A new polypyrrole/disulfide electrode studied by electrochemistry and the electrochemical quartz crystal microbalance

A new polypyrrole/disulfide electrode has been prepared by electrochemical oxidation of an aqueous potassium 2,5-dimercapto-1,3,4-thiadiazole (K(2)DMcT) solution at an electropolymerized polypyrrole chloride, PPy/Cl-, film electrode. The formation of this new electrode material was investigated by cyclic voltammetry and chronoamperometry in combination with the electrochemical quartz crystal microbalance (EQCM). Following cycling in a DMcT(2-) solution, the cyclic voltammogram of a PPy/Cl- electrode changed significantly as the common redox waves of the PPy/Cl- electrode were no longer observed. instead, a new set of anodic and cathodic waves appeared at more negative potential in addition to an irreversible anodic wave at about 0.2 V. The mass of a Au/PPy/Cl- electrode, immersed in a 50 mM DMcT(2-)/0.1 M KCl solution, increases much more than that of a bare Au electrode when the potential was switched from 0 to 0.5 V whereas no mass variation was noticed for a Au/PPy/Cl- electrode when DMcT(2-) was not present in the solution. In contrast, the cyclic voltammogram and the mass of a poly(N-methylpyrrole) chloride electrode did not change at all when the experiments are carried out in the same conditions as for the PPy/Cl- electrode. These results suggest that acid/base chemistry may occur between PPy and DMcT(2-) although hydrogen bond formation between the NH moiety on the pyrrole ring and the sulfur of the monobasic form of DMcT cannot be ruled out completely. This PPy-DMcT electrode has also been characterized by cyclic voltammetry, EQCM, and in-situ conductivity measurements. This new material has unique properties compared to PPy/Cl- and is characterized by a redox potential of about -0.45 V vs Ag/AgCl, and the oxidized material has a conductivity of 7.5 x 10(-2) S cm(-1). The EQCM study shows that, unlike the PPy/Cl- electrode for which the anions are the mobile species, cation transport is significant for the PPy-DMcT electrode during potential cycling although anions are also mobile to some extent. This has important consequence for lithium ion batteries application where lithium cation transport is required. The cyclic voltammetry and EQCM data of the PPy-DMcT electrode are consistent with a mechanism that involves a polymerization/depolymerization reaction of the DMcT species entrapped in the electrode matrix and whose species interact chemically with PPy.