Unlike gasoline-powered vehicles, which emit carbon dioxide (CO2), cars using hydrogen are emission-free. However, making hydrogen fuel involves burning natural gas that releases CO2 into the atmosphere.
To address this challenge, researchers at Stanford University focused on an emerging technology called photovoltaic water-splitting.
It 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.
But conventional solar electrodes made of silicon quickly corrode when exposed to oxygen — a key byproduct of water-splitting.
The team used bismuth vanadate — an inexpensive compound that absorbs sunlight and generates modest amounts of electricity — to overcome this problem, according to the study published recently in the journal Science Advances.
“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 Yi Cui of Stanford University.
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. As a result, visible light that could be used to generate electricity simply passes through the cell.
To capture sunlight before it escapes, Cui’s team turned to nanotechnology. The researchers created microscopic arrays containing thousands of silicon nanocones, each about 600 nanometers tall.
In an experiment, the team 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.