Method of the Four-Electrode Electrochemical Cell for the Characterization of Concentrated Binary Electrolytes: Theory and Application

A novel method based on a four-electrode cell to determine the transport properties of concentrated binary electrolytes is presented. The cell contains two potential sensors in addition to the working and counter electrodes. The sensors measure the closed-circuit as well as the open-circuit potential in response to an input galvanostatic pulse across the working and counter electrodes. An important advantage of this new method is that it requires only the application of a single pulse in addition to the appropriate concentration cell experiments. By fitting a suitable model to the data obtained from these experiments, the three independent transport properties of a concentrated binary electrolyte and thermodynamic factor can be determined. The proposed technique benefits considerably from the measurement of closed-circuit data for estimation of the transference number. A comprehensive 2D axisymmetric model based on concentrated-solution theory is developed to account for faradaic convection as well as the bipolar effect at the surface of the sensors. It is shown that the bipolar effect has negligible impact on the potential measurements under operating conditions relevant to these experiments. Consequently, a simpler ID model can be used in place of the 2D model to estimate the transport properties without any loss in accuracy.