Electrochemical cell overpotential

When a current I flows in an electrochemical cell, such as the one shown in Fig. 4.1, between the catalyst, or working electrode (W) and the counter electrode (C), then the potential difference Uwc deviates from its open-circuit value U c. The electrochemical cell overpotential t Wcis then defined from ... 

According to the fundamental theory of overpotential for aqueous electrochemical systems, the electrochemical cell overpotential is defined as the potential difference deviahon from the open-circuit value. The electrochemical cell overpotential is then defined as ... 

It must be emphasized that Equations (5.24) and (5.25) stem from the definitions of Fermi level, work function and Volta potential and are generally valid for any electrochemical cell, solid state or aqueous. We can now compare these equations with the corresponding experimental equations (5.18) and (5.19) found to hold, under rather broad temperature, gaseous composition and overpotential conditions (Figs. 5.8 to 5.16), in solid state electrochemistry ... 

V) is actually required to electrolyze water when platinum electrodes are used. Much contemporary research on electrochemical cells involves attempts to reduce the overpotential and hence to increase the efficiency of electrolytic processes. 

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

Hysteresis is not a problem of kinetics (which is treated in Chapter 8). Normally, the energy dissipated in an electrochemical cell varies as the square of the current, so the corresponding overpotential t] is proportional... 

Finite electrolyte conductivities and ionic current flow lead to ohmic voltage components in electrochemical cells. It is constructive at this point to review the effects of ohmic voltage contributions to driven and driving cells in the case of uniform current distributions. It will be shown that for each type of cell, the ohmic resistance lowers the true overpotential at the electrode interface for a fixed cell voltage even in the case of a uniform current distribution at all points on the electrode. 

The cyclic voltammograms for various sweep rates at 550°C are reported in Figure 46. According to the theory of cyclic voltammetry, the peak overpotential shift and peak current convey important information. A reversible redox reaction will not exhibit peak shifting with sweep rate change. In an aqueous electrochemical cell, the shift is generally caused... 

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

The processes taking place in an electrochemical cell during metal deposition can be modeled by a combination of three resistors in series, each associated with an overpotential given by the product of the applied current density and the relevant resistance ... 

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 ... 

Overpotential (q) - In an electrochemical cell, the difference between the potential of an electrode and its zero-current value. 

As explained earlier, an electrochemical cell requires at least two electrodes - an anode and a cathode - to enable a current to flow through it. The rate of electrolysis will depend on the kinetics of the two electrode reactions. It is usually essential to have an overpotential, rj, to increase the rate at which an electrode reaction occurs. The total cell voltage required to bring about chemical changes by electrolysis is given by ... 

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. 

In spite of its limitations for complex systems, the Wagner number gives a good qualitative idea of the current distribution in an electrochemical cell. Indeed, when the Wagner number is small (W << 1), the influence of overpotentials can be neglected and the current distribution is nearly the primary distribution. For larger values (- 0.01 < W < 100), the... 

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