A note on overpotential dependence of AC impedance data

Charge-transfer resistance [R-ct = (d eta/di)(eta=0)] and Tafel plots of current density (i) versus overpotential (eta) data are generally known to yield values of the energy-transfer coeffficient (alpha) and exchange current density (i(o)) of an electrochemical reaction. In the present investigation, the resistance (d eta/di)(eta not equal 0) that could be calculated by differentiating a wide range of i - eta curves was also shown to provide the values of alpha and i(o), by plotting ln(d eta/di)(eta not equal 0) against eta. Since alpha and i(o) could also be evaluated directly from the experimental DC polarization data, the procedure was not of significant importance.. Nevertheless, it was considered important in evaluating alpha and i(o) from AC impedance data, because the procedure was based on data analysis, which was much simpler than that reported in the literature. A cobalt electrode prepared from fine metal powder was used in 1 M KOH electrolyte and the hydrogen evolution reaction was studied by AC impedance at several potentials. The resistance values measured from the complex plane impedance diagram were plotted against the potential, and the values of alpha and i(o) were evaluated.