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Winter 2019

A Key Link in Harnessing the Power of Photosynthesis for Carbon-Neutral Fuel

Scientists design a molecule that helps to replicate photosynthesis for solar energy conversion

Evan Lafalce

Triarylamines (shown in the red rectangle) are chemically attached to a ruthenium polypyridyl molecule (shown in the blue rectangle). When light is absorbed by the ruthenium complex, it transfers electrons (blue arrows) that can be used to convert carbon dioxide (CO2) into fuel, mimicking one side of natural photosynthesis. The triarylamines allow the molecule to function in a similar manner to the water-splitting side of photosynthesis, producing hydrogen ions (blue arrows) and releasing oxygen gas (red arrows). Credit: Adapted with permission (see "More Information"). Copyright 2018, American Chemical Society

Photosynthesis is the way that plants use sunlight to produce food. Ultimately feeding the lifecycle of the entire biosphere, photosynthesis is the primary energy source of life on Earth. In this process, plants take in carbon dioxide from the atmosphere and then use the energy from the sun to convert the carbon dioxide into sugars that plants consume as food. Because the overall process turns carbon dioxide into potential biofuel building blocks and produces energy, there is a great interest among scientists to harness and replicate the mechanisms of natural photosynthesis in the laboratory. This field is referred to as artificial photosynthesis.

Artificial photosynthesis can benefit society in two ways. First, it has the potential to be used as a “green” energy source; that is, a way of producing energy that does not emit carbon dioxide. Secondly, it may be used for carbon dioxide sequestration, which is to pull the carbon dioxide right out of the atmosphere. Artificial photosynthesis may be a valuable tool to mitigate climate change due to the release of carbon dioxide during the burning of fossil fuels. The current challenge is to make this process efficient enough to be useful to society.

One of the biggest challenges in achieving artificial photosynthesis has been to efficiently split water into hydrogen and oxygen. Natural photosynthesis is controlled by a collection of molecules referred to as Photosystem II. When Photosystem II absorbs sunlight, it drives a two-sided reaction. On one side, water molecules are split into separate hydrogen ions and oxygen molecules. On the other side, electrons are transferred from Photosystem II to other molecules. The released hydrogen ions and transferred electrons are both necessary to chemically convert carbon dioxide from the atmosphere into sugars.

Researchers at the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED), formerly known as the UNC EFRC Center for Solar Fuels, have recently made a breakthrough that may significantly enhance artificial photosynthetic efficiency and practicality. They designed and studied molecules that help maintain the water-splitting side of the reaction. The scientists started with a molecule that is commonly used to absorb light and create the electron-transfer side of the reaction, known as ruthenium polypyridyl. They then chemically attached new molecules called triarylamines onto the ruthenium molecule. These additional triarylamine molecules performed in a similar manner as the molecules in natural photosynthesis do. Through detailed measurements, the scientists studied the way the molecule responds when exposed to light. The triarylamines helped to mimic the water-splitting side of the reaction of photosynthesis. Furthermore, the researchers directly demonstrate the ability of this molecule to produce electricity from sunlight in a photochemical cell.

The design and discovery of a molecule that can successfully replicate the behavior of water-splitting in natural photosynthesis is an important breakthrough, but it also may be just the beginning. The scientists still need to study how efficiently the molecules can produce biofuels in artificial photosynthetic devices and hope to optimize the properties of the molecules for this purpose. Fortunately, the triarylamines are a whole family of molecules that can be systematically tailored to achieve the best results. With this achievement, the researchers at the AMPED center have made a key step towards harnessing artificial photosynthesis and providing solutions to issues stemming from the use of carbon-based fuels.

More Information

Eberhart MS, LM Rader Bowers, B Shan, L Troian-Gautier, MK Brennaman, JM Papanikolas, and TJ Meyer. 2018. “Completing a Charge Transport Chain for Artificial Photosynthesis.” Journal of the American Chemical Society 140:9823. DOI: 10.1021/jacs.8b06740

Acknowledgments

This research was solely supported by the Center for Solar Fuels (now known as the Alliance for Molecular PhotoElectrode Design for Solar Fuels), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

About the author(s):

  • Evan Lafalce is a research assistant professor at the University of Utah, Department of Physics & Astronomy, working under Z. Valy Vardeny. He is a member of the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), where he studies the optical, electronic, and spin properties of hybrid organic-inorganic lead-halide perovskites. He obtained his Ph.D. in applied physics from the University of South Florida in Tampa, where he characterized organic solar cells and organic solar cell materials. He was born in Baltimore, Maryland.

Electron Manager Recruited for Artificial Photosynthesis

New molecule absorbs light and juggles electrons, beneficial for solar fuels

Mimicking photosynthesis in plant leaves could let us use sunlight to produce fuels. Scientists devised an electron-managing molecule to help that process along. Credit: Pixabay

In nature, plants use sunlight and carbon dioxide to create fuel via photosynthesis. If we could mimic photosynthesis on an industrial scale, we could use sunlight to make solar fuels. But we need a molecule that can manage the electrons in a key reaction. The natural one just won’t do. A team at the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED) built a hard-working electron manager. It keeps the key water-splitting reaction going; however, it isn’t perfect. It is a major step forward for solar fuels. AMPED is an Energy Frontier Research Center based at the University of North Carolina.  

More Information

Eberhart MS, LM Rader Bowers, B Shan, L Troian-Gautier, MK Brennaman, JM Papanikolas, and TJ Meyer. 2018. “Completing a Charge Transport Chain for Artificial Photosynthesis.” Journal of the American Chemical Society 140:9823. DOI: 10.1021/jacs.8b06740

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.