The energy storage materials account for only a fraction of a flow battery’s total cost. Vanadium, the material typically used now, costs about $80 per kilowatt-hour. But that’s high enough to make hitting the $100 target for the whole system impossible. Michael Aziz, a professor of materials and energy technologies at Harvard University who led the work, says the quinones will cut the energy storage material costs down to just $27 per kilowatt-hour. Together with other recent advances in bringing down the cost of the rest of the system, he says, this could put the DOE target in reach.
Studies indicate that one to two days' worth of storage is required for making solar and wind dispatchable through the electrical grid. To store 50 hours of energy from a 1-megawatt power capacity wind turbine (50 megawatt-hours), for example, a possible solution would be to buy traditional batteries with 50 megawatt-hours of energy storage, but they'd come with 50 megawatts of power capacity. Paying for 50 megawatts of power capacity when only 1 megawatt is necessary makes little economic sense.
Liquid energy: Novel energy storage materials flow from the white containers shown here into a fuel-cell like device in the foreground, where they generate electricity.
Nature - A metal-free organic–inorganic aqueous flow battery
The Harvard researchers still need to demonstrate that the new materials are durable enough to last the 10 to 20 years that electric utilities would like batteries to last, says Robert Savinell, a professor of engineering and chemical engineering at Case Western Reserve University. Savinell wasn’t involved with the Harvard work. He says initial durability results for the quinones are promising, and says the new materials “without a doubt” can be cheap enough for batteries that store days of electricity from wind farms. And he says the materials “can probably be commercialized in a relatively short time”—within a few years.
The researchers face competition from other startups developing cheaper flow batteries, such as EnerVault and Sun Catalytix.
As the fraction of electricity generation from intermittent renewable sources—such as solar or wind—grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output. In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form. Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts. Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. 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 0.6 W cm−2 at 1.3 A cm−2. Cycling of this quinone–bromide flow battery showed over 99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals. This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.
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