Anodes polarisation/overpotential

The anodic polarisation (or, in the example here, overpotential) of the electrode considered... 

To predict the local polarisation in a full-scale cell or stack at any point, its dependence on composition, pressure, and temperature of the gas flowing in the gas channel contacting the electrode must be known. In a large cell, these bulk gas properties vary from one point to the next. Electrode polarisation or overpotential - the difference between the local potential of the electrode under load and the potential at open circuit (equilibrium potential) - is also a local quantity because it depends not only on the bulk gas composition but also on the current density. In a large cell the current is usually distributed nonuniformly, as discussed in Sections 11.2-11.5. Similar to Eq. 7, one can express the local cell voltage under load, i.e., when current is passed, as the thermodynamic cell potential minus three loss terms the ohmic loss, the cathode polarisation, and the anode polarisation ... 

The predictions in Fig. 14 show more detailed stress profiles across the membrane under air/membrane/fuel gradients with initial steam/methane ratio = 0.5, relative degree of oxidation OiOstokh = 0.25, and different values of overpotential (rj — 0, 0.05, 0.1, 0.2 V). These predictions were obtained on combining Eqs. 19 and 23, for linear potential distance dependence across the membrane. The dependence of chemical expansion on oxygen partial pressure was taken from Fig. 6, and effects of anodic polarisation given by Eq. 14. Relevant elastic properties E — 187 GPa and v = 0.334 were taken from ref [102]. These predictions also show that overpotential imposes a gap t] = 0.25 (RT/F) x ln(pO"/p02... 

Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

Stern and Geary on the basis of a detailed analysis of the polarisation curves of the anodic and cathodic reactions involved in the corrosion of a metal, and on the assumption that both reactions were charge-transfer controlled (transport overpotential negligible) and that the /R drop involved in determining the potential was negligible, derived the expression... 

Thus, the polarisation data, cyclic voltammetric results and the a.c. impedance measurements all suggest that, when an Ru02/TiC>2 anode exhibits a high overpotential, this is a direct consequence of the surface depletion of Ru. This is also consistent with the estimated Re values of approximately 20 Q for the failed electrodes, in contrast to the known, much higher specific resistivity of Ti02 of... 

In the description given outlining electrochemical systems in which a current flows, key parameters include the variations of the anodic and cathodic polarisations (or overpotentials if applicable) as a function of current and time. These relationships are generally represented in the form of current-potential curves of an electrode, /= f(E), where E is the voltage between the electrode in question and a reference electrode . The experimental results can also be presented in the form of current density-potential curves. However, when the study concerns the whole electrochemical system and is not just focused on the working electrode, it is best to keep the current-potential representation I... 

If the reaction kinetics of the electrode is assumed to be very rapid, mass transfer and ohmic resistance are the dominant resistances. Assuming a reaction zone that coincides with the electrode-electrolyte interface, the diffusion fluxes in stationary operation can be expressed simply in terms of bulk gas partial pressures and gas-phase diffusivities. This is illustrated schematically in Figure 11.8, which compares anode- and cathode-supported cell designs for the simple case of a H2/O2 fuel cell. The decrease in concentration polarisation at the cathode, rjcc- is obvious in the case of an anode-supported cell, while the model shows that concentration polarisation at the anode, tiac is relatively insensitive to anode thickness. The advantage of the mass transfer-based approach is that analytical expressions are obtained for the polarisation behaviour. These are rather simple if activation overpotential is excluded but may still become elaborate in the case of an internally reforming anode where a number of reactions (discussed in Section 11.3) may occur simultaneously within the pores of the anode. 

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