Nanoporous carbon materials are poised for an important role in future energy-storage systems, according to a new simulation study by researchers from Korea and the United States. Through computational modeling, Youngseon Shim from Seoul National University and Hyung Kim from Carnegie Mellon University in the USA and the Korea Institute for Advanced Study1 have now obtained molecular-level insights into how carbon micropores, modeled as nanotubes, can combine with molten salt-like substances　known as ionic liquids to form high-performance supercapacitors — energy-storage devices with higher power output and faster response times compared to conventional batteries.

Carbon-based supercapacitors store massive amounts of charge by capturing negative and positive ions from an electrolyte solution in the pores of opposing electrodes. To optimize charge retention, scientists need precise control over the dimensions of the pores — a task ideally suited to the highly ordered structures of carbon nanotubes. “Carbon nanotubes can have a variety of pore sizes,” says Kim, “making these materials theoretically one of the best choices for next-generation supercapacitors.”

Fig. 1: A carbon nanotube of a certain size (center) allows in only one polarity of ion from an ionic liquid (left and right), forming a supercapacitor with high energy-storage capacity.From Ref. 1. Reproduced with permission. © 2010 ACS

Shim and Kim studied how carbon nanotubes interact with an ionic liquid composed solely of anions (negative ions) and cations (positive ions). As ionic liquids can supply large amounts of charge under a range of electrochemical conditions, they are attractive alternatives to conventional supercapacitor electrolytes. Through simulations of molecular dynamics, the researchers modeled how hundreds of cations and anions move around in the ionic liquid and into the pores of a charged nanotube electrode at room temperature (Fig. 1).

These simulations, sometimes comprising more than 20,000 atoms, were challenging because of the ‘sticky’ nature of ionic liquids. “Ionic liquids are hundreds to thousands of times more viscous than water,” explains Kim, “so it takes some time to generate good equilibrium statistics.”

The team found that certain nanotubes had just the right dimensions to permit only the anion or cation to enter the tube and fill it completely. This behavior, which they termed ‘exclusive internal solvation’, boosts the charge-storing capabilities of the system dramatically. Pores that were too small prevented ions from entering the nanotubes, while those that were too large drew in the whole anion–cation pair; both effects led to lower capacitance.

The next challenge, according to Kim, is modeling ion transport within the nanotubes to help improve the power performance of these remarkable energy storage devices.