How does the solution resistance influence the electrochemical behavior of porous electrodes?

Abstract

In our recent papers simplified models of the porous electrodes in the presence of the electroactive species were presented. In this work, these models are compared with the full model, which considers dc concentration and potential gradients in the pore and the ac effect by division of the pore length into small sections and addition of the solution resistance to the electrode impedance of each section. Simulations for different porosity, concentration, kinetics, and solution resistivity were carried out and allowed to estimate the influence of these factors on the observed complex plane impedance plots. Presence of the dc gradients changes the size of the two semicircles while the ac solution resistance causes formation of the high-frequency straight line at 45° on the complex plane plots. Porosity effects are larger when the total impedance is smaller, that is for higher currents and concentrations of electroactive species and faster kinetics. Estimation of the porosity parameter (Thiele modulus), ${\Phi }_0$, and the parameter v, which relates concentration and potential in pores, allows for the estimation of the porosity effects on the impedance.

In the absence of the concentration gradient (i.e.potential gradient only present), a straight line at 45° followed by one skewed semicircle is observed. Conditions for obtaining such impedances are discussed.