<p>Manganese oxides (MnO_x) represent a highly promising class of electroactive materials for supercapacitor applications. Nevertheless, their practical performance is frequently constrained by low electrical conductivity and a limited interfacial surface area. Integrating MnO_x with highly conductive materials onto porous conductive scaffolds presents an effective strategy to circumvent these limitations. In this work, a self-supported electrode was prepared by incorporating MnO_x within a three-dimensional (3D) polymeric matrix composed of poly(3,4-ethylenedioxythiophene) (PEDOT) and polyurethane (PU) doped with carbon black via a facile solution-based method. Benefiting from the high pseudo-capacitance of MnO_x, the outstanding mechanical flexibility of the polymer matrix, and their synergistic interaction including robust interfacial adhesion, rapid charge transfer, and efficient ion diffusion, the resultant electrode exhibited a high areal capacitance (Ca) of 373.1 mF cm^− 2 (~ 310.9&#xa0;F g^− 1) at 1&#xa0;mA cm^− 2. When integrated into a solid flexible symmetric device, it achieved a remarkable specific capacitance of 116.4 mF cm^− 2 at 1&#xa0;mA cm^− 2 and delivered an excellent energy density of 0.013 mWh cm^− 2 (at a power density of 0.45 mW cm^− 2). This study is expected to provide valuable insights for designing high-performance flexible supercapacitors (SCs) by integrating MnO_x into highly conductive 3D polymer frameworks via a simple, cost-effective approach.</p> Graphical abstract <p>A self-supporting 3D porous polymeric matrix of PBUE, composed of PEDOT and polyurethane doped with carbon black, was fabricated as a supportive electrode framework. Due to its porous architecture, MnO_x can be uniformly grown within PBUE, while PEDOT:PSS can fully infiltrate the composite structure, thereby forming a high-performance supercapacitor electrode.</p>