<p>Recent novel studies on improving the properties of electrode materials for energy storage devices primarily explore the synergistic potential of different nanomaterials, either in doped or composite forms, and eco-friendly approaches that use plant extracts for biosynthesis. This study reports comparative results from synthesizing oxides of cobalt (Co) and manganese (Mn) and Co/Mn nanocomposites, both fused with extracts from Vitex doniana, for supercapacitor applications. Structural characterizations confirmed the successful synthesis of the face-centered cubic Co₃O₄ and tetragonal MnO₂ phases, both of which are present in the nanocomposites with crystallite sizes of 15&#xa0;nm (Co₃O₄), 26&#xa0;nm (MnO₂), and ~ 16&#xa0;nm (composite), alongside corresponding variations in lattice parameters, strain (1.4–2.6 × 10^− 3), and dislocation density (1.4–4.4 × 10^− 3 nm^− 2). Morphological studies revealed flake-like grain structures within the nanocomposite, with analysis revealing flake-like morphologies and a reduced particle size (~ 16&#xa0;nm), indicative of enhanced surface area. UV–Vis studies showed bandgaps of 2.1&#xa0;eV (Co₃O₄), 2.4&#xa0;eV (MnO₂), and a reduced 1.7&#xa0;eV for the composite. The composite exhibited improved non-linear optical properties, including a higher third-order susceptibility (χ³) and a larger non-linear refractive index (n₂). The electrochemical studies yielded comparably better specific capacitance (1018&#xa0;F g^− 1 from CV and 1128&#xa0;F g^− 1 from GCD), lower resistances (Rs = 0.03 Ω, Rct = 2.40 Ω), energy and power densities (320.91 Wh kg^− 1 and 37.55&#xa0;W kg^− 1), Coulombic efficiency (81.5%), and excellent cycling stability (92.55% retention after 5000 cycles), better than values obtained for individual oxides on all measured parameters.</p> Graphical abstract <p></p>