Maize (Zea mays L.) fresh husk mediated biosynthesis of copper oxides: Potentials for pseudo capacitive energy storage

Abstract Copper oxide nanoparticles were synthesized using an eco-friendly, simple and cost effective green synthesis method mediated by maize ( Zea mays L.) fresh husk extract. Pure reddish cubic Cu 2 O nanoparticles were obtained for the first time via this green synthesis route. The Cu 2 O nanoparticles were thermally oxidized to pure monoclinic CuO nanoparticles at 600 °C. The phases of the copper oxides were confirmed from the X-ray diffraction (XRD) studies. The calculated crystal sizes are 39.2, 20.0 and 22.5 nm for the un-annealed, 300 °C and 600 °C annealed copper oxide nanoparticles, respectively. The values of the bandgap energies obtained from diffuse reflectance of the nanoparticles are 1.95, 1.30 and 1.40 eV, respectively, for the unannealed, 300 °C, and 600 °C annealed copper oxide nanoparticles. The electrochemical properties were studied using cyclic voltammetry (CV), galvanostatic charge discharge (GCD) cycles and electrochemical impedance spectroscopy (EIS). The unannealed reddish Cu 2 O nanoparticles are very stable after 2500 GCD cycles and gave the highest specific capacitance of 252 F g −1 at a scan rate of 5 mV s −1 . This shows that the reddish Cu 2 O nanoparticle is a promising material for electrochemical energy storage applications. An asymmetric device made using activated carbon anode and unannealed copper oxide as cathode gave a specific capacitance of 87 F g −1 at a current density of 0.125 A g −1 and high energy density of 39.3 Wh kg −1 at a power density of 737.0 W kg −1 .