Nanocomposite materials of CuO–NiO from Raphia hookeri seed for investigation of energy storage potentials

Abstract

In the present study, green synthesis of CuO nanoparticles (CuO NPs), NiO nanoparticles (NiO NPs) and CuO–NiO nanocomposites were prepared using the extract of Raphia hookeri seed (RHS) using hydrothermal method for electrochemical energy storage applications is reported. The RHS was extracted for their phytochemical screening. The nanomaterials were characterized for UV–Vis spectroscopy, scanning electron microscopy (SEM), transmission electrom microscopy (TEM), powered x-ray diffraction (pXRD), energy dispersive x-ray (EDX) spectroscopy and thermogravimetry analysis (TGA). Phytochemical screening showed presence of flavonoids, alkaloids, tannins, phenolics, terpenoids, saponins, glycosides and reducing sugars were found to which allowed the formation of nanoparticles. The corresponding UV–Vis spectroscopy revealed characteristic absorption peaks at 301 nm (CuO), 295 nm (NiO), and 299 nm (CuO–NiO) while the optical band gaps were equal to 2.29, 3.21 and 2.40 eV, respectively. SEM and TEM showed that the CuO, NiO and CuO–NiO are porous, not agglomerated, but with average particle sizes of 21.0 nm, 14.9 nm and 16.2 nm respectively. Powder X-ray diffraction data showed high crystallinity, with the CuO–NiO nanocomposite having an average crystallite size of 19.18 ± 5.40 nm, in accordance with the results of TEM investigations. The successful incorporation of Cu (47.30 wt.%), Ni (32.10 wt.%) and O (20.60 wt.%) in the nanocomposite was confirmed by EDX analysis. TGA showed decomposition temperatures of 379 °C (CuO), 394 °C (NiO) and 355 °C (CuO–NiO). The electrochemical test showed that the CuO–NiO had a specific capacitance value as high as 489.60 F g⁻¹ and an energy density of 0.272 Wh kg⁻¹, which were better than those of the CuO NPs (124.48 F g⁻¹ and 0.0692 Wh kg⁻¹) and NiO NPs (124.84 F g⁻¹ and 0.0694 Wh kg⁻¹). Furthermore, electrochemical impedance spectroscopy revealed that the electrode of CuO–NiO had the lowest charge-transfer resistance (Rct = 2.31 Ω), which in turn led to Rs = 0.29 Ω and Cdl = 126 µF, respectively, demonstrating good electron-transfer kinetics and charge storage at the interface. A synergistic effect between CuO and NiO is responsible for the improved electrochemical behavior and this suggests that CuO–NiO nanocomposites prepared from Raphia hookeri are promising electrode materials for advanced supercapacitor applications.