High Performance Flexible Supercapacitor Based on Single Precursor Derived NiFe2O4 Spinel with Tailored Cationic Distribution and Oxygen Vacancies in Acidic Medium

Spinel metal oxides are extensively studied for supercapacitors (SCs) in alkaline electrolytes, where charge storage capacity is limited by surface site availability due to surface reconstruction forming metal hydroxides/oxyhydroxides. The use of an acidic medium, which can boost the charge storage capacity of spinel oxides offering an additional channel of intercalation-deintercalation of protons, is underexplored. Moreover, the impact of chemical compositions and the cationic distributions is crucial on the electrocatalysis performance of spinel oxides, however, such a correlation is first time reported for charge storage properties of spinel ferrite NiFe₂O₄ nanoparticles (NFO NPs). Besides, a low-cost and scalable synthesis of NFO NPs involving the thermal decomposition of Ni-Fe glycolate, followed by controlled air-calcination is reported. Thus crafted NFO NPs-based device shows impressive specific capacitance (2112 F g⁻¹ at 10 A g⁻¹) in half-cell configuration. A flexible all-solid-state asymmetric device (full-cell) configuration depicts impressive energy density (20.7 Wh kg⁻¹), power density (4000 W kg⁻¹), gravimetric capacitance (140 F g⁻¹ at 2 A g⁻¹), and retention of its performance (≈75% after 10,000 charging/discharging cycles). The results depict a new insight toward the tuning of electronic and charge storage properties in NFO, which otherwise are predominately attributed to only the crystallite size and morphological effects.