Functionalized Carbon Nanotube Supported By Nickel Nanowire Array with Improved Rate Capability for High Power Electrochemical Capacitors

High surface area, high current-carrying capacity, and chemical robustness make carbon nanotubes (CNTs) promising electrode materials for electrochemical capacitors[1]. Moreover, functionalized CNT (f-CNT) can be uniformly dispersed into solvent to enable solution-based manufacturing of CNT electrodes. Appropriate functional groups can even introduce battery-like pseudocapacitance and increase the overall charge storage capacity of CNT electrodes [2]. However, functionalization has shown to decrease the electrical conductivity of CNT. [3] To achieve low internal resistance, and hence high rate capability, f-CNT active materials are deposited onto metal current collectors. During charging/discharging, electrons are transported from/to the CNT to/from the current collector. The interface between the f-CNT and the current collector is expected to play an important role in determining the electron transport resistance and overall the rate capability of the electrochemical capacitor. In this research, we worked to lower the internal resistance of f-CNT electrode by introducing nanostructured interface between f-CNT and nickel current collector. f-CNT are deposited onto different nickel current collectors:plain nickel foil with smooth surface and nickel nanowire arrays with nano-sized surface roughness. The rate performances of the resulting f-CNT/nickel electrodes are compared and the structure-performance relationship is identified.

In current study, f-CNTs are made by refluxing CNTs in concentrated HNO₃-H₂SO₄ (1:3 v/v). 0.5mg/ml f-CNT dispersion in water and electrophoretic deposition(EPD) are used to coat the surface of nickel foils with f-CNT (f-CNT/foil). Nickel nanowires were grown inside the pores of a polycarbonate filter with 13 mm diameter and 400nm pore size using electroplating for 25 min at 1.5 volts. Then, the filter was removed by chloroform to yield free standing nanowires on the nickel substrate. f-CNTs were coated onto the nanowires (f-CNT/nanowire) by similar EPD method.

Specific capacitance of 210 F/g of CNT at 1 mV/s scan rate in cyclic voltammetry test is achieved with f-CNT/foil electrode. CV test results show 60% capacity drop in f-CNT/foil electrode when the scanning rate was increased from 1 to 50 (mV/s) while capacitance drop is about 90% for nickel in the same scanning rate window. As a result, f-CNT/foil electrode shows a better rate capability in comparison with plain nickel electrode. Furthermore, Internal resistance of the f-CNT/foil electrode is measured with electrochemical impedance spectroscopy (EIS) and EIS results show the internal resistance is below 10 ohm. f-CNT/nanowire electrode is expected to provide even better rate capability behavior, thanks to larger contact area between CNTs and nickel, which makes the electron transport path shorter.

[1] Yan Wang, Zhiqiang Shi, Yi Huang, Yanfeng Ma, Chengyang Wang, Mingming Chen and Yongsheng Chen, “Supercapacitor Devices Based on Graphene Materials”, J. Phys. Chem. C, 2009. 113: p 13103–13107.

[2] Lee, S. W., Yabuuchi, N., Gallant, B. M., Chen, S., Kim, B. S., Hammond, P. T., & Shao-Horn, Y. (2010). High-power lithium batteries from functionalized carbon-nanotube electrodes. Nature Nanotechnology, 5(7), 531-537.

[3] Holzinger, M., Vostrowsky, O., Hirsch, A., Hennrich, F., Kappes, M., Weiss, R., & Jellen, F. (2001). Sidewall functionalization of carbon nanotubes.Angewandte Chemie International Edition, 40(21), 4002-4005.

[4] Jun Li, Quan Min Yang, Igor Zhitomirsky, “Nickel foam-based manganese dioxide–carbon nanotube composite electrodes for electrochemical supercapacitors”, Journal of Power Sources, 2008. 185: p.1569–1574.