A note on overpotential dependence of AC impedance data

Charge-transfer resistance [R-ct = (d eta/di)(eta=0)] and Tafel plots of cu rrent density (i) versus overpotential (eta) data are generally known to yi eld values of the energy-transfer coeffficient (alpha) and exchange current density (i(o)) of an electrochemical reaction. In the present investigatio n, the resistance (d eta/di)(eta not equal 0) that could be calculated by d ifferentiating 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) agains t eta. Since alpha and i(o) could also be evaluated directly from the exper imental DC polarization data, the procedure was not of significant importan ce.. 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 elect rode prepared from fine metal powder was used in 1 M KOH electrolyte and th e hydrogen evolution reaction was studied by AC impedance at several potent ials. The resistance values measured from the complex plane impedance diagr am were plotted against the potential, and the values of alpha and i(o) wer e evaluated.