Electrochemical capacitors (ECs) have been considered as one of the most promising electrochemical energy storage devices owing to their several advantages such as high power density and long cycle life. In ECs, charges are stored on the surface of porous electrodes, therefore, the available energy density strongly depends on the microstructure and surface chemistry of the electrodes. However, state-of-the-art ECs only exhibit an energy density of 4–5 Wh/kg by using the electric double layer capacitance, which is still considerably lower than that of Li-ion batteries.

In this presentation, we will show our recent results on enhancing the energy density of carbon-based electrodes for capacitive energy storage. Firstly, for replacing the conventional activated carbon in ECs, we developed high surface area carbons with a surface area of 3550 m²/g from polyacrylonitrile via a low-cost and scalable process.¹ The high surface area carbon exhibited a capacitance of 150 F/g in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) and 210 F/g in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The composite electrodes consisting of high surface area carbon and carbon nanotubes delivered a high capacitance of~ 170 F/g in symmetric configurations, and a high capacity of~ 150 mAh/g in asymmetric configurations against lithium metal with excellent rate-performance at practical mass loading and bulk densities.

Improving volumetric performance has been one of the major challenges in nanostructured carbon electrodes for capacitive energy storage. Recently, we developed a simple hydrothermal oxidation method for functionalizing carbon nanotube films using dilute nitric acid.² This method shows high efficiency in densifying as-assembled single-walled carbon nanotubes (SWNT) films from 0.63 g/cm³ to 1.02 g/cm³, as well as introducing a considerable amount of redox-active oxygen functional groups on the surface of the SWNTs. The functionalized SWNT electrodes deliver high volumetric as well as gravimetric capacities, 154 Ah/L and 152 mAh/g, respectively, owing to the surface redox reactions between the introduced oxygen functional groups and Li ions. In addition, the functionalized electrodes also exhibit a remarkable rate capability by retaining its high capacity of 94 Ah/L (92 mAh/g) at a high discharge rate of 10 A/g.

References:

(1) Gupta, K.; Liu, T.; Kavian, R.; Chae, H. G.; Ryu, G. H.; Lee, Z.; Lee, S. W.; Kumar, S. Journal of Materials Chemistry A 2016, 4, 18294.

(2) Liu, T.; Davijani, A. A. B.; Sun, J.; Chen, S.; Kumar, S.; Lee, S. W. Small 2016, 12, 3423.