Semi-artificial photosynthesis: Sunlight into fuel

Photosynthesis

Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules are constructed from water and carbon dioxide, and oxygen is released as a byproduct. The glucose molecules provide organisms with two crucial resources: Energy and fixed-organic-carbon.

Photosynthetic organisms, including plants, algae, and some bacteria, play a key ecological role. They introduce chemical energy and fixed carbon into ecosystems by using light to synthesize sugars. Since these organisms produce their own food-that is, fix their own carbon-using light energy, they are called photoautotrophs.

The chemical formula of photosynthesis is written as:

6H20 + 6CO2 ——-> C6H12O6 + 6O2

And can be translated as six water molecules plus six carbon dioxide molecules yields one molecule of sugar and six oxygen molecules.

It is one of the most important reactions on the planet because it is the source of nearly all of the world’s oxygen. Hydrogen which is produced when the water is split could potentially be a green and unlimited source of renewable energy.

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Solar Energy

The apparent incompatibility between the increasing energy demand, environmental awareness, and the excessive consumption of finite fossil fuels has spurred incessant research endeavors in seeking renewable and green energy resources to maintain the sustainability of our society. Solar energy, as an inexhaustible clean energy source that powers all the life on the Earth, is considered to be the most exploitable one. The conversion and utilization of solar energy for chemical fuel production and environmental remediation through artificial photocatalysis has been recognized to be an ideal scheme to address the worldwide energy and environmental concerns. Essentially, there is a need to transform the current world from a consumptive fossil fuel-powered mode into a sustainable “photons” mode. To enable the large-scale application of the photocatalysis technique, improving the solar energy utilization efficiency is crucial.

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Semi-artificial photosynthesis bridges the rapidly progressing fields of synthetic biology and artificial photosynthesis, and offers a platform to develop and understand solar fuel generation. Synthetic biology has vastly opened up the way that nature can be manipulated to streamline functionality and to build artificial biological systems, but its current complex machineries and metabolic pathways limit engineering flexibility. Artificial photosynthesis utilizes synthetic, often biomimetic, components to convert and store solar energy, but it is often constrained by inefficient catalysis and costly and/or toxic materials.

Solar-driven water splitting into H2 and O2 is the most prominent model reaction in artificial photosynthesis. Inefficient catalysis (particularly, kinetically slow O2 evolution and the formation of partially oxidized side products) is a major limitation in synthetic systems, and results in the requirement of large over-potentials and energy conversion losses. Oxygenic organisms convert solar energy using a photosynthetic Z scheme that contains two light absorbers, photosystem I (PSI) and photosystem II (PSII). In this tandem configuration, the first excitation in PSII drives water oxidation to O2 and produces a proton gradient, whereas the second excitation in PSI generates a low potential electron to drive CO2 fixation into sugars. Alternatively, H2 can be produced from microalgae and cyanobacteria via electron transfer from ferredoxin to a [FeFe]-hydrogenase ([FeFe]-2ase), which reduces protons to H2. The efficiencies for photo biological H2 production are low for several reasons. First, PSII and PSI overlap in light absorption and compete for a small fraction of the solar spectrum. Second, high light intensities limit the efficient electron flux upstream and downstream of PSII. Third, in vivo H2 production relies on O2-sensitive [FeFe]-H2ases, which prevents sustained water splitting. Fourth, CO2 fixation is preferred over proton reduction, which leads to low H2 yields. Overcoming these limitations offers scope to enhance H2 production with biological components.

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A new study, led by academics at St John’s College, University of Cambridge, used semi-artificial photosynthesis to explore new ways to produce and store solar energy. They used natural sunlight to convert water into hydrogen and oxygen using a mixture of biological components and manmade technologies.

The research could now be used to revolutionize the systems used for renewable energy production. A new paper, published in Nature Energy, outlines how academics at the Reisner Laboratory in Cambridge developed their platform to achieve unassisted solar-driven water-splitting. Their method also managed to absorb more solar light than natural photosynthesis.

The Experiment:

A semi-artificial system for the unassisted, light-driven water splitting with PSII and H2ase was presented. This PEC system does not require an external energy input as dual light absorption is realized by a tandem photoanode that consists of PSII wired to dye-sensitized titanium dioxide (TiO2), which provides a sufficient voltage to reduce protons using a H2ase cathode. This PEC design was inspired by dye-sensitized solar cells and it allows PSI to be replaced by a rationally designed diketopyrrolopyrrole (dpp) dye with absorption complementary to that of PSII. An efficient electronic communication between PSII and dpp was achieved by using the redox polymer poly-1-vinylimidazole-co-allylamine-Os(bipy)2Cl (POs), which bypasses possible limitations from an inefficient interfacial electron transfer. Simultaneously, the hydrogel character of the redox polymer provides a solvated environment for the biocatalyst. A hierarchically structured inverse opal TiO2 (IO-TiO2) scaffold was employed to provide a high surface area for the effective integration of polymer/PSII.

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Conclusion: The tandem system produced H2 and O2 from water with high Faradaic efficiencies in a 2:1 ratio and presents an effective strategy to construct biotic-abiotic interfaces. Future work will involve investigating other dyes and replacing TiO2 with a semiconductor with a more negative CB potential to enhance the driving force for a more efficient catalysis or CO2 reduction chemistry. Moreover, the study provides a blueprint to advance future semi-artificial systems capable of bias-free solar fuel and chemical synthesis and a toolbox to develop proof-of-concept model systems for solar energy conversion.

©BforBiotech by Bedadyuti Mohanty, Assistant Managing Editor by Profession and Bio-technologist by heart.

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