Ionomer water transport

The simple water charmel models can explain the ionomer peak and the small-angle upturn in the scattering data of fhe unoriented samples as well as of the oriented films. Interestingly, the helical structure of backbone segments is responsible for fhe sfabilify of fhe long cylindrical charmels. The self-diffusion behavior of wafer and protons in Nation is well described by the water channel model. The existence of parallel wide channels af high wafer uptake favors large hydrodynamic confributions to electro-osmotic water transport and hydraulic permeation. 

For typical catalyst layers impregnated with ionomer, sizes of hydrated ionomer domains that form during self-organization are of the order of 10 nm. The random distribution and tortuosity of ionomer domains and pores in catalyst layers require more complex approaches to account properly for bulk water transport and interfacial vaporization exchange. A useful approach for studying vaporization exchange in catalyst layers could be to exploit the analogy to electrical random resistor networks of... 

T.A. Zawodzinski, J. Davey, J. Valerio, and S. Gottesfeld. Water transport-properties of various fuel-cell ionomers. Abstracts of Papers of the American Chemical Society 205, 75-PMSE 1993. 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

Copolymers of tetrafluoroethylene and sulfonic acid functional per-fluorinated monomers (e.g., Nafion, Dow s perfluorosulfonic acid (PFSA)) have high water permeability. Water transport through these ionomer membranes has been investigated. The non-Fickian diffusion process is analyzed by a thermodynamic approach. The results provide some useful insights into the behavior of these materials as dehydration membranes. 

To effectively use ionomer membranes for dehydration applications it is necessary to understand water transport in these polymers. Molecular diffusion in swollen polymers does not follow the classical Fickian behavior. Fickian behavior is observed for diffusion of gases at low pressure through rubbery polymers at temperatures well above Tg. Under these conditions permeability is independent of gas pressure. Glassy polymers show pressure dependent permeabilities. These effects disappear at higher pressures and can be explained by dual mode theory. Similarly, permeabilities of vapors such as water in hydrophobic or mildly hydrophilic membranes are independent of water vapor pressure. 

Perluorosulfonated ionomer membranes have high water permeabilities and excellent selectivities for water over most gases and organic liquids. These membranes have been shown to be useful for dehydration applications. Water transport in these membranes is non-Fickian. Water permeability and solubility coefficient vary with water vapor pressure. Concentration dependent thermodynamic diffusion coefficients have been obtained by combining the steady state water permeability data and the equilibrium sorption data. These diffusion coefficients correspond to what would be obtained from an experiment with infinitismal partial pressure drop across the membrane. 

Okada, T. et al. 2002. Ion and water transport characteristics of perfluorosul-fonated ionomer membranes with H and alkali metal cations. Journal of Physical Chemistry B 106 (6) 1267-1273. 

Meier, F. and Eigenberger, G. 2004. Transport parameters for the modelling of water transport in ionomer membranes for PEM fuel ceU. Electrochim. Acta 49 1731-1742. 

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. 

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. 

The result is a thin-film, Nafion -bonded hydrophilic electrode in which the catalyst and ionomer are thoroughly mixed, but which lacks the passage for gas and water transport because it has no hydrophobic agent. So, this electrode is usually made very thin (5-10 pm) to avoid water flooding Fig. 2.14 shows the fuel cell performance of a thin-film electrode prepared by the above method. It can seen that the electrode exhibited a good performance with a Pt loading of just 0.13 mg cm. ... 

The hydrophobicity and hydrophilicity of the GDL and CL play complex and critical roles in water management within PEM fuel cells. Adequate hydrophilicity is necessary for better PEM fuel cell performance and extended lifetime. If the amount of water in the membrane is too low, the membrane conductivity will decrease, as will the fuel cell performance. However, if an electrode is too hydrophilic, the excess water cannot be removed efficiently liquid water floods the electrodes, interfering with mass transport of the reactant. Significant effort has been put into investigating water transport and water balance within PEM fuel cells. Research has found that the hydrophobic properties of electrodes can be controlled by the choice of carbon, the ionomer/ carbon ratios, the content of the hydrophobic agent, and the pretreatment and fabrication procedures. 

Nafion is an ionomer that has a hydrophobic tetrafluoroethylene (TFE) backbone and perfluoro alkyl ether (PFAE) side chains terminated with hydrophilic sulfonate salt [1 ], as shown in Fig. 1. It was introduced by DuPont in 1960 and has been used in a variety of applications such as an ion-selective membrane for the Qilor-Alkali process [5, 6], a water transport membrane for humidifiers, and as a strong acid catalyst [7]. Nafion was introduced as a polymer electrol34e membrane (PEM) in fuel cells in the early 1990s [4, 8]. Compared to hydrocarbon membranes, Nafion was chemically more robust in the fuel cell and the acid form has high proton conductivity. There have been many efforts to find alternative membrane materials... 

Saito, M., Arimura, N., Hayamizu, K., Okada, T. (2004) Mechanisms of ion and water transport in perfluorosulfonated ionomer membranes for fuel cells. The Journal of Physical Chemistry B, 108, 16064-16070. 

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