Electrical conductivity of magnetically stabilized fluidized-bed electrodes - Chronoamperometric and impedance studies

Fluidized-bed electrodes could offer an interesting way to increase the electrode surface area applicable in electrochemical processes when the problem of poor electrical contact within the particle bed could be overcome. We recently demonstrated, that the contacting can be improved by the use of magnetizable electrode particles and the superposition of a magnetic field. However, details of the magnetic influence on the charge transport are still mostly unknown. In this work, we investigate the electrodynamics of a fluidized bed electrode with and without the superposition of a magnetic field by means of chronoamperometry and electrochemical impedance spectroscopy (EIS). In the chronoamperometric studies two types of charge transfer mechanism can be distinguished by the slope of the resistance increase with increasing distance between the electrodes. In close proximity to the electrodes direct conductive charge transfer along statistically formed particle chains dominates. Because the probability of uninterrupted particle chains quickly diminishes with increasing length, above a certain distance of approx. 6 mm a second, so-called convective, charge transfer mechanism dominates. This mechanism is based on the transfer of electrons between colliding fluidized particles and corresponds with a substantially higher specific resistance. The conductive charge transfer mechanism can be enhanced by up to a factor of four applying a superimposed magnetic field, while the second mechanism shows only a weak field dependence. The presented equivalent circuit model and the magnetic field dependency of its parameters contribute to a deeper understanding of the novel magnetically stabilized fluidized bed electrode and demonstrate the usefulness of EIS measurements for the prediction of the effectiveness of a particle based electrochemical reactor.