Strategic Mass Optimization of MnO ₂ Nanostructures Synthesized via Coprecipitation: A Response Surface Methodology (RSM)-Guided Electrochemical Study

Manganese oxide (MnO 2 ) nanostructures (NSs) were synthesized via a coprecipitation method using MnCl 2 ·4H 2 O and KMnO 4 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-MnO 2 (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-MnO 2, 156.82 m 2 g –1 ). Electrochemical measurements (CV, GCD, and EIS) demonstrated superior performance for the optimized TB2 electrode, synthesized with 2 g KMnO 4, 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-MnO 2 NSs for advanced energy storage applications.