Hydrogen insertion reaction with restricted diffusion .1. Potential step-EIS theory and review for the direct insertion mechanism

Electrochemical impedance spectroscopy (EIS) theory is presented for the direct hydrogen insertion reaction with restricted diffusion. The limiting shapes of the Nyquist impedance plots are given and the changes in the impedance diagrams due to the double-layer capacitance are presented. The responses to small potential steps are also considered. All the limiting expressions for the current transients are given and a kinetic zone diagram is plotted in the time domain to determine the conditions of validity of these expressions. The correspondences between EIS diagrams and current transients due to small potential steps are shown. The results obtained for small potential steps are finally generalized to large steps when a Langmuir insertion isotherm is considered and the hydrogen diffusion coefficient is assumed to be constant.