Insight on the surface polarisation for non-blocking electrodes

A complete phenomenological description of the electrode polarisation and of the non-blocking character of electrodes is theoretically discussed. To do this, the role of the ions on the electrical response of an electrolytic cell submitted to an external field of small amplitude is investigated. We assume that the conduction current across the electrodes is limited by energy barriers which determine the activation energy of the corresponding electrochemical reactions responsible for the processes of charge transfer between the solution containing ions and the external circuit. This assumption implies that the boundary conditions for the conduction current on the electrodes contain two terms: one proportional to the surface electric field, as in the Ohmic model, and the other proportional to the surface variation of the bulk concentration of ions, as in the Chang-Jaffe model. We deduce, in the onemobile ion approximation, the expression for the electric impedance of the cell, in the shape of a slab, for mixed boundary conditions. Our expression contains, as particular cases, the Ohmic and Chang-Jaffe models. The equivalence between the general case considered in our analysis and previous models is discussed