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Autumn's leaves have mostly fallen here in Brooklyn, but nearby at the Massachusetts Institute of Technology (MIT), Daniel Nocera and his colleagues are making foliage they hope will have a long future.1 Instead of a natural leaf's tissues, chloroplasts, and vascular structures, these artificial leaves are each made from a thin sheet of semiconducting silicon with different materials bound to either side: a nickel-molybdenum-zinc alloy on one surface and a cobalt-based catalyst on the other.2 When submerged in water and exposed to sunlight, the solar cell's cobalt side releases oxygen from the H2O — a breakthrough first identified by Nocera and his team in 20083 — while the alloy side encourages hydrogen molecules to effervesce. Mimicking the water splitting that occurs during natural photosynthesis, these reactions can be elicited with artificial light, as illustrated by the following soundless video from the Nocera lab.
Researchers are hoping they will soon be able to collect, store, and redirect these ionic streams through fuel cells that can recombine the molecules back into water again, resulting in surges of electrical power. Unlike existing fuel cells, these systems could operate without hydrogen provided by fossil fuels,4 as they are totally solar powered, as well as having the advantages of being lightweight, wireless, and constructed from inexpensive, earth-abundant materials such as silicon, cobalt, borate, and nickel.5 These features make the technology very appealing to environmentalists, but the process is still far from being optimized. While MIT's artificial leaves redirect around 2.5% of solar energy into hydrogen production, a variation that uses wires to link the two catalytic materials has achieved an efficiency of 4.7%.6 Unfortunately both of these rates still fall far below the efficiencies of commercial solar cells, which typically now have rates of more than 10%.
Working with additional partners to improve this yield, this year the US Department of Energy established the Joint Center for Artificial Photosynthesis (JCAP) with the California Institute of Technology (Caltech) in Pasadena and the Lawrence Berkeley National Laboratory in Berkeley, California.7 With a budget of $122 million, the five-year project is hoping to produce new market-ready solar cells through the invention of advanced components including light absorbers, electrolytic catalysts, molecular linkers, and specialized membranes for separating oxygen and hydrogen. In order to generate useable power, these researchers, those at MIT, and many others will also need to design fuel cells for safely storing the separated ions and for recombining them to make electrical energy. Basically there's a lot to figure out before the technology could be integrated into vehicles and buildings, but after watching this video, I have to admit that artificial photosynthesis already leaves me impressed.
Video credit: YouTube video of artificial leaf splitting water produced by the Nocera lab and by Sun Catalytix.
1. Reece, S.Y., Hamel J.A., Sung, K., Jarvi , T.D., Esswein, A.J., Pijpers, J.J. & Nocera, D.G. (2011). Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science, 334, 645-648 PMID: 21960528
2. Chandler, D. "‘Artificial Leaf' Makes Fuel from Sunlight." MIT News. September 30, 2011.
3. Kanan, M. & Nocera, D. (2008). In Situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+. Science, 321, 1072-1075 DOI: 10.1126/science.1162018
4. Mulik, K. "Does Hydrogen Fuel Pose Environmental Problems?" PBS NewsHour. October 20, 2003.
5. Dincă M., Surendranath Y. & Nocera D.G. (2010). Nickel-borate oxygen-evolving catalyst that functions under benign conditions. PNAS, 107, 10337-10341 PMID: 20457931
6. Chandler, 2011.
7. N.V. "The Difference Engine: The Sunbeam Solution." Babbage blog by The Economist. February 11, 2011.
