Ionomer molecule

Here it i s assumed that only excess water causes swelling. The parameter p = y /V is the ratio of partial molar volumes of ionomer molecules and water and v is the number of polar head groups (SO3) per ionomer molecule. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Physical models of fuel cell operation contribute to the development of diagnoshc methods, the rational design of advanced materials, and the systematic ophmization of performance. The grand challenge is to understand relations of primary chemical structure of materials, composition of heterogeneous media, effective material properties, and performance. For polymer electrolyte membranes, the primary chemical structure refers to ionomer molecules, and the composition-dependent phenomena are mainly determined by the uptake and distribuhon of water. 

Note Some protein molecules may be classified as ionomer molecules... 

By using Equation 4, the effective ionic diameters, D, can be estimated. The initial slope of each curve in Figure 1 may be obtained by either a simple graphical method or a curve fitting method. The effective diameter is a measure of the distance of closest approach of the centers of the macrolons and reflects the range of interaction of ionomer molecules with other ionomer molecules. 

As suggested by Uchida et al., solvents used for ink preparation strongly affect the state of ionomer molecules, and may thus influence their distribution in CLs [39,221]. Depending on the dielectric constant e of the solvent, perfluorosulfonate molecules form either a solution (at e > 10), reversed micelles (3<8 < 10), or a precipitate (e < 3). The influence on cell performance of the dielectric constant of the solvent used for ink preparation was confirmed by Fernandez et al. [222]. 

As illustrated in Figure 2.1b, ideal locations of Pt particles are at the true triple-phase boundary, highlighted by the big star. Catalyst particles with nonoptimal double-phase contacts are indicated by the smaller stars. Pt gas interfaces are inactive due to the inhibited access to protons. Bulky chunks of ionomer on the agglomerate surface build the percolating network for proton conduction in secondary pores. Only individual or loosely connected ionomer molecules seem to be able to penetrate the small primary pores. It is unlikely that they could sustain notable proton conductivity. They merely act as a binder. Proton transport inside agglomerates, thus, predominantly occurs via water-filled primary pores, toward Pt water interfaces. 

How does the primary chemical architecture of ionomer molecules determine ionomer aggregation, formation of water containing pathways in the PEM, and water sorption properties of the PEM ... 

One should notice an important dilference between CGMD and DPD techniques. In comparison to the DPD technique, CGMD is essentially a multiscale method (parameters are directly extracted from classical atomistic MD), and it has a different force field handling scheme, as described below. Angular and dihedral interactions, which are ignored in DPD, are included in CGMD to account for the conformational flexibility of ionomer molecules. 

Ionomer molecules aggregate and phase-segregate into hydrophobic polymer domains and hydrophilic water-filled pathways at the nanometer scale. These phenomena are understood on the basis of scattering data (SANS, SAXS, USAXS). However, the interpretation of such data is still controversial. Structural pictures of the membrane morphology, focusing on sizes, distribution, and connectivity of water-filled nanochannels, are still going through revisions. Relatively well established is... 

Molecular simulations yield unrealistic morphologies (pore sizes, shapes, connectivity) if they employ insufficient representations of ionomer molecules. Results of simulations depend on interaction parameters that are provided as input. Parameters have to be acquired from fundamental modeling studies (DFT-based calculations) and experimental studies (e.g., adsorption studies). CGMD simulations offer a sound trade-off of computational efficiency and adequate structural representation. The coarse-grained treatment implies simplification in interactions, which can be systematically improved with advanced force-matching procedures, but it allows simulations of systems with sufficient size and sufficient statistical sampling. Structural correlations, thermodynamic properties and transport parameters of PEMs can be studied. 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

These parameters and conditions determine complex interactions between Pt nanoparticles, carbon support, ionomer molecules, and solvent, which control the catalyst layer formation process. Self-organization of ionomer and carbon/Pt in the colloidal ink leads to the formation of phase-segregated and agglomerated morphologies. The choice of a dispersion medium determines whether ionomer exists in solubilized, colloidal, or precipitated form. This influences the microstructure and pore size distribution of the CL (Uchida et al., 1996). It is believed that mixing of ionomer with dispersed Pt/C catalysts in the ink suspension, prior to deposition to form a CL, enhances the interfacial area of Pt with water in pores and with Nation ionomer. 

---