Electrodes overvoltage and

The surface layer formation is a crucial phenomenon as it affects the electrode charge transfer rate. Low-conductivity films determine high electrode overvoltages and lower battery performance. The charge transfer resistance can be monitored by impedance spectroscopy measurements, which represent a useful tool for the in-situ characterization of resistive and capacitive processes occurring in different time scales (1 mHz-100 kHz) [80]. 

Electrochemists use the term polarization for the algebraic sum of the electrode overvoltages and the ohmic voltage drop. 

Determination of individual electrode overvoltages and elimination of iR contribution. 

To circumvent high overvoltage and fouling problems encountered with the direct oxidation of NADH at conventional electrode (equation 6-11), much work has been devoted to the development of modified electrodes with catalytic properties for... 

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. 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

Under the condition of band edge level pinning, where the interfacial electron level of electrode relative to the redox electron level of redox particles is imchanged, the level differences of ej - ered and ej. - eqx remain constant irrespective of any change of the electrode potential. Consequently, the anodic transfer current of redox electrons, in(T ), in Eqn. 8—60 is independent of the overvoltage and remains equal to the exchange current ia.o as expressed in Eqn. 8-62 ... 

The transfer of anodic holes is associated with the following three processes the generation and transport of holes in the electrode the hole transfer across the compact layer and the diffusion of redox particles in aqueous solution. The total overvoltage, T], is the sum of the three overvoltages np sc for the generation and transport of holes in the electrode, ria for the transfer of holes across the electrode interface, and ii4ur for the diffusion of redox particles in the solution as defined in Eqn. 10-27 ... 

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). ... 

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. 

Figure 19.17. Overvoltage and distribution of voltage drops in cells (Jtiine, 1985). (a) Overvoltage of hydrogen on some metals, (b) Voltage distribution in two kinds of cells for electrolysis of brine, (c) Variation of voltage distribution with current density in the electrolysis of HC1. (d) Schematic of voltage profile in a bipolar cell with five pairs of electrodes.

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

Overvoltage is additional volt-/ V I age that must be applied in excess of the calculated voltage to carry out an electrolytic reaction. It is most important for those systems involving gases that impede the flow of electrons between electrode surfaces and reactants in solution. 

The electrochemistry of both the oxidized (NAD+) and the reduced (NADH) forms of the cofactor at naked electrodes is complicated by large overvoltages and... 

Numerical simulations reveal that this effective value Rif differs form Re if the measured (simulated) Rif is smaller than Rsw (Fig. 21a). These deviations are connected with the inhomogeneous potential distribution in the vicinity of a microelectrode yielding laterally varying electrode overvoltages for small electrode resistances (Fig. 21b). In microelectrode experiments, however, the ratio Reef / Rm is often large (cf. Sec. 3.4) and hence Ra x R f is usually a reasonable approximation. In liquid electrochemistry, similar effects are discussed in the context of primary (Rei = 0) and secondary (Re > 0) current distributions [268, 269]. 

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