Adsorption impedance Intermediates kinetics

Many reactions of industrial importance are electrocatalytic, i.e., they involve the specific adsorption of intermediates, for example hydrogen, chlorine, and oxygen evolution, oxygen reduction, and methanol or ethanol oxidation in fuel cells. Many different electrochemical techniques were used to study these reactions, and EIS is one of them, providing interesting kinetic and surface information. Certain model reactions will be presented in what follows with a detailed method of relating impedance parameters with mechanistic and kinetic equations. 

The above analysis shows that in the simple case of one adsorbed intermediate (according to Langmuirian adsorption), various complex plane plots may be obtained, depending on the relative values of the system parameters. These plots are described by various equivalent circuits, which are only the electrical representations of the interfacial phenomena. In fact, there are no real capacitances, inductances, or resistances in the circuit (faradaic process). These parameters originate from the behavior of the kinetic equations and are functions of the rate constants, transfer coefficients, potential, diffusion coefficients, concentrations, etc. In addition, all these parameters are highly nonlinear, that is, they depend on the electrode potential. It seems that the electrical representation of the faradaic impedance, however useful it may sound, is not necessary in the description of the system. The systen may be described in a simpler way directly by the equations describing impedances or admittances (see also Section IV). In... 

The kinetic parameter whose evolution governs the impedance is the concentration of the intermediate Bads in the adsorbed phase. If Q is the fraction of the electrode surface covered by this adsorbate and j3 is the superficial concentration for a complete coverage by Bads> by assuming a Langmuir adsorption isotherm, the surface concentration of Bads is cb = j30. 

The adsorption of reaction intermediates constitutes the most widely known cause of inductive phenomena in low-frequency impedance studies. Corresponding kinetic models represent a total reaction that combines multiple... 

---