TOKYO — A research group based in a Tokyo university has succeeded to produce hydrogen using a Gallium Nitride (GaN) device as a photocatalyst, which scientists believe could lead to a process that extracts hydrogen directly from water using only solar energy.
The research team is one of three groups running under the Nakamura Inhomogeneous Crystal Project sponsored by Japan Science and Technology Agency (JST). Shuji Nakamura, University of California Santa Barbara, who pioneered GaN based blue LED and blue laser development, directs the project.
Earlier this week, one of the other project groups announced progress in GaN device performance using new film crystallization techniques.
"We confirmed that nitride semiconductor can produce hydrogen from water," said Kazuhiro Ohkawa, associate professor of Tokyo University. He noted that a light-emitting semiconductor absorbs the same wavelength light that it emits. Relying on the same characteristics for photocatalysis reaction, the group succeeded to extract hydrogen by just exposing light on the nitride film dipped in water.
The patents related to this technology will be finalized next month, the first ones concerned with nitride semiconductor-based hydrogen production, Ohkawa said.
Ohkawa added that extracting hydrogen using a nitride semiconductor as a photocatalyst is environmentally conscious as the catalyst directly electrolyzes water into oxygen and hydrogen without producing carbon dioxide. Nitride semiconductors have no poisonous materials such as arsenic and phosphor.
He noted that other sources of so-called clean energy, such as methanol fuel cells, produce hydrogen but also release carbon dioxide. Using a solar battery to electrolyze water does not produce carbon dioxide, but is considered inefficient because it has to first convert light to electric energy, then electrolyze water.
In its research, scientists grew thin films of GaN and InGaN on sapphire substrates, then used a Xenon lamp to expose light on the nitride films. The nitride film and a platinum electrode in water are connected with a 1-volt power source.
While the photocatalysis theoretically do not require any power, Ohkawa said, "We apply the small voltage as a tap to urge the reaction."
Once exposed to light, oxygen generates at the nitride electrode and hydrogen generates at the platinum electrode. The conversion efficiency is 0.5 percent for the GaN film and 0.7 percent for the InGaN film. The experimental system using a 50-millimeter diameter GaN film produces several milliliters of hydrogen per hour.
Titan dioxide (TiO2), a widely-used photocatalyst, has a conversion efficiency of about 2 percent. Ohkawa is optimistic that nitride's conversion efficiency will exceed the level of TiO2 within the next year.
Doping Indium in GaN enables the nitride semiconductor to emit longer wavelength lights. If high quality InGaN film can be fabricated, the photocatalyst can utilize the wider frequency range of sunlight.
Research team scientists expect to raise conversion efficiency to 10 percent in three years. By using a solar battery for longer wavelength light, Ohkawa expects efficiency to reach 40 percent.