In Situ Ellipsometry and EIS Study of Potentiostatic Synthesis of Pseudocapacitive MnOx

Abstract

This work discusses the one-step potentiostatic growth of manganese oxide on stainless steel for pseudocapacitor electrodes. The electrode material was studied through in situ ellipsometry and electrochemical impedance spectroscopy, in order to correlate its microstructure with the capacitive response. Ellipsometry results show the formation of three layers during the potentiostatic synthesis of manganese oxide on stainless steel: the thickening of the native oxide of the substrate, and the growth of two distinct layers of manganese oxide. The inner layer is slightly more compact (>n) and more resistive (<k) than the outer one. The electrochemical characterization of modified electrodes showed that the increase in the growth potential (from 0.7 to 1.0 VSCE) leads to the formation of manganese oxide films with higher specific capacitance (from 64 to 330 F g−1 at 2 A g−1). This effect is due to an increase in the area of the active material and to a decrease in the thickness of the inner layer of the manganese oxide film. However, higher growth potentials also lead to an increase in film resistivity due to the increase in the density and thickness of the outer layer, which hinders the diffusion of ions through the film as evidenced by EIS.