Electrode pseudo-capacitance

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.

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—... 

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]. 

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... 

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... 

For a nonpolarizable double layer, the charges led in or out of the electrode from the outer circuit are equivalent to a capacitance C = dqH/dV). However, this would be a pseudo-capacitance because the charge penetrates the double layer. ads can be calculated from Eq. (10.5). 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

If the ring electrode of the rotating ring-disc system monitors the flux of the oxidized or reduced components of the redox couple, then deconvolution of both Faradaic and pseudo-capacitive currents may be done [85]... 

The surface state capacitance for t i CdTe-electrolyte interface is plotted as a function of electrode potential in Fig. 16 (the minimum was taken as the value at 0.2V NHE). The surface state capacitance decreases in the cathodic direction in the region -0.56 to -2.26V (NHE). Capacitance measurements at cathodic potentials less negative than -0.56V could not be carried out because of the onset of a C02 independent anodic dark current. Assuming (in consistence with other examples of pseudo capacitance behavior) that the capacitance-potential curve is symmetrical with respect to a maximum at -0.66V, the number of surface states was calculaed using the above equation. The number of surface states as a function of electrode potential, on the basis of this assumption, is shown in Fig. 17. Geometric area of the electrode was used to calculate the surface state density. Real surface area may be larger. 

Hydrogel Polymer Electrolytes for Pseudocapacitors and Hybrid ESs As mentioned in Section 2.2 and similar to the research trend in the field of aqueous electrolytes, considerable work has focused on the development of hydrogel electrolytes for ESs with enhanced charge storage capacity, such as pseudocapacitors and hybrid ESs including asymmetric ESs. The electrochemical behavior of the pseudo-capacitive electrode materials is directly influenced by the nature of the electrolytes. 

Wang, J. G., Y. Yang, Z. H. Huang, and F. Kang. 2013. Effect of temperature on the pseudo-capacitive behavior of freestanding Mn02 carbon nanofibers composites electrodes in mild electrolyte. Journal of Power Sources 224 86-92. 

For supercapacitor carbon electrodes, it will be further shown that (1) the developed surface area is responsible of an important electrical double-layer capacitance (2) both the oxygenated and nitrogenated functionalities may be involved in redox reactions with the electrolyte, which enhance capacitance through a pseudo-capacitive contribution. 

In general, two modes of energy storage are combined in electrochemical capacitors (1) the electrostatic attraction between the surface charges and the ions of opposite charge (electrical double layer) (2) a pseudo-capacitive contribution which is related with quick redox reactions between the electrolyte and the electrode [14,15]. Whereas the redox process occurs at almost constant potential in an accumulator, the electrode potential varies proportionally to the charge-exchanged dq m 2L pseudo-capacitor, what can be summarized by formula (Eq. 12.6) ... 

The electrical response of such a system is comparable to that of a capacitor. Being of faradic origin and non-electrostatic, this capacitance is distinguished from the double-layer one and is called pseudo-capacitance. In summary, the electrical double-layer formation is a universal property of a polarized material surface, and pseudo-capacitance is an additional property which depends both on the type of electrode material and electrolyte. Compared to the double-layer normalized capacitance ( 10 pF cm ), it has generally a high value (100-400 pF cm ), because it involves the bulk of the electrode and not only the surface. From a practical point of view, pseudo-capacitance contributes to enhancing the capacitance of materials and their energy density. 

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