Surface capacitance

An interesting special application has been proposed by Schlichthorl and Peter.31,41 It aims at deconvolution of electrochemical impedance data to separate space charge and surface capacitance contributions. The method relies on detection of the conductivity change in the semiconductor associated with the depletion of majority carriers in the space charge region via potential-modulated microwave reflectivity measurements. The electrode samples were n-Si(lll) in contact with fluoride solution. 

They used the constant capacitance model (surface capacitance of 18F/m2) to fit the following three sorption reactions to observed absorption edge data ... 

Literature data on the surface capacitance of gold for mercury vary from 0.7 to 1.5 pg Hg(0)/cm2 [15,18,19] the mean value corresponds to about 0.3 nm2 per mercury atom. This density is about twice less than the density of gold atoms in the crystal lattice, i.e., it corresponds to about a half of the monolayer. An increasing of temperature decreases the amount of the adsorbed mercury [14]. 

First results indicate a dependence of the surface capacitance of untreated carbon aerogels on their microstructure. Micro- and mesopores exhibit different storage capacitances (6.6 and 19.4 pF/cm in 1 M sulfuric acid, respectively).. An optimized thermal activation procedure of low density aerogels at 950°C in controlled CO2- atmosphere leads to an increase of the specific surface area and capacitance. On the other hand, the increase of the capacitive current after anodic oxidation in sulfuric acid is caused by electroactive surface groups, while the BET-surface area remains almost constant. 

In table 1 the densities and surface capacitances of various carbon aerogels are shown. The data reveal that the surface capacitance varies from one species to another.. According to Shi et al. the micro- and mesopore surface areas were separated and the total capacitance of the samples was split up in contibutions from both pore species [3]. The correlation can be expressed as... 

Density and surface capacitance of different carbon aerogels pyrolized at 1050°C. The surface capacitance was derived dividing the capacitance calcuted from the impedance data at 8.25 mHz by the BET surface area. 5-10% error must be assumed for the evaluated data. 

To sum up the results the deviation of the surface capacitance is a meaningful supplementation in characterizing the microstructure of porous conducting materials. Beyond that by dividing the total capacitance by the surface capacitance the determination of the surface area of samples with known pore size distribution is possible. This enables especially the characterization of ultra thin carbon films which can not be analyzed by-adsorption measurements because of their low mass. 

Since any change in the chemical composition of both surface and skeleton changes the capacitive behaviour but not necessarily the BET surface area the surface capacitance is an important parameter to control activation procedures on carbon electrodes (see chapter 3.2). 

The increase in both surface capacitance and density of the activated samples account for added functional groups on the surface, as indicated for the samples F and G in... 

According to tab. 2 the surface capacitance after C02-activation of the low density sample (F ) is reduced from 14.1 to 7,2 pF/cm. The value is close to the micropore capacitance derived in the previous chapter and demonstrates the microporous character of the sample, as expected for high surface area carbons [5]. The redox peaks at 0.9 V and 0.2 - 0.3 V appearing in the CV can be ascribed to quinone/hydrochinone-like groups in different chemical enviroments [5j. In addition the time constant of the charging process is almost the same for the activated and non-activated sample. 

The application of electrochemical investigations is an efficient method to characterize the microstructure of porous electrically conducting materials. Modifications in the chemical constitution of the skeleton which do not change the BET values can be identified by the electrochemically derived surface capacitance, as demonstrated in the case of anodically oxidized carbon aerogels. 

The resistor R and capacitor C shown in Table 16.1 can take on different meanings for different electrochemical systems. The resistance may, for example, be associated with the charge-transfer resistance of an electrochemical reaction, with the resistance of an oxide or porous layer, or with the electronic resistance of a semiconductor. The capacitor C may be associated with the double layer for an electrode in electrolyte, with surface capacitance of a film, or with the space-charge region of a semiconductor. The resistor Re may be associated with the Ohmic resistance of the electrolyte or with the frequency-independent resistance of a solid. 

The surface capacitance for the constant capacitance model is 1.06 F/m, The intrinsic constants to be used in the constant capacitance model calculation are tabulated in Problem 9. Intrinsic constants for the double-layer model are listed in the MINTEQ file feo-dim.dbs. You can either enter them individually in PRODEFA2, or simply attach that file to your problem. Assign all phosphate adsorption to the high-energy SOI sites. 

The number of surface states at a given electrode potential can be calculated without model assumptions from the differential surface capacitance, using the relation (35). 

Development of new capabilities is moving more and more in this direction, taking advantage of the power of these techniques to probe local non-averaged structure related properties (spectroscopy) simultaneously with beautiful topographic data. The drive is to show the one-to-one correspondence between structure and property. One such example is a new technique that will map surface capacitive variations that, like the EFM, could be used to evaluate high Tc devices. 

In the MCNS, the microporous nanospheres are interlinked to form a three-dimensional mesoporous network. Their exceptional performance could be also related with the presence of mesopores which provide a rapid mass transport of ions within the electrode, facilitating the charging and discharging of the double layer. This effect has been already consider when studying activated carbons, which surface capacitance was found to increase with the mesopore content The optimal proportion of mesopores for using activated carbons in EDLCs was found to be between 20 and 50% [57]. 

Molar concentration of species [M (molH ), molm ] Surface capacitance [Fm ]... 

Electromagnetic methods such as eddy current, capacitance, microwaves, and terahertz radiation are not traditional inspection methods for composites, but they can be used in some circumstances [36]. Microwaves (300 MHz—300 GHz, 1000—1 mm) and terahertz (300 GHz—3 THz, 1—0.1 mm) are applicable to fiberglass composite inspection and have been successful at the detection of damage and internal features [37—45]. However, electromagnetic radiation at these wavelengths does not penetrate conductive materials. For CFRP, which is mildly conductive, they are only useful for sensing very near the surface. Capacitance measurements can be used to measure dielectric property changes in composites such as moisture uptake or cure condition. 

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