Reaction pseudocapacitive

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. 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

The calibrated m/z = 44 and m/z = 60 ion currents were converted into the respective partial reaction faradaic currents as described above, and are plotted in Fig. 13.3c as dashed (m/z = 44) and dash-dotted (m/z = 60) lines, using electron numbers of 6 electrons per CO2 molecule and 4 electrons per formic acid molecule formation. The calculated partial current for complete methanol oxidation to CO2 contributes only about one-half of the measured faradaic current. The partial current of methanol oxidation to formic acid is in the range of a few percent of the total methanol oxidation current. The remaining difference, after subtracting the PtO formation/reduction currents and pseudocapacitive contributions as described above, is plotted in Fig. 13.3c (top panel) as a dotted line. As mentioned above (see the beginning of Section 13.3.2), we attribute this current difference to the partial current of methanol oxidation to formaldehyde. This way, we were able to extract the partial currents of all three major products during methanol oxidation reaction, which are otherwise not accessible. 

In supercapacitors, apart from the electrostatic attraction of ions in the electrode/electrolyte interface, which is strongly affected by the electrochemically available surface area, pseudocapacitance effects connected with faradaic reactions take place. Pseudocapacitance may be realized through carbon modification by conducting polymers [4-7], transition metal oxides [8-10] and special doping via the presence of heteroatoms, e.g. oxygen and/or nitrogen [11, 12]. 

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]. 

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 ... 

The plots we have shown, which are all based on simulated data, serve the purpose of illustrating the principles involved. Tlie results of real experiments are rarely so simple and easy to interpret. This is caused by two types of factors on the one hand, the reaction may not be as simple as is assumed in the model. The formation of adsorbed intermediates, for example, can lead to an adsorption pseudocapacitance. The corresponding equivalent circuit usually has two widely different time constants that show up as two semicircles, which could be partially overlapping. Surface heterogeneities are tantamount to different values... 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

In the treatment which follows, we assume that discharge of the doublelayer capacitance drives the reaction, and therefore use C = in Eq. (41). The effects of changes in coverage of the adsorbed intermediate are then taken into account by combining Eq. (41) with the kinetic equations for steps in the mechanism. In this method, no assumptions need then be made about the equivalent circuit or the nature of the pseudocapacitance, and the transient current during potential decay is not assumed to be equal to the steady-state current. The results then enable all three definitions of [Eqs. (46)-(48)] to be evaluated and compared, as illustrated in Fig. 10. 

Electrochemical capacitors based on carbon materials can be divided into doublelayer capacitors, where only a pure electrostatic interaction between ions and charged electrode surfaces occur, and supercapacitors, based on the occurrence of faradaic pseudocapacitance reactions (see Chapter 10). It is generally assumed that the charge is mainly stored, in porous carbons, in the double layer at the electrode/electrolyte... 

The reversible pseudocapacitance redox reaction might be represented as ... 

The peak current, and indeed the current at each point on the wave, is proportional to V, in contrast to the dependence observed for nemstian waves of diffusing species. The proportionality between i and v is the same as that observed for a purely capacitive current [see (6.2.25)], and this fact has led to some treatments of adsorption in terms of pseudocapacitances (32, 34). The area under the reduction wave, corrected for any residual current, represents the charge required for the full reduction of the layer, that is, nFATQ. The anodic wave on scan reversal is the mirror image of the cathodic wave reflected across the potential axis. For an ideal nemstian reaction under Langmuir isotherm conditions, and the total width at half-height of either the cathodic or anodic... 

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