Impedance technique potential modulation

As with alternating electrical currents, phase-sensitive measurements are also possible with microwave radiation. The easiest method consists of measuring phase-shifted microwave signals via a lock-in technique by modulating the electrode potential. Such a technique, which measures the phase shift between the potential and the microwave signal, will give specific (e.g., kinetic) information on the system (see later discussion). However, it should not be taken as the equivalent of impedance measurements with microwaves. As in electrochemical impedance measurements,... 

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-... 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

The refined method of analysis of co-dependent ER signal data set to obtain described in the last part of the previous section is in principle the combined use of ac impedance and the modulated spectroscopic signal. A similar calculation was reported by Yamada and Finklea and their colleagues [21, 71]. We describe below in detail the procedure for a concerted use of these two potential modulation techniques in the kinetic analysis. 

Potential modulation techniques are used frequently in electrochemistry. The most well-known potential modulation electrochemical technique is a.c. impedance spectroscopy, in which current modulation caused by a potential modulation is analyzed. The potential modulation technique has also been used for in-situ IR spectroscopy (EMIRS and SNIFTIRS), but its use was aimed only to subtract the solution background and to enhance the S/N ratio of the spectram. If the IR signal caused by a potential modulation is analyzed, some information on electrode dynamics could be obtained as in a.c. impedance spectroscopy. 

J. van de Lagemaat, N. G. Park, A. J. Frank, I nfluence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline Ti02 solar cells a study by electrical impedance and optical modulation techniques, J. Phys. Chem. B 2000, 104(9), 2044-2052. 

AC impedance technique is also effective to study the passive oxide. The passive oxide may have semiconductive property, so that the AC potential apphcation will induce the charge modulation in the oxide film. For example, when we consider n-type semi-condnctive oxide film rmder positive bias, i.e., nnder reverse bias condition, as shown in Fig. 8, a depression layer is formed in the oxide film. In the depression layer, space charge is extended. 

Nonfaradaic components associated with the uncompensated resistance between reference electrodes (7 ) and the double layer capacitance (Qi) can be accurately determined by AC impedance measurements. In this technique, a small AC potential perturbation is superimposed to the DC bias, and the resulting AC current is measured as a function of the frequency of modulation. The simplest circuit considered for a polarizable... 

Because most applications of (photo)elec-trochemical systems involve the transfer of electrons across an interface (Sect. 2.1.1), current density-potential techniques are commonly used in (photo)electrochemis-try. In this case, the difference in electrochemical potential of electrons across the interface of interest (accessible via the working electrode - reference electrode potential difference) and the current density through this interface are used as the perturbation and the response (or vice versa). Two approaches can be distinguished. When (quasi) steady state signals are used, one speaks of current density versus potential measurements whereas harmonically modulated signals, superimposed on a bias, are involved in electrochemical impedance spectroscopy (EIS). We introduce these two approaches on the basis of the kinetics of the simple system shown in Fig. 1. 

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