PEMs morphological studies

Efforts of polymer scientists and fuel cell developers alike are driven by one question What specific properties of the polymeric host material determine the transport properties of a PEM, especially proton conductivity The answer depends on the evaluated regime of the water content. At water content above kc, relevant structural properties are related to the porous PEM morphology, described by volumetric composition, pore size distribution and pore network connectivity. As seen in previous sections, effective parameters of interest are lEC, pKa, and the tensile modulus of polymer walls. In this regime, approaches familiar from the theory of porous media or composites (Kirkpatrick, 1973 Stauffer and Aharony, 1994), can be applied to relate the water distribution in membranes to its transport properties. Random network models and simpler models of the porous structure were employed in Eikerling et al. (1997, 2001) to study correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake, and proton conductivity, as will be discussed in the section Random Network Model of Membrane Conductivity. ... 

Morphological study for hybrid/composite manbranes is a very important method to investigate the microstructure and thus correlate with the properties of the composite polyelectrolytes. To better understand the structure-property relationship for PEMs, except for spectral analysis as mentioned earlier, microscopic studies such as field mission (FE) scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AEM) technologies are basically extensively utilized. 

The structure and properties of aliphatic PEMs are of vital importance for electrochemical device performances, which are, of course, the main concern of the researchers. Consequently, there are plenty of characterizations to evaluate aliphatic PEM structure and properties, such as spectral studies, morphological studies, physical properties, and ion conductivity. 

Studies on morphology and conclusions about observed levels of proton conductivity have also been carried out on PEMs other than Nafion and sulfonated poly(ether ketone). These include studies in which phenomenological examinations of relationships between conductivity and observed microstructure were carried out upon polymer systems where acid content was varied but the basic chemical structure was kept constant. In addition, other systems allowed... 

Proton conductivities of 0.1 S cm at high excess water contents in current PEMs stem from the concerted effect of a high concentration of free protons, high liquid-like proton mobility, and a well-connected cluster network of hydrated pathways. i i i i Correspondingly, the detrimental effects of membrane dehydration are multifold. It triggers morphological transitions that have been studied recently in experiment and theory.2 .i29.i ,i62 water contents below the percolation threshold, the well-hydrated pathways cease to span the complete sample, and poorly hydrated channels control the overall transports ll Moreover, the structure of water and the molecular mechanisms of proton transport change at low water contents. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

The structure of PEMs, in particular their phase-separated morphology at nm-scale, has been studied with a number of experimental techniques, including small- and wide-angle X-ray and neutron scattering, infrared and Raman spectra, time-dependent FTIR, NMR, electron microscopy, positron annihilation spectroscopy, scanning probe microscopy, and scanning electrochemical microscopy (SECM) (for a review of this literature see [31]). Structural studies of PEMs have mainly focused on Nafion. A thorough recent review on this particular membrane is provided in [32]. 

Studies of the dynamical behavior of water molecules and protons in PEMs rationalize the influence of random morphology and water uptake on effective physicochemical properties, that is, proton conductivity, water flux, and electro-osmotic drag. 

SAXS, SANS, porosimetry, and water sorption studies provide ample evidence for the dispersion in pore size and the evolution of the pore size distribution in the PEM upon water uptake. The changes in the pore space morphology upon water uptake translate into variations in transport properties of the PEM, as is well known (Eikerling et al., 1997, 2007a, 2008 Kreuer et al., 2004). There is, however, uncertainty regarding the mechanism of these macroscopic swelling phenomena. 

The effective PEM conductivity depends on the random heterogeneous morphology, namely, the size distribution and connectivity of the proton-containing aqueous pathways. Random network model of PEMs was developed in Eikerling et al. (1997). It included effects of the swelling of pores and the evolving connectivity of the pore network upon water uptake. The model was applied to study the dependence of membrane conductivity on water content and temperature. It could rationalize trends in... 

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. 

This section presents a review of atomistic simulations and of a recently introduced mesoscale computational method to evaluate key factors affecting the morphology of CLs. The bulk of molecular dynamics studies in PEFC research has concentrated on proton and water transport in hydrated PEMs (Cui et al., 2007 Devanathan et al., 2007a,b,c Elliott and Paddison, 2007 Jang et al., 2004 Spohr et al., 2002 Vishnyakov and Neimark, 2000, 2001). There has been much less effort in using MD techniques for elucidating structure and transport properties of CLs, particularly in three-phase systems of Pt/carbon, ionomer, and gas phase. 

Nafion membranes were modified by the in situ electrodeposition of polypyrrole inside the membrane pores on the anode side only, in order to prevent the crossover of methanol in the direct methanol fnel cell (DMFC) [86]. The modified membranes were studied in terms of morphology, electrochemical characteristics, and methanol permeability. FTIR and SEM confirmed the presence of the polypyrrole on the anode side of the Nafion membrane. SEM showed the polymer to be present both on the membrane surface and inside the membrane pores. It was found to be deposited as small grains, with two distinct sizes the smallest particles had a diameter of around 100 nm, whereas the larger particles had diameters of around 700 ran. Methanol permeability was determined electrochemically and was shown to be effectively reduced. Controlling the phase through blend or block co-polymer will be a good approach in PEM development for satisfying requirement in various DMFC systems. 

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