Water transport, Nafion

Xie, G. and Okada, T. 1995. Water transport behavior in Nafion-117 membranes. Journal of the Electrochemical Society 142 3057-3062. 

R.J. Bellows, M.Y. Lin, M. Arif, A.K. Thompson, D. Jacobson, Neutron imaging technique for in situ measurement of water transport gradients within nafion in polymer electrolyte fuel cells. J. Electrochem. Soc. 146, 1099-1103 (1999)... 

It is favorable for fuel cell operation when reduced methanol transport across the membrane is accompanied by proper water management. In particular, a low water crossover from the anode to the cathode is necessary to avoid flooding of the cathode. The dependence of water permeation on the membrane thickness is weak. Only a small decrease in water permeation is observed for the commercial Nafion membranes, whereas the thickness of the recast membranes has no significant influence on the water transport rate. In contrast, the effect of temperature on water permeation is strong. At 65°C, the rates are higher by a factor of 5 compared to those at 25°C. 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

For the application of these membranes to the electrolytic production of chlorine-caustic, other performance characteristics in addition to membrane conductivity are of interest. The sodium ion transport number, in moles Na+ per Faraday of passed current, establishes the cathode current efficiency of the membrane cell. Also the water transport number, expressed as moles of water transported to the NaOH catholyte per Faraday, affects the concentration of caustic produced in the cell. Sodium ion and water transport numbers have been simultaneously determined for several Nafion membranes in concentrated NaCl and NaOH solution environments and elevated temperatures (30-32). Experiments were conducted at high membrane current densities (2-4 kA m 2) to duplicate industrial conditions. Results of some of these experiments are shown in Figure 8, in which sodium ion transport number is plotted vs NaOH catholyte concentration for 1100 EW, 1150 EW, and Nafion 295 membranes (30,31). For the first two membranes, tjja+ decreases with increasing NaOH concentration, as would be expected due to increasing electrolyte sorption into the polymer, it has been found that uptake of NaOH into these membranes does occur, but the relative amount of sorption remains relatively constant as solution concentration increases (23,33) Membrane water sorption decreases significantly over the same concentration range however, and so the ratio of sodium ion to water steadily increases. Mauritz and co-workers propose that a tunneling process of the form... 

The electroosmotic transport coefficient for water through Nafion 295 and 1150 membranes is typical and is shown to be highly dependent on the anolyte concentration to the exclusion of all the other variables studied. The water transport coefficient varies almost linearly with anolyte concentration from 6 to 17 molar caustic, giving 2.9 to 0.8 moles/F, as shown in Figure 3. The sodium ion transport number goes through a maximum of 0.82 eq/F in the 7 to 13 molar caustic range (27). 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

A.G. Winger, R. Ferguson and R. Kunin, The electroosmotic transport of water across permselective membranes, J. Phys. Chem., 1956, 60, 556 B.R. Breslau and I.F. Miller, A hydrodynamic model for electroosmosis, Ind. Eng. Chem., Fundam., 1971, 10, 554-565 T. Okada, G. Xie, O. Gorseth, S. Kjelstrup, N. Nakamura and T. Arimura, Ion and water transport characteristics of Nafion membranes as electrolytes, Electro-chim. Acta, 1998, 43, 3741-3747. 

The protonic conductivity is strongly dependent on the membrane water content. In order to understand the water transport and swelling behaviour of PFSA membranes, we first examine the processes that take place as the membrane sorbs water molecules, focusing on Nafion, for which observations are more readily available. Due to similarities in morphology, other PFSA membranes are expected to exhibit similar behaviour. 

T. Okada, G. Xie, O. Gorseth, S. Kjelstrup, N. Nakamura, and T. Arimura, Ion and Water Transport Characteristics of Nafion Membranes as Electro-l5des, Electrochimica Acta, 43, 3741 (1998). 

The variation in literature values of Dchem has been addressed in a study of water transport though Nafion membranes [222]. Measurements of the diffusion of water were conducted wherein the membrane was exposed to liquid water on one side and dry flowing nitrogen on the other side, and water flux measurements were made across a Nafion 115-H membrane at 80 °C. Based on comparisons of their experimental data, the most accurate expression for the dependence of the Dchem ou water content was exhibited using Fickian diffusion coefficient data obtained from [208]. The following expression was developed, where Dchem has units of cm s ... 

FIGURE 4.8.30. Dependence of NaCl in caustic on the water transport across Nafion membrane [96]. (Reproduced with permission of the Society of Chemical Industry.)... 

NAFION incorporated with Si02, Ti02, or Zr(HP04)2 nanoparticles exhibits enhanced performance in PEMFCs compared to fiUer-free NAFION at elevated temperatures under low RFl. The water transport properties, which were investigated by PFGSE NMR, spin—lattice relaxation Ti, and NMR spectroscopy, revealed at least two distinct water environments. The enhanced water uptake relative to fiUer-free NAFION was attributed to alteration of the pore structure of the membrane [80]. 

Z. Zhang, K. Promislow, J. Martin, H. Wang, B.J. Balcom, Bi-modal water transport behavior across a simple NAFION membrane, J. Power Sources 196 (2011) 8525-8530. 

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. 

G. Q. Lu, F. Q. Liu, and C.-Y. Wang. Water transport through Nafion 112 membrane in DMFC s. Electrochem. Solid State Lett., 8 A1-A4, 2005. 

Besides the catalyst gradient in the catalyst layer, other components such as the hydrophobic agent (PTFE) and proton conductive polymer (Nafion) may also need to be adjusted in order to optimize gas/water transportation and electron/proton transfer. It can be expected that the catalyst layer adjacent to the gas diffusion layer side should be more hydrophobic to ensure much more of the reactants penetrates the inside of the electrode. While near the membrane side, more proton conductive polymer is needed to ensure a continuous network for proton conduction. Therefore, a non-uniform catalyst layer with a decreasing PTFE loading and an increasing Nafion content along the through-plane direction from GDL to membrane should be more efficient. 

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