<p>Sustainable, high-performance nanocomposite electrolytes are pivotal for solid-state energy-storage devices. In this work, an eco-friendly nanocomposite based on a poly(vinyl alcohol)/hydroxypropyl methylcellulose/carboxymethyl cellulose (PVA/HPMC/CMC) ternary blend reinforced with Co₃O₄ nanoparticles is developed via a solution casting method. Co₃O₄ nanoparticles were synthesized using a starch-assisted sol–gel route. XRD analysis showed the crystallinity to decrease from 33.9 to 19.5% which indicated the presence of larger amorphous regions favorable for ionic transport. FTIR spectra indicate strong interaction between polymer and filler. TEM analysis indicates well dispersed nanoparticles of nearly spherical shape with average size of 25.09 nm enhancing interface contact. The optical analysis via Tauc shows a narrowing of the band gap with increasing Co₃O₄ loading (0.0–2.4 wt.%): the direct gap reduces from 5.35 to 4.72 eV, while the indirect gap reduces from 4.63 to 3.63 eV. Dielectric and electrical measurements reveal significant improvement at 2.4 wt.% Co₃O₄, including the maximum dielectric permittivity, the maximum AC conductivity and the decrease of bulk resistance from 1.40 × 10⁸ Ω to 1.34 × 10⁶ Ω. Phase Bode analysis confirmed enhanced ionic conduction, reduced polarization, and faster charge-transfer dynamics, consistent with impedance spectroscopy results. Furthermore, the relaxation in frequency domain is faster, and the characteristic time decreases from τ ≈ 3.65 × 10⁻⁵ s to τ ≈ 2.48 × 10⁻⁶ s. The synergistic improvements are ascribed to nanoparticle-induced amorphization, interfacial polarization and charge carrier activation. Collectively, PVA/HPMC/CMC–Co₃O₄ nanocomposites emerge as promising electrolytes for next-generation solid-state capacitors.</p> Graphical Abstract <p></p>