Electrochemically Generated Magnetic Forces. Enhanced Transport of a Paramagnetic Redox Species in Large, Nonuniform Magnetic Fields

Abstract

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 ∇ B, which is proportional to the gradient of the magnetic field, arises from electrogeneration of paramagnetic molecules in a nonuniform magnetic field. F ∇ B is dependent on the magnetic field strength, its spatial gradient, and the magnetic properties of the redox-active molecules. F ∇ B and F 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 ∇ B to be investigated without interference from magnetohydrodynamic flows. Order-of-magnitude theoretical estimates of F MHD and F ∇ B are correlated with voltammetric data for the electrochemical reduction of nitrobenzene at a 25-μm-radius Pt microdisk electrode in a superconducting cryomagnet. Enhancements in the voltammetric limiting current as large as ∼400% (B = 9.4 T, ∇B = 0 T/m) and ∼100% (B = 6 T, ∇B ∼ 75 T/m) are associated with F MHD and F ∇ B, respectively.

Non-monotonic influence of a magnetic field on the electrochemical behavior of Fe78Si9B13 glassy alloy in NaOH and NaCl solutions

“…Under the magnetic field, Fe 3+ and Cl − ions are aggregated on the electrode; this aggregation can cause an increase in the viscosity of the electrolyte, and a maximum electrogeneration rate of molecules is possibly surpassed according to the arguments of Ragsdale et al [39]. Hence, the current density i Z2 decreases under the 0.02-T magnetic field, which is consistent with the variation of i P2 in NaOH solutions.…”

“…6). This result indicates that chloride ions in the low-concentration NaCl solu-tions pass the oxidation film more easily in the presence of a magnetic field, which can be explained by the MHD effect [39] and by the arguments of Lu et al [22]. In 0.05 mol/L NaCl solution, the tubers on the corroded glassy alloy surface become stepped pits under the magnetic field, the size of which are larger than those formed in the absence of a magnetic field (as shown in Fig.…”

Magnetic field effects on electric behavior of [Fe(CN)6]3−at bare and membrane-coated electrodes

“…Aogaki et al studied the MHD effect quantitatively by analyzing the flow of electrolyte solutions between electrodes [16]. The increase in electrolytic current under a steady magnetic field, however, arises from not only MHD effects but also magneto-convection, which may arise from the concentration gradient of the magnetic susceptibility under an inhomogeneous magnetic field or magnetic field gradient [17][18][19][20][21]. The magneto-convection affects the diffusion processes of redox species when an inhomogeneous magnetic field is applied, even when it is parallel to the electrolytic current.…”

Electrochemical Detection of Magnetic Convection: Effect of High Magnetic Fields on O2/O2− Electrode Reaction in Acetonitrile

“…As an alternative effect of magnetic fields on solutions, we can consider a magnetic force which is produced by the magnetic susceptibility of a substance and the gradient of magnetic field. Even in electrochemical fields, this effect of the magnetic susceptibility has become of interest in recent years [5,6]. The effect of a magnetic field on the dissolution process of paramagnetic oxygen into solutions is one example of such as magnetic force.…”