A molecule nearly identical to one in rhubarb may hold the key to the future of renewable energy. Researchers have used the compound to create a high-performance “flow” battery, a leading contender for storing renewable power in the electric utility grid. If the battery prototype can be scaled up, it could help utilities deliver renewable energy when the wind is calm and the sun isn’t shining.
Renewables are already big business, with wind and solar power accounting for about 7% of all new power-generating capacity installed worldwide. Many experts would love to see that number grow to limit carbon emissions from fossil fuels. But if utilities use wind and solar power to provide more than about 20% of their power, they run into trouble. Above that level, it’s hard for the backup sources to meet demand when the sun isn’t shining and the wind isn’t blowing.
Numerous technologies have been proposed to store excess renewable energy for later use. And many regions already pump water uphill and later run it through a generator to do just that. But that doesn’t work in the flatlands. Other options, such as conventional batteries, are still too expensive and have safety concerns.
Flow batteries are another option. Unlike conventional batteries, which pack the chemical reactants and electrodes together, flow batteries keep their reactants in separate tanks. Energy can be extracted, or fed into the reactants, simply by flowing the materials past two electrodes separated by a membrane. A full-scale pilot project of the leading flow battery contender, based on vanadium ions dissolved in water, is due to be completed next year in Japan for grid storage. But vanadium is expensive. The vanadium alone in a flow battery with the storage capacity to provide a kilowatt-hour of electricity now costs $81. Adding the other components raises the price to between $350 and $700 per kilowatt-hour. According to the U.S. Department of Energy, the cost target for a viable grid storage technology is about $100 per kilowatt-hour.
Hoping to get closer to that mark, a team led by Michael Aziz, a physicist at Harvard University, decided to explore organic molecules called quinones. The compounds have long been known for being adept at grabbing and releasing electrons, a key requirement for a battery material. And they are plentiful in plants and even crude oil, making them potentially cheap. So Aziz says he and his students started testing a few different types of quinones in a flow battery and got fair results. That prompted them to team up with theoretical chemists led by Alán Aspuru-Guzik of Harvard to calculate the properties of more than 10,000 quinone molecules. That’s where they hit upon the rhubarblike compound.
Aziz and his team incorporated it into their flow battery setup. In one tank they place the quinone, abbreviated AQDSH2, dissolved in water. In a separate tank they place Br2, or bromine liquid. To get electricity out, they pump the two liquids past adjoining electrodes separated by a thin proton-conducting membrane. At one electrode, each quinone molecule gives up two electrons and two protons. The electrons zip through an outside circuit to the opposite electrode, where they meet up with the protons that passed through the membrane. The partners then combine with a bromine atom to make molecules of HBr. To store energy, the researchers simply run the pumps in reverse and provide energetic electrons. That coaxes the hydrogens to break away from bromine atoms and reattach themselves to the quinone at the opposite electrode. In a paper published online today in Nature, Aziz and his colleagues show that the quinone flow battery not only works, but also appears stable in early testing and provides considerable power. And perhaps best of all, Aziz notes that the cost of the quinones and bromine is about one-third the cost of vanadium, making it potentially a far cheaper option.
“It’s a great new materials set,” says Mike Perry, a flow battery expert at United Technologies Research Center in East Hartford, Connecticut, whose company is developing advanced vanadium flow batteries. But bromine is highly caustic, Perry notes, which may add to the cost of a final flow battery that relies on it. Aziz says his group is exploring other organics to replace the bromine as well. If it works, the novel batteries could energize efforts to add renewable power to the grid and curb society’s reliance on fossil fuel-generated power.