Credit: Adapted From Nature Credit: Eliza Grinnell/Harvard U In this flow battery (schematic and photo) quinone-based (AQDS) and bromine-based electrolyte solutions are pumped from external tanks to an electrochemical cell to generate electrical power and store it.

Solar and wind power are sustainable energy sources. But they produce power only when the sun shines or wind blows. Powering city-sized electric grids reliably from intermittent sources requires storing electrical energy on an enormous scale.

Conventional batteries are not well suited to the task. Less common devices known as flow batteries offer potential advantages for storing electrical energy at the city scale. But their broad application has been limited by their dependence on costly, low-abundance compounds, such as redox-active metals and precious-metal catalysts. Commercial versions may now be closer at hand as a result of a study demonstrating a new type of low-cost, metal-free flow battery that uses only abundant, water-soluble compounds (Nature 2014, DOI: 10.1038/nature12909).

In flow batteries, the compounds undergoing electrochemical reactions that generate and store energy are housed in external storage tanks. The fluids (two types of electrolytes per device) are pumped through the electrochemical cell during battery operation. In conventional batteries, the electroactive species, electrodes, and other cell hardware are confined in a small space. Decoupling the chemicals from the hardware enables each component of flow batteries to be optimized independently.

That flexibility means that flow batteries can be designed to quickly store large amounts of energy in vast storage tanks when the sun suddenly bursts through the clouds. Likewise, they can provide energy at full power continuously when it is unexpectedly demanded. By contrast, conventional batteries drain quickly when operated at full output.

To advance flow batteries, a team based at Harvard University searched for uncommon electroactive compounds. “Redox-active metal species have been pretty well picked over, and none of them are ideal,” says Harvard’s Michael J. Aziz, one of the study’s team leaders.

Instead of searching for metals, the group looked for suitable organic compounds. The investigation led them to small, fused-ring quinones, which are related to energy-storage compounds found in plants and animals. According to team member Roy G. Gordon of Harvard, these compounds are abundant and inexpensive. What’s more, a large variety of derivatives are known and easily prepared.

The team used computational methods to screen more than 10,000 quinones and selected 9,10-anthraquinone-2,7-disulfonic acid (AQDS) for testing. They built an electrochemical test cell with inexpensive carbon electrodes and other common components and paired AQDS with a bromine electrolyte solution.

Aziz says the team expected they would need a fast-acting catalyst to drive the quinone-to-hydroquinone reversible reaction, but none was needed because of the high rate of reaction. They also tested the AQDS dihydroxy derivative and observed an 11% improvement in cell voltage.

When they submitted the work for publication, the group reported that the AQDS system survived 15 charge-discharge cycles with almost zero loss of charge capacity. Now they are up to more than 100 cycles with equally good results, Aziz says.