Supercapacitor Is Made From Cement, Carbon Black And Water

Researchers at MIT have combined cement, carbon black (a substance like a fine charcoal) and water to provide a low-cost energy storage system. They say that the new material could be particularly useful as storage for renewable energy sources by stabilising energy networks when fluctuations in renewable energy supply occur.

Combined, the three materials can make a supercapacitor. For example, the researchers say that in the future, the supercapacitor may be incorporated into the concrete foundations of a house, while adding minimal cost to the foundation and still providing the required structural strength. Another possibility is a concrete roadway that could provide contactless recharging for EVs as they travel along it.

The discovery was published in the journal PNAS, in a paper by MIT professors Franz-Josef Ulm, Admir Masic, and Yang-Shao Horn, and four others at MIT and at the Wyss Institute for Biologically Inspired Engineering.

The amount of energy that a capacitor can store depends on the total surface area of its conductive plates. The research team has found a method producing the cement-based material with a high internal surface area through a dense network of conductive material. This was achieved by introducing highly conductive carbon black into a concrete mixture along with cement powder and water, and letting it cure. The water naturally forms a branching network of openings within the structure as it reacts with cement, and the carbon migrates into these spaces to make wire-like structures within the hardened cement. These structures have a fractal-like structure, ending up with an extremely large surface area within a relatively small volume.

The material is then soaked in an electrolyte material, such as potassium chloride. The two electrodes are made of this material, separated by a thin space or an insulating layer. The two plates of the capacitor function like the two poles of a rechargeable battery of equivalent voltage: When connected to a source of electricity energy gets stored in the plates, and then when connected to a load, the current flows back out to provide power.

The team calculated that a block of nanocarbon-black-doped concrete that is 45 cubic meters in size would have enough capacity to store about 10 kilowatt-hours of energy, which is considered the average daily electricity usage for a household. A house with a foundation made of this material could store a day’s worth of renewable energy and allow it to be used whenever it’s needed. Supercapacitors can also be charged and discharged much more rapidly than batteries.

The team demonstrated the process by making small supercapacitors, about 1 cm across and 1 mm thick, that could each be charged to 1 volt, comparable to a 1-volt battery. They then connected three of these to light up a 3-volt LED. Having proved the principle, they now plan to build a series of larger versions, starting with ones about the size of a typical 12-volt car battery, then working up to a 45-cubic-meter version.

The system could be tuned by adjusting the mixture. For a vehicle-charging road, very fast charging and discharging rates would be needed, while for powering a home “you have the whole day to charge it up,” so slower-charging material could be used, Ulm says. Besides its ability to store energy in the form of supercapacitors, the same kind of concrete mixture can be used as a heating system, by simply applying electricity to the carbon-laced concrete.

The research team also included postdocs Nicolas Chanut and Damian Stefaniuk at MIT’s Department of Civil and Environmental Engineering, James Weaver at the Wyss Institute, and Yunguang Zhu in MIT’s Department of Mechanical Engineering. The work was supported by the MIT Concrete Sustainability Hub, with sponsorship by the Concrete Advancement Foundation.

PSD