Solar flow battery breakthrough combines PV generation and storage in one device

A US-led research collaboration including a team of PV scientists from Australia has produced an ultra-efficient and long-lasting solar flow battery – a way to generate, store and re-deliver renewable electricity from the sun in one device.

Chemists from the Song Jin lab at the University of Wisconsin-Madison said on Tuesday[1] that the new device was made of silicon solar cells combined with advanced solar materials and integrated with optimally designed chemical storage components.

Essentially, they combine the advantages of photovoltaic cells that convert sunlight into electricity with the advantages of flow batteries, which use tanks of chemicals that can react to produce electricity and be recharged by the solar cells.

The research team says that the solar flow battery produced through the collaboration has set a new record efficiency of 20 per cent, besting most commercially available silicon solar cells, and was 40 per cent more efficient than the previous record holder for solar flow batteries, also developed by the Jin lab.

The work was done in collaboration with researchers from the University of New South Wales and the University of Sydney in Australia, Utah State University, King Abdullah University of Science and Technology in Saudi Arabia and the City University of Hong Kong. It was published on July 13 in the journal Nature Materials.

Schematic illustration of an integrated solar flow battery. Image: Wenjie Li

Lead author of the study, UW–Madison graduate student Wenjie Li, said that while solar flow batteries were years away from commercialisation, they offered the potential to provide reliable electricity generation and storage for lighting, cell phones or other fundamental uses for homes in remote areas.

On the solar side, the team tapped halide perovskites, the solar conversion efficiency of which has dramatically increased from a few per cent to over 25 per cent in 10 years. To this, they added silicon, which remains key for making a stable device that can withstand the chemicals in a flow battery.

And this is where the Australian team came into play. Professor Anita Ho-Baillie (Uni Syd) and postdoctoral researcher Jianghui Zheng (UNSW) fabricated the perovskite-silicon solar cells with an additional protection layer on the silicon surface and shipped them to Wisconsin for testing.

“Our motivation for the design of the solar cell was to combine these two materials together so we have both high efficiency and good stability,” said Li.

For the flow battery, Li used theoretical modelling to select a pair of chemicals to operate at the ideal voltage based on the characteristics of the solar cell, maximising efficiency. The chemicals are readily available, cheap organic compounds which are dissolved in a benign water solution of table salt rather than strong acids.

Once the ideal recipe was settled upon, a team at Utah State University led by chemistry professor T. Leo Liu was put to work to provide the key matching chemicals.

“Thanks to a good match between the solar cell and the flow battery, the winning device maintained a high efficiency over hundreds of hours and hundreds of charge-discharge cycles while retaining most of its capacity,” the report said.

Overall, the research team found that the new system’s long life and 20 per cent efficiency made it the best solar flow battery device yet.

“That’s 20 per cent efficiency any time you like,” Professor Jin said. “You can use the solar electricity right away during the day and get 20 percent, or you can use it in the evening from storage and get 20 per cent.”

The challenge, now, is to transform the solar flow batteries into practical renewable-energy solutions, starting with increasing the size and scale of the current small prototypes produced by the research lab.

Greater robustness will also be necessary. The Jin lab says it is continuing to develop even more efficient solar flow batteries while also experimenting with practical trade-offs to reduce the cost of the devices.

“Our eventual goal, if we can make this practical, is to target solar home systems,” said Li. “People who don’t have electrical grid access could use this device to have reliable electricity.”

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