Redox pseudocapacitance

The main fault of electrodes based on ECPs is their limited chemical reproducibility and thus insufficient cyclability in aqueous electrolytes. Therefore, replacement of ECPs in PsCs by a redox couple based on organic monomers covalently bound to the carbon support of a highly dispersed carbon electrode is considered (Hashmi SA, et al 2004). Naturally, a synergistic effect between the EDL capacitance charging and the concurrent redox (pseudocapacitive) reaction occurs in this case. The latter, alongside with EDL capacitance, contributes to the overall capacitance of such a composite electrode. 

FIGU RE 1.7 Different types of reversible redox mechanisms that give rise to pseudocapacitance (a) underpotential deposition, (b) redox pseudocapacitance, and (c) intercalation pseudocapacitance. (August)m, V., P. Simon, and B. Dunn. 2014. Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy Environmental Science 7 1597-1614. Reproduced by permission of The Royal Society of Chemistry.)... 

The electrochemical quartz crystal microbalance was utilized to elucidate the deposition and intercalation behavior of PPy-CNT and PPy-chloride thin layer [421]. Interestingly, the capacitive charging current was observed on the massogram at the positive switching potential, and this result confirmed the redox pseudocapacitive behavior of PPy. 

As a final note to this section, instead of the interpretation of the low frequency capacitative region as a faradaic, redox pseudocapacitance, Jakobs et al [104] described Region B as a porous capacitor region. The impedance of a porous capacitor depends on frequency as... 

Tilak et al. [1992] made useful calculations of the impedance of a redox pseudocapacitance, Cp, (e.g. as for RUO2) for a planar electrode in comparison with that for a semiinfinite and an infinite pore in a porous electrode. The results were plotted as Nyquist diagrams (e.g. see Figures 17.14 and... 

Hence there can be a dispersion of the pseudocapacitance with fiequency of a modulation signal or of duration of a potentiostatic pulse. Figure 4.5.38 shows the broad range of dispersion of the redox pseudocapacitance of a RuOa electrode in aqueous H2SO4 at 298 K with AV frequency. 

Therefore admittance data can also be plotted in the complex plane (V versus F with (o implicit). Some researchers choose to display data in terms of the complex capacitance C( o>) here C( a>) = Y j(o)lj(o. The latter type of representation can be useful when examining the electrochemical response of electronically conducting polymer films. The low-frequency redox pseudocapacitance can be read directly from a plot of C" versus C at low frequency. 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

Due to their moderate specific surface area, carbon nanotubes alone demonstrate small capacitance values. However, the presence of heteroatoms can be a source of pseudocapacitance effects. It has been already proven that oxygenated functional groups can significantly enhance the capacitance values through redox reactions [11]. Lately, it was discovered that nitrogen, which is present in carbon affects also the capacitance properties [12]. 

For explaining this decay, it has to be considered that the pseudocapacitance of hydrous oxides like a-Mn02 nH20 is attributed to redox transitions with exchange of protons and/or cations with the electrolyte following the equation12 ... 

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

In Figure 4 the effect of prothrombin on the differential capacity of a PS monolayer is presented. The overall capacitance, which is proportional to the out-of-phase (quadrature) ac current, increases upon interaction with the prothrombin. Moreover, a pseudocapacitance peak (at —0.7 V relative to the N-AgCl electrode) characteristic of cystine-cysteine redox potential appears. The peak potential moves as expected toward more... 

This chapter intends to discuss the fundamental role played by carbons, taking particularly into account their nanotexture and surface functionality. The general properties of supercapacitors are reviewed, and the correlation between the double-layer capacitance and the nanoporous texture of carbons is shown. The contribution of pseudocapacitance through pseudofaradaic charge transfer reactions is introduced and developed for carbons with heteroatoms involved in functionalities able to participate to redox couples, e.g., the quinone/hydroquinone pair. Especially, we present carbons obtained by direct carbonization (without any further activation) of appropriate polymeric precursors containing a high amount of heteroatoms. 

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 (EDL) and (2) a pseudocapacitive contribution which is related with quick faradic charge transfer reactions between the electrolyte and the electrode [7,8], Whereas the redox process occurs at almost constant potential in an accumulator, the electrode potential varies proportionally to the charge utilized dr/ in a pseudocapacitor, which can be summarized by Equation 8.5 ... 

CNTs were also demonstrated to be a perfect support for cheap transition metal oxides of poor electrical conductivity, such as amorphous manganese oxide (a-Mn02 H20) [5,96], The pseudocapacitance properties of hydrous oxides are attributed to the redox exchange of protons and/or cations with the electrolyte as in Equation 8.13 for a-Mn02 H20 [97] ... 

Redox polymer film (pseudocapacitance) Polypyrrole, polythiophene, polyacene... 

Figure 11.4 presents typical linear sweep voltammetry (LSV) data for the ICA trimer film. It is clear from these data that profound changes occur in the redox behaviour of the ICA trimer film on cycling. The freshly prepared film shows a square voltammogram, indicating that the coat has a large pseudocapacitance, with little evidence of redox peaks (Fig. 

Figure 1. Impedance spectra (real vs. imaginary part) of an carbon aerogel and the corresponding equivalent circuit. The RC-circuit parallel to the double layer capacitance (circuit with dotted lines) corresponds to pseudocapacitances due to reversible redox-groups on the carbon surface [10]. The position x = 0 denotes the pore entrance of the cylindrical pore adjacent to the reference electrode. The corresponding frequencies are between 20 kHz and 8.25 mHz (region a to c).

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