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

The present work reports spray-deposited manganese oxide (MnO2) thin films annealed at different temperatures (200, 300, and 400 degrees C). X-ray diffraction shows the formation of a polycrystalline nature with a tetragonal crystal structure and a pyrolusite (beta) 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. beta-MnO2 annealed at 300 degrees C has a large surface area (35 m(2)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 mAcm(2)). At annealing temperatures of 200, 300, and 400 degrees 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 degrees C.