Silver-Doped Mesoporous Spinel NiCo2O4 Nanorods as Advanced Electrodes for Supercapacitors: From Material Design to Device-Scale Evaluation

This study explores the hydrothermal synthesis of NiCo₂O₄ and Ag-doped NiCo₂O₄ (Ag_xNi_1−xCo₂O₄, x = 0.0–0.5) electrodes as cost-effective materials for supercapacitor applications. Electrochemical analysis of all fabricated materials was conducted using a half-cell configuration, with measurements performed at various scan rates. As Ag concentration increased (for x ≤ 0.4), the specific capacitance also increased, reaching a maximum performance of 1501 F g⁻¹ for x = 0.4. This represents a 2.5-fold increase compared to its pristine counterpart. However, the specific capacitance value decreased by 21% when the Ag concentration was raised from x = 0.4 to x = 0.5. Performance decline is linked to the increment of pore size (20%) and the decrement in surface area (12%) in sample x = 0.5 compared to x = 0.4, as confirmed by BET analysis. Cyclic studies over 5000 cycles demonstrated retention capacities of 94% for x = 0 and 106% for x = 0.4. The x = 0.4 sample outperformed others, exhibiting the lowest solution-resistance (R_s = 0.5 Ω) and charge transfer resistance (R_ct = 0.9 Ω). The sample with the best performance, x = 0.4, has been chosen for fabrication of a 2-electrode system in both symmetric and asymmetric designs to evaluate device-level performance. The symmetric supercapacitor (SSC) showed specific capacitance of approximately 252 F g⁻¹ at current density of 1 A g⁻¹, maintaining 93.5% of its initial capacitance after 5000 cycles at 10 A g⁻¹. It delivered an energy density of 42 W h kg⁻¹ at a power density of 549 W kg⁻¹. Meanwhile, the asymmetric supercapacitor showed specific capacitance (178 F g⁻¹ at 1 A g⁻¹), and retaining 105% capacitance after 5000 cycles at 10 A g⁻¹, with energy and power densities of 67 W h kg⁻¹, 853 W kg⁻¹, respectively. The findings from various characterization techniques are thoroughly analyzed to draw the structure–property–performance correlations and presented in detail.