Ionomers structure

Bose, A. B., Shaik, R., and Mawdsley, J. Optimization of the performance of polymer electrolyte fuel cell membrane electrode assemblies Roles of curing parameters on the catalyst and ionomer structures and morphology. Journal of Power Sources 2008 182 61-65. 

Mauritz summarized a number of molecular models of ionomer structure, including those pertaining to Nafion, that had had been formulated up to 1996. Within the context of the title of this review, it should be appreciated that the results contribute to the state of understanding only if they are verifiable by careful experimentation. To be sure, theoretical predictions are welcome in the design of experiments and pointing the way toward useful applications. 

The ionomer structure is probably a combination of reacted and unreacted sites that may be part of other "chelated" sites. Thus, it is appropriate to consider an approximate structure such as that given below where all of the acrylate and tin groups are reacted. 

Grady, B.P. Goossens, J.G.P. Wouters, M.E.L. Morphology of zinc-neutralized maleated ethylene-propylene copolymer ionomers structure of ionic aggregates as studied by x-ray absorption spectroscopy. Macromolecules 2004, 37, 8585. 

Considerable progress has been made in the solution theory of poly-electrolytes. However, for the condensed-phase analogs of polyelectrolytes, ionomers, this is not the case. Eisenberg (1) has put forth an initial theory of ionomer structure that contains conceptual formalisms of general use. His theory has been consulted extensively in the work reported here. Ponomarev and Ionova (2) have attempted to construct a sophisticated statistical mechanical model to describe the thermodynamics of ionomers. Recently, Gierke (3) has described a theory of ion transport in the Nafion ionomer based on a specific molecular organization. 

We are attempting to computer model ion transport through Nations by generating sample membrane cubes. These cubes contain the structures determined from our theory of ionomer structure. Once a sample cube is generated, we computer simulate the migration of solvated anions (OH") and cations (Na+) through the membrane sample cube under the potential of an applied field. Electrostatic and diffusion interactions are computed with respect to the explicit structure of the sample... 

Asahi Kasei has developed [13-16] a modified membrane by treating the cathode side surface of the sulfonate ionomer to convert the sulfonate functional groups at and near the surface to carboxylate functional groups. It was found that the carboxyl ionomer of this structure was not sufficiently long-lived in the chlor-alkali environment and more stable carboxyl ionomer structures were developed. 

Spectroscopic data offer a more local view of the ionomer structure, and can be considered complementary to the small-angle scattering methods. Binding of paramagnetic cations in Nafion ionomers has been studied by multifrequency ESR and simulations (76). Self-assembling of ion-containing polymers, as swollen... 

At X > Xs, capillary effects control the equilibration of water with the polymer. In this regime, the values of molecular mobilities of protons and water approach the corresponding values in free bulk water, and hydrodynamic effects control transport phenomena. The PEM conductivity is described well by Equation 2.1. Highly functionalized polymer-water interfaces have a minor impact on transport mechanisms in this regime. An important consequence of this picture is that molecular-level studies of proton transport that account for details of ionomer structure are required strictly only for X <. At X >, it is sufficient to employ the well-established mechanism... 

Key structural characteristics determining these processes are atomistic surface structure and electronic structure of the catalyst, morphology of the pore network, surface structure and wettability of the support, catalyst nanoparticle shape and size, ionomer structure, mixed wettability of the composite layer, and, last but not least, the electrode thickness, Icl-... 

Final microstructures obtained reveal a high sensitivity of carbon agglomeration and ionomer structure formation to the wetting properties of carbon particles and the strength of ionomer-carbon interactions. While ionomer sidechains are confined in hydrophilic domains, with a weak contact to carbon domains, ionomer backbones are preferentially attached to the surface of carbon agglomerates for the given hydrophobic type of C. As expected, the correlation between hydrophilic species... 

A viable approach to optimize the dispersion and structure of ionomer in catalyst layers is, thus, to make the surface of PPG agglomerates sufficiently hydrophilic, as to achieve the favorable orientation of the ionomer film, depicted on the left-hand side of Figure 3.41. Essentially, in this configuration, water exists where it is needed, at the interface with Pt. The ionomer structure is optimally utilized to provide high proton concentration in the interfacial water film. At the same time, the secondary pore space remains hydrophobic and thus water-free. 

Simulations of physical properties of realistic Pt/support nanoparticle systems can provide interaction parameters that are used by molecular-level simulations of self-organization in CL inks. Coarse-grained MD studies presented in the section Mesoscale Model of Self-Organization in Catalyst Layer Inks provide vital insights on structure formation. Information on agglomerate formation, pore space morphology, ionomer structure and distribution, and wettability of pores serves as input for parameterizations of structure-dependent physical properties, discussed in the section Effective Catalyst Layer Properties From Percolation Theory. CGMD studies can be applied to study the impact of modifications in chemical properties of materials and ink composition on physical properties and stability of CLs. 

As discussed in the section Ionomer Structure in Catalyst Layers Redefined in Chapter 3, a theory of composition-dependent effective properties that incorporates recent insights into stmcture formation in CCLs is yet to be developed. At present, the relations presented in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3 do not account for agglomerate formation and skin-type morphology of the ionomer film at the agglomerate surface. Qualitative trends predicted by the simple structure-based catalyst layer theory should be correct, as confirmed by the results discussed in this section. 

One of the major parameters in the cathode catalyst layer affecting the cell performance is the oxygen solubility in the enclosing ionomer. It is important to employ an ionomer structure. 

To address this question we will currently introduce new results regarding the mechanism by which the material absorbs water and providing additional information on how the interior co-continuous (water and ionomer) structure is formed and what it looks like. Furthermore, supporting observations from dielectric spectroscopy measurements are provided as a direct means for... 

As shown in the table, 02 (Pt) increases with increasing RH, which suggests that an increase in the RH will speed up the electrode kinetics of the ORR in PEM fuel cells. This observed trend is consistent with other reported results [4-6,8-11,28,29]. (EPSA)c also increases with increasing RH (Table 8.1). These results can be attributed to the restructuring of the ionomer surface, as suggested by Uribe et al. [29]. The ionic clusters and channels will shrink at a low RH level, and the ionic channels can even collapse at a very low RH. As a result, some of the hydrophobic components in the ionomer structures come into direct contact with the Pt surface at a low RH level [14]. It has also been reported that a hydrophobic interfacial configuration can apparently lead to poor ORR reactivity [29]. 

A breakthrough occurred in the mid 1960s when Du Pont introduced copoly(ethylene/meth-acrylic acid) under the tradename Surlyn these copolymers were partially neutralized with sodium and zinc cations. These modified polyethylenes possess remarkable clarity and tensile properties superior to those of conventional polyethylene. The development of Surlyn was an important factor in stimulating research in ionomers. The Surlyn systems emphasized the versatility of ionomer structures and the unique properties available from the modification of the polyethylene backbone. Many of the features which are peculiar to ionomers were recognized at this time notably, the idea that multi-ion clusters would be formed due to the low dielectric constant of the hydrocarbon matrix. It was only with more detailed X-ray diffraction studies and mechanical property measurements that the morphology of these materials was gradually revealed. 

With high hydration in the electrolyte, a proton hopping, or Grotthuss mechanism [5] is observed, with concomitantly higher effective proton conductivity. In this mode of transport, protons hop from one H3O+ to another along a connected pathway in the ionomer structure. 

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