UNIST Organic Tandem Photocathode Splits Water at 7.9% Efficiency

• UNIST researchers built an all-organic tandem photocathode that splits water at 7.9% solar-to-hydrogen efficiency without external voltage.
• The team calls it the best result for an unbiased organic photocathode; a 320-cm² module also made hydrogen outdoors.
• UNIST's cost model gives $9.26/kg at 10 tons a day and $4.26/kg if efficiency reaches 16%.

Researchers at UNIST in South Korea have built an all-organic tandem photocathode that splits water at 7.9% solar-to-hydrogen (STH) efficiency without external voltage. UNIST announced the result on Oct. 26. The paper went online in Applied Catalysis B: Environment and Energy on Sept. 6.

The team at the Ulsan National Institute of Science and Technology includes professors Jin Young Kim, Jae Sung Lee and Hankwon Lim. A single organic absorber struggles to supply the 1.23 V needed to split water, so the team stacked two that absorb different wavelengths. The new part is the layer joining them: molybdenum oxide paired with a molecule called 2PACz. It replaces PEDOT:PSS, an acidic, moisture-absorbing material that damaged nearby layers.

The 7.9% comes from a 0.56-cm² lab cell. The team called it the highest for an unbiased organic photocathode. UNIST cited no independent certification. After 10 hours the cell kept 86% of its photocurrent. A version with the old interlayer fell to 36%.

The photocathode holds about 0.1% platinum by weight, and the oxygen electrode uses a nickel-iron-cobalt catalyst. Both sit in one potassium hydroxide bath, so no ion-exchange membrane is needed. A 320-cm² module made hydrogen outdoors, without bias.

The photocathode in numbers

• 7.9% unbiased solar-to-hydrogen efficiency on a 0.56-cm² device.
• 1.67 V onset potential versus the reversible hydrogen electrode.
• 571 times the area of the 0.56-cm² cell, in the 320-cm² outdoor module (list.solar calculation).
• $9.26 per kg, UNIST's estimate, is 9.3 times the DOE's $1-per-kg goal (list.solar calculation).
• 20% and 25%: the DOE's 2020 and ultimate STH targets for photoelectrode systems that use concentrated sunlight.

Cost is the main hurdle for direct solar fuels. UNIST's model puts hydrogen at $9.26 per kilogram for a 10-ton-a-day plant. That is close to a 10%-efficient photoelectrode's cost, because organic layers are cheaper to make. The U.S. Department of Energy's Hydrogen Shot, introduced in 2021, aims for $1 per kilogram within a decade. In Australia, Sparc Hydrogen's owners committed another A$1.3 million on Oct. 20 to test water-splitting modules.

Kim said the new interlayer solves the corrosion and instability that had degraded stacked organic electrodes.

The model also sketches a path for this solar technology. At 16% efficiency, which UNIST puts at about the theoretical limit, and 30 tons a day, hydrogen would cost $4.26 per kilogram. That is 54% below the current estimate (list.solar calculation).