Recently, the fast-growing population is a great concern for our modern generation. As the population rise more the need for clean and renewable energy will also follow for various types of portable electronic devices. Among different types of electrical energy storage devices, supercapacitors, also known as electrochemical capacitors have garnered huge attention due to their many advantages, such as long cycle life, high power density, cost-effectiveness, light weight, eco-friendly, broad operating temperature range, high rate capability and good cycling stability. According to the charge storage mechanism Supercapacitors are basically of two types, electrochemical double layer capacitors (EDLCs) and pseudocapacitors. Compared to the EDLC materials (mainly carbon based materials) the Pseudocapacitive materials (such as metal oxides and conducting polymers) exhibited much higher capacitance due to fast and reversible redox reactions1. This inspired the search for an extensive variety of electrode materials for pseudocapacitor. In general, transition metal oxides, such as RuO2, MnO2 , NiO, Co3O4, SnO2, TiO2, V2O5, CuO, Fe2O3, and WO3 etc., have exhibited high specific capacitance values with their redox reaction performances. But high resistivity of the metal oxides electrodes seems to be a great challenge for their practical applications as high performance energy storage devices. Mixing one metal oxide material with other metal oxide materials has been well established and proved as one of the best solutions to improve the electrochemical conductivity. Literatures said that electrochemical performance can be enormously improved by adjusting the morphology of the nanomaterials. Among various metal oxide composite materials, NiMn2O4 nanoparticle are very promising electrode materials for pseudocapacitors due to their low-cost, eco-friendly, good electrochemical properties, and high cyclic life. Herein, we have synthesized and optimized NiMn2O4 nanocomposites with Ni:Mn molar ratio (1:2) and the cycling performance of NiMn2O4electrode was examined giving maximum Specific capacitance value of 936.8 F/gm in 1M KCl electrolyte shown in figure-1 (b,c).
The specific capacitance (Cm) for NiMn2O4 electrode material was determined from the CV profiles measured at scan rate 2 mV/s, following the equation Cm= i/ 2mν , where m and ν are the mass of the electroactive material and potential scan rate and current (i) is obtained by integrating the area of the curves as it is defined by eq.(1),i(v) = ∫ i(v)dv/ (vc - va) ...... (1), where va and vc are the lowest and highest voltage of the potential range. The galvanostatic charge-discharge (GCD) cycling curves also have a nearly symmetric shape for various current densities [0.4mA/cm2 – 1mA/cm2], indicating that the composite has a good electrochemical capacitive characteristic and superior capacitive retention.
RESULTS AND DISCUSSION
NiMn2O4 nanoparticle was successfully synthesized by chemical process. Nanostructure studies were carried out using XRD and FTIR spectroscopy. The formation of the nanoparticle was inferred by field emission scanning electron microscopy (FESEM) and the average crystallite size has been observed 40 nm. The electrochemical response of the NiMn2O4electrodes, as an electrode material for supercapacitors, was found to be improved to a great extent. The maximum specific capacitance 936.8F/g is achieved in 1M KCl solution in 2mV/s scan rate with molar ratio of Ni/Mn 1:2.
The composite exhibits high Specific capacitance value and good cycling stability up to 5000 cycles (not shown in fig.). Hence, the synthesized NiMn2O4nanopartice was found to be the suitable promissing electrode material for energy storage applications.
- B.E. Conway, Electrochim. Acta, 38, 1249 (1993).