Electrosynthesis of Iron, Cobalt, and Zinc Microcrystals and Magnetic Enhancement of the Oxygen Reduction Reaction

Sparse metallic crystals of iron, cobalt, and zinc are produced by electrodeposition on polyaniline (PANI) coated gold electrodes. The crystal habit is determined by the composition of the electrolyte. Thin hexagonal crystallites of cobalt or zinc can grow with the basal plane parallel to the substrate, but acetic acid strongly affects the growth of the cobalt deposits, as it changes the crystal habit from hexagonal to hemispherical. Iron is electrodeposited on PANI in the form of submicrometer cubes in the presence of acetic acid, while in its absence micrometer-size hemispheres are produced. The magnetization curves for Fe and Co are correlated with the observed textures. Sparse heterogeneously structured surfaces with ∼2 μm hemispherical metal grains of the three metals were then used as electrodes to investigate the magnetic field effect on the oxygen reduction reaction. For zinc, the effect of a 360 mT applied magnetic field was a slight increase (3%) in the maximum oxygen reduction current, which is attributed to magnetohydrodynamic (MHD) flows produced by the interaction of the external magnetic field and the current distribution near the surface of the grains via the Lorentz force. Significantly larger increases, ∼12% and ∼8% for iron and cobalt, respectively, are attributed to enhanced convection in the vicinity of the ferromagnetic grains due to the magnetic field gradient force