Theoretical analysis of the pulse-clamp method as applied to neural stimulating electrodes

A mathematical model was developed to simulate potential pulse clamp experi ments at inert-electrode/aqueous solution interfaces in the absence of diox ygen or other adventitious redox active species. This model incorporates a potential invariant interfacial capacitor, a kinetically slow redox couple with parameters consistent with the H2O/H-2 reaction on polycrystalline Au in acid electrolytes as the only faradaic process involved, and diffusion a s the only mode of mass transport in solution phase. Numerical integration of the resulting system of differential equations was found to yield result s in good agreement with experimental data reported by Mortimer and co-work ers for Au in dearated sulfuric acid solutions. A detailed analysis of thes e calculations identified the fast and slow recoverable charges to be capac itive and the unrecoverable charges to be faradaic. The results obtained in dicated that for small overpotentials the charge is stored in the interfaci al capacitor, and that significant faradaic processes occur only when the o verpotential is large. Furthermore, during the delay, and despite the fact that no current flows through the external circuit, the capacitor discharge s via the faradaic reaction, increasing the total amount of product generat ed. More importantly, under the conditions selected for the simulations, no ne of the faradaic charge is recovered during the potential controlled stag e of the sequence. These results provide insight into the relationships bet ween stimulus parameters and charge injected into irreversible faradaic rea ctions, which may generate biologically harmful species. In general, as sti mulus pulse durations increase, unrecoverable charge increases. Also, as th e delay increases between the end of the primary and beginning of the secon dary pulse, unrecoverable. charge increases. Furthermore, based on the math ematical model used herein, the use of an electrode material with a small e xchange current density would allow greater overpotentials to be reached be fore the onset of significant faradaic reactions, and thus greater total ch arge injection prior to faradaic reactions. (C) 2001 The Electrochemical So ciety. All rights reserved.