Researchers at the Massachusetts Institute of Technology (MIT) have developed a new material using ubiquitous materials – cement and carbon black – that could form the basis for a novel energy storage system, the university reported.
The researchers published their findings July 31 in the peer-reviewed journal Proceedings of the National Academy of Sciences. The two materials were combined with water to form a supercapacitor, which can act as an alternative to batteries to store energy.
What is a capacitor?
Capacitors consist of two electrically conductive plates immersed in an electrolyte and separated by a membrane. When voltage is applied across the capacitor, charged ions accumulate on the plates with the opposing charge, creating an electric field between the plates. Capacitors can maintain this charge for long periods and then discharge it quickly when needed. Supercapacitors are simply capacitors that can store exceptionally large charges.
The limiting factor for the amount of power a capacitor can store is the total surface area of the conductive plates. The researchers were able to produce a cement-based material with an extremely high internal surface area through the introduction of carbon black, which is highly conductive, into a concrete mixture with cement powder and water and letting it cure. The water forms a fractal-like network throughout the overall structure as it reacts with the cement, and the carbon black naturally migrates into these channels to make wire-like structures within the hardened cement, MIT said.
The resulting material was soaked in potassium chloride – a standard electrolyte material – to provide the charged particles that accumulate on the carbon structures. Two electrodes made of the material and separated by a thin space or insulating layer created a powerful supercapacitor.
“The material is fascinating, because 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,” MIT Professor Admir Masic said. “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.”
How much energy storage are we looking at?
The carbon networks can be formed with as little as 3% carbon black in the mix by volume. The researchers calculated that a 45-cubic-meter block of the material would be able to store 10 kWh of energy, equivalent to the average daily electricity consumption for a household. For comparison, the median square footage of a single-family home in the U.S. in 2021 was about 2,273 sq ft. With foundation slab thicknesses typically ranging 6-10 inches, that would amount to 42.1-70.2 cubic yards of concrete.
What’s the trade-off?
There is a trade-off between storage capacity and structural strength, as higher levels of carbon black slightly weaken the concrete. For structural elements such as foundations for homes or wind turbines, the researchers found the sweet spot to be about 10% carbon black by volume. Higher percentages could be useful in applications where the concrete does not play a structural role. The researchers also envisioned a scenario in which the material is used in roadways, which could store electricity from adjacent generation sources and wirelessly recharge electric vehicles as they drive. The material could also be used for heating by applying electricity to the carbon-laced concrete.
When can we expect to see widespread use?
Of course, the researchers are not yet building and testing 45-cubic-meter blocks of the material, so don’t expect to see it in commercial use anytime soon. The initial experiments were conducted using small supercapacitors about one centimeter in diameter and one millimeter thick, three of which were connected to light up a 3-volt LED, but they believe it is a highly scalable system. The researchers are now planning to test progressively larger versions of the material, starting with supercapacitors about the size of a 12-volt car battery and working toward a 45-cubic-meter version.
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