The capacitance of the electric double layer of electrodes in molten salts

High exchange current electrodes are reversible electrodes with an exchange current density greater than 0.1 A cm(-2). Both in the time and frequency domains, their behaviour is predominantly inductive. Using the inductive model of high exchange current electrodes developed earlier, the double layer capacitance of a liquid Mg electrode in a melt composed of 20 mol% Of MgCl2 and 80 mol% of NaCl or KCl, is evaluated. As in aqueous electrolytes, the double layer capacitance is a sum of the compact and the diffuse layer capacitances. A model of the electric double layer of electrodes with high exchange current density is proposed, which allows for the calculation of the compact layer capacitance. It has been assumed in this calculation that the compact layer of the electric double layer is composed of a primary ionic shell, formed by specifically adsorbed chloride anions. The capacitance of such a layer is calculated as the Helmholtz capacitance, with the dielectric permittivity equal to 1. This capacitance is then corrected for the exchange current density. The corresponding amount of counterions to the specifically adsorbed anions in the primary ionic shell, is transferred to the neighbouring holes, creating the diffuse layer. The capacitance of the diffuse layer is thus calculated from the experimentally evaluated double layer capacitance and calculated by the present model compact layer capacitance. (C) 2002 Elsevier Science B.V. All rights reserved.