Fuel mediated defect engineering in nickel manganese oxide nanostructures for enhanced supercapacitor performance: Experimental and density functional theory insights

NiMn 2 O 4 nanoparticles were synthesized using urea, ascorbic acid, and dextrose as fuels to study the impact of fuel chemistry on structural evolution, defect-related electronic modulation, and electrochemical energy-storage performance. The fuel type significantly affected crystallinity, morphology, porosity, and surface chemical states, producing samples denoted as NMO:U, NMO:A, and NMO:D. The urea-derived NMO:U electrode had the highest specific capacitance of 293.72 Fg −1 at 1 Ag −1 , compared to 220.00 Fg −1 for NMO:D and 144.07 Fg −1 for NMO:A. It retained 86.43% capacitance and 98.75% coulombic efficiency after 10,000 charge-discharge cycles. Density functional theory (DFT + U) calculations on pristine and oxygen-deficient supercell models showed that oxygen-vacancy configurations introduce defect-derived electronic states near the Fermi level, providing theoretical insight into improved charge-transfer characteristics. The consistency between experimental observations and theoretical analysis suggests that NMO:U's superior electrochemical performance stems from the synergistic effects of improved crystallinity, mesoporosity, favourable morphology, and defect-related electronic modulation, rather than surface area alone. The asymmetric NMO: U//activated carbon device achieved a specific capacitance of 56.06 Fg −1 , 74.12% capacitance retention, and 99.6% coulombic efficiency, indicating stable proof-of-concept performance. This study introduces a fuel-mediated strategy to tailor the structural and electronic properties of NiMn 2 O 4 , emphasising the importance of experimental characterization and DFT analysis for understanding its electrochemical behaviour.