Mass transfer effects on the electropolymerization current efficiency of 3-methylthiophene in the magnetic field

3-Methylthiophene was chosen as a representative conducting polymer precursor, whose electropolymerization proceeds through the coupling of cation radicals. The density of the solution that contains electrogenerated 3-methylthienyl radicals and early oligomers is higher than the density of the surrounding solution, and at low (< 0.2 M) monomer concentrations the diffusion layer falls, compromising the electropolymerization current efficiency. Applying a homogeneous magnetic field (3.0 T) perpendicular to the electrode surface (theta= 0 degrees) produces concentration gradient paramagnetic forces (F-del C) that hold the diffusion layer in contact with the electrode. This gives time for more oligomers to get oxidized and for more cation radicals to couple so that the current efficiency increases from 0.028 to 0.037 at 0.05 M of monomer concentration, and from 0.051 to 0.071 at 0.1 M. With the magnetic field parallel to the electrode surface (theta= 90 degrees) Lorenz forces causing magneto-hydrodynamic convection, in combination with F-del C forces keeping the flow pattern in contact with the electrode, increase the current efficiency even more, to 0.048 at 0.05 M of monomer concentration, and to 0.076 at 0.1 M. At higher monomer concentrations (> 0.2 M), the rate of radical coupling is evidently fast enough so that, even in the absence of a magnetic field, no natural convection effects are observed and the current efficiency ( 0.7 - 0.8) is not affected by the magnetic field.