Promise for Onion-Like Carbons as Supercapacitors
Why it Matters: No single electrical storage technology works perfectly across all needed applications. The two most important storage technologies are batteries and capacitors. Batteries can store a lot of charge and last a long time, but have limited charge and discharge rates. Capacitors generally store less charge but have very fast (nearly instant) charge and discharge rates. There is a need to research technologies that can combine the optimal characteristics of both.
A capacitor is an object that stores electricity typically by separating two metal plates that have opposite electrical charge with a non-conducting substance in between. There is a special class of capacitors, called electrochemical capacitors, also called electric double-layer (EDL) capacitors or supercapacitors, that store energy in electrochemical media. In these devices there are no metal plates. In EDL systems positive and negative charges are separated by extremely short distances (on the order of just a few nanometers). This vanishingly-thin charge separation results in some key benefits vis-a-vis standard capacitors. Supercapacitors offer the usual fast charging and discharging rates of conventional capacitors but also have higher energy densities and the ability to sustain millions of cycles, thereby bridging the gap between batteries and conventional capacitors. Supercapacitors are currently available commercially and are used to power hybrid electric vehicles, portable electronic equipment and other devices.
Key Challenges: Researchers are looking for ways to improve supercapacitor technology, for example, to increase lifespan and to allow use in a wider variety of applications, especially applications involving microscale energy storage. There is an interest in incorporating new materials into supercapacitors, especially graphene and other novel forms of carbon, due to the unique electrical, thermal, mechanical, and chemical properties such materials have. However, there is very little fundemental understanding of how such materials behave in EDLs. Given that graphene can form stripes, tubes, balls, ribbons, and other useful shapes, an important question is how shape affects EDL electrical properties. Some experimentation has been done showing that onion-like carbons (OLCs), which consist of concentric graphene spheres, offer ultrahigh energy density and charging/discharging rates in supercapacitors but the physical origin of this phenomenon is still unknown.
Accomplishments: Researchers used molecular dynamics simulations 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 – a key result because it is unlike the bell or camel shape 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. This curvature effect had been observed experimentally but its origin was unknown.
Supercapacitors are not going to substitute for conventional lithium-ion batteries in most intances in the very near future. However, the novel and very promising features exhibited by OLCs in ionic liquids revealed in these simulations invite further experimental exploration to take advantage of these materials. More compact or more uniform onions and better electrolytes could boost performance, especially for applications that require large bursts of power, long lifetimes, and reasonable storage capacities. The study is a good example of how simulation and experiment interact and reinforce one another to advance the state-of-the-art in a key energy-related basic science.
Results from this work appeared in the March issue of American Chemical Society “Journal of Chemical Theory and Computation” and were selected for that issue’s cover story. This work was done as part of the DOE’s Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center (EFRC). Between Jan 10, 2011 and December 1, 2011 members of this FIRST group ran 2,909 jobs on the NERSC Franklin and Hopper systems, using a total of 1.7M hours. They use a variety of software made available by NERSC staff, including the VASP, NAMD, SIESTA, LAMMPS, and ABINIT electronic structure application codes. A companion paper by FIRST project members that explained the formation of OLCs from nanodiamonds (see below) also relied on NERSC computational resources and was published late in 2011.
Investigators: Dr. De-en Jiang (Oak Ridge National Laboratory); Guang Feng and Peter T. Cummings (Vanderbilt University).
More Information: J. Chem. Theory Comput. 2012, 8, 1058−1063.
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