusions of this manuscript.
1. The authors should explain clearly why “a higher oxidation resistance” is beneficial for HER performance. Also, please provide the experimental results to verify this statement.
2. The introduction part is not sufficient and attractive. At least, the global requirement for hydrogen and the hydrogen economics should be discussed to explain why green hydrogen is important. The authors should refer to this work (DOI: 10.1002/adma.202305074).
3. For Table 1, the authors should clearly explain how to obtain the results in Table 1 and what is the difference between Table 1 and Figure 1.
4. We can observe obvious XRD peak shift in Figure 1g and 2f, which should be ascribed to the existing structural strain in the lattice. The authors should analyze and compare the structural strain in the samples by referring to these papers (DOI: 10.1063/5.0083059; 10.1016/j.jechem.2023.03.033), which may affect the 111/100 facets ratio and HER performance.
5. The time for HER stability is too short, please re-measure the HER stabilities of samples for at least 10 h.
6. Besides the SEM after HER, the structural and electronic structural properties on samples after HER should be measured and compared as well. Therefore, XRD and XPS after HER are recommended.
Minor editing of English language required
Author Response
- The authors should explain clearly why “a higher oxidation resistance” is beneficial for HER performance. Also, please provide the experimental results to verify this statement.
In Fig. S3 and S4 XPS spectrum of Cu2O and Cu2O-rGO composites are shown, also in Table S2 a ratio of Cu(I)/Cu(II) can be found. In this section, the presence of the oxidation state Cu(II) is found in all Cu2O geometric particles. In addition, in the oxides with greater facet 100, a higher presence of Cu(II) can be seen, indicating a greater oxidation due to air. Greater resistance to oxidation leads to a more pristine oxide surface without impurities. In a catalysis process, the material surface plays a critical role in the performance of the catalyst. Therefore, for the HER reaction, the electrical conductivity, the crystallinity, and affinity towards H2 are highly affected by the copper(I) oxide surface composition.
- The introduction part is not sufficient and attractive. At least, the global requirement for hydrogen and the hydrogen economics should be discussed to explain why green hydrogen is important. The authors should refer to this work (DOI: 10.1002/adma.202305074).
A new paragraph was added in introduction in line 42, page 2/25, as follow; “Estimating the global demand for hydrogen in 2050 reveals an expected demand surpassing 500 Mt, emphasizing the urgency for expanding hydrogen production capabilities (Source: 10.1002/adma.202305074). Presently, gray hydrogen, derived from fossil fuel sources, holds a competitive advantage in terms of cost-effectiveness and well-established production processes, overshadowing green hydrogen, produced from carbon-neutral sources. The primary challenge lies in the relatively high production costs associated with green hydrogen, which currently stand at 4-5 times greater than those of gray hydrogen. These cost disparities can be mitigated through the utilization of scalable and readily available materials, such as oxides and perovskites, derived from abundant metals like iron (Fe), nickel (Ni), cobalt (Co), and copper (Cu).”
- For Table 1, the authors should clearly explain how to obtain the results in Table 1 and what is the difference between Table 1 and Figure 1.
To clarify the results in table 1, a new statement was added in line 229, pag 5/25, “These two parameters were computed using ImageJ software. To determine particle size, we measured the length of the edges of a minimum of 100 particles and calculated the average. Furthermore, we measured the area of both facets of individual particles to obtain the 100/111 facet ratio. This measurement was repeated for at least 20 particles to obtain an average value for 'r'.”
Figure 1 represents a 3D model of the geometric particles that are shown in the SEM images in the same figure.
- We can observe obvious XRD peak shift in Figure 1g and 2f, which should be ascribed to the existing structural strain in the lattice. The authors should analyze and compare the structural strain in the samples by referring to these papers (DOI: 10.1063/5.0083059; 10.1016/j.jechem.2023.03.033), which may affect the 111/100 facets ratio and HER performance.
In Figure 1g, no significant shift is observed in the XRD patterns of the geometric Cu2O. In contrast, Figure 2h reveals a noticeable shift in Cu2O peaks and the emergence of peaks associated with CuO. However, the images depicting the Cu2O-rGO composites in Figure 2 suggest that there is no alteration in size or reconstruction, except for the TO PVP-G composite. Given the limited significance of the TO PVP-G composite's performance, there is no imperative need to delve further into addressing this lattice strain. For the remaining Cu2O-rGO composites, this shift can be attributed to the physical characteristics of the aerogel (Source: 10.1016/j.proeng.2012.02.047) and a reduced oxide presence per unit volume.
- The time for HER stability is too short, please re-measure the HER stabilities of samples for at least 10 h.
The increase in current density during the chronoamperometry of all Cu2O geometries is illustrated in Figure 4C for a short-duration test. This phenomenon can be attributed to the size reduction or decoupling of particles, as depicted in Figure 6, resulting in a larger exposed surface area. However, due to the material's instability over a short testing period, a longer-term test lasting 10 hours would likely yield a similar conclusion. To enhance the stability of Cu2O samples, further future approaches and investigations are warranted.
- Besides the SEM after HER, the structural and electronic structural properties on samples after HER should be measured and compared as well. Therefore, XRD and XPS after HER are recommended.
The primary objective of this research was to investigate the morphological impact of Cu2O particulates on their performance in the Hydrogen Evolution Reaction (HER), an aspect that has not been previously explored. The oxidation states and their electrochemical stability have been previously examined and discussed in prior works (References: 10.1021/jp103437y, 10.1149/1.3089290) and have been duly referenced in the introduction
Some modifications were made to the section 3.4.4 as followed:
In line 735, page 18/25. It was added “A dynamic evolution can be found in Cu2O after an electrochemical process (10.1002/adfm.202111193). This dynamic evolution involves atomic rearrangement and a change in chemical state, particularly during the hydrogen evolution. The Pourbaix diagram of Cu species suggests that the dissolution and reduction of Cu+ occur at the pH and cathodic potential conditions utilized in HER.”
In line 741, page 18/25. It was added “Regarding the O-Cu2O catalyst, we detected the presence of smaller octahedral particles. This observation strongly implies that O-Cu2O is undergoing a dynamic process involving Cu+ dissolution and subsequent reconstruction into Cu2O.”
In line 748, page 18/25. It was added “In contrast, the S-G composite exhibited a more radical transformation, where the star-like particles were completely converted into irregular agglomerations of approximately 250 nm. This significant change suggests a distinct mechanism compared to the O-Cu2O particles. In this case, it appears that during the hydrogen evolution, the dissolution of Cu+ species takes place, followed by their diffusion into the reduced graphene oxide sheets, culminating in the creation and growth of nuclei.”
In line 761, page 18/25. It was added “It is advisable to utilize XPS to differentiate between Cu(0) and Cu(I) on the particle surface. Additionally, In-situ Raman spectroscopy (Reference: 10.1021/acscatal.6b00205) has proven to be a valuable technique for elucidating and identifying Cu phases during water splitting. Therefore, it should be considered for further investigation.”
Reviewer 2 Report
In this study, Ramirez-Ubillus et al. successfully synthesized Cu2O particles with various morphologies, including octahedron, truncated octahedron, cube, and star-like structures. They then investigated the impact of these particle morphologies on the hydrogen evolution performance. The authors discovered that the presence of more Cu+ terminated atoms on facet 111 resulted in a higher number of active sites for the hydrogen evolution reaction (HER). Additionally, they found that the performance could be further enhanced by loading Cu2O onto reduced graphene oxide. The authors conducted a comprehensive set of measurements and characterizations, and their conclusions are largely supported by the obtained results. Therefore, I recommend the publication of this work in J. Compos. Sci. after addressing the following concerns.
1. It is crucial to clarify whether the ESA measurement was performed on the fresh Cu2O samples or on the samples after the HER reaction. Since the authors have demonstrated that the Cu2O samples undergo structural evolution, it is essential to evaluate the ESA of the samples after the reaction.
2. On page 13, the authors state that " the S-Cu 2 O sample, which was expected to have a greater area due to its surface with more unsaturated sites, did not exhibit a significantly higher ESA". It would be valuable to provide an explanation for this unexpected observation.
3. During the HER experiments, it is important to analyze the oxidation state of the Cu2O samples, as both Cu and Cu2O are highly sensitive to air. This analysis will provide additional insights into the reaction mechanism and the stability of the catalyst.
4. Due to the dissolution of the surface layers of Cu2O in the acidic electrolyte and the resulting changes in surface structure, DFT simulations based on Cu2O (100) and (111) surfaces cannot well explain the morphological effects of Cu2O nanoparticles on the HER performance.
5. Some recent studies have focused on investigating the structural effects of electrocatalysts in various electrochemical reactions (10.1021/acsenergylett.1c01965; 10.1021/acscatal.2c03842) as well as the structural evolution of Cu-based electrocatalysts (10.1021/acscatal.6b00205; 10.1021/acscatal.3c01439; 10.1002/adfm.202111193). It would be beneficial to reference and discuss these relevant studies in order to provide a broader context for the findings presented in this paper.
6. The full name of some abbreviations should be provided, such as “PBS” in Table 4.
Author Response
- It is crucial to clarify whether the ESA measurement was performed on the fresh Cu2O samples or on the samples after the HER reaction. Since the authors have demonstrated that the Cu2O samples undergo structural evolution, it is essential to evaluate the ESA of the samples after the reaction.
In line 199, page 5/25, the term 'fresh catalyst' has been added to specify that pristine Cu2O samples were used for measuring the ESA. It is important to note that Cu2O geometric particles undergo a process of particle uncoupling, transitioning from larger particles (> 1000 nm) to smaller particles (< 500 nm), as illustrated in Figure 6. This size reduction leads to the conclusion that the effective surface area (ESA) can increase significantly, resulting in a higher exposed surface area. Furthermore, this increase in effective surface area corresponds with the observed rise in current density during chronoamperometry testing for all Cu2O geometric configurations, as depicted in Figure 4C.
- On page 13, the authors state that " the S-Cu2O sample, which was expected to have a greater area due to its surface with more unsaturated sites, did not exhibit a significantly higher ESA". It would be valuable to provide an explanation for this unexpected observation.
As stated in previous work cited (doi.org/10.1039/C5TA00218D), It is expected that corners and edges will have a greater number of unsaturated sites (10.1021/acscatal.2c03842), compared to a spherical shape. For the sample S-Cu2O, the square-based truncated pyramids onto hexapod surface, provide many more unsaturated sites per particle than the other geometries. However, by the ESA analysis S-Cu2O does not have as many electroactive sites as unsaturated sites.
In line 576, page 13/25. The paragraph was rewritten to “For the S-Cu2O sample, it is expected that corners and edges will have a greater number of unsaturated sites (10.1021/acscatal.2c03842). However, this composite did not manifest as many electroactive sites as unsaturated ones. Consequently, its performance in HER was inferior when compared to other Cu2O catalysts.”
- During the HER experiments, it is important to analyze the oxidation state of the Cu2O samples, as both Cu and Cu2O are highly sensitive to air. This analysis will provide additional insights into the reaction mechanism and the stability of the catalyst.
The authors would like to express their appreciation for the valuable suggestion. This study primarily focuses on exploring the morphological impact of Cu2O particulates on their performance in HER, a novel approach that has not been previously undertaken for this material. It is noteworthy that the oxidation states following the HER reaction and their electrochemical stability have already been investigated and documented by other researchers (References: 10.1021/jp103437y, 10.1149/1.3089290), and these findings have been duly referenced in the introduction section. Given that Cu2O is susceptible to corrosion, future research endeavors should indeed consider post-test analyses and delve deeper into strategies for mitigating this corrosion, all while avoiding the introduction of additional materials.
- Due to the dissolution of the surface layers of Cu2O in the acidic electrolyte and the resulting changes in surface structure, DFT simulations based on Cu2O (100) and (111) surfaces cannot well explain the morphological effects of Cu2O nanoparticles on the HER performance.
We appreciate the reviewer's insightful observation. It is indeed true that the dissolution of the surface layers of Cu2O in the acidic electrolyte can lead to changes in the surface structure, rendering it less representative of the ideal Cu2O (100) and (111) surfaces typically used in DFT simulations. Ideally, one would investigate the altered surface structure and then replicate these changes in simulations. However, the non-uniform nature of these changes poses a considerable challenge. Previous experimental research (Reference: DOI: 10.1021/acs.est.2c07845) has indicated that copper dissolution primarily occurs in the surface layer in aqueous environments. Furthermore, our findings suggest that the enhanced stability of molecular H2 on the Cu2O (100) surface is attributed to the surface oxygen rather than copper.
While the exact morphology of the synthesized Cu2O particles may be challenging to ascertain, our simulations can still provide valuable insights into the HER performance on different facets of Cu2O. Moreover, these calculations can serve as a foundation for demonstrating variations in hydrogen H2 affinity across different facets of other oxides or composites, offering insights into their electrochemical stability.
- Some recent studies have focused on investigating the structural effects of electrocatalysts in various electrochemical reactions (10.1021/acsenergylett.1c01965; 10.1021/acscatal.2c03842) as well as the structural evolution of Cu-based electrocatalysts (10.1021/acscatal.6b00205; 10.1021/acscatal.3c01439; 10.1002/adfm.202111193). It would be beneficial to reference and discuss these relevant studies in order to provide a broader context for the findings presented in this paper.
The authors are grateful for the suggestion. Some modifications were made to the section 3.4.4 as followed:
In line 735, page 18/25. It was added “A dynamic evolution can be found in Cu2O after an electrochemical process (10.1002/adfm.202111193). This dynamic evolution involves atomic rearrangement and a change in chemical state, particularly during the hydrogen evolution. The Pourbaix diagram of Cu species suggests that the dissolution and reduction of Cu+ occur at the pH and cathodic potential conditions utilized in HER.”
In line 741, page 18/25. It was added “Regarding the O-Cu2O catalyst, we detected the presence of smaller octahedral particles. This observation strongly implies that O-Cu2O is undergoing a dynamic process involving Cu+ dissolution and subsequent reconstruction into Cu2O.”
In line 748, page 18/25. It was added “In contrast, the S-G composite exhibited a more radical transformation, where the star-like particles were completely converted into irregular agglomerations of approximately 250 nm. This significant change suggests a distinct mechanism compared to the O-Cu2O particles. In this case, it appears that during the hydrogen evolution, the dissolution of Cu+ species takes place, followed by their diffusion into the reduced graphene oxide sheets, culminating in the creation and growth of nuclei.”
In line 761, page 18/25. It was added “It is advisable to utilize XPS to differentiate between Cu(0) and Cu(I) on the particle surface. Additionally, In-situ Raman spectroscopy (Reference: 10.1021/acscatal.6b00205) has proven to be a valuable technique for elucidating and identifying Cu phases during water splitting. Therefore, it should be considered for further investigation.”
- The full name of some abbreviations should be provided, such as “PBS” in Table 4.
Thank the reviewer for pointing that out. The abbreviation PBS was changed to Phosphate buffer solution in Table 4. Furthermore, all the abbreviations in the manuscript were checked to provide a correct reference.
Round 2
Reviewer 1 Report
This version can be accepted.
Reviewer 2 Report
This manuscript can be accepted.
