The mismatch between the availability of intermittent wind or sunshine and the variability of demand is a major obstacle to getting a large fraction of electricity from renewable sources. A growing number of engineers have focused their attention on flow battery technology, which reversibly converts chemical energy directly to electricity. Until now, flow batteries have relied on chemicals that are expensive or difficult to maintain, driving up the energy-storage costs. Now, a team of scientists and engineers from Harvard University has developed a metal-free flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.
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 its cost sets a high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts such as the platinum.
As reported in the January 9 issue of Nature (DOI:10.1038/nature12909; p. 195), the flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalysts.
“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,” said Roy G. Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS). “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.”
In this laboratory-scale organic molecule flow battery, energy-storing chemicals are dissolved in water and stored in containers in the background. To discharge the battery they are pumped through the energy conversion hardware in the foreground, converting chemical energy to electrical energy and becoming low-energy molecules. To recharge the battery, electrical energy is pushed into the energy-conversion hardware, converting low-energy chemicals into high-energy chemicals which are stored in these containers until electrical energy is needed again. The negative electrode is the left side of the battery and the positive electrode is the right side. In order to store more energy, the energy-conversion hardware does not need to change: only the container size and the amount of chemicals need to increase. Credit: Eliza Grinnell, Harvard School of Engineering and Applied Sciences.
Quinones are abundant in crude oil as well as in green plants, and the molecule the researchers used in their first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.
Flow batteries store energy in chemical fluids contained in external tanks—as with fuel cells—instead of within the battery container itself. The two main components—the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity), and the chemical storage tanks (which set the energy capacity)—may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at a lower cost than with traditional batteries.
Team leader Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at SEAS, said the next steps in the project will be to further test and optimize the system that has been demonstrated on the bench-top and bring it toward a commercial scale. “So far, we’ve seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles,” he said. He also expects to achieve significant improvements in the underlying chemistry of the battery system. “I think the chemistry we have right now might be the best that’s out there for stationary storage and quite possibly cheap enough to make it in the marketplace,” he said. “But we have ideas that could lead to huge improvements.”
