Electrochemical methods impedance

D. Electrochemical Impedance Methods for Determination of Polarization Resistance... 

The complications and sources of error associated with the polarization resistance method are more readily explained and understood after introducing electrical equivalent circuit parameters to represent and simulate the corroding electrochemical interface (1,16-20). The impedance method is a straightforward approach for analyzing such a circuit. The electrochemical impedance method is conducted in the frequency domain. However, insight is provided into complications with time domain methods given the duality of frequency and time domain phenomena. The simplest form of such a model is shown in Fig. 3a. The three parameters (Rp, Rs, and C d,) that approximate a corroding electrochemical inter-... 

Using the electrochemical impedance method, Freire et al. [9] studied the effect of Nafion membranes with different thicknesses under different operating conditions. Their fuel cells had an active area of 1 cm2 and were operated with H2/02 at atmospheric pressure. Four different Nafion membranes, 117, 115, 1135, and 112, with average thicknesses of 175, 125, 80, and 50 pm, respectively, were used in their study. The impedance measurements were carried out using a Solartron 1250 frequency analyzer with a Solartron 1286 electrochemical interface in the potentiostatic mode. The input signal had an amplitude of 10 mV. 

Once Rp is determined, icorr is evaluated with the polarization-resistance (or Stem-Geary) equation, Eq 6.28. The two-electrode method is a relatively simple and fast method for evaluating when compared with the standard polarization-resistance and electrochemical-impedance methods. 

Wittmann, M.W., R.B. Leggat, and S.R. Taylor. 1999. The detection and mapping of defects in organic coatings using local electrochemical impedance methods. J Electrochem Soc 146 (11) 4071. 

Despite these common features the techniques have been developed in specialized research fields that seldom cross. For brevity, this chapter will only describe the electrochemical impedance method in detail, with more cursory reference or other related techniques. Since the chapter is intended for novices in the field, theoretical aspects are kept to a minimum. Some practical hints for experimental work will be included. 

Another methodology consists in applying small amplitude perturbations where the steps or sweeps are sufficiently small to yield a Unear current-potential relationship (Figure 11.15). This approach is at the heart of electrochemical impedance methods. 

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