Capacitive Performance of MWCNTs Decorated with MnOx and Silver Nanoparticles As Electrodes in Neutral Electrolytes

Carbon nanotubes (CNTs) are considered as promising material for supercapacitor electrodes, due to good chemical and mechanical stability. Generally, pristine CNTs perform as a typical electric double layer capacitor (EDLC) when charging and discharging. However, the capacitance of pristine CNTs is only produced by simple physical adsorption of ions on the surface of CNTs, and the hydrophobicity of CNTs limits the electrical capacity, with extremely low specific capacitance .

Manganese dioxide (MnO₂), exhibiting high specific capacitance and chemical stability, is a promising material for use as a supercapacitor electrode. Moreover, MnO₂ considered as a potential material for replacing ruthenium oxides, because it is inexpensive and environmental friendly when compared with other transitional metal. Supercapacitor electrodes fabricated from CNTs and MnO₂ have been widely developed and studied to increase the specific capacitance. However, the poor conductivity of MnO₂ limits its capacitive characteristics at high rates of charging and discharging.

In this study, silver nanoparticles (AgNP, diameter of ~15nm) were chemical synthesized on multi-walled carbon nanotubes (MWCNTs) to form MWCNTs/AgNP composites (Figure 1a). Manganese oxides (MnO_x) were then electrodeposited on the surface of MWCNTs/AgNP composites to fabricate MWCNTs/AgNP/MnO_x electrodes for supercapacitors. The electrochemical behavior of MWCNTs/AgNP/MnO_x electrodes in neutral solutions was investigated, and the morphology of the composites was characterized by transmission electron microscopy (TEM).

The specific capacitance of MWCNTs deposited with MnO_x (MWCNTs/MnO_x) and silver particles (MWCNTs/AgNP) can be increased to 47 F g^-1 and 62.5 F g^-1 respectively, much higher than that of MWCNTs (~15 F g^-1) (Figure 1b). The capacitive performance of the MWCNTs/MnO_x electrode remains stable over 100 cycles of charging and discharging, while the specific capacitance of the MWCNTs/AgNP electrode decreases rapidly and stabilizes at ~7 F g^-1 after 40 cycles of charging and discharging (Figure 1c). The MWCNTs/AgNP/MnO_x electrode has the highest specific capacitance (~122 F g^-1), compared with the MWCNTs/AgNP electrode and MWCNTs/MnO_x electrode (Figure 1b), but its specific capacitance decreases to 45 F g^-1 after 100 cycles of charging and discharging (Figure 1c). The specific capacitance decrease in electrodes decorated with silver might be due to the flaking of Ag/Ag₂SO₄ from the electrodes during the redox reaction between silver and silver sulfate.

In conclusion, MWCNTs decorated with both AgNP or MnOx can improve the specific capacitance of the MWCNTs electrodes, while decorating MWCNTs with AgNP and MnOx together can attain significantly higher specific capacitance than with AgNP or MnOx respectively. The MWCNTs/MnO_x electrode possesses better cyclability than both the MWCNTs/AgNP electrode and the MWCNTs/AgNP/MnO_x electrode. The poor capacitive stability of electrodes decorated with silver might be the result of silver flaking from the electrode during the redox reaction between Ag and Ag₂SO₄.