Effect of thermal annealing on physiochemical properties of spray-deposited β-MnO2 thin films for electrochemical supercapacitor

Abstract The present work reports spray-deposited manganese oxide (MnO2) thin films annealed at different temperatures (200, 300, and 400 °C). X-ray diffraction shows the formation of a polycrystalline nature with a tetragonal crystal structure and a pyrolusite (β) MnO2 phase. X-ray photoelectron spectroscopy confirms the presence of Mn in the 4+ oxidation state. Field-emission scanning electron microscopy images reveal a porous structure with a granular-like morphology, and the sizes of the granular structures increase with the annealing temperature. β-MnO2 annealed at 300 °C has a large surface area (35 m2 g−1) and an average pore size (8.1 nm). Cyclic voltammetry (CV) of all the samples have a mirror symmetry, revealing the capacitive behavior and an excellent rate capability (91% at 1 mAcm−2 and 89% at 2 mAcm−2 to an initial current density of 0.5 mA cm2). At annealing temperatures of 200, 300, and 400 °C, the samples have specific capacitances of 230, 399, and 298 Fg−1 calculated at a current density of 0.5 mAcm−2 in 1 M aq. Na2SO4, respectively. The retention of the specific capacitance is approximately 91%, which is calculated up to 5000 consecutive CV cycles for the sample annealed at 300 °C.