Taking a Leaf from Nature's Book
The wireless device could hardly be simpler in its appearance, but it represents years of research: a small metal rectangle, as thin as a credit card, is dropped into a glass of ordinary water and placed in the sun. Within seconds, bubbles begin to flow—oxygen from one side, hydrogen from the other.
Daniel Nocera, a professor of energy and of chemistry, calls this device an artificial leaf, and indeed it carries out a simple version of the same trick that leaves perform every day: it converts sunlight into a chemical fuel. This counters one of solar energy’s biggest drawbacks—the sun doesn’t always shine—by making it possible to store the energy indefinitely and use it when needed.
The artificial leaf is a sheet of semiconducting silicon, the material used in most solar cells, coated on one side with a cobalt-based catalyst and on the other with a nickel-molybdenum-zinc alloy. The silicon turns incoming sunlight into electricity that chemically splits water molecules with the help of the catalysts. First the cobalt compound releases oxygen from a pair of water molecules, producing O2. That process causes hydrogen atoms to be released as ions; the nickel alloy catalyzes a reaction that recombines them with previously released electrons, yielding H2 molecules that can power a fuel cell.
Other attempts to use sunlight to split water have relied on corrosive solutions or on relatively rare and expensive materials such as platinum, and they required extra equipment like wires, controllers, or batteries. The new device, Nocera says, is wireless and made entirely of abundant, inexpensive materials. He and his colleagues discovered the potential of the cobalt compound for freeing oxygen from water in 2008, but the technology is not yet ready for commercial production, since systems to collect, store, and use the gases remain to be developed. “It’s a step,” he says. “It’s heading in the right direction.”
Ultimately, Nocera sees a future in which individual homes could be equipped with solar collection systems based on this principle. Panels on the roof could use sunlight to produce hydrogen and oxygen that would be stored in tanks and then fed to a fuel cell when electricity is needed. Such systems, he hopes, could be made simple and inexpensive enough to be widely adopted throughout the world, bringing reliable sources of electricity to areas that currently don’t have them.
Nocera’s ongoing research with the artificial leaf, he says, is directed toward “driving costs lower and lower” and improving the wireless system’s efficiency, which is now about 2.5 percent. Another line of research is to explore the use of alternative photovoltaic materials such as iron oxide, which might be even cheaper to work with than silicon. “It’s all about providing options for how you go about this,” he says.
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