Overpotential ultimate

It is usual to operate an aqueous-medium fuel cell under pressure at temperatures well in excess of the normal boiling point, as this gives higher reactant activities and lower kinetic barriers (overpotential and reactant diffusion rates). An alternative to reliance on catalytic reduction of overpotential is use of molten salt or solid electrolytes that can operate at much higher temperatures than can be reached with aqueous cells. The ultimate limitations of any fuel cell are the thermal and electrochemical stabilities of the electrode materials. Metals tend to dissolve in the electrolyte or to form electrically insulating oxide layers on the anode. Platinum is a good choice for aqueous acidic media, but it is expensive and subject to poisoning. 

The apparent agreement with theory has, however, to be taken with caution. All experiments up to now have been made in a relatively small overpotential interval and no more than one cusp, i.e., two slopes, of the log J/rfc curve has been observed. The determination of the number of atoms in the critical nucleus is also quite uninformative as a criterion for the validity of the small cluster model equation. Furthermore it does not lead to any conclusions about the free energy of nucleation or forces of interaction between the cluster atoms themselves, and eventually between them and the substrate which would be the ultimate goal of a nucleation rate study. 

One of the characteristic parameters of any anodic or cathodic reaction is a particular value of the overpotential rj which is arbitrarily named the ultimate overpotentiaL As shown in Fig. 2, at their respective ultimate overpotentials both w-EACOP-c and w-EACOP-a reach their boundary value of zero. 

As follows from Figs. 2 and 3, when either direction of the reaction in Eq. (1) occurs at the corresponding ultimate oveipotential, it is converted to a barrierless process. Hence, at the ultimate overpotential for... 

When the reaction in Eq. (1) occurs in the simplest isothermal cell, with Pc = Pa- 0.5, equal absolute values for the ultimate overpotentials are obtained, in accordance with perfect symmetry in the relationships for the cathodic and anodic directions. When Eqs. (31) are introduced into Eqs. (15), the corresponding ultimate current densities are obtained, representing the highest possible rates of the reaction considered. In the case Pc-Pa- Ju a symmetric with respect... 

It should be emphasized that for the reaction in Eq. (1), under the restrictions already mentioned, a pair of ultimate parameters (Ju,Vu) foj both directions, can alternatively be obtained from the intersection points of the extrapolated Tafel lines at various temperatures, as first observed by Post and Hiskey and schematically presented in Fig. 4. It may be concluded from Fig. 4 that the ultimate overpotentials possess the singular property of being temperature-independent. 

Model Development Returning to our ultimate goal of this chapter to analytically model the polarization curve, we now have an expression that includes the starting equilibrium voltage from the Nernst equation (the top of the waterfall ) and the departure from the waterfall resulting from activation overpotential on the anode and cathode ... 

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