Morphology polymer electrolytes

Higher-density currents result in water mass-transport limitations and the appearance of additional low-frequency impedance relaxation. This diffusion impedance due to water transport within the polymer electrolyte morphology is often represented by the finite diffusion processes affecting both the anodic (Zq ) and cathodic (Z ) processes (Figure 12-16B). An increase in anodic impedance is often related to partial drying out of the anode/membrane interface. 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

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. 

Cheng, X., Yi, B., Han, M., Zhang, J., Qiao, Y., and Yu, J. Investigation of platinum utilization and morphology in catalyst layer of polymer electrolyte fuel cells. Journal of Power Sources 1999 79 75-81. 

M. Neergat and A. K. Shukla. Effect of diffusion-layer morphology on the performance of solid-polymer-electrolyte direct methanol fuel cells. Journal of Power Sources 104 (2002) 289-294. 

Schematic depiction of the structural evolution of polymer electrolyte membranes. The primary chemical structure of the Nafion-type ionomer on the left with hydrophobic backbone, side chains, and acid head groups evolves into polymeric aggregates with complex interfacial structure (middle). Randomly interconnected phases of these aggregates and water-filled voids between them form the heterogeneous membrane morphology at the macroscopic scale (right).

Fig- 13.29. Distribution of particle sizes in 10 wt. % Pt-C electrocatalyst. Particle sizes are average diameters. (Reprinted from E. A. Ti-cianelli, M. N. Beery, and S. Srinivasan, Dependence of Performance of Solid Polymer Electrolyte Fuel Cells with Low Platinum Loading on Morphologic Characteristics of the Electrodes, J. Appl. Electrochem. 21 601 copyright 1991, Fig. 9. 

Clearly, a fundamental understanding of the key strac-ture/property relationships, particularly membrane morphology and conductivity as a function of polymer electrolyte architecture and water content - both in the bulk hydrated membrane and at the various interfaces within the membrane electrode assembly (MEA), can provide guidance in the synthesis of novel materials or MEA manufacturing techniques that lead to the improvement in the efficiency and/or operating range of PEMFCs. 

Thus, the first task provides a stracture/property relationship between morphology and polymer electrolyte architecture and degree of hydration. The second task aims to determine proton transport as a function of the morphology. Coimecting the two tasks together in sequence then leads to proton transport as a function of polymer electrolyte architecture and degree of hydration. 

K. Onishi, S. Sewa, K. Asaka, N. Fujiwara and K. Oguro, Morphology of electrodes and bending response of the polymer electrolyte actuator, Electrochim. Acta, 2000, 46, 737-743. 

Looking back, the only unequivocal membrane improvement, in spite of all these efforts, has been the reduction of thickness from 200 jjim in 1995 to <50 (jun in 2005. In terms of chemical or morphological modifications at the microstructural level, no definite recommendations could be discerned so far. The focus of the works reviewed herein has been exploring the fundamental relations between micromorphology and transport from micro- to macroscales for prototypical polymer electrolyte membranes and the understanding of their major principles of operation. 

Abstract Chemical structure, polymer microstructme, sequence distribution, and morphology of acid-bearing polymers are important factors in the design of polymer electrolyte membranes (PEMs) for fuel cells. The roles of ion aggregation and phase separation in vinylic- and aromatic-based polymers in proton conductivity and water transport are described. The formation, dimensions, and connectivity of ionic pathways are consistently found to play an important role in determining the physicochemical properties of PEMs. For polymers that possess low water content, phase separation and ionic channel formation significantly enhance the transport of water and protons. For membranes that contain a high... 

It is helpful to use a more restrictive definition that classifies polymers as those materials which are based on covalently bonded chain structures formed by repetition of similar units, in which the chains are of sufficient length to confer on the material some additional properties not possessed by the individual units. Some of these properties are morphological and have a considerable bearing on structure-conductivity relationships in polymer electrolytes, as will be discussed later. 

Figure 1.1 Mixed morphology in polymer electrolytes, schematically showing polymer chains which have aligned themselves into crystalline micelles within an amorphous matrix.

Itaconates of structure II in which n = 1-5 are amorphous polymers. Their Li salt complexes, however, retain the amorphous morphology only when n 2. Even the amorphous electrolytes show low conductivity apparently because of large increases in the Tg upon complexation with Li salts. It is apparent that the amorphous morphology of a polymer electrolyte should be complemented by high polymer fluidity in order to have good conductivity at ambient temperatures. 

The surface morphology of polyaniline was affected remarkably by the addition of polymer electrolyte such as poly(styrenesulphonate), poly(2-acrylamido-2-methyl-l-propanesulphonate) or poly(vinylsulphonate), becoming granular [54]. 

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