A new experimental approach to the time-constants of electrochemical impedance: frequency response of the double layer capacitance

Modeling of electrochemical impedance is, generally, realized by calculatin g the faradaic impedance on the basis of the reaction mechanism involving a dsorbed reaction intermediates, then connecting it in parallel to a constan t interfacial capacitance. If this procedure is largely used, one may consi der it arbitrary, and it is essential to verify whether the double layer ca pacitance is actually independent or not of dc and ac currents. In order to measure this coupling, an original experimental setup was devised that all owed measurement of the variation of double layer capacitance induced by a low frequency ac perturbing signal used for the electrode impedance measure ments Z(omega). The latter was measured between 100 Hz and 100 mu Hz as a c lassical way. The modulation of the double layer capacitance DeltaC(omega)/ DeltaE at 1 kHz was measured simultaneously by coupling a lock-in amplifier to a classical experimental setup. The electrochemical interface examined was a rotating disk of iron in 1 M Na2SO4 acidified at pH 4. A close correl ation between Z(omega) and DeltaC(omega)/DeltaE was found. This is, in our knowledge the first investigation of the modulation of the double layer cap acitance together with the impedance measurements. As for the very origin o f the relaxation time constant of the double layer capacitance, more detail ed experiments are necessary. (C) 2001 Published by Elsevier Science Ltd.