Fundamental principles of electrochemical ohmic heating of solutions

Ohmic drop and capacitive current, two major nuisances in electrochemical experiments, can be used to great advantage for heating solutions homogeneously within their bulk without significant electrolysis or corrosion of electrodes. This is achieved by submitting a two-electrode cell to an alternating potential with a sufficiently large frequency, so that only minimal charging of the cell double layers is possible. The potential difference across each double layer is thus maintained between the threshold values corresponding to the onset of anodic and cathodic Faradaic reactions, although potentials with the large peak-to-peak amplitudes required for a large heating power are applied to the cell. A simple analysis based on an ideal RC cell is adequate for delineating the fundamental principle of ohmic heating and for identifying the gross parameters controlling the system. Within the framework of this first order approximation, classical voltammetry provides a facile and rapid examination of various electrode materials so as to select the best electrodes for a given application. A method is then proposed to ensure in situ that a cell is operated safely. This is exemplified in the case of a NaCl solution and DSA electrodes. The same method affords all parameters required to elaborate an equivalent electrical circuit which adequately describes the cell under the conditions of its operation, since this is required for the final optimization of the process, viz., selection of the nature and frequency of the alternative potential input, as well as of the applied current density. (C) 1998 Elsevier Science S.A. All rights reserved.