Green Catalysts: Applied and Synthetic Photosynthesis^†

Round 1

Reviewer 1 Report

Green Catalysts: Applied and Synthetic Photosynthesis review

The manuscript presents an interesting topic in recent advances in the generation of photocurrents and the generation of solar fuels in biohybrid solar cells utilizing reaction centers along with the production of hydrogen as a fuel, however, in my opinion, revision in the manuscript should be done before considering it for publication. Please see my comments below:

1- In line 41 in the introduction the authors mentioned the advantages of photovoltaics, which is absolutely fine, however you miss the discussion of the recycling of PV panels after 25 years of operations. The global exponential increase in installations of Photovoltaic (PV) modules and the subsequent waste generated is an increasing concern. It is estimated that between 60-78 million tonnes of PV module waste will be generated by 2050. As of 2014, PV modules were added to the EU’s Waste Electrical and Electronic Equipment (WEEE) directive making it law; that manufacturers and suppliers are responsible for their end-of-life management. This, alongside the amount of rare and precious metals these modules contain, provides added incentive, both environmental and economical, to recycle these panels. Between 80-90% of the current PV market share is dominated by first-generation PV known as crystalline silicon (c-Si) PV modules. This needs to be discussed in the introduction.

2- Also, according to the title and the content of green catalysts for hydrogen production; a process based on waste materials should be considered such as the photocatalytic production of H₂ from glycerol, a by-product from the manufacture of bio-diesel over alumina catalyst (derived from foil waste) as a green route for the production of hydrogen as in the recent publication of Osman et al., “Exploring the photocatalytic hydrogen production potential of titania doped with alumina derived from foil waste”, International Journal of Hydrogen Energy 2020.

3- In line 726, authors mentioned “Artificial photosynthesis endeavors to mimic the overall chemistry of converting water (and CO2) to oxygen and a fuel (either H2 or reduced carbon species) with light using technological components”. Herein, the discussion of the global warnings mitigation approach through carbon sequestration should be mentioned, where the conventional mitigation technologies, negative emissions technologies (NETs) and radiative forcing geoengineering (RFG) techniques.

Author Response

Reviewer 1 Comments:

1. In line 41 in the introduction the authors mentioned the advantages of photovoltaics, which is absolutely fine, however you miss the discussion of the recycling of PV panels after 25 years of operations. The global exponential increase in installations of Photovoltaic (PV) modules and the subsequent waste generated is an increasing concern. It is estimated that between 60-78 million tonnes of PV module waste will be generated by 2050. As of 2014, PV modules were added to the EU’s Waste Electrical and Electronic Equipment (WEEE) directive making it law; that manufacturers and suppliers are responsible for their end-of-life management. This, alongside the amount of rare and precious metals these modules contain, provides added incentive, both environmental and economical, to recycle these panels. Between 80-90% of the current PV market share is dominated by first-generation PV known as crystalline silicon (c-Si) PV modules. This needs to be discussed in the introduction.

Thank you for this important point, and we have added mention of this to the introduction to help frame the necessity of emerging PV technologies more clearly! This can be seen in Section 1, line 45-52 “However, the overwhelming majority of current photovoltaics in use today are crystalline silicon solar cells, part of the first generation of photovoltaic technologies. These utilize rare and precious metals in their materials that require specific recycling requirements, and are estimated to generate over 50 million tons of waste by 2050. Clearly, further improvement on current photovoltaic technology is necessary for the future. Applied photosynthesis, being studied as an emerging photovoltaic technology, takes inspiration from and utilizes the fundamental mechanisms and components that Nature has already developed for biological photosynthesis, specifically the light reactions of biological photosynthesis.’

2. Also, according to the title and the content of green catalysts for hydrogen production; a process based on waste materials should be considered such as the photocatalytic production of H₂from glycerol, a by-product from the manufacture of bio-diesel over alumina catalyst (derived from foil waste) as a green route for the production of hydrogen as in the recent publication of Osman et al., “Exploring the photocatalytic hydrogen production potential of titania doped with alumina derived from foil waste”, International Journal of Hydrogen Energy 2020.

We thank you for this added reference! We add mention to it in line 751-753 of the manuscript “A completely inorganic photocatalytic method for production of molecular H₂ from glycerol has been recently published, utilizing aluminia-doped TiO₂, an example of a fully artificial photosynthetic catalytic process [162].” and have added that reference.

3. In line 726, authors mentioned “Artificial photosynthesis endeavors to mimic the overall chemistry of converting water (and CO2) to oxygen and a fuel (either H2 or reduced carbon species) with light using technological components”. Herein, the discussion of the global warnings mitigation approach through carbon sequestration should be mentioned, where the conventional mitigation technologies, negative emissions technologies (NETs) and radiative forcing geoengineering (RFG) techniques.

We have added a mention to the secondary benefits of carbon sequestration by CO₂ reduction for valuable product and fuel generation, thank you for the reminder to more explicitly state this as well. This is in the revised manuscript lines 762-765 “Reduction of CO₂ to more chemically complex valuable products and fuels has a secondary benefit of carbon sequestration, and could potentially help act as negative emissions technologies to help reduce atmospheric CO₂ concentrations, further boosting the potential benefits of these solar technologies.”

Reviewer 2 Report

This manuscript is a very nice overview on the generation of photocurrents and solar fuels in biohybrid solar cells. The role of different parameters such as electrode material and surface coverage, semiconductor material and architectures,  electron transfer pathways and redox mediators, electrolytes and working conditions are discussed as well as several reactions, among them hydrogen evolution and CO₂ reduction.

The structure is good and it is written very clear. The list of references is nearly complete and the work will be appeal to a wide audience. I do think that this paper is very interesting and can be published in Catalysts in its current version.

Author Response

Reviewer 2 Comments:

This manuscript is a very nice overview on the generation of photocurrents and solar fuels in biohybrid solar cells. The role of different parameters such as electrode material and surface coverage, semiconductor material and architectures, electron transfer pathways and redox mediators, electrolytes and working conditions are discussed as well as several reactions, among them hydrogen evolution and CO₂ reduction.  The structure is good and it is written very clear. The list of references is nearly complete and the work will be appeal to a wide audience. I do think that this paper is very interesting and can be published in Catalysts in its current version.

We thank you for the favorable review, and are glad you enjoyed the article and found it informative and interesting! Thank you for your time in reviewing this manuscript.

Reviewer 3 Report

In this very interesting review the authors discuss the research advances in understanding and exploiting photosynthesis as a catalytic process. 

Two minor comments:

In the section 2.1, this work should be mentioned: 10.1126/science.aar8313. It is a new form of photosynthesis that can be highly relevant for future research and development.

In section 3.1, this work should be mentioned: 10.1021/acs.biochem.6b00780. It is an important feature of Fe-Fe hydrogenases that can be very helpful for future  technology development.

Author Response

Reviewer 3 Comments:

In this very interesting review the authors discuss the research advances in understanding and exploiting photosynthesis as a catalytic process. 

We thank you for the positive review, and are glad you found it interesting and worthwhile of publication! Thank you for your time in reviewing this manuscript.

Two minor comments:

1. In the section 2.1, this work should be mentioned: 10.1126/science.aar8313. It is a new form of photosynthesis that can be highly relevant for future research and development.

Thank you for suggesting this reference to be included, we added it into section 2.1 of this review as suggested here in line 132 “These modifications have also been seen in nature, with photosynthetic organisms  utilizing novel forms of chlorophyll pigments with unique optoelectronic properties to make use of the portion of the UV-Visible spectrum that is available to them [6].”.

2. In section 3.1, this work should be mentioned: 10.1021/acs.biochem.6b00780. It is an important feature of Fe-Fe hydrogenases that can be very helpful for future technology development.

Thank you for the additional reference! This could be a potential new exciting advancement in the field of hydrogenase-mediated H₂ production. We added this  reference into the manuscript in section 3.1, at  line 443 “Recent studies have suggested the possibility of an in vitro method for stably and reversibly reversing O₂ inactivation of an [FeFe]-hydrogenase from the non-photosynthetic organism Clostridium beijerinckii (CbA5H), which may be able to be used with other [FeFe]-hydrogenases as well [74].”