MIT Makes Renewables Supercapacitor From Cement, Carbon-black Mixture
MIT researchers have demonstrated a supercapacitor that uses low-cost cement and carbon black—a development that may lead to inexpensive storage for renewable energy.
Practical power storage is a missing link slowing the adoption of intermittent and off-cycle power sources like solar, wind, and tides. MIT researchers recently demonstrated a new supercapacitor technology that may fill that need.
MIT's supercapacitor, made of cement, water, and carbon black, could give rise to low-cost systems that store intermittently renewable energy, such as wind or solar power. Image (modified) used courtesy of MIT
The MIT team was searching for a low-cost, readily-available material to use in supercapacitors. Some of their candidates—cement, carbon black, and water—show great promise and have been successfully demonstrated in a laboratory setting.
How the Supercapacitor Works
To understand how MIT researchers developed this prototype, it may be helpful to first review some of the working principles behind capacitor power storage.
Capacitors include two conductive plates immersed in an electrolyte separated by an insulated membrane. In response to a charge, positive ions from the electrolyte gather on the negative plate, and negative ions gather on the positive plate. The membrane prevents the ions from meeting in the middle and creates a charge potential that can be delivered quickly at a high current flow. One of the key driving factors determining a capacitor's power storage is the surface area of the conductive places. More surface area equals more ions collected, leading to higher current potential.
In MIT's carbon black/cement supercapacitor, the highly conductive carbon black was mixed into the concrete with the cement powder and water. The curing chemical process formed branching networks of openings in a fractal-like structure.
The fractal-like carbon black structure acts as the porous electrode. Image used courtesy of the Journal of Vacuum Science & Technology/Wikimedia Commons (CC BY-SA 4.0)
Since carbon black is hydrophobic (it repels water), it migrated to the openings, essentially self-assembling as a network of highly-conductive carbon threads. These threads acted as a conductor through the space and created a conductor with an extremely high surface area relative to the overall volume. The MIT team then soaked a concrete piece in potassium chloride, a common electrolyte, as a source of charged particles. That structure became an electrode, and the two of them, separated by an insulating membrane, made up the supercapacitor.
Next Steps for the Research Team: Scale
The capacitors in the experiment were fairly small at about 1 mm by 1 cm. Three of these coin-sized cells powered an LED for a few seconds. The team suggests that the size of the electrodes is easily scalable to 1-m thick or even larger.
The test setup at MIT used a small solar panel, three carbon black/cement supercapacitors delivering one volt each, and an LED. Image used courtesy of MIT
For their next step, the team will build a cell about the size of a typical car battery to demonstrate the ability to scale. Following this, they plan to develop a 45-cubic meter version that, according to their preliminary data, should provide 10 kWh—enough energy to power a typical single-family home.
Turning Any Concrete Surface Into a Supercapacitor
The higher the concertation of carbon black, the weaker the concrete, the researchers say. However, even at about 10% carbon black, the MIT team's prototype was strong enough for a home foundation or structural elements at the base of a wind turbine. While 10% carbon black is the sweet spot for structural use, non-structural or low-stress applications can store more power with a higher percentage of carbon black.
The team also noted that applying power to the material can generate heat. While the surface area of the carbon black structure determines the power capacity, tuning the mixture can also enable different charging and discharging rates. For example, an EV charging road could be tuned for very fast charge and discharge rates. A home foundation could be tuned to charge slowly over the day and discharge slowly through the night. With this study, the MIT team suggests that nearly any place concrete is used, with the exception of the most strength-critical applications, can be a potential supercapacitor.