Water-filled nanopore

The next level in Figure 1.16 shows a single water-filled pore with Pt deposits at pore walls. Typical pore sizes are in the range of 2-20 nm. Ultimately, all electrochemical reactions in PEFC must proceed at Pt/water interfaces, and it can be assumed that a large portion of these interfaces exists in water-filled nanopores. Gas-filled nanopores will not make a contribution to current generation. Single pore effects... 

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

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 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. 

Model OF A Water-Filled Nanopore with Charged Metal Walls... 

Water-filled nanopores with heterogeneous metal-solution interfaces... 

The transport properties of water-filled nanopores inside of agglomerates and the properties of the ionomer film at the agglomerate surface define local reaction conditions at the mesoscopic scale. These local conditions, which involve distributions of electrolyte phase potential, proton density (or pH), and oxygen concentration, determine the kinetic regime, under which interfacial electrocatalytic processes must be considered. Combining this information, a local reaction current can be found, which represents the source term to be used in performance modeling of the cathode catalyst layer. 

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... 

This intermediate dynamics are quite common in the HD-TG signal of homogenous matter as supercooled liquids and glass-formers [8,9], In these materials, this behavior is due to the presence of structural relaxation processes. Also in water filled nanoporous materials, an intermediate rising signal has been revealed in the HD-TG data and it has been addressed to the water flow processes inside the nanopores [10]. 

R. Cucini, A. Taschin, P. Bartolini and R. Torre, Acoustic, thermal and flow processes in a water filled nanoporous glasses by time-resolved optical spectroscopy, J. Mech. Phys. Solids Vol. 58, 2010, pp. 1302-1317. 

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