Cathodic overpotential

Thus, irrespective of r.ceii. a thermodynamic parameter, the rate will be controlled by the irreversibility of the reaction, which is reflected in the magnitudes of the anode and cathode overpotentials. 

Electrodeposition deposition of a metal or alloy onto a substrate by electrochemical reduction of its ions from an electrolyte under the application of a cathodic overpotential. 

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

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. 

The ESCR model allows us to derive from both Eq. (51) and the above experimental results an expression for the interchain free volume (od) left inside the polymeric structure after polarization at a given cathodic overpotential rjc ... 

This describes a semilogarithmic dependence between the overpotential for the opening of the polymeric structure (tjN) and the cathodic overpotential (ffc) at which it was closed. The experimental results (Fig. 56) fit Eq. (53). This equation also contains an asymptotic approach to the opening potential (rjN) when the cathodic potential of prepolarization increases. 

Thus, at constant temperature and at a constant sweep rate, the influence of the cathodic overpotential (tjc) on the peak overpotential (t]p) of the voltammogram obtained under conformational relaxation control of the polymeric structure is described by... 

Voltammetry performed at different sweep rates, keeping both the cathodic overpotential and the temperature constant, is predicted to have... 

As also intuitively obvious to electrochemists and as shown in Figures 5.8 to 5.16 the Uwr range of validity of Equation (5.18) is typically 1 V, although wider Uwr ranges of validity have also been reported.9 At high anodic or cathodic overpotentials significant deviations from Eq. (5.18) are observed and this is due to the destmction at one end and saturation at the other end of the effective backspillover ion double layer at the metal/gas interface. 

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.

Murase K, Uchida H, Hirato T, Awakura YJ (1999) Electrodeposition of CdTe films from ammoniacal alkaline aqueous solution at low cathodic overpotentials. J Electrochem Soc 146 531-536... 

The ORR at Pt metal sites has a rate dependence on potential described by a Tafel slope of 120 mV/decade (as determined for Pt model systems at high cathodic overpotential, where Pt is practically surface oxide-free [Parthasarathy et al., 1992a, b]). 

Equation (1.12) describes an ORR rate dependence on potential that derives from two dijferent redox potentials, one affecting the exponential part of the expression and the other affecting the pre-exponential part. The term depending on — q2/h o reflects the lowering of the activation energy at an active metal site by an increase in cathode overpotential, whereas the term depending on ( pt(H20)/Pt-OHads describes the fraction of active metal sites, (1/Z + 1), at some value of . Equation (1.12) suggests... 

A significant steady-state population of reduced surface sites is generated by the cathodic overpotential, as set by the fuel cell load. 

Fig. 22. Cathode overpotential for MCFC-based C02 separator. Inlet C02 = 0.50%, temperature = 915 K.

Fig. 11. The development of local cathodic overpotentials at a segmented horizontal plate electrode in forced laminar convection. [From Hickman (H3).]...

Figure 6.4 Distributions Wox and Wred at equilibrium (left) and after application of a cathodic overpotential.

For high anodic or cathodic overpotentials one of the partial current densities can be neglected ... 

FIGURE 3.6 Cathodic overpotential of some Mn-based perovskites at a current density of 0.1 A cut2 as a function of operating temperature. (From Ishihara, T. et al., J. Electrochem. Soc., 142 1519-1524, 1995. With permission.)... 

---