Electrochemical cell overvoltage

The electrodes are the typical and most important components of an electrochemical cell - especially the working electrode - which usually decide about the success of an electroorganic synthesis. Electrode materials need a sufficient electronic conductivity and corrosion stability as well as, ideally, a selective electrocat-alytic activity which favors the desired reaction. The overvoltages for undesired reactions should be high, for example, for the decomposition of the solvent water by anodic oxygen or cathodic hydrogen evolution. But, additionally, the behavior of electrodes can show unexpected and incomprehensible effects, which will cause difficulties to attain reproducible results. 

The thermal energy generated or absorbed by an electrochemical cell is determined first by the thermodynamic parameters of the cell reaction, and second by the overvoltages and efficiencies of the electrode processes and by the internal resistance of the cell system. While the former are generally relatively simple functions of the state of charge and temperature, the latter are dependent on many variables, including the cell history. 

Therefore, during the charge process of an electrochemical cell, a potential Fj, which is the sum of the cell equilibrium potential Voc, the cathodic overvoltage ( /e,c) Ihe anodic overvoltage ( /j. g) and the ohmic drop (Eq. (8)),... 

Overpotential, overvoltage, El Excess voltage necessary to produce current in a polarized electrochemical cell. 

Historically, Faraday observed that single-electrode half-cell potentials shifted from their equilibrium values when current passed through electrochemical cells. This deviation is referred to as overpotential or overvoltage. It is generally designated as q and is defined by the relationship ... 

Current flows through the corrosion system (electrochemical cell) only when the redox reaction is not at equihbrium. The difference between the operating electrode potential, E, and the equihbrium potential, e q, is defined as the electrode polarization, AE. Thus, the electrode polarization is a deviation from the equihbrium potential in the presence of current. When a cathodic current is imposed, the potential is displaced to the negative side, causing cathodic polarization to be negative. When an anodic current is apphed, polarization is positive. The electrode polarization and defined nature of the hmiting step is called electrode overpotential or overvoltage. 

The in-depth study of the polarization curve is developed in Chapter 1 for an electrochemical cell. Here we recall a semi-empirical expression of the voltage as a function of the current density, which can be interpreted as the reversible potential less the activation-, concentration- and Ohmic overvoltages ... 

The fist of publications [1-50] covers the period from 1958 to 1990, i.e., up to the very last years of the united Soviet Union. It includes only a small part of the publications and stiU reflects the wide variety of research on solid electrolytes at the IE US AS/IE UD AS and the journals publishing these results. There were many theoretical and experimental studies on electrochemical cells with solid electrolytes [1, 4, 5, 21, 39] and an extensive research on the phase composition of oxide ionic conductors [2] and their electric properties [3, 6, 8, 10, 13, 14, 17, 18, 20, 30, 34, 48]. Many papers were related to practical applications like sohd electrolyte degradation [33, 47] or application limits related to the electronic conductivity of the solid electrolytes [5, 30, 40, 43]. There were many publications on the implementation of different electrodes and on the kinetics of electrode processes [23,27, 31, 35, 36, 45], on investigations of the electrode overvoltage [7, 12, 25, 28], on impedance spectroscopy of solid electrolytes [19, 27], and on isotope exchange research [15,16]. The double layer and electrocapUlarity of solid electrolytes were studied in detail [9, 11, 19, 32, 44]. Systematic studies were performed on the thermo-EMF of different solid electrolytes [22,24,29], the EMF of electrochemical cells with solid electrolytes [26, 39], and the thermodynamics of oxygen in molten copper [41]. Applied research was focused on electrochemical oxygen pumps... 

The various possible electrode reactions at the cathode and at the anode in electrolytic cells have been shown in Table 6.2. It has been pointed before that the outcome of an electrolytic process can be made on the basis of knowledge of electrode potentials and of overvoltages. The selection of the ion discharged depends on the following factors (i) the position of the metal or group in the electrochemical series (ii) the concentration and (iii) the nature of the electrode. Examples provided hereunder deliberate on these aspects. 

The rate of electrochemical reactions is given by the cell current, that is, in principle, it can be controlled independent of the temperature (the required overvoltages are influenced by the temperature, however). But usually, electroorganic conversions include chemical reaction steps and therefore the temperature influence, especially on reaction kinetics and selectivity, is frequently similar to that of pure chemical reactions. Consequently, a constant temperature is desirable to achieve clearly defined conditions for the investigations. 

However, under working conditions, with a current densityj, the cell voltage E(j) becomes smaller than the equilibrium cell voltage eq, as the result of three limiting factors (i) the overvoltages Tia and T],- at both electrodes due to a rather low reaction rate of the electrochemical reactions involved (T] is deflned as the difference between the working electrode potential and the equilibrium potential , so that i = ( + T]), (ii) the ohmic drop J both in the electrolyte and interface resistances e and (iii) mass transfer limitations for reactants and products (Figure 1.2). ... 

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