ORR in Water-Filled Nanopores Electrostatic Effects

This section emphasizes the importance of local reaction conditions in nanopores of CLs. In this case, spatially varying charge distributions ions exert a major impact on electrochemical processes at internal pore surfaces. Such charge distributions, occurring in the region of the electrochemical double layer, invalidate the assumption of electroneutrality. In fact, the double layer concept itself becomes meaningless when the nominal thickness of the double layer, that is, the Debye length, is of the same order as the pore radius. 

The importance of proton distribution and transport in water-filled nanopores with charged metal walls is most pronounced in ionomer-free UTCLs (type II electrodes), cf. the main case considered in this section. In either type of CLs, proton and potential distribution at the nanoscale are governed by electrostatic phenomena. 

A model of the ORR in a single water-filled pore with charged walls of Pt will be presented. It affords the definition of an effectiveness factor, by which the performance of any nanoporous CL material could be evaluated. Furthermore, a remarkable conclusion is drawn in view of the coupling of ORR kinetics and metal corrosive dissolution. 

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As for effective kinetic parameters of the ORR that should be used in macroscopic models, it seems reasonable to assume that the reaction order for oxygen concentration will be yo2 = 1 for conditions of interest. The effective transfer coefficient of the ORR, ac, will transition through a sequence of discrete values between 1 and 0.5, as a function of electrode potential. The reaction order for proton concentration, yh+, depends strongly on the adsorption regime and, therefore, a prediction of the value is not trivial. The difference ac - yh+ is a key determinant of electrostatic effects in water-filled nanopores inside of catalyst layers, as discussed in the section ORR in Water-Filled Nanopores Electrostatic Effects. ... 

The model of water-filled nanopores, presented in the section ORR in Water-Filled Nanopores Electrostatic Effects in Chapter 3, was adopted to calculate the agglomerate effectiveness factor. As a reminder, this model establishes the relation between metal-phase potential and faradaic current density at pore walls using Poisson-Nernst-Planck theory. Pick s law of diffusion, and Butler-Volmer equation... 

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