Study on the Electrochemical Characteristics of Multiwalled CNTs Decorated by Various Oxygen Functional Groups (Non-Aqueous and Aqueous based Electrolytes)

Functionalized nano carbon have been reported as promising cathode materials for Li ion battery (LIB).^[1-2] Unlike the conventional Li intercalation into host material, the Li storage mechanism on this materials was thought to be the Faradaic reaction between surface functional groups and Li, C=O + Li+ + e- = C-O-Li.^[1-2] Introducing functional groups on  few walled carbon nanotubes (CNTs) by acid treatment resulted in Li storage capacity up to 120 mAh/g.^[2] However, the origin of which functional groups is responsible for such reaction still elusive. A better understanding on the electrochemical characteristics of functionalized nano carbon is not only important for LIB storage but also aqueous based electrochemical systems.

It is known that there are two major dominant surface groups present on functionalized CNTs; they are carboxyl and carbonyls.^[3] To be able to study the electrochemical characteristics of different oxygen functional groups, controlling the type of surface groups on CNTs is obligatory. Here, different kind of functional groups are introduced onto multiwalled CNTs (VGCFX) by varying different oxidizing agent, such as reflux under inorganic acid solution or reacting with organic acid (i.e. citric acid). Furthermore, by annealing at certain temperatures, we also could selectively attain the functional groups present on the surface of CNTs. The electrochemical studies of oxygen functional groups are carried out in non-aqueous (organic) and aqueous based electrolytes, to obtain comprehensive knowledge on the functional groups reactivity.

On the other hand, X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) are extensively used to examine the type and amount of surface groups. Fig. 1 shows the C1s XPS spectra of two of different samples, they are acid treated CNTs (HA) and citric acid treated HA (HA-CA). Briefly, acid treated CNTs was derived from refluxing pristine CNTs in the mixture of H₂SO₄/HNO₃, while citric acid treatment was conducted by mixing the HA with citric acid, followed by annealing in air at 300 ^oC. It can be seen from XPS scan, that HA-CA has an appeared peak relating to the carboxyl group (288.9 eV) than HA. This suggests that citric acid treatment is plausible to introduce carboxyl groups on the surface of multiwalled CNTs. The roles of the oxygen functional groups play in the electrochemical characteristics will be discussed in the presentation.

References

[1] S. W. Lee, N. Yabuuchi, B. M. Gallant, S. Chen, B. S. Kim, P. T. Hammond, and Y. S. Horn, Nature Nanotechnology, 5, 531 (2010)

[2] S. W. Lee, B. M. Gallant, Y. Lee, N. Yoshida, D. Y. Kim, Y. Yamada, S. Noda, A. Yamada, and Y. S. Horn, Energy Environ. Sci., 5, 5437 (2012)

[3] K. Waki, R. A. Wong, H.S. Oktaviano, T. Fujio, T. Nagai, K. Kimoto, and K. Yamada, Energy Environ. Sci., 7, 1950 (2014)