High double-layer capacitance, electrolyte

High double-layer capacitance. The capacitance C of a capacitor is proportional to the capacitance of an electrode, which is dependent on the type of electrolyte material chosen. An electrolyte material showing a high double-layer capacitance Cd for a given electrode is desired. 

Figure 1-13 displays the experimental dependence of the double-layer capacitance upon the applied potential and electrolyte concentration. As expected for the parallel-plate model, the capacitance is nearly independent of the potential or concentration over several hundreds of millivolts. Nevertheless, a sharp dip in the capacitance is observed (around —0.5 V i.e., the Ep/C) with dilute solutions, reflecting the contribution of the diffuse layer. Comparison of the double layer witii die parallel-plate capacitor is dius most appropriate at high electrolyte concentrations (i.e., when C CH). 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

Shiraishi, S., Kurihara, H., Okabe, K., et al. (2002). Electric double layer capacitance of highly pure single-walled carbon nanotubes (HiPco Buckytubes ) in propylene carbonate electrolytes. Electrochem. Commun., 4, 593-8. 

Progress in the accuracy of micromachined products by EMM started with the introduction of high-frequency ultrashort voltage pulses [8,9]. Thus, dissolution of metal is confined to the resolution of machining limited by the charging time constant, r = ps Ca h, of the double layer. Electrolyte resistance along with electrical double-layer capacitance constitutes an equivalent circuit as shown in Fig. 3.5. 

The EDLCs store charge electrostatically by using reversible adsorption of ions of the electrolyte on to high specific surface area materials, usually activated carbons. The charge separation occurs on polarisation at the electrode-electrolyte interface, which was first described by Helmholtz in 1853 as double layer capacitance. This is mathematically defined as ... 

Electrode impedance is an important parameter while fabricating the microelecffode array. Lower impedance is generally favoured for the stimulation material which enables high charge transfer capabilities. Looking closer to the elecffode-electrolyte interface the electrode impedance is dominated by the double-layer capacitance (Cd) as explained earlier, which is in series with the resistance of the electrolyte (Rg) [43], This acts like a high-pass filter with a cut-off frequency at ... 

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