Keys to their version of a 'flow battery' are: having no metals in the system to keep cost down, and achieving high cycle efficiency.
In a flow battery, electrolyte is stored away from the electrodes in two tanks, one for each of the ions that will react at the electrodes. When electricity is required, electrolytes are allowed to flow into a chamber containing the electrodes.
With this arrangement, storage capacity is proportional to the size of the tanks, and is completely decoupled from power generating capability, which is set by the electrode area within the reaction chamber.
Sponsored by the US Department of Energy grid-scale battery programme, the Harvard team has developed a quinone-bromide chemistry based around a particular quinone screened from 10,000 candidate quinones.
"The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow-battery technology now in development, but it sets a rather high floor on the cost per kWh at any scale. Other flow batteries contain precious metal electrocatalysts, such as the platinum used in fuel cells," said Harvard, claiming that the quinone cell "already performs as well as vanadium flow batteries with chemicals that are significantly less expensive."
“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.” said Harvard chemist Professor Roy Gordon.
This particular quinone is 9,10-anthraquinone-2,7-disulphonic acid (AQDS): "almost identical to one found in rhubarb", said Harvard. For the battery, it is dissolved in water and undergoes a redox reaction.
So far, the experimental battery has shown no signs of degradation after more than 100 charge-discharge cycles, and energy returned per cycle is >99% of charge energy.
In a paper in Nature, 'A metal-free organic–inorganic aqueous flow battery', the team said: "An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br2/Br? redox couple, yields a peak galvanic power density exceeding 0.6W/cm2 at 1.3A/cm2."
In the photo, clockwise from the left: Dr Changwon Suh, Prof Roy Gordon, Dr Brian Huskinson, Dr Suleyman Er, Dr Michael Marshak, Prof Alán Aspuru-Guzik, Prof Michael Aziz, Michael Gerhardt, and Lauren Hartle.