Scientists in China have constructed and tested a device that uses redox couples paired with a single triple-junction amorphous-silicon photoelectrode. When tested under a xenon lamp simulating one sun, the device achieved an average solar-to-electricity conversion efficiency of 4.2%.

December 24, 2025
Lior Kahana

A research team from China’s Nanjing Tech University (NanjingTech) has developed and constructed a novel anthraquinone-based solar redox flow battery (SRFB) device.

The system consists of redox couples known as 2,6-DBEAQ and K4[Fe(CN)6], paired with a single triple-junction amorphous-silicon photoelectrode. SRFBs are systems that combine a solar cell with a redox flow battery, enabling simultaneous capture of sunlight and storage of chemical energy.

“Among the redox couples utilized in SRFBs, aqueous organic anthraquinone derivatives have gained recognition as the favored materials for energy storage,” corresponding author Chengyu He told pv magazine. “Anthraquinone-based SRFBs typically employ redox couples such as AQDS and 2,6-DHAQ, operating in both strongly acidic and alkaline conditions. However, due to the corrosion of photoelectrodes and the instability of the paired redox couple, these SRFB devices exhibit relatively low solar-to-output electricity efficiencies.”

For the photocathode part of the device, the group cut commercial triple-junction amorphous silicon (3jn-a-Si) photovoltaic cells into 2 cm × 2 cm pieces. The cell comprises stacked amorphous silicon–germanium alloy junctions deposited on a stainless-steel substrate, topped with an indium tin oxide (ITO) layer. It is electrically connected via an external circuit to a carbon-felt counter-electrode.

The photocathode is in chemical contact with the catholyte containing 2,6-DBEAQ, which is reduced during photo-charging. In contrast, the anolyte containing K₄[Fe(CN)₆] is oxidized at the carbon felt electrode. The two electrolytes flow continuously from external tanks through the cell using a peristaltic pump. They are kept separated by a Nafion ion-exchange membrane that allows ion transport to maintain charge balance but prevents electrolyte mixing.

The performance of the SRFB device was assessed by a cyclic test, a repeated charge–discharge experiment. Before the measurements, the electrolytes were purged with Argon for 30 minutes to eliminate the dissolved oxygen. During charging, the photocathode was irradiated with a xenon lamp simulating 1-sun intensity at 100 mW cm2. The device was discharged at a current density of 10 mA cm-2 until the battery voltage dropped to 0.2 V.

“The SRFB device can be photo-charged without an external bias and discharged over 10 cycles, while the solar-to-output electricity efficiency reaches 4.3%,” Chengyu He stated. “The successful preparation of this SRFB device opens up new possibilities for the further development of advanced solar-to-chemical energy conversion technologies.”

The results were presented in “An aqueous anthraquinone solar redox flow battery for efficient conversion and storage of solar energy,” published in Electrochimica Acta.

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