Dependence of the polarization resistance on the voltage sweep rate

The dependence of the response of an electrochemical system, represented in the vicinity of its mixed potential by an equivalent electric circuit, on the potential sweep rate is examined. The study, under steady-state conditions, is based on the use of Fourier's analysis of a periodic signal and the mesh method. Besides its simplicity, the main advantage of the use of Fourier's series is that it makes it possible to prove that in the case of slowly varying signals, when the transient disappears, current and voltage are in phase. The results obtained using a triangular wave as voltage and considering the first thirty terms of its cosine series have shown that too high potential sweep rates give rise to a marked hysteresis of the current-voltage characteristic, whose extent mostly depends on the capacitance and polarization resistance values. As the hysteresis is associated with the presence of negative values of the current supplied by the voltage generator, it is difficult to maintain that this electric representation describes the response of an electrochemical system to an external excitation correctly. Lastly, the determination of the resistance seen by the voltage generator using RMS current and voltage values shows that the hysteresis is more marked when the values of the apparent polarization resistance are very small.