The inconsistency of renewable energy production is one reason why power grids are still reliant on fossil fuels. For example, solar panels cannot generate energy at night and wind turbines do not work when the air is still.
But MIT researchers believe their energy storage device, which is a supercapacitor rather than a battery, could provide cheap storage of electrical energy that could help even out the ebb and flow of renewable energy generation.
The technology could even be incorporated into the concrete foundation of a house, where it could store a full day's worth of energy while adding little cost to the construction of the foundations while still providing the structural strength required.
The researchers also envision a concrete roadway that could provide contactless recharging for electric cars as they travel over that road.
Capacitors typically consist of two electrically conductive plates immersed in an electrolyte and separated by a membrane. When a voltage is applied across the capacitor, positively charged ions from the electrolyte accumulate on the negatively charged plate, while the positively charged plate accumulates negatively charged ions.
The amount of power a capacitor can store depends on the total surface area of its conductive plates. 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.
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 have a fractal-like shape that has 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 (a kind of salt), 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 very powerful supercapacitor, the researchers found.
"The material is fascinating," said MIT professor Admir Masic, "because you have the most-used manmade material in the world, cement, that is combined with carbon black, a well-known historical material – the Dead Sea Scrolls were written with it.
"You have these at least two-millennia-old materials that when you combine in a specific manner you come up with a conductive nanocomposite, and that's when things get really interesting."
The research team believes that supercapacitors made of this material have the potential to aid in the global transition to renewable energy. They 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.
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.
