Inexpensive liquid-phase synthesized NiFe₂O₄ and Cu-substituted nickel ferrites for advanced energy storage

The synthesized Cu_xNi_1-xFe₂O₄ (x = 0.00–1.00) spinel ferrite nanoparticles via the polyol method were systematically investigated to understand the effect of Ni substitution on structural, morphological, and electrochemical properties for supercapacitor applications. While Ni substitution influences particle size, cation distribution, and redox activity, the Cu-rich composition (CuFe₂O₄, x = 1.00) exhibited the highest electrochemical performance, achieving a remarkable specific capacitance of 1540 F g⁻¹ at a scan rate of 1 mA/cm² in 1 M KOH, 85.67% capacitance retention over 5000 cycles, an energy density of 48 Wh kg⁻¹, and outstanding cycling stability. These enhancements are attributed to optimized cation distribution, reduced particle size (98 nm), and efficient ion transport in the Cu-rich lattice. Integration of CuFe₂O₄ into an asymmetric supercapacitor device has validated its practical applicability, delivering competitive energy and power densities. This study demonstrates that while Ni substitution provides valuable insights into structure–property relationships, CuFe₂O₄remains a superior candidate for next-generation high-performance and sustainable energy storage devices.