Conducting-Polymer Nanocomposites as Synergistic Supports That Accelerate Electro-Catalysis: PEDOT/Nano Co₃O₄/rGO as a Photo Catalyst of Oxygen Production from Water

Round 1

Reviewer 1 Report

Alsultan et al. demonstrated the use of conducting polymer nanocomposites in the generation of oxygen from water. The obtained results compared to commercial platinum catalysts, are very promising. The manuscript was carefully prepared without major editorial flaws. I can recommend this manuscript for publication in J. Compos. Sci. in its present form. 

Author Response

Reviewer 1

Alsultan et al. demonstrated the use of conducting polymer nanocomposites in the generation of oxygen from water. The obtained results compared to commercial platinum catalysts, are very promising. The manuscript was carefully prepared without major editorial flaws. I can recommend this manuscript for publication in J. Compos. Sci. in its present form. 

 

Thanks to reviewer 1  to accept the paper in its present form  !!!!!!

Reviewer 2 Report

The manuscript focused on the photocatalytic properties of Co₃O₄/reduced graphene oxide (rGO) for oxygen evolution reaction (OER). The authors developed a synergistic effect of poly(3,4-ethylenedioxythiophene) (PEDOT) and Co₃O₄/rGO as supporting materials with improved photocatalytic activity. The catalyst characterization was performed detailed and photocatalytic activity was relatively good, however the synergistic effect of PEDOT and Co₃O₄/rGO was hardly to understand in the manuscript. For these reasons, I can recommend this manuscript to be published but it still needs a considerable revision. The authors should address the following points:

 

Major comments

 

1. The Co₃O₄ photocatalyst, including with conductive carbon supports, was well known and already used for OER. The synergistic effect of PEDOT and Co₃O₄/rGO was very important to show the novelty of this manuscript. Please add the more effective discussion for the synergistic effect of PEDOT and Co₃O₄/rGO in the revised manuscript.

 

2. In Figure 7, the resistance at low frequency of the PEDOT/Co₃O₄/rGO was lower than that of the Pt/FTO even without the light irradiation. It is well known that Pt shows excellent electrocatalyst for OER. I can’t agree with the PEDOT/Co₃O₄/rGO was superior catalyst to Pt for OER in the dark. I think that there are other factors to be considered.

 

3. Corresponding Q2, please add the current efficiency for OER in the manuscript.

 

4. Please add the incident photon to current efficiency of PEDOT/Co₃O₄/rGO for OER.

 

5. It would be useful to include UV spectrum of PEDOT/Co₃O₄/rGO in the manuscript or SI.

 

6. Why the current density in Figure 3 was three orders smaller than that of Figure 2?

 

7. The y-axis label of Figure 1 and 2 were lacking.

 

Author Response

Response to reviewer 2

1. The Co₃O₄ photocatalyst, including with conductive carbon supports, was well known and already used for OER. The synergistic effect of PEDOT and Co₃O₄/rGO was very important to show the novelty of this manuscript. Please add the more effective discussion for the synergistic effect of PEDOT and Co₃O₄/rGO in the revised manuscript.

 

Response

Thanks for your highlighted point. PEDOT has the same roles that discussed in the previous paper in the PEDOT/ Co₃O₄/CNT study.  The only difference was using rGO instead of CNT. A more effective discussion of the synergistic effect of rGO was added and is highlighted.

 

 

2. In Figure 7, the resistance at low frequency of the PEDOT/Co₃O₄/rGO was lower than that of the Pt/FTO even without the light irradiation. It is well known that Pt shows excellent electrocatalyst for OER. I can’t agree with the PEDOT/Co₃O₄/rGO was superior catalyst to Pt for OER in the dark. I think that there are other factors to be considered

     Response

Thanks for this comment. The labeled (i) in figure 7 c was in the wrong order with (ii) label they was swapped by mistake. However the table one showed more correct details. The labeled was correct now thanks.

For PEDOT/Co3O4/rGO films, the chemical material was previously coated by Pt layer via sputter coating. We mean the FTO glass was previously coated by Pt via sputter coating before we deposited the chemical material on FTO substrate. For this reason, PEDOT/Co3O4/rGO films showed lower frequency than pure Pt/FTO

We trust the above changes address the reviewer’s comments.

 

 

3. Corresponding Q2, please add the current efficiency for OER in the manuscript

Response

The Faradaic Efficiency of PEDOT/ Nano-Co3O4/rGO was added and is highlighted.

 

4. Please add the incident photon to current efficiency of PEDOT/Co₃O₄/rGO for OER.

 

Response

Unfortunately we do not have the necessary equipment to perform this measurement.

5. It would be useful to include UV spectrum of PEDOT/Co₃O₄/rGO in the manuscript or SI

Response

The UV spectrum of PEDOT/Co₃O₄/rGO was added in the SI.

6. Why the current density in Figure 3 was three orders smaller than that of Figure 2?

 

Response

The current density in Figure 3 was three orders smaller than that of Figure 2 because in Figure 2 the chemical catalytic material was deposited on FTO slide, while in Figure 3 the chemical catalytic material was deposited in FTO slide that had been previously sputter-coated with Pt.

7. The y-axis label of Figure 1 and 2 were lacking.

 

 

Response

The y-axis labels of Figures 1 and 2 have been fixed.

 

Reviewer 3 Report

This article describes deals with composites of Co3O4 and reduced graphene oxide which are embedded in a matrix of the polymer PEDOT, with a view to photo-electric water splitting. The preparation process itself followed a method applied for analogous composites comprising multiwall carbon nanotubes instead of graphene oxide, yet it is not evident how much graphene oxide was used in this process compared to the carbon nanotubes and if the quantity of produced material was the same as in the previously described protocol with carbon nanotubes.

 

Further, the sources of the monomer EDOT, the Co3O4 and the reduced graphene oxide are not indicated in the section Materials and Methods, and no information of the particle size and particle size distribution of Co3O4 is provided. While it is claimed that the size of Co3O4 is in the nanometer region, this has not been shown.

 

While the prepared materials were properly characterized with a variate of methods, the primary idea of this study, the usage of the materials as useful electro-catalysts for oxygen production or water splitting, respectively, is not reliable. The authors emphasize in the first paragraph of the Introduction that renewable energy will be produced at very low cost in the near future. Hence he system presented here does not offer significant advantage compared to direct electrolysis of water without catalysts. In fact industrial representatives recently strongly favoured direct electrolysis of water compared to photo-electric catalysis due to its simple nature.

 

Moreover it is unclear how much oxygen is produced with the system described in this manuscript per hour and gram of catalyst. It is claimed in the conclusions that oxygen evolution rate is “excellent” but the quantity of produced oxygen is not indicated.

 

Also, comparison of the system prepared in this manuscript with platinum is not sound. It is stated that platinum is generally “the best industrial catalyst”. However, this catalyst does not apply for many important industrial processes, such as Ziegler-Natta catalysis or Fischer-Tropsch catalysis. In fact, platinum does not serve as an industrially applied photo-electric catalyst for water splitting, thus the addressed comparison is misleading. In fact, a number of photo-electric catalysts for water splitting have ben reported in the literature, but this is not reflected in the citations in the Introduction, and the system described here has not been discussed adequately in the light of the previous research in this field.

 

As another basic problem in the presented system is the limited stability of PEDOT towards intense sunlight. By necessity, a system with a photo-electric catalyst has to be exposed to sunlight.

 

As another point to be addressed, it is mentioned in the Abstract as well as in the text that oxygen gas was the only product of the water splitting reaction. This is very strange as it then remains unclear what happened to the hydrogen atoms of water once the oxygen atom has been consumed for O2 production. By necessity, there must be another reaction product containing the hydrogen atoms of the converted water molecules.

As a summary of the above comments, I have to regret that I have to recommend rejection of this manuscript due to lack of significance in the area of oxygen production from water.

 

 

 

Author Response

Reviewer 3

1. This article describes deals with composites of Co3O4 and reduced graphene oxide which are embedded in a matrix of the polymer PEDOT, with a view to photo-electric water splitting. The preparation process itself followed a method applied for analogous composites comprising multiwall carbon nanotubes instead of graphene oxide, yet it is not evident how much graphene oxide was used in this process compared to the carbon nanotubes and if the quantity of produced material was the same as in the previously described protocol with carbon nanotubes.

 

 

Response

 

Carbon-based material including  CNT or rGO were thoroughly used in water splitting system due to their conductivity, durability, and synergetic effects. One of the reasons to use rGO instead of CNT in PEDOT/nano Co3O4 /rGO is not to make comparison between them, but rather to find a carbon material that improves long-term durability. This was achieved, with the system using rGO working at high efficiency for more than 50 hours, instead of 42 hours using CNT in previous work.

 

2. Further, the sources of the monomer EDOT, the Co3O4 and the reduced graphene oxide are not indicated in the section Materials and Methods, and no information of the particle size and particle size distribution of Co3O4 is provided. While it is claimed that the size of Co3O4 is in the nanometer region, this has not been shown.

 

Response

The monomer EDOT can be converted to PEDOT by vapour phase polymerization at 60 °C.^1-3 There have been many publications about EDOT to PEDOT. For Co₃O₄ nano particles, we used material with nominal particle size of 25 nm provided by Skyspring Nanomaterials. This is now indicated in the manuscript.

 

3. While the prepared materials were properly characterized with a variate of methods, the primary idea of this study, the usage of the materials as useful electro-catalysts for oxygen production or water splitting, respectively, is not reliable. The authors emphasize in the first paragraph of the Introduction that renewable energy will be produced at very low cost in the near future. Hence he system presented here does not offer significant advantage compared to direct electrolysis of water without catalysts. In fact industrial representatives recently strongly favoured direct electrolysis of water compared to photo-electric catalysis due to its simple nature.

 

Response

Both direct electrolysis and photoelectrolysis are new applications of renewable energy. Direct electrolysis needs expensive, corrosion-resistant materials  with high efficiency to produce H₂ and O₂ gases. In addition, the use of external higher voltage. Photo electrolysis depends on using sun-light or artificial light to lower the required external voltage and uses cheaper materials, such carbon-based materials. In our system we used very cheap material with 8-fold the durable activity of Pt. Also less external voltage was used (0.8 V instead of 1.23 V).

 

4. Moreover it is unclear how much oxygen is produced with the system described in this manuscript per hour and gram of catalyst. It is claimed in the conclusions that oxygen evolution rate is “excellent” but the quantity of produced oxygen is not indicated.

 

Response

In the SI we provide the results of gas chromatography that proved pure O₂ gas was produced from the system. Also we added Faradaic Efficiency of PEDOT/ Nano-Co₃O₄/rGO that showed the oxygen gas generation efficiency in dark and light. The oxygen peaks was collect from GC peaks and calibrated with pure O₂ gas to determine the efficiency.

5. Also, comparison of the system prepared in this manuscript with platinum is not sound. It is stated that platinum is generally “the best industrial catalyst”. However, this catalyst does not apply for many important industrial processes, such as Ziegler-Natta catalysis or Fischer-Tropsch catalysis. In fact, platinum does not serve as an industrially applied photo-electric catalyst for water splitting, thus the addressed comparison is misleading. In fact, a number of photo-electric catalysts for water splitting have ben reported in the literature, but this is not reflected in the citations in the Introduction, and the system described here has not been discussed adequately in the light of the previous research in this field.

 

Response

The comparison of the system prepared in this manuscript with platinum was done in Figure 2 (c) The pt -coated FTO showed very low current efficiency about 80 (µA/cm²)  at experiment condition while our system produced very high current density as can be seen in Figure 2 (b). Pt is not used as a photo-electro catalyst because it does not absorb light. We only try to find system to electrolysis water better than Pt for industrial application in near future. Our system is really better than Pt in oxidizing water with higher efficiency. We added short sentences about PEDOT light absorption property. Much recent research has used conductive polymer for water splitting with synergetic effects and light absorbance (see references from 4-23).

6. As another basic problem in the presented system is the limited stability of PEDOT towards intense sunlight. By necessity, a system with a photo-electric catalyst has to be exposed to sunlight.

 

 

Response

 

According to the literature and the experimental works in lab the PEDOT is not affected by light under 50 W hydrogen lamp.  We kept the system 10 cm away from the light source. In addition we placed a special filter between the light source and the film surface. This filter only passes the visible light between 400-800 nm, and the heat-generating IR cannot pass. Even at 1500 W the system can work. Finally the film was submerged in room temperature electrolyte, which should have helped to keep the film at room temperature.

7. As another point to be addressed, it is mentioned in the Abstract as well as in the text that oxygen gas was the only product of the water splitting reaction. This is very strange as it then remains unclear what happened to the hydrogen atoms of water once the oxygen atom has been consumed for O2 production. By necessity, there must be another reaction product containing the hydrogen atoms of the converted water molecules.

 

Response

Water-splitting includes electrolysis to generate O₂ and H₂ . Water splitting research can involve the study of oxygen evolution reaction and/or hydrogen evolution reaction. In our system we used the film as anode that generating only oxygen gas while the counter electrode, we used Pt mesh which is already generating hydrogen gas. We studied photo- electrochemical oxygen generation on our material deposited on the anode. In future we can apply the chemical material on the counter side to investigate photo-electrochemical hydrogen production. In addition, we can apply our chemical material on both sides (anode and cathode) to investigate O2 and H2 generation at same time. We added sentences that describe the water splitting reaction and equation as general information.    

 

 

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The manuscript has been revised well. I think this manuscript could potentially be acceptable after some corrections have been done.

Author Response

Reviewer 2

1. The manuscript has been revised well. I think this manuscript could potentially be acceptable after some corrections have been done.

 

Response

Thank you for your useful comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

The following points of my last review have not been answered adequately:

 

It is not evident how much graphene oxide was used in this process compared to the carbon nanotubes and if the quantity of produced material was the same as in the previously described protocol with carbon nanotubes.

 

Only the source of Co3O4 (Skyspring Nanomaterials) has been indicated in the section Materials and Methods but not the sources of the monomer EDOT and the reduced graphene oxide.

 

The usefulness of the catalytic system used in this manuscript is still not evident. The authors focus on oxygen production, however, the technically interesting product form water splitting is hydrogen. Further, photocatalysis is about 1000 times less efficient for water splitting than direct hydrolysis of water , see e.g. s. Shiva Kumar, V. Himabindu, Mater. Sci. Energy Technol.  2019, 2, 442-454. Also, photocatalysis only operates properly in the presence of sun light, and the cobalt in the Co3O4  is considered more and more problematic due to toxicity; accordingly there is, for example, a strong tendency to omit cobalt in batteries.

 

It is still not indicated in the manuscript how much oxygen is produced with the system described in this manuscript per hour and gram of catalyst. It is claimed in the conclusions that oxygen evolution rate is “excellent” but the quantity of produced oxygen is not indicated, and the production hardly excellent compared to direct electrolysis, and not even a comparison with the best other photocatalytic water splitting systems is evident.

 

Comparison of the system prepared in this manuscript with platinum is still not sound. It is stated that platinum is generally “the best industrial catalyst”. However, this catalyst does not apply for many important industrial processes, such as Ziegler-Natta catalysis or Fischer-Tropsch catalysis. In fact, platinum does not serve as an industrially applied photo-electric catalyst for water splitting, thus the addressed comparison is misleading. In fact, a number of photo-electric catalysts for water splitting have been reported in the literature, but this is still not reflected in the citations in the Introduction, and the system described here has still not been discussed adequately in the light of the previous research in this field.

 

The laboratory tests with the hydrogen lamp are by far not sufficient to prove that the PEDOT system may withstand intense sun light for years. A proper xenon lamp test would be required for technical applications.

 

All in all, unfortunately my comments have been met only marginally so I regret that I still have to recommend rejection of this manuscript essentially for the reasons addressed already in my first review.

Author Response

Reviewer 3’s comments are given in bold. Our response is provided underneath.

 

1. It is not evident how much graphene oxide was used in this process compared to the carbon nanotubes and if the quantity of produced material was the same as in the previously described protocol with carbon nanotubes. 

 

We are, respectfully, a little confused by the reviewer’s comment. We stated rather clearly in Section 3.2 of the paper that 2 mg of graphene oxide (later converted to rGO) was found to be optimum and incorporated in the test samples (4.2 cm² each). The quantity of carbon nanotubes in the previous paper was also 2 mg per test sample (4.2 cm²). We further reported the elemental analysis of the catalytic coating, giving a specific mol percentage for graphene oxide. We do not understand what the reviewer wants us to provide?

 

2. Only the source of Co3O4 (Skyspring Nanomaterials) has been indicated in the section Materials and Methods but not the sources of the monomer EDOT and the reduced graphene oxide.

We are, again, respectfully a little confused. As explicitly noted in section 2.1, PEDOT was prepared as described previously in reference 4. That reference provides full details of PEDOT fabrication, including the origin of the monomer EDOT, its purification, and all the steps involved in making the PEDOT. As also noted in section 2.1, the GO that was included in the coatings was later reduced to rGO as described previously. We don’t understand why, contrary to common author guidelines, these procedures need to be described again in the present manuscript? If the reviewer can please explain, then we would be happy to try to accommodate them.

 

3. The usefulness of the catalytic system used in this manuscript is still not evident. The authors focus on oxygen production, however, the technically interesting product form water splitting is hydrogen. Further, photocatalysis is about 1000 times less efficient for water splitting than direct hydrolysis of water , see e.g. s. Shiva Kumar, V. Himabindu, Mater. Sci. Energy Technol.  2019, 2, 442-454. Also, photocatalysis only operates properly in the presence of sun light, and the cobalt in the Co3O4 is considered more and more problematic due to toxicity; accordingly there is, for example, a strong tendency to omit cobalt in batteries.

 

We, respectfully, again don’t understand the reviewer’s comments. The technically interesting product is certainly the hydrogen that is produced. But, to make hydrogen, oxygen evolution must simultaneously be produced at the other electrode. Oxygen production is, by far, the more demanding and difficult process to catalyse. Thus, while the 2-e^- hydrogen evolution reaction (HER) has an E^o of 0 V, the 4-e^- oxygen evolution reaction (OER) has an E^o of 1.23 V, which is vastly greater. Accordingly, we have focused on maximizing the rate of the OER as a means of maximizing the rate of the HER. This is a standard approach that is very widely used in the field. We do not understand why the reviewer would object to this and why they would want anything else? If the reviewer can explain what it is that they want, then we would be happy to try to accommodate them.

 

We also, respectfully, don’t understand the reviewer’s point that photocatalysis is less efficient than direct hydrolysis. Photocatalysis can occur constantly and indefinitely, so long as the sun is shining on the photocatalyst. By contrast, hydrolysis can only occur while there is a suitable reactant to facilitate it. When the reactant runs out, the hydrolysis stops. The fact that sunlight facilitates water splitting is precisely why thousands of scientists are presently working on photocatalytic water-splitting. Is the reviewer saying that all of this work is a waste of time and those scientists should be focusing on hydrolysis? We are quite mystified by these comments and would appreciate a further explanation.

 

The same goes for the reviewer’s comment regarding cobalt and Co₃O₄. This material is widely used as a spinel catalyst in industry, including in water electrolyzers. It is manufactured in volume and worldwide cobalt production is growing fast. Is the reviewer seriously suggesting that it should not be studied as an OER catalyst? We do not understand the reviewer’s point and would appreciate further clarification.

 

4. It is still not indicated in the manuscript how much oxygen is produced with the system described in this manuscript per hour and gram of catalyst. It is claimed in the conclusions that oxygen evolution rate is “excellent” but the quantity of produced oxygen is not indicated, and the production hardly excellent compared to direct electrolysis, and not even a comparison with the best other photocatalytic water splitting systems is evident

 

With respect, the amount of oxygen was provided in the revision. We indicated that the Faradaic efficiency for oxygen production was 61.2-62.1%, which means that this percentage of the electrons travelling through the circuit were converted into oxygen gas, which we physically collected in our apparatus. This is the conventional way of reporting gas production in photocatalytic systems, where the quantity of electrons passing through the cell is more accurately determined than the absolute weight of catalyst. For the record, that equates to 1.3 mmol/min in the dark and 1.7 mmol/min in the light, which figures could have been calculated from the Faradaic efficiency and have now been included in the Supplementary Material.

 

We, respectfully, also submit that that rate is “excellent”. As noted in Figure 2, by comparison, the precious metal, platinum, which has historically been widely used as the oxygen-generating anode in industrial alkaline water electrolyzers (but is now far too expensive for such an application), produces an order of magnitude less oxygen for the same geometric area, at the same applied voltage and pH (12). Thus, we have a nanocomposite, Earth-abundant electrocatalyst that is far more active than the best precious metal catalyst for the reaction. We do not understand what the reviewer finds so objectionable about this?

 

We also do not understand why it would be necessary for us to compare this result with other photocatalysts? We have made a comparison with the ‘Rolls Royce’ of oxygen-generating catalysts. Why would a comparison with other photocatalysts be relevant, especially where such other photocatalysts have only been tested under a wide variety of non-comparable conditions?  By contrast, we have tested Pt under precisely the same conditions. If the reviewer could explain, we will try to understand and respond accordingly.

 

The rates of oxygen generation are certainly less than in alkaline water electrolyzers, but such electrolyzers utilize extremely corrosive conditions that are not amenable to sustained photocatalysis. As such, they are irrelevant to this project, which seeks to develop systems that utilize very mild, non-corrosive (pH 12) conditions.

 

5. The laboratory tests with the hydrogen lamp are by far not sufficient to prove that the PEDOT system may withstand intense sun light for years. A proper xenon lamp test would be required for technical applications.

 

We, respectfully, do not understand why the reviewer expects us to prove that a PEDOT system will withstand intense sunlight for years. That may be relevant to making a commercial product, but it has no bearing on a research project such as that reported here. There are, literally, thousands of scientific papers on conducting polymers in light-activated applications. None has addressed this question. Why does ours need to? If the reviewer can explain then we will be happy to try to respond.

 

6. All in all, unfortunately my comments have been met only marginally so I regret that I still have to recommend rejection of this manuscript essentially for the reasons addressed already in my first review.

 

As noted above, we are a little confused and mystified by the reviewer’s comments. We cannot understand the objections. But, if the reviewer can help us to comprehend their point of view, we will, nevertheless, try to address it.