Cathodic Discharge Controlled by Dissolution Kinetics

A theoretical analysis is presented for the constant-current discharge of a thin- layer cathode with control by dissolution and diffusion of the electro-active material. The treatment involves the solution of Fick's second law of diffusion for an electrolyte layer which is bounded on one side by a planar elec-trode of a metal to be electrodeposited and on the opposite side by a planar surface of a partial ly soluble compound of that metal. It is shown that the po-tential at the metal surface may reach a steady state within a few minutes after the current is applied in an initially saturated solution. There is a critical current density, however, at which a transition occurs and the potential ap-proaches infinity. The dissolution rate constant and other parameters of the system may be determined by several procedures involving the use of tran-sient data. This electrode configuration also provides a model of the inner structure of a porous battery cathode. Experimental results on cadmium fluo-ride in aqueous 4M KF indicated a dissolution rate constant of 8 x 10-9 mole