Cement and carbon black (similar to very fine charcoal), two of humanity's most ubiquitous historical materials, could form the basis of a new, low-cost energy storage system, according to a new study. The technology could enable energy networks to remain stable despite fluctuations in renewable energy supplies, thereby promoting the use of renewable energy sources such as solar, wind and tidal power.
MIT engineers have used an ancient and abundant material to create a "supercapacitor" that can store large amounts of energy. Made from just cement, water and carbon black (similar to charcoal powder), the device could serve as the basis for an inexpensive system for storing intermittent renewable energy, such as solar or wind. Image source: Franz-JosefUlm, AdmirMasic and Yang-ShaoHorn
Researchers have discovered that the two materials can be combined with water to create supercapacitors (an alternative to batteries) that provide electrical energy storage. The MIT researchers who developed the system say, for example, that their supercapacitors could eventually be installed in the concrete foundations of homes, allowing them to store an entire day's worth of electricity with little to no additional (or subtracted) cost from the foundation and still provide the needed structural strength. The researchers also envision a concrete pavement that could provide contactless charging for electric vehicles as they pass over it.
MIT professors Franz-Josef Ulm, Admir Masic, and Yang-Shao Horn, along with four other professors from MIT and the Wyss Institute, describe this simple yet innovative technique in a paper recently published in the Proceedings of the National Academy of Sciences (PNAS).
In principle, a capacitor is a very simple device consisting of two conductive plates immersed in an electrolyte, separated by a thin film. When a voltage is applied across a capacitor, the positively charged ions in the electrolyte collect on the negatively charged plates, and the positively charged plates collect negatively charged ions. Since the film between the plates blocks the migration of charged ions, the separation of charges generates an electric field between the plates, and the capacitor becomes charged. The two plates can hold the pair of charges for a long time and release them quickly when needed. A supercapacitor is a capacitor capable of storing extremely large electrical charges.
The amount of electricity a capacitor can store depends on the total surface area of its conducting plates. The key to the new supercapacitor developed by the research team lies in a method of producing a cement-based material that has an extremely high internal surface area due to the presence of a dense, interconnected network of conductive materials within its volume. The researchers achieved this by introducing highly conductive carbon black into the concrete mixture along with cement powder and water and allowing it to solidify. As the water reacts with the cement, it naturally forms a branching network of openings in the structure, and the carbon migrates into these spaces, forming a wire-like structure in the hardened cement.
These structures have a fractal-like structure, with larger branches growing into smaller branches, which grow into smaller branches, and so on, ultimately creating an extremely large surface area within a relatively small volume. This material is then soaked in a standard electrolyte material such as potassium chloride, a salt, which accumulates charged particles on the carbon structure. The researchers found that two electrodes made of this material separated by a thin layer of space, or insulation, could create a very powerful supercapacitor.
Because this new "supercapacitor" concrete retains its strength, a house with a foundation built on this material can store a day's worth of energy generated by solar panels or windmills and use it whenever needed. Image source: Franz-JosefUlm, AdmirMasic and Yang-ShaoHorn
The two plates of a capacitor are like the poles of a rechargeable battery of equal voltage: like a battery, energy is stored in the plates when connected to a power source, and then when connected to a load, current flows back, providing electrical energy.
"This material is fascinating," Masic said, "because you have the most commonly used man-made material in the world, cement, combined with carbon black, which is a well-known historical material -- the Dead Sea Scrolls were written in it. You have these materials that are at least two thousand years old, and when you combine them in a specific way, you create a conductive nanocomposite, and that's where things get really interesting."
"As the mixture sets and solidifies, water is systematically consumed through cement hydration reactions, and this hydration reaction fundamentally affects the carbon nanoparticles because they are hydrophobic (repellent to water). As the mixture evolves, the carbon black self-assembles into a connected conductive thread. This process is easily replicable, using materials that are inexpensive and available anywhere in the world. The amount of carbon required to achieve the carburized network is very small, accounting for only 3% of the volume of the mixture."
Supercapacitors made from this material have huge potential to help the world transition to renewable energy. The main sources of emission-free energy - wind, solar and tidal power - are output at irregular times, often inconsistent with peak electricity demand, so ways to store electricity are crucial. "There is a huge need for large-scale energy storage devices. Existing batteries are too expensive and rely mainly on materials such as lithium, which is in limited supply, so there is an urgent need for cheaper alternatives." Ulm said: "This is where our technology is very promising, because cement is everywhere."
The team calculated that a 45 cubic meter (or yard) block of nanocarbon black-infused concrete (equivalent to a cube about 3.5 meters in diameter) would be enough to store about 10 kilowatt hours of energy, which is equivalent to the average daily electricity usage of a household. Because concrete retains its strength, a house with a foundation made of this material can store a day's worth of electricity from solar panels or windmills and have it ready for use when needed. Moreover, supercapacitors charge and discharge much faster than batteries.
After a series of tests to determine the most effective ratio of cement, carbon black and water, the team created small supercapacitors, about the size of some button batteries, about 1 centimeter in diameter and 1 millimeter thick. Each can be charged to 1 volt, equivalent to a 1-volt battery. They then connected three of these cells and demonstrated their ability to light up a 3-volt light-emitting diode (LED). Having proven the principle, they now plan to build a series of larger versions, starting with one about the same size as a typical 12-volt car battery and then scaling up to a 45 cubic meter version to demonstrate its ability to store a house's worth of electricity.
They found that there was a trade-off between the material's storage capacity and its structural strength. By adding more carbon black, the resulting supercapacitor can store more energy, but the concrete will be slightly weaker, which could be useful in applications where concrete does not serve a structural role or where concrete's full strength potential is not needed. They found that for applications such as foundations or wind turbine base structural members, the "sweet spot" is about 10% carbon black in the mix.
Another potential application for carbon-cement supercapacitors is in the construction of concrete pavements that can store energy generated by solar panels on the roadside and then deliver the energy to electric cars driving along the road using the same technology used to wirelessly charge mobile phones. Companies in Germany and the Netherlands are already developing a related car charging system, but using standard batteries.
The researchers say initial uses for the technology could be in isolated homes, buildings or shelters off the grid that could be powered by solar panels attached to cement supercapacitors.
The system is highly scalable because the energy storage capacity is a direct function of the electrode volume. "You can scale from a 1-millimeter-thick electrode to a 1-meter-thick electrode, and by doing that, you can basically scale the energy storage capability from lighting up an LED for a few seconds to powering an entire house," Ulm said.
Depending on the properties required for a specific application, the system can be tuned by adjusting the mixture. For car charging roads, very fast charging and discharging speeds are required, while for home power supply there is a full day to charge, so slower charging materials can be used.
So this is really a versatile material. In addition to storing energy in the form of supercapacitors, the same concrete mixture can also be used as a heating system by simply passing electricity through the carbon-containing concrete.
Ulm sees this as "a new way of looking at the future of concrete as part of the energy transition".