Nanostructured composite electrode based on manganese dioxide and carbon vulcan–carbon nanotubes for an electrochemical supercapacitor

Chemical manganese dioxide (CMD) was synthesized by reaction between KMnO₄ (Merck) and MnSO₄ (Sigma Aldrich, >99%). The carbon powder was first added in deionized water and kept stirring for 20 min to obtain a homogenous suspension. Then KMnO₄ and MnSO₄ with a molar ratio of 3 : 2 were continuously added into the suspension and all the mixture was stirred for 1 h at 80 °C. The CMD product was finally washed many times by distilled water and dried at 120 °C for 48 h. Here we study the effect of carbon types on structure, morphology and electrochemical properties of composite electrodes. Mesoporous carbon black (MC), carbon vulcan-XC72R (VC), noir and CNTs were purchased from Sigma-Aldrich, Carbot and PINACO-Vietnam.

Samples were analyzed by powder x-ray diffraction (XRD) using a PANalytical X'Pert MPD diffractometer with CuKα radiation, 0.05° step and 8 s per step counting time to minimize noise. The morphology and the distribution of grain size were determined using a field emission scanning electron microscopy (FE-SEM) ZEISS ULTRA 55 instrument equipped with energy dispersive x-ray (EDX) spectroscopy analysis.

The composite electrode was prepared by dipping graphite substrate in the mixture of MnO₂/C powder, isopropanol and PTFE with a weight ratio MnO₂/C: isopropanol: PTFE = 65:30:5. The electrode was dried at room temperature for 24 h and then at 120 °C for 1 h.

All electrochemical measurements of materials were performed in a three-electrode cell including MnO₂/C electrode, platinum mesh and Ag/AgCl (0.198 V) (Biologic) as working, counter and reference electrode. The cyclic voltammetry (CV) measurement was carried out on an Autolab PGSTAT302N Potentiostat/Galvanostat in 2 M NaCl from 0 to 1 V at a scan rate 50 m V s⁻¹.

Figure 1 illustrates the XRD patterns of MnO₂/C composite electrodes with different types of carbon: carbon vulcan, mesoporous carbon and carbon vulcan/CNTs at 30 wt% content. The diffraction peaks at 2θ (CuKα) = 37.2°, 42.7°, 58°, 78°, 90° correspond to the structure of epsilon manganese dioxide (ε-MnO₂), which is a defective structure of γ-MnO₂ [13]. The MnO₂/carbon vulcan composite electrode with 5% CNTs addition exhibits the same structure (figure 1(b)). Thus, the MnO₂/C structure is not affected by carbon content and types of carbon. However, the synthesis conditions such as solution concentration and temperature can influence the structure of electrode materials [14, 15].

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As shown in figure 2, the morphology of CMD based composite electrodes was analyzed by SEM images in changing types of carbon. MnO₂ sample prepared by chemical reaction exhibited nanostructure with mono-dispersed flower-like particles (figure 2(a)). With all MnO₂/C samples, the particles have random shapes when using carbon noir, mesoporous carbon or carbon vulcan as seen in figures 2(b), (c) and (d). In addition, a wide distribution of particle sizes was also obtained. However, the addition of 5 wt% CNTs for MnO₂/carbon vulcan gives the homogenous and fine particles of diameters about 50–70nm (figure 2(e)).

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The electrochemical properties of MnO₂/C samples were characterized by cyclic voltammetry in 2.0 M NaCl solution in the potential window of 0–1 V versus Ag/AgCl at different scan rates. The specific capacitance (SC) of MnO₂/C was calculated by the following equation [15]:

$$\begin{equation} SC = \frac{Q}{{\Delta Em}}, \end{equation} \tag{ 1 }$$

where SC is the specific capacitance, Q is charge (C), ΔE is the voltage window (1V) and m is the active material mass (g).

Figure 3(a) shows the CV curves of CMD based composite electrodes in 2.0 M NaCl solution at a scan rate 50 mV s⁻¹. Without the carbon additional, the SC of MnO₂ active material was 35 F g⁻¹. At 30 wt% content, the SC of MnO₂/C electrodes are 67, 88 and 144 F g⁻¹ for carbon Noir, mesoporous carbon and carbon vulcan, respectively. Indeed, in using various types of carbon, the SC of MnO₂/C changes due to the variation of specific surface area (figure 3(b)). It could be explained that the carbon performs not only as a conductive agent but also as the template supporting dispersion of MnO₂ nanoparticles and stabilization of MnO₂ structure [10, 12, 13].

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Figure 4(a) shows the CVs of MnO₂/carbon vulcan-CNTs based composite electrodes in 2.0 M NaCl solution at a scan rate 50 mV s⁻¹ when CNTs addition changes from 0 to 2.5, 5 and 7.5%. When CNTs content increases, the SC of MnO₂/C increases and reaches the highest value at 5 wt% CNTs (199 F g⁻¹). When the addition of CNTs varies from 5 to 7.5%, the SC of MnO₂/C electrode does not change (figure 4(b)). Thus, the presence of CNTs increases the SC due to stability of defective structure and improvement of charge transfer between material and electrolyte [16].

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Figure 5 shows the charge–discharge capacitance at different cycles calculated from CV curves. It can be seen that MnO₂/C composite electrodes exhibit high electrochemical reversibility and cycling stability after 300 cycles. With MnO₂/carbon vulcan, the SC gradually reduced and lost nearly 20% initial SC after 300 cycles. In contrast, the specific capacitance of MnO₂/carbon vulcan-CNTs (5%) showed excellent cycling performance and lost only 10% after 300 cycles. This behavior can be explained by good ionic conductivity network between MnO₂ particles and conducting additives using CNTs. Indeed, CNTs facilitate fast electron transport on the electrode surface and develop good electrolyte–electrode interface due to the high active surface area of homogenous nanometric particles [17, 18].

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