Enhanced performance of CoFe2O4 supercapacitors through synergistic interaction of Co2+ and Fe2+

Abstract

Hybrid supercapacitors incorporating multivalent ions have emerged as innovative electrochemical energy storage systems due to their high energy and power densities. Despite their promising attributes, the full potential of these devices remains unexplored because of the complex electrochemical interactions of multivalent ions within electrode materials, which impede widespread adoption in hybrid supercapacitors. A thorough investigation is conducted into the long-term electrochemical behavior of CoFe₂O₄ electrodes in the presence of multivalent ions (Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺ transitions) and K⁺ ion electrolytes. The correlation of the electrochemical behavior with detailed analyses of elemental composition, surface and structural morphology, and the electronic structure evolution of the CoFe₂O₄ heterostructures, in contrast to pristine Fe₂O₃ and Co₃O₄ electrodes, is examined. The CoFe₂O₄ electrode possesses a high specific surface area and porosity, facilitating greater electrolyte penetration and an increase in electroactive sites, achieving a high specific capacitance of 1231 F/g at 10 mA/cm² compared to Co₃O₄ and Fe₂O₃. This is attributed to its transformation into nanoflakes and nanosheet morphology, which promotes efficient K⁺ ion intercalation and deintercalation. A symmetric supercapacitor configured with CoFe₂O₄ electrodes achieves 230 F/g at 10 mA/cm², signifying an energy density of 25.88 Wh/kg and a power density of 281.25 W/kg, with 90.9 % capacitance retention across 5000 cycles. Furthermore, an asymmetric supercapacitor integrated with AC and CoFe₂O₄ electrodes achieves the highest energy density of 39.76 Wh/kg while maintaining 96.6 % of its initial capacity after 5000 cycles. The improved performances of the electrodes are well matched with the first principle-based density functional theory (DFT) outcomes. This research advances the understanding of multivalent ion charge storage mechanisms, offering critical insights for enhancing hybrid supercapacitor performance.