Equivalent mass electrode processes

The equivalent mass needed for electrolytic calculations can be found by inspecting the balanced half-reaction for the electrode process. For instance, the reduction of Cu2+ is... 

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. 

The Nyquist plots obtained were fit to multiple equivalent circuits to determine what model works the best. As the authors had already determined earlier the importance of cathode potential and oxygen availability, it could easily be anticipated that the equivalent circuit should include at least one charge transfer and one mass transfer resistance, in addition to the Ohmic resistance between the working and the reference electrodes. Even so, changing the cathode potential and oxygen flow rates clearly confirmed which circuit should be used. We thus suggest that any time EIS measurements are done to understand electrode processes, such studies be performed to confirm the validity of the equivalent circuit used to model the data. 

Figure 11.13 illustrates a basic equivalent circuit to represent a general electrochemical reaction. Rs represents the electric resistance, which consists of the ionic, electronic, and contact resistances. Since the electronic resistance is typically much lower than the ionic resistances for a typical fuel cell MEA, the contribution of the electronic resistance to Rs is often negligible. Cj is the double-layer capacitance associated with the electrode-electrolyte interfaees. Since a fuel cell electrode is three-dimensional, the interfaces include not only Arose between Are surfaces of the electrodes and the membrane but also those between the catalysts and the ionomer within the electrodes. Ret is the resistanee associated with the charge transfer process and is called charge transfer resistanee. Z is called the Warburg impedance it deseribes the resistance arising from the mass transport processes. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

Electrochemical reactions consist of electron transfer at the electrode surface. These reactions mainly involve electrolyte resistance, adsorption of electroactive species, charge transfer at the electrode surface, and mass transfer from the bulk solution to the electrode surface. Each process can be considered as an electric component or a simple electric circuit. The whole reaction process can be represented by an electric circuit composed of resistance, capacitors, or constant phase elements combined in parallel or in series. The most popular electric circuit for a simple electrochemical reaction is the Randles-Ershler electric equivalent... 

A second important property of Eq. (149) is that it provides an estimate of the rate, in terms of a characteristic time 6, associated with mass transfer. Indeed, this is the time 9 needed for a molecule to reach the electrode, that is, to cover the space interval in which the molecular concentration differs from that in the bulk. In transient methods this time is identical to that elapsed since the beginning of the experiment, provided that it is lower than tmax = conv/2D. For steady-state methods, the length to be covered is (Sconv and thus from Eq. (149) it follows that 9 = 5conv/2D. The rate of mass transfer can be defined as 1 /9, since it is obviously equivalent to a first-order process (see Chapter 3 for a demonstration of this point). Yet in light of the previous discussion, it is preferable to think in terms of a characteristic time 9 associated with a given electrochemical method rather than in terms of mass transfer rate, although this intuitive latter notion was extremely worthwhile up to this point. ... 

Does a soil-fluid-chemical system behave as an active electrochemical system or a passive electrical conductor under the influence of a DC electric field This is a fundamental question of significant implications. The evaluation criterion that can be used to differentiate the two systems of completely different nature is vested in Faraday s laws of electrolysis, as the transfer of electrons from the electrodes to the system and vice versa in an ideal electrochemical system is invariably associated with chemical reactions obeying Faraday s laws of electrolysis (Antropov, 1972). The two important fundamental laws of electrolysis can be simply expressed as follows (a) the amount of chemical deposition is proportional to the quantity of electric charges flowing through the system in an electrolytic process, and (b) the masses of different species deposited at or dissolved from electrodes by the same quantity of electric charges are directly proportional to their equivalent weights (Crow, 1979). 

Fig. 14.7 Comparison of an experimental (solid lines) (scan rate 0.1 V s ) cyclic voltammogram obtained when an array of solid trans-Mn microparticles is adhered to a 1 mm diameter GC disk electrode (high-mass ease) that is placed in contact with [C4mim][PF6] and simulated data (open circle) that represents a cyclic voltammogram for the case where tra s-Mn is dissolved in bulk ionic liquid. The simulated voltammogram was calculated for a reversible one-electron-transfer process using D = 9.1x 10 cm s for both trans-Mn and [trans-Mn], electrode area 0.00857 cm, uncompensated resistance (R = 4,500 fl, and temperatine (T) = 293 K and has been normalized to the peak current of the experimentally obtained voltammogram in order to demonstrate the shape and peak potential equivalence of voltammograms obtained from adhered and dissolved material. Adapted with permissimi frtnn Zhang et al.. Anal. Cltem. 2(X)3, 75, 6938-6948 [23]. Copyright 2013, American Chemical Society...

EIS changed the ways electrochemists interpret the electrode-solution interface. With impedance analysis, a complete description of an electrochemical system can be achieved using equivalent circuits as the data contains aU necessary electrochemical information. The technique offers the most powerful analysis on the status of electrodes, monitors, and probes in many different processes that occur during electrochemical experiments, such as adsorption, charge and mass transport, and homogeneous reactions. EIS offers huge experimental efficiency, and the results that can be interpreted in terms of Linear Systems Theory, modeled as equivalent circuits, and checked for discrepancies by the Kramers-Kronig transformations [1]. 

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