Gel-Synthesized Zirconium Diboride (ZrB ₂ ) Powders for All-Solid-State Symmetric and Zinc-Ion Hybrid Supercapacitor

This study reports the synthesis and electrochemical evaluation of gel-synthesized ZrB₂ for all-solid-state symmetric supercapacitors. The prepared ZrB₂ powders were characterized using XRD, SEM, FTIR, and XPS analyses. Electrochemical performance was tested using cyclic voltammetry (CV) in 1 M Na₂SO₄ electrolyte in both positive and negative voltage ranges at a wide range of scan rates from 5 to 500 mV/s. Specific capacitances ranged from 18 to 55 F/g in the positive region and 19-73 F/g in the negative region. Dunn's method analysis revealed that pseudocapacitive contributions dominated at low scan rates (80%), while electric double-layer capacitance (EDLC) contributions dominated at high scan rates (70%), indicating hybrid energy storage behavior. A solid-state symmetric supercapacitor cell was assembled using ZrB₂ electrodes with PVA/Na₂SO₄ gel electrolyte, exhibiting a low internal resistance of 2.79 Ω·cm² and an operational voltage of 1.4 V with rectangular CV profiles. Galvanostatic charge-discharge (GCD) tests showed an energy density of 5.0 Wh/kg and a power density of 5600 W/kg at 8 mA cm^-2. The cell exhibited good cyclic stability of 90.7% after 5000 cycles. Additionally, a gel-derived ZrB₂ electrode was successfully applied in a solid-state zinc-ion hybrid supercapacitor configuration, demonstrating a high specific capacity of 230 mAh/g at 0.9 C and excellent reversibility with 87% Coulombic efficiency. These results confirm the versatility of gel-synthesized ZrB₂ for both symmetric and hybrid energy storage systems, making it a promising electrode material for multifunctional solid-state devices.