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

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.