Electrodeposition-induced Fe-coordinated gradient dual-network hydrogel electrolyte for carbon-based symmetric supercapacitors

Hydrogel electrolytes for quasi-solid-state carbon-based supercapacitors need to balance structural stability, ion transport, and electrode/electrolyte interfacial compatibility, which often constrain each other. Here, we developed a carboxymethyl chitosan (CMCS)–sodium l-aspartate (AspNa)–Fe gradient dual-network hydrogel electrolyte through an electric-field-induced electrodeposition strategy. In this system, CMCS/AspNa formed a hydrophilic polymer framework; 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl, hereafter abbreviated as EDC) promoted the formation of amide-related covalent linkages, providing stable skeletal support; and Fe–O coordination was introduced through the in situ release of Fe species from a sacrificial Fe anode. During electrodeposition, electric-field-driven migration coupled with concentration diffusion produced a non-uniform Fe distribution, leading to a spatially heterogeneous coordination/covalent dual network within the hydrogel. FTIR, XPS, and SEM/EDS analyses revealed amide-related covalent linkages, Fe–O coordination, and a gradient Fe distribution, supporting the formation of the gradient dual-network structure. Among the samples with different EDC contents, the hydrogel prepared with 2.0% (w/v) EDC showed a suitable balance between compressive stability and ion transport. The AC//AC symmetric supercapacitor assembled with this hydrogel electrolyte delivered a device-level specific capacitance of 22.81 F g⁻¹ at 0.5 A g⁻¹ and retained over 90% of its initial capacitance after 10,000 cycles at 1 A g⁻¹, with a Coulombic efficiency close to 100%. These results highlight the role of the electrodeposition-induced gradient coordination/covalent network in coupling mechanical support, ion transport, and interfacial stability in quasi-solid-state carbon-based supercapacitors.