Effect of the Formation of a Charged Intermediate on the Electroreduction of Cobalt(III) Trioxalate Complexes to Cobalt(0) Atoms

Abstract

Kinetic relationships for the recharge of a cobalt(III) trioxalate complex on a mercury electrode are derived for electrode potentials of 0 to –1.5 V vs. point of zero charge (PZC) of mercury. This wide range includes high negative potentials at which the recharge of Co(C₂O₄)₃ ^3– and discharge of Co²⁺ occur simultaneously. The contribution of the reaction Co(II)/Co(0) (whose kinetic parameters are known) into the overall reduction of Co(C₂O₄)₃ ^3– to metallic cobalt is calculated. Migration fluxes of species present in solution are shown to be insignificant; hence, these can be neglected in the calculations. Relationships, which permit the determination of a partial polarization curve for the Co(III)/Co(II) recharge from the overall rate of the Co(III)/Co(0) process, are found. A quantitative evidence for the appearance of a secondary current drop at potentials near –0.7 V vs. PZC is obtained. The drop is caused by variations in the outer Helmholtz plane potential because of the commence of discharge of Co²⁺.