Diffusion overvoltage

Concentration overvoltage (reaction overvoltage and diffusion overvoltage) If a... 

As the resistance of mass transport increases, a diffusion overvoltage becomes significant and the total overvoltage t) is distributed to both interfadal ovmvoltage i1h and difiusion overvoltage T)di r as stressed in Eqn. 8-31 ... 

The diffusion overvoltage according to this model can be evaluated using... 

The concentration changes at the electrode surface due to mass transport limitations are responsible for the concentration overvoltages. When a reduction process takes place (e.g. Zn + + 2e Zn), a concentration of the oxidized species at the electrode surface (Cox,e) lower than that in the bulk makes the current, at a given potential, lower than that in the absence of an ion diffusion limitation, and to achieve the same current value an overvoltage (concentration overvoltage) must be imposed. This concentration (or diffusion) overvoltage can be calculated from Eq. (16) ... 

Figure 6.15 Steady state current density, igg, as a function of for Ag OPD in the system Au(100)/0.1 M AgC104 + 0.5 M HCIO4 at T = 298 K [6.161]. The overvoltage 7 is corrected by ohmic drt and diffusion overvoltage assuming Df - = 2 x 10 cm s and a diffusion layer thickness

Concentration polarization occurs when the rate of transport of reactive species to the electrode surface is not sufficient to maintain the current required by Equation 22-16. When concentration polarization begins, a diffusion overvoltage appears. 

When the mass-transport process cannot meet the demand for reactant, the IK drop in Equation 22-16 becomes smaller than the theoretical value, and a diffusion overvoltage appears that just offsets the decrease in H. Here, we consider an electrolytic cell to which we apply a negative voltage to produce a reduction at the cathode. We assume that the anode is nonpolarized, Thus, with the appearance of concentration polarization, Equation 22-16 becomes... 

The charge transfer coefficient (determined by chronoamperometric elimination of diffusion overvoltage) is... 

Most metal sulfides and certain metal oxides are electronic conductors and are capable of establishing corrosion and galvaific couples in aqueous solutions. Similarly, metals react by well-established kinetic patterns involving corrosion couples displaying charge transfer or diffusion overvoltages or combinations of these depending on the metal. 

Transport of reactants and products, controlled by the diffusion overvoltage... 

This diffnsion overvoltage leads to the Nemst impedance. Operating the fuel cell with oxygen (po, = 1), no diffusion overvoltage for X = 0.5 occnrs. Therefore, the open cell voltage, given by Eq. (86), is very sensitive to the water vapor content in the fuel gas and can be nsed to determine the water vapor in the fnel gas quantitatively. 

The diffusion overvoltage due to the cathode diffusion is then given by ... 

The formnlation of (< 0 in generator convention) is obtained by inverting the formnla for diffusion overvoltage, whose expression is of the type ln(l- / //,TO) ... 

In case of quasi-reversible processes when the rate of the reaction is controlled by both diffusion and the charge transfer steps, it is necessary to assess the influence of such kinetic parameters as the exchange current density (/q) and the charge transfer coefficient (a). Furthermore, additional variants to be considered appear related to different mechanisms of the electrochemical process. Therefore, in the simplest case, it is most convenient to analyze reversible processes when -> oo and only the diffusion overvoltage ri is observed in the system. Despite the fact that characteristics of the latter processes differ quantitatively from the first one, general peculiarities remain the same. 

The difference between surface and bulk concentrations depends on the regularities of mass transport and manifests itself in the fact that the certain diffusion overvoltage arises in the system. Use of the rotating disc electrode (RDE) makes it possible to carry out the experiments under different intensities of forced convection and to control the intensity of diffusive mass transport. This provides the means to eliminate the diffusive part of the overvoltage promoting the determination of kinetic parameters. 

Elimination of diffusion overvoltage becomes readily available when the electrochemical system can be treated as linear. This is true in the neighborhood of the equilibrium potential where the overvoltage is sufficiently low. Let us consider... 

Transport of reactants to the electrode surface (diffusion, migration) diffusion overvoltage, rij)... 

After correcting for the effect of non-negligible fuel utilisation, the cell resistance is still significantly smaller when measured with 20% water in the feed than with 5%. This reflects a gas composition dependence of some of the loss terms in Eq. (2). In reference [45], it is argued that the observed composition dependence is primarily due to the composition dependence of the diffusive losses on the anode side (diffusion overvoltage), and it is shown how one may utilise characteristics obtained with different water vapour/hydrogen ratios to assess the magnitude of the diffusion loss [45]. 

The cathodic current density for the reduction of an oxidized species Ox gets more negative with decreasing potential and finally levels off to a constant value when is reached (Figure 1.23). Introducing the concentration in front of the electrode in Nernst s equation yields Equation 1.121 for the diffusion overvoltage i1d-... 

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