ELECTROCATALYTIC DEPOSITION OF POLYPYRROLE IN THE PRESENCE OF BROMIDE

Bromide ions (typically c = 1 mM) are oxidized in acetonitrile electrolytes at platinum anodes in two steps, well separated by about 300 mV, leading to tribromoanions 1 and to bromine 2, respectively. These basic electrochemical reactions interfere quite differently with pyrrole 3, typically c = 0.1-0.01 M, added to the system. 1 oxidizes 3 to bromopyrroles, indicated by an amplification of the limiting current density of the first wave by a factor of three. No polypyrrole is deposited. In the course of the second wave, 2 oxidizes 3 to the radical cation 4. The rate determining step in the formation of polypyrrole is a dimerization of 4 moieties, and the strong electrostatic shielding effect via complexation of 1 with 4 leads to a catalytic enhancement of the polymerization rate by more than a factor of 10 with respect to conventional systems. Regeneration of 1 in the course of these processes gives a large current amplification in the second wave. The overall reaction leads to polypyrrole, which forms an electrode coating. The material has improved properties, for example electronic conductivities above 100 S cm-1. The current efficiency is nearly 100%, but only if the convection is weak. This 'negative stirring effect' is discussed in detail.