Surface films, capacitance

The use of galvanostatic transients enabled the measurement of the poten-tiodynamic behavior of Li electrodes in a nearly steady state condition of the Li/film/solution system [21,81], It appeared that Li electrodes behave potentio-dynamically, as predicted by Eqs. (5)—(12), Section III.C a linear, Tafel-like, log i versus T dependence was observed [Eq. (8)], and the Tafel slope [Eq. (10)] could be correlated to the thickness of the surface films (calculated from the overall surface film capacitance [21,81]). From measurements at low overpotentials, /o, and thus the average surface film resistivity, could be measured according to Eq. (11), Section m.C [21,81], Another useful approach is the fast measurement of open circuit potentials of Li electrodes prepared fresh in solution versus a normal Li/Li+ reference electrode [90,91,235], While lithium reference electrodes are usually denoted as Li/Li+, the potential of these electrodes at steady state depends on the metal/film and film/solution interfaces, as well as on the Li+ concentration in both film and solution phases [236], However, since Li electrodes in many solutions reach a steady state stability, their potential may be regarded as quite stable within reasonable time tables (hours —> days, depending on the system s surface chemistry and related aging processes). 

Electrochemical impedance spectroscopy (EIS) provides indirect information about the surface phenomena of all kinds of electrodes [32]. The high-frequency part of impedance spectra of electrodes is usually attributed to surface phenomena such as Li-ion migration through surface films, surface film capacitance, and interfacial charge transfer [33]. However, it should be noted that EES provides very ambiguous information. A special skill, as well as experience, is needed for a reliable assignment of spectral features to the time constants of a complicated electrochentical system such as that of composite electrodes [34]. 

The effects of alternating currents are much less of a corrosion danger than those of direct currents. Experiments on steel have shown that during the positive half wave [34-37] only about 1 % contributes to the dissolution of iron according to Eq. (2-21). The remaining 99% is involved in the discharge of capacitances, of redox systems (e.g., Fe /Fe in surface films) or in the evolution of Oj by... 

Figure 18, taken from Ref. 77, describes several models proposed for the Li electrodes in solutions, their equivalent circuit analogs, and the expected impedance spectra (presented as Nyquist plots). Assuming parallel plate geometry for the solid electrolyte interface, as well as knowledge of the surface species involved from spectroscopy (and thus their dielectric constant, which is around 5 for many surface species formed on Li, including R0C02Li, Li2C03, LiF, ROLi, etc. [186]), it is possible to estimate the surface film s thickness from the electrode s capacitance (calculated from the model fitted to the spectra) ...

It is all but impossible to prepare any semiconductor electrode without some surface film being present. The III/V semiconductors, for example, will normally possess oxide films whose thickness will vary from less than 10 A to more than 40 A after exposure to air and similar observations have been reported for silicon [77], Although the capacitance of these films will normally be considerably larger than that of the depletion layer, the film may affect the a.c. response both by virtue of the analysis leading to eqn. (72) and, if Css becomes sufficiently large, that the impedance of the depletion layer falls to a value comparable with that of the film. If the film has a finite resistivity, which may be ionic in character, then the equivalent circuit takes the form... 

The logo for the 2004 International Symposium on Impedance Spectroscopy, shown in Figure 1, was intended to evoke the lessons of the blind men and the elephant. The multiple loops resemble the Nyquist plots obtained in some cases for the impedance of corroding systems influenced by formation of surface films. The low-frequency inductive loop was deformed to evoke the image of the elephant s trunk, and the capacitive loops resemble the head and body of the elephant. 

Deposition and etching of surface films and removal of a sacrificial layer under the structural film can produce many different shapes. The most common shape is a capacitive structure that is attached to the single-crystal silicon substrate at an anchor and is free to move above the surface (Fig. 5.2.3). 

A corroding metal can be represented by the equivalent circuit, shown in Fig. 5.26, consisting of Ra, which includes the resistance of the electrolyte, the surface films, and circuit leads. The polarization resistance at the interface is in parallel with the double-layer capacitance Cji. 

Thus, a potential-dependent smooth background in IR spectra can be due to modulation of concentration/population of free carries and surface states. In contrast to common capacitance measurements, IR spectra of free carrier can provide information on the space charge capacitance of the semiconductor electrode under accumulation as well as under depletion conditions, without interference from the surface state capacitance. The DA-potential plots under depletion conditions allow measurements of the flat-band potential, while absorption of charge carriers in conducting polymer films can be used for estimating the film conductivity. It is also possible to follow the filling of surface states and relate these energy levels to the chemical composition of the interface. 

Rittersma ZM, Splinter A, Bodecker A, Benecke W (2000) A novel surface-micromachined capacitive porous silicon humidity sensor. Sens Actuators B 68 210-217 Sim J-H, Cho C-S, Kim J-S, Lee J-H, Lee J-H (1998) Eight beam piezoresistive accelerometer fabricated by using a selective porous silicon etching method. Sens Actuators A 66 273-278 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255 52-58... 

To some extent the influence of double layer charging currents may be reduced by subtracting from the I-E curve for the test solution, the l-E curve for the electrolyte solution in the absence of the electroactive species. This technique assumes that the double layer capacitance/potential curve is unchanged by the presence of electroactive species. A better approach may be to use a microelectrode. With solid electrodes, changes in oxidation state of the surface lead to similar distortions of the I-E response and then the problem is even more difficult as the surface film may effect the rate of other electron transfer processes. 

Ellipsometry is an experimental technique employed primarily for measurements of optical spectra of highly absorbing solids and of surface films on solid or liquid substrates. It is used mostly in a reflection mode. In electrochemical interfacial systems, the main use of ellipsometry has been for measurement of optical properties and the thickness of surface films formed on the electrode surface. These observations complement other interfacial information (e.g. electric charge associated with film growth, changes in interfacial capacitance following growth of film and impedance associated with the film conversion process). 

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