Herein, a novel advanced electrode substrate, electrochemically modified graphite paper (ECM-GP) having a foam-like structure, was developed via the green electrochemical oxidation/exfoliation of pristine graphite paper (PGP). The exfoliation technique greatly enhanced the specific surface area from 28.6 m² g⁻¹ for PGP to 560.9 m² g⁻¹ for ECM-GP with a pore volume of 0.766 cm³ g⁻¹. This structural improvement not only enhanced its capacitance (from 108 mF cm⁻² for PGP to 156 mF cm⁻² for ECM-GP) but also improved its charge storage kinetics from diffusion-controlled to a capacitive nature. These amazing characteristics of ECM-GP allowed it to be extended as an advanced electrode substrate by electrochemically depositing MoO₂ nanoparticles, resulting in a high capacitance value of 1409 mF cm⁻² (19.6 F cm⁻³) at a current density of 2 mA cm⁻² with 33.7% retention at a current density of 30 mA cm⁻². This capacitance value is very high compared to that of previously reported MoO₂-based electrodes and other differently designed electrodes. Electrochemical impedance spectroscopy indicated low charge transfer resistance in ECM-GP and low contact resistance between ECM-GP and MoO₂. The designed solid-state symmetric supercapacitor (SSC) using the MoO₂-decorated ECM-GP electrode showed a high energy density of 0.212 mW h cm⁻² (1.41 mW h cm⁻³) with a power density of 98.46 mW cm⁻² (665.41 mW cm⁻³).