<p>Electrodes that exhibit both high energy and power densities are highly desirable for the development of supercapacitors competitive with rechargeable batteries. In this study, a novel&#xa0;mesoporous phosphorus-doped graphitic carbon nitride (P-g-C_₃N_₄) anchored with magnetic Fe_₃O_₄ was synthesized via a facile (NH_₄)_₂HPO_₄-assisted solvothermal method&#xa0;and confirmed through&#xa0;material characterization. X-ray photoelectron spectroscopic (XPS) analysis revealed a strong interaction between carbon in the P-g-C_₃N_₄ and iron, while vibrating sample magnetometry (VSM) confirmed the superparamagnetic behaviour imparted by Fe_₃O_₄. The energy storage performance of the Fe_₃O_₄/P-g-C_₃N_₄ nanocomposite as a supercapacitor electrode material was evaluated using a modified carbon paste electrode. It demonstrated a remarkable specific capacitance of 3470&#xa0;mF/cm² at a current density of 3&#xa0;mA/cm², significantly higher than that of the individual components. This enhancement is attributed to several factors, including high surface area, mesoporosity, improved electron transport due to P-doping, and the pseudocapacitive contribution from the Fe_₃O_₄. The Fe_₃O_₄/P-g-C_₃N_₄ nanocomposite showed good cycling stability, retaining 74.1% of its initial capacitance and delivering a coulombic efficiency of 93.3% over 1000 cycles in the three-electrode system. To assess practical applicability, a symmetric coin cell supercapacitor was assembled using the Fe_₃O_₄/P-g-C_₃N_₄ electrodes, which exhibited a high specific capacitance of 246.7&#xa0;F/g at 0.2&#xa0;A/g and an impressive capacitance retention of 113.9% after 10,000 cycles at a high current density of 15&#xa0;A/g. The device delivered a maximum energy density of 27.7&#xa0;Wh/kg and a power density of 2250&#xa0;W/kg. The ability to power a green LED further demonstrated the practical potential of the developed electrode material.</p> Graphical abstract <p></p>