Discussion of the potential step method for the determination of the diffusion coefficients of guest species in host materials: Part I. Influence of charge transfer kinetics and ohmic potential drop
Résumé
Theoretical expressions are given for the output response of ion-insertion electrodes to a potential step assuming linear diffusion, restricted (blocking) diffusion conditions and possible limitations by insertion reaction kinetics. The effects of ohmic potential drop are also investigated. It is shown that slow interfacial charge transfer cannot be distinguished from ohmic drop effects, in contrast to impedance diagrams where ohmic drop and charge transfer effects can be separated. The influence of potential step amplitude is discussed. Chronocoulometric analysis is dealt with considering diffusion controlled processes as well as mixed control conditions. The error in the determination of the chemical diffusion coefficient of a guest species from chronoamperometric data, when using the limiting Cottrell equation in the short-time range or the exponential decay of current in the long-time domain, is evaluated in relation to insertion reaction kinetics and ohmic potential drop. Determination of the diffusion coefficients by curve fitting is also envisaged using the current versus time and charge versus time relationships. Finally, previous results in the electrochemical literature are discussed in the light of the theoretical derivations proposed in this paper.