Making an energy-storing supercapacitor out of cement, water, and black carbon is a novel idea that combines materials that aren’t normally associated with energy storage devices. Supercapacitors, also known as ultracapacitors or electrochemical capacitors, store energy by electrostatic charge separation. They are well-known for their high power density and quick charging and discharging capabilities.
Engineers have developed a’supercapacitor’ that can store large amounts of energy and is made of ancient, abundant materials. The device, which is made of cement, water, and carbon black (which resembles powdered charcoal), could serve as the foundation for low-cost systems that store intermittently renewable energy, such as solar or wind energy.
Two of humanity’s most ubiquitous historical materials, cement and carbon black (which resembles very fine charcoal), may form the basis for a novel, low-cost energy storage system, according to a new study. The technology could facilitate 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.
The two materials, the researchers discovered, can be combined with water to form a supercapacitor, an alternative to batteries that could provide electrical energy storage. For example, the MIT researchers who developed the system claim that their supercapacitor could eventually be incorporated into a house’s concrete foundation, where it could store a full day’s worth of energy while adding little (or no) cost to the foundation and still providing the necessary structural strength. The researchers also envision a concrete roadway that could provide contactless charging for electric vehicles as they travel along it.
The simple but innovative technology is described in a forthcoming paper 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.
The material is fascinating, because you have the most-used human-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.
Admir Masic
In principle, capacitors are very simple devices that 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 electrolyte ions accumulate on the negatively charged plate, while negatively charged ions accumulate on the positively charged plate. Because the membrane between the plates prevents charged ions from migrating across, the separation of charges creates an electric field between the plates and charges the capacitor. The two plates can store this pair of charges for an extended period of time and then deliver them quickly when needed. Supercapacitors are simply capacitors that can store extremely large amounts of charge.
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 due to a dense, interconnected network of conductive material within its bulk volume.
The researchers accomplished this by incorporating carbon black, a highly conductive material, into a concrete mixture along with cement powder and water and allowing it to cure. As the water reacts with the cement, it naturally forms a branching network of openings within the structure, and the carbon migrates into these spaces to form wire-like structures within the hardened cement. These structures have a fractal structure, with larger branches sprouting smaller branches, and those sprouting even smaller branchlets, and so on, resulting in an extremely large surface area contained within a relatively small volume.
The material is then immersed in a common electrolyte, such as potassium chloride, a type of salt that provides the charged particles that accumulate on the carbon structures. The researchers discovered that two electrodes made of this material separated by a thin space or an insulating layer form a very powerful supercapacitor.
The capacitor’s two plates function similarly to the two poles of an equivalent voltage rechargeable battery: When connected to an electrical source, such as a battery, energy is stored in the plates, and when connected to a load, electrical current flows back out to provide power.
“The material is fascinating,” Masic says, “because you have the most-used human-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. 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.”
“The water is systematically consumed through cement hydration reactions as the mixture sets and cures,” he says, “and this hydration fundamentally affects carbon nanoparticles because they are hydrophobic (water repelling).” “The carbon black is self-assembling into a connected conductive wire,” he says as the mixture evolves. The process is simple to replicate, using materials that are inexpensive and widely available around the world. And, according to Masic, the amount of carbon required to achieve a percolated carbon network is very small – as little as 3% by volume of the mix.
Ulm believes that supercapacitors made of this material have great potential to aid in the world’s transition to renewable energy. Wind, solar, and tidal power, the primary sources of emissions-free energy, all produce their output at variable times that do not always correspond to peaks in electricity usage, so methods of storing that power are critical. “There is a huge need for large energy storage,” he says, and existing batteries are too expensive and rely on materials such as lithium, which is in short supply, so cheaper alternatives are desperately needed. “That’s where our technology is extremely promising, because cement is ubiquitous,” Ulm explains.
The team calculated that a block of nanocarbon-black-doped concrete that is 45 cubic meters (or yards) in size — 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. And, supercapacitors can be charged and discharged much more rapidly than batteries.
Following a series of tests to determine the most effective cement, carbon black, and water ratios, the team demonstrated the process by fabricating small supercapacitors the size of some button-cell batteries, about 1 centimeter across and 1 millimeter thick, that could each be charged to 1 volt, comparable to a 1-volt battery. They then connected three of these to show that they could light up a 3-volt light-emitting diode (LED). They plan to build a series of larger versions, beginning with ones about the size of a typical 12-volt car battery and progressing to a 45-cubic-meter version to demonstrate its ability to store a house’s worth of power.
There is a tradeoff between the storage capacity of the material and its structural strength, they found. By adding more carbon black, the resulting supercapacitor can store more energy, but the concrete is slightly weaker, and this could be useful for applications where the concrete is not playing a structural role or where the full strength-potential of concrete is not required. 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, they found.
Another potential use for carbon-cement supercapacitors is in the construction of concrete roadways that could store energy generated by solar panels alongside the road and then deliver that energy to electric vehicles traveling along the road using the same technology used for wirelessly rechargeable phones. Companies in Germany and the Netherlands are already working on a similar type of car-charging system that uses standard batteries for storage.
According to the researchers, the technology’s initial applications could be for isolated homes, buildings, or shelters that are not connected to the grid and could be powered by solar panels attached to the cement supercapacitors.
