Overpotentials variation

Mechanical strain of Pd and Pt has been observed [278] to give rise to overpotential variations. Such an effect has been studied to gain insight into the factors responsible for the relationship between surface modifications and electrocatalytic activity. The analysis has shown that, of the various factors scrutinized including the possible variation in the interatomic distance, the most probable one is a change in ionic specific adsorption. 

Figure 26 exhibits the polarization curves in terms of the cathode overpotential variation with current density for the CL27 obtained from the 3-D, single-phase DNS model prediction,25,27 the experimental observation25,27 and the liquid water transport corrected 1-D macrohomogeneous model.27 The polarization curve refers to the cathode overpotential vs. current density curve in the... 

Cyclic voltammetry offers several advantages, which include rapid acquisition of the current-overpotential variations under quasi-steady-state conditions and the ease with which the experiment can be performed. A block diagram of the circuit used for this method is presented in Fig. 4.3.16. 

When a certain amount of a reactant gas is added to an electrode at the open-circuit state, the addition changes the partial pressures of the cell and determines Eqcv according to Eq. 8.13. When the same amounts of reactant gas are added at a polarization state, the voltage is varied by overpotential according to Eq. 8.14. Thus the gap (AVa) between the voltage shift at the open-circuit state (A a) and at a polarization state (AVp a) is overpotential variation due to the addition. 

As the Nemst equation suggests, concentration variations in the electrolyte lead to potential differences between electrodes of the same kind. These potential differences are concentration polarizations or concentration overpotentials. Concentration polarizations can also affect the current distribution. Predicting these is considerably more difficult. If concentration gradients exist, equations 25 and 27 through 29 must generally be solved simultaneously. 

The cathodic overpotential tjc controls the compactness of the polymeric structure included in the constant a of the equation through AH. Any variation in rjc promotes a change in the current required to oxidize the system at any time because a is contained by the two terms of Eq. (43). Figure 42 shows both theoretical and experimental chronoamperograms. 

Taking into account the variation in the oxidized area as a function of the overpotential, and the counter-ion flows, the charge consumed during the potential sweep in those regions where the structure was previously opened under conformational relaxation control, is given by... 

Figure 8.48. Variation of rco2 (B), rHcHO (D)s and current (A) with cathodic overpotential during CH3OH oxidation on Ag/YSZ. T=500°C, inlet po2=5 kPa, and inlet PCH3OH =5 kPa.52 Reprinted with permission from Academic Press.

Wagner, 7 Wolkenstein, 279 Work function and absolute potential, 353 and electrochemical promotion, 138 and cell potential, 138, 218 Helmholtz equation, 24 of metals, 139 measurement of, 138 spatial variations, 222 variation with coverage, 24 Working electrode as catalyst, 9 overpotential of, 123... 

The variation of overpotential with time is given by the expression... 

This equation gives the relationship between the current density i and the charge-transfer overpotential rj in terms of two parameters, the exchange current density Iq and the transfer coefficient a. Eigure 6.7 depicts the variation of the partial current densities and the net current density with overpotential. It can be seen that for large... 

Figure 6.7. Variation of partial current densities (dashed line) and net current density (solid line) with overpotential ry.

The relationship between q and I, the reaction rate of an electrode reaction, is expressed by the Butler-Volmer equation, whose model describes a linear variation of the activation energy with the applied overpotential [22]. Hence,... 

Equation (7.17) is the Tafel equation and expresses the way in which the applied potential difference operates to enhance the reaction rate [22]. Since the unit of q is volts, the units of a and b are also volts a is called the Tafel intercept, that is, the overpotential at 7 = 1 (which depends on the units of 7, A or mA or pA) b is known as the Tafel slope, that is, the variation of q per decade of current. 

In electrochemistiy, it is envisaged that reactions can fall into two classes, depending on the physical nature of the forces that cause their departure from the equilibrium state (see Table 7.14). If the variations are simply due to concentration changes at the interface, the departure from equilibrium may be small (e.g., T deviation from equilibrium can be represented by equations that are thermodynamic in origin. Such a situation arises in electrochemistiy in transport control and the associated concentration overpotential. 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

The variation of the overpotential with the current density for the reaction of hydrogen evolution on a mercury cathode in diluted sulfuric add at 25 °C is ... 

Correspondingly, a typical value for AG°/ES [cf Eq. (9.3)] is 0.5 so that (0r /3 In i) = (2RT/1.5F) = 1.3(RT/F). Although observed values of this coefficient vary from RT/4F to 2RT/F, and sometimes above this, the figure for the majority of electrochemical reactions is very near 2RT/F and thus the formation of the rate— overpotential relation to which this Weiss-Marcus harmonic energy variation theory gives rise is not consistent with experiment (Fig. 9.26). 

In deriving eqn. (80), limitations due to mass transport at the interface were not considered. Strictly speaking, this is not realistic and as the reaction rate increases with overpotential in each direction a variation of the concentrations of reactant and product at the surface operates and concentration polarization becomes more important. Each exponential expression in eqn. (80) must be multiplied by the ratio of surface to bulk concentrations, ci s/ci b. The effect of mass transfer in electrode kinetics has been discussed in Sect. 2.4. 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

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