Solvothermally synthesized Dual-phase Mn3O4-MnO2/ZnO nanocomposites exploit synergistic Mn3+/Mn4+ redox activity and ZnO conductivity for high-energy supercapacitors. The heterostructure achieves 967 F/g specific capacitance (electrode) and 346.68 F/g device capacitance at 1 A/g, with energy densities of 108.8 Wh/kg (electrode) and 39.01 Wh/kg (device). The symmetric cell retains 99.7% capacitance over 50 cycles, enabling carbon-free energy storage.

Abstract

The integration of dual-phase manganese oxides (MnO₂-Mn₃O₄) with zinc oxide (ZnO) nanocomposites presents a compelling strategy to advance supercapacitor electrodes by synergizing the redox activity with structural stability. Here, solvothermally synthesized MnO₂-Mn₃O₄/ZnO nanocomposites (1:1, 1:2, 2:1 by wt%) are systematically investigated for their structural, optical, and electrochemical properties. X-ray diffraction (XRD) confirms the coexistence of tetragonal Mn₃O₄ and MnO₂ phases alongside the hexagonal ZnO phase, whereas FTIR and EDX validate the interfacial bonding and stoichiometric purity. Scanning electron microscopy (SEM) reveals spherical and agglomerated morphologies, and XRD computes reduced crystallite sizes (14.3 nm for the 2:1 composite), enhancing the surface area for charge storage. Optical analyses unveil bandgap modulation (2.16–3.95 eV) driven by type-II heterojunction formation, suppressing recombination, whereas photoluminescence (PL) spectra highlight defect-mediated transitions critical for charge carrier dynamics. Electrochemically, the MnO₂-Mn₃O₄/ZnO (2:1) nanocomposite achieves a specific capacitance of 967 F/g at a current density of 1.5 A/g, an energy density of 108.8 Wh/kg, and a low charge-transfer resistance of 377.4 Ω, outperforming individual MnO₂-Mn₃O₄ and ZnO components. A symmetric supercapacitor device delivers 39 Wh/kg energy density with 99.7% capacitance retention over 50 cycles, attributed to Mn³⁺/Mn⁴⁺ redox synergies and the conductive framework of ZnO. This work establishes dual-phase MnO₂-Mn₃O₄/ZnO as a scalable and high-performance electrode material, bridging redox-driven energy storage with structural resilience for next-generation supercapacitors.