Electrochemical impedance spectroscopy electrode-electrolyte interfaces

When we begin to investigate an electrochemical system, we normally know little about the processes or mechanisms within the system. Electrochemical impedance spectroscopy (EIS) can be a powerful approach to help us establish a hypothesis using equivalent circuit models. A data-fitted equivalent circuit model will suggest valuable chemical processes or mechanisms for the electrochemical system being studied. From Chapter 1, we know that a fuel cell is actually an electrochemical system involving electrode/electrolyte interfaces, electrode reactions, as well as mass transfer processes. Therefore, EIS can also be a powerful tool to diagnose fuel cell properties and performance. 

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. 

Most often, the electrochemical impedance spectroscopy (EIS) measurements are undertaken with a potentiostat, which maintains the electrode at a precisely constant bias potential. A sinusoidal perturbation of 10 mV in a frequency range from 10 to 10 Hz is superimposed on the electrode, and the response is acquired by an impedance analyzer. In the case of semiconductor/electrolyte interfaces, the equivalent circuit fitting the experimental data is modeled as one and sometimes two loops involving a capacitance imaginary term in parallel with a purely ohmic resistance R. 

Because of the resistance to ion flow at the electrode-electrolyte interface, normal measurement of total ionic conductivity is not possible in polymer electrolytes. In order to overcome this problem the conductivity measurements are carried out by the ac impedance spectroscopy method, which minimizes the effects of cell polarization. The measurements are often made with the electrolyte sandwiched between a pair of electrochemically inert electrodes made of platinum or stainless steel. The detailed methodology of impedance spectroscopy is reviewed thoroughly elsewhere [45-47]. 

Electrical double layer EDI). Favorable electron-transfer capabilities make ionic hquids good conductive media and vahd substitutes for conventional electrolytes. Electrolytic properties of ionic hquids were studied to determine the capacitance-layer thickness relationship of the EDL by electrochemical impedance spectroscopy (EIS). EIS data combined with supporting SFG analysis indicate that the EDL formed by ionic hquids at the electrode-ioitic liquid interface follows the Helmholtz model and corresjtonds to a Helmholtz layer of one ion thickness [35,36]. 

The combination of the electrochemical and SPR techniques can provide multidimensional information on the properties and characteristics of the electrode surface and has proven to be useful. Hence, electrochemical methods, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), which present advantages such as high sensitivity and simplicity, are very effective to monitor the characteristics of electrode/electrolyte interfaces [4]. 

In this section the following principal assumptions were made. First, it was assumed that the surface concentrations and potentials were given by their equilibrium or dc steady state values. In other words, there was supposed to be no barrier preventing or slowing down the transfer of matter across the electrode-electrolyte interface. In general, of course, this will not be true, and the impedance associated with the interface forms a very important aspect of impedance spectroscopy as applied to electrochemical situations. This is in contrast to applications where the only interest lies in bulk effects. The interfacial impedance, due to both the storage and the dissipation of energy, will be addressed in Section 2.1.4. 

Electrochemical methods are well adapted for characterizing the corrosion behavior of coated metals in solution. Because of the high resistance of organic coatings, ac methods are generally more suited than dc polarization methods. In electrochemical impedance spectroscopy (EIC) one measures the response of the coated electrode to a small amplitude ac perturbation as a function of frequency (Chapter 5). The interpretation of the measured frequency response, in principle, requires a physical model. However, for coated metals useful information is more easily obtained by representing the metal-coating-electrolyte interface by an electrical circuit (equivalent circuit). 

The two preceding electroanalytical techniques, one in which the measured value was the current during imposition of a potential scan and the other a potential response under an imposed constant current, owe their electrical response to the change in impedance at the electrode-electrolyte interface. A more direct technique for studying electrode processes is to measure the change in the electrical impedance of an electrode by electrochemical impedance spectroscopy (EIS). To relate the impedance of the electrode-electrolyte interface to electrochemical parameters, it is necessary to establish an equivalent circuit to represent the dynamic characteristics of the interface. 

Electrochemical impedance spectroscopy (EIS), also known as AC impedance spectroscopy, is a very powerful technique for characterizing the behaviors of electrode-electrolyte interfaces. Initially, EIS was used to determine doublelayer capacity subsequently, it has been used for more complicated processes, such as metal corrosion [21-24] and electrodeposition [25-27], and to characterize the electrical properties of materials and interfaces. With the developments in PEM fuel cells during recent years, EIS has been widely used for PEM fuel cell diagnosis and the electrochemical characterization of PEM fuel cell materials and components [17,28-35]. 

Electrochemical Impedance Spectroscopy (EIS) is a powerful nondestructive method to characterize biomaterials. Electrode materials, solution resistance, elec-trode/electrolyte interface impedance, charge transfer resistance, and surface rough-ness/double layer capacitance can be measured and their frequency response properties can be determined in a fast frequency scan. 

An ideal electrode-electrolyte interface with an electron-transfer process can be described using Randle equivalent circuit shown in Fig. 2.7. The Faradaic electron-transfer reaction is represented by a charge transfer resistance and the mass transfer of the electroactive species is described by Warburg element (W). The electrolyte resistance R is in series with the parallel combination of the double-layer capacitance Cdi and an impedance of a Faradaic reaction. However, in practical application, the impedance results for a solid electrode/electrolyte interface often reveal a frequency dispersion that cannot be described by simple Randle circuit and simple electronic components. The interaction of each component in an electrochemical system contributes to the complexity of final impedance spectroscopy results. The FIS results often consist of resistive, capacitive, and inductive components, and all of them can be influenced by analytes and their local environment, corresponding to solvent, electrolyte, electrode condition, and other possible electrochemically active species. It is important to characterize the electrode/electrolyte interface properties by FIS for their real-world applications in sensors and energy storage applications. 

Usually, the ionic conductivity is much smaller than that of the electrons. To determine it, the convenient tool is the complex impedance technique, because it requires very small current (prevents heating) and very small ionic motion. The a.c. method is called electrochemical impedance spectroscopy (EIS) because the impedance spectrum measured in a wide frequency range evaluates the performance of batteries and characterizes the various elements such as electrode, electrolyte and electrolyte/electrode interface. First, let us consider the ionic conductivity in a solid electrolyte. The complex impedance due to the Li-motion is ... 

Thus, in the metal/YSZ systems of solid-state electrochemistry, AC-impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-500 pF cm-2 of superficial electrode surface area and of the order 2-10 pF cm-2 when the electrode roughness is taken into account and, thus, the true metal/gas interface surface area is used, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore AC-impedance spectroscopy... 

The construction of a cell permitting both FTIR measurements and electrochemical impedance measurements at buried polymer/metal interfaces has been described [266]. Ingress of water and electrolyte, oxidation (corrosion) of the aluminum metal layer, swelling of the polymer and delamination of the polymer were observed. A cell suitable for ATR measurements up to 80°C has been described [267]. The combination of a cell for ATR measurements with DBMS (see Sect. 5.8.1) has been developed [268]. It permits simultaneous detection of stable adsorbed species and relatively stable adsorbed reaction intermediates (via FTIR spectroscopy), quantitative determination of volatile species with DBMS and elucidation of overall reaction kinetics. An arrangement with a gas-fed electrode attached to the ATR element and operated at T = 60°C has been reported [269]. In this study, the establishment of mixed potentials at an oxygen consuming direct methanol fuel cell in the presence of methanol at the cathode was investigated. With infrared spec-... 

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