An operando ORP-EIS study of the copper reduction reaction supported by thiourea and chlorides as electrorefining additives

Additives are used in metal electroreduction processes to obtain a smooth and optimal metal deposit. However, the mechanism with which these additives influence the metal deposition is poorly understood. Operando odd random phase electrochemical impedance spectroscopy (ORP-EIS) experiments have been performed to study the effect of a chloride-thiourea additive mixture on the reduction of copper ions in a sulfuric acid solution. This additive mixture is crucial in the copper electrorefining process. An optimal experimental design and a well-founded methodology is applied to study this complex additive mixture. The ORP-EIS data is studied to analyze non-stationarities and non-linearities and the time-varying impedance is resolved when necessary. Two equivalent circuits are proposed to model the impedance data and the model parameters are linked to physical processes in the system. The use of the time-varying impedance when studying thiourea in a plating process and the use of distributed circuit elements in the EIS model is discussed.