Factors Affecting Double-Layer Capacitance

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. 

In fact, due to the electrode surface status, electrolyte structure, and their interaction at the interface, different electrode materials and different electrolytes have different double-layer differential capacitances. In particular, when electrolyte ions (both inorganic and organic) are strongly adsorbed on the electrode surface, the differential capacitance of the double-layer is significantly affected. 

For a chosen electrode material, different types and sizes of electrolyte ions have different interactions with the electrode surface, resulting in different strengths of adsorption or for a chosen electrolyte, different electrode materials have different affinities to the electrolyte ion, leading to different 

Capacitances of Carbon Electrode Materials and Electrolytes at Room Temperature 

Materials Specific Surface Area (mCgi) Density (gxm ) Aqueous Electrolyte Eg Fxm Organic Electrolyte F.g Fxm  

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Based on the above discussion, there are a number of factors that affect the double-layer behavior and the corresponding EDL capacitance, such as the concentration and size of ions, the ion-specific adsorption, the ion-solvent interaction, and the solvent in the electrolytes. The thickness of the EDL is typically on the order of several angstroms in aqueous solution. Since the distance separating the charges in an EDL is extremely small, the specific capacitance (capacitance normalized by the effective surface area) of EDL can reach a very high value in the aqueous electrolyte. In contrast, the specific capacitance of a typical parallel-plate capacitor is quite small... 

In conclusion, it appears that few metal-molten salt systems behave in the ideally polarizable sense generally associated with the mercury/aqueous solution interface at 298 K. Possible exceptions include some noble liquid metal/melt systems such as mercury/molten nitrates and lead/molten halides at low temperatures (<773 K), but only when the molten electrolyte is extensively purified. Otherwise, systems need to be analyzed as complex impedances, using ac or pulse techniques, to determine whether the minimum interfacial capacitance is affected by extensive factors, leading to parallel pseudocapacitances and Faradaic components. The range of potentials and measuring frequencies for which the interface approaches ideally polarizable behavior also needs to be established. It now seems clear that the multilayer ionic model of charge distribution at the metal/melt interface is more pertinent to molten media than the familiar double layer associated with aqueous solutions. However, the quantitative theories derived for the former model will have to be revised if it is confirmed that the interfacial capacitance is, indeed, independent of temperature in the ideally polarizable region. 

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