Electrochemical Mechanism Insights from Mott-Schottky Measurements: Application to Europium in Molten Oxide

Abstract

Electrochemical impedance measurements are commonly conducted at a constant direct current (DC) potential while sweeping the frequency of the perturbation. The mainstream plots used to discuss data are then Nyquist and Bode plots, on which fitting curves for the postulated equivalent circuit are represented. Unfortunately for high temperature electrochemists interested in faradaic reactions in molten electrolytes with a high concentration of electroactive species [1] or a very high conductivity [2], this method rarely provides meaningful results due to data that are often noisy and difficult to interpret at potentials away from the open-circuit potential (OCP). Methods employing a dynamic sweep of the DC potential at a fixed frequency of perturbation experimentally provide more reliable insight into electrochemical impedance phenomena in molten electrolytes. The archetypical example is alternating current (AC) voltammetry in its various forms, such as sine- or square-wave voltammetry [3][4]. However, most modern potentiostats offer only one means of conducting DC potential sweep impedance measurements, the so-called Mott-Schottky (MS) method. This method is not truly dynamic from a DC potential perspective, and is traditionally used for capacitance measurements and characterization of semi-conductor electrodes. The present work shows a procedure to extract faradaic data from MS data, inspired from the background subtraction method previously proposed for AC-polarography [5]. Nyquist-type representations are derived from the MS data, enabling a connection to mainstream impedance analysis methods. An example of this procedure, an investigation of the electrochemistry of europium ions in molten oxide, is provided. [1] A. Kisza, Polish J. Chem., 67 [1993] 885-894 [2] B. Savova-Stoynov and Z.B. Stoynov, Journal of Applied Electrochemistry, 17 [1987] 1150-1158 [3] A.M. Bond et al., Anal. Chem., 77 (9) [2005] 186-195 [4] C. Montel, C. Russel and E. Freude E, Glastech Ber, 61, [1988], 59–63 [5] A.M. Bond et al., J. Electroanal. Chem., 222 (1) [1987] 35-44.