Spectroelectrochemical investigations of cation-insertion reactions at sol-gel-derived nanostructured, mesoporous thin films of manganese oxide

Sol-gel-derived manganese oxide can readily be prepared as highly porous, nanostructured materials, including ambigel or aerogel monoliths and thin films supported on conductive, transparent glass. The electrochromic properties of manganese oxide, a mixed electron-cation conductor, provide an independent measure of the electronic state of the manganese in the oxide during the electrochemically driven cation-insertion process. Sol-gel-derived thin films of birnessite-type manganese oxide, NadeltaMnO2. xH(2)O, which is a layered polymorph of MnO2, were characterized by spectroelectrochemistry. During nonaqueous electroinsertion reactions of Li+, Mg2+, and tetrabutylammonium cation, the spectral and electrochemical signatures are temporally distinguished and electrochemical site specificity can also be discerned. The rate of coloration of the oxide (recovery of the Mn(IV) state) is approximately 4-fold more rapid than the rate of decoloration as Mn(III) centers are electrogenerated and electrolyte-derived cations are concomitantly inserted. The electrochromic efficiency for Li+ insertion into thin films of birnessite-type manganese oxides is 36 cm(2) C-1.