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Turning Sunlight Into Liquid Fuel

For millions of years, green plants have employed photosynthesis to capture energy from sunlight and convert itinto electrochemical energy. A goal of scientists has been to develop an artificial version of photosynthesis that can be used to produce liquid fuels from carbon dioxide and water.

Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have now taken a critical step towards this goal with the discovery that nano-sized crystals of cobalt oxide can effectively carry out the critical photosynthetic reaction of splitting water molecules. Researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory  are now in the process of a major breakthrough towards artificial photosynthesis. They are quite hopeful about the properties of nano-sized crystals of cobalt oxide.

They are banking on cobalt oxide that can effectively carry out the crucial photosynthetic reaction of splitting water molecules. Artificial photosynthesis will not add up to the green house gases and hence global warming. This will be a renewable resource for transportation energy. The idea is to create an artificial leaf that can duplicate the few steps of photosynthesis. That leaf can capture the solar photons and have a catalytic system in place that can oxidize water.

Heinz Frei and Feng Jiao have published the findings of their study in the journal Angewandte Chemie. This research was carried out with help of the Helios Solar Energy Research Center (Helios SERC), a scientific program at Berkeley Lab under the direction of Paul Alivisatos. They are concentrating on developing fuels from sunlight. Frei is the deputy director of Helios SERC. Heinz Frei, who is a chemist with Berkeley Lab’s Physical Biosciences Division, explains, “Photooxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artificial photosynthesis system – it provides the electrons needed to reduce carbon dioxide to a fuel.” He again emphasized why he is putting lots on effort into cobalt oxide, “Effective photooxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with solar flux in order to avoid wasting those photons.

Clusters of cobalt oxide nanocrystals are sufficiently efficient and fast, and are also robust (last a long time) and abundant. They perfectly fit the bill.” Fredi also explained why they are not very keen on using iridium oxide for artificial photosynthesis. He stated though iridium oxide is efficient and fast enough for light absorption and a good catalyst but this metal is least abundant metal on earth. Hence it is not very practical to use it on commercial scale. He says, “We needed a metal that was equally effective but far more abundant.” First they tried to take the manganese-based organometallic complexes for artificial photosynthesis, which plants use in Photosystem II. But manganese-containing compounds were water insoluble and not very robust. Fredi and his team paid attention to cobalt oxide which is a highly abundant material and fit for commercial use.

Cobalt oxide also dissolves in water. But it was not a success story right from the beginning. The micron-sized particles of cobalt oxide were ineffective and slow to act as catalysts. Then Frei and Jiao turned to nano-sized cobalt oxide. “The yield for clusters of cobalt oxide (Co3O4) nano-sized crystals was about 1,600 times higher than for micron-sized particles,” said Frei. “And the turnover frequency (speed) was about 1,140 oxygen molecules per second per cluster, which is commensurate with solar flux at ground level (approximately 1,000 Watts per square meter).” The next big step, however, will be to integrate the water oxidation half reaction with the carbon dioxide reduction step in an artificial leaf type system.

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