| The electrochemical processes occurring on gold in sulfuric acid under pulsed conditions that are typical of neural stimulation were studied in the absence and presence of oxygen using the pulse-clamp technique. As the electrode was pulsed cathodically from +750 mV RHE, using 50 μC/cm2 of injected charge; (i) the initial 10 to 15 μC/cm2 of injected charge was reversibly stored by the double layer capacitance, resulting in an electrode potential excursion to the range of +150 to −110 mV, (ii) the next 20 to 30 μC/cm2 of injected charge was then accommodated by double layer charging plus an undetermined process consuming approximately 1 μC/cm2, likely due to an adsorbed reactant, resulting in a potential excursion to approximately −300 mV, the potential where irreversible water reduction began to occur, (iii) when the potential reached approximately −540 mV, all further injected charge went into water reduction. In electrolyte equilibrated to 100% oxygen, the unrecoverable charge was similar to the deoxygenated case, but the electrode potential was less negative than in the deoxygenated case. Oxygen reduction products appear to have been reversible at +750 mV during a 100 μsec current pulse. If an open-circuit period was introduced between the pulse and application of the reversal potential, unrecoverable charge accumulated more rapidly in the presence of oxygen than in its absence. If the starting and ending clamp potential was increased to +1.1 V, in the oxygen-saturated electrolyte all charge was recoverable during the pulse or up to 19.9 msec following the pulse.; These results indicate that during biphasic pulsing, the secondary anodic phase discharges the double layer capacitance through the stimulator circuitry rather than allowing faradaic reactions to discharge the capacitance. The anodic phase terminates reduction processes that otherwise occur during the interpulse interval of monophasic pulsing. Further, if the electrode potential is clamped to a sufficiently positive value, e.g. 1.1 V, reduced species generated during the stimulus phase may be re-oxidized. These results suggest that for maximum safety, biphasic pulses are important, but the pulse need not be charge balanced. Further, for maximum performance, electrode potentials should be controlled during the stimulation process. |
