Preparation and performance comparison of supercapacitors based on nanocomposites of MnO2 with cationic surfactant of CTAC or CTAB by direct electrodeposition

Abstract

Supercapacitors based on nanocomposites of manganese oxide (MnO2) with cationic surfactant of cetyltrimethylammonium bromide (CTAB) or cetyltrimethylammonium chloride (CTAC) are prepared by direct electrodeposition. The obtained nanocomposites of MnO2: CTAB or MnO2: CTAC are compared by surface morphologies, specific capacitances and cycling stabilities. The scanning electron microscopy (SEM) images show that MnO2: CTAC is composed of nano sheets, while MnO2: CTAB changes morphology from the mixture of nano sheets and nanoparticles to big blocks when CTAB concentration increases. The Brunauer Emmett and Teller (BET) specific area of MnO2: CTAC is larger than that of MnO2: CTAB prepared at the same surfactant concentration. Maximal BET specific areas are obtained at concentration of 50mM for both surfactants. From transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images, both of the nanocomposites of MnO2: CTAC and MnO2: CTAB show polycrystalline laminar structures in nanoscale. The Raman spectrum indicates that birnessite structure of MnO2 is obtained for both nanocomposites. The maximum of specific capacitance of MnO2: CTAC is calculated to be 347F/g, 3 times larger than that of MnO2: CTAB. The chemical redox reaction of Mn2+ with Br2 molecules from Br− anion electrooxidation leads to low surface area and small specific capacitance of MnO2: CTAB. Due to protection by the alkyl chains of surfactants coating on surface of MnO2, the retention of capacitance after 1500 charge/discharge cycles by MnO2: CTAC is 90%, which is better than those by pristine MnO2 (60%) and MnO2:CTAB (80%). The electrochemical impedance spectroscopy (EIS) shows that the charge transfer resistance of MnO2: CTAC and MnO2: CTAB films are slightly larger than that of pristine MnO2. The dependence of specific capacitance and frequency reveals that MnO2: CTAC can provide more effective mass for pseudocapacitive reactions.