Adsorption impedance Pseudocapacitance

The dependence of the impedance associated with on frequency can be visualized with the aid of the equivalent circuit shown in Fig. 121(b). Here the adsorption pseudocapacitance is part of a more complete circuit, showing the other circuit elements we have already... 

The difference in behavior of pyrolytic graphite, examined in Ref. 2, and the glassy carbon investigated in this more recent work, is probably done to the different electrocatalytic surfaces that these materials present. Thus, impedance measurements we have made indicate little adsorption pseudocapacitance for adsorbed Br species at glassy carbon whereas, at pyrolytic graphite, an adsorption capacitance is measurable. The results depend on edge or basal-plane exposure. 

In a paper by Kemer and Pajkossy [2002], anion adsorption rates have been measured by impedance spectroscopy at Au (111), for SO4, Cr, Br and T. Cr ion adsorption is very fast, so the equivalent circuit for the process not only has to include a Faradaic pseudocapacitance and its corresponding reaction resistance, but also a Warburg element for the anion diffusion. The interfacial capacitance is then plotted in the Nyquist plane as real vs. imaginary capacitance components, C and... 

The sorption impedance depends on charge associated with specific adsorption and/or desorption of charged species at the interface, primarily in the compact portion of the double layer. Hence the adsorption process results in an additional "pseudocapacitive effect" that can be broadly presented as ADs ADsoHP ADsmp " hh and being adsorption capaci-... 

In another study of DMFC anodes, shovm in Figure 16.10, the complex-plane impedance plots were studied as a function of the current density applied. The diameters of the semicircles were found to decrease with increasing current density, as expected, but the new feature observed is an inductive branch of the curves. This can be modeled, of course, by adding an inductive element to the equivalent circuit representation, in series with the Faradaic resistance, but the physical origin of this added circuit element is still open for debate. There is a tendency to associate it with sluggish adsorption of CO, formed as an intermediate in the oxidation of methanol. However, unlike the adsorption pseudocapacitance, which is well understood (cf Section 11.2), there is no theory for the dependence of the pseudoinductance on potential, coverage or any other measured parameter. 

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