Analysis of diffuse-layer effects on time-dependent interfacial kinetics

We investigate the subtle effects of the diffuse charged layer on interfaci al kinetics by solving the governing equations for ion transport (Nernst-Pl anck) with realistic boundary conditions representing reaction kinetics (Bu tler-Volmer) and compact-layer capacitance (Stern) in the asymptotic limit epsilon = lambda (D)/L-->0, where lambda (D) is the Debye screening length and L is the distance between the working and counter electrodes. Using the methods of singular perturbation theory, we derive the leading-order stead y-state response to a nonzero applied current in the case of the oxidation of a neutral species into cations, without any supporting electrolyte. In c ertain parameter regimes, the theory predicts a reaction-limited current sm aller than the classical diffusion-limited current; this over potential eff ect is not due to ohmic drop effects in the bulk of the cell but rather to antagonist processes involved in the surface charge transfer and diffuse la yer charging respectively. We demonstrate that the charging of diffuse char ge, since it is intimately coupled to the surface reaction and cannot be co nsidered independently, plays a fundamental role in nonequilibrium surface reactions when the transport of one of the reacting species is coupled to t he total interfacial response of the compact and diffuse layers. (C) 2001 E lsevier Science B.V. All rights reserved.