<p>A ternary nanocomposite polyaniline/acetylene black (AB)/manganese dioxide (MnO₂) was synthesized via in situ oxidative polymerization of aniline in the presence of AB and MnO₂. Detailed structural and morphological analyses revealed the deposition of polyaniline onto the surfaces of AB and MnO₂. This coating of PANI on the surfaces of AB and MnO₂ is likely due to strong interfacial synergy, possibly through hydrogen bonding and π–π stacking interactions among the components. The ternary nanocomposite exhibits a remarkable specific capacitance of 404.2 F g⁻¹ at a current density of 0.5 A g⁻¹ and retains over 82% of its capacitance even at a high current density of 5 A g⁻¹, indicating excellent rate capability. A symmetric device was fabricated using a PVA/H₂SO₄ gel as the electrolyte, which demonstrated a specific capacitance of 366 F g⁻¹ at 0.5 A g⁻¹ and exhibited good cycling stability, retaining 86% of its initial capacitance after 8000 charge–discharge cycles at 5 A g⁻¹. Furthermore, the device achieved a high energy density of 24.9 W h kg⁻¹ at a power density of 700 W kg⁻¹, maintaining 18.3 W h kg⁻¹ even at 7&#xa0;kW&#xa0;kg⁻¹. The ternary nanocomposite exhibited excellent interfacial synergy and good electrochemical properties, delivering high specific capacitance, superior rate capability, good cycling stability, and high energy density, demonstrating its strong potential for advanced energy storage devices.</p> Graphical abstract <p></p>