Faradaic pseudocapacitance reactions

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... 

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 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]. 

The enhancement of specific capacitance for carbon materials is generally realized by the usage of pseudocapacitance effects, which depend on the surface functionality of carbon and on the presence of electro-active species (termed supercapacitors) [162-164]. Several modifications (i.e., oxidation of carbon for increasing the surface functionality, formation of carbon/conducting polymers composites or insertion of electroactive particles of transition metals) can be carried out to increase the pseudocapacitance that arises fi om faradaic reactions. However, the enhancement of the capacitance connected with faradaic reactions of surface groups often contributes to an increase in the self-discharge of the capacitor due to the instability of the functionalities [165]. 

The enhancement of the double layer capacitance of carbon electrodes may be connected with the pseudocapacitance effects that appear at the carbon surfece. This is attributed to faradaic reactions involving electron transfer reactions of functional groups at the carbon surface. 

This kind of capacitance, of faradaic and not electrostatic origin, is distinguished from the double-layer capacitance and called pseudocapacitance . It can originate when fast redox reactions take place between the electrode and the electrolyte. In this process, capacitances of 100-400 pF cm can be obtained [9] because the bulk of the electrode is involved, and not only the surface as for EDLC. 

Capacitance of a carbon material can be greatly enhanced through pseudocapacitive effects by additional faradaic reactions. Different possibilities have been considered in order... 

The introduction of different heteroatoms in the carbon network has been already considered for changing the redox properties which can further influence the electrochemical performance [78-83], The foreign atoms modify the electron donor/acceptor character of the graphene layers, and are consequently expected to affect the charging of the electrical double Isyer and to give pseudocapacitance faradaic reactions. 

Pseudocapacitors store charge based on reversible (faradaic) charge transfer reactions with ions in the electrolyte. For example, in a metal oxide (such as RUO2 or I1O2) electrode, charge storage results from a sequence of redox reactions. Electrochemical capacitors (ECs) based on such pseudocapacitive materials will have both faradaic and nonfaradaic contributions. The optimization of both EDLCs and pseudocapacitors depends on understanding how features at the nanoscale (e.g. pore size distribution, crystaUite or particle size) affect ion and electron transport and the fundamental properties of electrochemical interfaces. 

The contribution of faradaic and double-layer capacitances in the response of DG-structured V2O5 can be estimated by comparing the CV curves of devices that use RTIL electrolyte with and without lithium salt, shown in Fig. 5.14b. Since V2O5 does not react with either of the ions of pure RTIL, the lithium-free experiment tests the EDLC response. With lithium salt added, the significant increase in capacitive current and the appearance of peak pairs indicates that redox reactions are taking place. These faradaic processes are kinetically facile and thus considered pseudocapacitive, but phase transitions may occur. Although it is difficult to distinguish between redox and intercalation pseudocapacitance, the latter is likely to be present in DG bicontinuous materials. 

Thin films of oxides and sulfides of transition metals, HS2, LiMn204, LiNi02, LiCo02 with the thickness of 5-15 pm deposited onto supports of A1 or stainless steel were studied as electrodes for PsCs. The electrolyte was the solution of LiAsF in PC. The faradaic process determining pseudocapacitance was the reaction of Li intercalation into the matrix of a transition metal oxide. The capacitance of the studied PsCs was 400 pF/cm for FiNi02 and LiCo02 and 1500 pF/cm for 1182. 

Adsorption pseudocapacitance can come from two-dimensional surface reactions that involve faradaic desorption and adsorption of an electroactive species from the electrolyte at a metal surface [69], One good example is the adsorption/desorption of hydrogen (H) at Pt in the acid solution, following the reaction [41,69]... 

The ESs mentioned above consist of two electrodes with the same type of capacitive materials made from either EDL capacitive materials or pseudocapacitive materials (symmetrical configuration). In order to further increase the operating potential window, energy, and power density, a new type of ES has been developed, which is known as hybrid capacitors. With extensive achievements in this area, various types of hybrid ESs have been developed. Generally, hybrid capacitors utilize both the EDL capacitance and faradaic reaction to store charges. The hybrid capacitors reviewed in this book include (1) ESs based on composite electrodes made from both EDL capacitive materials and pseudocapacitive materials (2) asymmetric ESs with one EDL electrode and another pseudocapacitive or battery-type electrode and (3) asymmetric ESs with one pseudocapacitive electrode and another rechargeable battery-type electrode. 

Similarly, using C03O4 as pseudocapacitive materials in the KOH electrolyte, the surface faradaic reaction is involved with OH- ions adsorption/desorption or inser-tion/extraction accompanied by the charge transfer process. This can be expressed as follows [127,128] ... 

Recently, redox-active electrolytes have been proposed to be used for ESs to provide addition pseudocapacitive contribution, which utilized the faradaic reactions originated from the electrolyte to store charge [905-907]. Therefore, both the electrode materials and electrolytes contribute to the overall capacitance [908]. 

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