Bifacial solar water-splitting device brings hydrogen fuel closer to reality
A slim, low-cost, printable device could bring solar-generated hydrogen fuels closer to reality. The two-sided device has a photovoltaic side that converts light to electricity, while the other efficiently splits water into hydrogen and oxygen. The device converts 13% of solar energy falling on it into hydrogen. Current state-of-the-art solar water-splitting systems have comparable efficiencies, but the new devices are more stable and should cost much less.
Researchers have for years been trying to develop a cheap, efficient process to split water using sunlight and catalysts in order to get clean-burning hydrogen fuel. The simplest way to do that is to connect a solar cell to an electrolysis cell, which splits water by passing current through it. More recently, some researchers have tried to combine the two devices, by coating conventional solar cells with catalysts.
So far, such photocatalytic systems based on III–V compound semiconductors such as gallium arsenide have shown the highest known solar-to-hydrogen efficiencies of over 14%. But they rapidly corrode in the presence of the salt or acid electrolytes typically added to water to increase the efficiency of electrolysis, and become unusable after a few hours.
Conventional semiconductor devices also have other limitations because they are grown layer-by-layer on wafers. This is expensive, and leaves only the top surface of the solar cell available for depositing the metal catalyst. This means the metal layer has to be an ultra-thin transparent film in order to let light through at the expense of the amount of charge it transports. “Application of solar water-splitting technology has been severely hindered by high material cost and poor stability of semiconductor electrodes,” says Jongseung Yoon, a professor of chemical engineering and materials science at the University of Southern California.
To overcome those issues, Yoon and his colleagues borrowed a technique used to make flexible electronic circuits. They made thin-film GaAs solar cells on a sacrificial Al0.95Ga0.05As layer grown on top of a GaAs wafer, and then etched away the sacrificial layer with dilute hydrochloric acid, releasing the devices from the wafer. Using a polydimethylsiloxane stamp, they transferred the devices to a transparent glass slide so that one of the solar cell’s electrodes was flush against the glass. On the other electrode the researchers deposited a small area of platinum catalyst and then coated the surrounding area with a polyimide layer as a liquid-proof barrier.
The device, reported in Nature Energy, is laid metal-side down on water to operate. Light goes through the glass and hits the solar cell side, while the metal side underneath electrolyzes water. “So we can have two material interfaces responsible for light absorption and catalysis, independently,” Yoon says. “And now we can add layers to further increase both efficiency and stability.”
On the sun-facing electrode against the glass, for example, the researchers added a silicon nitride anti-reflection coating to increase sunlight absorption. The relatively thick platinum layer, meanwhile, blocks the protons in the electrolyte from reaching the semiconductor surface and inducing corrosion, increasing their lifetime.
The device converted sunlight to hydrogen fuel with 13.1% efficiency for up to eight days; standard GaAs photocatalytic devices corrode and degrade after 2–3 days. It should also cost much less than conventional devices, since lift-off technique uses just a few micrometers-thick layers of GaAs instead of the entire wafer, which can be reused to make more devices.
“This is an important advance as it demonstrates the applicability of microprinting [stamping] techniques to fabricate complex photoelectrochemical devices over potentially large areas from high-quality single crystal semiconductors,” says Shannon Boettcher, a professor of chemistry and biochemistry at the University of Oregon. The printing approach addresses some of the key hurdles to solar fuel systems, he says, and performance and lifetime of the reported devices is particularly impressive.
To be cost competitive with fossil fuels, solar water-splitting systems need to have 15–25% efficiencies. Yoon says that using their design with multi-junction solar cells made of different semiconductors should allow them to reach 20% efficiency.
Read the abstract in Nature Energy.