Electrical conductivity double-layer capacitance

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

The electric double-layer capacitance is almost linear to the accessible surface area of the electrolyte ions. Additionally, the chemically/electrochemically stability, electric conductivity, and adequate commercial price are necessary for the EDLC electrode, so the activated carbons are suitable as practical electrode materials. The EDLC has been commercialized as a memory back-up device since the 1970s because of its high cycle efficiency and its long cycle life. Recently, the EDLC is also being considered to be one of the promising systems for... 

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]. 

The discussion about Equations (2.16) and (2.19) shows that the differential capacitance of the double-layer is mainly dependent on the charge (z ), the electrolyte concentration (C°), the solvent used (s,.), and the temperature (T), but does not depend on the types of electrolytes or electrode materials and their structures. It may be true that as long as an electrode is electrically conductive, the differential capacitance should be similar if other conditions are the same. However, if the electrode is a semiconductive material, the net charge accumulated at the electrode will have a diffuse distribution near the interface at the electrode side. 

It is assumed that the IPMC is composed of an IP film sandwiched between two perfectly conductive metal electrodes. The linearized PNP model is used to describe the dynamics of the electric potential and the concentration of the mobile counterions within the polymer. In the case of the fiat electrodes, by solving the partial differential equation based on the PNP model, the equivalent circuit, which is composed of the following lumped capacitances - the double-layer capacitance, Cj, the bulk capacitance Q, and the bulk conductance, Si (see Fig. 6a) - can be obtained (Aureli and Porfiri 2012). 

Unlike porous amorphous carbons, the high ratio of the external surface area to the total surface area of CNTs provides fast adsorption/desorption of electrolyte ions associated with the process of the formation of the electric double layer due to no ion-sieving effect occurring (Arulepp et al. 2006). Sorption of ions onto external surface area of CNTs makes the double-layer capacitance of CNT-based actuators less dependent on the ionic hquid species (ion dimensions) than the capacitance of amorphous carbon-based actuators, where the ion transport into the pores depends on the pore size and the size of electrolyte ions. The frequency dependence of generated strain has been attributed to the elecfrochemical kinetics different deflection amplitudes are the result of different ionic conductivities of EL species (Imaizumi et al. 2012). 

Sivakkumar and coworker have investigated that electrochemical behavior (especially conducting behavior) CNT/PANI nanocomposites-based cell much better than of that pure polyaniline nanofiber-based cell [46-48]. It is ultimate because of very efifective electric double-layer capacitance in the CNT-based electrode. During the study, it has been found that the current passed by the PANl/CNT composite-based device is higher than that of pure PANI nanofiber, which means that the capacitance of the nanocomposites is larger than that of flie pure PANI nanofiber. This indicates a synergistic effect from the combined contributions from PANI and CNT [46] (Fig. 6). 

High-Frequency Resistance A more sophisticated approach to measurement of the ohmic losses is the HFR measurement, as discussed in the previous section. In this approach, an AC signal is superposed on the DC from the fuel cell. At very high fiequencies, the AC will render the various electrochemical double-layer capacitances to zero, and only the purely ohmic resistance will be measured. Typical frequencies high enough to successfully use this technique are > 1 kHz for PEFCs. This approach only measures the path of least resistance however, and care should be taken to properly understand results. For example, in the electrodes, the HFR will normally measure the electrical resistance only, since it is generally much less than the ionic loss in the mixed conductivity stracture. Therefore, HFR could not generally be used to determine electrode ionomer performance. 

The existence of Galvani potentials between two different conducting phases is connected with the formation of an electric double layer (EDL) at the phase boundary (i.e., of two parallel layers of charges with opposite signs, each on the surface of one of the contacting phases). It is a special feature of such an EDL that the two layers forming the double layer are a very small (molecular) distance apart, between 0.1 and 0.4nm. For this reason EDL capacitances are very high (i.e., tenths of pF/cm ). 

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