Supercapacitors are rechargeable energy storage devices that deliver more power for their size than similar-sized batteries. They also recharge quickly, and they last for hundreds to thousands of recharging cycles. Today, they are used in hybrid cars’ regenerative braking systems and for other applications. Advances in supercapacitor technology could make their use widespread as a complement to, or even replacement for batteries used in household electronics.
The challenge has been producing more efficient and durable electrodes. Electrodes attract ions to the surface of the supercapacitor, where that energy becomes available to use. Ions in supercapacitors are stored in an electrolyte solution. An electrode's ability to deliver stored power quickly is determined in large part by how many ions it can exchange with that solution; the more ions it can exchange, the faster it can deliver power.
A long-lasting electrode for supercapacitors was developed that is more than ten times more efficient than other designs. The electrode design provides the same amount of energy storage, and delivers as much power as similar electrodes, despite being much smaller and lighter. In experiments, it produced 30 percent better capacitance — a device's ability to store an electric charge — for its mass compared to the best available electrode made from similar carbon materials, and 30 times better capacitance per area. It also produced ten times more power than other designs, and retained 95 percent of its initial capacitance after more than 10,000 charging cycles.
The electrode was designed to maximize its surface area, creating the most possible space for it to attract electrons. The device design was inspired by the structure of trees, which absorb ample amounts of carbon dioxide for photosynthesis because of the surface area of their leaves. To create the branch-and-leaves design, the researchers used two nanoscale structures composed of carbon atoms. The “branches” are arrays of hollow, cylindrical carbon nanotubes, about 20 to 30 nanometers in diameter, and the “leaves” are sharp-edged, petal-like structures, about 100 nanometers wide, made of graphene — ultra-thin sheets of carbon. The leaves are then arranged on the perimeter of the nanotube stems. The leaf-like graphene petals also give the electrode stability.
The engineers then formed the structures into tunnel-shaped arrays; the ions that transport the stored energy flow through the arrays with much less resistance between the electrolyte and the surface to deliver energy than they would if the electrode surfaces were flat.
The electrode also performs well in acidic conditions and high temperatures, both environments in which supercapacitors could be used.