Using reciprocal derivative chronopotentiometry as a technique to determine safe charge injection limits of electrodes used for neural stimulation.

We used reciprocal derivative chronopotentiometry (RDC) with platinum electrodes of 50 microm diameter in 0.15 M phosphate buffered saline solution to identify the various electrochemical processes occurring at the electrode during biphasic current pulsing. RDC allowed to determine the limits of water hydrolysis based on the specific (dt/dE)-E data representation employed in this technique resulting in curves similar to the voltammetric i-E response. Current stimulation was performed by either varying the pulse amplitude or pulse width. We found that the limits for H(2) and O(2) evolution for constant-amplitude pulses lied at 0.51 mC/cm(2) and 0.67 mC/cm(2), respectively, while for constant-width pulses they occurred at slightly lower values of 0.49 mC/cm(2) and 0.61 mC/cm(2), respectively. We could also extract values for the anodic and cathodic overvoltages associated with gas evolution. The cathodic overvoltage for H(2) evolution was 1.43 V for both constant-amplitude and constant-width pulses, while the anodic overpotentials for O(2) evolution were 2.45 V in the first and 2.24 V in the latter case. These values are clearly larger than the gas evolution limits generally found with steady-state voltammetry.