Artificial Photosynthesis: Will It Feed the Hydrogen Economy?

April 11, 2011

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In 1913, poet Joyce Kilmer famously wrote:

"Poems are made by fools like me, But only God can make a tree."
Almost a hundred years later, a tree is still a marvelous thing, but scientists are making progress in mimicking photosynthesis, the solar-energy conversion process that takes place in the leaves of trees and other green plants. Artificial-photosynthesis technologies might soon be harnessed to collect sunlight and convert it into fuels to help supply the world's energy needs.

As reported a few days ago by A. Witt (see "Deriving Innovation from Nature: Is the Leaf the Next Energy Muse?"), Massachusetts Institute of Technology (MIT) researcher Daniel Nocera claims to have developed an artificial leaf. The invention, he believes, could provide a key technology for the emerging hydrogen economy and to provide cheap energy in developing countries.

Green leaves are like solar-power machines that grow in your back yard -- they capture sunlight and use its energy to convert water (H2O) and carbon dioxide (CO2) into oxygen and organic compounds -- carbon-based chemical fuel. Of particular interest to scientists who have been engaged in the quest for an artificial leaf is the key "water-splitting" process that takes place during natural photosynthesis -- that is, the splitting of water into its basic components, oxygen and hydrogen. With artificial photosynthesis, these gases could be stored and used to power fuel cells.

In the solar industry today, photovoltaic (PV) cells are commonly employed to capture solar energy and convert it directly into electricity. That technology is expensive because it requires purified silicon. In addition, PV technologies suffer from inherent inefficiencies: Energy generated has to be used in real time or stored in a battery. These obstacles have held down the market adoption of PV technology. A solar process producing chemical fuel could offer important advantages.

According to a paper developed by participants at "Powering the World With Sunlight," a 2009 symposium in Germany, an efficient artificial-leaf technology could solve the crucial problem of cost and storage in active-solar energy. At that symposium, Nocera presented some of the findings of his MIT group up to that time. According to the paper,

"Nocera stated that if chemists could develop an affordable, efficient means of using solar energy to split water and produce hydrogen fuel, society would be able to power the world simply by splitting, every second, one-third the amount of water in MIT’s swimming pool."
Nocera envisions a world in which a simple device based on the artificial leaf could be used on a household level in developing lands. On March 27, 2011, at the National Meeting of the American Chemical Society (ACS) in Anaheim, Calif., Nocera announced:
"A practical artificial leaf has been one of the Holy Grails of science for decades. We believe we have done it. The artificial leaf shows particular promise as an inexpensive source of electricity for homes of the poor in developing countries. Our goal is to make each home its own power station. One can envision villages in India and Africa not long from now purchasing an affordable basic power system based on this technology."
In spite of all the use of the word "leaf," don't expect to see the delicate curved, veined, green leaf you'll encounter on a walk through the woods. Nocera's artificial version is based on a commercially-available PV cell and is "fashioned from silicon, electronics, and catalysts," says the ACS. It's "about the shape of a poker card but thinner." In other words, look for something shiny, gray, and rectangular.

The quest for an artificial leaf is nothing new. In 1971, for example, researchers studying photosynthesis at Queen's University in Kingston, Ontario, used the term "artificial leaf" in an article in Nature.

Scientist John Turner of the U.S. National Renewable Energy Laboratory in Boulder, Col., developed a working artificial leaf about 10 years ago, says ACS. Although highly efficient at mimicking photosynthesis, Turner's device was built with rare and expensive metals and had a lifespan of only a day because of its instability.

In a current effort by the Joint Center for Artificial Photosynthesis (JCAP), about 200 scientists and engineers from labs and universities in California are seeking technologies for converting sunlight into liquid fuels. According to The Economist,

"The JCAP team uses a carpet-like structure of microfibers made of a silicon-based semiconductor similar to those employed in photovoltaic solar panels. But instead of generating electricity, the charge-carriers produced by the semiconductor drive the catalytic process for splitting water into hydrogen and oxygen. Special membranes vent the oxygen away, while collecting the hydrogen."
The JCAP group also hopes to solve the other side of the artificial photosynthesis problem: processing CO2 to convert its carbon into fuel and to help reduce levels of CO2 as a greenhouse gas in the atmosphere. So far, though, the group has not made it over the catalyst hurdle.

That's where Nocera's innovation stands out: Nocera has solved key problems through discovery of new inexpensive catalysts made from nickel and cobalt, which can make artificial photosynthesis practical and cost-efficient. ACS says that "Nocera’s leaf is about 10 times more efficient at carrying out photosynthesis than a natural leaf" and that Nocera "is optimistic that he can boost the efficiency of the artificial leaf much higher in the future."

Nocera's discovery has generated interest from the private sector. In January, Indian industrial giant Tata Group announced that it was investing $15 million to fund Nocera's research with the aim of using water to produce hydrogen as an automobile fuel.

Tata's announcement says that the company and Nocera plan to organize a startup company to build a specialized "can" -- a container "that can store hydrogen in a compressed form and fit it into a car for [use] as an alternative fuel cost-effectively." The company, which includes auto manufacturing in its many businesses, wants to use the technology to develop cars that will run on water.

Hydrogen figures significantly in the U.S.'s energy plans, especially for transportation fuel. A report from the U.S. Department of Energy (DOE) and Department of Transportation (DOT) estimates that fuel cell vehicles (FCVs) "could provide more than twice the efficiency of conventional vehicles and have the potential to reduce our dependence on oil while substantially reducing emissions of air pollutants and greenhouse gases" and that "oil savings could be 5.3 mbpd (million barrels per day) by 2050 assuming a 37% market penetration of light duty fuel cell vehicles."

Artificial photosynthesis technologies might never produce anything as huggable as the tree that Joyce Kilmer extolled, but they could provide a great deal of value in meeting the needs of an energy-hungry world.

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