Water plasma functionalized CNTs/MnO2 composites for supercapacitors

Abstract

A water plasma treatment applied to vertically-aligned multiwall carbon nanotubes (CNTs) synthesized by plasma enhanced chemical vapour deposition gives rise to surface functionalization and purification of the CNTs, along with an improvement of their electrochemical properties. Additional increase of their charge storage capability is achieved by anodic deposition of manganese dioxide lining the surface of plasma-treated nanotubes. The morphology (nanoflower, layer, or needle-like structure) and oxidation state of manganese oxide depend on the voltage window applied during charge-discharge measurements and are found to be key points for improved efficiency of capacitor devices. MnO2/CNTs nanocomposites exhibit an increase in their specific capacitance from 678 Fg(-1), for untreated CNTs, up to 750 Fg(-1), for water plasma-treated CNTs.

Figure 1

SEM images of untreated CNTs (a) and wpCNTs (b). Inset graphs show top view of the CNTs mat.

Figure 2

SEM images of wpCNTs/MnO₂ before electrochemical cycling (a), wpCNTs/MnO₂ after electrochemical cycling (0-1 V) (b), and wpCNTs/MnO₂ after electrochemical cycling (0.1–0.8 V) (c). Inset graphs show top view of the CNTs mat.

Figure 3

Raman spectra of (A) wpCNTs/MnO₂ (75 W, 135 Pa) before cycling, (B) wpCNTs/MnO₂ (75 W, 135 Pa) after 2000 cycles (between 0-1 V), and (C) wpCNTs/MnO₂ (75 W, 135 Pa) after 2000 cycles (between 0.1–0.8 V). Inset figure shows Raman spectra of (D) untreated CNTs and (E) wpCNTs (75 W, 135 Pa).

Figure 4

XPS spectra of Mn 2p and O1s (a) wpCNTs/MnO₂ (75 W, 135 Pa) before cycling (b) wpCNTs/MnO₂ (75 W, 135 Pa) after 2000 cycles (between 0-1 V), and (c) wpCNTs/MnO₂ (75 W, 135 Pa) after 2000 cycles (between 0.1–0.8 V).

Figure 5

(a) Cyclic voltammograms obtained at a scan rate of 10 mVs⁻¹ for untreated MWCNTs/MnO₂ and wpCNTs/MnO₂ under different plasma conditions ((A): untreated, (B): 75 W, 135 Pa, (C): 140 W, 135 Pa, (D): 10 W, 135 Pa). (b) Cyclic voltammograms of nanocomposite wpCNTs/MnO₂ (10 W, 135 Pa) at different scan rates, 10, 20, 30, 40, 50, 100, and 150 mVs⁻¹.

Figure 6

3-dimensional graph of the specific capacitance of water plasma-treated CNTs/MnO₂ electrodes with respect to rf-power and water pressure.

Figure 7

(a) Charge/discharge curves at different current densities from 1.75, 0.70, 0.52, and 0.35 mA cm⁻², as indicated by the arrow, for wpCNTs/MnO₂ (75 W, 135 Pa). (b) Galvanostatic charge/discharge cyclic stability in the 0.1–0.8 V potential.