Copying Nature's Radical Fuel Production
A major challenge in renewable energy is devising sustainable ways of making fuel. This is exactly what photosynthesis does, converting solar energy (sunlight) into electrochemical energy by using electrons donated from water, and making oxygen as a byproduct. The ability of plants, microbes, and other natural photosynthetic organisms to generate relatively long-lived chemically reactive molecules, or oxidizing radical species for short, and using them to split water so the resultant positive and negative charges can be used in producing bio-energy has inspired the design of an artificial system for use in solar fuel production at the Center for Bio-Inspired Solar Fuel Production (BISfuel).
The Power of Photosynthesis: One of the first stages of photosynthesis involves photosystem II (PSII), a protein that provides electrons for water splitting and is activated by light. The light is captured using molecules known as "chlorophylls." The captured light energizes electrons that are transferred through a variety of molecules. The energized electrons are replaced by splitting or oxidizing water into molecular oxygen, protons and electrons. If harnessed correctly, PSII could be used as a major energy source. The ability of proteins such as PSII to create oxidizing radical species, which split water and release electrons, capable of surviving long enough to be transported within the organism is one of the key secrets that allow the conversion of light into bio-energy.
Anything You Can Do, I Can Do Better: The BISfuel artificial system – a molecular triad – transfers electrons to oxidize water just as quickly as PSII's oxidizing radical species, rivaling natural photosynthesis. However, the formation of oxidizing radical species to split water is very challenging. Water is highly stable; it requires power from the radical species to allow it to be oxidized.
These radical species produced need to survive long enough to get the job done, and not undergo any chemical reactions. The researchers extended the triads lifespan by protein-radical interactions that are involved in the transfer of electrons and protons for oxidizing water. Elaboration of synthetic models able to mimic these complex protein/radical interactions is a very important, but challenging, step in the development of artificial photosynthetic devices.
Thus, an artificial system for use in artificial photosynthetic fuel production must not only mimic the electron transfer of the natural system, but the final charge separated state, needed for the oxidation of water, should also survive long enough that it would be capable of splitting water.
Designing an Artificial Mimic: Inspired by nature, the team used molecules called porphyrins, which are artificial alternatives to the chlorophylls in natural systems, that are attached to each other. These triad molecules were designed and synthesized in such a way that upon exposure to light, they transfer electrons and protons, mimicking PSII, and producing radical species with enough power to split water.
What You See Is What You Get: Using a trio of studies, the team showed that the synthesized triad mimics the electron transfer of PSII and that the final charge separated state was thermodynamically capable of water oxidation. More importantly, the radicals produced yielded a long enough lifetime to allow the triads to be coupled to artificial catalysts which were capable of splitting water.
Such photocatalytic units could be coupled to semi-conducting electrodes to incorporate them into complete systems for solar fuel production. The research group believes that the system reported is an important step forward in the assembly of artificial photosynthetic systems able to produce fuel from sunlight.
Megiatto, Jr. JD, A Antoniuk-Pablant, BD Sherman, G Kodis, M Gervaldo, TA Moore, AL Moore, and D Gust. 2012. "Mimicking the Electron Transfer Chain in Photosystem II with a Molecular Triad Thermodynamically Capable of Water Oxidation." Proceedings of the National Academy of Sciences of the United States of America. DOI: 10.1073/pnas.1118348109
This work was sponsored by the Center for Bio-Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.