One layer, two layers, etc. An introduction to the EIS study of multilayer electrodes. Part 1: Theory

In this theoretical EIS study of the redox reaction Fe3+ + e− ↔ Fe2+ occurring at multilayer electrodes, four points are dealt with in detail: firstly, a recursive formulation allowing the exact calculation of the impedance whatever the number of layers is given (‘Russian dolls’ behaviour of the mass-transport functions). Secondly, the study of the two bilayer electrodes Pt/layer 1/layer 2 and Pt/layer 2/layer 1 shows that the layer order drastically influences the electrochemical response of the system, and proves that a multilayer electrode cannot be simply treated as a monolayer electrode. Thirdly, it is shown that all the key parameters (thicknesses, diffusion coefficients, and partition coefficients) cannot be only determined by EIS, another combined physicochemical technique being indispensable. Fourthly, an exhaustive study of bilayer electrodes in quiescent solution is made and all the possible impedance diagram shapes are plotted.