Thursday, May 16, 2013

Hydrogen energy the chloroplast way: solar-to-fuel with the artificial leaf

With atmospheric carbon dioxide recently hitting a record 400 parts per million, the discovery of alternative renewable energy sources has taken on added urgency. One effort is the so-called "artificial leaf," a photosynthetic system that uses light energy to split water molecules and produce hydrogen. Researchers at Lawrence Berkeley National Lab have recently published details of their new nanowire-based system that mimics the way plant chloroplasts transport charged particles.

The artificial leaf's titanium dioxide and silicon nanowires are arranged in an array that actually resembles a microscopic forest of straight pines. The key to achieving good solar-to-fuel conversion efficiency is the integration of the components - the nanowire semiconductors that absorb light, an interfacial layer, and co-catalysts for the water splitting reaction - in a structure that resembles and functions like a chloroplast.

Plants are so efficient at turning sunlight into sugars partly because of what is termed the "Z-scheme": the daisy chain of molecules that deliver a charged electron from a chloroplast to molecular energy production in the cell. The artificial leaf uses the Z-scheme, too, but with the silicon nanowires responsible for the hydrogen generation and the titanium dioxide nanowires contributing to the formation of by-product oxygen. The use of two semiconductor materials allows for a large part of the sunlight spectrum to be harnessed (the silicon works off visible light and the titanium dioxide uses UV), while the forest-like array of nanowires increases the surface area for the solar-to-fuel reactions, which are helped along by embedded catalysts.

The artificial leaf has a conversion efficiency of 0.12 percent, comparable to that of natural photosynthesis. To be commercially viable, the efficiency number will have to get into the single digit percentages, and companies like MIT spin-off Sun Catalytix have already chosen to refocus their efforts away from artificial leaf tech. Replacing the current-limiting titanium dioxide anode in the system is the Berkeley researchers' next target for improving conversion efficiency.

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