Effect of Surface Area and Loading Amount on the Super-Capacitance of MnO₂/CNTs Composites

The rapid growth of the global economy has led to a significant worldwide increase in the consumption of fossil fuels over last decades, thereby producing two major associated issues:the depletion of existing fossil fuel reserves and the affiliated environmental problem. Supercapacitors, also called as electrochemical capacitors, have attracted great attention for the application in future energy storage devices due to their high power density, long cycle life, fast charge-discharge rates, and low maintenance cost. To further enhance the energy density of supercapacitors, it is a crucial issue to design favorable micro/nano-structures for electrodes with fast ion and electron transport and high utilization rate of transition metal oxides/hydroxides. In this work, multi-walled carbon nanotubes (CNTs) and potassium permanganate (KMnO₄) were used as raw materials to obtain MnO₂/CNTs composites with different contents of MnO₂ enwrapped on CNTs by a facile synthetic method. The composition, morphology, specific surface area and pore volume of electrode materials were examined via field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and isothermal adsorption of nitrogen. The electrochemical properties of composite materials were characterized. Results showed that the MnO₂ content had a significant effect on the fiber diameter, layer thickness and specific surface of the composite materials. The composite material possessed the best performance with the highest specific capacitance (199 F/g) and normalized specific capacitance (255 F/g) of MnO₂ when the mass ratio of potassium permanganate to carbon nanotubes was 10 in the synthesis reaction. The normalized MnO₂ specific capacitance in the MnO₂/CNT composites was compared, since it could directly reveal the utilization ratio of active materials. Our findings underscore the need for considering a broad range of MnO₂/CNT combinations when optimizing a particular functional property of electrode materials. We believe that the manipulation of the structural and capacitive properties of composites via varying the MnO₂/CNT combination may, in fact, be one of the most overlooked and important aspects of these novel materials.