Synergistic enhancement of charge transport, dielectric behavior, and impedance response in Cu/ZnO dual-nanofiller-loaded NaAlg/CMC biopolymer nanocomposites for biodegradable solid polymer electrolytes

ZnO nanoparticles were synthesized via a sol-gel method and then incorporated as Cu/ZnO dual nanofillers into sodium alginate (NaAlg)/carboxymethyl cellulose (CMC) biopolymer composite films using the solution-casting technique. The structural, dielectric, electrical, and impedance characteristics of the prepared films were systematically evaluated. XRD analysis showed that Cu/ZnO incorporation progressively disrupted the crystalline domains of the NaAlg/CMC matrix, with the optimum effect observed at 4.0 wt.% filler content. FTIR spectra indicated strong intermolecular interaction between the functional groups of the polymers (–OH/–COO⁻) and the nanofillers through hydrogen bonding and electrostatic coordination. SEM images showed improved dispersion of nanofillers at lower concentrations, while higher loading led to partial agglomeration. The dielectric parameters ( ε ′ and ε ″) strongly depended on the filler content, consistent with Maxwell–Wagner–Sillars (MWS) polarization arising from charge accumulation at the polymer–nanofiller interfaces. The AC conductivity increased with Cu/ZnO incorporation and reached a maximum value of ~2.1 × 10⁻⁵ S.cm⁻¹ at 4.0 wt.% filler loading at high frequency (10 MHz). The Nyquist plots exhibited depressed semicircles, indicating a non-Debye response associated with the bulk resistance of the polymer matrix and interfacial charge-transport processes. Overall, the optimized NaAlg/CMC:Cu/ZnO nanocomposite exhibited enhanced dielectric and electrical properties, indicating its potential as a biodegradable solid polymer electrolyte platform for future energy-storage and electronic applications.