The kinetics of the reduction of thionyl chloride

Abstract The rate, reaction mechanisms, and the catalysis of the electrochemical reduction of thionyl chloride from anhydrous media, are discussed. Voltammetric measurements at stationary plane and rotated disc electrodes, using glassy carbon, graphite, or metal electrodes, have been made using thionyl chloride alone or at high dilution in supporting electrolytes. Porous carbon electrodes have been used to study stoichiometry and to determine how electrolyte concentration affects capacity. Chemical analysis of cells after partial or complete discharge has established that without catalysis, the products are sulfur, sulfur dioxide, and lithium chloride. In both acid and neutral electrolytes, the reduction behaves as though it were diffusion controlled, whether the cathodes are porous carbon electrodes or rotated disc electrodes. The mechanism of the reduction likely involves several electrochemical and chemical steps, proceeding through unstable and metastable intermediates. The reduction is irreversible because the species initially produced rapidly decomposes. On decomposition, one of the intermediates produces thionyl chloride. The overpotential at either porous or nonporous electrodes can be lowered, and the capacity of porous electrodes increased, by high surface area carbons or by the use of catalysts. It is not known whether catalysts change the overall discharge stoichiometry. In basic electrolytes, the first step in the reduction on a carbon surface appears to be first order with respect to the concentration of chloride ion and to involve only one electron. The species produced by the initial reduction is stabilized by chloride ion. As the chloride ion concentration is lowered, the initial reduction begins to involve more than one electron.