EIS study of hydrogen insertion under restricted diffusion conditions: I. Two-step insertion reaction

The theoretical expressions for the Faradaic impedance and electrode impedance are derived for indirect (two-step) insertion of hydrogen in metals and alloys, irrespective of the mass transport conditions in the electrode. Next, impermeable boundary (restricted diffusion) conditions are considered. The determination of the hydrogen concentration, diffusion coefficient and kinetic parameters from EIS data is discussed. The thermodynamic and kinetic conditions for which it is possible to distinguish between the direct (one-step) insertion mechanism and the indirect (two-step) mechanism on the basis of EIS data are analysed. Previous results in the electrochemical literature are discussed in relation to the theoretical derivation in this paper.