Electrochemical Intercalation of Alkali-Metal Ions into Birnessite-Type Manganese Oxide in Aqueous Solution

A thin layer electrode of birnessite-type manganese oxide was prepared by brushing a mixed solution of KOCOCH<INF>3</INF> and Mn(OCOCH<INF>3</INF>)<INF>2</INF> on a platinum substrate, followed by heating at 1073 K. The chemical composition of the electrode was K<INF>x</INF><INF></INF>MnO<INF>y</INF><INF></INF> (x = 0.33 and y ~ 2) with an interlayer spacing of c<INF>0</INF> = 0.697 nm. The positive-potential going sweep on the electrode in an aqueous phase caused the deintercalation of K⁺ with an increase in c<INF>0</INF>. The quasi-reversible intercalation of K⁺ occurred with a subsequent negative-potential going sweep in a 0.2 mol/dm³ KCl solution. The electrochemical measurements suggested that K⁺ is not electrochemically active in the deintercalation/intercalation reaction but H⁺ is. The reaction proceeds based on a mechanism consisting of an electrochemical reaction (the redox reaction between Mn³⁺ and Mn⁴⁺) and an ion-exchange reaction between K⁺ and H⁺. The intercalation experiments in various alkali-metal chloride solutions showed the intercalation capacity to be in the order of Na ~ K > Li > Rb > Cs.