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US researchers have created a metal-free battery that uses organic chemicals found in rhubarb that they say is cheap and scalable enough to be used in grid applications to store electricity from wind farms and solar PV installations.
A paper, published this month in Nature, reports the use of abundant organic carbon-based molecules called quinones in a flow battery by a team of scientists and engineers at the Harvard School of Engineering and Applied Sciences.
Quinones are abundant in crude oil and green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb.
Researchers synthesised and screened some 10,000 quinone molecules in search of the best candidates for the battery. The battery was then designed, built and tested. The quinones are dissolved in water, which prevents them from catching fire.
Many experts believe that flow batteries offer the best solution for electricity storage in grid applications where there is a large amount of intermittent generation from wind turbines or solar PV. Flow batteries use external tanks, which are scalable in size, to store chemical fluids that are combined in a separate hardware unit and produce an electrochemical reaction.
The external tanks can be as large as required, permitting more energy to be stored at lower costs. However the active components of electrolytes in most flow batteries are metals, such as vanadium or platinum, which makes them expensive and drives up the energy storage costs.
The flow battery developed by the Harvard team performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.
Roy Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, said: "The whole world of electricity storage has been using metal ions in various charge states but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy."
"With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good."
The university said the technology could be used commercially for large wind farms or solar PV installations, where large storage tanks would be used, or domestically, where a small tank would be used.
Research leader, Michael Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at Harvard, said the next steps in the project will be to further test and optimise the system that has been demonstrated in the laboratory to develop a commercial demonstrator. This is being done in partnership with Connecticut-based firm Sustainable Innovations, which is developing low cost electrochemical cell design and system architecture for energy storage applications. The company plans to develop a flow battery unit the size of a horse trailer, that can be connected to solar panels on the roof of a commercial building, within the next three years.
Aziz said: “The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind. A safe and economical flow battery could play a huge role in our transition from fossil fuels to renewable electricity. I'm excited that we have a good shot at it."