Direct Electrodeposition of Polypyrrole on Aluminum and Aluminum Alloy by Electron Transfer Mediation

The direct electrodeposition of electroactive conducting polymers on active metals such as iron and aluminum is complicated by the concomitant metal oxidation that occurs at the positive potentials required for polymer formation. In the case of aluminum and its alloys, the oxide layer that forms is an insulator that blocks electron transfer and impedes polymer formation and deposition. As a result, only patchy nonuniform polymer films are obtained. Electron transfer mediation is a well-known technique for overcoming kinetic limitations of electron transfer at metal electrodes. In this work, we report the use of electron transfer mediation for the direct electrodeposition of polypyrrole onto aluminum and onto Al 2024-T3 alloy. This report focuses on the use of Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt) as the mediator, although catechol appears to function in a similar manner. Depositions were carried out under galvanostatic conditions at current densities of 1 mA/cm2. The mediator reduced the deposition potential by nearly 500 mV compared to deposition performed in the absence of mediator (where Tiron was replaced by p-toluene sulfonic acid sodium salt). Polypyrrole formation and deposition appears to occur with 100% current efficiency and uniform films are obtained. Results of the characterization of these films by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, conductivity measurements, and adhesion measurements are presented.