Power harvested from the Sun and wind is pouring into electricity grids by the gigawatt. That makes it ever more important to find an efficient and convenient way to store renewable energy for those times when the breeze dies or the skies cloud over.

“Now we have a good chance of solving that problem,” says Michael Aziz, a materials scientist at Harvard University in Cambridge, Massachusetts. His solution is a flow battery that packs a high energy density with no need for the expensive metals found in other models.

Flow batteries work by pumping different chemical broths over two electrodes separated by a membrane. The chemicals exchange protons across the membrane and shuttle electrons around the circuit that connects the electrodes, which discharges the battery. Reversing the reaction recharges the cell.

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