14 April 2010
Electricity generated directly from photosynthesis
by Kate Melville
Stanford University scientists have "wired up" algae to harness a tiny electric current directly from the plant during photosynthesis; an achievement which could lead to the highly efficient generation of bioelectricity with no carbon byproducts, the researchers say. "We believe we are the first to extract electrons out of living plant cells," said WonHyoung Ryu, whose paper on the work appears in Nano Letters. (Ryu, formerly of Stanford, is now a professor at Yonsei University in Seoul, South Korea)
Ryu's team developed a unique, ultra-sharp nanoelectrode made of gold, specially designed for probing inside cells. They gently pushed it through the algal cell membranes, which sealed around it, and the cell stayed alive. From the photosynthesizing cells, the electrode collected electrons that had been energized by light and the researchers generated a tiny electric current.
Photosynthesis takes place in chloroplasts, the cellular powerhouses that make sugars and give leaves and algae their green color. In the chloroplasts, water is split into oxygen, protons and electrons. Sunlight penetrates the chloroplast and lifts the electrons to a high energy level, and a protein promptly grabs them. The electrons are passed down a series of proteins, which successively capture more and more of the electrons' energy to synthesize sugars until the entire electron's energy is spent.
In Ryu's experiment, the researchers intercepted the electrons just after they had been excited by light and were at their highest energy levels. They placed the gold electrodes in the chloroplasts of algae cells, and siphoned off the electrons to generate the tiny electrical current. The result, the researchers say, is electricity production that doesn't release carbon into the atmosphere. The only byproducts of photosynthesis are protons and oxygen.
Ryu isn't getting too excited yet, however. "We're still in the scientific stages of the research," he said. "We were dealing with single cells to prove we can harvest the electrons." He explained that they were able to draw from each cell just one picoampere, an amount of electricity so tiny that they would need a trillion cells photosynthesizing for one hour just to equal the amount of energy stored in a AA battery.
In addition, the cells die after an hour. Ryu said tiny leaks in the membrane around the electrode could be killing the cells, or they may be dying because they're losing out on energy they would normally use for their own life processes. One of the next steps would be to tweak the design of the electrode to extend the life of the cell, Ryu said.
Harvesting electrons using Ryu's method is more efficient than burning biofuels, as most plants that are burned for fuel ultimately store only about 3 to 6 percent of available solar energy. His process bypasses the need for combustion, which only harnesses a portion of a plant's stored energy. Electron harvesting in this study was about 20 percent efficient. Ryu said it could theoretically reach 100 percent efficiency one day.
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Source: Stanford University