MIT boffins build battery alternative out of cement, carbon black, water

Imagine your home's foundation was its own energy-storing supercapacitor

Researchers at MIT claim to have found a novel new way to store energy using nothing but cement, a bit of water, and powdered carbon black – a crystalline form of the element.

The materials can be cleverly combined to create supercapacitors, which could in turn be used to build power-storing foundations of houses, roadways that could wirelessly charge vehicles, and serve as the foundation of wind turbines and other renewable energy systems – all while holding a surprising amount of energy, the team claims. 

According to a paper published in the Proceedings of the National Academy of Sciences, 45 cubic meters of the carbon-black-doped cement could have enough capacity to store 10 kilowatt-hours of energy – roughly the amount an average household uses in a day. A block of cement that size would measure about 3.5 meters per side and, depending on the size of the house, the block could theoretically store all the energy an off-grid home using renewables would need.

"You have the most-used man-made material in the world, cement, that is combined with carbon black, that is a well-known historical material – the Dead Sea Scrolls were written with it," said MIT Associate Professor of Civil and Environmental Engineering, Admir Masic.

"You have these at least two-millennia-old materials that when you combine them in a specific manner you come up with a conductive nanocomposite, and that's when things get really interesting," Masic added.

It's really super!

Capacitors are able to store energy, but not in the same way as batteries.

While batteries rely on the conversion of chemical energy to electrostatic charge to store and release energy, capacitors store energy in an electric field between conductive, separated plates. The more perfect the insulator in the gap between the two conductors, the more (equal but opposite) charge can be stored in each conductor. Supercapacitors – which use ion-permeable membranes including graphene as the separator – function similarly. 

When connected to a source of electricity, energy is stored in the plates, but when it is connected to a load, the current flows back out to give power.

The capacity of a capacitor or supercapacitor is largely, but not solely, determined by the surface area of its plates. The MIT researchers explained that the material they've explored has an exceptionally high internal surface area thanks to the way the carbon black and water interact.

"The hydration reactions of cement in the presence of carbon generate a fractal-like electron-conducting carbon network that permeates the load-bearing cement-based matrix," the authors note. In essence, a block of this carbon-rich cement has highly-conductive carbon black wires running through it which drastically increase the surface area, and thus storage capacity.

Masic said that as the mixture cures, water is absorbed into the cement. Carbon black, which is highly hydrophobic, can't be dispersed in the same way, thus "the carbon black is self-assembling into a connected conductive wire." 

Just three percent of the mixture has to be carbon black for the hardened cement to act as a supercapacitor, but the researchers found that a 10 percent carbon black mixture appears to be ideal. Beyond that ratio, the cement becomes less stable – not something you want in a building or foundation. The team notes that non-structural use could allow higher concentrations of carbon black, and thus higher energy storage capacity. 

The team has only built a tiny one-volt test platform using its carbon black mix, but has plans to scale up to supercapacitors the same size as a 12-volt automobile battery – and eventually to the 45 cubic meter block. 

Along with being used for energy storage, the mix could also be used to provide heat – by applying electricity to the conductive carbon network encased in the cement, MIT noted.

"There is a huge need for big energy storage," MIT civil and environmental engineering professor, and head of MIT's concrete sustainability hub, Franz-Josef Ulm, noted. Ulm, a co-author on the paper, explained that supercapacitors made using cheaper, more abundant materials could be a huge aid in helping the world transition to renewable energy, and eliminate the need for expensive and scarce materials like lithium. 

"Our technology is extremely promising, because cement is ubiquitous," Ulm said. 

'The engineering starts now'

The MIT team said that its new technology is easily made and scaled, and Ulm told The Register that "the engineering starts now" in terms of getting the product to market. He noted that the process need the expertise of materials scientists, electrical engineers, structural engineers and architects working together to accomplish – no easy task.

Within six to nine months, Ulm's team wants to produce the 12V auto battery equivalent, which he said would serve as "an elemental brick for storing energy in homes." If everything is successful, "the first energy independent home prototype could be available in maybe 18 months," Ulm told us. 

Ulm believes it will take around three years of development for the carbon black cement to be used for energy-storing foundations or in the superstructure of wind turbines. It would take around the same time to design and build a contactless electric vehicle charging spot with the new material – or even roads built of the stuff.  

"If this is successful, an additional 2–3 years of development [would be needed] for dynamic charging along highways," Ulm noted. This isn't entirely far-fetched – small-scale tests of inductive EV charging are already being planned in Germany and Detroit, Michigan – but with traditional forms of energy storage as opposed to cement supercapacitors.

More generally, it'll take a decade until the tech can be fully implemented, Ulm opined. And that's "provided all players" – like the construction industry, energy operators and government regulators – can "come together."

Given the hostility to climate change initiatives from one side of the US political establishment, that might be a tough process. ®

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