Electrocatalytic deposition of polypyrrole in the presence of bromide

Bromide ions (typicallyc=1 mM) are oxidized in acetonitrile electrolytes at platinum anodes in two steps, well separated by about 300 mV, leading to tribromoanions1 and to bromine2, respectively. These basic electrochemical reactions interfere quite differently with pyrrole3, typicallyc=0.1-0.01 M, added to the system.1 oxidizes3 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 oxidizes3 to the radical cation4. The rate determining step in the formation of polypyrrole is a dimerization of4 moieties, and the strong electrostatic shielding effect via complexation of1 with4 leads to a catalytic enhancement of the polymerization rate by more than a factor of 10 with respect to conventional systems. Regeneration of1 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.