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 t he direct hydrogen insertion reaction with restricted diffusion. The l imiting shapes of the Nyquist impedance plots are given and the change s in the impedance diagrams due to the double-layer capacitance are pr esented. The responses to small potential steps are also considered. A ll the limiting expressions for the current transients are given and a kinetic zone diagram is plotted in the time domain to determine the c onditions of validity of these expressions. The correspondences betwee n EIS diagrams and current transients due to small potential steps are shown. The results obtained for small potential steps are finally gen eralized to large steps when a Langmuir insertion isotherm is consider ed and the hydrogen diffusion coefficient is assumed to be constant.