ELLIPSOMETRIC STUDY OF THE CHARGE/DISCHARGE PROCESS OF MANGANESE OXIDE FILMS

M.A. PEREZ ; H.M.VILLULLAS; M.LOPEZ TEIJELO; B.A.LÓPEZ DE MISHIMA

Varsovia

Congreso; 51st Annual Meeting International Society Electrochemistry; 2000

International Society Electrochemistry

Electrolytic manganese dioxide is well known as an excellent electrode material and is the mostsuitable form of manganese dioxide as active material in alkaline cells. The main problem limiting the rechargeability is caused by irreversible structural changes accompanying the oxidation/reduction process in aqueous solutions leading to the formation of electrochemical inactive compounds. In this work the electrochemical and optical behaviour of thin films of manganese oxide are studied combining cyclic voltammetry and in-situ ellipsometry. Manganese oxide films of different thicknesses were deposited on a platinum substrate from 0.1M MnSO4 + 0.017M H2SO4 (pH 2) solutions by applying potentiostatic pulses and the response during charge/discharge cycles was studied in an alkaline borate solution (pH 9.2).The ellipsometric results for films of different thicknesses electroreduced at -0.5V showchanges in the optical constants as compared with those of oxidized samples leading to a decrese of both, the refractive index and the extinction coefficient.In order to interpret the optical data, two-layer models with an interface sweeping inward fromthe electrolytic interface or outward from the substrate interface, or a one-layer model with therefractive index of a homogeneous film varying continuously during the conversion process wereemployed.The ellipsometric behaviour during the charge/discharge process can be correlated with the potendiodynamic results where a loss of charge with cycling is obtained. The optical response is reversible for thin oxide films while a pronounced irreversibility is observed for thick films. Apossible explanation for the irreversibility is advanced.