The difficulty of the kinetics determination for very fast electrochemical reaction by using impedance spectroscopy (EIS) is related to the fact that the charge transfer resistance for this process is much smaller than the solution resistance. Moreover, for very fast reactions only a low frequency part of the spectra can be observed when potentiostat is coupled with frequency response analyzer in a classical three electrode arrangement. In such case, the high frequency part of the spectra is very often entirely covered by artifacts related to a relatively low time constant of the potentiostat. A special experimental setup is needed for such experiments, in which impedance is measured in stand-alone mode of the frequency response analyzer. The solution resistance can be minimized by using twin electrode arrangement, in which two identical electrodes are facing each other [1]. However, at high frequency part of the impedance spectra a distinct frequency dispersion occurs due to an unequal and frequency dependent current distribution. Therefore a special attention has to be devoted to control the geometry of the electrode arrangement. Unequal current distribution can be eliminated by using recessed electrodes [2]. However, even for recessed electrodes, a distinct frequency dispersion can be observed at kHz region. This effect is more pronounced in the presence of specific adsorption [3]. To date its origin has not been fully understood.  

In this study high frequency impedance measurements (up to 1 MHz) have been carried out in a twin electrode and hanging meniscus configurations. Recessed electrodes have been used for the first time to study the kinetics of hydrogen electrosorption at Pt(111) at high frequency range. Hydrogen absorption in palladium thin layers (50 nm) covered by platinum monolayers (1ML/Pd) have also been investigated as model systems involving diffusion processes related to hydrogen absorption. In the 1ML/Pd system, the hydrogen insertion is particularly fast [4], the resistance of the charge transfer, related to this process, is in the order of 0.02 Ωcm². For these model electrochemical processes the frequency dispersion at high frequency rage will be discussed.

Literature:

[1] B. Losiewicz, R. Jurczakowski, A. Lasia, Electrochim. Acta, 80 (2012) 292-301

[2] Frateur, I.; Huang, V. M. W.; Orazem, M. E.; Pebere, N.; Tribolletd, B.; Vivier, V., Electrochim. Acta 53 (2008) 7386-7395.

[3] T. Pajkossy, Z. Phys. Chem., 226 (2012) 935-943

[3] P. Połczyński, R. Jurczakowski, J. Power Sources, 2016 in press