<p>The interfacial structure at the electrode-electrolyte interface is a critical determinant of the performance of electrochemical supercapacitors. Although grafting polyelectrolyte (PE) brushes onto electrodes is a promising strategy to enhance charge storage, the role of dielectric contrast and its coupling with PE microstructure remains to be studied. In this work, a classical density functional theory (cDFT) is employed to elucidate the synergistic effects of dielectric contrast and PE microstructure on supercapacitor performance. The theory explicitly accounts for image-charge interaction induced by dielectric discontinuity, chain correlation of the PE, excluded volume effect, and electrostatic correlation. The results reveal that increasing the grafting density and chain length of the PE brushes significantly enhances the capacitance by providing more immobile charge sites and expanding the effective double-layer region, which facilitates stronger counterion adsorption. It is found that the dielectric contrast between the electrode substrate and solvent, which is often oversimplified in idealized models, plays a decisive role. A higher dielectric permittivity of the substrate weakens the shielding effect of the polarization field within the electrode. Consequently, the external electric field provides a stronger influence on the electrolyte, promoting a denser accumulation of counterions at the interface. Crucially, a high-dielectric substrate mitigates the repulsive image-charge interaction, effectively transforming it into an attractive force that further adsorbs charge near the electrode. This effect becomes particularly pronounced at high surface potentials. These results provide fundamental insights into the optimization of high-performance SCs using PE coatings.</p>