Impedance measurements quasi-reference electrode

Fig. 3.15 Schematic diagram of a frequency response analyzer (FRA), showing the proper connections to the potentiostat for impedance spectroscopy and Mott-Schottky measurements. The function of the quasi-reference electrode is explained in Sect. 3.6.5...

A quasi-reference electrode (QRE) is a general phrase for any poorly defined or unpoised RE. A QRE typically consists of an inert metal wire such as platinum or gold. A silver wire can also be used, so long as silver ions do not interfere with the measurements. The potential should ideally be steady, but the actual potential is not predictable if it is unpoised. Some methods to circumvent the ill-defined potential are to compare the potential to a conventional RE, or to include an electrochemical couple as an internal standard (see Section 4.6.2). A QRE is a low-impedance RE, but is also a polarizable electrode. 

Because most applications of (photo)elec-trochemical systems involve the transfer of electrons across an interface (Sect. 2.1.1), current density-potential techniques are commonly used in (photo)electrochemis-try. In this case, the difference in electrochemical potential of electrons across the interface of interest (accessible via the working electrode - reference electrode potential difference) and the current density through this interface are used as the perturbation and the response (or vice versa). Two approaches can be distinguished. When (quasi) steady state signals are used, one speaks of current density versus potential measurements whereas harmonically modulated signals, superimposed on a bias, are involved in electrochemical impedance spectroscopy (EIS). We introduce these two approaches on the basis of the kinetics of the simple system shown in Fig. 1. 

Measuring DMFC cathode impedance is difficult for two reasons. First, it is hard to achieve tme steady-state DMFC operation. Typically, the cell potential slightly varies with time (drift). Another problem is that in a cell without a reference electrode, the cathode impedance cannot be measured directly. Usually, the impedance of the whole cell is measured first. The oxygen on the cathode is then replaced by hydrogen, and the impedance of this quasi-half cell (anode impedance) is subtracted from the whole-cell curve to obtain the cathode spectrum. 

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