Pseudocapacitance faradic

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 energy capacity of ECs arises from either double-layer capacitance for electric doublelayer capacitors (EDLCs) or pseudocapacitance for redox capacitors [2, 3]. The energy storage mechanism of EDLCs is based on non-faradic phenomena in electric double layer formed at an electrode/electrolyte interface. In regard to electrode active materials for EDLCs, carbon materials such as activated carbons have been most widely used [4] because of their reasonable cost, good electrical conductivity, and high specific surface area. However, there is a limitation in their specific capacitance the gravimetric capacitance of most carbon materials does not linearly increase with an increase in the specific surface area above 1,200 m g [5]. 

On the other hand, pseudocapacitance arises from a faradic reaction at an electrode surface as an oxidation or reduction process, which can provide higher capacitance and/or higher power capability than carbon electrodes utilized in conventional EDLCs. Electrode materials that exhibit such pseudocapacitive storage are typically organic materials such as conducting polymers and transition metal oxides [2, 4]. 

To increase the capacitance of ESs, some electrochemically active materials are explored for electrode use to provide much higher pseudocapacitance than double-layer capacitance. Pseudocapacitive charge storage fundamentally differs from the electrostatic mechanism that governs double-layer capacitance. For pseudocapacitance, a faradic charge transfer in the electrode porous layer occurs through a thermodynamically and kinetically favored electrochemical reduction-oxidation (redox) reaction [1]. 

For example, in an acidic medium, carbon electrodes exhibit stronger stability in the negative region due to hydrogen electrosorption [17], In this approach, the carbon anode seems to work well with respect to another pseudocapacitive cathode material with overpotential in the positive regime to boost performance and the potential window. Furthermore, to determine acceptable materials it is important to understand the charge regimes, identify operable characteristics, and be able to quantify resistances due to faradic components in the system. 

In Section 3.2.1, the ideal coupling of double-layer and pseudocapacitance was discussed briefly. In practice, one important factor to consider when looking to optimize capacitance is the interaction between faradic charge transfer and double-layer charge storage. In general, it has been shown theoretically that coupling between the two capacitive types is possible [1,33,34]. 

Analysis based on electrochemical impedance spectroscopy (EIS also called AC impedance spectroscopy) allows estimation of frequency behavior, quantification of resistance, and the ability to model equivalent circuits (ECs) of ES systems. The fundamental EC for a double-layer circuit, as discussed in Chapter 2, contains series resistance and double-layer capacitance. In addition, there is often a faradic parallel resistance from impurities in the carbon. In the pseudocapacitive case, the faradic resistance is a related reciprocal of the overpotential-dependent charge transfer [2,21]. 

As discussed in Chapter 3, two parallel processes within the electrode layer of a pseudocapacitor contribute to the overall capacitance. The first is doublelayer charging and discharging, and the second is from faradic electrochemical reactions. This kind of device is called a pseudosupercapacitor, because the electrode charging-discharging process involves electrochemical reactions giving rise to pseudocapacitance. For an ideal electrode layer inside a... 

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