Old materials produce brand-new supercapcacitor

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. This 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 the supply of renewable energy.

The two materials can be combined with water to make a supercapacitor — an alternative to batteries — for storing electrical energy. As an example, the researchers from Massachusetts Institute of Technology (MIT) who developed the system propose that their supercapacitor could eventually be incorporated into the concrete foundation of a house. Here, it could store a full day’s worth of energy while adding little (or no) to the cost to the foundation and still providing the needed structural strength. The researchers also envision a concrete roadway that could provide contactless recharging for electric cars as they travel over that road.

The researchers report this simple but innovative technology in a paper in the Proceedings of the National Academy of Sciences.

Capacitors are in principle very simple devices, consisting 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 negatively charged ions accumulate on the positively charged plate.

Since the membrane sandwiched between the two plates blocks charged ions from migrating across, this separation of charges creates an electric field between the plates, and the capacitor becomes charged. The two plates can maintain this pair of charges for a long time and then deliver them very quickly when needed. Supercapacitors are simply capacitors that can store exceptionally large charges.

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 the researchers is a method for producing a cement-based material with an extremely high internal surface area due to the presence of 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 the cement, and the carbon migrates into these spaces to form wire-like structures within the hardened cement. These structures have a fractal-like structure, with larger branches sprouting smaller branches, and those sprouting even smaller branchlets, 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, 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 two plates of the capacitor function just 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 electrical current flows back out to provide power.

“The material is fascinating,” says MIT professor Admir Masic, “because you have the most-used manmade 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.”

As the mixture sets and cures, he says, “the water is systematically consumed through cement hydration reactions, and this hydration fundamentally affects nanoparticles of carbon because they are hydrophobic (water repelling)”. As the mixture evolves, “the carbon black is self-assembling into a connected conductive wire”.

The process is easily reproducible, with materials that are inexpensive and readily available anywhere in the world. And the amount of carbon needed to achieve a percolated carbon network is very small – as little as 3% by volume of the mix.

Supercapacitors made of this material have great potential to aid in the world’s transition to renewable energy. The principal sources of emissions-free energy – wind, solar and tidal power – all produce their output at variable times that often do not correspond to the peaks in electricity usage, so ways of storing that power are essential.

“There is a huge need for big energy storage,” says MIT professor Franz-Josef Ulm, and existing batteries are too expensive and mostly rely on materials such as lithium, whose supply is limited, so cheaper alternatives are badly needed. “That’s where our technology is extremely promising, because cement is ubiquitous.”

The team calculated that a block of nanocarbon-black-doped concrete that is 45m³ in size — equivalent to a cube about 3.5m 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. Supercapacitors can also be charged and discharged much more rapidly than batteries.

After a series of tests used to determine the most effective ratios of cement, carbon black and water, the team demonstrated the process by making small supercapacitors, about the size of button-cell batteries (1cm across and 1mm thick), which could each be charged to 1 volt, comparable to a 1-volt battery. They then connected three of these to demonstrate their ability to light up a 3-volt light-emitting diode (LED). Having proved the principle, the researchers 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 45m³ version to demonstrate its ability to store a house-worth of power.

They also found, however, that there is trade-off between the storage capacity of the material and its structural strength. By adding more carbon black, the resulting supercapacitor can store more energy, but the concrete is slightly weaker. 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% carbon black in the mix.

Another potential application for carbon-cement supercapacitors is for building concrete roadways that can store the energy produced by solar panels alongside the road and then deliver that energy to electric vehicles traveling along the road using the same kind of technology used for wirelessly rechargeable phones. A related type of car-recharging system, but using standard batteries for storage, is already being developed by companies in Germany and the Netherlands.

According to the researchers, the technology might initially be used for isolated homes or buildings or shelters far from grid power, which could be powered by solar panels attached to the cement supercapacitors.

Ulm says that the system is very scalable, as the energy-storage capacity is a direct function of the volume of the electrodes. “You can go from 1mm-thick electrodes to 1m-thick electrodes, and by doing so basically you can scale the energy-storage capacity from lighting an LED for a few seconds, to powering a whole house.”

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 a slower-charging material could be used.

“So, it’s really a multifunctional material,” 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.

Ulm sees this as “a new way of looking toward the future of concrete as part of the energy transition”.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.