Energy Storage

Supercapacitors and Pseudocapacitors

Electrochemical devices, such as batteries and capacitors, are the best known and perhaps the most promising energy storage systems. Electric double layer capacitors (EDLCs), also known as supercapacitors, have received a significant experimental attention because they can achieve a higher energy density than conventional capacitors and offer a better power performance than batteries. A wide variety of different electrode materials and electrolytes are currently being scrutinized to improve on efficiency and practicality of EDLCs. Using computational methods, we study EDLCs based on microporous carbon electrodes in room-temperature ionic liquids and organic electrolytes. Our recent analysis via MD shows that internal solvation of micropores of nano- and subnano-meter scale plays a critical role in specific capacitance normalized to the pore surface area. This explains nicely the anomalous behavior of capacitance with the pore size, observed in experiments. Our current thrust is to quantitate how electrode properties such as size and shape of carbon micropores and electrolyte properties, e.g., ion size, density and conductivity, control the energy and power densities of EDLCs. We are also investigating how to model pseudocapacitance, which involves partial charge transfer between the electrode and electrolyte, in the framework of MD.


Further reading

Y. Shim, H. J. Kim and Y. Jung, Graphene-based Supercapacitors in the Parallel-Plate Electrode Configuration: Ionic Liquid versus Organic Electrolyte, Faraday Discuss., Advance Article.

Y. Shim, Y. Jung and H. J. Kim, Graphene-based Supercapacitors: A Computer Simulation Study, J. Phys. Chem. C, ASAP.

Y. Shim and H. J. Kim, Nanoporous Carbon Supercapacitors in an Ionic Liquid: A Computer Simulation Study, ACS Nano 4, 2345–2355 (2010) and highlighted in NPG Asia Materials 2 (2010).

Liquids in Nano-confinement

benzene 88 benzene 99

The structural, dynamic and thermodynamic properties of liquids confined in nano-scale environments can differ substantially from those of the bulk phase. For instance, phase behaviors, such as melting and glass transition temperatures, of the liquids vary considerably with both the size of, and interactions with, their nano-environment. Another example is exclusive internal solvation of small nano-porous electrodes via counter-ions, which is mainly responsible for the anomalous behavior of specific capacitance observed in experiments. To gain fundamental understanding of differing effects of nano-confinement, we are currently studying a variety of different liquids, such as RTILs and benzene, trapped in carbon nanotubes or between graphene sheets.

Further reading

Y. Shim, Y. Jung and H. J. Kim, Carbon Nanotubes in Benzene: Internal and External Solvation, Phys. Chem. Chem. Phys. 13, 3969–3978 (2011).

Y. Shim and H. J. Kim, Solvation of Carbon Nanotubes in a Room-Temperature Ionic Liquid, ACS Nano 3, 1693–1702 (2009).