We've written before about the idea of using concrete for energy storage – back in 2021, a team from the Chalmers University of Technology showed how useful amounts of electrical energy could be stored in concrete poured around carbon fiber mesh electrodes, with mixed-in carbon fibers to add conductivity.

MIT's discovery appears to take things to the next level, since it does away with the need to lay mesh electrodes into the concrete, and instead allows the carbon black to form its own connected electrode structures as part of the curing process.

This process takes advantage of the way that water and cement react together; the water forms a branching network of channels in the concrete as it starts to harden, and the carbon black naturally migrates into those channels. These channels exhibit a fractal-like structure, larger branches splitting off into smaller and smaller ones – and that creates carbon electrodes with an extremely large surface area, running throughout the concrete.

Two of these branches, separated by an insulating layer or a thin space, work happily as the plates of a supercapacitor once the whole thing's been bathed in a standard electrolyte, like potassium chloride.

Supercapacitors, of course, can charge up and discharge almost immediately, so power density and output is generally much higher than you'd get with a standard lithium battery.

Energy density is lower, and there's a tradeoff to be made between how much energy is stored volumetrically and how strong you need your concrete to be, since adding more carbon black both boosts energy storage and weakens the final concrete.

But the great thing here is that this energy storage device doesn't need to be small; concrete tends to get used in bulk. An average American 2,000-sq-ft (185.8-m2) home built on a reasonably standard five-inch-thick (13-cm) concrete slab uses about 31 cubic yards (~24 m3) of concrete. Add more if you've got a driveway or a concreted garage, and significantly more again if the house is built using concrete walls or columns.