A few years ago, a team at MIT researched and published a paper on using concrete as an energy-storage supercapacitor (MIT engineers create an energy-storing supercapacitor from ancient materials) (also called an ultracapacitor), which is a battery based on electric fields rather than electrochemical principles. Now, the same group has developed a battery with ten times the storage per volume of that earlier version, by using concrete infused with various materials and electrolytes such as (but not limited to) nano-carbon black.
Concrete is the world’s most common building material and has many virtues, including basic strength, ruggedness, and longevity, and few restrictions on final shape and form. The idea of also being able to use it as an almost-free energy storage system is very attractive.
By combining cement, water, ultra-fine carbon black (with nanoscale particles), and electrolytes, their electron-conducting carbon concrete (EC3, pronounced “e-c-cubed”) creates a conductive “nanonetwork” inside concrete that could enable everyday structures like walls, sidewalks, and bridges to store and release electrical energy, Figure 1.
Figure 1 As with most batteries, schematic diagram and physical appearance are simple, and it’s the details that are the challenge. Source: Massachusetts Institute of Technology
This greatly improved energy density was made possible by their deeper understanding of how the nanocarbon black network inside EC3 functions and interacts with electrolytes, as determined using some sophisticated instrumentation. By using focused ion beams for the sequential removal of thin layers of the EC3 material, followed by high-resolution imaging of each slice with a scanning electron microscope (a technique called FIB-SEM tomography), the joint EC³ Hub and MIT Concrete Sustainability Hub team was able to reconstruct the conductive nanonetwork at the highest resolution yet. The analysis showed that the network is essentially a fractal-like “web” that surrounds EC3 pores, which is what allows the electrolyte to infiltrate and for current to flow through the system.
A cubic meter of this version of EC3—about the size of a refrigerator—can store over 2 kilowatt-hours of energy, which is enough to power an actual modest-sized refrigerator for a day. Via extrapolation (always the tricky aspect of these investigations), they say that 45 cubic meters of EC3 with an energy density of 0.22 kWh/m3 – a typical house-sided foundation—would have enough capacity to store about 10 kilowatt-hours of energy, the average daily electricity usage for a household, Figure 2.
Figure 2 These are just a few of the many performance graphs that the team developed. Source: Massachusetts Institute of Technology
They achieved highest performance with organic electrolytes, especially those that combined quaternary ammonium salts—found in everyday products like disinfectants—with acetonitrile, a clear, conductive liquid often used in industry, Figure 3.
Figure 3 They also identified needed properties for the electrolyte and investigated many possibilities for this critical component. Source: Massachusetts Institute of Technology
If this all sounds only like speculation from a small-scale benchtop lab project, it is, and it isn’t. Much of the work was done in cooperation with the American Concrete Institute, a research and promotional organization that studies all aspects of concrete, including formulation, application, standardized tests, long-term performance, and more.
While the MIT team, perhaps not surprisingly, is positioning this development as the next great thing—and it certainly gets a lot of attention in the mainstream media due to its tantalizing keywords of “concrete” and “battery”—there are genuine long-term factors to evaluate related to scaling up to a foundation-sized mass:
- Does the final form of the concrete matter, such a large cube versus flat walls?
- What are the partial and large-scale failure modes?
- What are the long-term effects of weather exposure, as this material is concrete (which is well understood) but with an additive?
- What happens when an EC3 foundation degrades or fails—do you have to lift the house and replace the foundation?
- What are the short and long-term influences on performance, and how does the formulation affect that performance?
The performance and properties of the many existing concrete formulations have been tested in the lab and in the field over decades, and “improvements” are not done casually, especially in consideration of the end application.
Since demonstrating this concrete battery in structural mode lacks visual impact, the MIT team built a more attention-grabbing demonstration battery of stacked cells to provide 12-V of power. They used this to operate a 12-V computer fan and a 5-V USB output (via a buck regulator) for a handheld gaming console, Figure 4.
Figure 4 A 12-V concrete battery powering a small fan and game console provides a visual image which is more dramatic and attention-grabbing. Source: Massachusetts Institute of Technology
The work is detailed in their paper “High energy density carbon–cement supercapacitors for architectural energy storage,” published in Proceedings of the National Academy of Sciences (PNAS). It’s behind a paywall, but there is a posted student thesis, “Scaling Carbon-Cement Supercapacitors for Energy Storage Use-Cases.” Finally, there’s also a very informative 18-slide, 21-minute PowerPoint presentation at YouTube (with audio), “Carbon-cement supercapacitors: A disruptive technology for renewable energy storage,” that was developed by the MIT team for the ACI.
What’s your view? Is this a truly disruptive energy-storage development? Or will the realities of scaling up in physical volume and long-term performance, as well as “replacement issues,” make this yet another interesting advance that falls short in the real world?
Check back in five to ten years to find out. If nothing else, this research reminds us that there is potential for progress in power and energy beyond the other approaches we hear so much about.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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