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 μm 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 H₂ and O₂ evolution for constant-amplitude pulses lied at 0.51 mC/cm₂ and 0.67 mC/cm₂, respectively, while for constant-width pulses they occurred at slightly lower values of 0.49 mC/cm₂ and 0.61 mC/cm₂, respectively. We could also extract values for the anodic and cathodic overvoltages associated with gas evolution. The cathodic overvoltage for H₂ evolution was 1.43 V for both constant-amplitude and constant-width pulses, while the anodic overpotentials for O₂ 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.