Electrochemically generated magnetic forces. Enhanced transport of a paramagnetic redox species in large, nonuniform magnetic fields

Electrochemically generated magnetic forces at a disk-shaped ultramicroelectrode have been investigated in large, nonuniform magnetic fields. Two sources of magnetic force are simultaneously operative in the electrochemical experiment, both having a significant influence on molecular transport of the electrochemical reactants and products. First, the magnetohydrodynamic (MHD) force, F-MHD, described by the Lorentz equation, arises from the diffusion of electrogenerated ions in the magnetic field. The magnitude of F-MHD is dependent upon the strength and orientation of the magnetic field. Second, the gradient magnetic force, F-del B, which is proportional to the gradient of the magnetic field, arises from electrogeneration of paramagnetic molecules in a nonuniform magnetic field. F-del B is dependent on the magnetic field strength, its spatial gradient, and the magnetic properties of the redox-active molecules. F-del B and P-MHD may be experimentally decoupled and investigated by variation of the field homogeneity and the electrode orientation. Specifically, F-MHD is negligibly small when the surface of the ultramicroelectrode is oriented perpendicular to the magnetic field, thus allowing F-del B to be investigated without interference from magnetohydrodynamic flows. Order-of-magnitude theoretical estimates of F-MHD and F-del B are correlated with voltammetric data for the electrochemical reduction of nitrobenzene at a 25-mu m-radius Pt microdisk electrode in a superconducting cryomagnet. Enhancements in the voltammetric limiting current as large as similar to 400% (B = 9.4 T, del B = 0 T/m) and similar to 100% (B = 6 T, del B similar to 75 T/m) are associated with F-MHD and F-del B respectively.