Making Electricity With Artificial Leaves Made From Crystalline Silicon
March 5, 2013

Making Electricity With Artificial Leaves Made From Crystalline Silicon

Lee Rannals for — Your Universe Online

Researchers writing in the Proceedings of the National Academy of Sciences claim they have developed a solar-to-fuel roadmap for crystalline silicon, opening up a potential future system that efficiently harvests sunlight to make storable fuel.

A new analysis helps lay out the roadmap for researchers to improve the efficiency of an "artificial leaf," which is a concept of using a silicon solar cell to harness sunlight the way real leaves do. Real leaves can split water into hydrogen and oxygen and can convert the energy of sunlight directly into storable chemical form, without needing any external connections. An artificial leaf system could use sunlight to produce a storable fuel, like hydrogen, instead of electricity for immediate use. This fuel can be used to generate electricity through a fuel cell or other device.

MIT researchers are following up on a 2011 project that produced a "proof of concept" of an artificial leaf.

Their technology uses a standard silicon solar cell, which converts sunlight into electricity, and a chemical catalyst. This helps create an electrochemical device that uses an electric current to split atoms of hydrogen and oxygen from the water molecules surrounding them. With their system, they hope to produce an inexpensive, self-contained system that could be built from abundant materials.

“What´s significant is that this paper really describes all this technology that is known, and what to expect if we put it all together,” said former MIT graduate student Casandra Cox, who is now at Harvard. “It points out all the challenges, and then you can experimentally address each challenge separately.”

Their original concept had low efficiencies, converting less than 4.7 percent of sunlight into fuel. However, the latest analysis shows efficiencies of 16 percent or more can be possible using single-bandgap semiconductors, like crystalline silicon.

In order to obtain high solar-to-fuel efficiencies, the researchers must be able to combine the right solar cells and catalyst. The team wrote about an approach that allows for each component of the artificial leaf to be tested individually, then combined.

Associate professor of mechanical engineering Tonio Buonassisi said their simulations have a framework to determine the limits of efficiency that are possible with the artificial leaf system. He says that limit is about 16 percent, for silicon solar cells, and 18 percent for gallium arsenide cells.

He said their analysis lays down a roadmap for development and helps to identify a few "levers" that can be worked on.

“Some of the most impactful papers are ones that identify a performance limit,” Buonassisi says.

Researchers wrote in the American Chemical Society's journal Accounts of Chemical Research last year of the first practical artificial leaf. The team wrote their new device is made from inexpensive materials, and employs low-cost engineering and manufacturing processes.

“Considering that it is the 6 billion nonlegacy users that are driving the enormous increase in energy demand by midcentury, a research target of delivering solar energy to the poor with discoveries such as the artificial leaf provides global society its most direct path to a sustainable energy future,” said Daniel G. Nocera, a researcher on the project.