Carbon trees make effective supercapacitor electrodes

Mechanical engineers from the Henry Samueli School of Engineering and Applied Science at the University of California, Los Angeles (UCLA) and four other institutions have designed a super-efficient and long-lasting electrode for supercapacitors. The device's design was inspired by the structure and function of leaves on tree branches and is more than 10 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% better capacitance – the 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 10 times more power than other designs and retained 95% of its initial capacitance after more than 10,000 charging cycles. The new electrode design is reported in a paper in Nature Communications.

Supercapacitors are rechargeable energy storage devices that deliver more power for their size than similar-sized batteries. They also recharge quickly and can last for hundreds to thousands of recharging cycles. Today, they're 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, the more familiar batteries consumers buy every day for household electronics.

Engineers know that supercapacitors can be made more powerful, but one challenge has been producing more efficient and durable electrodes. Supercapacitor electrodes attract ions, which store energy, to the surface of the supercapacitor, where that energy becomes available for 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 the electrolyte: the more ions it can exchange, the faster it can deliver power.

Knowing that, the researchers designed their electrode to maximize its surface area, creating the largest possible space for attracting ions. They drew inspiration from the structure of trees, which are able to absorb ample amounts of carbon dioxide for photosynthesis because of the surface area of their leaves.

"We often find inspiration in nature, and plants have discovered the best way to absorb chemicals such as carbon dioxide from their environment," said Tim Fisher, the study's principal investigator and a UCLA professor of mechanical and aerospace engineering. "In this case, we used that idea but at a much, much smaller scale – about one-millionth the size, in fact."

To create the branch-and-leaves design, the researchers used two nanomaterials composed of carbon atoms. The ‘branches’ are arrays of hollow, cylindrical carbon nanotubes, about 20nm to 30nm in diameter. The ‘leaves’ are sharp-edged petal-like structures, about 100nm wide, made of graphene – ultra thin sheets of carbon. The leaves are arranged on the perimeter of the nanotube stems, where they also confer stability to the electrode.

The engineers formed these structures into tunnel-shaped arrays. When the energy-transporting ions flow through these arrays, they experience much less resistance between the electrolyte and the surface than 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.

This story is adapted from material from UCLA, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.