DGIST Cuts Solar Hydrogen Electrode Coating Voltage to 4 V From 200 V

• DGIST and Kookmin University coated a TiO2 photoanode with AgBiS2 nanocrystals in water at 4 V, down from 200 V or more.
• The lead- and cadmium-free electrode gave 3.0 mA/cm² under one sun and about ten times the photocurrent of single-material electrodes.
• It is lab-scale: it kept 65.1% of its performance after two hours, and no solar-to-hydrogen efficiency was given.

Researchers at DGIST and Kookmin University in South Korea have coated a titanium dioxide solar hydrogen electrode with silver bismuth sulfide (AgBiS₂) nanocrystals in water at 4 V. Earlier methods needed 200 V or more. DGIST published the English version of its statement on Oct. 29, after a Korean release on Oct. 16. The paper appeared online in the journal Small on Sept. 3.

Most photoelectrodes for solar hydrogen use titanium dioxide, which is stable but absorbs only ultraviolet light. AgBiS₂ absorbs visible and near-infrared light and contains no lead or cadmium. The teams of Jongmin Choi at DGIST and Younghoon Kim at Kookmin replaced the long surface molecules on the nanocrystals with 3-mercaptopropionic acid, which lets the particles disperse in water. Electrophoretic deposition in that solution coated the electrode in three minutes, without the surface damage and oxidation by-products seen at high voltage.

Under one-sun AM 1.5G light, the TiO₂–AgBiS₂ photoanode delivered 3.0 mA/cm² at 0.6 V versus the reversible hydrogen electrode, the paper's abstract says. DGIST puts the photocurrent at about ten times that of conventional single-material electrodes. Gas analysis confirmed hydrogen. Neither the abstract nor the release gives a solar-to-hydrogen (STH) efficiency.

The electrode in numbers

• 4 V deposition voltage in water, against 200 V or more in organic solvents for earlier methods.
• 50 times lower process voltage, at minimum (list.solar calculation).
• 3 minutes to coat the electrode.
• 3.0 mA/cm² photocurrent density at 0.6 V vs. RHE under one sun.
• 65.1% of initial performance kept after 2 hours of continuous light, a loss of 34.9% (list.solar calculation).
• $1 per kilogram is the US Department of Energy's Hydrogen Shot target, an 80% cut that implies a $5/kg starting point (list.solar calculation).

The work is at lab scale. DGIST said the tests ran in an electrolyte containing sodium sulfide and sodium sulfite, so electrolyte conditions and full-system efficiency still need checking. In October, another Korean group at UNIST reported an all-organic tandem photocathode that splits water at 7.9% STH without external voltage on a 0.56-cm² cell.

"The significance of this study lies in achieving high hydrogen production efficiency through a safe and clean water-based, low-voltage process," Choi said in the statement.

DGIST gave no date for practical use. The next steps, it said, are to cut degradation during long operation, coat larger areas evenly and test the electrode under practical operating conditions.