Application of electrochemical quartz crystal microbalance technique to hydrogen/lithium insertion into MnO2/LiCoO2 electrode in aqueous/non-aqueous solution

The hydrogen insertion/desertion into/from MnO2 electrode in aqueous solution and the lithium intercalation/deintercalation into/from LiCoO2 electrode in non-aqueous solution have been investigated by using in-situ electrochemical quartz crystal microbalance (EQCM) technique combined with cyclic voltammetry (CV) and galvanostatic charge-discharge experiment. In the case of the MnO2 electrode, the combined cyclic electrogravimetric and CV results indicated that the redox potentials at the transition in oxidation state of manganese ion measured on the cathodic scan are satisfactorily in accord in value with those thermodynamic equilibrium potentials calculated in Pourbaix diagram. The positive/negative slope with a constant value in the plot of mass change rate vs. potential means that the reaction is inclined to proceed in the direction of an oxidation/reduction between two phases. From the electrogravimetric curves obtained simultaneously with galvanostatic discharge curves, the discrepancy between the charge and mass variations was discussed in relation with the hydrogen-induced stress. In the case of the LiCoO2 electrode, the cyclic electrogravimetric data obtained simultaneously with CV indicated that neither solvent nor any of other species but lithium ions are intercalated into and deintercalated from the electrode. From the cyclic electrogravimetric curve obtained simultaneously with galvanostatic charge-discharge curve, the discrepancy between the charge and mass variations was discussed in relation with the change of the molar volume and surface roughness of the electrode during the lithium intercalation and deintercalation.