New ways to produce clean hydrogen fuel using sunlight

Stanford scientists have developed a new way to produce hydrogen fuel using sunlight, in an effort to tackle one of the world's biggest energy challenges - clean fuel for transportation.
Hydrogen fuel has long been touted as a clean alternative to gasoline.
"Millions of cars could be powered by clean hydrogen fuel if it were cheap and widely available," said Yi Cui, an associate professor at Stanford University in the US.
Unlike gasoline-powered vehicles, which emit carbon dioxide (CO2), hydrogen cars themselves are emissions free.
However, making most hydrogen fuel involves natural gas in a process that releases CO2 into the atmosphere.

To address the problem, researchers focused on photovoltaic water splitting. This emerging technology consists of 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, a key byproduct of water splitting. Several research teams have reduced corrosion by coating the silicon with iridium and other precious metals.
Researchers presented a new approach 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," said Cui.

"However, the performance of this material remains well below its theoretical solar-to-hydrogen conversion efficiency," Cui said.
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 nanometres or less, making it virtually transparent.
As a result, visible light that could be used to generate electricity simply passes through the cell.
To capture sunlight before it escapes, researchers created microscopic arrays containing thousands of silicon nanocones, each about 600 nanometres tall.
They 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 per cent, already matching the theoretical maximum rate for a bismuth vanadate cell.
The study was published in the journal Science Advances.