Silicon nanocones improve hydrogen fuel production from water

Silicon nanocones improve hydrogen fuel production from water

Technology News |
Researchers at Stanford University (Stanford, CA) have used silicon nanocones to produce a low cost photovoltaic cell for producing hydrogen from water for fuel cells.
By Rich Pell

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“Millions of cars could be powered by clean hydrogen fuel if it were cheap and widely available,” says Yi Cui, an associate professor of materials science and engineering at Stanford.

Cui and his colleagues at the Stanford Institute for Materials and Energy Sciences have focused on photovoltaic water splitting, using a solar-powered electrode immersed in water. When sunlight hits the electrode, it generates an electric current that splits the water into its constituent parts, hydrogen and oxygen.

Conventional solar electrodes made of silicon quickly corrode when exposed to oxygen, and other research teams have reduced corrosion by coating the silicon with iridium and other precious metals. Instead, Cui and his colleagues are using bismuth vanadate, an inexpensive compound that absorbs sunlight and generates modest amounts of electricity.

“Bismuth vanadate has been widely regarded as a promising material for photoelectrochemical water splitting, in part because of its low cost and high stability against corrosion,” says Cui. “However, the performance of this material remains well below its theoretical solar-to-hydrogen conversion efficiency.” 

Bismuth vanadate absorbs light but is a poor conductor of electricity. To carry a current, a solar cell made of bismuth vanadate must be sliced very thin, 200 nanometers or less, making it virtually transparent and allowing visible light to generate electricity as it passes through the cell.

The researchers created microscopic arrays containing thousands of silicon nanocones, each about 600 nanometers tall. “Nanocone structures have shown a promising light-trapping capability over a broad range of wavelengths,” Cui says. “Each cone is optimally shaped to capture sunlight that would otherwise pass through the thin solar cell.”

In the experiment, Cui and his colleagues deposited the nanocone arrays on a thin film of bismuth vanadate. Both layers were then placed on a solar cell made of perovskite, another promising photovoltaic material. When submerged, the three-layer tandem device immediately began splitting water at a solar-to-hydrogen conversion efficiency of 6.2 percent, already matching the theoretical maximum rate for a bismuth vanadate cell.

“The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability,” says Cui. “Although the efficiency we demonstrated was only 6.2 percent, our tandem device has room for significant improvement in the future.”

www.stanford.edu 

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