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Chemistry

Artificial photosynthesis

Lab-made molecules mimic the energy production mechanism of plants

Léo RamosImagine a metal plate coated in a lab-made material, submerged in a bottle of water, and that starts to produce and store energy in the form of hydrogen gas, simply because it’s out in the sun. “We are thinking of a world where water is used as fuel,” says chemist Jackson Megiatto from the University of Campinas (Unicamp). The device is not yet ready for large-scale use, but according to the researcher, neither is it science fiction anymore. “A body of knowledge is being built, for obtaining energy from the sun and water in the near future.” Hydrogen is an important energy source, not only on account of its efficiency, but also because it generates no pollutants when used as fuel. But producing the gas has been a major challenge. In partnership with researchers from Arizona State University (ASU) and the University of Pennsylvania, Megiatto came one step closer to solving the problem: reproducing the reaction that breaks down water molecules in the presence of sunlight, in the laboratory.

Plants, algae, and certain bacteria have the unique capability of producing energy from water and sunlight, and they all do it the same way: photosynthesis, involving complex molecules and chemical reactions that are not yet completely understood. When activated by sunlight, these natural molecules are able to break down one of nature’s most stable molecules, H2O, into its oxygen and hydrogen components. “This stability of water is so strong that when we try to reproduce the process, our molecules get broken down before water does,” Megiatto explains.

The project’s biggest innovation is the design of the photo-active molecules and nanoparticulate catalysts, which mimic the natural photosynthetic system that plants have been using for millions of years to accumulate the energy that sustains most life on Earth. The results were published in two papers in PNAS in 2012, and more recently online in Nature Chemistry on February 9, 2014.

After studying what is presently known about natural photosynthesis, Megiatto went into the laboratory and successfully synthesized a more robust group of molecules, called perfluoroporphyrins, that behave similarly to P680, a naturally occurring cofactor in plants. To imitate the protein structure of the natural system that is directly involved in the water breakdown process, it was also necessary to add a phenol group to the porphyrin. “When excited by sunlight, porphyrin steals an electron from the phenol group, generating a chemical species with sufficient energy to break down water molecules,” describes the chemist from Unicamp, who conducted the work during his time as an associate researcher at ASU and the Center for Bio-inspired Solar Fuel Production (BisFuel), created in 2009 with a $14 million investment from the U.S. Department of Energy.

The team monitored electron transfers between porphyrin and phenol using a technique known as electron paramagnetic resonance spectroscopy. “The technique detects only the free electrons in molecules, and not those involved in chemical bonds in the material,” Megiatto explains. They observed responses very similar to those obtained when a natural photosynthetic system is submitted to the same analysis, indicating a similarity in how these compounds transport electrons when exposed to sunlight.

46-48_Molecula-H2O_217INFOGRAPHIC: Ana Paula Campos; ILLUSTRATION: Alexandre Affonso“Up until now, no material had been able to transfer electrons so similarly to the natural system,” celebrates the chemist. Although the results were achieved in 2011, the group made a point of running exhaustive tests before publishing them, to make sure that they could be reproduced. They also analyzed the new material using other techniques. It worked. “The material has been synthesized in Arizona even by undergraduate students, and the results are always the same,” says the researcher.

The material developed by Megiatto is already being used in photosynthetic devices that work like tiny, water-based power plants. The plan is to connect them to fuel cells. Preliminary tests have shown, however, that the system is not yet efficient enough for large-scale energy production. Additional laboratory studies will be necessary for refining the energy production system.

Upon completing this work, Megiatto was about to sign on as a professor at BisFuel when he heard about an opening at Unicamp’s Chemistry Institute. He decided to return to Brazil. From here, he maintains an ongoing collaboration with the American group through integrated research, online meetings and, in the future, a student exchange program between the Brazilian and American laboratories, for upcoming stages of the study that will require the use of specific equipment located in either country.

In the near future, Megiatto wants to find a way of improving the performance of the porphyrin-based material and the efficiency of the photochemical process, aiming ultimately to reduce the cost of energy production. His plan is to make the porphyrin and phenol molecules organize themselves spontaneously like pieces in a construction set, instead of needing to be chemically bound to each other. He explains that it’s necessary to find out how to “talk” to these chemicals dispersed in a solution: “You there, come over here, hold hands with that other molecule over there…”. Once again, this is not science fiction, but part of a science called supramolecular chemistry. “The costs would drop significantly and efficiency would improve,” predicts the chemist, if his project succeeds.

Far from standing alone in his quest for fossil-free fuels, Megiatto was invited in late February 2014 to explain the new technology and discuss future alternatives at the Delft University of Technology in the Netherlands. He also talked about artificial photosynthesis at the multidisciplinary meeting Frontiers of Science, organized in England by the Royal Society, FAPESP, and the Brazilian Academy of Sciences. If joint initiative is all it takes, then the days of plants doing all the photosynthesis are numbered.

Scientific articles
MEGIATTO, J. D. et al. A bioinspired redox relay that mimics radical interactions of the Tyr-His pairs of photosystem II. Nature Chemistry. on-line. Feb. 9, 2014.
MEGIATTO, J. D. et al. Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation. PNAS. V. 109, p. 15.578-583. 2012.
ZHAO, Y. et. al. Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator. PNAS. v. 109, p. 15.612-616. 2012.

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