MIT Researchers Develop Supercapacitor Made of Cement, Carbon Black and Water

They say genius is 1 percent inspiration and 99 percent perspiration. Yet in the case of MIT’s energy-storing “supercapacitor” concrete, made simply of cement, carbon black and water, I’d say it’s 99 percent inspiration.

Yes, you read that correctly. A cement/carbon black (which resembles powdered charcoal) functioning supercapacitor. Who’d a thunk it.?

Better still, this isn’t a theoretical exercise—it’s for real—albeit it is now only a lab–sized project. As described in the July 31 edition of the journal PNAS in a paper by MIT professors Franz-Josef Ulm, Admir Masic and Yang-Shao Horn, the energy storage capacity of this space-filling carbon black network has a high specific surface area that can store charge. 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 for Biologically Inspired Engineering 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.

Why It Matters

What are the implications for cement/carbon black supercaps? For starters, the environmental footprint of worldwide cement and concrete production amounts to roughly 8 percent of the worldwide CO₂ emissions. Here, the MIT team sought to redeem cement’s image by using it as the base for their supercapacitor’s electrodes.

The resulting properties--energy storage capacity and high energy rate capability—point to the opportunity for employing these structural concrete-like supercapacitors for bulk energy storage in both residential and industrial applications. The device could form the basis for inexpensive systems that store intermittently renewable energy, such as solar or wind energy. Potential applications range from intermittent energy storage for wind turbines to energy self-sufficient shelters to self-charging roads for electric vehicles.

This project could prove important to the use of renewable energy sources such as solar, wind and tidal power by allowing energy networks to remain stable despite fluctuations in renewable energy supply. A house with a foundation made of this material could store a day’s worth of energy produced by solar panels or windmills, while adding little (or nothing) to the cost of the foundation and still providing the needed structural strength. Concrete is already used to construct roads, so the researchers envision a concrete roadway that could provide contactless recharging for electric cars as they travel over that road.

Depending on the properties desired for a given application, 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.

How It Works

How does placing nanoparticles of carbon black into wet cement transform it into a supercapacitor? According to the researchers, as the mixture sets and cures the water is systematically consumed through cement hydration reactions, and this hydration fundamentally affects the nanoparticles of carbon because they are hydrophobic (water repelling). As the mixture evolves, the researchers conclude, the carbon black self-assembles into what is essentially a connected conductive wire.

The key to the new supercapacitors developed by this team comes from a method of producing a cement-based material with an extremely high internal surface area due to a dense, interconnected network of conductive material within its bulk volume. The researchers achieved this by introducing carbon black—which is highly conductive—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 are said to have a fractal-like structure, with larger branches sprouting smaller branches and so on, ending up with an extremely large surface area within the confines of a relatively small volume. The material is then soaked in a standard electrolyte material, such as potassium chloride, which provides the charged particles that accumulate on the carbon structures. Two electrodes made of this material, separated by a thin space or an insulating layer, form a powerful supercapacitor.

The researchers noted that as the cement mixture cured, the water was absorbed and left behind a branching network of tunnels that the carbon black filled. The end result is that the cement paste is filled with a large surface area of conductive, wire-like tunnels, without expanding the overall volume of the electrode.

Carbon black is a highly conductive material. To build their supercapacitor, the team mixed together a paste made of cement and water and then introduced a small amount of carbon black. The amount of carbon needed is very small—as little as 3 percent by volume of the mix —to achieve the carbon network. For applications such as a foundation or structural elements of the base of a wind turbine, the “sweet spot” is around 10 percent carbon black in the mix, the researchers found.

For these supercapacitors, the carbon network results from competition between particle aggregation of hydrophobic carbon black, and water demand by the hydration reactions of hydrophilic cement. The authors explain this as follows: The hydration of anhydrous clinker (Cement and clinker are not the same material. Cement is a binding material used in construction whereas clinker is primarily used to produce cement) dissolves calcium oxide, freeing calcium ions to react with water to form various cement hydration products. Further, in the carbon-cement composites, the aggregation of nonpolar carbon black occurs via attractive Van der Waals interactions (a van der Waals interaction occurs when adjacent atoms come close enough that their outer electron clouds just barely touch) and the ionic strength of free calcium ions is reduced via hydration reactions in the high pH cement environment.

What’s Next?

Additional development work will need to be performed. In the lab researchers fabricated button-size capacitors capable of holding 1 volt of charge and determined that the capacitor was able to maintain its storage capacity with minimal loss over 10,000 charge-discharge cycles. Three of the 1V supercapacitors were also able to charge a 3V LED.

The team calculated that a block of nanocarbon-black-doped concrete that is 45 cubic meters (equivalent to a cube about 3.5 meters across) would have enough capacity to store about 10 kilowatt-hours of energy, which is considered the average daily electricity usage for a household. Since the concrete would retain its strength, a house with a foundation made of this material could store a day’s worth of energy produced by solar panels or windmills and allow it to be used whenever it’s needed.

Industry veterans know that going from a lab experiment to a work-a-day commercial product is difficult and takes time. In future they 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 to demonstrate its ability to store a house-worth of power.