Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Lorentz forces are in almost all cases an inevitable consequence, when electrochemical reactions are performed under the influence of a magnetic field. The reason for this fact is that it is quite difficult to actually guarantee parallelism of magnetic and electric fields everywhere in the electrochemical cell.
The current contribution focuses on cases which are essentially axial symmetric. While the main parts of the electric and magnetic fields are parallel to the axis, an azimuthal Lorentz force is generated by radial components of either the electric or the magnetic field. The Lorentz force acting in circumferential direction drives primarily azimuthal flows. However, pressure differences due to these primary flows as well as the nonuniform Lorentz force density distribution itself give rise to secondary flows which, together with the primary flow, can lead to complex and sometimes unexpected flow patterns. The matter is complicated even more by the action of buoyancy originating from the density changes of the electrolyte solution due to the electrode reactions. Since the flow, i.e. the momentum transfer, determines mass transfer to a good extend electrochemical reactions under mass transfer control are usually influenced by magnetic fields. This fact has been known for a long time and is often referred to as “MHD–effect” in the electrochemical literature. However, often the seeming simplicity suggested by this term is misleading since, as denoted above, the Lorentz force driven flow is frequently rich in features [1].
We use particle image velocimetry (PIV) as well as synthetic schlieren, i.e. background oriented schlieren (BOS), to study velocity and concentration gradient fields in electrochemical cells. On the basis of these measurements, the flow in the cells and its consequences for the concentration distributions and the reactions are discussed. Examples include the retainment of buoyant electrolyte near circular electrodes as described in [2], the reversal of the secondary flow direction depending on the electrode radius [3] and the interplay of gravity and Lorentz forces in cylindrical cells with horizontal electrodes.
References
[1] Mutschke G, Cierpka C, Weier T, Eckert K, Mühlenhoff S and Bund A. ECS Transactions 13, 16, 9 (2008).
[2] Weier T, Eckert K, M¨uhlenhoff S, Cierpka C, Bund A and Uhlemann M. Electrochem Comm 9, 2479 (2007).
[3] Cierpka C, Weier T, Gerbeth G, Uhlemann M and Eckert K. J Solid State Electrochem 11, 687 (2007).