Examples impedance spectroscopy measurements

The electrical properties of liquid crystalline phthalocyanines have been determined for example by complex impedance spectroscopy measurements [10a,72]. Thus it was possible to observe that liquid crystalline phthalocyanines show a slight increase in conductivity when going from the solid to the mesophase. The conductivities of these compounds are low ca.5 x 10 ° S/cm) [75]. Higher conductivities are obtained if crown ether phthalocyanines (CEPcM) are aggregated by adding metal pricate salts (M = K +, Rb +, Cs+ (Trt 10 ° to lO " S/cm)... 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Unfortunately, impedance has amassed quite a few different names. For example, it is common now to see the acronym EIS , meaning electrochemical impedance spectroscopy. Strictly speaking, impedance is not a form of spectroscopy because the sample is not absorbing photons of any kind. A further linguistic problem is that AC impedance is tautologous since the word impedance itself implies an AC measurement. In this present discussion, we will simply say impedance . 

Conventional two-electrode dc measurements on ceramics only yield conductivities that are averaged over contributions of bulk, grain boundaries and electrodes. Experimental techniques are therefore required to split the total sample resistance Rtot into its individual contributions. Four-point dc measurements using different electrodes for current supply and voltage measurement can, for example, be applied to avoid the influence of electrode resistances. In 1969 Bauerle [197] showed that impedance spectroscopy (i.e. frequency-dependent ac resistance measurements) facilitates a differentiation between bulk, grain boundary and electrode resistances in doped ZrC>2 samples. Since that time, this technique has become common in the field of solid state ionics and today it is probably the most important tool for investigating electrical transport in and electrochemical properties of ionic solids. Impedance spectroscopy is also widely used in liquid electrochemistry and reviews on this technique be found in Refs. [198 201], In this section, just some basic aspects of impedance spectroscopic studies in solid state ionics are discussed. 

Dispersion — Frequency dispersion results from different frequencies propagating at different speeds through a material. For example, in the electrochemical impedance spectroscopy (EIS) of a crevice (or porous) electrode, the solution resistance, the charge transfer resistance, and the capacitance of the electric double layer often vary with position in the crevice (or pore). The impedance displays frequency dispersion in the high frequency range due to variations in the current distribution within the crevice (pore). Additionally, EIS measurements in thin layer cells (such as electro chromic... 

These circuits naturally have a frequency-dependent impedance, and it is this that is measured in impedance spectroscopy experiments. The components of the circuit also determine the response of the reaction in the real time domain to any dc perturbation, for example, an electrical pulse or termination of a prior steady current (potential-relaxation experiment). 

Regression problems in impedance spectroscopy may become ill-conditioned due to improper selection of measurement frequencies, excessive stochastic errors (noise) in the measured values, excessive bias errors in the measured values, and incomplete frequency ranges. The influences of stochastic errors and foequency range on regression are demonstrated by examples in this section. The issue of bias errors in impedance measurement is discussed in Chapter 22. The origin of stochastic errors in impedance measurements is presented in Chapter 21. 

To determine numerical values for the different elements of the equivalent circuit they have to be separated, for example, by electrochemical impedance spectroscopy (EIS). Similar to the above-described lock-in measurement a small ac signal of a few mV is superimposed to the electrode potential. The resulting current and its phase shift are then measured as a function of the frequency. Typical impedance spectra of thin oxide films on aluminum are shown in Fig. 17. At high frequencies (10 — lO Hz) the capacitors act as shorts and only the electrolyte resistor determines the impedance, which is typically 10 Ohm for concentrated electrolytes and independent of the electrode. At the lowest frequencies, for example, 10 Hz or below, current flow through the capacitors is impossible and the impedance of the system is given by the sum of the 3 resistors in the current path. The... 

Conductometric sensors measure the change in conductivity of a selective layer in contact with two electrodes upon its interaction with the analyte. Conductometric sensors are often based on field-effect devices. For example, capacitance sensors such as the above-mentioned field-effect capacitor [5] belong to this group. Capacitive detection was also employed in conjunction with imprinted electropolymerized polyphenol layers on gold electrodes [36]. The sensitive layer was prepared by electropolymerization of phenol on the electrode in the presence of the template phenylalanine. The insulating properties of the polymer layer were studied by electrochemical impedance spectroscopy. Electrical leakages through the polymer layer... 

In essence, only 1% of OM in the paint, which has to form a 20 (jim primer layer containing a nanosized complex OM particle network, provides a drastic change in the metal surface behavior it stops corroding. This can be measured, for example, by scanning the voltage potential and by impedance spectroscopy [76]. Here, the nanostructures cause the following macroscopic consequences ... 

---