Comprehensive characterization of Co/Cr, Fe/Cr, and Mg/Cr -codoped ZnO nanoparticles by solution combustion method for improved battery-type supercapacitor performance

Zinc oxide (ZnO) holds promise for energy storage applications, but its bulk form suffers from low electrical conductivity and limited active surface area. To address these limitations, we synthesized cobalt/chromium (Co/Cr), iron/chromium (Fe/Cr), and magnesium/chromium (Mg/Cr) co-doped ZnO nanoparticles via the solution combustion synthesis (SCS) method. Co-doping induced lattice expansion, reduced crystallite size, and improved structural uniformity, as revealed by Rietveld refinement. Structural and morphological analyses using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) further corroborated these findings. The successful incorporation of dopants was confirmed through X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Optical studies revealed a red shift in the absorption edge and a reduction in the optical band gap to 2.91 eV, particularly with Co/Cr doping. Electrochemical testing revealed that Co/Cr-ZnO achieved an impressive specific capacity of 337.9 C g⁻¹ at 1 A g⁻¹, outperforming undoped ZnO by a remarkable factor of 16.9. Additionally, the material demonstrated excellent stability, retaining 71 % of its capacitance after 5000 cycles. These findings underscore the potential of Co/Cr-ZnO as an efficient cathode material for hybrid supercapacitors.