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Researchers from the Massachusetts Institute of Technology (MIT) have harnessed two of the world’s most ubiquitous materials, concrete and carbon black, to develop a novel energy storage system. The technology could play a pivotal role in accelerating the energy transition by offering a low-cost, scalable, and readily available solution to address the intermittent nature of renewables.

 

"Concrete and carbon black are materials that humans are very familiar with and have known how to process for millennia," says Admir Masic, Associate Professor of Civil and Environmental Engineering at MIT. "With these two materials, we end up with an extremely sophisticated nano-composite that can solve future bottlenecks associated with our transition from non-renewable to renewable energy sources."

 

Researchers envision that the composite material could be incorporated into everyday construction projects, turning ordinary cement into a renewable electricity-conducting material with almost no additional costs. For example, they imagine roads capable of recharging electric vehicles, concrete home foundations that provide renewable energy at night, and wind turbine superstructures that serve as bulk intermittent energy storage.

 

Surprisingly, such advanced ideas start with a remarkably simple process. By combining just three ingredients — cement, water, and carbon black — an energy storage device known as a supercapacitor is naturally formed.

 

Compared to conventional batteries, which store energy through chemical reactions and degrade over time, supercapacitors store energy electrostatically between two conductive surfaces. This results in faster charging and discharging capabilities, and a longer lifespan that has virtually unlimited charge cycles without any decline in performance.

 

The amount of energy that a supercapacitor can store depends in part on the size of its conductive surfaces. The larger the surface area, the more electrical charge it can hold. In the novel supercapacitor, researchers discovered that carbon black, a highly conductive powder, was the key to increasing surface area and, subsequently, capacitance.

 

Carbon black is hydrophobic, explains Masic. When added to the mixture of water and cement, the water constrains the carbon into isolated clumps. But once the cement starts hydrating, an essential step for the hardening and setting of a cement mixture, the water is consumed. This frees space for the carbon nanoparticles to move around, and they migrate to areas with less water.

 

Eventually, the carbon nanoparticles form a fractal structure. Fascinatingly, the structure meets the exact characteristics needed for a high-efficiency supercapacitor. This includes an electron-conductive network and ample surface area for ions to accumulate — both of which are provided by the carbon black — along with space for the electrolyte (a water-based salt solution to carry the charged particles throughout the supercapacitor), which exists in the concrete’s innate porosity.

 

"The interplay between the hydrophobic carbon black nanoparticles and the hydrophilic cement leads to this natural self-assembly of a kind of nano-wiring within the system," Masic says.

 

"It looks a little bit like a tree branch," adds Franz-Josef Ulm, Professor of Civil and Environmental Engineering at MIT and study coauthor. "Imagine branches that split off into smaller and smaller parts. The bottom, in essence, is where you apply the charge from the photovoltaic cell."

 

In experiments with different mix proportions of cement, water, and carbon black, researchers found that they only needed a small amount of the carbon powder — about 3% by volume of the mix — to form the conductive network. At around 10% by volume, they observed a "beautifully developed" network of branches throughout the material, Ulm says.

 

However, above 12%, researchers found that the concrete experienced significant strength loss. Although higher proportions of carbon black theoretically offer more energy storage, the design challenge for future applications will be to find the optimal material mix that satisfies both capacity and structural requirements, according to Masic. For instance, structures like wind turbines and house foundations have varying requirements for concrete strength, necessitating different mixtures.

 

At present, researchers have demonstrated a proof-of-concept of the technology by making small supercapacitors, about 1 mm thick, comparable to a 1-V battery. They then connected three of these together to light a 3-V LED.

 

But beyond the laboratory, the research suggests that the new technology has the potential to function as a bulk energy storage solution for residential and industrial applications. A key finding of the study shows that the carbon-cement supercapacitor is scalable. As the system increases in size, it can store the same amount of energy per unit of volume.

 

Next, researchers are working on larger versions of the technology, including a supercapacitor that is equivalent to a 12-V battery. For future scale-up purposes, they calculated that 45 m3 of the material would be enough to store 10 kWh of energy — the typical amount required to power a household each day.

 

"What really excites me is this bioinspired design," says Masic. "How we can invent new shapes for our houses, walls, and foundations that will incorporate efficient supercapacitors and energy storage solutions that were unavailable until now and can be done anywhere in the world."

 

Chanut, N., et al., "Carbon-Cement Supercapacitors as a Scalable Bulk Energy Storage Solution," Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2304318120 (2023)