Passage IV

NATURAL SCIENCE: This passage Is adapted from the article "Reinventing the Leaf”by Antonio Regal ado (@2010by Scientific American, a division of Nature America, Inc.).

Nathan S. Lewis has been giving a lecture on the energy crisis that is both terrifying and exhilarating. To avoid potentially debilitating global warming, the chemist says civilization must be able to generate more than 10 trillion watts of clean, carbon-free energy by 2050. That level is three times the U.S.'s average energy demand of 3.2 trillion watts.

Before Lewis's crowds get too depressed, he tells them there is one source of salvation: the sun pours more energy onto the earth every hour than human kind uses in a year. But to be saved, humankind needs a radical breakthrough in solar-fuel technology: artificial leaves that will capture solar rays and chum out chemical fuel on the spot, much as plants do. We can burn the fuel, as we do oil or natural gas, to power cars, create heat or generate electricity, and we can store the fuel for use when the sun is down.

Lewis's lab is one of several that are crafting prototype leaves, not much larger than computer chips, designed to produce hydrogen fuel from water, rather than the glucose fuel that natural leaves create. Unlike fossil fuels, hydrogen burns clean. Other researchers are working on competing ideas for capturing the sun's energy, such as algae that has been genetically altered to pump out biofuels, or on new biological organisms engineered to excrete oil. All these approaches are intended to turn sunlight into chemical energy that can be stored, shipped and easily consumed. Lewis argues, however, that the man-made leaf option is the most likely to scale up to the industrial levels needed to power civilization.

Although a few lab prototypes have produced small amounts of direct solar fuel—or electrofuel, as the chemicals are sometimes called-the technology has to be improved so the fuel can be manufactured on a massive scale, very inexpensively. To power the U.S., Lewis estimates the country would need to manufacture thin, flexible solar-fuel films, instead of discrete chip­ like devices, that roll off high-speed production lines the way newsprint does. The films would have to be as cheap as wall-to-wall carpeting and eventually cover an area the size of South Carolina.

Far from being a wild dream, direct solar-fuel technology has been advancing in fits and starts ever since President Jimmy Carter's push for alternative energy sources during the l970s oil shocks. Now, with a new energy and climate crunch looming, solar fuel is suddenly gaining attention.

In photosynthesis, green leaves use the energy in sunlight to rearrange the chemical bonds of water and carbon dioxide, producing and storing fuel in the form of sugars. "We want to make something as close to a leaf as possible," Lewis says, meaning devices that work as simply, albeit producing a different chemical output. The artificial leaf Lewis is designing requires two principal elements: a collector that converts solar energy (photons) into electrical energy (electrons) and an electrolyzer that uses the electron energy to split water into oxygen and hydrogen. A catalyst—a chemical or metal— is added to help achieve the splitting. Existing photovoltaic cells already create electricity from sunlight, and electrolyzers are used in various commercial processes, so the trick is marrying the two into cheap, efficient solar films.

Bulky prototypes have been developed just to demonstrate how the marriage would work. Engineers at a Japanese automaker, for example, have built a box that stands taller than a refrigerator and is covered with photovoltaic cells. An electrolyzer, inside, uses the solar electricity to break water molecules. The box releases the resulting oxygen to the ambient air and compresses and stores the remaining hydrogen, which the automaker would like to use to recharge fuel-cell cars.

In principle, the scheme could solve global warming: only sunlight and water are needed to create energy, the by-product is oxygen, and the exhaust from burning the hydrogen later in a fuel cell is water. The problem is that commercial solar cells contain expensive silicon crystals. And electrolyzers are packed with platinum, to date the best material for catalyzing the water-splitting reaction, but it costs $1,500 an ounce.

Lewis calculates that to meet global energy demand, future solar-fuel devices would have to cost less than $1 per square foot of sun-collecting surface and be able to convert 10 percent of that light energy into chemical fuel. Fundamentally new, massively seal­ able technology such as films or carpets made from inexpensive materials are needed.