Outer potential overpotential

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

For the outer-sphere Co(NH3)63+ reduction, the SERS and current-potential data are closely compatible in that the SERS intensities drop sharply at potentials towards the top of the voltammetric wave where the overall interfacial reactant concentration must decrease to zero. Some discrepancies between the SERS and electrochemical data were seen for the inner-sphere Cr(NH3)sBr2 and Cr(NH3)sNCS2+ reductions, in that the SERS intensities decrease sharply to zero at potentials closer to the foot of the voltammetric wave. This indicates that the inner-sphere reactant bound to SERS-active sites is reduced at significantly lower overpotentials than is the preponderant adsorbate. (15)This suggests that SERS-active surface sites might display unusual electrocatalytic activity in some cases. 

The search for such curved Tafel plots has yielded some well-documented examples where essentially straight Tafel lines are observed, even when slight curvature is predicted from eqn. (37). In particular, this is the case for proton reduction [73] and the outer-sphere reduction of some Cr(III) aquo complexes [34] at mercury electrodes over wide overpotential ranges (> 600 mV). However, the former reaction is not an outer-sphere process with symmetrical reactant and product parabolae to which eqn. (37) should apply, but rather involves the formation of an adsorbed hydrogen atom intermediate. The influence of such a mechanistic feature upon the rate-potential behavior is unclear even now [74]. The Cr(III)/Cr(II) aquo couple at mercury has also been examined over wide ranges of anodic as well as cathodic over-potentials [75]. In contrast to the cathodic behavior, marked... 

The membrane overpotential is related to the fact that an electric field is necessary in order to maintain the motion of the hydrogen protons through the membrane. This field is provided by the existence of a potential gradient across the cell, which is directed in the opposite direction from the outer field that gives us the cell potential, and thus has to be subtracted. The overpotential in membrane is calculated from the potential equation (3.42). 

The symbol for the fuel cell and electrolysis cell is derived from the battery symbol the longer and shorter lines represent, respectively, the cathode and anode, and the dashed line represents the electrolyte. An arrow drawn in the direction of positive current flow points toward an electrolyte with negative charge carriers, as in the manner of the transistor symbol. Galvanic and electrolytic cells are distinguished by the location of the positive terminal a positive terminal at the cathode indicates a galvanic cell, while a positive terminal at the anode indicates an electrolytic cell. The outer box represents the system enclosure, which may or may not be open. The values of potential and overpotential are consistent with Table 2. 

Example 8.2 A pipeline is buried in humid soil has an outer diameter of 30.48 cm (12 inches). If the longitudinal resistance (Ra) and the leakage resistance (Rl) of the structure are 1.31x10 ohm/cm and 3,048 ohm.cm, respectively, calculate a) the characteristic resistance, b) attenuation coefficient, c) the overpotential ratio at 1 Km from a drainage point, and the potential gradient at 1 Km. 

It is well known that anions influence the overpotential much more strongly than the potential of the outer Helmholtz plane. On the other hand. Parsons[385] observed that while equating An to the... 

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