An Ellipsometric Study of Manganese Oxide Films

Abstract

The electrodeposition of manganese oxide films onto a platinum substrate was investigated by means of in situ ellipsometry. In the thickness range from 0 to 150 nm, the anodic oxide behaves as an isotropic single layer with optical constants that are independent of thickness. Deviations at higher thickness are explained in terms of anisotropic properties of the film. The electroreduction of thin films ͑up to ca. 150 nm͒ in an alkaline electrolyte leads to a decrease in both the refractive index and the extinction coefficient and is accompanied by a thickness increase of ca. 10%. The Mn͑IV͒ to Mn͑III͒ conversion takes place from the oxide/electrolyte interface inwards. © 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1825951͔ All rights reserved. The wide spread use of MnO 2 for battery electrodes has continued over the years and has been extended today to nonaqueous lithium cells. [1][2][3][4] Manganese dioxide can exist as various polytypes denoted ␣-, ␤-, ␥-, ramsdellite, ␦-and -forms, with the ␥-form, referred to as nsutite, being the most active electrochemically. Among the samples referred to as ␥-MnO 2 and depending on their origin, natural MnO 2 , chemical MnO 2 ͑CMD͒, and electrolytic MnO 2 ͑EMD͒ can be distinguished. The differences are associated to subtle structural changes. Electrolytic MnO 2 films have been prepared using galvanostatic and potentiostatic methods. 5,6 Amorphous hydrous manganese oxides anodically deposited from MnSO 4 solutions of different pH show an acceptable capacity, high reversibility, and high pulse-power properties for electrochemical supercapacitors. 7,8 For battery-active manganese dioxides used in primary cells ͑␥-and -MnO 2 ), 9 the electroreduction process during the first cycle in alkaline electrolytes has been extensively studied. Kozawa and coworkers observed that the reduction process depends on KOH concentration. 9,10 Amarilla et al. 11 have studied the influence of KOH concentration on the MnO 2 redox mechanism using step potential electrochemical spectroscopy ͑SPECS͒ and X-ray diffraction ͑XRDS͒. In 1 M KOH the voltamperometric and XRD data show that the redox mechanism can be described as a H ϩ /e Ϫ insertion/ desinsertion process with good reversibility. For strongly alkaline KOH solutions, a different redox mechanism is observed after the first cycle and a loss of electrochemical activity is also noticed.In general, the reduction of ␥-MnO 2 occurs in two potential regions. The first step corresponds to a homogeneous reduction while the second step is a heterogeneous reduction process. investigated the reduction of electrodeposited manganese dioxide ͑EMD͒, birnessite, and Bi-birnessite electrodes and found that each type of manganese dioxide underwent a homogeneous reduction followed by a heterogeneous reduction stage.The electrochemical process of insertion/desinsertion of H ϩ was investigated by studying its influence on the evolution of the crystallographic structure of ␥-MnO 2 in EMD samples during the discharge and recharge processes of an alkaline batt...