Strategic Mass Optimization of MnO₂ Nanostructures Synthesized via Coprecipitation: A Response Surface Methodology...

Manganese oxide (MnO₂) nanostructures (NSs) were synthesized via a coprecipitation method using MnCl₂·4H₂O and KMnO₄ precursors. The effects of precursor concentration, calcination temperature, and reaction time were systematically optimized using Response Surface Methodology (RSM) based on a Box-Behnken design (BBD). Analysis of variance (ANOVA) identified calcination temperature and reaction time as a key factor governing the electrochemical response. Structural characterization (XRD and FTIR) confirmed the formation of ε-MnO₂ (akhtenskite) as the dominant phase, while SEM and EDX verified the morphology and elemental composition. BET analysis revealed a high surface area for the optimized TB2 sample (ε-MnO₂, 156.82 m² g^-1). Electrochemical measurements (CV, GCD, and EIS) demonstrated superior performance for the optimized TB2 electrode, synthesized with 2 g KMnO₄, and calcined at 200 °C for 3 h. It delivered a specific capacitance of 113.996 F/g at 10 mV s^-1 and an areal capacitance of 317.84 mF cm^-2 at 0.5 A g^-1. A maximum energy density of 2.4 W h kg^-1 (11.04 mW h cm^-2) was achieved at a power density of 372.40 W kg^-1 (1787.5 mW cm^-2). After 200 cycles, it retained 78.3% capacitance retention and 112% Coulombic efficiency were maintained, highlighting the potential of ε-MnO₂ NSs for advanced energy storage applications.