Hydrogen fuel has long been touted as a clean alternative to gasoline. Automakers began offering hydrogen-powered cars to American consumers last year, but only a handful have sold, mainly because hydrogen refueling stations are few and far between.
“Millions of cars could be powered by clean hydrogen fuel if it were cheap and widely available,” says Yi Cui, associate professor of materials science and engineering at Stanford University.
Unlike gasoline-powered vehicles, which emit carbon dioxide, hydrogen cars themselves are emissions free. Making hydrogen fuel, however, is not emission free: Today, making most hydrogen fuel involves natural gas in a process that releases carbon dioxide into the atmosphere.
To address the problem, Cui and his colleagues have focused on photovoltaic water splitting.
Clean and cheap hydrogen
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.
Finding an affordable way to produce clean hydrogen from water has been a challenge. 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.
In the journal Science Advances, Cui and his colleagues present 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,” says Cui, who is also an associate professor of photon science at SLAC National Accelerator Laboratory. “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. 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.
Credit: Wei Chen and Yongcai Qiu
“Nanocone structures have shown a promising light-trapping capability over a broad range of wavelengths,” Cui explains. “Each cone is optimally shaped to capture sunlight that would otherwise pass through the thin solar cell.”
The researchers 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.”
Source: Stanford University
