Hierarchically Porous NiCuFe₂O₄ Coupled with Li⁺‑Ion Cubic Garnet-Based Electrolyte for Solid-State Supercapacitors

A hierarchically porous NiCuFe₂O₄ electrode material was synthesised using a NaCl hard-templating route and integrated with a Li_6.75La₃Zr_1.75Nb_0.25O₁₂ (LLZNO) garnet-type solid electrolyte infiltrated with ∼6 wt % 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF₄) to construct solid-state supercapacitors (SSCs). The salt-templated ferrite preserves a cubic spinel framework with a reasonably high surface area (∼140 m² g^-1) and abundant Ni, Cu, and Fe redox centres. In a three-electrode configuration with a 2 M KOH electrolyte, the HT-NiCuFe₂O₄ electrode delivers a high specific capacitance of 914 F g^-1 and shows predominantly diffusion-controlled, battery-type Faradaic charge storage. The electrode facilitates multivalent redox processes involving Ni²⁺/Ni³⁺ and Fe²⁺/Fe^3+, as verified by cyclic voltammetry and XPS. In a symmetric configuration operated at 40 °C, the assembled device (mass loading per electrode ∼1 mg/cm²) delivers a notably high specific energy of ∼115 Wh kg^-1, with a specific power of ∼3000 W kg^-1 at 2.0 V/8 mA. The SSCs assembled with HT-NiCuFe₂O₄ electrodes and LLZNO/EMIM BF₄ electrolyte demonstrate moderate cycling stability, exhibiting 56% (450 to 254 F g^-1) capacitance retention after 10,000 galvanostatic charge-discharge cycles at 1.5 V/3 mA. These results highlight NaCl hard-template NiCuFe₂O₄ as a promising redox-active electrode for solid-state supercapacitors.