Polymer electrolytes, charge transport

One of the first and most often cited PEM fuel cell models is the Bernardi-Verbrugge model [5]. This is a one-dimensional model that treats the cathode gas diffusion electrode bonded to a polymer electrolyte and transport of neutral and charged species within. The model results in a set of differential equations, which once solved allow determination of species concentration profiles, spatial dependence of the pressure and potential drop, and identification of various contributions to the total potential drop. The model is based on simplifying assumptions such as ... 

It is known that organoboron halide-imidazole complexes dissociate diming equilibrium 56 however, charges disappear upon dissociation. In such a matrix, mobile ions should not originate from the matrix. Therefore, the polymer electrolytes composed of boron halide-imidazole complexes were considered to be appropriate for selective ion transport. 

Most automotive fuel cells use a thin, fluorocarbon-based polymer to separate the electrodes. This is the proton exchange membrane (PEM) that gives this type of fuel cell its name. The polymer provides the electrolyte for charge transport as well as the physical barrier to block the mixing... 

The structures and charge transport mechanisms for polymer electrolytes differ greatly from those of inorganic solid electrolytes, therefore the purpose of this chapter is to describe the general nature of polymer electrolytes. We shall see that most of the research on new polymer electrolytes has been guided by the principle that ion transport is strongly dependent on local motion of the polymer (segmental motion) in the vicinity of the ion. 

The electrolyte concentration is very important when it comes to discussing mechanisms of ion transport. Molar conductivity-concentration data show conductivity behaviour characteristic of ion association, even at very low salt concentrations (0.01 mol dm ). Vibrational spectra show that by increasing the salt concentration, there is a change in the environment of the ions due to coulomb interactions. In fact, many polymer electrolyte systems are studied at concentrations greatly in excess of 1.0 mol dm (corresponding to ether oxygen to cation ratios of less than 20 1) and charge transport in such systems may have more in common with that of molten salt hydrates or coulomb fluids. However, it is unlikely that any of the models discussed here will offer a unique description of ion transport in a dynamic polymer electrolyte host. Models which have been used or developed to describe ion transport in polymer electrolytes are outlined below. 

We have already discussed ion association in Section 6.2. In that section we referred to evidence for the existence of ion clusters from static techniques such as IR, Raman, EXAFS and X-ray diffraction. In this section we examine ion association from the point of view of dynamics, concentrating in particular on electrochemical measurements which reveal the presence of ion clusters. Because ion association is so intimately connected to the transport of matter and charge through polymer electrolytes, it seems appropriate to consider these two topics in the same section. 

A fundamental fuel cell model consists of five principles of conservation mass, momentum, species, charge, and thermal energy. These transport equations are then coupled with electrochemical processes through source terms to describe reaction kinetics and electro-osmotic drag in the polymer electrolyte. Such convection—diffusion—source equations can be summarized in the following general form... 

A series of articles were published by Ennari et al. on MD simulation of transport processes in Poly(Ethylene Oxide) and sulfonic acid-based polymer electrolyte.136,137 The work was started by the determination of the parameters for the ions missing from the PCFF forcefield made by MSI (Molecular Simulations Inc.), to create a new forcefield, NJPCFF. In the models, the proton is represented as a hard ball with a positive charge. Zhou et al. used the similar approach to model Nation.138 The repeating unit of Nafion (Fig. 17) was optimized using ab initio VAMP scheme. The protons were modeled with hydronium ions. Three unit cell or molecular models were used for the MD simulation. The unit cell contains 5000 atoms 20 pendent side chains, and branched Nafion backbone created with the repeating unit. Their water uptakes or water contents were 3, 13, or 22 IEO/SO3, which correspond to the room temperature water uptakes at 50% relative humidity (RH), at 100% RH, and in liquid water respectively.18 The temperature was initially set at a value between 298.15 and 423.15 K under NVE ensemble with constant particle number, constant volume (1 bar), and constant energy. 

These experiments address a number of issues which are of importance for immobilized polymer layers in general. First of all, the layer structure and thickness is strongly dependent on the nature of the contacting electrolyte. Therefore, in cases where liquid-polymer interfaces are envisaged to allow for particular applications, this interaction needs to be considered in detail. As discussed below in Chapter 5, in order to understand the charge transport properties of these layers the interplay between polymer layer and contacting liquids needs to be considered seriously. 

Cooper pairs which are formed by two electrons.) Although the -> conductivity of the electronically conducting phase is a critical factor in all electrochemical experiments and applications, electrochemists are mostly interested in the ionic charge transport in electrolyte solutions or surface layers [i-iii]. Mixed, electronic and ionic conductivity occurs, e.g., in polymer-modified electrodes [ix], and in many -> solid electrolytes (see also... 

As occurring for thin films of redox polymers, there are three elements of porous electrode behavior crucial to its performance in electrocatalysis the transport of a solution reactant to the catalytic sites within the porous system, the transport of electrolyte charge-balancing ions, and the electron transport across the solid, a process responsible for the regeneration of the initial oxidation state of the catalyst. 

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