Electrochemistry of gold in aqueous sulfuric acid solutions under neural stimulation conditions

The electrochemistry of gold in 50 mM aqueous sulfuric acid solutions under conditions typical of neural stimulation (cathodic-first current pulsing at 50 Hz) was studied in oxygen-free and oxygenated solutions. As the electrode was pulsed from +0.75 V vs. a reference hydrogen electrode using a 100 μs, -500 mA/cm² pulse in oxygen-free solutions, (i) the initial 10 μC/cm² of injected charge was stored in the double layer, (ii) the next 20-30 μC/cm² was accommodated by double-layer charging plus an undetermined process consuming about 1.8 μC/cm², and (iii) when the electrode potential reached approximately -0.52 V, water reduction consumed all further charge. In oxygen-saturated solution, the unrecoverable charge (associated with irreversible reactions) was not significantly different from the deaerated situation throughout the pulse period, but the electrode potential was less negative, consistent with oxygen reduction. Oxygen reduction appears reversible during a 100 μs pulse, provided the electrode is clamped back to +0.75 V immediately after the pulse. If an open-circuit period was introduced between the current pulse and application of the reversal potential, unrecoverable charge accumulated more rapidly in oxygen-saturated solution, suggesting that processes occurring during the interpulse interval of neural stimulation may produce potentially damaging reaction products.