Manganese Oxide Ion-Sieves. (2). Electrochemical Host-Guest Reactions.
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Electrochemical host-guest reactions of manganese oxides were reviewed by illustrating an electroinsertion of Li<SUP>+</SUP> into a Pt/λ-MnO<SUB>2</SUB> electrode and an electrointercalation of alkali-metal ions into a birnessite-type manganese oxide electrode in aqueous solutions.<BR>A thin film electrode of spinel-type manganese oxide, Pt/λ-MnO<SUB>2</SUB>, is prepared by the brush-ing-heating methods, followed by the electrochemical extraction of Li<SUP>+</SUP> from the electrode. Equilibrium potential of the Pt/λ-MnO<SUB>2</SUB> electrode shows a near-Nernstian response to Li<SUP>+</SUP> and not to other alkali metal or alkaline earth metal ions. The selectivity of the Pt/λ-MnO<SUB>2</SUB> electrode for Li<SUP>+</SUP> is markedly high, even compared to that of organic ionophores for a lithium ionselective electrode. The equilibrium potential was stable and pH-independent over a wide pH range (pH 4 to 9.5) in a solution with the Li<SUP>+</SUP> concentration above 5 mmol/dm<SUB>3</SUB>. Cathodic and anodic potential sweeps cause an insertion and extraction of Li<SUP>+</SUP> into/from the Pt/λ-MnO<SUP>2</SUP> electrode. The electrochemical insertion/extraction reaction originates from the redox reaction between trivalent and tetravalent manganese. The interfacial charge-transfer process is independent of the composition <I>x</I> in Li<SUB>x</SUB>Mn<SUB>2</SUB>O<SUB>4</SUB>, and the exchange current density is almost constant irrespective of <I>x</I> and relatively high (1.1×10<SUP>-3</SUP> A/cm<SUP>2</SUP>), indicating the fast ion transfer between the solid and liquid phases, whereas the chemical diffusion coefficient of lithium depends greatly on x because of the effect of the thermodynamic factor. The chemical diffusion coefficient of lithium is in the range of 10<SUP>-12</SUP> to 10<SUP>-10</SUP> cm<SUP>2</SUP>/s. The solid state diffusion of Li<SUP>+</SUP> is a rate-deter-mining step in the electroinsertion.<BR>A thin film electrode of birnessite-type manganese oxide is prepared with the chemical composition of K<SUB>x</SUB>MnO<SUB>y</SUB>, (<I>x</I>=0.33 and <I>y</I>-2) and an interlayer spacing <I>c</I><SUB>0</SUB> of 0.697 nm. The anodic potential sweep in an aqueous solution causes the deintercalation of K<SUP>+</SUP> with an increase in <I>c</I><SUB>0</SUB> due to the intercalation of H<SUB>2</SUB>O. The quasi-reversible intercalation of K<SUP>+</SUP> occurs by the subsequent catholic potential sweep in a 0.2 mol/dm<SUP>3</SUP> KCl solution. The electrochemical measure-ments show that K<SUP>+</SUP> is not electrochemically active in the deintercalation/intercalation reaction but H<SUP>+</SUP> is. The reaction proceeds based on the mechanism consisting of an electrochemical reaction (the redox reaction between Mn<SUP>3+</SUP> and Mn<SUP>4+</SUP>) and an ion-exchange reaction between K<SUP>+</SUP> and H<SUP>+</SUP>. The intercalation experiments in various alkali-metal chloride solutions show the intercalation capacity in order of Na-K>Li>Rb>Cs.
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