Common cement and carbon black pave the way for a foundational tech for efficient future energy storage.

Channeling the power of the ancients, a team at the Massachusetts Institute of Technology announced they have developed a supercapacitor from two of humanity's most ubiquitous historical materials – cement and carbon black.

Possibly the foundation of 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 renewable energy supply.

These materials, when combined with water, can make a supercapacitor that could provide the much-needed energy storage of the future.

In short, batteries and capacitors do a similar job – storing electricity – but in completely different ways.

Batteries have two electrical terminals (electrodes) separated by a chemical substance called an electrolyte. When power is switched on, chemical reactions take place involving both the electrodes and electrolyte, resulting in a reaction that releases electrical energy.

However, once the chemicals have been depleted, the reactions stop, and the battery dies. In a rechargeable battery, such as in a lithium-ion power pack, the reactions can "reverse," – resulting in charging and discharging hundreds of times before the chemicals are entirely exhausted.

Capacitors, on the other hand, use static electricity or electrostatics rather than chemistry to store energy.

Inside a capacitor, there are two conducting metal plates with an insulating material between them. Charging a capacitor is a bit like rubbing a balloon on your head and making your hair stand up. Positive and negative charges build up on the plates, and due to the plate between them, no discharge can occur, and thus the energy remains – being stored for later use.

A good separating plate, called a dielectric, allows a capacitor of certain sizes to store more charge at the same voltage than less capable dielectrics, making them more efficient as charge-storing devices.

Capacitors have many advantages over batteries: they weigh less, generally don't contain harmful chemicals or toxic metals, and can be charged and discharged near indefinitely without ever wearing out.

But they have a big drawback too – pound for pound, their basic design prevents them from storing anything like the same amount of electrical energy as batteries.

Enter Super

A supercapacitor or ultracapacitor differs from convention in two important ways – the plates have a much larger area, and the distance between them is much smaller.

Like an ordinary capacitor, a supercapacitor has two plates that are separated. The plates are typically made from metal coated with a porous substance such as powdery activated charcoal, which effectively gives them a bigger area for storing much more charge.

Imagine if electricity behaved like water, where an ordinary capacitor is like a cloth that can soak up only a tiny spill. A supercapacitor's porous plates, however, are more like a sponge that can absorb many times more of the water. More porosity, more electricity can be stored.

So how can cement and carbon black (half-combusted coal, coal tar, vegetable matter, or petroleum products) provide the adequate absorption of a super sponge?

"The material is fascinating," said Admir Masic, co-author of the research paper. "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, "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," he says. The process is easily reproducible, with materials that are inexpensive and readily available anywhere in the world. And the amount of carbon needed is very small - as little as 3 percent by volume of the mix - to achieve a percolated carbon network," the co-author said.

Described in the journal PNAS, this simple but innovative technology could be the future foundation of energy storage, literally.

Super-homes

In an example presented by the researchers, this new supercapacitor could potentially be incorporated into the very concrete foundations of a home, where it could store a full day's worth of energy while adding little or even no cost to the foundation and still provide the structural strength to meet code.

"Supercapacitors made of this material have great potential to aid in the world's transition to renewable energy," said Franz-Josef Ulm, another co-author of the paper.

The principal sources of emissions-free energy, wind, solar, and tidal power, all produce their output at variable times that often do not align with the peaks in electricity usage, so ways of storing that power are essential.

"There is a huge need for big energy storage," added Ulm.

This includes a practical and less costly alternative to existing batteries, which are expensive and mostly rely on materials such as lithium that are in short supply due to the enormous quantities needed for electric vehicle batteries.

"That's where our technology is extremely promising, because cement is ubiquitous," Ulm says.

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 (11.5 feet) 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.

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 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 demonstrate their ability to light up a 3-volt LED.

Having proved the principle, the team now plans to build a series of larger versions, starting with ones about the size of a typical 12-volt car battery, then work their way up to a 45-cubic-meter version to demonstrate its ability to store a house-worth of power.

Narrowing down the recipe

Refining the composition, the researchers found there is a tradeoff between the storage capacity of the material and its structural strength.

By adding more carbon black, the resulting supercapacitor can store more energy, but ultimately makes the concrete slightly weaker.

While this could be useful for applications where concrete does not play 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," they found, is around 10% carbon black in the mix.

Another potential application for carbon-cement supercapacitors is for building concrete roadways that could store energy produced by solar panels alongside the road and then deliver that energy to electric vehicles traveling using the same kind of technology used for wirelessly rechargeable phones.

For example, a related type of car-recharging system is already being developed by companies in Germany and the Netherlands but using standard batteries for storage.

Foreseeing initial uses of the technology might be for isolated homes or buildings, or shelters far from grid power, which could be powered by solar panels attached to the cement supercapacitors, the technology is remarkably scalable.

"You can go from 1-millimeter-thick electrodes to 1-meter-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," said Ulm.

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, said the MIT professor.

"So, it's really a multifunctional material," he added. 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.

Regardless of its early stage, Ulm sees this as "a new way of looking toward the future of concrete as part of the energy transition."