Preparation and supercapacitive performance of pinecone-like NiMoO4/MnO2 composite material

Abstract

Supercapacitors, also called electrochemical capacitors or ultracapacitors, have attracted increasing attention owing to their high specific capacitance, high power density, long lifecycle, fast charge-discharge ability, wide working temperature range, and environmental friendliness for mobile electronics, power grids, and hybrid electric vehicles. The electrode is the most important part of supercapacitors; therefore, the electrode material is the chief factor that determines the properties of supercapacitors. To enhance the performance of a supercapacitor, particularly its specific energy while retaining its intrinsic high specific power, several researchers have focused mainly on improving the properties of electrode materials. The major classes of materials applied for supercapacitors include various forms of carbon, transition metal oxides, and conductive polymers. Compared to the carbon materials and conducting polymer materials, transition metal oxides can achieve a much higher specific capacitance because of their high theoretical capacitance, well-defined electrochemical redox activity, low cost, and abundant resources. In particular, binary metal oxides, such as NiMoO4, MnMoO4, and CoMoO4, have been extensively studied as pseudocapacitor electrode materials because of their good electronic conductivity and rich redox reactions. In this study, pinecone-like NiMoO4/MnO2 composite materials were successfully synthesized using a facile hydrothermal method. Na2MoO4·2H2O, NiSO4·6H2O, and MnO2 were used as raw materials. The as-products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge-discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results show that when the optimal content of MnO2 reaches 10%, the obtained NiMoO4/MnO2 composite materials exhibits a pinecone-like porous morphology, with the particle size ranging from 200 to 600 nm. The results show that NiMoO4/MnO2 composite materials have excellent electrochemical properties. The discharge specific capacitance of NM0, NM5, NM10, NM15, and NM20 composites with corresponding MnO2 contents of 0%, 5%, 10%, 15%, and 20% are 260, 248, 650, 420, and 305 F·g-1, respectively, at a current density of 1 A·g-1. When the current density is up to 10 A·g-1, the initial discharge specific capacitance is 102 F·g-1. After 100-week cycles, the discharge specific capacitance of the NM10 sample is still 147 F·g-1. The improvements can be mainly attributed to the introduction of MnO2 in the NiMoO4/MnO2 composite materials to overcome the shortcomings of single NiMoO4.

Keywords

• composite material
• pinecone-like
• nimoo4/mno2
• hydrothermal method
• specific capacities