Capacitance pseudo

The electrical double layer has been studied at the interface of acidified (pH = 3) KCIO4 and K2SO4 solutions in contact with an Sn solid drop electrode with an additionally remelted surface (SnDER).616 The E, is independent of ctl as well as of the electrolyte. Weak specific adsorption of CIO4 at SnDER is probable around <7 = 0. This view is supported by the high value of/pz for SnDER/H20 + KCIO4 (fpz = 1 -27). A value of fpz = 0.99 for SnDER/H20 + K2S04 indicates that the surface of SnDER is geometrically smooth and free from components of pseudo-capacitance.616... 

D and fractional exponent a (Table 15) show that the surface of electrochemically polished Cd electrodes is flat and free from components of pseudo-capacitance. The somewhat higher values of D for electrochemically polished high-index planes and for chemically treated electrodes indicate that the surface of these electrodes is to some extent geometrically and energetically inhomogeneous. However, the surface of chemically treated Cd electrodes, in comparison with the surface of mechanically polished or mechanically cut electrodes, is relatively... 

Figure 16. General transmission-line model for a conducting polymer-coated electrode. CF is the faradaic pseudo-capacitance of the polymer film, while Rt and Rt are its electronic and ionic resistance, respectively. R, is the uncompensated solution resistance.

The adsorption pseudo-capacitance, Cg, is dominated by the factor ddjdE and hence a plot of C9 versus E gives direct information about the coverage. Figure 4 shows a Cg-E plot for aniline at mercury in aqueous solu-... 

We found an equivalent electrical circuit that fits best the LixC6 electrode behavior at high frequency. The circuit consists of a resistor R in parallel with a constant phase element (CPE). The latter is defined with a pseudo-capacitance Q and a parameter a with 0< a <1 [6], The impedance of... 

This proportionality to the scan rate is reminiscent of double-layer charging, leading to the appellation pseudo-capacitance, reflecting the fact that a Faradaic type of current is exchanged between the electrode and the molecules attached to the surface. 

In some cases, the kinetics of the redox charge— discharge reactions can proceed almost as quickly and reversibly as EDL charging. Thin film redox electrodes, based on the lithium intercalation/inser-tion principle such as Li4Ti50i2, exhibit high reversibility and fast kinetics. The Ru02 materials deposited on carbon show pseudo-capacitive charge—... 

Chemical capacitance. When the mechanism involves significant involvement of the bulk, accumulation of reactive intermediates not only involves surface species but oxidation and reduction of the bulk. This can be detected as an anomalously high effective capacitance, often referred to as a chemical (or pseudo) capacitance. This capacitance can be as large as 0.1 — 1 F/cm and thus easily detected by current-interruption or impedance techniques. Thus, capacitance is a strong indicator (independent of resistance) as to what degree the interface, surface, and/or bulk are playing in the... 

The peak current is hence proportional to the sweep rate with an adsorption pseudo-capacitance of... 

Chemisorption supposes breaking of chemical bonds in the reactant and formation of bonds with the electrode surface with charge transfer across the interface the nature of this process is pseudo-capacitive [5]. 

Electrochemical processes which pass through a surface intermediate species will exhibit a pseudo-capacitive current response (11) This form of behavior is observable in Figure 1(d). 

Indeed, at high bias (Figure 1(d)) the transient current response appears to be solely determined by this pseudo-capacitance, for the capacitance values determined from these scans ( vlO F) are two orders of magnitude larger than typical semiconductor space charge capacitances (12). 

The metal-water interaction has also been suggested [373] to play a role in determining the extent and strength of hydrogen adsorption. The metal-water interaction is potential dependent in particular it decreases as the potential is made more cathodic. Thus, pseudo-capacitances were observed at higher overpotential than 0.3 V because of the appearance of a considerable amount of adsorbed hydrogen. 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

As shown in Fig. 14, the cathode potential changes abruptly across the H2/air-front. This fact warrants the inclusion of the pseudocapacitance into the previous steady-state kinetic model.12 It is clear that the electrode s pseudo-capacitance can supply protons in transient events and thereby reduce the cathode carbon-support corrosion rate in the case of fast moving H2/air- ronts. Figure 18... 

Therefore, for equal H2/air-front residence times, the pseudo-capacitive model would suggest lower rates of carbon-support oxidation, i.e., lower rates of C02 formation for the stop process if compared to the start process, which is consistent with on-line C02 measurements of the air exiting the cathode flow-field during H2/air-front start-stop events, as shown in Fig. 16. 

The pseudo-capacitive effect can be incorporated in the coupled kinetic and transport model through Eqs. (19) and (20). Here we choose to illustrate the effect through the kinetic model for simplicity. With considering the pseudo-capacitive current density, the kinetic model becomes... 

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

Figure 20. The impact of a faster H2/air-front moving through die anode flow-field. 100% relative carbon loss is defined as the localized damage prediction when no pseudo-capacitance is considered in die model. Significantly less carbon corrosion is expected at the anode inlet region as the speed of H2/air-front increases but much less benefit at the anode outlet region.

Since the time scales for establishing local H2 starvation events are on the order of seconds or 10 s of seconds,11,14 pseudo-capacitive effects will not be important. 

Note that this capacitance is linked to the electron transfer reaction and therefore has a faradaic origin and is not related to the double-layer charging process (this last capacitance corresponds to a pure capacitor see Sect. 6.4.1.5). In this sense, it has been called pseudo-capacitance [56]. The normalized current i/rCv is a ratio of capacitances since, from Eq. (6.161), y ev = (Icv/v)/(Qf F/RT)) = Ccv/Cf [48, 57],... 

A persistent question regarding carbon capacitance is related to the relative contributions of Faradaic ( pseudocapacitance ) and non-Faradaic (i.e., double-layer) processes [85,87,95,187], A practical issue that may help resolve the uncertainties regarding DL- and pseudo-capacitance is the relationship between the PZC (or the point of zero potential) [150] and the point of zero charge (or isoelectric point) of carbons [4], The former corresponds to the electrode potential at which the surface charge density is zero. The latter is the pH value for which the zeta potential (or electrophoretic mobility) and the net surface charge is zero. At a more fundamental level (see Figure 5.6), the discussion here focuses on the coupling of an externally imposed double layer (an electrically polarized interface) and a double layer formed spontaneously by preferential adsorp-tion/desorption of ions (an electrically relaxed interface). This issue has been discussed extensively (and authoritatively ) by Lyklema and coworkers [188-191] for amphifunctionally electrified... 

---