Organosulfur/Conducting Polymer Composite Cathodes I. Voltammetric Study of the Polymerization and Depolymerization of 2,5-Dimercapto- 1 ,3,4-thiadiazole in Acetonitrile

This paper describes general trends in the redox potential and solubility of a dimercaptan, 2,5,-dimercapto-1,3,4thiadiazole (DMcT), and its various derivatives in acetonitrile, and illustrates how these trends manifest in the cyclic voltammetry observed at glassy carbon electrodes. Attention is focused on the polymerization/depolymerization processes which are the origins of the excellent charge-storage capabilities of electrodes based on the DMcT family of compounds. The extent of oligomerization/polymerization of DMcT at glassy carbon is shown to increase with increasing overpotential and/or oxidation time, as judged from its subsequently observed quasi-stable depolymerization wave. Oxidation of dimer DMcT results in extensive precipitation onto the electrode, relative to that observed for oxidation of monomer DMcT, as expected in light of solubilities observed for those compounds in acetonitrile. The protonation state of the compounds is shown to have a considerable effect on both their solubility and, as reported previously in part, redox potential. Similarly, chemical coupling of the redox processes of these compounds to their protonation state and, for DMcT, to a disproportionation reaction known to occur for its oxidation product(s) complicates analysis of the system as a whole. However, from consideration of all of the observed trends, a general picture emerges illustrating the redox character of DMcT during polymerization and depolymerization. Most importantly, it is shown that the electrochemical irreversibility typical of those processes can be avoided by careful control of the protonation state and solubility of the electroactive species, allowing the measurement of quasi-reversible redox couples. That and other results are discussed in the context of the use of DMcT in secondary battery electrodes.