Electrode, capacitance conditioning

Quantitative measurements are best carried out at much lower light intensities than those responsible for the large effects illustrated in Fig. 5. It is desirable to avoid, as far as possible, the photoelectrochemical oxidation or reduction of the film on the time scale of the measurements and this generally restricts incident power densities to less than 10-4 Wcm"2. Since the photocurrents generated by such low levels of illumination are too small to be measured directly, it is necessary to use a lock-in amplifier in conjunction with a mechanical chopper in the experimental arrangement shown in Fig. 6. The sensitivity of the photocurrent spectrometer is usually determined by the noise current arising from the electrode capacitance and the noise voltage in the system. Under favourable conditions, it is possible to measure photocurrents as small as 10 10 A. For typical illumination inten-... 

Under depletion conditions there is a relation between 1/C f. and the potential, where Csc is the semiconductor electrode capacitance. For n-type semiconductors the following relation is found ... 

Figure 13 shows a typical electrochemical response of graphite and disordered carbon electrodes (a, b, respectively), related to the diffusion and accumulation of hthium in the bulk carbon particles. The differential capacitance of these electrodes is nicely reflected by slow scan cyclic voltammetry. As already discussed in detail [105-107], the peaks in the CV of Figure 13a (4 sets of redox peaks) reflect phase transition tetween Li-graphite intercalation stages (indicated in the figure), and they correspond to the plateaus in Figure 11a Their shape depends on the resolution of these experiments. The resolution of the voltammetric response of these electrodes depends on the thickness of the electrode, the resistance of the surface films, and the potential scan rate [108]. The best resolution in electrochemical studies of these systems is obtained in experiments with single particles [109-110]. Such experiments, however, are difficult and require special apparatus. Using composite electrodes, a condition for meaningful results, is a situation in which the electrodes are thin and the solution reaches the entire active mass, and, in fact, aU the particles interact in parallel with both the current collector and solution species. In such a situation, the composite electrodes can be considered as an array of microelectrodes, and then toe resolution of the measurements and their reliability are high.

The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then measured by comparison or substitution methods in an electric bridge circuit. From these measurements and the dimensions of the specimen, dielectric constant and loss factor are computed. 

The differential capacitance method cannot be used for reactive metals, such as transition metals in aqueous solutions, on which the formation of a surface oxide occurs over a wide potential re ion. An immersion method was thus developed by Jakuszewski et al. 3 With this technique the current transient during the first contact of a freshly prepared electrode surface with the electrolyte is measured for various immersion potentials. The electrode surface must be absolutely clean and discharged prior to immersion.182-18 A modification of this method has been described by Sokolowski et al. The values of obtained by this method have been found to be in reasonable agreement with those obtained by other methods, although for reactive metals this may not be a sufficient condition for reliability. 

Measurement of the differential capacitance C = d /dE of the electrode/solution interface as a function of the electrode potential E results in a curve representing the influence of E on the value of C. The curves show an absolute minimum at E indicating a maximum in the effective thickness of the double layer as assumed in the simple model of a condenser [39Fru]. C is related to the electrocapillary curve and the surface tension according to C = d y/dE. Certain conditions have to be met in order to allow the measured capacity of the electrochemical double to be identified with the differential capacity (see [69Per]). In dilute electrolyte solutions this is generally the case. 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

In the measurements, one commonly determines the impedance of the entire ceU, not that of an individual (working) electrode. The cell impedance (Fig. 12.13) is the series combination of impedances of the working electrode (Z g), auxiliary electrode (Z g), and electrolyte (Z ), practically equal to the electrolyte s resistance (R). Moreover, between parallel electrodes a capacitive coupling develops that represents an impedance Z parallel to the other impedance elements. The experimental conditions are selected so that Z Z g Z g. To this end the surface area of the auxiliary electrode should be much larger than that of the working electrode, and these electrodes should be sufficiently far apart. Then the measured cell impedance... 

The question of the allowed sign of C was and remains a topic of discussion with significant contradictions. We suggest here that a major reason for these contradictions is that theoretical calculations for electrified interfaces are more easily carried out assuming a uniform electrode charge. Most studies have used this condition and, on some occasions, the restriction of cr-control took its toll. And those were exactly the situations where negative capacitance was predicted. 

As was discussed in section 2.1.1, electrocapillarity measurements at mercury electrodes, which have well-defined and measurable areas, allow the double-layer capacitance, CDL, to be obtained as Fm-2. Bowden assumed that the overpotential change at the very beginning of the anodic run in H2-saturated solution was a measure of the double-layer capacity. The slope of the E vs. Q plot in this region was taken as giving 1/CDL, and this gave 2 x 10 5 F. He then assumed that, under these same conditions, the double-layer capacity, in Fm-2, of the mercury electrode is the same. This gave the real surface area of the electrode as 3.3cm 2, as opposed to its geometric area of I cm2. 

The history of the observation of anomalous voltammetry is reviewed and an experimental consensus on the relation between the anomalous behavior and the conditions of measurement (e.g., surface preparation, electrolyte composition) is presented. The behavior is anomalous in the sense that features appear in the voltammetry of well-ordered Pt(lll) surfaces that had never before been observed on any other type of Ft surface, and these features are not easily understood in terms of current theory of electrode processes. A number of possible interpretations for the anomalous features are discussed. A new model for the processes is presented which is based on the observation of long-period icelike structures in the low temperature states of water on metals, including Pt(lll). It is shown that this model can account for the extreme structure sensitivity of the anomalous behavior, and shows that the most probable explanation of the anomalous behavior is based on capacitive processes involving ordered phases in the double-layer, i.e., no new chemistry is required. 

Moreover, despite the many advances in electrochemical measurement and modeling, our understanding of SOFC cathode mechanisms remains largely circumstantial today. Our understanding often relies on having limited explanations for an observed phenomenon (e.g., chemical capacitance as evidence for bulk transport) rather than direct independent measures of the mechanism (e.g., spectroscopic evidence of oxidation/reduction of the electrode material). At various points in this review we saw that high-vacuum techniques commonly employed in electrocatalysis can be used in some limited cases for SOFC materials and conditions (PEEM, for example). New in-situ analytical techniques are needed, particularly which can be applied at ambient pressures, that can probe what is happening in an electrode as a function of temperature, P02, polarization, local position, and time. 

In the second experimental approach, the Cgo molecules are deposited on top of an insulating self-assembled monolayer, thus creating a double barrier tunnel junction coimected in series and sharing an electrode [66, 67]. Under these conditions current steps in the I-V graph are observed, because when a potential is applied the capacitances of each junction has to be charged to a threshold potential before an electron can tunnel through the junction and when it is favorable for an electron to sit in the middle electrode the amount of current that flows through the junctions increases [68],... 

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