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 Li6.75La3Zr1.75Nb0.25O12 (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+/Fe3+, 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.