(Nanocarbons Division Richard E. Smalley Research Award Address) Artificial Photosynthesis Utilizing Nanocarbons
Authors: Fukuzumi Shunichi 
Published: February 2020

Sustainable and clean energy resources using solar energy alternative to fossil fuels are urgently required in order to stop global warming and solve global energy and environmental issues. This award lecture focuses on our long-term developments in artificial photosynthesis for solar fuel production and its usage by utilizing nanocarbon materials.
We have developed a number of photosynthetic reaction center models composed of metalloporphyrins and fullerenes with comparable charge-separated state lifetimes with that of the natural photosynthetic reaction center.[1-3] The light-harvesting unit and the charge-separation unit have been combined by utilizing coordination bonds, hydrogen bond, electrostatic interactions and p-p interactions.[4,5] The efficient photosynthetic reaction center models have been successfully combined with the catalytic water reduction unit to attain the most efficient photocatalytic hydrogen evolution system.[6-8]
The activation of hydrogen has also been achieved by developing the first structural and functional model of hydrogenases,[9] which enables to replace precious Pt catalyst by much more earth-abundant metal catalyst for hydrogen evolution.[10] With regard to water oxidation, we have developed efficient and robust water oxidation catalysts using earth-abundant metal oxides nanoparticles.[11] We have also clarified the significant role of redox-inactive metal ions on enhancement of electron-transfer reactions of high-valent metal-oxo complexes, which is classified as metal ion-coupled electron transfer,[12,13] in relation with the pivotal role of Ca2+ in the oxygen evolving complex (OEC) in Photosystem II.[14,15]
By combination of the catalytic four-electron water oxidation and the photocatalytic two-electron reduction of O2 using solar energy, we have succeeded in production of hydrogen peroxide (H2O2) as a solar fuel from water and O2 in the air.[16,17] We have also achieved efficient photocatalytic production of H2O2 from the most earth abundant seawater instead of precious pure water and O2 in the air using a two-compartment photoelectro-chemical cell with WO3 as a photocatalyst for water oxidation and a cobalt chlorin complex supported on a glassy-carbon substrate for the selective two-electron reduction of O2.[18] The highest solar energy conversion efficiency was determined to be 6.6% under simulated solar illumination adjusted to 0.05 sun after 1 h photocatalytic reaction (0.89% under 1sun illumination) when WO3 was replaced by the surface modified BiVO4 with iron(III) oxide(hydroxide) (FeO(OH)) as a water oxidation catalyst in the photoanode.[20]
We have also developed one-compartment H2O2 fuel cells using H2O2 produced in seawater as a solar fuel and earth-abundant metal catalysts.[18,20,22] Thus, the combination of the photocatalytic H2O2 production from seawater and O2 using solar energy with one-compartment H2O2 fuel cells provides on-site production and usage of H2O2 as a more useful and promising solar fuel than H2.[18]

References

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