Cathodic Activation Polarisation

I ifliire 9.4 Srliematir of a porous MIRC electrode with possible reaction pathways and involved species for the oxygen reduction reaction for SOh C application (adsorbed oxygen species OJ ad) 0 ,.  

Theoretical aspects of porous MIEC electrodes, both using single-phase and two-phase materials, have been analysed by many authors [18,27,30-34]. While the particulars of the models vary from model to model, general features of the porous MIEC electrodes can be summarised as follows (1) Gaseous species 

While the general features of several of the models are similar, the particular analytical expressions, wherever available, vary widely depending upon the details of a given model. In what follows, some of the equations from the work of Tanner et al for composite cathodes are given to illustrate the role of various parameters [27]. In the low current density limit, over which the Butler-Volmer equation can be linearised, the effective charge transfer or the polarisation resistance (activation polarisation only) for cathode interlayer thickness greater than the critical thickness can be given by [22] 

A well-defined increase of the effective electrolyte surface area can also be achieved by a structured electrolyte surface. Sintering separate 8YSZ particles onto the electrolyte substrate and covering the increased surface area by an electrochemically active thin porous film cathode via metal-organic-deposition 

Eigare 9.6 Cathode electrolyte interface structures (a) standard interface with smooth electrolyte surface and re.sirlcled numher of active reaction sites and (It) structured electrolyte surface with nanoporous MOD thin film cathode layer leading to an enhanced reaction zone with improved performance and duralhlity. 

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Cathodic and anodic activation polarisations, in light of MIEC electrodes, are discussed below. 

Activation polarisation is the voltage drop due to the sluggishness of reactions occurring at the electrode-electrolyte interfaces. Several processes are necessary for electron transfer to take place, especially at the cathode. Because LSM has little ionic conductivity, these processes are localised at the TPBs. Recently, it has become common to use MIEC (composite or single-phase) cathodes to spread the TPB and extend the reaction zones this has had a beneficial effect on reducing the activation polarisation and allowed better SOFC performance at lower temperatures. 

The final terms in Eq. (7), T ca, ilcc. ilAa and tiac- are the cathode activation, cathode concentration, anode activation, and anode concentration polarisations, respectively. In general, their dependence on the current density is nonlinear, although at low polarisation they may be approximated by linear relationships. 

At higher pressure and current density, the diffusion polarisation at the cathode decreases and reversible cell potential increases. At the cathode, increased oxygen and water pressures decrease activation polarisation. As shown in Fig. 2.5, an increase of 44 mV is observed when pressure and temperature are increased by 2.9 atm and 15°C, respectively. An increase in temperature exhibits a beneficial effect on cell performance because the polarisation losses diminish with increased temperature. The relationship between voltage gain and temperature change is given by ... 

Cyclic voltammetry (adsorption, monolayers) Potentiodynamic polarisation (passivation, activation) Cathodic reduction (thickness) Frequency response analysis (electrical properties, heterogeneity) Chronopotentiometry (kinetics)... 

Fig. 1.40 Schematic anodic polarisation curve for a passivatable metal (solid line), shown together with three alternative cathodic reactions (broken line). Open-circuit corrosion potentials are determined by the intersection between the anodic and cathodic reaction rates. Cathode a intersects the anodic curve in the active region and the metal corrodes. Cathode b intersects at three possible points for which the metal may actively corrode or passivate, but passivity could be unstable. Only cathode c provides stable passivity. The lines a, b and c respectively could represent different cathodic reactions of increasing oxidizing power, or they could represent the same oxidizing agent at increasing concentration.

Fig. 1.41 Schematic anodic polarisation curves for a passivatable metal showing the effect of a passivating agent that has no specific cathodic action, but forms a sparingly soluble salt with the metal cation, a without the passivating agent, b with the passivating agent. The passive current density, the active/passive transition and the critical current density are all lowered in b. The effect of the cathodic reaction c, is to render the metal active in case a, and passive...

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Polarisation from an external source may also affect the range of passivity. Cathodic polarisation may depress the potential from the passive to the active region (see Fig. 3.14) and thus care should be taken to avoid contact with any other corroding metal. Anodic polarisation, on the other hand, can stabilise passivity provided that the potential is not increased into the range of transpassivity (see Fig. 3.14) and anodic protection is quite feasible. 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

Polarise all cathodic areas to open circuit potential of most active anode areas. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

MTU describes a double-layer cathode a first layer of lithium-treated NiO and a second layer of cerium-activated lithium cobaltite. The objectives are reduced polarisation resistance and longer life. No performance details are given. 

As more OH radicals are present in the case of an active anode than for an active cathode, chemical etching is also more important. Consequently, the surfaces are smoother than those obtained by cathodic machining [63,120]. However, when using anodic polarisation, the tool-electrode will be anodically dissolved resulting in high tool wear. 

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