Why Onion-Like Carbons Make High-Energy Supercapacitors

The two most important electrical storage technologies are batteries and capacitors. Batteries can store a lot of energy, but have slow charge and discharge rates. Capacitors generally store less energy but have very fast (nearly instant) charge and discharge rates, and last longer than rechargeable batteries. Developing technologies that combine the optimal characteristics of both will require a detailed understanding of how these devices work at the molecular level. 

Electrical double-layer (EDL) capacitors, also called supercapacitors or ultracapacitors, offer the usual fast charging and discharging rates of conventional capacitors but also have higher power density, high capacitance, and excellent durability, thus bridging the gap between batteries and conventional capacitors. Commercial supercapacitors are currently used to power electric vehicles, portable electronic equipment, and other devices.

Carbon-based materials are the most widely used electrodes for supercapacitors, and researchers looking for ways to improve supercapacitor technology are especially interested in graphene because of its unique electrical, thermal, mechanical, and chemical properties. But given that graphene can form stripes, tubes, balls, ribbons, and other useful shapes, an important question is how shape affects EDL electrical properties.

Experiments have shown that onion-like carbons (OLCs), which consist of concentric graphene spheres, offer ultrahigh energy density and charging/discharging rates in supercapacitors, but the experiments did not reveal the physical origin of this phenomenon. Researchers in the DOE’s Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center designed molecular dynamics simulations, using a variety of software made available by NERSC staff, to explain the relationship between capacitance and electrode potential in supercapacitors consisting of an OLC electrode suspended in a room-temperature ionic liquid.

By varying the radius of the OLC spheres, they were able to understand the influence of electrode curvature and size. Simulations showed that the surface charge density in OLCs increases almost linearly with the potential applied at the OLC’s electric double layer. This leads to a nearly flat differential capacitance-versus-potential curve—unlike the bell or camel shaped curves observed for planar electrodes—and could potentially inspire the design of supercapacitors with much more stable capacitive performance. The simulations also explained why the capacitance of the OLC increases as the size, and thus curvature, of the OLC decreases. More compact or more uniform onions and better electrolytes could boost performance, especially for applications that require large bursts of power, long lifetimes, and high storage capacities.

Publication: Guang Feng, De-en Jiang, and Peter T. Cummings, “Curvature Effect on the Capacitance of Electric Double Layers at Ionic Liquid/Onion-Like